The sacroiliac joint

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Chapter 14 The sacroiliac joint

Chapter contents

Structure

Bones
Cartilage
Articulation
Ligaments
Capsule
Innervation

Age changes

Biomechanics

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.

The essence of the sacroiliac joint is that it is a stress-relieving joint. This can be appreciated by imagining what would happen if the sacroiliac joint did not exist.

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.

Structure

Bones

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

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Figure 14.1 A lateral view of the sacrum showing the contours of its articular surface. ‘+’ denotes an elevation; ‘−’ denotes a depression.

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Figure 14.2 A posterior view of the sacroiliac joints showing the sinuous shape of the joint cavities.

Particularly evident is a major depression on the sacral surface, on the S2 segment, which receives a major prominence from the ilium, the latter being known as Bonnaire’s tubercle.10

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).

image

Figure 14.3 Transverse sections of the sacrum showing how, at upper sacral levels (S1), the sacrum is wider posteriorly than anteriorly, but at lower sacral levels (S2.5) the sacrum is wider anteriorly than posteriorly.

Cartilage

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

Articulation

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.

Ligaments

The most important ligament of the sacroiliac joint is the interosseous sacroiliac ligament. This ligament is a dense, thick collection of short collagen fibres that connect the ligamentous surface of the sacrum with that of ilium. It lies deep in the narrow recess between the sacrum and ilium dorsal to the cavity of the joint. The full thickness of the ligament is most clearly evident in transverse sections of the sacrum and ilium (Fig. 14.4). Gray’s Anatomy11 recognises deep and superficial parts of this ligament, each divided into superior and inferior bands. The superior superficial band connects the superior articular processes and lateral crest of the first two sacral segments to the neighbouring ilium, and is highlighted as the short posterior sacroiliac ligament.

image image

Figure 14.4 The interosseous sacroiliac ligament. (A) Posterior view. (B) Axial view.

The cardinal function of the interosseous sacroiliac ligament is to bind the ilium strongly to the sacrum, thereby securing the bony interlocking mechanism.

Behind the interosseous ligament lies the posterior sacroiliac ligament proper. This consists of several fascicles of different lengths which connect the intermediate and lateral crests of the sacrum to the posterior superior iliac spine and the posterior end of the inner lip of the iliac crest. Those fibres from the third and fourth sacral segments are longer than the others and constitute the long posterior sacroiliac ligament (Fig. 14.5).11

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Figure 14.5 The short and long posterior sacroiliac ligaments.

The short posterior sacroiliac ligament is disposed to act in concert with the interosseous ligament to bind the ilium to the sacrum. Moreover, its posterior location allows it to prevent posterior flaring or diastasis of the joint. The long posterior sacroiliac ligament has a more longitudinal orientation and prevents backward rocking (counternutation) of the sacrum with respect to the ilium.17

The anterior sacroiliac ligament covers the ventral aspect of the joint. It consists of long, transversely orientated fibres that extend from the ala and anterior surface of the sacrum to the anterior surface of the ilium.

The fibres are attached to these bones for considerable distances beyond the margins of the joint (Figs. 14.4, 14.6). Like the interosseous ligament, this ligament binds the ilium to the sacrum but its anterior location enables it to prevent anterior diastasis of the joint.

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Figure 14.6 The anterior sacroiliac ligaments.

Remote from the joint proper are the sacrospinous and sacrotuberous ligaments. These ligaments are orientated to prevent upward tilting of the lower end of the sacrum (nutation) by anchoring it to the ischium.

The sacrospinous ligament takes a broad origin from the lateral edge of the sacrum below the sacroiliac joint. Its fibres converge on the ischial spine. The sacrotuberous ligament arises from the posterior superior iliac spine, where it blends with the long posterior sacroiliac ligaments; from the transverse tubercles of the lower sacral segments; and from the lateral margin of the sacrum, where it blends with the sacrospinous ligament. From this broad origin, the ligament narrows but broadens again to attach to the medial margin of the ischial tuberosity. The superficial fibres of the ligament are continuous with the tendon of the biceps femoris.11

Capsule

There is scant mention in the literature of the structure of the capsule of the sacroiliac joint. This is because the joint is so intimately surrounded by thick ligaments. Under these circumstances, it is difficult, if not impossible, to discern where a ligament ends and a capsule begins. For this reason, authors report that the posterior capsule is rudimentary or absent18 and that the anterior sacroiliac ligament is a thickening of the anterior capsule.11

Innervation

Various descriptions but few data have been published on the innervation of the sacroiliac joint. Even with modern studies, it is unclear the extent to which the joint is innervated from the front and from the back and which neural segments are involved.

