Clinical anatomy and biomechanics

Ioannis Paneris

The sacroiliac joints (SIJs) are designed mainly for stability, with their function being to transmit and dissipate load from the trunk to the lower extremities and vice versa during weight-bearing activities (Cohen, 2005). Although, for many decades, clinicians considered the SIJ to be immobile, this notion has been overturned over the recent years (Forst et al, 2006; Pool-Goudzwaard et al, 1998). Research has shown that the SIJ is a synovial joint where motion occurs as coupling with 6° of freedom. In fact, the advantage of a mobile SIJ is that it is more efficient as a shock absorber and as a provider of proprioception (Pool-Goudzwaard et al, 1998).The amount and direction of movement of the SIJ is not clearly agreed amongst researchers owing to the employment of different methodological approaches. Recently, the development of Roentgen Stereophotogrammetric Analysis (RSA) has led to more robust studies in the small movements of the SIJ (Goode et al, 2008).

Standing and sitting

In the normal bipedal standing posture, the line of gravity from the upper body falls posteriorly to the centre of the acetabula. In return the ground reaction force, transferred through the hip joints falls anteriorly to the SIJ. The interaction of these forces rotates the ilia posteriorly on the SIJs (DonTigny, 1985, 1990). This anterior rotation of the sacrum has been shown in both cadaveric (Wang & Dumas, 1998) and in vivo (Sturesson et al, 2000a) studies. In addition to the posterior rotation, the ilia rotate laterally, opening the superior part of the joint, and translate anteriorly and to a lot lesser degree inferiorly (Wang & Dumas, 1998).

The forward motion of the sacral promontory into the pelvis about a coronal axis is called ‘nutation’ (Lee, 1999, p. 49). Nutation increases the tension of the interosseous and short dorsal sacroiliac ligaments as well as the sacrotuberous ligament which increase compression of the SIJ and thus contribute to the stability of the joint (Pool-Goudzwaard et al, 1998).

Further to the ligamentous tension, the shape and morphology of the joint is a significant factor that contributes to stability. During the second and third decades of life, the articular surfaces become irregular and uneven with duller and rougher cartilage (Bowen & Cassidy, 1981). These changes reflect the dynamic development of the joint and allow for reduced load to be exerted on the ligaments when bearing the upper body (Vleeming et al, 1990). DonTigny (1990) states that in the lordotic posture of the spine, as well as in lifting, the line of gravity falls anteriorly to the centre of the acetabula which leads to posterior rotation of sacrum. This posterior sacral rotation is termed ‘counter-nutation’ and is a ‘position of vulnerability’ as the joint is less stable (Lee, 1999, p. 62). However, DonTigny (1990) claims that counter-nutation limits the caudal glide of the sacrum. This could possibly be due to the effect of the long dorsal sacroiliac ligament that limits counter-nutation and due to its attachments working together with the sacrotuberous ligament to stabilize the SIJ (Pool-Goudzwaard et al, 1998).

Similarly to standing, sitting upright should also result in nutation of the sacrum. A study (Snijders et al, 2004) has shown that in sitting conditions, where there is lumbar flexion and posterior rotation of the pelvis, counter-nutation of the sacrum is the result. In contrast, maintaining and/or supporting the lumbar lordosis in sitting results in nutation of the sacrum.

Lumbar flexion/extension

Lee (1999, p. 62) states that during the first 60° of forward bending of the trunk the nutation of the sacrum can be felt to increase. However, in some subjects, slight counter-nutation occurs towards the end of the range (Lee, 1999, pp. 62–63). The study of Jacob and Kissling (1995) mixed results of the rotation of the sacrum leading to the conclusion that ‘during forward flexion, the sacrum is as likely to nutate as to counter-nutate’ (Jacob & Kissling, 1995). An explanation for this phenomenon is offered by Lee (1999, p. 62) who states that when during forward flexion, the extensibility of the biceps femoris, sacrotuberous ligament and the deep lamina of the posterior layer of the thoraco-dorsal fascia has been reached, the relative flexibility of the sacrum becomes less than one of the innominates. Continuation of anterior rotation of the innominates on the femoral heads, after this point, leads to relative posterior rotation of the sacrum on the innominates and thus counter-nutationutation. Lee (1999, p. 63) claims that during extension, the thoraco-lumbar spine extends while the innominates posteriorly rotate at the hip joints, thus the nutation of the sacrum should not reverse.

