TKA Balancing

Laxity in Normal Knees

Before I begin our quest of balancing an arthritic knee, let us first look at the normal knee. There are various studies in normal subjects and nonarthritic cadavers using radiographs, MR scans, navigation, and robotics with different stresses applied to determine what can be defined as “envelopes of passive knee joint motion” (p. 705). All these studies show that in the normal knee, medial and lateral laxity are not equal, and this difference varies with flexion. In general, the lateral gap is greater than the medial gap, and the difference increases with flexion. Even in cadaveric studies, it has been shown that there is an asymmetry of medial and lateral gaps right through from extension up to flexion; this gap is greater in flexion than in extension. There are no studies that suggest that these gaps are equal medially and laterally and that the flexion and extension gaps are equal. This is true not only of varus-valgus but also of anteroposterior and rotational laxity. Whether gaps in a prosthetic knee should mimic the normal joint or seek to establish equal gaps is a subject of debate.

With stress radiographs using a tensioner with 15 Nm torque, varus laxity of 2.8 degrees, and valgus laxity of 2.3 degrees in full extension in healthy subjects was reported. (Applying elementary trigonometry, 1 degree roughly equals 1 mm over the length of a normal femur or tibia.) When knees were flexed to 90 degrees and imaged in neutral and under a varus-valgus stress in an open MRI system, the lateral joint gap opened by 6.7 mm with a varus stress, whereas the medial joint gap opened by only by 2.1 mm, indicating that the lateral joint gap is significantly lax. One study has shown that the mean varus-valgus limits are smallest at 0 degrees flexion and increase nearly linearly with flexion; the varus limit increases more rapidly than the valgus limit. The ranges of the varus and valgus limits are both largest at 90 degrees flexion (2.6 and 1.1 degrees, respectively). A robotic study in cadavers has shown that the native medial and lateral gaps are tightest in extension at 1.3 and 2.2 mm, respectively. These gaps increase by 3 to 4 mm with 0 to 20 degrees of flexion and then plateau. The lateral native gap is 1.3 mm larger than the medial gap throughout the range. Generally, larger differences in laxity of more than 2 to 3 degrees between medial and lateral gaps over the entire range are not seen.

Effect of Stability on Patient-Reported Outcome Measures

Residual imbalance of 2.8 and 1.3 degrees varus in extension and flexion, respectively, was not associated with significant differences in early clinical results of postoperative ROM and the subscales of (symptoms, patient satisfaction, patient expectation, functional activities) of Knee Society Score (KSS) among the groups categorized according to the varus-valgus gap angle and the laxity. However, excessive intraoperative medial joint laxity of ≥4 mm at 90 degrees flexion decreased patient satisfaction at 1 year.

Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores of patients with <3 mm gaps showed worse scores for two functional items demanding knee flexion (bending to floor and getting on/off toilet), and 3 to 4 mm laxity at 90 degrees might be necessary to carry out daily life activities. Anteroposterior (AP) laxity measured with an arthrometer at 60 degrees knee flexion significantly correlated with Knee Injury and Osteoarthritis Outcome Score (KOOS) pain score. AP laxity of ≥7 was significantly associated with a subjective feeling of instability in TKA patients.

Similarly, it has been suggested that a controlled flexion gap increase of 2.5 mm may have a positive effect on postoperative flexion and patient satisfaction after TKA. In contrast, WOMAC scores were better in TKAs with a medial-lateral balanced (<2 mm) gap as were outcomes in terms of physical functioning, bodily pain, social functioning, Oxford and Knee Society scores at 6 months, and improved social functioning scores at 2 years with flexion-extension gap differences of ≤2 mm. Knees in which the difference between varus and valgus laxity was <2 degrees had greater ROM by 10 degrees whereas those with >2 mm difference had lesser ROM by 9 degrees. Smaller medial gaps at 60 and 90 degrees of flexion have been shown to play an important role in achieving medial pivot motion with tibial internal rotation; moreover, tibial internal rotation provides a better flexion angle after PS-TKA. Patients with medial-lateral instability <5 at 30 degrees at 2 years postoperatively had superior KSS functional knee scores and 36-item Short Form Health Survey (SF-36) scores than those with greater instability.

Thus there appears to be some degree of variation in the reported optimum laxity required for superior outcomes. It would appear that the medial side is key in varus knees, especially if activities involve deep flexion. It needs to be balanced to approximately within 2 mm from 0 to 90 degrees; the lateral gap especially in flexion can be more lax but not >4 mm.

