CHAPTER 19 Fixed-Bearing Uni
Long-Term Outcomes
Historical Background
Modern UKA implants have evolved from the early designs of MacIntosh and McKeever.1 These prostheses, introduced in the 1950s and 1960s, were metallic hemiarthroplasty implants designed to resurface only the tibial plateau. Early reports of both implants were encouraging; however, metallic hemiarthroplasty never gained popularity due to early loosening and to the advent of metal-to-plastic cemented arthroplasty.2
Classic Indications
In 1989, Kozinn and Scott3 published their classic article detailing the selection criteria for appropriate candidates for UKA. This involves consideration of the patient’s age, weight, occupational and recreational demands, range of motion, extent of angular deformity, and intra-articular pathology of the knee. According to their criteria, patients over 60 years old with a low-demand lifestyle are the best candidates. Patients should not be obese; ideally, patients should weigh less than 82 kg (180 pounds). They should have minimal pain at rest, a preoperative range of motion of at least 90°, and no more than a 5° flexion contracture. The angular deformity in the coronal plane should be less than 15° (10° of varus to 15° of valgus) and must be passively correctable to neutral after removal of tibial osteophytes. Intraoperatively, examination of the patellofemoral joint and opposite femoral compartment should not reveal exposed subchondral bone. In addition, the best results are obtained with intact cruciate ligaments. Patients with generalized inflammatory arthropathy are not candidates for UKA. Chondrocalcinosis is considered a relative contraindication for this procedure. Avascular necrosis is not contraindicated as long as adequate healthy bone is available to support the implants.
Expanding Indications: Patient Age, Activity, and Weight
Traditionally, UKA has been used to treat elderly, low-demand patients. However, the indications may be expanding. Pennington et al.4 in 2003 reviewed the results of UKA in patients 60 years of age or younger. These were all physically active patients. At the time of surgery, all patients were employed and participated in high-demand activities. Forty-five UKAs were reviewed at a mean of 11 years. Only three knees were revised. For the remaining 42 UKAs, 93% were rated as excellent. Survivorship was calculated at 92% at 11 years.
Similarly, Parratte et al.5 in 2009 reviewed 35 UKAs in patients less than 50 years old. They reported good clinical results and survivorship (80% at 12 years), but also noted that polyethylene wear is the major concern following UKA in this younger age group.
Kozinn and Scott3 believed that weight in excess of 82 kg should be a contraindication for a UKA. In support of this criterion, Berend et al.6 reported that body mass index (BMI = kg/m2) greater than 32 predicted early failure and reduced survivorship. The 82-kg cutoff weight limit continues to be challenged, however. Some cautiously suggest that the cutoff weight limit for a UKA could be raised to 90 kg.7 Naal et al.8 reported that BMI had no association with early clinical outcome or implant failure. Tabor et al.9 also suggested that youth and obesity should not be considered contraindications to UKA. In fact, in their analysis, obese patients had better survivorship than nonobese patients at 20 years.
UKA Long-Term Results (Table 19–1)
Much of the published literature combines medial and lateral UKAs. One of the earliest long-term outcome studies of fixed-bearing UKAs was by Marmor in 1988.10 This included 60 Modular Marmor arthroplasties, 53 medial UKAs, and 7 lateral UKAs. At a minimum of 10 years’ follow-up, patients maintained a 70% satisfactory result.
Table 19–1 Long-Term Results of Studies Including Both Medial and Lateral Fixed-Bearing Unicompartmental Knee Arthroplasty
Over the last decade, there have been numerous studies demonstrating the excellent long-term outcomes and survivorship of more modern fixed-bearing UKAs. Squire et al.11
