Biomechanics of the Sacroiliac Joint: Part I−−Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain Ali Kiapour, Amin Joukar, Hossein Elgafy, Deniz U. Erbulut, Anand K. Agarwal and Vijay K. Goel Int J Spine Surg published online 30 December 2019 http://ijssurgery.com/content/early/2019/12/30/6077 This information is current as of September 23, 2021. Email Alerts Receive free email-alerts when new articles cite this article. Sign up at: http://ijssurgery.com/alerts The International Journal of Spine Surgery 2397 Waterbury Circle, Suite 1, Aurora, IL 60504, Phone: +1-630-375-1432 © 2019 ISASS. All RightsDownloaded Reserved. from http://ijssurgery.com/ by guest on September 23, 2021 International Journal of Spine Surgery, Vol. 00, No. 00, 0000, pp. 000–000 https://doi.org/10.14444/6077 ÓInternational Society for the Advancement of Spine Surgery Biomechanics of the Sacroiliac Joint: Part I—Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain ALI KIAPOUR, PHD,1,2 AMIN JOUKAR, MS,1 HOSSEIN ELGAFY, MD,1 DENIZ U. ERBULUT, PHD,1 ANAND K. AGARWAL, MD,1 VIJAY K. GOEL, PHD1 1Engineering Center for Orthopaedic Research Excellence (E-CORE), Departments of Bioengineering and Orthopaedics, The University of Toledo, Toledo, Ohio; 2Department of Neurosurgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts ABSTRACT Background: The sacroiliac joints (SIJs), the largest axial joints in the body, sit in between the sacrum and pelvic bones on either side. They connect the spine to the pelvis and thus facilitate load transfer from the lumbar spine to the lower extremities. The majority of low back pain (LBP) is perceived to originate from the lumbar spine; however, another likely source of LBP that is mostly overlooked is the SIJ. This study (Parts I and II) aims to evaluate the clinical and biomechanical literature to understand the anatomy, biomechanics, sexual dimorphism, and causes and mechanics of pain of the SIJ leading to conservative and surgical treatment options using instrumentation. Part II concludes with the mechanics of the devices used in minimal surgical procedures for the SIJ. Methods: A thorough review of the literature was performed to analyze studies related to normal SIJ mechanics, as well as the effects of sex and pain on SIJ mechanics. Results: A total of 65 studies were selected related to anatomy, biomechanical function of the SIJ, and structures that surround the joints. These studies discussed the effects of various parameters, gender, and existence of common physiological disorders on the biomechanics of the SIJ. Conclusions: The SIJ lies between the sacrum and the ilium and connects the spine to the pelvic bones. The SIJ transfers large bending moments and compression loads to lower extremities. However, the joint does not have as much stability of its own against the shear loads but resists shear due the tight wedging of the sacrum between hip bones on either side and the band of ligaments spanning the sacrum and the hip bones. Due to these, sacrum does not exhibit much motion with respect to the ilium. The SIJ range of motion in flexion-extension is about 38, followed by axial rotation (about 1.58), and lateral bending (about 0.88). The sacrum of the female pelvis is wider, more uneven, less curved, and more backward tilted, compared to the male sacrum. Moreover, women exhibit higher mobility, stresses/ loads, and pelvis ligament strains compared to male SIJs. Sacroiliac pain can be due to, but not limited to, hypo- or hypermobility, extraneous compression or shearing forces, micro- or macro-fractures, soft tissue injury, inflammation, pregnancy, adjacent segment disease, leg length discrepancy, and prior lumbar fusion. These effects are well discussed in this review. This review leads to Part II, in which the literature on mechanics of the treatment options is reviewed and synthesized. Biomechanics Keywords: sacroiliac joint, anatomy, biomechanics, sexual dimorphism, causes of pain, surgery BACKGROUND across the joint that support and limit the movement of the SIJ. The tight wedging of the sacrum between One of the most overlooked sources of low back the hip bones and the band of ligaments surround- pain (LBP) is the sacroiliac joint (SIJ) due to its ing the SIJs provide resistance to shear loads. complex nature and the fact that presumed radicular Several of the largest and most powerful muscles 1 pain syndromes could be linked to the SIJ. The SIJ of the body also surround the SIJ; however, these is the largest axial joint in the body that connects the muscles don’t produce its active movements. spine to the pelvis and thus transfers load between The sacrum can move with respect to the ilium in the lumbar spine and the lower extremities.2,3 6 degrees of freedom, although this motion is However, the joint does not have as much stability minimal, and the joint’s hyper- or hypo-mobility against the shear loads. There are several ligaments may cause pain emanating from the joint region. Downloaded from http://ijssurgery.com/ by guest on September 23, 2021 Copyright 2019 by International