In the final instalment 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.
Caudocranial assessment
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 diagnosed with a DSIL injury and treated conservatively.
Lateral view
When looking from the side, a bum high posture, within which the tuber sacrale are at the height of, or higher than, the withers, may be considered to be a pain-relieving posture due to sacroiliac pain (12), putting the horse out of balance (6), making hindlimb engagement more difficult.
The angle between the ilium and ischium (coxal angle) can also have implications on performance and injury (8). A large coxal angle is beneficial for jump racing as it provides a larger area for the gluteal musculature and decreases the risk of pelvic fracture (8).
Palpation
Horses with suspected sacroiliac dysfunction are more sensitive to palpation of the middle gluteal and biceps femoris muscles (15) and to dorsoventral palpation of the tuber sacrale (12). The same study showed that the majority of horses with sacroiliac pain (86%) had complimentary causes of lameness, mainly in the front feet, hindlimb proximal suspensory or thoracolumbar spine. The most common clinical finding at rest was limited flexibility of the thoracolumbar region, with no difference between the horse which just had sacroiliac pain, and those which had multiple orthopedic issues.
A 2014 study (16) showed that ventral pelvic rotation resulted in a steeper hoof in the opposite forelimb, as opposed to a wider hoof on the forelimb on the same side, further indicating a more holistic approach at treating the entire body, and not just the initiating cause.
The thoracolumbar region
Palpation
The most prevalent clinical signs at rest associated with thoracolumbar back pain are limited flexibility and pain on palpation (17). Facet osteoarthritis usually affects 2-5 sites at the caudal thoracic spine, over multiple joints, and that the location of kissing spines often corresponded to these lesions, with the severity of clinical signs worse in horses with kissing spines (17). This area is at increased risk for pathological change as it is a region of high mobility (dorsoventrally and laterally) (18), resulting in increased load through this junction.
Palpation should be performed before and after exercise, and repeatable signs of discomfort should be considered of importance as the horse becomes accustomed to the examination.
Signs of ill fitting tack
As the saddle fit and its kinematics have a significant influence on the way the horse moves (19), an ill-fitting saddle will affect the biomechanics of the back and its muscular development negatively, resulting in muscular atrophy and discomfort (20).
Although different qualified saddlers generally follow the same guidelines when fitting saddles (21), the quality of the saddle will depend on the owner’s budget, and the effort they put into its maintenance. Saddles with a narrow tree tend to tip the seat backwards and forward when the tree is wide (22). Asymmetric saddles result in uneven pressure distribution (23) and although a thicker pad or numnah can create some temporary comfort, new pressure points can be created, exacerbating the issue (24).
Recognizing focal patches of white hair, muscle wastage behind the withers, asymmetrical hair wear and asymmetrical sweat patches after ridden work may be some of the signs indicative of poor saddle fit, and should urge the owner to immediately seek assistance from a qualified saddler.
Finally, ensuring that the bridle is not too tight, and that the horse is wearing the appropriately sized bit may be more difficult to do as outward signs are rarely present. Assessing the corners of the mouth for abrasions and that the bit is not pinching the lips, regularly cleaned and not developing any sharp edges if it is not made out of metal should be important considerations.
References
Jackson, M., Fürst, A., Hässig, M. and Auer, J., 2007. Splint bone fractures in the horse: a retrospective study 1992–2001. Equine Veterinary Education, 19(6), pp.329-335.
Dyson, S., 2007. Diagnosis and Management of Common Suspensory Lesions in the Forelimbs and Hindlimbs of Sport Horses. Clinical Techniques in Equine Practice, 6, pp.179-188.
Routh, J., Strang, C., Gilligan, S. and Dyson, S., 2020. An investigation of the association between hindlimb conformation and suspensory desmopathy in sports horses. Equine Veterinary Education, 32(S10), pp.183-192.
Gnagey, L., Clayton, H. and Lanovaz, J., 2006. Effect of standing tarsal angle on joint kinematics and kinetics. Equine Veterinary Journal, 38(7), pp.628-633.
Abdelgalil, A., Hassan, E. and Torad, F., 2020. Cow hock: A normal tarsal conformation in donkeys (Equus asinus) ). Equine Veterinary Education, 32(S10), pp.193-198.
