The Canine Sacroiliac Joint: 1. Classification and Morphometrics

The canine sacroiliac joint (CSIJ) is becoming increasingly relevant in the clinical evaluation of lameness and lumbosacral pain syndromes. However, its detailed anatomical characteristics remain poorly understood. This study aimed to characterise the intra-articular structures of the CSIJ histologically, establish a substantiated joint classification and provide a comprehensive morphometric assessment of all joint surface areas. In 29 dogs without known spinal disorders, the synovial surface area (SSA), extrasynovial surface area (ESA) and total surface area (TSA) were quantified bilaterally using a two-dimensional measurement approach. Surface areas were proportionally analysed and normalised to crown-rump length (CRL). The intra-articular structures were predominantly composed of dense connective tissue with fibrocartilaginous components. Based on these findings, the CSIJ represents a combined joint comprising a caudoventral diarthrosis and a craniodorsal symphysis in this population. Morphometric analysis revealed pronounced bilateral symmetry, while iliac surface areas were significantly larger than sacral surface areas (p < 0.001). In relative terms, the sacral SSA contributed a greater proportion to the TSA. All measured surface areas showed strong correlations with CRL (R = 0.78-0.86). Sex-related differences were observed exclusively in SSA, with males exhibiting larger values. In conclusion, this study integrates detailed histological characterisation with comprehensive morphometric analysis to delineate the anatomical properties of the CSIJ.