Shedding more pounds helps preserve knees in overweight, obese individuals

Middle-aged and older adults with overweight or obesity should aim to reduce at least 7.5 percent of their weight, as opposed to keeping it stable, to reduce the risk of having their knee replaced, according to a study. Furthermore, weight gain is detrimental, as it puts them at risk of hip replacement.

“Our results extend previous research which highlighted the benefits of weight loss on knee osteoarthritis (OA) symptoms … [and] joint function in individuals with overweight and knee OA,” the investigators said, adding the such a benefit followed a dose-response pattern. [Ann Rheum Dis 2007;66:433-439; Arthritis Care Res 2016;68:1106-1114]

In the study, a total of 23,916 and 24,537 participants (body mass index [BMI] ≥25 kg/m²) from the 45 and Up Study (a population-based cohort in New South Wales, Australia) were included in the analysis of TKR and THR for OA, respectively.

Over a mean follow-up of 5.2 years, 2,139 (8.9 percent) participants lost >7.5 percent in weight, 1,655 (6.9 percent) lost >5–7.5 percent, 4,430 (18.5 percent) gained >5 percent, and most (65.6 percent) kept their weight stable (≤5 percent change) in the TKR cohort. The respective proportions in the THR cohort were 9.1 percent, 6.9 percent, 18.6 percent, and 65.5 percent.

There were 1,009 (4.2 percent) participants who underwent TKR and 483 (2.0 percent) THR. Multivariable Cox regression analysis showed that compared with stable weight, weight loss of >7.5 percent was associated with a significant reduction in the risk of TKR (hazard ratio, 0.69, 95 percent confidence interval, 0.54–0.87) but not of THR. [Int J Obesity 2021;45:1696-1704]

In a subgroup analysis, losing ≥7.5 percent of weight conferred protection only for the overweight, suggesting that a higher percentage of weight loss should be achieved for the obese.

Although weight loss was not associated with the risk of total hip replacement (THR), weight gain heightened the risk of THR.

Hip not the same as knee joint

The null association between weight loss and THR risk, according to the investigators, may be due to the hip joint being less sensitive to obesity and weight change than the knee joint. [Ann Rheum Dis 2007;66:158-162; Arch Orthop Trauma Surg 2018;138:99-103]

“It is possible that the reduction in mechanical loading of the joint resulting from weight loss is different in the knee and the hip,” given that these two body parts have different anatomy, they said. That is, the knee is a hinge joint, and the hip is a ball-and-socket joint.

“Changes in force on a misaligned hinge joint … may be magnified by two to three times compared with a normal hinge joint, due to the small area that the forces act on. In contrast, a misaligned ball-and-socket joint … might be less sensitive to changes in force, as they are distributed over a larger area compared to the knee,” the investigators explained, adding that these facts definitely do not negate the notion that individuals with overweight or obesity should avoid further weight gain.

Losing weight and keeping it off to avoid knee and hip replacement altogether has its merits, since TKR and THR are not without risk, as with all invasive procedures. For one, both procedures are associated with serious adverse events, including sepsis, pulmonary embolism, and cardiac complications. And then for some, joint pain can persist even 12 months after knee and hip replacement. [Anesth Analg 2012;115:321-327; BMJ Open 2012;2:e000435]

“Therefore, joint replacement treatment should be reserved as the last treatment option for those with end-stage OA when conservative management has failed,” according to the investigators. “The present study suggests a weight loss target of >7.5 percent to reduce the risk of TKR for OA,” a target that hasn’t been highlighted in OA clinical management guidelines.

Finally, the investigators called for further studies to address the current one’s limitations. These include the fact that OA at baseline was defined by self-reported treatment for OA, lack of data on whether OA affected the knees, the hips or both, and the inability to explore the confounding effect of important risk factors for receiving TKR and THR, among others.