In 1936, Pitkin and Pheasant19 lamented the lack of any description of the innervation of the sacroiliac joint in textbooks of anatomy. Indeed, this situation has still not changed.11 Instead, Pitkin and Pheasant relied on the 19th-century German literature to assert that the joint was innervated anteriorly by direct branches of the lumbosacral trunk, the superior gluteal nerve and the obturator nerve; posteriorly it was innervated by branches of the S1 and S2 dorsal rami.

Modern authors have endorsed this pattern, reporting a posterior innervation from the lateral branches of the posterior rami of L4 to S3, and an anterior innervation from the L2 to the S2 segments.18 However, the literature that they cite to this effect does not consist of authoritative anatomical studies. Others, referring to the same literature, describe a posterior innervation from L3 to S3 but emphasise that the principal segments are L5 to S2 but without citing any literature to this effect.20

Formal, modern anatomical studies provide conflicting conclusions. A German study21 stipulated a posterior innervation exclusively from the S1 and S2 dorsal rami but expressly denied an anterior innervation from either the sacral plexus or the obturator nerve. A Japanese study, however, reported a posterior innervation from the L5 and sacral dorsal rami and an anterior innervation from the L5 ventral ramus and S2 ventral ramus.22

Age changes

The future sacroiliac joint becomes apparent during the second month of fetal development, as a strip of mesenchyme between the cartilages of the future ilium and sacrum.23 Cavitation of this mesenchyme proceeds progressively and achieves its full extent by the seventh month. A synovial membrane appears by 37 weeks.12

In embryos, the articular surfaces remain smooth and flat. The anterior capsule is thin and lax, but the interosseous ligament is well developed.12

During the first 10 years of life, the joint enlarges in size but its surfaces remain flat; the anterior capsule thickens.12 Corrugation of the joint surfaces starts to develop during the second decade, starting with a depression along the articular surface of the sacrum matched by a reciprocal ridge along the ilium.12

With increasing age, the prominence of the iliac ridge increases but the surfaces of the articular cartilages start to exhibit superficial fibrillation and erosion, particularly on the iliac side.12 At the joint margins, the capsule and synovium become thicker and more fibrous, and osteophytes start to appear. By the fifth and sixth decades these changes are more marked. The articular cartilage loses its thickness and becomes fibrillated and eroded. Debris and fibrous tissue connecting the bones fill the joint cavity.12 Sinuous corrugations along the joint are established by the sixth decade9 but it is not clear when these start to develop.

By the eighth decade, osteophytes are large and interdigitate. Intra-articular fibrous adhesions are plentiful. The thickness of the articular cartilage is reduced to less than 1 mm on the sacrum and less than 0.5 mm on the ilium.12

These developmental changes predicate the nature and ranges of possible movements of the sacroiliac joint. By retaining a flat surface, young joints are able to glide in all directions. The advent of depressions on the sacral surface restricts possible movements to nodding movements along the longitudinal axis of the curved auricular surface. The range of available movement, however, decreases as fibrous ankylosis increases in the joint and as osteophyte formation increases.

Biomechanics

A variety of difficulties have faced investigators intent on studying sacroiliac movement. One has been ‘generalisability’. Studies on cadavers allow selected movements to be studied, but these movements may not be ones that occur in vivo. Studying a single isolated sacroiliac joint exaggerates the possible movements because the clamping effect of the other ilium has been removed. Fixing the entire pelvis in order to study pure sacroiliac motion eliminates the effects of pubic movement or deformation of the ilium when the sacrum is loaded.

For a realistic appreciation of sacroiliac movement, studies need to be conducted in living individuals but then technical difficulties obtain. Radiographically, the sacroiliac region is very complicated: the few landmarks are restricted to the sagittal plane and most margins of the bones under study are curved transversely. This makes landmark recognition essentially impossible. What appears to be the same point in lateral X-ray views is not necessarily the same point in a second view of a new position of the sacrum and pelvis. Errors in landmark recognition subsequently propagate into large errors in the geometry used to measure angular movement and the location of axes of rotation.

A further problem is sample size. A study of one specimen is inadequate, for it is not evident whether the specimen studied is typical or an extreme example.

Because of these problems, studies based on plain radiographs24 or single specimens25 are, in principle, unreliable and, at best, only indicative of what might be found using more reliable techniques or larger numbers of specimens. To ensure accurate landmark recognition, implanted devices are required. These establish unique landmarks. However, even then the implanted landmarks must be three-dimensional and multiple, lest rotations out of the plane of view exaggerate or reduce the apparent movement in the plane of view.

