Introduction

Unicompartmental knee arthroplasty (UKA) has been used for several decades to treat arthritis of a single compartment of the tibiofemoral joint that has otherwise failed nonoperative management. The concept is an appealing one: replace only the worn part of the knee joint. The perceived advantages of a UKA compared with a total knee replacement are considered to be a shorter recovery, an earlier return to function, and an increased range of motion. Fixed-bearing UKA has proven successful over the years. Favorable outcomes require proper patient selection, restoration of limb alignment, and careful surgical technique. The previous rigid patient selection criteria for an ideal surgical candidate have been slowly expanding over recent years. Fixed-bearing UKA has been shown to provide excellent patient satisfaction and functional results. Careful review of long-term results is important to justify the continued role of UKA in the treatment of single-compartment knee arthritis.

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

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.

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 reviewed 140 consecutive Marmor cemented UKAs with an all-polyethylene tibial component inserted between 1975 and 1982 and published their long-term outcomes. At final follow-up, 34 patients (48 knees) were alive and available for review. Only four patients (four knees) were lost to follow-up. There was a minimum of 15 years’ follow-up on 29 patients (40 knees). At this length of follow-up, 12.5% of UKAs required revision. The survivorship analysis showed encouraging results. Using revision surgery for any reason as an end point, survivorship was 84% at 22 years. Using revision surgery for aseptic loosening as an end point, survivorship was 93% at 22 years. Disease progression in the contralateral compartment and tibial subsidence with wear were the major long-term problems. Overall satisfaction, however, was excellent in these patients. In 2002, Argenson et al.¹² reviewed 160 UKAs (145 medial and 15 lateral) with an average of 5.5 years’ follow-up (range, 3–9.3 years). Mean patient age was 66 (range, 35–88). Average Hospital for Special Surgery (HSS) score improved from 59 (range, 10–90) to 96 (range, 50–100). Ninety-two percent of these knees were graded as excellent. The 10-year survivorship was 94% with revision for any reason or radiographic loosening as the end point.

Naudie et al.¹³ in 2004 reported on their results from 113 medial UKAs. They noted excellent pain relief and restoration of function with a mean follow-up of 10 years. With revision or radiographic loosening as an end point, survivorship was 93% at 5 years and 86% at 10 years. Mean Knee Society knee and function scores improved at final follow-up to 93 and 80 points, respectively. In 2005, Berger et al.¹⁴ reported on 62 consecutive UKAs with a minimum of 10 years’ follow-up. Mean postoperative HSS score was 92, with 92% good to excellent results. The 10-year survival rate was 98%. In addition, Berger et al.¹⁵ reported that, at 10 years’ follow-up, patellofemoral symptoms were present in only 1.6% of patients. However, at 15 years’ follow-up, 10% of patients had moderate to severe patellofemoral symptoms. Although in this series progressive patellofemoral arthritis was the primary mode of failure, clinical results and survivorship were excellent. O’Rourke et al.¹⁶ in 2005 reported on a series of 136 consecutive UKAs with a minimum 21 years’ follow-up. With revision for any reason as the end point, survivorship was 84% at 20 years and 72% at 25 years. The patients most at risk for revision were younger at the time of surgery. Although the clinical and functional scores in this group were relatively low at this long-term follow-up, it is important to note that this was a group of elderly patients with multiple medical and orthopaedic comorbidities. Steele et al.¹⁷ reviewed the Bristol database to determine survivorship beyond 10 years of a fixed-bearing UKA. Although the mean Bristol knee score fell from 86 to 79 during the second decade, survivorship was satisfactory into the second decade (85.9% at 20 years and 80% at 25 years).

A review of the literature highlights the important role of implant design in fixed-bearing UKA outcomes. A paper by Eickmann et al.¹⁸ in 2006 reviewed 411 fixed-bearing medial UKAs placed by a single surgeon in the 1980s to 1990s. There was a 9-year survivorship of 80%. This result was highly dependent on implant design. Nine-year survival improved to 94% when tibial component thickness was greater than 7 mm and polyethylene shelf age was less than 1 year. Factors that were associated with revision included younger patient age, thinner tibial component, increased polyethylene insert shelf age, and implant system utilized.

