34th Annual Scientific Meeting proceedings

Greyhounds race around an oval track comprising of two straights and two 180-degree bends. The racing surface is sand that is banked on the bends and the dogs always race in an anticlockwise direction. Racing injuries are common and are often specific to the breed. In the majority of cases external trauma is not a factor in the aetiology but injury is caused by a failure of the musculoskeletal structures to withstand the forces developed during a race. Specific injuries are often over-represented in one limb.

As Greyhounds always race in the same direction around an oval track the forces vary between the left and right limbs and the physiological response is to strengthen the structures affected by increased force. This is called asymmetric adaptive remodelling, and it affects both the skeleton and the soft tissues (Johnson and others 2000). The response is slow to occur and requires regular training and racing over a period.

It occurs in many of the distal limb bones (Emmerson and others 2000, Johnson and others 2001, Lipscombe and others 2001) when bones are subjected to cyclic asymmetrical loading (Johnson and others 2000). Microfractures appear in the bone leading to bone resorption followed by new bone formation, indicated by increases in bone mineral density with a consequent increase in strength to adapt to the new workload (Muir and others 1999). The adaptive response is relatively slow, and a sudden overload can cause fracture.

Fatigue fractures in thoroughbred horses are reported to be the end stage of a progressive disease process that can take weeks or months to result in ultimate failure (Wright and others 2024). Changes in the third metacarpal (canon) bone are identifiable by radiography and CT and could be used in screening programmes to find horses at risk of catastrophic fracture (Doust 2024). This has led to recent interest in observing these lesions in the tarsi of racing greyhounds with a view to advising on the rest period and rehabilitation to prevent fracture. The major site for study of fatigue fracture in greyhounds is the tarsus that can suffer a loss of 35% of bone density prior to collapse (Hercock, 2010).

The most common fractures occur in the right tarsus, right accessory carpal bone, and right third metatarsal bone (MT3) resulting from compressive, tensile and torsional forces.

Treatment of these fractures can be a challenge due to the comminution and complexity with the choices between surgical reconstruction, conservative management or a combination of the two. This decision is made on the expectation of a return to competitive racing or a pain free retirement.

External coaptation can be fraught with complications from ischaemic necrosis as the sighthound breeds have thin skin and little cushioning fat. In addition, because of the kennelling arrangements, the dressings are prone to becoming wet often from urine. Many fractures require surgical intervention, but cases of surgical site infection and implant loosening are common with 51% requiring explantation in a study of 116 procedures (Biggo and others 2023).

Fractures of the tarsus
Fracture of the right central tarsal bone (CTB), usually associated with concomitant fractures of other tarsal bones, is the most common stress fracture. Diagnosis is by radiography or CT and classification is on the position and number of fracture lines (Boudrieau, R. J. and others (1984).

The CTB lies on the medial aspect of the tarsus, articulating with all the other tarsal bones. The weightbearing axis of the tarsus runs from the tibia, talus, CTB, third tarsal bone (T3) into the third metatarsal bone (MT3). The dorsal aspect is in compression and the plantar aspect with the attachment of the plantar ligament to its plantar process, in tension. As the dog leans when running around the bend the aspect of the limb on the inside of the bend will have increased loading of the lateral aspect of the left tarsus and the medial aspect the right tarsus through the CTB with resultant compression fractures and collapse of the CTB.  Tarsal varus with dorsal bowing occurs with compressive forces transferring to the medial aspect of T4 and tensile forces affecting the lateral calcaneus, T4 and MT5 with resultant concomitant fractures in these bones. This is exasperated by the torsional forces on all the tarsal bones resulting in rotation on their helical axes and translation (Tan 2018).

