Mobile vs. fixed bearing unicondylar knee arthroplasty: A randomized study on short term clinical outcomes and knee kinematics
Introduction
The clinical results of unicondylar knee arthroplasty (UKA) have improved significantly over the past 30 years thanks to improved implant designs, new materials, advances in surgical techniques and proper patient selection. Recent studies have shown that 10-year survivorship of above 90% has been achieved [1], [2], [3], [4]. UKA has been regarded as a reliable procedure in the treatment of unicompartmental osteoarthritis.
However, whether a fixed or a mobile miniscal bearing UKA should be used is still controversial. Direct comparison has been made in several studies [5], [6], [7], however, the reported results have been variable. While Emerson [5] found a better component survivorship in the mobile bearing UKAs, Gleeson [6] revealed that patients receiving a fixed bearing UKA had better knee function and pain relief. Yet in another study by Confalonieri [7], no significant difference in clinical outcomes was found.
This randomized prospective study was conducted to further document the comparison between fixed and mobile bearing UKAs. We focused on knee kinematics, development of radiolucencies at the tibial component and clinical outcomes two years after the surgery. Our hypothesis was that the mobile bearing prosthesis generates closer to normal knee kinematics and therefore has a better functional outcome as well as improved tibial component fixation.
Section snippets
Patient recruitment
Between May 2001 to June 2003, 56 knees in 48 patients (34 male, 14 female, mean age of 72 years) undergoing unicompartmental knee arthroplasty for medial compartmental osteoarthritis (OA) were randomized into a fixed (Miller/Galante, Zimmer, Warsaw, USA) or a mobile meniscal bearing (Oxford, Biomet, UK) knee prosthesis. The inclusion criteria followed those outlined by Kozinn and Scott [8]: Noninflammatory osteoarthritis of the medical compartment, a mechanical axis deformity < 10° varus or 5°
Kinematic analysis
Knee kinematics was evaluated for the first 22 knees (11 fixed and 11 mobile bearing knees) at 2 years postoperative. The reason to have 22 knees examined for kinematics was due to a power calculation of detecting a 0.3 mm difference in translation and 3° difference in rotation with a power of 80% at 0.05 significant level. Kinematics assessment was conducted with the patients standing on the examined leg performing knee flexion of 0°, 30°, 60°, 90°. Radiographs were taken respectively at each
Missing data
Two mobile bearing knees were revised before 2 years: one for early infection and another for aseptic loosening of the tibial component. Two patients with three UKA (one mobile and two fixed bearings) died of unrelated courses before 2 year follow-up. While the basic information including age, gender, BMI was presented for these four patients (Table 1), they were excluded from the comparisons of 2 year results.
Statistics
Statistics was performed using SPSS. Mann–Whitney U-tests or Chi-square test were used to compare the two groups regarding patient demographics, radiographic assessment and clinical outcomes. ANOVA were used to compare the overall differences in kinematics between the two groups. Differences at each knee flexion position were also compared using Mann–Whitney U-tests. A p < 0.05 was regarded as statistically significant.
Kinematics
The mobile bearing knees showed a larger and an incremental increase in tibial internal rotation than the fixed bearing knees (4.3°, 7.6°, 9.5° vs. 3.0°, 3.0°, 4.2°, respectively, at 30°, 60°, 90° of knee flexion) (Fig. 3), with significant difference at 90° (Mann–Whitney U-test, p = 0.043). The medial femoral condyle in the mobile bearing knees remained 2 mm from the initial position in relation to the tibia during knee flexion, whereas it moved 4.2 mm anteriorly from the initial position in the
Discussion
Knee joint kinematics has been used as an indicator to evaluate the performance of the replaced joint after knee joint arthroplasty [11], [12], [13]. While it is still not clear if the restoration of normal knee kinematics is possible or necessary in knee joint arthroplasties, mimicking a normal knee function after joint arthroplasties may be expected to ensure a lasting and good functioning joint [12]. Normal knee kinematics during knee flexion is characterized as 5–10° tibial internal
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