This is borne out by a cadaveric study, using calibrated extensometers sutured to the LCL and sMCL, that found that the strains in both ligaments in the replaced knee are different from those in the native knee. The MCL becomes tighter in the native knee and the LCL relaxes with flexion. Both ligaments were found to be stretched in extension; in flexion the MCL was found to be relaxed, but the LCL was tight, suggesting that measured resection techniques may overstuff the joint. In another study in which transducers were attached to collateral ligaments in the native knee the MCL slackened 2 mm, whereas the LCL slackened 7 mm with flexion. Post-TKA the MCL slackened a further 3 mm and the LCL a further 4 mm during flexion. A 5 degree external rotation of the femoral component slackened the MCL by a further 2 mm and tightened the LCL by 2 mm; the opposite effect was found during 5 degrees internal rotation. Others have reported that a wide range of coronal plane laxity values are associated with highly satisfied TKA subjects and that a controlled flexion gap increase of 2.5 mm may have a positive effect on postoperative flexion and patient satisfaction after TKA. Another study determining the tibiofemoral forces and collateral ligament strain for variations in flexion and extension gaps noted that small variations in gaps had minimal effects on the soft tissue tension between 15 and 100 degrees of flexion. However, increasing the flexion gap by as little as 2 mm may reduce tibiofemoral forces beyond 90 degrees of flexion and a looser flexion gap decreases soft tissue tension beyond 120 degrees of flexion.

It is not inconceivable that it may be the postoperative kinematics of the prosthetic knee that may be responsible for patient satisfaction on patient-reported outcome measures (PROMS). In fact, a 2020 study reported that during closed kinetic chain movements, patients with poor PROM scores after TKA experience more anterior translation on the medial side followed by a medial midflexion instability and less posterior translation on the lateral side in deep flexion than patients with good PROM scores.

Given this background information regarding gaps, some of it conflicting and contradictory, the next sections of this chapter will consider relevant aspects of the patient’s history, clinical examination, and imaging that may have a bearing on balancing gaps during TKA. The surgeon often has a proclivity to bypass these steps and head directly to surgery. The surgeon is in danger of remaining oblivious about key issues that may call for a change in the surgeon’s technique to address special nuances that exist between one knee and the other, often even in the same patient ( Fig. 9.2 ).

Preoperative Planning

• 1.

  Relevant history

  • a.

    Prior soft tissue trauma, which may have led to a PCL or MCL injury that may have a bearing on implant type

  • b.

    Prior fracture with intra- or extraarticular malunion

  • c.

    Sudden worsening of pain above or below the knee signifying a possible stress fracture (wherein, for example, a long stem tibial component may be indicated; cutting the tibia orthogonal to its long axis would be mandated regardless of one’s views on kinematic or anatomical cuts. This would therefore impact flexion and extension gaps.) (See Fig. 9.3 .)

    

  • d.

    Prior surgery (such as high tibial ostomy [HTO], ligament reconstruction) that may be responsible for altered anatomy, kinematics, adhesions, and changes in patellar height (e.g., patella baja)

• 2.

  Preoperative clinical assessment

  • Flexion deformity (FD) is often masked by the leg lying in external rotation. The knee must be rotated internally and examined with the patella pointing upwards so that the FD manifests ( Fig. 9.4 ).

    

  • Hyperextension may not be evident as the leg is lying flat on the examining couch, and it will be missed unless the leg is passively lifted with a hand supporting the ankle ( Fig. 9.5 ). It will need a different technique of balancing than a knee with an FD.

    

  • Varus-valgus stress examination with the knee flexed 10 to 20 degrees gives a very good indication if the deformity is correctable ( Fig. 9.6 ). In full extension the posterior capsule is taut, making assessment of stability and correctability difficult; it is relaxed in slight flexion so mediolateral laxity can be better assessed.

    

  • Patellar tracking especially in valgus knees should be assessed; maltracking will need to be addressed with appropriate releases.

  • Tibial torsion is also masked in patients with a flexion and varus deformity. To detect intorsion, one needs to rotate the leg such that the patella points upward; if the foot and malleoli are internally rotated, it suggests tibial intorsion ( Fig. 9.7 ). Incorrect tibial tray rotational placement may affect knee kinematics.

    

  • Hip ROM has a bearing on ligament balancing; an arthritic or stiff hip or one with altered anteversion may affect orientation of the distal femur and influence femoral component orientation.

  • Neurological examination: weakness of quadriceps or gastrocnemius muscle (as after a stroke) may result in hyperextension, requiring a more constrained implant. Patients with severe combined valgus and flexion deformity are at risk of developing a common peroneal nerve palsy postoperatively, and the neurological status of the lower limb needs to be documented initially.

