Anterior Cruciate Ligament Primary Reconstruction: Diagnosis, Operative Techniques, and Clinical Outcomes

Effect of Anterior Cruciate Ligament and Lateral Structures on Anterior Tibial Translation and Internal Tibial Rotation Limits

The function of the ACL is discussed in Chapter 3 . Additional information regarding the restoration of ACL function related to surgical techniques and graft placement issues are discussed in this section. The ACL is the primary restraint to anterior tibial translation, providing 87% of the total restraining force at 30 degrees of knee flexion and 85% at 90 degrees of flexion. The iliotibial band (ITB), mid medial capsule, mid lateral capsule, medial collateral ligament (MCL), and fibular collateral ligament (FCL) provide a combined secondary restraint to anterior tibial translation. The posteromedial (PM) and posterolateral (PL) capsule structures provide added resistance with knee extension. The secondary restraints may become deficient after an ACL injury or repeat giving-way episodes resulting in increased symptoms.

The ACL resists the coupled motions of anterior tibial translation and internal tibial rotation along with the lateral structures. In our laboratory, Wroble and colleagues measured in ACL-deficient knees the simulated effect of lateral soft tissue injuries on internal tibial rotation and anterior tibial translation. The experimental design consisted of a six degrees of freedom (DOF)-instrumented spatial linkage and loading of the knee joint under defined loads of 100-N anteroposterior (AP), 15 N-m varus-valgus, and 5 N-m internal-external tibial rotation torque. The limits of knee motions were determined in the intact knee and then after sectioning the ACL, ITB, lateral capsule (anterolateral ligament), popliteus tendon, popliteofibular ligament, and PL capsule.

The effect of sectioning the ACL first, followed by the ITB and lateral capsule, is shown in Figure 7-2 . Two distinct responses were measured. In the first set of knees (see Fig. 7-2, A ), there were only moderate increases in anterior tibial translation after ACL sectioning and negligible further increases after sectioning the ITB and lateral capsule. However, in the second set of knees (see Fig. 7-2, B ), much larger increases were noted in anterior tibial translation after sectioning the ACL, and even greater increases were found after sectioning the ITB and lateral capsule.

These findings are compatible with clinical tests after ACL injury in which some knees have only moderate increases in anterior tibial translation, whereas other knees have much larger increases. This concept is discussed in Chapter 3 , using the analogy of the bumper model to simulate the resistance of the secondary restraints on motion limits once the ACL is removed. Clinically, an ACL graft would be expected to be subjected to greater in vivo forces in the second set of knees without the unloading provided by relatively “tight” secondary restraints. The increase in the magnitude of anterior translation and internal rotation in these physiologically loose knees after loss of the anterolateral structures results in a grade III pivot shift phenomena. These knees require a high-strength, securely fixated ACL graft to resist these major displacements, and autografts are highly recommended in these grossly unstable knees, with preference for a bone-patellar tendon-bone (B-PT-B) graft.

The effect of sectioning the lateral and posterolateral structures (PLS) is shown in Figure 7-3 . Sectioning of the FCL or PLS (popliteus tendon, popliteofibular ligament, PL capsule) produced small but statistically significant increases in anterior tibial translation at low flexion angles. The combined ACL-ALS-FCL (where ALS are the anterolateral structures comprising the ITB and anterior and middle portions of the lateral capsule) sectioned knee had large increases in anterior tibial translation, with the greatest changes occurring at higher flexion angles.

The effect of sectioning the ACL and lateral structures on the limits of internal tibial rotation are shown in Figure 7-4 . Sectioning of the ACL produced only a small increase in the final internal rotation limit. Sectioning of the ALS and the FCL produced sequentially larger increases in internal tibial rotation. In select revision knees that have large increases in internal tibial rotation, the data support the role of a lateral extraarticular procedure discussed later to unload the ACL graft and restore stability.

The conclusions from this study support the ACL as the primary restraint to anterior tibial translation. The data show that the ultimate limit for internal tibial rotation is resisted by the lateral extraarticular structures that are tightened by the internal tibial rotation. This is an important concept because clinical and biomechanical studies frequently attempt to measure increases in internal tibial rotation after ACL surgery in an effort to quantify ACL graft function. Accordingly, this approach would not be expected to be successful. When the ITB and lateral capsule are physiologically slack, the FCL and ACL assume increased importance in limiting internal rotation. In summary, ACL function is ideally described by the restraining effect of lateral and medial tibial plateau translation and not by internal rotation limits. A positive pivot shift test represents the effect of abnormal compartment translations and anterior subluxation of both tibiofemoral compartments.

The motions that occur during the pivot shift maneuver are shown in Figure 7-5 . At the beginning of the tests, the lower extremity is simply supported against gravity. After ACL disruption, both anterior tibial translation and internal tibial rotation occur as the femur drops back and externally rotates with subluxation of the lateral tibiofemoral compartment. This position is accentuated as the tibia is lifted anteriorly. At approximately 30 degrees of knee flexion, the tibia is pushed posteriorly, reducing the tibia into a normal position with the femur. From position C to position A (see Fig. 7-5, B ), the knee is extended to again produce the subluxated position.

The effect of the ACL in providing rotational stability is shown in Figure 7-6 . After ACL sectioning, there is an increase in medial and lateral compartment translation as the center of rotation shifts from inside the knee joint to outside the medial compartment. The medial ligamentous structures influence the new center of rotation. With injury to the medial ligament structures, the center of rotation shifts so far medially that a tibial rotational displacement is essentially lost, resulting in a gross anterior subluxation of both compartments. The data show the important surgical concept of restoring injured medial and lateral ligament structures to restore anterior knee stability in conjunction with the ACL.

There have been numerous in vitro and in vivo studies published over the past decade on ACL function in regard to providing rotational knee stability. Based on the results of these studies, authors have provided recommendations for ACL single- and double-bundle graft reconstructions, graft placement, and tensioning. These investigations involved a simulated Lachman test and, more importantly, the pivot shift test, which correlates with patient subjective giving-way symptoms. In our opinion, there are problematic errors in many studies that the surgeon should be aware of in terms of application of the results to surgical treatment of ACL ruptures. For example, nearly all published studies describe rotational knee stability according to the changes in degrees of internal tibial rotation, without measuring the center of tibial rotation or the position or subluxation of the tibiofemoral compartments. We believe that providing only the changes in degrees of tibial rotation, without specification of the medial and lateral tibiofemoral compartment position or subluxation, is an inadequate descriptor of rotational knee stability. In fact, after ACL rupture, there is a change of only a few degrees of internal tibial rotation as long as the lateral extraarticular structures remain intact as already described.

A second problem we have identified in a majority of in vivo and in vitro pivot shift studies is the loading profile used to induce anterior tibial subluxation. Studies commonly report, in the pivot shift simulation, an abnormal anterior tibial translation after ACL sectioning that is only half that obtained in the Lachman test. However, under clinical conditions, a positive pivot shift test produces greater anterior tibial subluxation than the Lachman test. Accordingly, these prior studies were conducted under low knee displacement conditions in which the loading profile involved an internal rotation-valgus condition. There was an absence of a simultaneous combined anterior tibial loading, which is the major reason the clinical pivot shift test induces anterior tibial subluxation. A prior published study from our institution that studied orthopedic surgeons performing a pivot shift test on an instrumented lower extremity reported that two types of clinical pivot shift loadings were induced: (1) a predominant internal rotational-valgus torque; and (2) a combined anterior tibial loading with internal rotation-valgus torque. The combined pivot shift loading test produced the greatest amount of anterior tibial subluxation of both tibiofemoral compartments. The predominant internal rotational-valgus torque loading constrained anterior tibial subluxation by reducing the medial tibiofemoral compartment translation.

