Decreased bone strength in HLA-B27 transgenic rat model of spondyloarthropathy

Objective. To investigate the nature of osteopenia/osteoporosis in spondyloarthropathy, an inflammatory disorder, using the HLA-B27 transgenic rat model. Methods. HLA-B27 transgenic rats were housed individually and sacrificed at the peak of their disease (8-month-old). The spine and femurs were removed and stored in saline at −20°C until analysis. The bone structure and strength were determined using a micro-computed tomography (micro-CT) device (Scanco Medical) and mechanical testing (Instron 5543). Vertebral bodies and femurs were scanned to determine trabecular structural properties in terms of bone volume (BV/TV), trabecular thickness, and spacing. After scanning, the mid-shaft femurs were subjected to a 3-point bending test (along anterior-posterior direction), the femoral necks were tested in bending, and the vertebral bodies (L4) were tested in compression. Structural (ultimate/yield load, stiffness) and apparent material (ultimate/yield stress, modulus) strength parameters were then determined. Results. The majority of the bone structural and strength parameters were significantly lower (P < 0.05) in the HLA-B27 transgenic rats as compared with control littermates. Micro-CT data suggested that the transgenic animals had lower BV/TV and trabecular thickness in their vertebral bodies. The poor trabecular structure observed in HLA-B27 rats is also indicative of the poor biomechanical strength properties in the vertebral bodies as well. Conclusion. The HLA-B27 transgenic rats develop bone fragility similar to that seen in spondyloarthropathy and may be an important model for the study of osteoporosis in spondyloarthropathy.