The role of excess weight in Osteoarthritis (OA)

It has long been recognised that there is a strong association between increased BMI (Body Mass Index) and the onset of osteoarthritis of the knee. Whether the effect is attributed to the overload of mechanical stress exerted on the joint or as a result of metabolic and inflammatory processes, or both, the fact still remains that epidemiological studies have shown that those with a BMI > 30 are over 4 times more likely to have osteoarthritis of the knee than those with a healthy BMI (i.e. less than 25)¹.
More recently the link between obesity and the rate of progression of knee osteoarthritis has also been recognised. In fact a UK study has shown that the strongest independent predictor of escalation from non-severe to severe knee pain is obesity (OR 2.1, 95% CI 1.2, 3.6)^2.

Not surprisingly then, the chances of a patient progressing to the point of requiring knee replacement (TKR) surgery is also linked with increased weight³.

>>> For every 10kg increase in weight, the chances of requiring surgery increases by 58%.
>>> For every 5 unit increase in BMI, the chances of requiring surgery increases by 88%.
>>> For every 10cm increase in waist, the chances of requiring surgery increases by 62%.

Weight loss can help delay the need for surgery

Losing weight has been shown to improve both symptoms and function in those with knee osteoarthritis. An average weight loss of just 5% in an overweight and obese adult can equate to an 18% improvement in function as measured by the WOMAC index⁴.

Bearing in mind the limited lifespan of a prosthetic joint, the longer a patient can delay their primary TKR, the better. By losing weight, and as a result, reducing the pain and immobility associated with osteoarthritis, patients can delay their need for primary TKR surgery - often by up to several years. 

The benefits of losing weight prior to surgery

Excess weight and BMI have been shown to adversely affect the longevity of primary total knee replacement but also to result in an increased need for, and to result in worse outcome scores following, revision surgery^5,6. What this tells us is that obese recipients of a total knee replacement will inevitably need a revision surgery sooner than non-obese patients.

It should also be noted that infection rates are much higher in obese patients than non-obese patients⁷. In fact patients with a BMI greater than 40 had a 3 fold higher risk of joint infection⁸.  In addition, adverse events and complications during or post-surgery have also been shown to be higher in obese patients (22.6% in obese, 35.1% in morbidly obese) than in non-obese patients (14.2%)⁹.

References

1. Ettinger, W.H., et al., Long-term physical functioning in persons with knee osteoarthritis from NHANES. I: Effects of comorbid medical conditions. J Clin Epidemiol, 1994. 47(7): p. 809-15.
2. Jinks, C., et al., Predictors of onset and progression of knee pain in adults living in the community. A prospective study. Rheumatology (Oxford), 2008. 47(3): p. 368-74.
3. Messier, S.P., et al., Exercise and dietary weight loss in overweight and obese older adults with knee osteoarthritis: The arthritis, diet, and activity promotion trial. Arthritis & Rheumatism, 2004. 50(5): p. 1501-1510.
4. Christensen, R., A. Astrup, and H. Bliddal, Weight loss: the treatment of choice for knee osteoarthritis? A randomized trial. Osteoarthritis Cartilage, 2005. 13(1): p. 20-7.
5. Foran, J.R., et al., The outcome of total knee arthroplasty in obese patients. J Bone Joint Surg Am, 2004. 86-A(8): p. 1609-15.
6. Mulhall, K.J., et al., Adverse effects of increased body mass index and weight on survivorship of total knee arthroplasty and subsequent outcomes of revision TKA. J Knee Surg, 2007. 20(3): p. 199-204.
7. Dowsey, M.M. and P.F. Choong, Obese diabetic patients are at substantial risk for deep infection after primary TKA. Clin Orthop Relat Res, 2009. 467(6): p. 1577-81.
8. Malinzak, R.A., et al., Morbidly obese, diabetic, younger, and unilateral joint arthroplasty patients have elevated total joint arthroplasty infection rates. J Arthroplasty, 2009. 24(6 Suppl): p. 84-8
9. Dowsey, M.M., et al., The impact of pre-operative obesity on weight change and outcome in total knee replacement: a prospective study of 529 consecutive patients. J Bone Joint Surg Br, 2010. 92(4): p. 513-20.