For reasons not apparent, pivot shift loading with a predominant internal rotation-valgus torque, but without a coupled anterior tibial loading, has been the typical loading profile in nearly all cadaveric studies on ACL function and surgical reconstructions even though this limits anterior tibial subluxation of the medial compartment. In Figure 7-7 , the results of a typical cadaveric knee specimen are shown under two pivot shift loading conditions. Three points are important. In the two simulated pivot shift tests, there is a greater anterior tibial subluxation of the lateral and medial compartment, with the pivot shift subluxation produced by a coupled anterior translation-internal rotation during the valgus loading as the knee approaches extension (pivot shift [PS] 4), compared with an absence of the anterior tibial loading component (PS3). Secondly, high tibial internal rotation torques in the pivot shift reduces the medial tibiofemoral compartment (see Fig. 7-7 ), constraining the amount of medial and central compartment translation. Third, an analysis of rotational knee stability requires knowledge of the resulting subluxations (translations) of the medial and lateral tibiofemoral compartments (see Chapter 3 ). We recommend that future in vitro and in vivo pivot shift studies use loading profiles of coupled anterior tibial translation and internal tibial rotation, along with valgus loading, to induce a maximum anterior tibial subluxation of both tibiofemoral compartments. Further, rotational torques should remain low to not constrain the maximum subluxation of the medial compartment.

Division of Anterior Cruciate Ligament Into Anteromedial and Posterolateral Bundles

Controversy exists in published studies on the division of the ACL into two distinct fiber bundles. Some authors provide evidence of both an anatomic and functional division, whereas others doubt this division exists and argue that ACL fiber function is too complex to be artificially divided into two bundles. In some studies, the anteromedial (AM) bundle is identified functionally at its femoral location as the proximal half of the attachment (knee in extension) that tightens with knee flexion. The PL bundle is identified as the distal half of the ACL femoral attachment that tightens with knee extension. The PL bundle is described to relax with knee flexion, as the ACL femoral attachment changes from a vertical to a horizontal structure. The problem is that this classical description of a reciprocal tightening and relaxation of the AM and PL bundles occurs only under low anterior loading conditions. With substantial anterior tibial loading, and particularly with a coupled motion of anterior translation and internal tibial rotation, the majority of the ACL fibers are brought into a load-sharing configuration to a differing percentage as is presented later.

We believe the characterization of the ACL into two fiber bundles represents a gross oversimplification not supported by biomechanical length-tension laboratory studies. Zavras and coworkers published a comparative study on previously published isometric points for the ACL and concluded that the ACL isometric zone was high and proximal in the ACL attachment, close to the posterior end of the Blumensaat line. These data, along with other studies, focused on placing grafts in the proximal aspect of the ACL attachment, in a so-called isometric position. The problem with applying published isometric data to the clinical situation is that the data are valid only for knee flexion-extension and do not indicate the most effective ACL graft position for controlling knee rotational loading as occurs during the pivot shift test.

The length-tension behavior of ACL fibers is primarily controlled by the femoral attachment in reference to the center of femoral rotation, the coupled motions applied, the resting length of ACL fibers, and tibial attachment locations. In Chapter 3 , the concept of an isometric transition zone or contour plot is presented. The zone represents a central transition zone in which fibers undergo 2 mm of length change during knee flexion and extension. This zone is not an isometric zone because the length change is not zero. All ACL fibers anterior to the zone lengthen with knee flexion, whereas the posterior fibers lengthen with knee extension. Under loading conditions, fibers in both the AM and PL divisions contribute to resist tibial displacements. ACL fibers in the PL division attach in part posterior to the transitional zone and lengthen with knee extension. Note that the function of the ACL fibers is determined by the anterior-to-posterior direction (knee at extension) as well as the proximal-to-distal femoral attachment. Placement of a graft in an anterior or posterior position has a large effect in producing deleterious lengthening and graft failure. The obvious example is a graft placed anterior to the “resident's ridge,” which is anterior to the femoral ACL attachment. The femoral proximal-to-distal division of the ACL used in two-bundle descriptions of fiber length in knee flexion and extension oversimplifies the functional behavior of the ACL fibers, which are more dependent on their anterior and posterior femoral attachments.

The division of the ACL into two bundles was historically based on the tibial attachment site and not on a corresponding femoral attachment site. Recent studies project the tibial bundles onto two corresponding femoral sites. In these studies, to be described later, we usually divide the ACL femoral attachment site at a midpoint into a corresponding AM and PL bundle. In our opinion, there is not at present convincing anatomic data to support a division of the ACL into two separate bundles, although one study reported a bony ridge between the two femoral attachment bundles. Because of the lack of a clear anatomic division of the ACL into two bundles, there is discrepancy among authors on anatomic descriptions of the ACL bundles and recommendations for the surgical technique on tibial and femoral tunnels for two bundle graft reconstructions.

Tibial and Femoral Footprint Anatomy

Hwang and associates performed a systematic review of the ACL tibial footprint in which 19 studies were analyzed. The authors noted wide variability in the reported size and shape of the native tibial footprint ( Fig. 7-8 ). As a general guideline, on a lateral radiograph, the AM bundle was located at one third the anterior-to-posterior distance, the PL bundle was located at one half this distance, and the ACL center was located midway between ( Fig. 7-9 and 7-10 ). The authors mentioned that although the line described by Amis and Jakob was parallel to the medial plateau and the line described by Staubli and Rauschning was perpendicular to the tibial axis, both lines yielded approximately the same results ( Table 7-1 ). The approximate arthroscopic landmarks of the reviewed studies are shown in Table 7-2 . These measurements need to be adjusted according to the relative size of the knee joint and potential anatomic variations. Even so, these measurements allow identification of the anterior two thirds of the ACL footprint for a 10-mm tunnel that avoids any portion of the tunnel occupying the posterior one third (PL bundle portion), ensuring that the tunnel remains anterior to the posterior lateral meniscus attachment.

TABLE 7-1

Studies That Used Lateral Knee Radiographs to Describe Location of Tibial Insertions of ACL AM and PL Bundles

From Hwang MD, Piefer JW, Lubowitz JH. Anterior cruciate ligament tibial footprint anatomy: systematic review of the 21st century literature. Arthroscopy. 2012;28:728-734.

Reference	Center of AM Bundle (Mean ± SD [Range])	Center of PL Bundle (Mean ± SD [Range])
Amis & Jakob
Kasten et al. (2010)	35% ± 4% (23%-42%)	48% ± 4% (39%-58%)
Doi et al. (2009)	34.6% ± 4.3% (24.0%-42.9%)	38.5% ± 4.3% (28.6%-48.1%)
Colombet et al. (2006)	36% ± 3.8% (NA)	52% ± 3.4% (NA)
Staubli & Rauschning
Iriuchishima et al. (2010)	31% ± 3% (NA)	50% ± 3% (NA)
Zantop et al. (2008)	30% (NA)	44% (NA)

The lateral knee radiograph method for describing the anatomic centrum of the anterior cruciate ligament (ACL) and its anteromedial (AM) and posterolateral (PL) bundles may be based on either the proximal tibial line described by Amis and Jakob or the line described by Staubli and Rauschning (see Fig. 7-13 ). The findings described by one of these two methods are summarized by published articles, with regard to the center of each bundle (and range when reported), as the percent distance along the line from anterior to posterior. In sum, both lines yield similar results; the center of the AM bundle of the ACL is approximately one third of the anterior-to-posterior distance along either line, and the center of the PL bundle is approximately 40% to 50% of the anterior-to-posterior distance along either line.

NA , not available; SD , standard deviation.