  • Flatfeet cause lateral deviation of the weight-bearing axis of the limb. It is more common in patients with valgus deformity ( Fig. 9.8 ), though it is frequently seen in patients with varus deformity. In the latter the surgeon may use this knowledge to slightly undercorrect the deformity, which would thus require a lesser release. Also the use of the second metatarsal for judging tibial jig placement may induce an error in rotational placement of the tibia tray because forefoot abduction is present in a severe flatfoot deformity.

    

  • Gait: In addition to observing for flatfeet, it is important for the surgeon to determine whether there is a major adductor thrust ( Fig. 9.9 ) during the stance phase in varus knees. This would signify increased laxity of the lateral structures often with severe medial tibial bone loss and will need to be factored into the balancing technique.

    

• 3.

  Radiographic assessment

  Weight-bearing long hip-to-ankle radiographs along with routine weight-bearing AP, lateral, and skyline views are valuable. A systematic assessment of radiographs should be undertaken. These need to be viewed carefully for the following elements:

  • Valgus correction angle ( Fig. 9.10 ) should be noted if conventional surgery is being performed. This can often vary from 2 to 11 degrees.

    

  • Osteophyte presence, location, and size should be noted. The larger the osteophyte, the more likely that a lesser release may be required ( Fig. 9.11 ).

    

  • Reactive bone remodeling should be sought on the radiograph. Its presence denotes significant deformity and the possibility that reduction osteotomy may be deployed to balance gaps ( Fig. 9.12 ).

    

  • Bone loss or a stress fracture that may need special attention (i.e., augment, graft, sleeve, cone, stem). If a longer stem is required, this may dictate whether a classical 90 degree tibial cut should be performed over a 3 degree or more varus tibial cut to prevent stem tip abutment against the lateral tibial cortex.

  • Extraarticular deformity in the femur or tibia should be identified as these knees may have to be addressed differently to align and balance them; this is discussed later. Lateral views of the length of the femur and tibia are especially mandated to determine presence and severity of extraarticular sagittal plane deformity.

  • Intracapsular deformity is discussed later.

  • Amount of tibial subluxation and extent of lateral opening often imply severe stretching of the lateral structures ( Fig. 9.13 ). The balancing technique will need to take this into account. By the same token, one should look for medial stretching in a valgus knee.

    

  • Patella tracking has been alluded to above.

  • Prior implant such as a staple, anchor, interference screw suggests a prior ligament reconstructive surgery and must alert the surgeon to possible ligament damage.

• 4.

  CT scan

  May be ordered if

  • there are bowing, torsional, and intracapsular deformities in the femur or tibia.

  • there is posttraumatic OA with a concern about ligament avulsion from bone, bone loss, and fracture union.

  • prior HTO was performed with change in tibial slope and rotational malunion.

• 5.

  MR scan

MR scan is useful in situations where the integrity of the MCL/PCL is in doubt.

• 6.

  Technical goals

  • a.

    Coronal and sagittal limb alignment: the surgeon has to select and aim for classic, anatomical, kinematic, or constitutional alignment or a variant of these.

  • b.

    Components: the surgeon should define the acceptable limits of component placement in all three planes and resection thicknesses; they should decide on the type of implant (i.e., CR, PS, constrained, hinge) that is likely to be required and the type of instruments (i.e., conventional, patient-specific instrumentation [PSI], navigation, robot) to be used for this purpose.

  • c.

    Gaps: The surgeon should also be clear regarding (1) technique of gap balancing (measured resection, gap balancing, or a variation); (2) aims in terms of acceptable flexion and extension gap differences—equal and symmetrical medially and laterally or unequal and asymmetrical and, if so, by how many mm or degrees; and (3) method of assessment of gaps and their symmetry.