The plantar ligament has its origin on the distal calcaneus with insertions on the plantar processes of CTB, fourth tarsal bone (T4) and T3 merging into the tarsal fibrocartilage over the proximal metatarsals. Torsional and excessive tensile forces on the plantar ligament cause degenerative changes seen as enthysiophytes at its attachments, avulsion fractures and ectopic mineralisation, having the speculated effect of stiffening the ligament and reducing the dorsal translation of the tarsal bones. This in turn leads to the remodelling of the dorsal cortices.

Current work is looking at the effect of centrodistal arthrodesis, as has been reported in the border collie breed (Guilliard 2005), to prevent rotation at these joints decreasing the load on the ligament.

Calcaneal fractures, usually concomitant with CTB fractures, can be lateral slab from the medial collapse, comminuted with multiple fragments and avulsion fracture of the origin of the plantar ligament.

T3 fractures, often with second tarsal bone (T2) fracture, tend not to involve the other tarsal bones.

Treatment of tarsal bone fractures
Most fractures involving multiple bones are both comminuted and articular. The goal is to achieve alignment and stability, while accurate reduction of the fragments is desirable but not essential. Healing is usually rapid and some joint ankylosis inevitable.

Conservative management is acceptable if there is no tarsal collapse with minimal fragment displacement, especially if the dog’s racing career is over. With types 3 to 5 CTB fractures the medial fragment is mobile and palpable but accurate reduction requires surgery.

Reduction of the displaced medial fragment is key to alignment and is often difficult to achieve requiring the gap to be manually opened and the bone forcibly reduced with bone forceps. A mediolateral lagged screw into T4 maintains reduction. If the screw does not tighten due to fracture of T4 then supplementary support is necessary usually with a medial or dorsomedial plate. A positional screw if used, will tighten on the cis fragment but its strength in the trans fragment (T4) is unknown.

Lag screws are used on lateral calcaneal slab fractures after reduction of the CTB. Comminuted fractures with collapse, and proximal intertarsal joint luxation (plantar ligament avulsion), require a lateral tarsometatarsal plate with good plate contouring (Ost and others 1987).

T3 and concomitant T2 fractures require lag screw fixation in both competitive dogs and in retired dogs with a degree of tarsal collapse (Guilliard 2010).

Stress fractures of the metacarpal and metatarsal bones
The most common fracture is to the right MT3 that lies below the load axis of the tarsus and is subjected to the same compressive, tensile and rotational forces. The radiographic dorso-plantar view shows hairline transverse fractures at the proximal third of the bone but on the mediolateral view there is usually a displaced dorsal slab under which is a transverse fracture of the trans cortex with displacement of the distal bone. Plate repair is described but a survey of 15 dogs treated conservatively showed 100% return to racing in 12 weeks with no loss of form (Guilliard 2010).

Single metacarpal bone fractures occur mainly in the left MC5 and the right MC2, the most loaded of the four metacarpal bones that are on the inside of the track when negotiating the bend (Guilliard 2012). Surgical treatment can be successful using lag screws and an external fixator or a plate.

Carpal bone fractures
The most common carpal fracture is to the right accessory carpal bone (ACB) and has been classified into 5 types (Johnson 1987) corresponding to avulsion fractures of the various ligaments and tendon from tensile and rotational forces. The ACB articulates with the ulnar carpal bone (UCB) and distal ulna with ligament attachments to the UCB, fourth carpal bone (C4) and ulna. From the lateral body there are ACB-metacarpal bone 4 and 5 ligaments with the tendon of the flexor carpi ulnaris attaching to the free end of the ACB. Avulsion fractures can be found at the origins of all the ligaments and at the insertions of the articular ligaments.

The most common fracture is the type1 with an avulsion of the ACB-C4 ligament and surgical repair with a lag screw is described. However, most surgeons opt for surgical removal of the fragment giving a reported success rate of about 50% (Chico 1992). Cases of multiple fragments carry a poorer prognosis, but most regain good function as retirees. Type 5 comminuted fractures require external coaptation healing rapidly without the need for pancarpal arthrodesis. Some have returned to racing.