TABLE 7-2

Arthroscopically Useful Landmarks for Identifying ACL Tibial Footprint or Centrum of ACL Tibial AM or PL Bundles

Modified from Hwang MD, Piefer JW, Lubowitz JH. Anterior cruciate ligament tibial footprint anatomy: systematic review of the 21st century literature. Arthroscopy. 2012;28:728-734.

Reference	Key Findings
Iriuchishima et al. (2010)	Distance between center of AM and PL footprints and anterior margin of PCL is 23 ± 4 mm and 12 ± 3 mm, respectively.
Purnell et al. (2008)	Distance from posterior fibers of ACL to anterior margin of PCL is 16.5 ± 2 mm (range, 12.7-19.1 mm).
Siebold et al. (2008)	Posterior horn of lateral meniscus is adjacent to posterior border of PL footprint; centrum of PL footprint is 5 mm anterior.
Heming et al. (2007)	Centrum of tibial footprint is 15.0 mm anterior to tibial notch of PCL.
Edwards et al. (2007)	Centrum of tibial footprint is 15 ± 2 mm (range, 11-18 mm) from anterior PCL; center of PL bundle is 10 ± 1 mm (range, 8-13 mm) and AM is 17 ± 2 mm (range, 13-19 mm) from anterior PCL.
Luites et al. (2007)	Centrum of AM bundle is one fourth interspinous distance from ML intercondylar eminence; PL bundle centrum is 4 mm more lateral, approximately halfway between spines; tibial footprint centrum as a whole is two fifths ML (interspinous distance).
Cuomo et al. (2006)	Anterior border of ACL tibial footprint is 22 ± 3 mm (range, 16-27 mm) anterior to “over the back position” (ridge just anterior to PCL); posterior border is 6 ± 2 mm (range, 2-8 mm) anterior to ridge.
Colombet et al. (2006)	Mean distance between AM bundle and anterior border of PCL is 17.5 ± 1.7 mm.

ACL , Anterior cruciate ligament; AM , anteromedial; ML , medial to lateral; PCL , posterior cruciate ligament; PL , posterolateral.

Edwards and coworkers defined the ACL tibial attachment in 55 cadaveric specimens. In Figure 7-11 , the tibial bony landmarks are depicted. The “over-the-back” ridge (posterior interspinous ridge [RER]) corresponds to the interspinous ridge just anterior and proximal to the posterior cruciate ligament (PCL) attachment. Figure 7-12 presents these authors' recommendations for best-fit central and two-tunnel positions. The center of the ACL attachment was 15 ± 2 mm (range, 11-18 mm) from the RER at 36% of the anteroposterior (AP) depth of the tibia. The center of the PL bundle anterior from the RER was 10 ± 1 mm (range, 8-13 mm) and the AM bundle was 17 ± 2 mm (range, 13-19 mm), corresponding to 29% and 46% of the tibial depth, respectively. These authors were not able to define two separate anatomic ACL bundles. Instead, they defined two functional bundles by observing the tightening and slackening behavior of the ACL fibers during flexion and extension.

Stabuli and Rauschning reported the center of the ACL tibial attachment at 43% of the AP depth, which extended from 25% to 62% of the tibial width. These important landmarks provide a reference to measure lateral radiographs before and after ACL graft placement to avoid a posterior and vertical graft placement ( Fig. 7-13 ). Siebold and associates performed a cadaveric study in 50 knees and documented the ACL tibial insertion using a digital image analysis system and divided the ACL into AM and PL bundles. The average AP length of the ACL tibial attachment was 14 mm (range, 9-18 mm). The average male ACL tibial insertion area was 130 mm ² versus the average female knee of 106 mm ² . The average AP length of the ACL footprint in females was 14 mm (range, 9-18 mm) and in males, 15 mm (range, 12-18 mm). The average width of the ACL in all knees was 10 mm. The study concluded that two tibial bone tunnels cannot be placed at the anatomic centers of the AM and PL bundles because the space is too narrow and the tunnels would overlap. This highlights the difficulty of the ACL double-bundle technique in recreating the native ACL fiber attachment.

Forsythe and colleagues quantified the position of AM and PL bone tunnels in ACL reconstruction using arthroscopic dissection and computed tomography (CT) images in cadaveric knees ( Figs. 7-14 and 7-15 ). Three-dimensional models were created from the CT scans and aligned into an anatomic coordinate system. The tibial and femoral measurements were similar to those published by other studies ( Table 7-3 ).

TABLE 7-3

Summary of Studies on Tibial and Femoral Positions of the AM and PL Bundles and Bone Tunnels

From Forsythe B, Kopf S, Wong AK, et al. The location of femoral and tibial tunnels in anatomic double-bundle anterior cruciate ligament reconstruction analyzed by three-dimensional computed tomography models. J Bone Joint Surg Am. 2010;92:1418-1426.

REFERENCE	TIBIAL MEASUREMENTS				FEMORAL MEASUREMENTS WITH QUADRANT METHOD
	ANTERIOR TO POSTERIOR (%)		MEDIAL TO LATERAL (%)		PARALLEL TO BLUMENSAAT LINE (%)		PERPENDICULAR TO BLUMENSAAT LINE (%)
	AM	PL	AM	PL	AM	PL	AM	PL
Tsukada et al. (2008)	37.6	50.1	46.5	51.2
Forsythe et al. (2010)	25	46.4	50.5	52.4	21.7	35.1	33.2	55.3
Colombet et al. (2006)					26.4	47.6	25.3	32.3
Zantop et al. (2008)					18.5	29.3	22.3	53.6

AM , Anteromedial; PL , posterolateral.

In 2012, Piefer and associates conducted a systematic review of studies that described the location of the ACL femoral footprint. Using the 20 articles selected for analysis, the authors provided general recommendations regarding the arthroscopic identification of the femoral footprint ( Table 7-4 ). Already appreciated was the ACL location, which was noted to be in the posterior third of the lateral wall, posterior to the lateral intercondylar ridge (resident's ridge). The anatomic centrum of the ACL femoral footprint was 43% of the proximal-to-distal length of the lateral femoral intercondylar notch wall and femoral socket radius plus 2.5 mm anterior to the posterior articular cartilage margin. There is a well-known anatomic variability of size and shape in the majority of knees in which the ACL fibers can be visualized ( Table 7-5 and Fig. 7-16 ).

TABLE 7-4

Arthroscopically Useful Landmarks for Identifying ACL Femoral Footprint or Centrum or AM or PL Bundles by Bony Landmarks, Angle to Femoral Anatomic Axis, and Distance From Articular Cartilage Margins

From Piefer JW, Pflugner TR, Hwang MD, Lubowitz JH. Anterior cruciate ligament femoral footprint anatomy: systematic review of the 21st century literature. Arthroscopy. 2012;28:872-881.