Interrelationship Between Bone Cuts, Joint Line, and Stability

Each of the surgeon’s decisions will have ramifications in the actual process of achieving balance. Aiming for hip-knee-ankle (HKA) axis of 177 to 180 degrees in a varus knee tight medially in flexion and extension by resecting the proximal tibia in 2 to 3 degrees varus while orienting the distal femoral cut orthogonal to its mechanical axis may result in lesser or perhaps even no release being required to achieve balanced extension and flexion gaps ( Fig. 9.14A ). A rectangular extension gap can also be achieved by cutting the distal femur in varus and externally rotating the femoral component further ( Fig. 9.14B ) while keeping the tibial cut orthogonal. After a varus distal femoral cut and correctly rotating the femoral component, the extension gap would be balanced but the lax flexion gap would be lax laterally ( Fig. 9.14C ). The dashed line represents an orthogonal bone cut. By measured resection of the posterior femoral condyles ( Fig. 9.14D , dashed lines ), there will be relative internal rotation of the femoral component and a 3- to 4-mm lateral laxity at 90 degrees if no medial release is performed. To obtain a rectangular and balanced flexion gap with the component oriented parallel to the transepicondylar axis (TEA), medial release may be required to establish equivalence of medial and lateral gaps or the femoral component must be placed in excessive external rotation ( Fig. 9.14E ). It becomes immediately apparent that it would be expedient for the surgeon to enunciate their goals at the outset, rather than struggle with the various options intraoperatively. Use of PSI can help the surgeon, as will the use of navigational and robotic software.

These goals are intricately connected with the establishment of the obliquity of the articular plane of the prosthetic joint commonly referred to as joint line . Any cut that removes relatively more from the medial femur throws the knee into varus; a deeper medial cut on the tibia would do the same (see Fig. 9.14A ). An excessively deep cut in the medial tibia might be balanced with a deep lateral cut on the femur to keep the overall tibiofemoral alignment correct. However, this combination would lead to undesirable obliquity of the joint line. In the coronal plane in normal controls the joint line is typically in 3 degrees varus, but it can vary widely, with almost half being >3 degrees and even up to 10 degrees. Likewise, in flexion if the axis of the posterior femoral condyles does not have a proper “neutral” rotational orientation, then as the condyles articulate with the tibial plateau in flexion, the femur goes into abnormal rotation or the tibia is positioned in abnormal varus or valgus attitude. Although properly oriented cuts from the perspective of joint line may imply normal alignment, they do not automatically recreate or maintain proper ligament balance.

Excess distal femoral resection ( Fig. 9.15 ) draws the MCL, LCL, and PCL closer to the distal joint line. As a result, it leads to relative laxity in extension, with the same or tighter stability in flexion. Distal migration of the femoral joint line is shown in Fig. 9.16 . It leads specifically to relative tightness in extension and possibly failure to achieve full extension. Anterior displacement leads to laxity in flexion ( Fig. 9.17 ); posterior displacement leads to relatively excess tightness in flexion ( Fig. 9.18 ).

Effect of Altering the Joint Line on the PCL

A femoral joint line that is more proximal places the PCL so that it is either lax in full extension or tightens abnormally as the knee moves into flexion ( Fig. 9.19 ). With the anterior-to-posterior vector of the PCL, the effect of its tightening should be understood. The posterior cruciate protects against abnormal posterior translation of the tibia. Its tightening pulls the tibia forward. Excessive tightening of this ligament can, in the extreme, cause the tibia to be pulled forward so that it dislocates.

Because of its somewhat anterior origin in the intercondylar notch, pathologic tightening of the PCL in extension is rarely a big problem. This is because ligaments originating more anteriorly from the femur will tend to tighten with flexion, whereas those originating more posteriorly tend to relax with flexion and tighten more with extension.

A femoral component that is too anterior would lead to relative laxity of the PCL in flexion, whereas a component that is too posterior would lead to excessive tightness in flexion.

On the tibial side, things are simpler. Displacement of the prosthesis joint line deeper into the tibia leads to increased laxity in all aspects of motion ( Fig. 9.20 ). Similarly, recession of ligament attachment from the tibia leads to laxity in all aspects of the motion cycle, whereas distal advancement leads to tightening throughout the ROM.

The Coronal Plane

The two variations in limb alignment in the coronal plane, varus and valgus deformities, can be considered as mirror images of one another. The words concave and convex are used to refer to the medial or lateral aspect of the deformed joint with varus or valgus changes. In the case of a varus deformity concave refers to the medial aspect and convex refers to the lateral aspect. The following discussion will examine more closely the overall bony contribution to deformity.

Bone Status

It is imperative to locate where the bony deformity resides, as this will have a bearing on the soft tissue sleeve and therefore on the principles of restoring both alignment and balance. Full-length radiographs are invaluable in determining the location of deformity. More recently, phenotypes of varus and valgus arthritic knees have been described; these may be valuable in planning and performing surgery.

Type E 0: Articular deformity

The entire arthritic process, bone loss and deformity, is confined to the articular surfaces of femur and tibia with no extraarticular deformity beyond ( Fig. 9.21 ). The TEA (i.e., the approximate attachment of the ligament origins) is normal as is its orientation with the femoral shaft and mechanical axes.

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