Abstract
Introduction
There are several advantages of unicompartmental knee arthroplasty (UKA) in the treatment of isolated compartment osteoarthritis (OA) compared to the conventional total knee arthroplasty. Although various series report similar survivorship results, the national registries tend to show higher revision rates among the UKA. Persisting, unexplainable pain is a leading cause for UKA revision surgery. Therefore it is essential to investigate the various patient specific characteristics which might influence outcome following UKA in order to minimize revision rates and optimize clinical outcomes. The purpose of this study is to evaluate the influence of the various individual patient factors, including pre-operative radiographic parameters, on the outcome following UKA.
Methods
168 consecutive patients who underwent robot assisted UKA (MAKO Tactile Guidance System, MAKO Surgical Corporation, Ft. Lauderdale, FL, USA) were included. The investigated pre- and/or postoperative parameters included gender, BMI, age, type of tibial implant (inlay versus onlay), laterality, state of OA (i.e. Kellgren and Lawrence grade) of the operated and non-operated compartment and mechanical axis alignment.
Pre-operatively and at a minimum of 1 year (average 1.97 years, range 1 – 4.2 years) following surgery, patients were asked to complete the Western Ontario and McMaster Universities Arthritis Index (WOMAC) questionnaire. It is subdivided in three separate scales (i.e. pain, stiffness and function). A score of 0 represents the best possible outcome and a score of 100 the worst. A p-value <0.05 was considered statistically significant.
Results
144 (85.7%) knees underwent medial UKA and 24 (14.3%) knees underwent lateral UKA. The average age of the patients at the time of surgery was 63.5 years (range 43.2 – 91.1 years) old. 68.9% of patients had a BMI < 30 m2/kg and 31.3% had a BMI > 30 m2/kg. Tibial inlay implants were used in 40 (23.8%) cases and tibial onlay implants were used in 128 (76.2%) cases.
Evaluating the separate factors, we noted no significant pre-operative WOMAC differences (figure 1). Age, KL-grade and type of tibial implant had a significant post-operative influence on outcome. Younger patients had significant inferior WOMAC scores (15.9±15.5) compared to older patients (9.9±10.4, p=0.015), including all three WOMAC domains (figure 2). Patients with an onlay implant had a significant better WOMAC score (11.8±12.1) than patients with an inlay implant (17.6±11.8, p=0.032). Furthermore, those patients experienced less pain than patients who received an inlay implant (9.5±12.6 versus 17.1±19.1, p=0.007). Patients with a more severe state of pre-operative OA of the operated compartment had significant better outcome than patients with a pre-operative mild state of OA (figure 3). All other evaluated parameters did not show any significant differences post-operatively.
Conclusion
Our data suggest that pre-operative care should be taken with the significant influence of age and the state of OA on the clinical results following UKA. Furthermore our data discourages the future use of tibial inlay implants.