Reference	Bony Landmarks	Angle to Femoral Anatomic Axis (Degrees)	Distance of Centrum or Footprint to Articular Cartilage Margin
Heming et al. (2007)	NA	28.8	Footprint is 4 mm from posterior *
Purnell et al. (2008)	Lateral intercondylar resident's ridge is anterior border	34.9 ± 3.7	Footprint is 3.5 ± 0.9 mm from posterior *
Siebold et al. (2008)	AM is 3.6 ± 1.2 mm posterior to “over the top” position; distance between AM and PL centers is 7 ± 2 mm	12	PL centrum is 6 ± 3 mm from “shallow” (distal) articular cartilage
Yasuda et al. (2004)	Anatomic centrum of AM bundle is 5-6 mm distal from “back” of femur	30	PL centrum is 5-8 mm from posterior
Ferretti et al. (2007)	Lateral intercondylar ridge is anterior border; bifurcate ridge separates AM and PL bundles	NA	NA
Steckel et al. (2010)	Bundles are oriented more horizontally when knee is flexed past 90 degrees	19 when knee is flexed to 90	NA
Luites et al. (2007)	Centrum is 7.9 ± 1.4 mm shallow (distal) along notch roof and 4.0 ± 1.3 mm posterior to notch roof; footprint includes notch roof in 31 of 35	NA	NA
Takahashi et al. (2006)	AM bundle is 4.1 ± 0.8 mm from notch roof perpendicular to Blumensaat line; PL bundle is 11.3 ± 1.8 mm from notch roof perpendicular to Blumensaat line	NA	AM bundle is 7.6 ± 1.5 mm (24.5% ± 4.7%) from posterior margin of article surface; PL bundle is 7.0 ± 1.4 mm (22.9% ± 4.7%) from posterior margin of articular surface
Edwards et al. (2008)	AM bundle extends to posterior proximal notch	37 ± 16 to femoral axis	Measured parallel to femoral axis: AM centrum is 4.3 ± 1.1 mm from posterior at 10:30 o'clock position; PL centrum is 8.9 ± 2.1 mm from posterior at 10 o'clock position
Zantop et al. (2008)	AM bundle is 5.3 ± 0.7 mm from roof of notch	NA	AM centrum is 18.9 ± 0.5 mm from shallow cartilage margin; PL centrum is 6.5 ± 0.9 mm from shallow (distal) cartilage margin and 5.8 ± 0.6 mm from posterior cartilage margin
Colombet et al. (2006)	Center of AM bundle is 1.8 ± 1.3 mm from over-the-top position	NA	Footprint is 2.5 ± 1.1 mm from posterior articular cartilage * ; distal border of footprint is 2.8 ± 1.5 mm from distal margin of articular cartilage
Iwahashi et al. (2010)	Anterior border in line with posterior cortical border of femoral diaphysis resident's ridge	Parallel to roof of intercondylar notch	Footprint extends to posterior cartilage margin *
Hara et al. (2009)	Lateral intercondylar resident's ridge is anterior border	NA	NA

AM, Anteromedial; NA, not available; PL, posterolateral.

* Descriptions of the distance from the posterior edge of the footprint as a whole to the posterior border of the articular cartilage.

TABLE 7-5

ACL Femoral Footprint Insertion Size (Proximal to Distal by Anterior to Posterior) and Shape for Footprint as a Whole or by Individual Bundle

From Piefer JW, Pflugner TR, Hwang MD, Lubowitz JH. Anterior cruciate ligament femoral footprint anatomy: systematic review of the 21st century literature. Arthroscopy. 2012;28:872-881.

Reference	Size (mm)	Shape
Heming et al. (2007)	18.4 × 9.5	NA
Purnell et al. (2008)	12.9 ± 0.1 × 7.6 ± 1.4	NA
Steckel et al. (2010)	NA	Semilunar
Siebold et al. (2008)	15 ± 3 × 8 ± 2	Long oval
Yasuda et al. (2004)	NA	Egg shaped
Ferretti et al. (2007)	17.2 ± 1.2 × 9.9 ± 0.8	Segment of a circle with straight anterior border and convex posterior border
Luites et al. (2007)	NA	Oval
Takahashi et al. (2006)	AM: 11.3 ± 1.6 × 7.5 ± 1.3 PL: 11 ± 1.7 × 7.6 ± 1.0	Elliptic
Edwards et al. (2008)	14 ± 2 × 7 ± 1	Variable
Mochizuki et al. (2006)	AM: 9.2 ± 0.7 × 4.7 ± 0.6 PL: 6.0 ± 0.8 × 4.7 ± 0.6	Oval
Colombet et al. (2006)	13.9 ± 9.5 × 9.3 ± 7.1	Variable
Iwahashi et al. (2010)	17.4 ± 0.9 × 8 ± 0.5	Oval
Stijak et al. (2009)	14.4 ± 2.3 × 6.8 ± 0.7	NA

AM , Anteromedial; NA , not available; PL , posterolateral.

These data indicate that a 10-mm drill would produce a central point at the 43% proximal-to-distal line (just proximal to the midpoint). Assuming a 5-mm radius, plus a 3-mm back wall (to the articular cartilage bone margin), there would be an 8-mm point for the posterior articular cartilage margin. We prefer arthroscopic identification of the ACL femoral footprint compared with the various radiographic techniques shown in Table 7-6 . The goal is to avoid a distal ACL footprint location (thereby producing a short graft) but to place the ACL graft into the proximal two thirds of the native footprint. This means the surgeon should select a point just proximal to the 43% line. We recommended the use of standard anatomic descriptions (anterior, posterior, proximal, and distal), rather than terms such as shallow, deep, high, and low.

TABLE 7-6

Radiographic Description of Anatomic Centrum of ACL Femoral Footprint, by Bundle or as a Whole, by Method of Description

From Piefer JW, Pflugner TR, Hwang MD, Lubowitz JH. Anterior cruciate ligament femoral footprint anatomy: systematic review of the 21st century literature. Arthroscopy 28:872-881, 2012.

Reference	Center of Anteromedial Bundle (%)	Center of Posterolateral Bundle (%)	Center of Anterior Cruciate Ligament Footprint (%)
Bernard & Hertel Method
Colombet et al. (2006)	26.4 ± 2.6 × 25.3 ± 4.2	32.3 ± 3.9 × 47.6 ± 6.5	29.35 × 36.45
Iriuchishima et al. (2010)	15 ± 6 × 26 ± 8	32 ± 9 × 52 ± 5	23.5 × 39
Forsythe et al. (2010)	21.7 ± 2.5 × 33.2 ± 5.6	35.1 ± 3.5 × 55.3 ± 5.3	28.4 × 44.25
Zantop et al. (2008)	18.5 × 22.3	29.3 × 53.6	23.9 × 37.95
Tsuda et al. (2010)	25.9 ± 2 × 17.8 ± 2.9	34.8 ± 2 × 42.1 ± 3.9	30.35 × 29.95
Pietrini et al. (2011)	21.6 ± 5.6 × 14.2 ± 7.7	28.9 ± 4.6 × 42.3 ± 6	25.25 × 28.25
Guo et al. (2009)	NA	NA	43.1 ± 4.6 × 38.3 ± 1.3
Steckel et al. (2010)	NA	NA	26.9 ± 3.5 × 27.5 ± 3.2
Mean	21.5 × 23.1	32 × 48.8	28.5 × 35.2
Mochizuki Method
Mochizuki et al. (2006)	28.3 ± 2.1 × 28 ± 2	59.8 ± 4.1 × 53 ± 2	44.05 × 40.5
Edwards Grid Method
Edwards et al. (2007)	21 × 24	27 × 57	24 × 40.5
Takahashi Method
Takahashi et al. (2006)	31.9 × 26.9	39.8 × 53.2	35.85 × 40.05

The Bernard-Hertel method describes the centrum as the percent distance along the Blumensaat line (from proximal and posterior to distal and anterior) by the percent distance along a line perpendicular to the Blumensaat line (from proximal and anterior to distal and posterior). The Mochizuki method describes the centrum as the percent distance from proximal to distal by the percent from anterior to posterior (referencing the roof of the intercondylar notch). The Edward method describes the centrum as the percent distance from shallow (proximal and posterior) to deep (distal and anterior) by the percent from proximal and anterior to distal and posterior (referencing the roof of the intercondylar notch). The Takahashi method describes the centrum as the percent distance from proximal to distal by the percent from anterior to posterior (referencing the roof of the intercondylar notch).

ACL, Anterior cruciate ligament; NA, not available.

