Background/Purpose:  Individuals with chronic knee pain often develop central and/or peripheral sensitization (altered pain processing of the nervous system). Whether improvements in knee pain are accompanied by resolution of pain sensitization is not clear. For example, obesity is related to knee pain, and musculoskeletal pain improves after massive weight loss. As obesity is also associated with systemic low-grade inflammation, which itself may contribute to pain sensitization, weight loss may theoretically lead to a reduction in pain sensitization. However, whether this actually occurs is not known. We sought to determine if knee pain and sensitization improve after massive weight loss in individuals undergoing bariatric surgery.

Methods: Individuals for the current study were part of the Osteoarthritis Before and After Bariatric Surgery (OABS) Study. Individuals aged 25-60 were included if they had BMI ≥35, knee pain on most days of the month and were approved for bariatric surgery (laparoscopic gastric bypass surgery or laparoscopic sleeve gastrectomy). Subjects had knee pain assessed at baseline and one year later in the knee that was more painful at baseline (index knee) using the WOMAC (0-20; 0=no pain) and VAS (0-100; 0=no pain). A pressure algometer applied at 0.5kg/s was used to assess pressure pain threshold (PPT) at the index patella (indicator of peripheral and/or central sensitization) and the right wrist (indicator of central sensitization) as the point at which the pressure first changed to slight pain. Three PPT trials at each anatomical site were averaged. Higher PPTs represent less sensitization or decreased pain sensitivity. To determine the mean change in measures of pain and pain sensitization before and after surgery, we performed a paired t-test.

Results: To date 23 individuals completed baseline and follow-up visits (mean (SD) age 45.7 (8.2), mean BMI 41.6 (3.4), 86% female). The mean (SD) weight loss was 31.1 (10.3) kilograms (% weight loss 28.1). There were significant mean improvements after surgery for all pain and sensitization measures (see Table).

Conclusion: Similar to previous findings, we found a reduction in knee pain severity after weight loss. For the first time, we report that persons who had bariatric surgery and experienced a major reduction in weight experienced a reduction in pain sensitivity, as evidenced by significant improvements in PPT. 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-relation-of-massive-weight-loss-to-changes-in-knee-pain-and-sensitization/