Society for the Advancement of Spine Surgery. Biomechanics of the Sacroiliac Joint and thus Part II narrates the mechanics of treatment options specific to SIJ. ANATOMY The SIJ, the largest axial joint in the body, connects the spine to the pelvis, which allows load transfer between the lumbar spine and the lower extremities.2,3 The SIJ lies between the sacrum and the ilium, spanning about 1 to 2 mm in width and a 10 joint on either side of the sacrum is held together by Figure 1. Articular surfaces of the sacroiliac joint. a fibrous capsule (Figures 1 and 2). The sacral side of the joint is covered with hyaline Sexual dimorphism exists in the pelvis. Men tend to cartilage that is thicker (1.18 mm) than iliac have a relatively long and narrow pelvis, with a cartilage (0.8 mm), which appears more fibro- longer and more conical pelvic cavity than that of 9 4 cartilaginous. The iliac cortical bone is thicker women. These sex differences also reflect in the than the sacral cortical bone (0.36 mm vs. 0.23 mm). biomechanics of the joint: the female SIJ has higher Thus, cartilage and cortical bone thicknesses have a mobility, and more stresses, loads, and pelvis- reverse relationship. No relationship between age 5 ligament strains, compared to the male SIJ. and cortical bone thickness was observed by Dall et Recent studies have reported a higher prevalence al.10 Regarding bone density, iliac cancellous bone of LBP due to the SIJ, with some reports having density was greater than sacral bone in all anterior, estimated that the SIJ is the actual source of pain in central, and posterior parts of the joint. 15% to 30% of cases.6–8 From an anatomical perspective, pathologic changes and injuries specific Ligaments to different SIJ structures can result in SIJ pain. Several ligaments depicted in Figure 2 support These changes or disorders include, but are not and limit the movement of the SIJ (see 7, Figure 2). limited to, capsular and ligamentous tension, hypo- These ligaments include the interosseous sacroiliac or hypermobility, extraneous compression or shear- ligament (8), the posterior and anterior ligaments (5 ing forces, micro- or macro-fractures, soft tissue and 6), and the sacrotuberous (4), sacrospinous (3) 2 injury, and inflammation. and iliolumbar ligaments (1 and 2). The interosseous Due to the high importance of the SIJ to LBP, ligament (8), also known as the axial ligament, this Part I review addresses basic anatomy, function, connects the sacrum and ilium at the S1 and S2 biomechanics, and mechanisms of pain in the SIJ. levels. The long posterior sacroiliac ligament (5) is The review leads to the need for treatment options quite strong and consists of multiple bundles that Figure 2. (a) Posterior view, (b) anterior view and (c) sacroiliac joint cut in transverse plane. 1, 2: superior and inferior iliolumbar ligaments, respectively; 3: sacrospinous ligament; 4: sacrotuberous ligament; 5: posterior sacroiliac ligaments; 6: anterior sacroiliac ligaments; 7: sacroiliac joint; 8: interosseous ligament.11 Downloaded from http://ijssurgery.com/ by guest on September 23, 2021 International Journal of Spine Surgery, Vol. 00, No. 00 0 Kiapour et al. Table 1. Sacroiliac joint (SIJ) ligaments, locations, and functions.10 Ligament Location Primary Restraint Posterior ligaments (5) Posterior superior iliac spine to sacral tubercles Sacral extension Long ligament Short ligament Sacrotuberous (4) Posterior superior iliac spine and sacrum to ischial tuberosity Sacral flexion Sacrospinous (3) Apex of the sacrum to ischial spine Sacral flexion Anterior ligament (6) Crosses ventral and caudal aspect of SIJ Sacral flexion, axial rotation Interosseous (8) Between sacrum and ilium dorsal to SIJ Sacral flexion, axial rotation Iliolumbar (1 and 2) Transverse process of L5 to the iliac tuberosity and crest Lateral side bending Ventral band Ventral band Dorsal band Forward flexion Sacroiliac part Dorsal band pass from the lateral crest of the sacrum to the muscles of the body. These muscles include the posterior superior iliac spine and the posterior end erector spinae, psoas, quadratus lumborum, pirifor- of the iliac crest. The anterior sacroiliac ligament (6) mis, abdominal obliques, gluteals, hamstrings, and is a thin ligament. It is weaker than the posterior pelvic floor muscles (levator ani and coccygeus ligament and runs over the joint obliquely from muscles). While they do not act directly on the SIJ, sacrum to ilium. The sacrotuberous ligament (4) is the muscles that cross the joint act on the hip or the located at the inferior-posterior part of the pelvis lumbar spine.12,13 Movements of the SIJ are and runs from sacrum to the ischial tuberosity. The indirectly produced by gravity and muscles acting sacrospinous ligament’s (3) attachment is behind on the trunk and lower limbs rather than active that of the sacrotuberous ligament, and it connects movements of the sacrum.11 However, pelvic floor the outer edge of the sacrum and coccyx to the muscles (levator ani and coccygeus muscles) act to ischia of the ilium.