In this regard, studies using single wires or rods implanted into each of two moving bones26 are unreliable because they do not account for three-dimensional movement. Furthermore, thin rods are subject to deformation and displacement by overlying skin, fascia and muscles.27

The most reliable results are obtained either by implanting tiny tantalum spheres into the bones of the pelvis and studying their relative movements by biplanar radiography, or by rigidly fixing onto the bones external devices that bear markers whose three-dimensional orientation can be determined.

The first study to use implanted spheres examined four patients moving from supine and prone positions to positions of standing, standing on one leg, and standing with maximum lumbar lordosis.28 In essence, the range of sacroiliac motion observed in these manoeuvres, whether around a transverse, longitudinal or anteroposterior axis, was less that 2°, and was less than 1° in most cases. A later study of 25 patients, using the same technology confirmed these results.29 Mean rotations were less than 2° and ranges were less than 4°. The same picture has been borne out in studies of intact pelvises in cadavers.30,31

A study using rigidly fixed external devices in 21 volunteers examined the range of motion of the sacroiliac joint during forward flexion of the trunk, during maximum extension of the trunk and standing on one leg.32 The mean ranges of motion reported were less than 1°, with standard deviations of not more than 0.9°. However, these results are somewhat illusory. They constitute the mean magnitude of the motion, irrespective of direction. During flexion of the trunk, the sacrum was just as likely to flex as to extend around its transverse axis.32 Consequently, in a sample of individuals, the true mean range of motion of the sacroiliac joint is 0°, with as many individuals exhibiting up to 1° of rotation of the sacrum backwards as forwards, for the same excursion of the trunk. Thus, although the sacroiliac joints move, their direction of movement is irregular.

Subsequent, more recent, studies have examined the movements of the sacroiliac joint in a variety of postures and movements, such as the reciprocal straddle position,33 and standing hip flexion.34 They show that the range of movement is essentially only about 1°. Furthermore, even after therapeutic manipulation, the joint shows no change in position or movement.35

None of these data makes anatomical sense if one looks to the sacroiliac joint for primary movements. Its movements are feeble in magnitude and irregular in direction. Moreover, there are no muscles designed to act on the sacroiliac joint to produce active, physiological movements. All muscles that cross the joint are designed to act on the hip or the lumbar spine.

However, the nature of sacroiliac movements makes perfect sense in terms of the joint being a stress-relieving joint. In this regard the studies of Lavignolle et al. although not definitive, are nonetheless indicative.36

These investigators obtained biplanar radiographs of the sacroiliac region in individuals lying prone and alternatively flexing one hip while extending the other. The radiographs were used to determine the location of the instantaneous axes of rotation of the sacroiliac joint.

Instead of conventional axes passing transversely, longitudinally or sagittally through the joints, these investigators found that the axes of movement of the sacroiliac joints passed obliquely across the pelvis. For flexion, the axis passed backwards from the pubic symphysis to the greater sciatic notch. For extension, the axis passed from the pubic symphysis through the pelvis between the ischium and coccyx (Fig. 14.7). Given these axes, it is evident that during flexion and extension of the lower limbs, the sacrum undergoes complex movements. During flexion of the hip the ipsilateral ilium glides backwards and downwards across the sacrum and compresses against it, pivoting at the pubic symphysis. During extension, the ilium glides forwards and flares away from the sacrum.

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Figure 14.7 (A) Anterior (B) and top views of the location of the axes of movement of the sacroiliac joints during flexion and extension of the lower limbs.

(Based on Lavignolle et al. 1983.36)

None of these movements is produced by active movements of the sacrum. They are imposed on the sacroiliac joint by the mass of the trunk acting on the sacrum and tension exerted on the ilium from the muscles of the lower limbs. The nature of movements at the sacroiliac joint is fully consistent with the pelvis being distorted in three dimensions, and with the sacroiliac joint being designed to relieve the stress on the pelvic ring.

References

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2 Cotty P, Fouquet B, Mezenge C, et al. Insufficiency fractures of the sacrum. J Neuroradiol. 1989;16:160-171.

3 Davies AM, Evans NS, Struthers GR. Parasymphyseal and associated insufficiency fractures of the pelvis and sacrum. Br J Radiol. 1988;61:103-108.

4 De Smet AA, Neff JR. Pubic and sacral insufficiency fractures: clinical course and radiologic findings. Am J Roentgenol. 1985;145:601-606.

5 Lourie H. Spontaneous osteoporotic fracture of the sacrum: an unrecognized syndrome of the elderly. JAMA. 1982;248:715-716.

6 Peris P, Guanabens N, Pons F, et al. Clinical evolution of sacral stress fractures: influence of additional pelvic fractures. Ann Rheum Dis. 1993;52:545-547.