Ross, M., 2010. Conformation and lameness. In: M. Ross and S. Dyson, ed., Diagnosis and Management of Lameness in the Horse, 2nd ed. St. Louis, Missouri: Elsevier WB Saunders, pp.15-32.
Sullins, K., 2011. The tarsus and tibia. In: G. Baxter, ed., Adams & Stashak's Lameness in Horses, 6th ed. UK: Wiley-Blackwell, pp.930-987.
Weller, R., Pfau, T., Verheyen, K., May, S. and Wilson, A., 2006. The effect of conformation on orthopaedic health and performance in a cohort of National Hunt racehorses: preliminary results. Equine Veterinary Journal, 38(7), pp.622-627.
Dyson, S., Berger, J., Ellis, A. and Mullard, J., 2018. Development of an ethogram for a pain scoring system in ridden horses and its application to determine the presence of musculoskeletal pain. Journal of Veterinary Behavior, 23, pp.47-57.
Hibbs, K., Jarvis, G. and Dyson, S., 2020. Crooked tail carriage in horses: Increased prevalence in lame horses and those with thoracolumbar epaxial muscle tension or sacroiliac joint region pain. Equine Veterinary Education,.
Munroe, G., 2009. The Clinical Examination. In: F. Henson, ed., Equine Back Pathology, 1st ed. Wiley-Blackwell, pp.63-72.
Barstow, A. and Dyson, S., 2015. Clinical features and diagnosis of sacroiliac joint region pain in 296 horses: 2004-2014. Equine Veterinary Education, 27(12), pp.637-647.
Denoix, J., 1996. Ligament injuries of the axial skeleton in the horse: Supraspinal and sacroiliac desmopathies. In: Dubai International Equine Symposium. California, USA: Rantanen Design, pp.273-86.
Tomlinson, J., Sage, A. and Turner, T., 2003. Ultrasonographic abnormalities detected in the sacroiliac area in twenty cases of upper hindlimb lameness. Equine Veterinary Journal, 35(1), pp.48-54.
Varcoe-Cocks, K., Sagar, K., Jeffcott, L. and McGowan, C., 2006. Pressure algometry to quantify muscle pain in racehorses with suspected sacroiliac dysfunction. Equine Veterinary Journal, 38(6), pp.558-562.
Ireson, A. and Cunliffe, C., 2014. An Investigation Into the Relationship of Pelvic Misalignment with Forelimb Hoof Size. Equine Veterinary Journal, 46, p.42.
Girodroux, M., Dyson, S. and Murray, R., 2009. Osteoarthritis of the thoracolumbar synovial intervertebral articulations: Clinical and radiographic features in 77 horses with poor performance and back pain. Equine Veterinary Journal, 41(2), pp.130-138.
Denoix, J., 1999. Spinal Biomechanics and Functional Anatomy. Veterinary Clinics of North America: Equine Practice, 15, pp.27-60.
Mackechnie-Guire, R., Mackechnie-Guire, E., Fisher, M., Mathie, H., Bush, R., Pfau, T. and Weller, R., 2018. Relationship Between Saddle and Rider Kinematics, Horse Locomotion, and Thoracolumbar Pressures in Sound Horses. Journal of Equine Veterinary Science, 69, pp.43-52.
Von Peinen, K., Wiestner, T., Von Rechenberg, B. and Weishaupt, M., 2010. Relationship between saddle pressure measurements and clinical signs of saddle soreness at the withers. Equine Veterinary Journal, 42, pp.650-653.
Guire, R., Weller, R., Fisher, M. and Beavis, J., 2017. Investigation Looking at the Repeatability of 20 Society of Master Saddlers Qualified Saddle Fitters’ Observations During Static Saddle Fit. Journal of Equine Veterinary Science, 56, pp.1-5.
Greve, L. and Dyson, S., 2015. Saddle fit and management: An investigation of the association with equine thoracolumbar asymmetries, horse and rider health. Equine Veterinary Journal, 47(4), pp.415-421.
Arruda, T., Brass, K. and De La Corte, F., 2011. Thermographic Assessment of Saddles Used on Jumping Horses. Journal of Equine Veterinary Science, 31(11), pp.625-629.
Greve, L., Murray, R. and Dyson, S., 2015. Subjective analysis of exercise-induced changes in back dimensions of the horse: The influence of saddle-fit, rider skill and work quality. The Veterinary Journal, 206(1), pp.39-46.
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