The final decision to proceed with unicompartmental arthroplasty was made at the time of surgery. Age, weight, and activity level specifically were not determining factors in the decision for unicompartmental replacement. A median parapatellar arthrotomy was used, and all cartilage surfaces were evaluated. Small areas of cartilage loss on the patella, usually seen on the medial facet, were acceptable. Full-thickness cartilage loss on the weight-bearing surface of the lateral compartment and notable inflammatory synovitis were considered contraindications to proceeding with unicompartmental replacement. A total knee replacement was performed in such cases.
Plain radiographs and the Knee Society clinical scoring instrument were used to assess the outcome at each follow-up visit. The Knee Society instrument consists of two separate scales: a knee score and a function score12. All data were collected prospectively. All knees were evaluated with plain radiographs with standard views, with use of the modified Knee Society zones9, to assess for lucencies and osteolysis. In addition, the radiographs were assessed to detect component subsidence, loosening, and progressive degeneration in the remaining joint compartments. Knee alignment was assessed with use of a preoperative and an immediate postoperative 90-cm standing anteroposterior radiograph, in order to determine the location of the mechanical axis with respect to the center of the tibial surface, as described by Kennedy and White13. Zones 1 and 2 are on the medial side of the tibial eminence, and zones 3 and 4 are on the lateral side of the tibial eminence. Zone C is the central part of the tibial plateau. At the final follow-up evaluation, patients who were not able or were unwilling to return for an examination were interviewed by telephone with use of a standard questionnaire, which was based on the knee scoring instrument.
Preoperative and postoperative alignment was compared with the Student t test. Correlation of bearing size to final alignment was determined by the Pearson coefficient. For all tests a p value of <0.05 was considered significant. Survivorship analysis was by the Kaplan-Meier method.
For the fifty-five knees, the mean preoperative Knee Society knee and function scores were 43 and 56 points, respectively. In addition, the mean preoperative alignment was 1.6° of varus (range, 8° of varus to 7° of valgus alignment), and the mean postoperative alignment was 5.6° of valgus (range, 2° of varus to 13° of valgus alignment). Immediately postoperatively, thirty-three of the fifty-five knees had the mechanical axis crossing through the intercondylar notch of the tibia (zone C), and forty-seven knees had the axis crossing the central 50% of the proximal aspect of the tibia (zones 2, C, and 3) (Fig. 2).
At the time of the final follow-up of the original series of fifty-one patients (fifty-five knees), fifteen patients (sixteen knees) had died with the prosthesis intact. These fifteen patients had been followed for an average of 5.1 years (range, 1.2 to 6.2 years) prior to death. Two patients (three knees) were lost to follow-up at 4.2 and 5.2 years, with the implants functioning well. Seven knees in six patients had been revised, at an average of 7.6 years (range, 1.0 to 11.2 years). The remaining twenty-nine knees in twenty-eight patients continued to function at the time of the latest follow-up. This final group of living patients had an average duration of follow-up of 10.4 years, (range, 2.6 to 14.3 years). Of the twenty-nine knees, thirteen had no recent follow-up radiograph and the patients were contacted by telephone, extending the average period of clinical follow-up to 11.8 years (range, 8.8 to 14.3 years). None of the contacted patients reported any changes in implant function. Clinical scoring was updated by combining the subjective symptoms with the most recent functional status contained in the medical record. At the time of the final follow-up, twenty-eight patients had mean Knee Society knee and function scores of 75 and 90 points, respectively.
Six of the knees that had a revision arthroplasty underwent conversion to a total knee prosthesis. Of the six knees, five were revised for progression of arthritis; these included one knee that was revised for acute inflammatory polyarthritis at one year and four that were revised for progression of arthritis in the lateral compartment, with progressive valgus, at an average of 10.2 years. The four knees with lateral compartment progression all had small tibial bearings, with an average preoperative alignment of 4° of varus and an average postoperative alignment of 5° of valgus (range, 2° to 8° of valgus). Immediately postoperatively, the mechanical axis crossed the center zone (zone C) in three of the knees, and it crossed in zone 2 in the fourth, on the medial side of the tibia, and was therefore in slight varus. None of the knees that had lateral compartment arthritis progression were overcorrected into valgus alignment at the time of the initial surgery.
One knee had femoral component loosening that required revision at 3.7 years. Only one size of femoral component was available for the phase-2 components, and the femoral component that loosened was undersized by current sizing recommendations.
No failure was due to subsidence, tibial component loosening, or polyethylene wear. Radiographic zone analysis at the time of the final follow-up, with the radiographic methods used, showed five femora with one radiolucent zone each. On the tibial side, with five zones for analysis, eleven components had a radiolucent line in one zone, four components had a lucent line in two zones, three components had a lucent line in four zones, and two components had a complete radiolucent line around the tibial component but without migration. Only one of these two patients had mild pain but did not desire revision. None of the radiolucent lines demonstrated a divergent pattern, and no osteolysis was seen around any of the components in this study, suggesting a stable interface appearance, and no tibial components were thought to be clinically loose.
