Anatomy and structure
The sacroiliac (SI) joint lies between the ventral surface of the wing of the ilium and the dorsal surface of the wing of the sacrum. The articular surface of the sacrum is covered with hyaline cartilage, while the articular surface of the ilium is covered in fibrocartilage. This combination of cartilage types makes the SI an atypical synovial joint. The shape of the joint surfaces varies from L-shaped to C-shaped and can be flat or concavo-convex (Goff et al., 2008).
Supporting ligaments
The ligaments supporting the joint capsule include the following:
- The interosseous ligament, which is dorsal to the synovial region of the joint running between the wing of the sacrum and the wing of the ilium;
- The ventral sacroiliac ligament, a thin but strong ligament on the ventral surface of the joint;
- The dorsal sacroiliac ligament (DSIL), which has two parts: the dorsal or short portion of the ligament runs from the dorsal portion of the tuber sacrale to the spinous processes of the sacrum and is cord-like. The lateral or long portion of the dorsal SI ligament is sheet-like and runs from the caudal tuber sacrale and ilial wing to the lateral sacral crest;
- The broad sacrotuberous ligament (BSTL) runs from the lateral aspect of the sacrum and the transverse processes of the first caudal vertebrae to the tuber ischium in a broad sheet. While this ligament does not directly cross the joint surface of the sacroiliac, it does serve as a strong point of connection to the neuromuscular system, with direct insertions of the semimembranosus and gluteus medius muscles to this ligament. It also provides tracts for the cranial gluteal nerve, sciatic nerve, and the cranial gluteal artery and vein.
Movement and function of the SIJ
The morphology of the SI joint is of a nature to withstand compressive load together with translation of the joint surfaces (Goff et al., 2008). According to Goff et al. (2008), the SI joint may not be as subject to full weight bearing as other joints are, with the sling structure of the SI ligaments and their connection to the surrounding musculature reducing the compressive loads on the joint while allowing translation of the joint, which promotes the transfer of propulsive forces from the hindlimb.
Instability vs DJD – a continuum of dysfunction
Goff (2022) proposes a continuum of equine SIJ dysfunction. The continuum starts with hypermobility of the SI joint and ends with hypomobility as a result of degenerative changes and fibrosis of the joint.
Hypermobility of the SI joint refers to a functional instability, or an increase in the neutral zone of the joint. In this situation, the stabilising soft tissues, including ligaments and muscles, fail to provide adequate or appropriate resistance to movement in the joint. The neutral zone refers to a region of physiological movement where stabilising structures offer little resistance to movement. In the human vertebral column, the neutral zone has been shown to increase, or to include a larger range of motion, when dysfunction or weakness is present (Panjabi, 1992). This loss of stability has also been referred to as ‘microinstability’ (Brolinson et al., 2003).
The SI joint is functionally stabilised by the surrounding ligaments and muscles of the joint. The role of these structures in maintaining functional stability is to provide neuromotor control (Goff et al., 2008). By evaluating the size of the neutral zone of the SIJ, we can start to understand the level of neuromotor control.
The presence of microinstability or hypermobility in the SI will predispose the joint to pain, dysfunction, degenerative changes and fibrosis as the osseus structures adapt to create stability in the joint. This leads us to the other end of the scale of dysfunction, where hypomobility may be seen, evident in a decrease in the neutral zone (Goff, 2022).
Functional instability differs from clinical instability and is therefore difficult to test post-mortem. The above continuum of SI dysfunction does not include the presentation of trauma-related SI dysfunction or injury.
Clinical signs of dysfunction
Horses will usually present SIJ dysfunction in one of two categories. In the first, horses are usually still in work, presenting with difficult-to-define clinical signs, including poor performance and gait abnormalities. Additional signs can include:
- an altered gait, such as plaiting of the hindlimbs at a trot, or bunny hopping in a canter;
- difficulty maintaining balance during static unilateral weight bearing on a hind limb, especially with the addition of perturbations;
- hypertonicity of the epaxial muscles, leading to stiffness and reduced mobility of the thoracolumbar region during work;
- asymmetry of the pelvic musculature, with or without muscle atrophy;
- asymmetry of the bony landmarks of the pelvis;
- pain or tension on palpation of the dorsal sacroiliac ligament and middle gluteal muscle, with hypertonicity in the epaxial muscles;
- and poor impulsion.
Other signs can include:
- poor connection with the bit;
- poor canter quality;
- bucking or kicking out with a hind limb during canter; and
- worsening of clinical signs during ridden work in comparison to groundwork.
