The feline sacroiliac joint: An overview of its innervation and morphology

The sacroiliac joint (SIJ) serves as a central interface for force transmission from the hindlimbs to the axial skeleton in animals of all species. Its specialized morphology reflects this functional requirement, comprising a smooth auricular surface and a rugose tuberosity covered by interosseous ligaments. In both human and equine medicine, the SIJ is recognized as a primary source of varied back pain. Despite its biomechanical and clinical significance, very limited data exist in small animal medicine concerning the structure, innervation, and clinical relevance of the SIJ. The present study investigated the morphological variation and innervation of the SIJ in the domestic cat (Felis catus). Macroscopic assessment of the joint's innervation was performed on 40 specimens, and 20 dorsal sacroiliac ligaments were analyzed microscopically for the presence of nervous tissue. Additionally, the individual components of the joint were scanned three-dimensionally and measured. The auricular surfaces were subsequently categorized into distinct morphological types. Current investigations have revealed that the dorsal branches of the seventh lumbar and the first sacral spinal nerves course dorsally within the dorsal sacroiliac ligaments, which was confirmed microscopically. Analysis of the size and shape of the articular surfaces showed that the auricular surface can be classified into five different morphological types. No evidence of sexual dimorphism was detected. However, interindividual variation in the size of specific joint components was observed. These findings provide an initial anatomical basis for a better understanding of the SIJ of domestic cats. This enhanced understanding can help optimize diagnostic or therapeutic approaches in small animal medicine. The results therefore may contribute to a preliminary framework for future clinical studies.