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Swedish Obese Subjects: Surgery results in increased incidence of knee arthroplasty

An analysis from the Swedish Obese Subjects (SOS) study has reported that bariatric surgery did not normalise the increased risk of knee and hip osteoarthritis in patients with obesity but was associated with an increased incidence of knee arthroplasty, compared to the control group. The paper, ‘Bariatric surgery, osteoarthritis and arthroplasty of the hip and knee in Swedish Obese Subjects – up to 31 years follow-up of a controlled intervention study’, was published in the journal, Osteoarthritis and Cartilage.

Although obesity is a major risk factor for osteoarthritis of the hip and knee, whether bariatric surgery is associated with a reduced risk of hip and knee osteoarthritis and a reduction of the need for arthroplasty is not clear. Therefore, the study authors examined the long-term effect of obesity and bariatric surgery on incidences of osteoarthritis and arthroplasty of hip and knee. Using data from the landmark SOS study, the researchers hypothesised that bariatric surgery would be associated with less osteoarthritis and less need for arthroplasty based on the large weight loss induced by bariatric surgery.

The SOS study is a prospective controlled intervention study including patients with obesity, one group treated with bariatric surgery, and a matched control group of patients receiving usual obesity care at their primary health care centre. The control group (n=2037) was created from eligible applicants who chose not to undergo bariatric surgery and received usual care. The patients in the per protocol surgery group (n=2007) were treated with either gastric banding (n=376), vertical banded gastroplasty (n=1365) or gastric bypass (n=266). The surgeon decided type of surgical procedure. The SOS reference cohort included 1,135 participants (524 men and 611 women) were included and underwent a baseline examination and assessment of anthropometry, blood biochemistry and filled in questionnaires similar to those in the SOS study.

Figure 1: Kaplan-Meier estimates of cumulative incidence of osteoarthritis and arthroplasty of the hip in the SOS intervention study and SOS reference cohort. The upper panels illustrate osteoarthritis of the hip (A), and arthroplasty of the hip (B). The lower panels illustrate arthroplasty of the hip in participants without hip pain at baseline (C), and arthroplasty of the hip in participants with hip pain at baseline (D).


The SOS reference cohort had a higher proportion of men (46.2%) and a lower BMI at baseline (25.2 (SD 3.8)) vs. the SOS surgery group (29.2% men; BMI 42.4 (SD 4.5)) and the SOS control group (29.1% men; BMI 40.1 (SD 4.7)). A larger proportion of participants reported pain in the knee (38.9%) and hip (24.5%) at baseline in the surgery group vs. the control group (knee 25.7%; hip 18.5%) and SOS reference cohort (knee 19.2%; hip 11.6%). The surgery group had a lower risk of hip osteoarthritis diagnosis during follow-up vs. the control group, but the incidence of hip arthroplasty for the surgery group was similar to that of the control group.

Compared to the reference cohort, both the surgery and control groups showed an increased risk of being diagnosed with hip and of undergoing hip arthroplasty (Figure 1). A similar risk of being diagnosed with knee osteoarthritis was observed in the surgery and control groups. A higher incidence of knee arthroplasty was observed in the surgery group vs. the control group. Both the surgery group and the control group displayed a higher risk of knee osteoarthritis and of knee arthroplasty vs. the SOS reference cohort.

“Bariatric surgery was associated with a reduced incidence of hip osteoarthritis compared to a usual care control group with obesity” the authors write. “However, this did not result in a reduced incidence of hip arthroplasty. We found a similar incidence of knee osteoarthritis in the bariatric surgery group and the control group, but unexpectedly, treatment with bariatric surgery was associated with higher incidence of knee arthroplasty. However, compared to the general population in the reference cohort, osteoarthritis and arthroplasty of the hip and knee were more common among patients with obesity treated with bariatric surgery or usual care.”

The authors noted that additional studies are needed to confirm these results.

To access this paper, please click here


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