Edwards and coworkers described the anatomic locations of the ACL femoral attachments of the AM and PL bundles in a companion study in 22 cadaveric knees using a measurement grid system shown in Figure 7-17 . The authors reported a wide variation in the size and shape of the ACL attachment and bundle arrangement that was selected ( Fig. 7-18 ). The femoral attachment was oriented at 37 degrees to the long axis of the femur. The authors reported that the AM bundle extended to the posterior proximal limit of the femoral notch between the 10:30 and 11:30 o'clock positions. The PL bundle was located between the 9:00 and 10:30 o'clock positions. They reported that a 6-mm graft tunnel had the best fit if the AM bundle was located at 11 o'clock, 6 mm from the posterior outlet, and the PL bundle was located at 10 o'clock, 9 mm from the posterior outlet. They noted that other studies had different recommendations for the placement of two femoral tunnels in the double-bundle technique.

Zantop and colleagues performed a cadaveric study in 20 knees that outlined the AM and PL bundle locations at the tibial and femoral sites. The authors arrived at different conclusions from those of Siebold and associates. The tibia ACL division into the AM bundle was oriented from medial lateral, occupied the anterior one half of the ACL footprint, and was aligned with the anterior horn of the lateral meniscus. The center of the PL bundle was located 11 mm posterior and 4 mm medial to the anterior insertion of the lateral meniscus. The center of the AM bundle was located at 30% and the PL bundle was located at 40% on the lateral transtibial line.

Rue and associates, in a cadaveric study, used a transtibial-drilled femoral tunnel, placed at the 10:30 position, and determined the location of a single-graft femoral tunnel in relation to the AM and PL bundles. The authors placed the guide pin for the tibial tunnel approximately 7 mm anterior to the PCL, which would represent a posterior one-third tibial attachment. They reported that the transtibial drilling of the femoral tunnel resulted in the AM bundle occupying an average of 32% (range, 3%-49%) of the ACL attachment and the PL bundle occupying an average of 26% (range, 7%-41%) of the ACL attachment. In addition, the remainder of the area of the 10-mm tunnel did not overlap the ACL footprint. The wide ranges reported in this study for the location of the femoral tunnel in terms of the native ACL femoral attachment highlight the problems of using the tibial tunnel to drill the femoral tunnel.

Heming and colleagues reported the ACL tibial and femoral footprints in 12 cadaveric knees ( Fig. 7-19 ). Note in Figure 7-19, C , the orientation of the clock face places the 9 to 3 o'clock horizontal line at the base of the femoral condyles and not within the center of the notch, which is typically used by other authors. A guide pin placed between the ACL anatomic femoral and tibial attachment centers (as in a transtibial drilling technique) was only possible if the tibial tunnel started in a medial position close to the joint line that would be too short to be functional for ACL grafts. The study concluded that transtibial drilling techniques resulted in a more vertical graft orientation.

Anterior Cruciate Ligament Function Defined by the Role of Anteromedial and Posterolateral Divisions

Biomechanical studies have reported marked differences in the function of the AM and PL bundles. Sakane and coworkers, in one of the first cadaveric robotic studies, reported the PL bundle provided the primary restraint to anterior tibial translation at low flexion angles. Increasing forces were reported in the AM bundle with knee flexion; these remained lower than the calculated in situ forces of the PL bundle. However, subsequent studies have shown an increased role of the AM bundle and decreased role of the PL bundle in limiting anterior tibial translation.

The role of the ACL bundles in restraining rotational motions was examined by Zantop and associates in a robotic study. A 134-N anterior tibial load or a combined 10 N-m valgus and 4 N-m internal tibial torque were applied to cadaveric knees. Sectioning the PL bundle at 30 degrees of flexion resulted in an approximate 7-mm increase in anterior tibial translation, whereas sectioning the AM bundle produced at 60 degrees an approximate 9-mm increase in anterior translation. Similar data were reported at low flexion angles for the combined rotational loads. These data imply a near-absence of function of the remaining “intact” bundle that is not supported by other studies. For example, Markolf and colleagues, in a cadaveric study, reported that cutting the PL bundle resulted in an increase of only 1.1 mm at 10 degrees of flexion and 0.5 mm at 30 degrees of flexion. These authors questioned the need to reconstruct the PL bundle in ACL reconstructions.

Gabriel and coworkers, in a robotic cadaveric experiment, removed all of the soft tissues to produce a femur-ACL-tibia specimen. For anterior tibial loading ( Fig. 7-20 ), the AM bundle was reported to be nearly equal to the PL bundle at 15 degrees of flexion, with increasing AM forces with knee flexion. Under the rotation loads of valgus and internal tibial rotation, the forces in the AM bundle exceeded those of the PL bundle ( Fig. 7-21 ).

Li and associates used MRI and fluoroscopic images of subjects performing a lunge to create 3-dimensional models in which the AM and PL attachments were outlined. The study reported that the AM and PL bundles reached their maximum elongation between full extension and 30 degrees of flexion and did not demonstrate a reciprocal behavior with knee flexion-extension.

Giron and colleagues, in a cadaveric experiment, determined that a “deep” (proximal) femoral tunnel position could be achieved with either one of three techniques (double incision, transtibial, or AM portal). However, Arnold and coworkers showed in cadaveric experiments that the ACL is entirely attached to the lateral femoral wall and that this attachment position was not able to be accessed through a transtibial tunnel (40% AP tibial tunnel measurement). The transtibial tunnel method resulted in the guide pin and drill hole placed at the junction of the proximal ACL attachment and femoral roof ( Fig. 7-22 ).

Mae and associates, in a cadaveric knee study, used a robotic simulator to study the effect of single- versus two-femoral tunnel ACL graft reconstruction. A single tibial tunnel was placed in the center of the ACL tibial attachment. The locations of the single and two tunnels were described at the 1 o'clock and 2:30 positions. The data for AP translation and in situ graft forces at 44-N graft pretension showed little difference between the single- and double-bundle grafts, both of which produced a mild to moderate overconstraint that limited anterior tibial translation. The study reported on the load-sharing between the normal ACL bundles ( Fig. 7-23 ), which demonstrated the PL bundle functioned more at low flexion angles, equal to the AM at 10 degrees of flexion, with a further increase in AM bundle function with increasing knee flexion. A study of load-sharing between the two bundles of a simulated pivot shift type of loading was not performed.

Yagi and colleagues compared single- and double-bundle ACL hamstring reconstructions. A double-looped single-bundle hamstring graft was placed at approximately the 11 o'clock position and tensioned to 44 N, whereas each bundle of the double-bundle graft was tensioned to 44 N. The double-bundle graft had a total of 88 N of tension versus 44 N for the single-graft reconstruction. In response to a 134-N anterior load at 30 degrees of flexion, the data showed an unexplained residual anterior tibial translation (intact, 6.4 mm ± 2.4 mm: single bundle, 10.2 ± 2.5 mm), which would not be anticipated from a single graft in terms of resisting translation. The finding of a lax single graft most likely explains the reported decreased ability of the single graft to resist the combined rotation-translation motions.

Petersen and coworkers concluded that the location of the ACL double-bundle grafts in clinical studies varied markedly at both the femoral and tibial sites. In a cadaveric study, Petersen et al. found that a two-tunnel tibial technique provided an advantage in resisting combined rotatory loads, simulating the pivot shift (anterior tibial translation, 7.5 mm versus 9.5 mm, 30 degrees of flexion, 5 N-m internal rotation, 10 N-m valgus torque). Of note, the single tibial tunnel and graft were placed 7 to 8 mm anterior to the PCL; the authors stated that this was a position recommended by some surgeons. As previously discussed, the posterior tibial tunnel position results in a more vertical ACL graft, with portions of the graft possibly even posterior to the native ACL tibial attachment. The addition of a second graft placed in a more anterior tibial position within the ACL footprint would be expected to provide better resistance to the loading conditions. Although the data do not provide evidence for a double-bundle ACL procedure, it does provide evidence to avoid placement of a single ACL graft in the posterior one third of the ACL tibial attachment.

