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DIAGNOSIS AND TREATMENT


surfaces - whether they are smooth or rough and whether there is a lubricant present. This is termed the friction coeffi- cient. The sacroiliac joint has a variety of innate characteristics and mechanisms to maximise friction and therefore stability. These elements have been described using the terms ‘form closure’ and ‘force closure’.


MECHANISMS OF FRICTION Form closure - the osteoarticular- ligamentous system These are the passive components of sta- bility - structures that either passively


limit range of movement and/or, more importantly, serve to increase the friction coefficient or assist in the production of compression (see figure 6). They include: ■ The nature of the cartilage - this is the only joint in the body where hyaline cartilage opposes fibrocartilage and this directly increases the friction coefficient.


■ The shape of the joint - the joint is rel- atively flat and smooth until the sec- ond decade when, seemingly timed to coincide with increased adolescent growth, the joint develops a convolut- ed and twisted shape, with inter-digi- tated ridges and grooves (see figure 3).


Figure 3: Cross section of the joint surface of the sacrum


Sacrospinous ligament


Sacrotuberous ligament


■ The surrounding ligaments (see figures 4 and 5) - the ligaments of this joint contribute to stability in a unique way. Their radial arrangement means that in one direction of joint movement they (more or less) all tighten, and in the other direction all slacken. When they tension, they draw the ilium and sacrum into one another, compressing and therefore stabilising the joint. This is called the self-braced position of the SIJ and occurs at end range sacrum nutation/innominate posterior rotation (see figure 6). In the opposite direc- tion of movement the ligaments by and large become lax and the stabilising compression is released. In fact only one ligament is put on tension - the long dorsal sacroiliac ligament. This may explain why it is commonly tender in syndromes of SIJ instability.


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Figure 4: Ligaments of the sacroiliac joint (anteri- or view)


Iliolumbar ligament


Sacrotuberous ligament


Force closure - the myofascial system These are the active, dynamic components of stability ie. the muscles and their attachments. They can contribute to pelvic stability in three ways: 1. by directly compressing the joint 2. by producing or maintaining self- braced positions of the joint


3. by directly increasing ligament tension. ©2003 Primal Pictures Ltd


Figure 5: Ligaments of the sacroiliac joint (poste- rior view)


8


The ‘inner core’ muscles of transversus abdominis and multifidus can stabilise the joint, as can gluteus maximus, biceps femoris, the erector spinae and latissimus dorsi. Other muscles may contribute in function - anatomists researching the bio- mechanics of this joint point to connec- tions between these more superficial structures, calling them ‘slings’ as shown in figures 7a and 7b.


The muscles of the slings are connected by fascia. Tension from muscle contraction is transferred throughout the slings and, owing to their alignment, this tension helps compress the SIJ. This implies that muscles quite distant from the pelvis can play some role in its stability, and also suggests an additional mechanism of ‘passive’ stability - elastic tension devel- oped in the fascia which is also compres- sive. It can be seen from the alignment of the slings that rotational activities eg. gait, running, throwing, will create elastic tension in this fascia which is potentially stabilising (9).


DIFFERENTIAL DIAGNOSIS In the absence of radicular signs and symptoms there is no clear way to differ- entiate SIJ-mediated pain from lumbar spine-mediated pain. Symptoms, aggra- vating and easing factors may be similar and there is little evidence that clinical differentiation tests are specific enough to be useful (3,11,12). According to injection studies, however, the most com- mon referral pattern of SI joint-mediated pain is to the buttock, just adjacent to the SIJ producing pain (13).


It has been proposed that the active straight leg raise test may be indicative of SIJ problems (5,6,14,15). In this test the patient is asked to actively produce hip flexion and to report on: 1. the perceived effort in lifting the leg 2. the production of symptoms. The rationale for the test is that as the hip flexors attach to the anterior aspect of


BOX 1. MYOFASCIAL SLINGS INFLUENCING THE SIJ


Posterior oblique ■ latissimus dorsi ■ thoracolumbar fascia ■ gluteus maximus ■ fascia lata of thigh ■ lateral retinacular of knee ■ peronei ■ tibialis anterior


Posterior longitudinal ■ erector spinae ■ sacrotuberous ligament ■ long head of biceps femoris


Anterior oblique ■ oblique abdominals ■ adductors


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