Surgery for osteoarthritis
Morbidly obese patients with knee pain and osteoarthritis may be candidates for knee surgery. There is no role for joint salvaging procedures such as arthroscopic interventions or osteotomies. From a mechanical perspective, each kilogram of excessive weight loads the knee by four times. Thus our morbidly obese patient Millie, who weighs about 50 kg more than her ideal body weight, will load her knee with an additional 200 kg and not surprisingly will have pain and decreased function.
In comparative studies, there is limited information on knee replacement surgery in patients who are morbidly obese. Obese patients tend to have a similar magnitude of knee improvement following surgery as nonobese patients but they start, and end, at a much lower level of function (including walking distance, the use of walking aids, psychological scores and use of global pain relief).
For nonobese young patients with osteoarthritis and knee pain, the goals are to optimise their conservative management, essentially to bide time and reduce their time-dependant risk of wear-related revision knee surgery. However, they have a dramatic erosion of their quality of life that must be risk-weighted against their likelihood of outliving a joint replacement. For disabled young patients, the timing of their first joint replacement is a strategic plan many decades in length, often anticipating further revision surgeries in mid and later life. There are no strict age-related limits for joint replacement surgery. Younger patients predictably wear their joint replacements at a substantially faster rate and have a greater life expectancy, and will therefore require more revision surgery. In Australia, patients aged 65 to 77 years have a knee replacement failure rate of 4.2% at 10 years after the surgery compared with 11.4% at 10 years for those aged under age 55 years.4
Obese patients considering knee replacement surgery are strongly advised to optimise their risk profile, especially to minimise the risk of infection and perioperative mortality. Morbidly obese patients have a greater prevalence of deep prosthetic infection (three to nine times) compared with controls. Successful treatment of an infected joint replacement is profoundly more complex and dramatically less successful in obese patients. Also, the primary surgery is more complex in obese patients, who are at risk of wound healing problems, ligament damage, reduced range of motion and perioperative medical complications.
The risk–benefit profiles of morbidly obese patients are very different from those of nonobese patients. Their surgical risks are dramatically increased in the perioperative period but their limited demands and dramatically shortened life expectancies probably mitigate their long-term revision risks compared with similarly aged non-obese patients. As the lifespan of an obese patient is decreased by eight to 10 years, Millie’s projected life expectancy at her present age of 38 years is reduced to a further about 38 years, which is similar to that of a 50-year-old Australian woman (from life tables; http://aga.gov.au/publications/life_table_2010-12/). Although Millie would overload her replaced knee should she have the surgery, her decreased activity profile would minimise the number of cycles (steps) per year. Embarking upon the challenges of knee surgery is consequently more akin to the challenges faced by a less active and older patient with multiple comorbidities.
When to refer to an orthopaedic surgeon
The management of patients with osteoarthritis and knee pain will usually involve a multidisciplinary team of which the orthopaedic surgeon is likely to be the last to intervene, when a knee replacement is required. Wherever possible, patients should be educated and optimised before considering a referral for orthopaedic surgery. Most episodes of acute knee pain will settle with time and the use of walking aids and analgesics, and rarely need surgical review.