Gait

During ambulation the innominates follow the direction of motion of the femurs causing intra-pelvic torsion between the innominates and the sacrum (DonTigny, 1990; Lee, 1999, p. 64; Smidt et al, 1997). The right innominate rotates posteriorly while the left rotates anteriorly following the extended right femur. The opposite motion of the innominates causes rotation of the sacrum to the right which is counteracted by the contralateral rotation of the trunk (DonTigny, 1990; Lee, 1999, p. 64). The intra-pelvic torsion causes nutation on the heel strike side and counter-nutationutation on the toe off side.

This arrangement seems to be theoretically sound as the nutation on the one side is essential to prepare the low limb for heel strike. However, the study of Smidt et al (1995) produced results that do not agree with the above assumption, in that only 10 out of their 32 healthy subjects demonstrated the expected pattern on reciprocal straddle position while 8 subjects demonstrated the internominate position changed in the same direction in both straddle positions.

The findings of the study of Smidt et al (1995) were not replicated by the later study of Sturesson et al (2000b) which experimentally confirmed the patterns of movement expected theoretically. In the Sturesson et al study, 6 subjects with posterior pelvic pain showed a uniform pattern in which changing from left leg extension to flexion, with parallel right leg flexion to extension, produced left innominate nutation and right counter-nutation with sacral rotation to the left around a vertical axis (with the exception of one person) and, in some subjects, rotation of the sacrum towards the left innominate on a sagittal axis where there was compression of the upper part of the left SIJ.

There is also a lack of agreement between the above studies regarding the magnitude of the motions observed, with the values reported by Smidt et al (1995) to be five times higher than the results of Sturesson et al (2000b). In fact, the results of Smidt et al (1995) imply that movements of the symphysis pubis range from 2.5 to 8 cm (Sturesson et al, 2000b). Perhaps the differences between the studies is that Smidt et al (1995) used bony landmark palpation and marking which is likely to lead to errors in determining the range of motion (Hungerford et al, 2004). In contrast, the study of Sturesson et al (2000b) utilized Roentgen stereophotogrammetric analysis (RSA) (radiostereomatic analysis) which is regarded as the gold standard in evaluating small joint and tendon mobility (Goode et al, 2008). During the swing phase, the non-weight-bearing innominate is moving with the lower limb from anterior rotation to posterior rotation. In other words, the innominate moves from a position of counter-nutation to a position of nutation ready to accept the ground reaction force on heel strike while, at the weight-bearing side, the innominate rotates anteriorly to a position of counter-nutation (Lee, 1999, p. 70).

Studies on the mobility of the SIJs during standing hip flexion have shown that both innominates rotate posteriorly (with few exceptions) while the sacrum rotates slightly to the side of hip flexion and rotates slightly on a sagittal axis gapping the top part of the SIJ on the side of hip flexion (Sturesson et al, 2000a). Similar findings were reported in the study of Hungerford et al (2004) where, during single leg flexion in standing, the non-weight-bearing innominate rotated posteriorly, there was gapping of the superior part of the joint and rotation of the anterior part of the innominate medially. Additionally, the innominate translated superiorly, anteriorly and laterally. This motion pattern was observed consistently in subjects with or without SIJ dysfunction and irrespective of side of hip flexion.

On the weight-bearing asymptomatic side, there was posterior rotation of the innominate, gapping of the superior part of the SIJ and rotation of the anterior innominate medially. The innominate rotated posteriorly, superiorly and medially (Hungerford et al, 2004). This pattern of translation can be attributed to the action of the multifidus and transversus abdominis whose action has been found to increase the stiffness of the SIJ (Richardson et al, 2002). On the weight-bearing symptomatic side the innominate rotated anteriorly and translated inferiorly and posteriorly, which is a pattern associated with inefficient intra-pelvic compression (Hungerford et al, 2004). The study of Mens et al (1999) reported that symptomatic SIJs have the tendency to exhibit excessive anterior rotation when the leg is hanging down with weight bearing on the asymptomatic leg over a block. Further, anterior rotation of the symptomatic SIJ is exhibited during supine active straight leg raising of the ipsilateral leg which is an indication of impaired stability of the joint.