Yamamoto and associates conducted a cadaveric robotic study that compared a single ACL graft placed in the 10 o'clock position on the lateral wall with an anatomic double-bundle reconstruction. This is one of the few published studies in which a single-graft lateral wall placement was used, avoiding a more proximal graft placement. Similar loading conditions were used as in the other robotic studies (anterior tibial load 134 N, combined rotatory load 10 N-m valgus and 5 N-m internal tibial torque). There was no statistical difference in the anterior tibial translation or in situ force at 30 degrees of knee flexion between the intact ACL, the single bundle, or the anatomic double-bundle reconstructions. Under the combined rotatory loading conditions, there were no differences in anterior tibial translation, internal tibial rotation, and ACL in situ graft forces, which is in direct contrast to other robotic experiments.

Markolf and colleagues measured in cadaver knees a simulated pivot shift in the intact knee and then after single-bundle and double-bundle ACL reconstruction. The single-bundle reconstruction (placed at the anatomic AM bundle site) restored mean tibial rotations and lateral plateau displacements to levels similar to those of the intact knee, whereas the double-bundle reconstructions reduced coupled rotations and displacements to levels less than those in the intact knee. The authors concluded that the overconstraint induced by the double-bundle reconstructions has unknown clinical consequences and that the need for the added complexity of this procedure is questionable.

Cuomo and coworkers reported on the effects of tensioning single- and double-bundle ACL reconstructions in cadaveric knees instrumented with a six-DOF system under defined loading conditions (anterior translation, 90 N; 5 N-m internal rotation torque). Different flexion positions were used for tensioning each of the grafts in the double-bundle construct. The single graft was tensioned at 20 degrees of knee flexion. All grafts had sufficient tension applied to restore normal intact knee translation. Tensioning each of the grafts individually (AM bundle at 90 degrees, PL bundle at 20 degrees, varying which was tensioned first) resulted in overconstraining AP translation. In contrast, tensioning both the AM and PL grafts simultaneously at 20 degrees provided the best match for the intact knee AP translation throughout knee flexion. The data show the difficulty in tensioning two ACL grafts at the time of surgery in terms of matching the intact ACL load-sharing among fibers. With anterior loading, both the AM and PL fibers participate in load-sharing, but at different percentages as already discussed. For single-bundle ACL grafts placed within the center of the femoral and tibial attachments, higher overall graft tension occurred under anterior loading compared with the two graft bundles tensioned at 20 degrees of flexion. Importantly, the data show that tensioning of the AM and PL grafts under low tensions (20 N) provided the best results. The single graft required 38 ± 27 N to restore the native knee laxity.

Scopp and associates, in cadaveric experiments, reported that a more oblique femoral tunnel placement (60 degrees from vertical) was more effective in resisting internal rotational torques (difference 4.4 degrees, 6.5 N-m) than a “standard” tunnel placement (30 degrees from vertical). The femoral tunnel was drilled using a transtibial technique and, most likely, a posterior tibial tunnel placement. Anterior tibial translation values were not restored to normal (increased 2.5 mm and 2.2 mm, standard and oblique femoral tunnels).

Simmons and colleagues, in cadaver experiments, showed the deleterious effects of a more vertical tibial tunnel (70 degrees and 80 degrees) with impingement against the PCL and higher graft tension. A more oblique tibial tunnel and femoral tunnel in the 60-degree coronal plane did not impinge against the PCL. The study recommended a posterior tibial tunnel placement and more proximal ACL femoral attachment with use of a transtibial drilling technique, again showing from a historical perspective the studies in the literature in which a more vertical graft orientation results.

Arnold and coworkers showed in cadavers that the passive ACL tension flexion-extension tension curve was not reproduced by femoral tunnels placed at the 10 and 11 o'clock positions, but was reproduced by grafts in the 9 o'clock positioned tunnels. However, a less ideal posterior ACL tibial tunnel was used in this experiment.

In summary, a majority of in vitro robotic studies compare a double-bundle graft construct with a hybrid proximal femoral and posterior tibial orientation of a more vertical single-bundle graft construct and not a centrally placed femoral and tibial graft construct. Therefore, the published data apply specifically to this type of single-graft construct and show as expected that a vertical single graft is not positioned to resist rotational loading. It may be concluded that there is sufficient experimental data to recommend this hybrid graft orientation be avoided in clinical practice.

There are incomplete in vitro data on the comparison of an anatomic single-graft construct (within central tibial and femoral tunnels) with a double-bundle construct. There are also incomplete experimental data on the function of individual regions of ACL fibers in resisting coupled motions as occurs in the pivot shift phenomena, which is an important area for future study ( Fig. 7-24 ). Other biomechanical studies reviewed show that a single ACL graft placed within the anatomic femoral and tibial attachments (avoiding a high femoral and posterior tibial position) restore rotatory stability and question the need for a double-bundle procedure.

One theoretical advantage of a double-graft construct tensioned appropriately is that there is initially less graft tension in each of the graft strands compared with the overall tension in a single graft. Stated differently, a single graft will always exhibit much higher graft tension to resist anterior loading than two graft arms in which load-sharing occurs. There is also the theoretical advantage of tensioning the two graft strands at different knee flexion positions to exhibit a different percent sharing of the overall tension than a single graft. From an experimental standpoint, either a single-graft or double-graft construct can be tensioned to restore normal motion limits; however, this may occur at the expense of high graft tensions, particularly in a single-graft construct. Thus, the advantage of a double-graft construct (either ACL or PCL) is to restore normal knee motion limits under the lowest graft tensile loads, which is advantageous during graft healing and remodeling. A graft under lower loads has the theoretical advantage of less risk in stretching out with return of the abnormal motion limits. In addition, an ACL or PCL graft under higher tension, under cyclic loading conditions, is at high risk of graft stretching and failure. Again, it should be noted that either one or two ACL grafts does not restore native ACL fiber length-tension properties.

Our Robotic Studies on Anterior Cruciate Ligament Anteromedial Bundle and Posterolateral Bundle Function and Single Anterior Cruciate Ligament Graft Reconstruction

We conducted a series of robotic cadaveric in vitro studies on the kinematic function of the AM and PL bundles of the ACL. The studies involved a six-DOF robotic testing protocol on intact knees, knees in which either the AM or PL bundle was sectioned, and knees in which complete ACL sectioning was performed. As previously described, the robotic testing protocol involved, for the first time, the simulation of the pivot shift event using four DOF; namely, anterior tibial translation with internal tibial rotation with valgus loading of the limb during knee flexion-extension to induce maximum anterior tibial subluxation. The studies also involved, for the first time, digitization of the tibial plateau to determine medial and lateral tibiofemoral compartment translations and centers of tibial rotation in the simulated pivot shift test.

The results showed both ACL bundles functioned synergistically to resist medial and lateral compartment subluxations during the simulated Lachman and pivot shift tests. The AM bundle provided more restraint to anterior tibial translation under both tests than the PL bundle. Both ACL bundles had to be sectioned to produce tibiofemoral compartment subluxations during pivot shift loading ( Figs. 7-25, 7-26, and 7-27 ). Neither bundle contributed to resisting internal rotation during pivot shift loading tests. We concluded that an ACL graft designed to duplicate the AM bundle restores medial and lateral compartment translations under multiple loading conditions; however, the PL bundle only provides a back-up restraint at low flexion angles. Neither ACL bundle resists internal tibial rotation or, in its absence, allows a positive pivot shift subluxation. Furthermore, the PL bundle is not the primary restraint for providing rotational knee stability, as has been quoted in the literature.

