The Canine Sacroiliac Joint: 2. Common Surface Variants

The canine sacroiliac joint (CSIJ) is crucial for transferring load between the axial skeleton and the pelvic limbs. However, the morphology of this joint beyond the primary articulation is not well characterized. This second study on the CSIJ aimed to systematically describe common morphological variants, including synovial surface area (SSA) shape types, non-fused ossification centers (NFOC), and accessory sacroiliac joints (ASIJ), and to assess their morphometric characteristics and potential biomechanical relevance. A total of 29 canine pelves (n = 58 CSIJ) were examined. SSA morphology was categorized, NFOC were identified based on characteristic surface interruptions of the articular surface, and ASIJ were documented in terms of frequency, localization, and size. Seven distinct SSA shape types were identified, with auricular, bifoliate, and phrygian cap-like configurations being the most prevalent. Only one-third of the examined joints exhibited the commonly described auricular or crescent shape. NFOC were present in 27.60% of CSIJ and occurred predominantly in dogs with a crown-rump length (CRL) exceeding 740 mm. Accessory articulations were detected in 74.10% of CSIJ. These were frequently bilateral and, in some cases, presented as multiple structures within a single joint. The ASIJ represented independent diarthrodial joints lined with hyaline cartilage. Their additional articular surface areas (AASA) correlated with body size and overall CSIJ dimensions. In conclusion, the examined CSIJ specimens demonstrated considerable anatomical variability extending beyond the primary articular surfaces. Variations in SSA morphology, the presence of NFOC, and the occurrence of ASIJ appear to represent frequent physiological variants rather than rare anomalies. Recognition of these features is essential for accurate anatomical characterization and imaging interpretation and provides a structural basis for future biomechanical and clinical investigations of the CSIJ.