Introduction

Unicondylar knee arthroplasty (UKA) has progressed through two separate time phases since the original designs were developed in the early 1970s. The first phase was fraught with problems related to the prosthetic designs and patient selection.^1–4 The results were good to excellent for the first 10 years after the surgery in the hands of the designing surgeons. In the second decade the results did tend to taper off and were not as good as the reports of total knee arthroplasty (TKA).^5,6 It was difficult for the standard orthopaedic surgeon to reproduce the findings of the designers, and interest decreased in the late 1980s and early 1990s. Insall’s data showed that only 6% of knees satisfied the criteria for UKA, and he favored TKA as the procedure of choice.⁷

Repicci introduced the limited surgical approach (minimally invasive surgery, or MIS) for UKA in the early 1990s, and interest in the procedure increased by the year 2000.^8–13 Newer designs appeared, and the Oxford mobile-bearing UKA became very popular both in Europe and in the United States.^14,15 With this new wave of interest, surgeons looked to improve the clinical results and reviewed the patient selection criteria, the surgical approach, and instruments. If the incorrect patient is chosen, the result will be compromised despite excellent surgical technique and prosthetic design. This chapter outlines the factors involved in the choice process that should lead to a more satisfactory overall result.

History

It is important to understand the patient’s complaints and disability secondary to the arthritic knee. The underlying cause of the arthritis should become evident during the course of the interview. Inflammatory arthritis is not typically acceptable for UKA because the synovial reaction in the knee tends to involve all of the compartments of the knee in an equal fashion, and partial replacement will not adequately address the problem. Previous history of infection, obesity (with a body mass index > 33 or a weight > 225 pounds), and multiple ligament injury to the knee are relative contraindications. The patient should be able to identify the location of the pain on the joint line either medially or laterally. If the patient either cannot localize the pain or is confused about it, the procedure should not be considered. Patellofemoral symptoms are a relative contraindication, and if there are more symptoms with stair climbing than on level surfaces, UKA is probably not indicated. While the reports using a mobile-bearing UKA tend to ignore or deemphasize the importance of the patellofemoral joint, other authors have indicated that this area can lead to significant symptomatology and compromise of the result.

If the opposite knee has been replaced, the surgeon should evaluate the result with the patient. If the result of the previous surgery is excellent, the same procedure should certainly be considered for the other knee because the excellent result becomes the standard for comparison and will be difficult to equal and certainly more difficult to exceed. If the first result is equivocal, the choice for the second side is much easier. The pain should be localized and should be aggravated with activity and better with rest. If the pain is much worse with rest and at night during sleep, the diagnostic evaluation should be even more thorough to be sure that there is no other underlying condition, such as infection or inflammatory arthritis. If the patient has not had any previous replacements, the opposite side should also be evaluated at the same time with the same questions and discussion.

Laboratory Tests

There are some pertinent tests for UKA in order to better guarantee the clinical result. If the erythrocyte sedimentation rate and C-reactive protein are both elevated, the possibility of underlying infection should certainly be ruled out. Patients who are seropositive for the inflammatory arthritides (rheumatoid arthritis, lupus, and gout) should not have a UKA because of the prevalence of the synovitis in the entire joint space.

Physical Examination

The examination should include inspection of the gait and, then, the full evaluation of both lower extremities. There should be a component of antalgia to the gait, and any thrust of the femur on the tibia through the stance phase should especially be noted. As the deformity progresses either in the varus or the valgus knee, the collateral ligament on the compressed side of the joint shortens and the ligament on the tension side lengthens. This ultimately leads to shifting of the tibia beneath the femur with impingement of the lateral tibial spine against the lateral femoral condyle in the varus knee and impingement of the medial tibial spine against the medial femoral condyle in the valgus knee (Fig. 2–1). The shift of the tibia correlates with a lateral thrust of the femur on the tibia through the stance phase of gait in the varus knee and a medial thrust in the valgus knee (Fig. 2–2). This finding is a relative contraindication to UKA and should alert the examiner to correlate the physical finding with the standing anteroposterior radiograph.

Figure 2–1 The tibia has translocated beneath the femur with impingement of the lateral tibial spine into the lateral femoral condyle.