Revision Anterior Cruciate Ligament Injuries

Radiographic Tunnel Positioning

A malpositioned femoral tunnel is typically either too anterior and/or too vertical, and a malpositioned tibial tunnel is typically too anterior. On a lateral radiograph, the tibial plateau and Blumensaat line may be divided from anterior to posterior into four equal quadrants. The tibial tunnel should enter the joint in the posterior third of the second quadrant, and the femoral tunnel should enter in the most posterior quadrant. Taking 0% as the anterior and 100% as the posterior extent of Blumensaat line, a femoral tunnel more than 40% anteriorly along Blumensaat line is considered excessively anterior. Femoral tunnels located at least 60% posteriorly along Blumensaat line and tibial tunnels at least 20% posteriorly along the tibial plateau have demonstrated improved clinical outcomes, whereas those with more anterior femoral or tibial tunnels are associated with increased failure rates and inferior clinical outcomes. Outcomes following revision for excessively anterior femoral tunnel malpositioning primary are improved compared to revision for another cause.

Femoral tunnel malpositioning in the coronal plane can also contribute to graft failure. On an AP or PA radiograph, fixation along the anterior rather than lateral cortex may indicate a “vertical” femoral tunnel ( Fig. 99.2 ). A double-blinded study assessing the results of high and low femoral wall position showed improved International Knee Documentation Committee (IKDC) scores for the low position group. The tibial tunnel should penetrate the articular surface at the midpoint of the plateau on an AP or PA radiograph. A cadaveric study using landmarks to determine anatomic tunnel location demonstrated the sagittal tibial tunnel angle to be 75 degrees and the coronal angle to be 65.7 degrees. A tibial tunnel angle greater than 75 degrees in the coronal plane may result in a loss of flexion and increased sagittal (AP) laxity.

Graft Impingement

A hyperextension lateral radiograph is the best method to assess for anterior graft impingement, which is associated with increased rates of effusion, lack of extension, and increased failure rates. The Multicenter ACL Revision Study (MARS) group demonstrated that approximately 51% of patients undergoing revision reconstruction had no graft impingement, 47% had some degree of impingement, and 2% had complete impingement with the tibial tunnel completely anterior to Blumensaat line. A study defined the distance posterior to the anterior edge of the tibia that minimizes the risk of anterior graft impingement, and if the center of the tibial tunnel at the articular surface was at least 22 to 28 mm posterior to the anterior edge of the tibia, then no graft impingement resulted. Another study demonstrated that if the tibial tunnel is positioned in the posteromedial portion of the native ACL footprint, then graft impingement does not occur.

Advanced Imaging

Computed tomography (CT) may be obtained to provide a more detailed assessment of existing tunnel location, tunnel dilation, and bone quality ( Fig. 99.3 ). Excessively posterior femoral tunnel placement may result in posterior wall blowout, limiting fixation options at the time of revision surgery to those relying on lateral cortical fixation or necessitating bone grafting followed by staged ACL reconstruction. The authors routinely obtain magnetic resonance imaging (MRI) preoperatively to assess for concomitant meniscal, chondral, and ligamentous injury, and it can also be useful to assess graft integrity and tunnel dilation. The authors routinely obtain MRI preoperatively. However, its utility may be compromised by artifact if metallic implants were used for fixation during the prior reconstruction.

Causes of Failure of Anterior Cruciate Ligament Reconstruction

Instability following ACL reconstruction may be categorized into early (less than 6 months postoperative) or late (greater than 6 months postoperative) instability. Early laxity generally results from technical errors, failure of graft incorporation, loss of graft fixation, premature return to activity/sport, overly aggressive rehabilitation, or a combination of these factors. Late instability generally results from traumatic rerupture and less often from technical errors. A less common cause of late instability is failure to address concomitant ligamentous pathology at the time of primary reconstruction. With appropriate surgical technique and rehabilitation, primary ACL grafts are at no greater risk of rupture compared with the contralateral uninjured ACL.

Patients undergoing revision reconstructions report trauma resulting in recurrent instability in approximately 55% to 70% of cases, most often from a noncontact injury during sports. However, at the time of revision, the MARS surgeons have deemed the cause of failure to be traumatic in only one-third of cases, technical error in approximately one-quarter of cases, biologic failure (lack of graft incorporation) in 7% of cases, and multifactorial in 31%. Up to 53% of patients undergoing revision reconstruction have some degree of technical error contributing to graft failure, and among these patients, 80% have femoral tunnel malposition. Injury resulting in rupture of a well-positioned and well-fixed graft is nearly as frequent as failure due to incorrect femoral tunnel placement. Interestingly, two decades ago, failure due to malpositioned tunnels was two to three times more likely than traumatic rerupture of a well-positioned, well-healed graft. These differences may be attributable to higher-demand patients expecting a return to their previous activity level, overaggressive rehabilitation protocols that may prevent adequate graft healing, and improved surgical techniques resulting in more consistently accurate tunnel placement during primary reconstruction.

Tunnel Malpositioning

Tunnel malpositioning is the most common technical error during primary and revision ACL reconstruction and can result in excessive graft forces and strain resulting in inadequate graft incorporation, graft loosening, and atraumatic graft failure. Occurring three times more frequently than tibial tunnel malpositioning, excessively anterior placement of the femoral tunnel is the most frequent technical error. This is, in part, due to difficulties in visualizing the native femoral footprint and results in a short graft with excessive graft tension in flexion and an initial loss of knee flexion. During rehabilitation and return to sport, recurrent stretching of the graft leads to laxity and eventual failure. Conversely, excessively posterior femoral tunnel placement results in excessive graft tension in extension with laxity in flexion. A vertical femoral tunnel may provide sagittal (AP) stability but will result in rotational instability.

