Weight loss following bariatric surgery was associated with improvements in pain sensitization among obese patients with chronic knee pain, a year-long study found.

One year after bariatric surgery, the pressure pain threshold increased by 38.5% at the patella and by 30.9% at the wrist among patients who had undergone either laparoscopic roux-en-y gastric bypass or sleeve gastrectomy, whereas no changes in the pressure pain threshold were observed among patients who had medical/lifestyle management, according to Joshua J. Stafanik, PhD, MSPT, of Northeastern University in Boston, and colleagues.

Obese individuals typically report more musculoskeletal pain than do those of normal weight, particularly at the knees, which has been attributed to mechanical stresses from excess loading at the weight-bearing joints and also to the release of adipokines from adipose tissue and the resulting low-grade joint inflammation.

But it has not been established whether additional pain relief at sites beyond the knee is provided by changes in central and peripheral pain sensitization, defined as “increased responsiveness of the peripheral and central nervous systems to nociceptive input,” according to the authors. So they sought to examine this possibility among individuals seen at the Nutrition and Weight Management Center at Boston Medical Center.

They recruited 87 participants who met the eligibility criteria for bariatric surgery, which were a BMI of 35 plus a weight-associated comorbidity, or a BMI >40. They all had knee pain on most days of the previous month.

Those receiving medical management were prescribed a low-fat diet of 1,200 to 1,800 calories per day plus medications such as phentermine, lorcaserin, and bupropion/naltrexone. Exercise, including walking 30 minutes daily, also was encouraged.

Knee pain was evaluated according to the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and pressure pain thresholds were assessed with a hand-held algometer applied at a rate of 0.5 kg/sec to the radioulnar joint and index patella.

The radioulnar joint of the right wrist was considered a control site, as it is not usually affected by osteoarthritis. The researchers also recorded the number of joints other than the knees with frequent pain in the past month.

A lower pressure pain threshold reflects higher sensitivity to pain.

A total of 45 patients in the surgery group and 22 in the medical group completed the 1-year follow-up.

Among the surgery group, mean age was 43.8, 97.8% were women, and baseline BMI was 42.1. Their WOMAC pain score was 9.5 out of 20, and the average number of painful joints was 7.1. The pressure pain thresholds were 346.5 kilopascals at the patella and 335.6 at the wrist.

At 1 year, the mean weight loss in the surgery group was 32.7 kg (about 72 lbs) compared with 4.6 kg in the medical management group.

Mean change in WOMAC pain score from baseline in the surgery group was -4.9, which was statistically significant (P<0.0001), while the -1.5 change in the medical management group was not significant. The change from baseline in number of painful joints was -2.3 in the surgery group (P=0.002), but +0.9 in the medical management group.

At 1 year, the change in pressure point threshold was +133.3 at the patella and +103.8 at the wrist, compared with changes of -56.4 and +44.4 in the medical group.

Changes in weight correlated with differences in WOMAC pain (r=0.50, P<0.0001) and in pressure point threshold at the patella (r=-0.33, P=0.006), although not with changes in pressure point threshold at the wrist (r=-0.04, P=0.77). There also was a moderate inverse correlation between WOMAC pain changes and changes in pressure point thresholds at both patella (r=-0.4, P=0.007) and wrist (r=-0.4, P=0.002).

“In the presence of sensitization, nociceptors respond to stimuli that they would normally not respond to. However, due to neuroplasticity, removal of the stimuli that contribute to sensitization may normalize nociceptor functioning,” they explained.

They also pointed out that other factors also may contribute to changes in pain after weight loss surgery, such as increased physical activity and improvements in mood and quality of sleep. In addition, further research will be needed to account for the potential influence of inflammatory markers such as C-reactive protein and metabolic factors on pain following bariatric surgery.