Lateral UKA Results (Table 19–2)

As lateral compartment osteoarthritis of the knee is relatively rare, results of fixed-bearing UKA for lateral osteoarthritis are limited. Although many series contain results of both medial and lateral unicompartment arthroplasty, there are few published studies of isolated lateral compartment arthroplasty. The limited data suggest that the indications for lateral compartment arthroplasty are not commonly met. Ashraf et al.¹⁹ in 2002 published their experience with the St. Georg Sled in 88 knees. With mean clinical follow-up of 9 years (range, 2–21 years), 15 knees had revision surgery. The 10-year survival rate was 83%. At 15 years, the survival rate was 74.5%. The authors pointed out that these results are acceptable since 4 of the 15 revisions were due to breakage of the implant. After redesign of the implant in 1988, there were no further revisions due to implant failure. At 10 years’ follow-up, 78% of patients had excellent or good results. The authors noted that the surgeries were performed by multiple surgeons, suggesting that these results are reproducible.

Pennington et al.²⁰ in 2006 discussed the concept of the “screw-home” mechanism in relation to proper positioning of the tibial component. The authors stressed the importance of positioning of the tibial component in 10–15° of internal rotation. This modification to the tibial component positioning was made after multiple observations of the femoral component overriding the anterior-medial aspect of the tibial component with knee extension. Placing the tibial component in 10–15° of internal rotation allows centerline articulation of the components throughout the entire range of motion. They further reported on 29 consecutive lateral unicompartmental arthroplasties using this modification, with a mean follow-up of 12.4 years (range, 3.1–15.6 years). There were no revisions, and the HSS score was excellent or good in all cases.

In 2007, Sah and Scott²¹ published their midterm results on 49 lateral UKAs through a medial approach.²² The average follow-up was 5.2 years (range, 2–15 years). Surgeries were performed for either primary osteoarthritis or posttraumatic arthritis. Mean postoperative Knee Society scores improved to 89 and there were no revisions. Of note, the mean postoperative knee and function scores were significantly higher for patients with primary osteoarthritis than patients with posttraumatic arthritis. Interestingly, while the authors noted that 102 knees were originally scheduled for lateral unicompartmental arthroplasty, only 49 of the knees actually received a lateral unicompartmental arthroplasty. For the remaining 53 cases, the initial plan was aborted after direct visualization of all compartments, and the case was converted to a total knee arthroplasty. In the authors’ opinion, the familiar medial arthrotomy used in this study allows for an easy conversion to total knee arthroplasty if necessary.

Argenson et al. ²³ in 2008 reported long-term results of 40 lateral unicompartmental arthroplasties with an average 12.6-year follow-up. Indications for surgery included a preoperative range of knee flexion over 100°, full extension, and a stable knee in the frontal and sagittal planes. In the later years of the study, they preformed preoperative varus/valgus stress radiographs to evaluate the medial compartment and to determine correction of the deformity. At final follow-up, 62% of patients were enthusiastic, 24% were satisfied, and 14% reported no change or were not satisfied. There were only four revisions for symptomatic osteoarthritis progression. Survivorship of the implants was 92% at 10 years and 84% at 16 years.

Conclusion

A review of the literature regarding fixed-bearing UKA confirms the importance of proper patient selection. While there has been some cautious expansion of traditional indications in terms of patient age, activity, and weight, published studies continue to emphasize the importance of alignment, preoperative range of motion, and limited disease in other compartments. In general, mild to moderate disease appears to be well tolerated in the patellofemoral joint, and the majority of authors feel an intact anterior cruciate ligament is an advantage for a fixed-bearing UKA. Careful surgical technique (good cementation, removal of loose cement and bone fragments, and appropriate implant-to-implant alignment) is paramount. Utilization of a well-designed implant with accurate instrumentation, modern polyethylene design, and good fixation is important. Recent studies of fixed-bearing UKA demonstrate long-term (over 10 years) survival rates of 90%. Patient selection criteria are paramount, and improved surgical technique, instrumentation, and implants contribute to the longevity of the fixed-bearing UKA. The relatively recent surge of interest in “minimally invasive” surgery has led to an increase in the use of UKA. Currently there are inadequate data to comment on how that will influence UKA results. Further study is necessary.