Indications for Anterior Cruciate Ligament Reconstruction in Anterior Cruciate Ligament– Deficient Patients Undergoing Unicondylar Knee Arthroplasty

Introduction and Pathophysiology

Anterior cruciate ligament (ACL) rupture is commonly associated with osteoarthritis (OA) of the knee. There are two pathologies to ACL rupture in knee OA. ACL rupture can be secondary to primary OA, or it can be the primary problem (e.g., traumatic rupture) leading to secondary OA.

In the first case, knee OA appears to be the initiating disease. Typically OA starts as anteromedial OA. In these instances, in the presence of intact ligaments, the tibial erosion is located anteriorly and centrally on the medial plateau, giving rise to the term anteromedial osteoarthritis . As the degenerative process progresses, the tibial erosions extend more posteriorly on the tibial plateau, with progressive degeneration in the ACL. Harman et al. demonstrated that in ACL-deficient (ACL-D) knees, wear was found to be 4 mm more posterior along the tibial plateau than knees with functioning ACL. Mullaji et al. and Moschella et al. also demonstrated similar findings. With the extension of tibial plateau wear posteriorly, the femur no longer rides on normal cartilage during flexion. Medial collateral ligament (MCL) develops structural shortening, as it is no longer stretched out to its normal length during knee flexion. This sequence of progression will eventually result in a fixed intra-articular deformity. This is also associated with attrition of the ACL and subsequent progression of arthritis to the lateral compartment. The most susceptible knee ligament to attrition is the ACL. ACL is an intrasynovial intra-articular ligament, thus putting it at risk of nutritional insufficiency in chronic synovitis. Moreover, the development of osteophytes around the femoral intercondylar notch causes impingement of the ACL, compromising its blood supply and eventually leading to attrition of the ligament. This patient group can be labeled as those with primary OA and secondary ACL rupture.

In the second case, OA can develop in a patient with traumatic ACL rupture being the primary problem. The incidence of OA after ACL injury ranges from 33% to 70%. Rupture of the ACL leads to increased instability of the knee and recurrent excessive and unphysiological tibiofemoral joint translation. These abnormal knee kinematics cause microfracture of cancellous bone and fissuring of overlying cartilage. Primary rupture of the ACL can also accelerate degeneration through alteration of the biochemical condition of the knee joint to favor cartilage catabolism. Analysis of the knee synovial fluids in patients with ACL rupture or meniscal tears revealed elevated inflammatory cytokines, keratin sulfates, stromelysin, and tissue inhibitor of metalloproteases, which will accelerate degeneration of the cartilage. In such cases, the cartilage defect and bony erosion on the tibial plateau tend to be central and posterior on the tibia (posteromedial OA). This is likely to be due to recurrent episodes of giving way, in which posterior femoral subluxation in the medial compartment places a heavy load on the posterior meniscus and posterior articular cartilage of the tibia, producing meniscal tears and the development of arthritis. In some cases the rest of the knee joint remains essentially intact, with no shortening of the MCL. This is probably because in extension the intact distal femoral cartilage is in contact with intact anterior tibial cartilage, so the varus deformity is corrected and the MCL retains its normal length. This patient group can be labeled as those with primary ACL rupture and secondary OA.

Surgical Options

There is a huge armamentarium of surgical options at the surgeon disposal for these patients: arthroscopic débridement with or without ACL reconstruction (ACLR), high tibial osteotomy (HTO) with or without ACL construction, unicondylar knee arthroplasty (UKA) with or without ACLR, and total knee arthroplasty (TKA).

The patients with primary OA and secondary ACL rupture tend to be older with more advanced knee arthritis instead of anteromedial osteoarthritis (AMOA). In these cases, TKA is frequently the most appropriate treatment option. In a small proportion of these cases, we have implanted UKA without ACLR, and the details are provided in subsequent paragraphs.

The patients with primary ACL rupture and secondary OA tend to be younger, more active, participate in higher demand activity, and would prefer to preserve their native knee. Moreover, unlike the first group of patients whereby the knee tends to be stiff, these patients often complain of instability in addition to significant knee pain. Thus other than alleviation of the knee pain, the treatment option also has to address the issue of knee instability. Arthroscopic procedure alone tends to give only temporary relief of symptoms. Reconstruction of the ACL may be indicated in patients complaining primarily of instability, but does not address the degenerative process. HTO with or without ACLR may improve the patient’s symptoms but does not completely abolish pain, and lateral compartment OA tends to be a common sequelae. ACLR (simultaneous or staged) is the preferred treatment option in young and active patients with primary ACL rupture and secondary OA.

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