We conducted a robotic cadaveric in vitro study that found that a single ACL graft placed into the anatomic center of the femoral and tibial attachment sites restored normal tibiofemoral compartment translations and rotations under simulated Lachman and pivot shift testing conditions. The study involved a six-DOF robotic testing protocol on intact knees, ACL-sectioned knees, and ACL-reconstructed knees. The reconstruction restored rotational stability as defined by normal motion limits and normal medial and lateral compartment translations under simulated pivot shift loading conditions ( Fig. 7-28 ).

The results of this study support the recommendations in this chapter on the use of a single ACL graft instead of a double-bundle ACL graft construct. This finding is supported by Markolf and associates who studied the effect of single- and double-bundle ACL grafts in cadaveric knees. As described in this chapter, prior robotic pivot shift studies on double-bundle ACL reconstructions used loading profiles that did not involve a coupled anterior tibial translation and internal rotation to induce maximum anterior tibial subluxations, and, in addition, did not measure resulting tibiofemoral compartment translations or subluxations. We question the validity of these studies in terms of the conclusions that a single ACL graft does not restore knee rotational stability or that a double-bundle ACL construct is required.

Clinical Measurement of Anterior Cruciate Ligament Graft Function During and After Anterior Cruciate Ligament Surgery

The assessment of ACL graft function must take into account the restoration of the normal coupled motion limits of anterior tibial translation and internal tibial rotation during the pivot shift phenomenon, which strongly correlates with patient giving-way symptoms. During KT-2000 (MEDmetric) testing, only anterior tibial translation is assessed. If the knee joint has a 3-mm or lower increase in central anterior tibial translation over the opposite knee, it can be assumed that there is no positive pivot shift because this amount of constraint to anterior tibial translation also limits anterior subluxation of the lateral tibial plateau. Conversely, if a greater than 5-mm increased anterior tibial translation exists, there may occur a positive pivot shift test and patient complaints of giving-way. The problem is in knees that demonstrate 3 to 5 mm of increased anterior tibial translation, which may represent 20% to 30% of patients in clinical investigations, especially when allografts are used. This mild to moderate increase in anterior tibial translation results in a mildly positive Lachman test with a hard endpoint. However, these knees may demonstrate a positive pivot shift and giving-way symptoms. Since the pivot shift test is highly subjective and variable among examiners (see Chapter 3 ), an author may report a successful result based on the KT-2000 (anterior tibial translation) or a pivot shift test, whereas another examiner may grade the knee as a failure, based on the method in which the pivot shift test is performed. There is a pressing need for clinical examination methods that simulate the pivot shift phenomenon and the resultant subluxations (in millimeters) of the medial and lateral tibiofemoral compartments.

Bull and associates were among the first authors to report intraoperative measurement of tibial translations and rotations using a 3-dimensional motion analysis system. In the operative setting, there was a marked variability in knees in the amount of medial and lateral tibial plateau displacements during the pivot shift test. Robinson and colleagues performed an ACL double-bundle reconstruction using computerized navigation techniques in 22 patients. The exact location of ACL grafts and the loads applied by the examiner are variables still to be determined.

In a frequently quoted study, Tashman and coworkers devised a unique methodology of dynamic in vivo measurements of patients who had undergone ACL reconstruction. The investigators used a biplanar radiographic system to measure tibiofemoral displacements while the patients ran downhill on a treadmill. The reconstructed knees showed an increase in mean values of external tibial rotation (3.8 ± 2.3 degrees) and adduction (2.8 ± 1.6 degrees). The effect of these small differences from a clinical standpoint is unknown and future studies are required that involve more dynamic rotational movements in comparison to straight-ahead running to measure the rotational kinematics of the knee joint. The specific ACL graft femoral and tibial tunnel placement was not identified in this study.

Ristanis and associates assessed 11 patients who had undergone B-PT-B reconstruction using kinematic data from a six-camera optoelectronic system obtained during a jump landing and pivoting maneuver. The ACL reconstruction did not restore tibial rotation to normal values of the opposite uninvolved extremity or controls. The femoral tunnel was placed through an AM approach with the knee joint flexed to 120 degrees to achieve a 10 to 11 o'clock lateral wall position. The tibial tunnel was placed into the center of the ACL footprint. The authors noted that a limitation of the study was the movement of skin markers that may not have accurately represented true tibiofemoral joint translations.

Monaco and colleagues evaluated the effect, on internal tibial rotation, of a single-bundle ACL reconstruction combined with an extraarticular procedure compared with a double-bundle ACL reconstruction. Computerized navigation instrumentation was used in the operating room before and after the ACL procedures. The addition of the PL bundle after the AM bundle did not have an effect in reducing AP translation or the limit of internal tibial rotation, and there were no significant differences between the single- and double-bundle reconstructions. The extraarticular reconstruction significantly reduced the internal tibial rotation limit, as would be expected. The tibial placement of the graft was performed with the guide pin 7 mm anterior to the PCL insertion into the posterior ACL attachment. The authors did not determine the effect of either graft configuration on the coupled motions of the pivot shift test but only on the limit of internal tibial rotation alone.

Our conclusions of single-graft versus double-bundle graft ACL studies are summarized in Table 7-7 . The hypothesis presented is that an ideally placed single-graft construct (central femoral and tibial ACL attachments, avoiding a vertical graft orientation) provides control of the abnormal pivot shift motions, avoiding the necessity and added complexity of a double-bundle graft reconstruction.

Recommended Tibial and Femoral Anterior Cruciate Ligament Graft Locations

Given the variation in ACL anatomic shapes among specimens, it is important during surgery to outline the individual size and shape of the ACL attachment for each knee. This can be done in a primary ACL reconstruction, but usually not in revisions. The cadaveric studies provide important anatomic reference landmarks. Because of the variation in ACL attachment shapes, it would be expected that there would exist variation among surgeons on the locations chosen for both single- and double-bundle ACL graft locations. Newer computer navigation techniques have been studied to aid in the placement of ACL grafts; however, it is still unknown at present which anatomic points to select for graft tunnels that would correlate with clinical stability results.

The important landmarks for the ACL tibial attachments are the medial tibial spine, RER of the proximal PCL fossa, and the attachment of the lateral meniscus. The PCL is a poor soft tissue landmark for the posterior extent of the native ACL attachment. It should be noted that some authors have advocated a guide pin and tibial tunnel placement 6 to 8 mm from the PCL, which would place the tibial tunnel in the posterior portion of the ACL. In some knees, the tibial tunnel would be posterior to the native ACL attachment and just a few millimeters from the RER or interspinous ridge. This posterior tibial tunnel produces a near-vertical ACL graft that would not be expected to resist rotational loads in obliterating the pivot shift phenomenon, as already discussed.

The ACL tibial attachment location we recommend for a single graft is directly adjacent to the lateral meniscus anterior horn attachment as shown in Figure 7-29 . The ACL attachment can be easily mapped at surgery based on the anatomic references provided. In some knees, the anterior extent of the ACL attachment may be obscured by soft tissues, and in these cases, the RER or posterior interspinous ridge of the PCL fossa is an important landmark. The center of the ACL will be 16 to 20 mm anterior to the RER. The guide pin is most ideally placed eccentric and 2 to 3 mm anterior and medial to the true ACL center, because the ACL graft displaces to the posterior and lateral aspect of the tibial tunnel. The tunnel places the majority of the graft within the central tibial attachment and avoids the posterior attachment location. It is important that graft impingement against the anterior intercondylar notch does not occur because the circular graft may occupy a portion of the native flattened ACL tibial attachment. An anterior notchplasty is required, particularly in knees with an A -shaped notch. In many ACL revision knees, the bony ridge posterior to the ACL attachment (“no-man's land,” Fig. 7-30 ) has been disrupted with a prior graft tunnel that extends 1 to 2 mm from the RER and requires a bone graft before the revision procedure. To avoid a vertical graft orientation, it is important that during the ACL tibial tunnel preparation (primary or revision surgery), the tibial drill does not inadvertently penetrate into or beyond the posterior one-third ACL attachment and adjacent RER.

