A Systematic Review of the Normal Sacroiliac Joint Anatomy and Adjacent Tissues for Pain Physicians.

BACKGROUND: The sacroiliac joint (SIJ) forms a complex joint and has shown to be underappreciated in its involvement with lower back pain. Research efforts have intensified on SIJ anatomy and biomechanics because of its predisposing position to pain and dysfunction in individuals suffering from lower back discomfort. Previous work has focused on SIJ anatomy including bone and joint structure, innervation, as well as biomechanics and the treatment of SIJ pain. However, to date, no review exists describing the range of 'normal' anatomic features of the SIJ. OBJECTIVES: To describe the normal appearance of the SIJ and adjacent tissues, as opposed to 'abnormal' conditions involving SIJ morphology. It will also identify key areas that require further study because of lacking information or disagreement. STUDY DESIGN: A systematic literature review. SETTING: The research took place at the University of Otago, New Zealand. All published research on 'normal SIJ anatomy' available from MEDLINE, OVID, Scopus, Web of Science, PubMed, and Science Direct were included, available until December 2018, in English, French, and German. Subject areas included bony landmarks, joint type, bone morphology, ligamentous attachments, muscular and fascial relationships, blood supply, fatty infiltration, and morphologic variation. METHODS: Articles met the selection criteria if they contained specific information on SIJ anatomy, including bone morphology and architecture, ligaments, muscle attachments, innervation, vasculature, and the presence of fat. Biomechanics and kinematics related keywords were used as the literature often couples these with the anatomy. Keywords of individual articles were named as 'structures of interest.' RESULTS: A total of 88 primary and 101 secondary articles were identified in the time frame from 1851 to 2018. Primary articles provided quantitative data and detailed anatomic descriptions. Secondary articles did not focus specifically on the anatomy of the SIJ. Although research appeared to be in general agreement on bony landmarks, joint type, myofascial attachments, vasculature, and innervation of the SIJ, there was only part consensus on ligament attachments and cartilage structure. Information regarding bone density of the articulating surfaces of the SIJ is lacking. Despite its potential clinical significance, fatty infiltration within the joint lacks research to date. LIMITATIONS: Only the given databases were used for the initial search. Keyword combinations used for this review may not have been inclusive of all articles relevant to the SIJ. Work in languages other than the ones listed or work that is not available via the internet may be missing. CONCLUSIONS: This study provides an overview of normal SIJ structures, including all neuromusculoskeletal elements related to the joint. There is a lack of knowledge on the SIJ ligaments warranting further investigation. Furthermore, there are discrepancies in relation to the nomenclature, layers, attachment sites, and on the topographical relationships between ligamentous tissues and nerves. Subsequent studies on the quantification of fat and bone density in the SIJ have been suggested. These could be useful radiologic parameters to assess the condition of the joint clinically. This review may provide insight into the clinical signs and abnormal biomechanical features of the joint for the purposes of treating SIJ pain. KEY WORDS: Bone density, bony landmarks, fat infiltration, innervation, ligaments morphology, muscles, sacroiliac joint, vasculature.