References

1. Boudrieau, R. J., Dee, J. F. & Dee, L. G. (1984a) Central tarsal bone fractures in the racing Greyhound: A review of 114 cases. Journal of the American Veterinary and Medical Association 184, 1486-1491
2. Chico A. (1992) Accessory carpal bone fracture in greyhounds, assessment of prognostic indicators and outcome following surgical management by fragment removal. Masters dissertation. University of Glasgow
3. Dee, J.F., Dee, L.G. & Piermattei, D.L. (1976) Classification, management and repair of central tarsal bone fractures in the racing Greyhound. 12, 398-405
4. Emmerson, T.D., Lawes, J.L., Goodship, A.E., Rueux-Mason, C. & Muir, P. (2000) Duel-energy X-ray absorptiometry measurement of bone- mineral density in the distal aspect of the limbs in racing Greyhounds. American Journal of Veterinary Research 61, 1214- 1219
5. Guilliard, M.J. (2005) Centrodistal joint lameness in dogs. Journal of Small Animal Practice 46, 199-202
6. Guilliard, M.J. (2010) Third tarsal bone fractures in the racing Greyhound. Journal of Small Animal Practice 51, 635-641
7. Guilliard, M.J. (2012) The nature, incidence and response to treatment of injuries to the distal limbs in the racing Greyhound. Thesis submitted in accordance with the requirements of the Royal College of Veterinary Surgeons for the Diploma of Fellowship
8. Hercock, C.A. (2010). Specialisation for fast locomotion: performance, cost and risk. Thesis for the degree of Doctor in Philosophy. The University of Liverpool, United Kingdom.
9. Irandoust, S., L.M. O’Neil, C.M. Stevenson, F.M. Franseen, P.H.L. Ramzan, S.E. Powell, S.H. Brouts, S.J. Loeber, D.L. Ergun, R.C. Whitton, C.R. Henak, and P. Muir. 2024. Comparison of radiography and computed tomography for identification of third metacarpal structural change and associated assessment of condylar stress fracture risk in Thoroughbred racehorses. Equine Veterinary Journal:14131
10. Johnson, K. A. (1987) Accessory carpal bone fractures in the racing greyhound. Veterinary Surgery 16, 1, 60-64
11. Johnson, K.A., Muir, P., Nicoll, R.G. & Roush, J.K. (2000) Asymmetric adaptive modelling of central tarsal bones in racing greyhounds. Bone 27, 257-263
12. Lipscombe, V.J., Lawes, T.J., Goodship, A.E. & Muir, P. (2001) Asymmetric densitometric and mechanical adaptation of the left fifth metacarpal bone in racing greyhounds. Veterinary Record 148, 308-311
13. Ost, P.C., Dee, J.F., Dee, L.G. & Hohn, R.B. (1987) Fractures of the calcaneus in racing greyhounds. Veterinary Surgery 16, 53-59
14. Muir, P., Johnson, K.A., & Ruaux-Mason C.P. (1999) In vivo matrix microdamage in a naturally occurring canine fatigue fracture. Bone 25, (5): 571-576
15. Tan, C (2018). The Effect of Loading, Plantar Ligament Disruption and Surgical Repair on Canine Tarsal Bone Kinematics, Sydney School of Veterinary Science Faculty of Science, The University of Sydney February 2018, A thesis submitted to fulfil requirements for the degree of Doctor of Philosophy.
16. Wright I., Minshall G., Young N. & Riggs C. (2024) Fractures in Thoroughbred racing and the potential for pre-race identification of horses at risk. Equine Vet J. 56: 424-436

Further reading: Yore, P., Katakasi, J. and Larratt, D. (2024) Save Your Dog’s Hock! – Tips for Trainers, C & T Control & Therapy Series, Issue 314; March 2024, Centre for Veterinary Education, The University of Sydney, NSW, Australia. https://www.greyhoundtarsalscreening.com.au/tarsal-screening