Yudu - sportEX medicine - 1999-2010

SACROILIAC JOINT JOINT

muscle responses in the glutei, quadratus lumborum and multifidus (the abdominals were not examined), indicating the likely regulatory function of the sacroiliac joint over the activation of the spinal and gluteal muscles (4). It is therefore possi- ble that injuries to the sacroiliac joint, or dysfunctions of movement that alter nor- mal joint and capsulo-ligamentous load- ing, may disrupt normal proprioceptive feedback to these muscles and change their recruitment patterns.

Of course the relationship is likely to be reciprocal - disruption of normal gluteal and stabilising muscle activity through some other cause is very likely to upset SIJ function. This may help explain the observation that patients with clinically diagnosed SIJ dysfunction commonly demonstrate disrupted hip control, poor gluteal activation, poor balance and poor lower limb alignment control. In this way, sacroiliac dysfunction is commonly seen combined with hip symptoms or with any one of a multitude of lower limb disorders that relate to a dysfunction in the control of alignment such as patellofemoral syndrome, iliotibial band problems and problems relating to an excess or lack of foot pronation.

This type of complementary muscle dys- function was recently confirmed in a study by Hungerford et al, who showed that SIJ- pain patients have timing deficits in their recruitment of internal oblique, multifidus, gluteus maximus and biceps femoris (5). Internal oblique, multifidus and gluteal activation was delayed and biceps femoris activity accelerated in the SIJ pain patients compared with controls during transfer of weight to the painful side in standing. In an earlier study O’Sullivan showed that SIJ pain patients produce poor pelvic floor and abnormal diaphragm activity compared with pain-free subjects on the performance of an active straight leg raise manoeuvre (6). Muscle activity improved when the SIJ was manually compressed.

STRUCTURE AND FUNCTION The pelvic girdle has been described as

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Superior articular surface

ASIS

Iliac fossa

PSIS AIIS

Body of ischium

PIIS Body of pubis

Ischial tuberosity

Figure 2: Bony landmarks of the innominate bone

being ‘the crossroads of the body and its architectural centre’ - a poetic yet appro- priate description of the function of the sacrum. The sacrum is situated at the junction where the vertical forces are transferred down through the spine and the transverse forces transferred from the two innominate bones between which it is wedged. The sacroiliac joint must absorb the large twisting moments developed within the pelvis by rotational activities and particularly by gait, and resist the shear induced by the even larger vertical forces transferred down through the spine and up through the lower limbs.

The loads are considerable. The vertical forces are estimated to be between 5 and 17 times body weight, depending on activity (7). The torsional forces of gait are of a sufficient magnitude to fracture the osteoporotic pelvis in situations where the joint is relatively ankylosed and its movement limited. Interestingly, these fractures run vertically through the sacrum in the same alignment as, and just adjacent to, the sacroiliac joint, a testa- ment to its necessity (2).

The best estimates of motion come from investigations that track the movement of implanted metal spheres using x-ray or CT scans. Studies on cadavers reveal an aver- age passive range of movement of between 5° and 8°, with extreme values ranging from 3° to 17° (2). In vitro stud- ies of the same type reveals an average 2° of rotation and two millilitres of transla- tion on weight-bearing activity (8). It has also been established from cadaver stud- ies that the joints stay mobile throughout life into quite respectable old age (9).

The function of the sacroiliac joint explains its design. The joint’s ligamen- tous support system is well-suited to the absorption of torsional forces but liga- ments are poorly suited for supporting static postural loads of such magnitude as they creep, deform and stretch. This requires there to be some bony locking mechanism.

This bony joint stability is referred to as self-bracing or self-locking. Biomechanical research supports the notion that the SIJ is stabilised by a com- plex modified friction mechanism. Operating like a form of clutch, it is fric- tion between the two opposing surfaces that prevents excessive joint translation (2,9,10). As is obvious when you slide two hands together, resistance to translation is dependent upon compressive forces (how forcefully the hands are pressed into one another) and the nature of the two

Auricular surface

Sacral hiatus

Figure 1: Bony landmarks of the sacrum Iliac crest

The sacroiliac joint is a synovial joint sur- rounded by the body’s most comprehen- sive ligamentous system. The pelvic ring is completed anteriorly by the pubic sym- physis - a very stiff structure that has a fibrocartilagenous disk interposing the hyaline cartilage-covered pubic bones and supported by strong ligaments. The SIJ starts life as a fairly flat and mobile struc- ture but various adaptions in its develop- ment through life increase its stability.

MOTION AND STABILITY The sacroiliac joints move as they adjust to the forces imposed upon the pelvis. The position of the joint is not under discrete voluntary control but the alignment of body weight, the ground reaction force and the activity of the surrounding mus- culature all play a role in determining its position and movement.

Auricular surface