Introduction

Unicompartmental knee arthroplasty (UKA) is an attractive alternative to tibial osteotomy and total knee replacement (TKR) in certain patients. Advantages of UKA include more retained native bone and soft tissues, including both cruciate ligaments in most cases, and less bone resection on both the femur and the tibia. Smaller incisions, less pain, and earlier return of function are also associated with UKAs. The fact that the conversion from a failed UKA to a TKR is usually easier than a conversion of a failed osteotomy to a TKR is another reason that surgeons find UKA an appealing option in the patient with isolated femoral-tibial compartment disease. It is generally agreed that the UKAs are more challenging than TKRs. With that in mind, there are several important considerations when planning a UKA.

The single most important determinant in a good outcome following a UKA is patient selection (Fig. 27–1). Most authors refer to the traditional selection criteria originally written by Kozinn and Scott.¹ These criteria say that UKAs should not be used in patients with more than a 10° fixed flexion contracture or varus or valgus deformity, and they should be used predominantly in thin, elderly, low-demand patients. Flexion deformity is usually considered the most important of these exclusions. More recently, there has been a cautious expansion of the indications to include younger^2,3 and heavier^4–6 patients. While the data are still relatively short term, there are increasing reports of the successful use of this concept in young (less than 60 years of age) patients at midterm follow-up. If this procedure can provide reasonable outcomes at 10 years, the thinking goes, then this relatively less involved procedure will have served as a “time buyer” until a further conversion to a TKR is performed. There is a growing appeal of thinking of a UKA as a patient’s first arthroplasty for the younger patient and a last arthroplasty for an elderly patient. Another reason for the increased use of UKA in younger patients is that the concept is an appealing one to these patients and, with the increased use of the Internet and direct-to-patient marketing, patients in this age category are increasingly aware of these surgical options and frequently seek out these procedures. Weight has been a concern with UKA procedures but, like use in younger patients, there are reports showing that excess weight, in and of itself, is not a contraindication. These reports are still in the midterm, but the traditional criterion of an 85- to 90-kg limit is being challenged.

Figure 27–1 Radiograph showing isolated disease in AP view of the knee.

There are several other important aspects to patient selection. These include the status of the anterior cruciate ligament (ACL), the amount of disease present in the other compartments, the presence of crystalline disease and other inflammatory arthropathies, and location of a patient’s pain. Most surgeons feel that a functioning ACL is important, particularly with mobile-bearing devices. The amount of disease in other compartments is controversial. Most surgeons will accept up to Grade 3 Outerbridge damage but not Grade 4, but some surgeons ignore cartilage damage in the patellofemoral joint entirely. The damage often seen on the medial aspect of the lateral femoral compartment due to irritation of the tibial spine is usually ignored (Fig. 27–2). The presence of calcium pyrophosphate dihydrate crystals on radiographs or at surgery or the finding of an inflammatory synovium at the time of arthrotomy would be considered by most to be a contraindication to proceeding with a UKA. Some surgeons feel that, for a patient to be a UKA candidate, he or she should be able to point with one finger to the area of pain (medial femoral-tibial joint for medial UKA). Patients with more anterior knee pain or those who complain of pain while going up or down stairs are, in their minds, less ideal candidates for this type of surgery. This idea, however, is also being challenged and now many surgeons are not as concerned with the location of the pain as they are with the preoperative radiographs and examination. Another important factor in patient selection is the patient’s understanding of the concept and the procedure. Patients looking for the most predictable surgical management of their knee disease are probably best served by having a TKR despite the isolated nature of their disease. On the other hand, many patients find the idea of a less invasive procedure, where only the diseased part of the knee is replaced, an appealing one even if it does not have as good long-term follow up as a TKR. It is important that the patient understand the differences and the pros and cons of both approaches.

Figure 27–2 Open arthrotomy demonstrating Grade IV changes in the patellofemoral joint: not an ideal candidate for a UKA.

Once appropriate patient selection has occurred, UKA preoperative planning is important. A UKA should be thought of as replacing what has worn away. It should not be used to correct a significant malalignment or deformity. A general sense of the patient’s knee is important. Has the patient always had a varus knee? If so, it is important not to overcorrect the joint line. Using the anteroposterior (AP) radiograph, the surgeon should plan the level of tibial resection to be essentially perpendicular to the long axis of the tibia, although slight undercorrection is acceptable (Fig. 27–3). Because only one part of the joint is being replaced, it is imperative that the lateral radiograph be evaluated so the UKA will reproduce the preexisting posterior tibial slope. Failure to do so will lead to loosening and implant failure. The range of posterior tibial slope has been measured to be anywhere from 0° to 22° (Fig. 27–4).

Figure 27–3 AP view of the knee. The surgeon should aim for perpendicular or slight undercorrection of the knee.

Figure 27–4 Lateral view of the knee. Beware of variability of the posterior tibial slope.

Technique

There are several surgical techniques for UKA implantation that can be utilized with various devices. An extramedullary, tibia-first approach is described here as it is the most commonly used technique. The sequence for this procedure is as follows:

At each step of the procedure there are several keys to success.