Unicompartmental knee arthroplasty in outliers

The ACL-deficient knee

A deficient ACL has been said to be a contraindication to UKA based on criteria that were established in the late 1980s. The rationale for this is that a fixed-bearing UKA that imparts little or no anteroposterior (AP) constraint will be unable to restore AP stability to the knee. There are some reports that claim short-term success in these patients if an ACL reconstruction is performed prior to or in conjunction with the arthroplasty. ^, Recovery, however, with such a combined procedure(s) can be protracted. The authors, however, have had success performing a fixed-bearing medial UKA in ACL-deficient knees as long as the patient’s tibial wear pattern has not progressed posteriorly beyond the mid-third of the tibial plateau and the patient has no subjective complaints of instability. As noted in Chapter 3 , the normal tibial wear pattern in medial osteoarthritis is anterior (see Fig. 3.6 A). As the disease progresses in an ACL-deficient knee, the wear pattern moves posteriorly ( Fig. 9.1 ). Once it reaches the posterior third of the plateau, stability cannot be restored and long-term success is unlikely. In a mobile-bearing construct, bearing dislocation is also possible.

If a fixed-bearing UKA is performed in an ACL-deficient knee with a mid-third plateau wear pattern, it is important to apply little or no posterior slope to the tibial resection to discourage posterior subluxation, posterior polyethylene wear, and potential tibial loosening. The authors prefer a slope of zero to no more than 3 degrees in these patients ( Fig. 9.2 ).

Osteonecrosis

Isolated spontaneous osteonecrosis of the medial femoral condyle with secondary unicompartmental arthritis can be successfully treated with a UKA as long as there is adequate healthy bone remaining after femoral component preparation ( Fig. 9.3 ). This can often be predicted preoperatively by obtaining an MRI to assess the surrounding bone structure of the condyle. The status of the remaining bone of the opposite condyle can also be assessed as UKA is contraindicated in multifocal disease such as associated with lupus erythematosus, high-dose steroid usage, and Caisson’s disease.