7 Ries T. Detection of osteoporotic fractures with radionuclides. Radiology. 1983;146:783-785.

8 Weber M, Hasler P, Gerber H. Insufficiency fractures of the sacrum. Twenty cases and review of the literature. Spine. 1993;18:2507-2512.

9 Vleeming A, Stoeckhart R, Volkers ACW, et al. Relation between form and function in the sacroiliac joint. Part I. Clinical anatomical aspects. Spine. 1990;15:130-132.

10 Wilder DG, Pope MH, Frymoyer JW. The functional topography of the sacroiliac joint. Spine. 1980;5:575-579.

11 Williams PL, editor. Gray’s Anatomy, 38th ed, Edinburgh: Churchill Livingstone, 1995.

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14 Paquin JD, van der Rest M, Marie PJ, et al. Biochemical and morphologic studies of cartilage from the adult sacroiliac joint. Arthritis Rheum. 1983;26:887-895.

15 McLauchlan GJ, Gardner DL. Sacral and iliac articular cartilage thickness and cellularity: relationship to subchondral bone end-plate thickness and cancellous bone density. Rheumatology. 2002;41:375-380.

16 Vleeming A, Volkers ACW, Snijders CJ, et al. Relation between form and function in the sacroiliac joint. Part II. Biomechanical aspects. Spine. 1990;15:133-135.

17 Vleeming A, Pool-Goudzwaard AL, Hammudoghlu D, et al. The function of the long dorsal sacroiliac ligament: its implication for understanding low back pain. Spine. 1996;21:556-562.

18 Bernard TN, Cassidy JD. The sacroiliac joint syndrome. Pathophysiology, diagnosis, and management. In: Frymoyer JW, editor. The Adult Spine. Principles and practice. New York: Raven Press; 1991:2107-2130.

19 Pitkin HC, Pheasant HC. Sacrarthogenetic telalgia I. A study of referred pain. J Bone Joint Surg. 1936;18:111-133.

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21 Grob KR, Neuhuber WL, Kissling RO. Innervation of the human sacroiliac joint. Z Rheumatol. 1995;54:117-122.

22 Ikeda R. Innervation of the sacroiliac joint: macroscopical and histological studies. Nippon Ika Daigaku Zasshi. 1991;58:587-596.

23 Schunke GB. The anatomy and development of the sacro-iliac joint in man. Anat Rec. 1938;72:313-331.

24 Weisl H. The movements of the sacroiliac joints. Acta Anat. 1955:80-91.

25 Frigerio NA, Stowe RR, Howe JW. Movement of the sacro-iliac joint. Clin Orthop. 1974;100:370-377.

26 Colachis SC, Worden RE, Bechtol CO, et al. Movement of the sacroiliac joint in the adult male. Arch Phys Med Rehab. 1963;44:490-498.

27 Vleeming A, Stoeckhart R, Snijders CJ. General introduction. In: Vleeming A, Mooney V, Snijders C, et al, editors. First Interdisciplinary World Congress on Low Back Pain and its Relation to the Sacroiliac Joint. Rotterdam: European Conference Organizers; 1992:3-64.

28 Egund N, Olsson TH, Schmid H, et al. Movements in the sacroiliac joints demonstrated with roentgen stereophotogrammetry. Acta Radiol. 1978;19:833-846.

29 Sturesson B, Selvik G, Uden A. Movements of the sacroiliac joints: a roentgen stereophotogrammetric analysis. Spine. 1989;14:162-165.

30 Miller JAA, Schultz AB, Andersson GB. Load–displacement behaviour of sacroiliac joints. J Orthop Res. 1987;5:92-101.

31 Vleeming A, van Wingerden JP, Dijkstra PF, et al. Mobility in the sacroiliac joints in the elderly: a kinematic and radiologic study. Clin Biomech. 1992;7:170-176.

32 Jacob HAC, Kissling RO. The mobility of the sacroiliac joints in healthy volunteers between 20 and 50 years of age. Clin Biomech. 1995;10:352-361.

33 Sturesson B, Uden A, Vleeming A. A radiostereometric, analysis of the movements of the sacroiliac joints in the reciprocal straddle position. Spine. 2000;25:214-217.

34 Sturesson B, Uden A, Vlemming A. A radiostereometric analysis of movements of the sacroiliac joints during the standing hip flexion test. Spine. 2000;25:364-366.

35 Tulberg T, Blomberg S, Branth B, et al. Manipulation does not alter the position of the sacroiliac joint. A Roentgen stereophotogrammetric analysis. Spine. 1998;23:1124-1129.

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