Sacro-Iliac Joint Anatomy

Sacro-iliac disease

The sacro-iliac joint is a DIARTHROSIS   an atypical synovial joint. It comprises the auricular (ear-shaped) surface of the sacrum and the ilium. The ilial surface is covered by fibrocartilage and the sacral surface is covered by hyaline cartilage. The joint surfaces are seperated by a small amount of synovial fluid.

Biomechanics of the SI joint:

•Functions to transfer hind limb propulsive force to the thoracolumbar spine

•Assisted by the musculoligamentous sling

•The ligamentous structure acts as a site of attachment for major muscle groups of the vertebral column, abdomen, hind limb and fascia of the sacrocaudalis or tail-head group

MRI image showing a cross-section through the sacro-iliac region

Ok, so the images above give a good pictorial impression of the location of the sacro-iliac joint. As you can see, it is a deep structure, not easy to assess; it can’t be palpated(touched) and it is difficult to image the region. Ultrasound images can be obtained through the rectal wall and scintigraphy or bone scanning can show signs of inflammation. We can block the area with local anaesthetic and see if the horse moves better or similarly, we can medicate the region with corticosteroids and see if there is improvement. Neither of these techniques is straightforward due to the position of the joint. Ultrasound-guided needle placement is the most accurate way to deliver drugs to the area, but it is not possible to be sure that the joint itself is penetrated. Many vets believe that depositing drugs close to the region is good enough especially as it is likely the sacro-iliac disease includes significant soft-tissue pain in the surrounding area as there is a complex ligamentous structure to the area.

The characteristics of sacro-iliac pain are loss of power from the hind limbs, low foot flight, scuffing of the toes on the ground due to a reduced range of movement of the hind limb. Some horses display pain on deep palpation of the muscles overlying the region. Riders often report that the horse is reluctant to go forward when ridden, disunites in the canter and generally displays poor rideability compared with normal. With severe sacro-iliac luxation following a fall or other trauma,the symmetry of the pelvis will be altered when viewed from behind; one of the tuber sacrale will appear higher than the other. Sacro-iliac pain can be secondary to other causes of hind limb lameness particularly proximal suspensory desmitis.

Because of the location of the sacro-iliac joint and the difficulties in making an accurate assessment of the region, there is scope for over diagnosis and therefore unnecessary treatment is often carried out. These cases should only be managed by experienced equine vets who know what they are doing.The rehabilitation of the horse with true sacro-iliac pain requires a very specialized program and is best supervised by a facility with many years experience such as Peasebrook.