In the second presentation of SI dysfunction, horses suffer from chronic degenerative changes to the joint over time. All the above clinical signs apply, with marked changes and abnormalities to the gait as well as noticeable asymmetry of the muscles and bony landmarks of the pelvis (Goff, 2022).
Evaluation and testing
As with any condition, full evaluation is important and must include subjective and objective data analysis with gait analysis. When looking for SI dysfunction, a ridden evaluation may be incorporated into gait analysis for a more accurate overview of the gait.
Additional manual therapy tests include:
- palpation of the short dorsal sacroiliac ligament for pain;
- palpation of the middle gluteal muscles for pain and trigger points;
- compression of the sacroiliac joint by compressing the tuber sacrale and tuber coxae together; and
- ventrally displacing the sacrum (Goff et al., 2008).
Goals of treatment and rehabilitation
An effective treatment plan for SI dysfunction and rehabilitation will rely on the evaluation findings of the specific patient. Broadly, a treatment programme will aim to manage pain and restore functional movement to the affected joint. This can be done through manual therapy techniques such as manipulation and mobilisation, followed by therapeutic exercise that restore neuromotor control, muscle length, and the strength and functional stability of the pelvic region (Goff, 2022).
Rest vs exercise in the treatment of SIJ
Functional stabilisation and retraining for appropriate neuromuscular control are fundamental components of rehabilitating SI dysfunction. Rest can decrease or reduce both of these components, and should therefore be used with caution.
Developing an effective therapeutic exercise programme will involve thinking about the severity and location of the pain, and the degree of functional instability or stiffness present in the SI joint (Goff, 2022).
We need to select exercises that target the stabilising system as well as the hindlimb propulsive muscles.
Dosing of exercises will be determined by the specific muscle type and fatiguability of the muscles we wish to target.
The number of repetitions of an exercise will be determined by the onset of fatigue in the individual patient.
We can customise the speed and timing of repetitions to try to mimic the desired activity we want to improve. For example, if we want to improve the quality of the canter, we can perform a caudal weight shift with the forelimb raised to activate the hamstring muscle group in a rhythm that reflects the timing of a canter stride (Goff, 2022).
To target the muscles of stabilisation, we want to consider the multifidus and the sacrocaudalis dorsalis muscles, the psoas muscle group, the abdominal muscles and the diaphragm.
These muscles have a higher proportion of slow twitch muscle fibres with low levels of fatigue. We therefore want to incorporate slow contractions with long hold durations in our exercises, as well as fewer repetitions of each exercise.
Exercises that we could consider include:
- dynamic mobilisation exercises;
caudal weight shifting;
- backing up hills;
- pelvic flexion reflexes;
- sternal elevation reflexes;
- trotting pole work to target the diaphragm and abdominal muscles.
To target the hindlimb propulsion muscles, we need to consider primarily the gluteal and hamstrings muscle groups. These muscles include a higher proportion of fast-twitch muscle fibres that fatigue more quickly. Exercises should be performed with quick strong contractions, short holds, and a higher number of repetitions.
Exercises we may consider include:
- caudal weight shifting with the forelimb raised;
- trot and canter pole work;
- raised walk poles;
- backing up a hill;
- hill work at a walk or trot.
While we may want to target one muscle group over another at some point during rehabilitation, for SI dysfunction we want to rehabilitate overall neuromuscular control, which involves the coordinated contraction and use of both stabilising and propulsive muscle groups, as well as the muscles of the limbs.
Exercises to consider include:
- sternal lifting with lateral bending of the trunk;
- poles on a circle;
- poles on a slope;
- lateral tail pull at standing or walking; and
- all previously mentioned exercises.
Training aids such as the Equiband system can be incorporated into ridden and in-hand work to increase the challenge to the abdominal and hindquarter muscles.
Resources
Assessment and Treatment of the SIJ, a webinar presented by Lesley Goff.
References
Brolinson, P., Kozar, A., Cibor, G. (2003). Sacroiliac joint dysfunction in athletes. Current Sports Medicine Reports 2, 47–56.
Goff, L.M. (2022). Therapeutic exercises for equine sacroiliac joint pain and dysfunction. Vet Clin North Am Equine Pract, 38(3):569–584.
Goff, L.M, Jeffcott, L.B., Jasiewicz, J., McGowan, C.M. (2008). Structural and biomechanical aspects of equine sacroiliac joint function and their relationship to clinical disease. The Veterinary Journal 176, 281–293.
Panjabi, M. (1992). The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. Journal of Spinal Disorders 5, 390–396.
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