The sacroiliac joint: an overview of its anatomy, function and potential clinical implications

Abstract

This article focuses on the (functional) anatomy and biomechanics of the pelvic girdle and specifically the sacroiliac joints (SIJs). The SIJs are essential for effective load transfer between the spine and legs. The sacrum, pelvis and spine, and the connections to the arms, legs and head, are functionally interrelated through muscular, fascial and ligamentous interconnections. A historical overview is presented on pelvic and especially SIJ research, followed by a general functional anatomical overview of the pelvis. In specific sections, the development and maturation of the SIJ is discussed, and a description of the bony anatomy and sexual morphism of the pelvis and SIJ is debated. The literature on the SIJ ligaments and innervation is discussed, followed by a section on the pathology of the SIJ. Pelvic movement studies are investigated and biomechanical models for SIJ stability analyzed, including examples of insufficient versus excessive sacroiliac force closure.

Fig. 1

(A) Unpaired sacrum and ilium showing the interindividual variation of the auricular part of the sacrum and ilium (articulated line). On the iliac side a more C-like form of the auricular SIJ is visible. Contrary, on the sacrum a L-form is present. The arrows indicate the position of the sacral concavity and the iliac tuberosity located dorsal to the auricular part of the SIJ (axial joint). (With permission from the Willard Carreiro collection.) (B) Paired sacrum and ilium. The SIJ is folded open posteriorly by dissecting all major ligaments. Notice (A) the concavity of the sacral auricular part, and (B) the corresponding auricular iliac part. (C) The iliac tuberosity of the axial joint and (D) the sacral concavity of the axial joint covered with rough cartilage. Notice parts of the interosseous ligaments (arrows).

Fig. 2

(Top left) Pelvis in erect posture. View of the pelvis from the ventrolateral side. (Bottom left) Dorsolateral view of the sacrum. The position of the axial joint is indicated, made up from the smaller cavity of the sacrum, corresponding with a generally larger iliac tubercle. (Top right) Showing the different angles of S1-S3 between left and right sacral articular surface. (Bottom right) Sacral articular surface at the right side. The different angles reflect the propeller-like shape of an adult SIJ. (With permission from Vleeming collection.)

Fig. 3

Ventral view of the thin anterior capsule of the SIJ (VSI). The iliolumbar ligaments are clearly visible (ILL), also the anterior long spinal ligament (ALL). (With permission from the Willard Carreiro collection.)

Fig. 4

Differences in the geometry of the auricular and axial areas. Corresponding numbers depict both the right and the left SIJ. Note the large intra- and interindividual SIJ differences (reprinted with permission from Bakland & Hansen, 1984).

Fig. 5

(A) Dorsal overview of the lumbopelvic area. The investing superficial fasciae over the muscles are removed. The superficial lamina of the posterior lumbar fascia is indicated as PLFsl. Note the thickness of the fascia over the sacrum and its geometry, forming part of the composite of the TLF over the sacrum. Gmax, gluteus maximus; PLFsl, superficial lamina of the posterior lumbar fascia. (Figure used with permission from the Willard Carreiro collection.) (B) The superficial lamina of the PLF. Notice the increased and specific density patterns of the superficial lamina over L4-L5 and sacrum. A, fascia of the gluteus maximus; B, fascia of the gluteus medius; C, fascia of external oblique; D, fascia of latissimus dorsi; 1, increased density of the lamina over the posterior superior iliac spine (PSIS); 2, sacral crest; LR, multidirectional thickening of the lamina over the lateral raphe. The connections of the mm. transverses abdominus and obliquus internus to the lateral raphe are located under the latissimus muscle. (Reproduced from Vleeming et al. , with permission from Spine.) (C) The deep lamina of the PLF. Notice the overall fiber direction of the deep lamina in relation to the superficial fascia (A). The deep lamina. B, Fascia of the gluteus medius; E, connections between the deep lamina and the underlying aponeurosis of the erector spinae and multifidi muscles. Notice the increased and specific density patterns of the deep lamina over L4-L5, especially covering lower lumbar multifidi and sacrum. The more caudal part of the deep lamina fuses with the STL; F, fascia of the internal oblique; G, fascia of the serratus posterior inferior; H, STL; 1, the posterior superior iliac spine (PSIS); 2, sacral crest; LR, lateral raphe formed by the aponeurosis of both the internal oblique and the transversus muscle connecting to the deep lamina. (Reproduced from Vleeming et al. , with permission from Spine.)

Fig. 6

Dorsal overview of the lumbosacral spine. The multifidus are removed. The posterior superior spine is indicated (PSIS). The long dorsal ligament is indicated (LDL). The ischial tuberosity is visible (IT) and the sacrotuberous diverges craniomedially. (With permission from the Willard Carreiro collection.)

Fig. 7

A dorsal overview of the deep dorsal SIJ ligaments after removing fascia and muscles and the STLs. The short posterior sacroiliac ligaments are indicated (SPSIL). (With permission from the Willard Carreiro collection.)

Fig. 8

Frontal sections of the sacroiliac joint (SIJ) of embalmed male specimen. S indicates the sacral side of the SIJ. (A) and (B) concern a 12-year-old boy; (C)-(I) concern a specimen older than 60 years. Arrows are directed at complementary ridges and depressions. They are covered by intact cartilage, as was confirmed by opening the joints afterwards. (With permission from the Vleeming collection.)

Fig. 9

Components of the composite over the lumbosacral spine. The superficial lamina of the PLF (1), which is dissected at (2) where the deep lamina of the PLF becomes visible. The deep lamina (2) is dissected and the aponeurosis tendon of the erector trunci and multifidi becomes visible (APO). (With permission from the Willard Carreiro collection.)

Fig. 10

The erector muscles are dissected and the multifidus muscle becomes clearly visible after removing the superficial and deep lamina of the PLF and aponeurose over the multifidus. COMP, indicates the remaining blended parts of the strong fascial composite and aponeurosis over the sacrum. (With permission from the Willard Carreiro collection.)

Fig. 11

Cross-section of the SIJ on the level of S1. Force application indicated, mainly by the transverse and internal oblique muscles (Fo), producing tension dorsally both to the SIJ ligaments and the composite of the TLF; (Fi); a larger reaction force ensues (Fj). Lateral tuberculum sacrum (1), ventral part of the ilium (2), auricular part of the SIJ (3), dorsal SIJ ligaments (4, 6), anterior SIJ ligaments (5), LDL (7), composite of TLF with aponeurosis and muscles visualized as a clamp (8). (Adapted after Snijders et al. .)