Abnormal Bone Geometry at the Metacarpal Bone Shaft of Rheumatoid Arthritis Patients with Maintained Muscle-bone Relationship

Objective: Metacarpal juxtaarticular bone is altered in rheumatoid arthritis (RA). However, a detailed analysis of disease-related geometric adaptations of the metacarpal shaft is missing. The aim of the present study was to assess the role of RA disease, forearm muscle cross-sectional area (CSA), age, and sex on bone geometry at the metacarpal shaft.

Methods: In 64 RA patients and 128 control subjects, geometric properties of the third metacarpal bone midshaft and forearm muscle CSA were measured by peripheral quantitative computed tomography (QCT). Linear models were performed for cortical CSA, total bone CSA, polar stress-strain index (SSI; a surrogate for a bone's resistance to bending and torsion), cortical thickness, and the metacarpal index (MI; cortical CSA/total CSA), with the explanatory variables muscle CSA, age, RA status, and sex.

Results: Forearm muscle CSA was associated with cortical and total metacarpal CSA and polar SSI. RA group status was associated with all bone parameters except cortical CSA. There was a significant interaction between RA status and age, indicating that the RA group had a greater age-related decrease in cortical CSA, cortical thickness, and the MI.

Conclusion: Bone geometry of the metacarpal shaft is altered in RA patients compared to healthy controls. Whereas bone mass of the metacarpal shaft is adapted to forearm muscle mass, cortical thickness and the MI are reduced, but outer bone shaft circumference and the polar SSI increased in RA patients. These adaptations correspond to an enhanced aging pattern in RA patients.