Abstract
Background
In the late 1980's Michael Freeman conceived the idea that knee replacement would most closely replicate the natural knee joint, if the medial Tibio-Femoral articulation was configured as a “ball-in-socket”. Over the last three decades, medial rotation and medial pivot designs have proved successful in clinical use. Freeman's final iteration of the medial ball-in-socket concept was the Medial Sphere knee. We report the three-year survivorship, clinical outcomes, patient reported outcome measures (PROMs) and radiographic analysis of this implant in a multi-centre, multi-surgeon, prospective observational study.
Methods
Patients awaiting total knee replacement were recruited by four centres. They had no medical contraindication to surgery, were able to provide informed consent and were available for follow-up. Primary outcome was implant survival at six months, one, two, three and five years. Secondary outcomes were patient reported outcome measures: Oxford Knee Score (OKS), Euroqol (EQ-5D), International Knee Society Score (IKSS), IKSS Functional score and Health State score, complications and radiographic outcomes. Radiographic analysis was undertaken using the TraumaCad software and data analysis was undertaken using SPSS.
Results
To date, 328 female and 202 male patients with a mean age 66.9 years and mean body mass index 30.0 were recruited. Three year Kaplan-Meier survivorship analysis of cumulative failure showed an implant survival of 99.46% (95% confidence interval 100 – 96.74), when deaths and withdrawals were treated as censored data. Twelve patients withdrew (2.26%), seven died (1.32%) and two knees were revised (0.38%).
The mean EQ5D, Health State Scores, OKS, IKSS & IKSS Function scores at three years improved significantly from pre- operative scores (Health State Score: 9.91 (65.59 pre-op to 75.50); OKS: 18.82 (19.90 pre-op to 38.72); IKSS: 39.87 (44.39 pre-op to 92.09); IKSS Function Score: 35.03 (49.42 pre-op to 84.45). The mean improvement of EQ5D at three years was: 0.34 (0.48 pre-op to 0.82).
Discussion
Survival of the GMK Sphere to three years in this study was over 99%. Risk of revision compares favourably with UK National Joint Registry (NJR) data. The improvements that are seen in patient reported outcome measures reflect an enhancement in patient function and quality of life.
Conclusion
At three years follow-up, the implant demonstrates satisfactory survival and outcomes. Patient matching and evaluation of more cases, at more time points will allow outcome comparison with other implant options.