One bearing had to be replaced at three years because of osseous impingement in full extension, causing pain and an effusion. The anterior edge of the bearing was striking the femur in full extension. Bearing replacement, with removal of the impinging bone, eliminated the effusion and the pain, with the original metal components intact at eleven years. There were no bearing dislocations in this series.
With the small numbers of subjects in this study, no correlation could be identified between the size of the bearing and the alignment of the knee (Pearson coefficient, r = 0.009). Therefore, by restoring soft-tissue tension of the collateral and cruciate ligaments, optimal mechanical alignment of the knee was consistently achieved.
With use of the Kaplan-Meier method, survivorship at ten years was 85% with revision surgery for any reason as the end point (Fig. 3), 90% with revision for progression of the arthritis in the lateral side as the end point, and 96.3% with component loosening as the end point.
We found that medial unicompartmental knee arthroplasty with the Oxford prosthesis can restore medial soft-tissue balance and can consistently restore the alignment of the limb through the center of the knee. Our patients had no tibial component failure or failure due to polyethylene wear or osteolysis. To our knowledge, polyethylene wear has not been reported as a cause of failure of the Oxford knee1,10,14,15, supporting the mechanical and material benefits of the mobile-bearing design.
Limb alignment is recognized as a determinant of survivorship of medial unicompartmental knee replacement. Hernigou and Deschamps16 found, with a fixed-bearing design, that varus alignment with a mechanical axis of <170°, as measured from the hip to the knee to the ankle on long radiographs, leads to polyethylene failure, and overcorrection into valgus alignment with a mechanical axis of >180° leads to lateral compartment failure. Argenson et al.3, despite showing excellent ten-year survivorship of 94% with a contemporary fixed-bearing unicompartmental knee design, reported two failures from polyethylene wear in a series of 160 arthroplasties.
Failure in our study occurred primarily because of progression of arthritis in the lateral compartment, which we believe was not related to overcorrection of the alignment at the time of surgery. In order to achieve stability of the mobile bearing, there is a tendency to use the largest possible bearing, with the potential unintended consequence of overcorrecting the knee into valgus alignment, thereby overloading the lateral compartment and predisposing the knee to late degenerative arthritis of the lateral compartment. With the numbers studied, we could not identify a correlation between bearing size and postoperative alignment, suggesting that there was, in fact, no tendency to use an overly large bearing. The bearing sizes we used were predominantly small, which allowed removal of minimal amounts of tibial bone. Only three of the fifty-five bearings had >6 mm of polyethylene (mean bearing thickness, 4.5 mm). Progression of arthritis in the lateral compartment noted in this study appears to have been due to the natural progression of the underlying arthritic disease. This was also observed by Kumar and Fiddian14 who noted that it is not always possible to predict which knees will show progression of the underlying arthritis. This observation emphasizes the importance of not performing a medial unicompartmental arthroplasty in a knee that has early degenerative changes of the lateral compartment. Progression of patellofemoral arthritis was not noted to be a mode of failure in this study.
Survivorship analysis in this study is consistent with that published in the literature for this implant. Carr et al.10 reported on 121 knees with 99% survivorship at eight years. Murray et al.15 reported on 143 knees with 98% survivorship at ten years. Svärd and Price17 reported on 124 knees with 95% survivorship at ten years, and Kumar and Fiddian14 reported on 100 knees with 85% survivorship at eleven years.
The Oxford meniscal bearing is completely free-moving, and a major concern for all mobile-bearing designs has been dislocation of the bearing. Except for one study of the records of 699 arthroplasties in the Swedish Registry, in which Lewold et al.1 documented sixteen dislocations attributed to malposition of the components and soft-tissue imbalance with subsequent maltracking of the meniscal bearing, dislocations have been infrequent10,15. The consequence of the study by Lewold et al. was the recommendation that all surgeons undergo specific training on the Oxford knee surgical technique. In a separate study from Sweden, Svärd and Price reported only three dislocated bearings in 124 knees with a ten-year survivorship of 95%17. No bearing dislocations occurred in the present series.
We found that, following Oxford medial unicompartmental replacement, the overall mechanical alignment of the knee was consistently through the center of the tibial plateau. While progression of arthritis in the lateral compartment was the most common reason for revision in the present series, the knees that progressed had not been placed into excessive valgus at the time of surgery but rather were in neutral or slight varus alignment.
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants of less than $10,000 from Biomet. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Biomet). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Texas Center for Joint Replacement, Plano, Texas
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