Chapter 14 The sacroiliac joint
The sacrum has two unique roles. In a longitudinal direction, it lies at the base of the vertebral column and therefore supports the lumbar spine. Consequently, all longitudinal forces delivered to the lumbar spine are ultimately transmitted to the sacrum. Meanwhile, in a transverse direction, the sacrum is an integral part of the pelvic girdle. It is wedged between the two iliac bones and constitutes the posterior wall of the pelvis. This relationship enables it to transmit forces from the vertebral column sideways into the pelvis and thence into the lower limbs. Conversely, forces from the lower limbs can be transmitted through the pelvis to the sacrum and thence to the vertebral column.
The sacrum, however, is not fused with the rest of the pelvis. Rather, it forms a joint on each side with the corresponding ilium; but although the structure of the sacroiliac joint is well known, its purpose has been a source of contention.
At one extreme, conservative authorities have essentially dismissed the joint as having no functional significance on the grounds that it exhibits little or no movement. On the other hand, others have portrayed the joint as having important primary movements that can and should be assessed clinically like any other joint of the body.
Both views are in error. Despite its size, the sacroiliac joint cannot be considered like any other major joint of the body. Its ranges of movement are very small and it is not endowed with muscles that execute active movements of the joint. Structurally and functionally, the sacroiliac joint is more like the intertarsal joints of the foot, which do not exhibit active movements but which, nonetheless, move passively.
If the sacrum was fused with the rest of the pelvis, the pelvis would be a solid ring of bone. But this ring would be exposed daily to large twisting forces, particularly during walking. When the right lower limb is extended, the pelvis on that side would tend to twist forwards. For example, tension in the iliofemoral ligament would draw the anterior ilium downwards, thereby rotating the right pelvis clockwise, if viewed from the right. Meanwhile, if the left lower limb was flexed, the left half of the pelvis would be twisted backwards. For example, tension in the hamstrings would draw the ischium forwards, causing the left pelvis to rotate anticlockwise, if viewed from the right. As gait continues, the alternating flexion and extension of the lower limbs would impart alternating twisting forces on the pelvis around its transverse axis.
This effect can be modelled by holding a pretzel in two hands and twisting it around its long axis in alternating directions. Eventually the pretzel will snap. The same occurs clinically. Insufficiency fractures of the sacrum occur in elderly individuals, particularly females, in whom the sacroiliac joint is relatively ankylosed and in whom the sacrum has been weakened by osteoporosis. Under these conditions the torsional stresses, normally buffered by the sacroiliac joint, are transferred to the sacrum, which fails by fracture. Conspicuously and strikingly, these fractures run vertically through the ala of the sacrum parallel to the sacroiliac joint.1–8
This phenomenon indicates the need for, and role of, the sacroiliac joint. The joint is placed strategically in the pelvic ring at the site of maximum torsional stress in order to relieve that stress. In teleological terms, a solid ring of bone will not work; it will crack, and the sacroiliac joint is there in anticipation of that crack.
From these observations, the design features of the sacroiliac joint emerge. On the one hand, it must allow movements imposed on the pelvis by twisting forces from the lower limbs, but the movements need not be major in amplitude; it need only be that the twisting forces are absorbed into ligaments and thereby reduce the tendency of the pelvic ring to fracture. At the same time, the sacroiliac joint must be strong and stable in order to transmit the forces from the vertebral column to the lower limbs. A loose joint dependent on ligaments would simply creep under static body weight, let alone under the forces incurred during movements of the trunk. To this end, a bony locking mechanism can be used so as to spare the ligaments from static and longitudinal loads.
The structure of the sacroiliac joint can, therefore, be anticipated. For its longitudinal functions, it will exhibit osseous features that lock it into the pelvic ring. For its antitorsion functions it will exhibit, in a parasagittal plane, a planar surface that can allow gliding movements, but it will be strongly reinforced by ligaments that both retain the locking mechanism and absorb twisting forces.
The first design imperative is to lock the sacrum into the pelvis. To this end, the articular surface of the sacrum presents an irregular contour, marked by ridges, prominences, troughs and depressions (Fig. 14.1). These are matched by reciprocal depressions, troughs, prominences and ridges on the ilium, so that the bones can lock into one another. This gives the sacroiliac joint a sinuous appearance in frontal view (Fig. 14.2).9
A further feature, noted in textbooks of anatomy11 but not verified by modern quantitative studies, pertains to the plane of the joint. The articular surface of the sacrum is twisted from above downwards. Opposite the S1 segment, the dorsal edge of the articular surface projects slightly further laterally than the ventral edge. Conversely, at the S3 segment, the ventral edge projects slightly more laterally than the dorsal edge. Because of this, when viewed in transverse section, the sacrum is wedge shaped but in opposite directions at opposite ends of the sacroiliac joint. At the S1 segment, the posterior width of the sacrum is greater than its anterior width. Conversely, at the S3 segment, the anterior width is greater than its posterior width (Fig. 14.3).
The cartilages differ on the sacrum and ilium. The sacral articular cartilage is normally white and smooth, and has the features of typical hyaline cartilage.12 Its thickness ranges from 1 to 3 mm.13 The iliac cartilage is duller in appearance and is marked by dense bundles of collagen, which give it the appearance of fibrocartilage,12 but histologically and biochemically it is nonetheless hyaline in nature.14 It is usually less than 1 mm thick. Its cell density, however, is greater than that of the sacral cartilage.15 Meanwhile, the subchondral plate of the ilium is some 50% thicker than that of the sacrum.15
The reasons for these differences between the sacral and iliac cartilages has not been established. One contention, however, is that the sacral cartilage is designed for transmitting forces (from the spine to the pelvis) whereas the iliac cartilage is designed to absorb them.15
When articulated between the two ilia, the sacrum is held firmly in place by bony locking mechanisms. The interlocking contours of the sacrum and ilium prevent downward gliding of the sacrum under body weight. Indeed, the friction coefficient of the sacroiliac joint is larger than that of the knee and is considerably greater in proportion to the prominence of the ridges and depressions of the articular surface.16
Furthermore, the sacrum is set obliquely between the ilia such that its anterior end leans forwards. Consequently, under vertical loads it tends to tilt forwards and downwards, rotating around Bonnaire’s tubercle; the wedge shape of the sacrum opposes this. If the sacrum rotates forwards, the wider posterior end of the S1 segment will move inferiorly and will tend to separate the ilia. Meanwhile, the wider anterior end of the S3 segment will move upwards and also will tend to separate the ilia.
None of these movements will occur, however, if the sacrum remains clamped between the two ilia. If the ilia press against the sacrum, the engaged corrugations will prevent the sacrum from sliding downwards. If the ilia are prevented from separating, the wedge shape of the sacrum will not allow it to rotate forwards. Critical to keeping the ilia locked against the sacrum are the ligaments of the sacroiliac joint.