Important landmarks for the femoral attachment are the posterior articular cartilage, the Blumenstaat line, and identification of the ACL attachment on the lateral femoral wall of the notch (in the regions outlined on the grid systems). The goal is to locate the tunnel in the central to proximal thirds to maintain ACL graft length, and within the central direct ACL native insertion that occurs through fibrocartilage and not within the posterior indirect insertion of fibers adjacent to the femoral articular cartilage edge. A tunnel distance (posterior or deep) from the femoral condyle articular cartilage of 3 mm prevents an excessive posterior graft placement that would be slightly lax with increasing knee flexion (flexion laxity). Again, the emphasis is made that no ACL fibers extend to the intercondylar roof; all of the fibers are on the lateral wall. Tsukada and coworkers reported that the osseous ridge on the lateral femoral wall of the intercondylar notch (resident's ridge) had major positional and dimensional variations and should not be used as a reference for femoral tunnel creation.

Bird and associates reported in a cadaveric study that, with the knee at 90 degrees of flexion, the central location of the ACL attachment was 50% of the AP depth of the femoral condyle (junction articular cartilage) along the lateral bifurcate ridge. The possible error in this technique is the distance that the central point is measured along the bifurcate ridge to avoid a tunnel adjacent to the femoral articular cartilage. Piefer and colleagues, in a systematic review of 20 ACL femoral footprint publication, noted differences in techniques among studies; however, these investigators arrived at recommended arthroscopic osseous landmarks on the lateral wall of the intercondylar notch ( Fig. 7-31 ) which are very useful for the arthroscopic surgeon. These include, when possible, identification of the native ACL attachment, the resident's ridge or intercondylar ridge, the bifurcate ridge, the notch roof where no ACL fibers attach, and the articular cartilage junction of the lateral femoral condyle. We first place the knee at 30 degrees (see Fig. 7-30 ), making sure the radius of the tunnel plus 3 mm is followed. For example, a 10 mm drill hole would have a radius of 5 mm plus 3 mm, or 8 mm from the articular cartilage. We measure tunnel placement at both 30 degrees and 90 degrees of knee flexion and each method is different in terms of measurements as discussed. The question regarding the ideal femoral graft placement at time zero (for example, proximal two thirds, central two thirds) and its effects on subsequent biologic remodeling cannot be answered and still requires more kinematic and clinical studies. It is our impression based on current knowledge that there would be no difference between these two femoral placements that would affect final clinical results. With femoral tunnel placement try to ensure the placement is not too anterior or posterior or distal in the femoral footprint.

Hensler and associates conducted a study on the ACL femoral tunnel morphology induced by use of an AM portal drilling technique with knee hyperflexion. From a review of the literature, the authors determined that the average femoral ACL insertion size was 8.9 mm wide, 16.3 mm long, and 136 mm ² in area. The femoral tunnel aperture was influenced by drill-bit diameter, transverse drill angle, and knee flexion angle ( Fig. 7-32 ). The femoral tunnel aperture best matched the native ACL footprint at 102 degrees of knee flexion, whereas at 130 degrees of flexion, a mismatch in area of 13.5% was calculated ( Fig. 7-33 ). Even so, the higher knee flexion angle of 115 degrees to 120 degrees lessens the chance of a posterior femoral wall blow-out. This study shows how the AM accessory portal drilling of the femoral ACL tunnel requires clinical experience and precise guide pin placement. The surgeon must determine that the exiting guide pin position is in the midlateral plane, above the femoral epicondyle, and maintain a 3- to 4-mm osseous posterior rim from the tunnel wall to the femoral articular cartilage. An angled flexible guide pin and drill are useful in larger patients when the AM portal drilling technique is selected because it is difficult to achieve 120 degrees of knee hyperflexion.

For single grafts, we recommend a central anatomic ACL placement with the femoral guide pin 2 to 3 mm above the midpoint of the proximal-to-distal length of the ACL attachment (30 degrees of knee flexion) and 8 mm from the posterior articular cartilage edge (see Fig. 7-30 ). This will produce a 10-mm tunnel, leaving a 3-mm thick posterior tunnel wall. The senior author prefers to define the ACL attachment at 20 degrees to 30 degrees of flexion with the arthroscope in the AM portal. After the femoral site is marked, the knee can be placed in 120 degrees of flexion if an AM portal arthroscopic drilling technique is selected. A 9- to 10-mm diameter tunnel occupies the central ACL attachment, leaving only the most proximal and distal few millimeters of the attachment not occupied by a graft. It is important that the ACL femoral tunnel not be placed too far posteriorly because this produces excessive graft tension with knee extension. In addition, the graft should not be too distal at its femoral attachment because this shortens the intraarticular graft tibiofemoral length. The location for a single-tunnel ACL reconstruction is more distal than recommendations for a 1 o'clock tunnel that replaces only the proximal portion of the ACL femoral attachment. However, it has not been determined in clinical studies if a difference exists between the 1 o'clock and central femoral graft placement sites.

In Figure 7-34, A , the hybrid graft position of a proximal femoral and posterior tibial position is shown, which we do not recommend. The preferred central anatomic tibial and femoral position is presented in Figure 7-34, B , and the femoral position is presented in Figure 7-34, C . In our opinion, it would not be possible from a clinical standpoint to examine the difference in knee stability (pivot shift and Lachman tests and subjective symptoms) between a central femoral ACL graft location (that occupies the central two thirds, single graft) and a proximal femoral ACL graft location (that occupies the proximal two thirds, single graft). Both graft locations are described as anatomic and avoid placing the graft in the distal two thirds that would represent a shortened ACL construct. A recent study suggested that a central femoral anatomic placement, achieved using an AM approach, was associated with a higher failure rate compared with grafts placed through a transtibial approach (5.16% and 3.20%, respectively). However, this study only used revision surgery in the calculation of failure and patients were followed only for a mean period of 22 months postoperatively.

Graft Selection

The principles of ACL surgery are summarized in Table 7-8 . Currently, there is no standard graft choice for ACL reconstruction. Autograft tissue sources include B-PT-B, quadriceps tendon-patellar bone (QT-PB), and STG tendons. We prefer B-PT-B autogenous grafts in athletes, a recommendation supported by several long-term studies and in investigations showing a higher rate of ACL primary reconstruction failure after allografts compared with autografts, especially in younger patients. A B-PT-B autograft is not recommended if there is associated patellofemoral arthritis (grade 2B classification, see Chapter 44 ), anterior knee pain, or history of patellar subluxation or dislocation. A B-PT-B autograft is not performed when patient issues suggest a decreased ability to follow the more involved rehabilitation program and cope with the initial pain related to this graft. In recreational athletes and more sedentary patients, a four-strand or six-strand STG autograft is recommended. Modern STG graft fixation methods have increased the success rate, and the rehabilitation postoperatively is easier on the patient. There is a suggestion in the literature of increased STG failure rates with associated physiologic posterolateral laxity. Although allografts offer technical ease and reduced donor site pain, there are risks of disease transmission, a biomechanically inferior graft, and biologic reaction to irradiation and chemical sterilization processing. A recent meta-analysis demonstrated that, when compared with low-dose irradiated allografts (<2.5 mrad), nonirradiated allografts produced superior stability (Lachman, pivot shift, KT-2000 tests), had a lower risk of revision, and higher Lysholm scores. The orthopedic surgeon should understand the sterilization techniques used by the tissue bank and use grafts supplied by banks accredited by the American Association of Tissue Banks.