Lateral compartment arthroplasty

Indications

The indications for lateral unicompartmental knee arthroplasty (UKA) are similar to those for medial UKA. The incidence of its need, however, is much less than that for medial replacement. In most experienced UKA practices, lateral arthroplasty comprises about 10% of UKAs.

The patient is more likely to be female and will present with a valgus knee alignment along with lateral knee pain and radiographic changes limited to the lateral compartment. The anterior cruciate ligament (ACL) should be intact. The radiographic presence of chondrocalcinosis is more frequent in lateral versus medial disease and does not contraindicate UKA as long as there is no history of chronic or recurrent active synovitis. As the wear pattern in lateral osteoarthritis is posterior on both the femur and the tibia, a standard standing anteroposterior (AP) radiograph may still show a joint space in end-stage disease. A posteroanterior (PA) standing flexed view may be necessary to demonstrate the loss of joint space and should be obtained routinely ( Fig. 8.1 ).

For the knee to be a candidate for lateral UKA, deformity should be less than 12 degrees of anatomic valgus alignment (or less than 5 to 7 degrees of mechanical axis valgus). Deformity beyond this is often associated with medial collateral ligament laxity, which is a contraindication to lateral UKA ( Fig. 8.2 ).

Surgical principles

The procedure for a lateral UKA is technically more difficult than for medial UKA, in part because it is performed less frequently. Patellar impingement is more likely because the patella normally favors articulating on the lateral rather than the medial femoral condyle in high flexion, making it critical that the leading edge of the femoral component is recessed into the trochlear cartilage at surgery.

As opposed to the wear pattern in medial osteoarthritis, which is anterior and peripheral, the wear pattern in lateral disease is posterior. This fact makes it even more important that the ACL is intact and functioning and also leads to the recommendation that little or no posterior slope be applied to the tibial resection to discourage posterior tracking of the femoral component on the tibial component polyethylene ( Fig. 8.3 ). This tendency for posterior wear and component tracking also means that mobile-bearing articulations are discouraged from being used in lateral arthroplasty to avoid posterior subluxation or dislocation of the bearing. Clinically, a physical examination can elicit bony crepitus if the knee is gently ranged between 30 and 60 degrees of flexion while a valgus stress is being applied. This posterior wear pattern also explains why some patients with significant lateral arthritis have acceptable standing and walking tolerance but have major complaints when their knee is loaded in flexion, such as when negotiating hills or stairs.