Obesity and arthritis of the knee Study (OAKS): study protocol for a prospective cohort study examining the treatments and outcomes of people with obesity and severe knee arthritis.

Abstract

People with body mass index (BMI) ≥35 kg/m² have an approximately 19-fold increased risk of undergoing total knee replacement (TKR); however, many UK integrated care boards (37%) have restrictive policies which limit access to TKR for people based on BMI. Therefore, access to both surgical and non-surgical treatments varies widely, exacerbating existing health inequalities. It remains unclear how decisions about offering TKR are made in people with severe knee osteoarthritis, which weight-loss interventions are provided in practice and how different management pathways relate to patient outcomes among individuals with high BMI. This study will recruit 400 participants with severe Kellgren-Lawrence grade four knee osteoarthritis from eight secondary care centres in England. All participants, irrespective of BMI, will provide baseline clinical data and patient-reported outcome measures (PROMs), enabling characterisation of baseline associations between BMI, knee function and body image.A prespecified subgroup of participants with BMI ≥35 kg/m² (minimum n=105) will undergo longitudinal follow-up at 6 months, 12 months and 24 months, including repeat BMI measurement, PROMs and detailed data on access to surgical and non-surgical interventions, including weight-loss strategies and TKR. For those with BMI ≥35 kg/m², statistical modelling will be used to explore associations between baseline factors and longitudinal outcomes including Oxford Knee Score and weight change at 12 months (n≥105). Structural equation modelling will be used to quantify associations between BMI and knee pain/function mediated by psychosocial factors using data from all participants (n=400). A nested qualitative study of surgeons and patients will explore obstacles and preferences in the management of severe knee osteoarthritis. The study received ethical approval from the West of Scotland REC 5 (Ref: 24/WS/0146) on 10 October 2024. Results will be disseminated through peer-reviewed publications. ISRCTN42984928.