Bariatric surgery results in cortical bone loss

Abstract

Background: Bariatric surgery results in bone loss at weight-bearing sites, the mechanism of which is unknown.

Methods: Twenty-two women (mean body mass index 44 kg/m(2); aged 45 years) who underwent Roux-en-Y gastric bypass (n = 14) and restrictive procedures (n = 8) had measurements of areal bone mineral density by dual-energy x-ray absorptiometry at the lumbar spine, total hip (TH), femoral neck (FN), and one third radius and trabecular and cortical volumetric bone mineral density and microstructure at the distal radius and tibia by high-resolution peripheral quantitative computed tomography (HR-pQCT) at baseline and 12 months postoperatively.

Results: Mean weight loss was 28 ± 3 kg (P < .0001). PTH rose 23% (P < .02) and 25-hydroxyvitamin D was stable. C-telopeptide increased by 144% (P < .001). Bone-specific alkaline phosphatase did not change. Areal bone mineral density declined at TH (-5.2%; P < .005) and FN (-4.5%; P < .005). By HR-pQCT, trabecular parameters were stable, whereas cortical bone deteriorated, particularly at the tibia: cortical area (-2.7%; P < .01); cortical thickness (-2.1%; P < .01); total density (-1.3%; P = .059); cortical density (-1.7%; P < .01). In multivariate regression, bone loss at the TH and FN were predicted by weight loss. In contrast, only PTH increase predicted cortical deterioration at the tibia. Roux-en-Y gastric bypass patients lost more weight, had more bone loss by dual-energy x-ray absorptiometry and HR-pQCT than those with restrictive procedures, and had declines in cortical load share estimated by finite element analysis.

Conclusions: After bariatric surgery, hip bone loss reflects skeletal unloading and cortical bone loss reflects secondary hyperparathyroidism. This study highlights deterioration of cortical bone loss as a novel mechanism for bone loss after bariatric surgery.

Figure 1.

A, Percent change in aBMD by DXA 1 year after bariatric surgery presented as mean with SE. *P < .05 compared with baseline. B, Association between weight loss and bone loss at the total hip and femoral neck.

Figure 2.

Percent change in vBMD and microarchitecture by HR-pQCT 1 year after bariatric surgery presented as mean with SE. *P < .05 compared with baseline. Ct.Ar, cortical area; Tb. Ar, trabecular area; Tot. Ar, total area; Tot.D, total density; Ct.D cortical density; Tb.D, trabecular density; CT.Th cortical thickness; Tb.N, trabecular number.

Figure 3.

Association between change in PTH and cortical area, cortical thickness, cortical density, and total density by HR-pQCT at the tibia. Ct.Ar, cortical area; Ct.Den, cortical density; Ct.Th, cortical thickness; Tot.D, total density.