Static Assessment - Splint bones, hocks, tail, pelvis, back and tack. (Part 3)
In the final installment of this blog series, I delve into the splint bones, hocks, tail, pelvis, back and tack.
Splints The splint bones are palpable, and become more fine distally. The majority of splint bone fractures are due to external trauma, and almost a third are undiagnosed for up to 3 weeks, with lameness or discomfort on palpation not always present (1). Early recognition can ensure that the appropriate treatment is undertaken, especially as callus formation may lead to suspensory ligament injury (1) and suspensory branch desmitis (2).
Image 1: Radiograph showing a proximal composite splint bone fracture.
The hocks A horse’s hock consists of 10 bones and 4 joints, providing a common source of lameness. The limb should ideally be assessed with ‘the plantar aspect of each metatarsus perpendicular to the ground, aligned with the tuber ischii’ (3). Lateral assessment When looking from the side, the average hock angle has been calculated as 159.1° (3). This angle can be accurately calculated from precisely obtained photographs, with the angle created by two lines bisecting at the talus, starting proximally from the fibular head and distally from the lateral condyle of the third metatarsal bone (3). Horses with tarsal angles over 165° (therefore, straighter hocks) had lower shock absorbance and were linked to increased risk of degenerative joint disease (4) and proximal suspensory desmitis (3). Horses with tarsal angles under 155° generally have a cranially sloping metatarsus, resulting in increased joint flexion when standing, and this too may result in thick arthritis (4).
Image 2: Hock angle of 162° is within normal range, however, this should be tracked to ensure that increased strain is not being put through the suspensory apparatus.
The term ‘cow-hocked’ is reserved for horses with a valgus deformity of the point of the hocks. This means that the point of the hock is drawn towards the midline.
Although this is considered to be normal in donkeys (5), it is always abnormal in horses (6), predisposing horses to hock arthritis due to increased stress on the medial aspect of the tarsal joints (7). In my personal experience, this may be uni- or bilateral, and often due to muscular weakness, and I have seen improvement in this position following both exercise and successful treatment with intra-articular medication.. A lateral deviation of the hock has been linked with both an increased risk of pelvic fracture and digital tendon sheath effusion (8).
The tail The tail plays a role in spatial awareness and posture. Tail clamping to the buttocks, swishing and swirling during a dynamic assessment occurs significantly more in horses with musculoskeletal pain (9). Crooked tail carriage, both at rest and in work, is more common in lame horses, particularly in horses with hindlimb lameness, sacroiliac disease and thoracolumbar epaxial muscle tension (10). The direction of the crooked tail was not related to the predominantly lame limb.
Image 3: Tail held to the left.
The pelvis Symmetry The following bony and muscular asymmetries may have orthopedic implications (11):
Unilateral muscle wastage - particularly of the gluteals, indicative of hindlimb lameness on that limb
Elevation of one tuber sacrale (hunter’s bump) with or without muscle wastage - damage to the dorsal sacroiliac ligament, sacroiliac disease or stress fractures to the wing of the ilium (particularly in racehorses).
Lowered tuber sacrale on one side with muscle wastage - chronic sacroiliac disease.
Lowered tuber coxae or ischii - damage to the respective structure, generally a fracture.
In my personal experience, it is often difficult to differentiate between a lowered or an elevated tuber sacrale without significant muscle wastage. I have found that horses with lowered tuber sacrale generally have a squared off toe on that hindlimb, however this is not usually present when one tuber sacrale is elevated. It is also essential to remember that even in the absence of pelvic asymmetry, sacroiliac discomfort can still be present (12). It is also important to consider the stabilization of the pelvis provided by the dorsal sacroiliac ligament (DSIL). The paired DSIL consists of a cord-like portion which runs from the dorsal aspect of the tuber sacrale to the lateral aspect of the sacral spinous processes and a triangular, lateral portion which covers the sacrocaudal muscles, acting to stabilize the sacroiliac joint (13). Up to 33% of horses with DSIL injuries have asymmetry of the tuber sacrale (14).
Images 4 and 5: Note the increasing swelling in the left hindquarters compared to the right in image 4, and the position of the tuber ischium on the left compared to the right in image 5. Ischial fracture was diagnosed based on pelvic asymmetry, crepitus and pain on palpation.
Image 6: Note the elevation of the right tuber sacrale. No obvious muscle wastage was present. This horse was diagn