An excessively anterior tibial tunnel results in graft impingement against the intercondylar notch with loss of terminal extension. Excessively posterior tunnel placement results in graft impingement against the PCL and a vertical graft. This initially results in a loss of terminal flexion, with eventual graft laxity if full flexion is achieved. Errant placement of a tibial tunnel too far medially or laterally can also result in graft impingement on the intercondylar notch, potentially causing articular cartilage injury.

Untreated Ligamentous Laxity

Of patients undergoing revision ACL reconstruction, 3% to 31% of patients may have had unrecognized collateral ligament instability or malalignment contributing to graft failure. Untreated varus malalignment can result in varus thrust and graft attenuation. Similarly, untreated posterolateral or posteromedial corner injuries may result in excessive forces within the graft and early failure; these injuries should be addressed before or during revision ACL reconstruction. In the setting of combined ACL and PCL injury, reconstruction of the ACL prior to addressing PCL insufficiency will predictably lead to ACL failure. As the medial meniscus is a secondary constraint to anterior tibial translation, the graft within a medial meniscus-deficient knee experiences increased forces that contribute to early failure. Meniscal transplant should be considered in these patients.

Primary Reconstruction Graft Choice

Numerous studies and meta-analyses of primary hamstring and patellar tendon autograft ACL reconstructions have demonstrated average failure rates of 3.6%, with no difference in failure rates between these autografts. However, allograft reconstructions have a three- to five-times higher likelihood of graft failure compared with autograft. This may be attributable, in part, to patients having relatively less postoperative pain and a more rapid postoperative rehabilitation following allograft reconstruction, thereby returning to a higher activity level prior to adequate graft healing. Allograft processing may also increase the risk of graft failure, as irradiated grafts have demonstrated a significantly higher failure rate compared with nonirradiated grafts.

Concomitant Intra-Articular Pathology

Knees undergoing revision ACL reconstruction have a higher incidence of chondral injuries and meniscal tears compared to primary reconstruction. In one study, 90% of knees undergoing revision reconstruction had a meniscus or chondral injury and greater than 50% demonstrated both. Similarly, MARS group reported modified Outerbridge grade 2 or higher lesions in 73% of patients undergoing revision, with concurrent meniscal and cartilage damage in 57%.

Meniscus Injury

Although is it known that the medial meniscus plays a critical role in limiting anterior tibial translation in an ACL-deficient knee, it remains unknown if there is a critical amount of medial meniscus that, if removed, predisposes the reconstruction to failure. Previous partial meniscectomy, but not meniscal repair, is associated with a higher incidence of articular cartilage lesions at the time of revision. The MARS group demonstrated an overall 74% incidence of meniscal injury at the time of revision reconstruction, similar to that at the time of primary reconstruction. The prevalence of medial meniscus tears at the time of revision is 40% to 46%, higher than at the time of primary reconstruction. Conversely, there is actually a decreased incidence of new, untreated lateral meniscal tears at revision compared with primary reconstruction. Thus patients undergoing revision due to recurrent instability appear to be continuing to put the medial meniscus at risk for further injury.

Articular Cartilage Injury

Inferior patient-reported outcome scores have been reported following revision ACL reconstruction compared with primary reconstruction, and this is associated with an increased incidence and severity of chondral lesions at the time of revision. Although it is unlikely that these lesions affect stability in the reconstructed knee, increased chondrosis at the time of revision may have detrimental effects on clinical outcomes despite appropriate surgical technique, graft healing, and adequate clinical laxity. Even when controlling for meniscus status (prior meniscectomy versus no prior meniscectomy), there is an increased risk of lateral and patellofemoral compartment chondrosis at the time of revision compared with primary reconstruction. There is a strong association between ACL deficiency and acceleration of degenerative changes, and the status of the articular cartilage at the time of revision reconstruction may be one of the most important predictors of a successful clinical outcome. Patients undergoing delayed revision reconstruction (greater than 6 months following onset of symptomatic instability) have a markedly higher incidence of articular cartilage degeneration compared with earlier revisions (53% vs. 24%), and the prevalence of advanced degenerative changes is also significantly higher in the delayed group. Early restoration of stability may lead to reduced secondary articular cartilage damage and a return to previous activity levels, and it has been suggested that revision reconstruction should be performed within 6 months of graft failure to minimize the risk of degenerative changes and arthritis progression.

Evaluation of Tunnels

Prior tunnel placement may be categorized in one of three ways: (1) accurate and not requiring redirection, (2) completely inaccurate in a location that will not interfere with new tunnel creation, or (3) overlapping, such that the prior tunnel and a properly placed tunnel will partially overlap. Partially overlapping tunnels are generally the most challenging of these three because they require the surgeon to adjust his or her technique due to enlarged tunnels. The scope should be inserted into each tunnel to assess tunnel size and the quality of the surrounding bone to determine the need for grafting and the best method of fixation. The tibial tunnel can be viewed by placing the scope directly into the tunnel through the previously made tibial skin incision. The femoral tunnel can be visualized by placing the scope through the previous anteromedial portal.

Prior Fixation

Bioabsorbable implants are generally difficult to remove and may be left in place and overreamed after guide pin placement. However, metal interference screws must be removed if interfering with the planned tunnel location. In the rare case that properly placed tunnels can be made independent of prior tunnels and metallic implants, the implants may be left in place ( Fig. 99.4 ). If metallic hardware is removed, the size and location of the resulting defect must be considered. The defect may necessitate grafting, either as part of a single-stage of two-stage revision, or the use of larger bone blocks with a patellar tendon autograft or various bone block allograft options.