Discussion

Introduction/purpose:
Incomplete longitudinal fractures of the proximopalmar aspect of the third metacarpal bone (McIII) have been attributed to both acute trauma (Ross et al. 1988; Pinchbeck and Kriz 2001) or fatigue damage secondary to repetitive loading (referred to subsequently as repetitive stress-related bone injury) (Dyson 1988; Lloyd et al. 1988; Pleasant et al. 1992; Riggs 1994; Pinchbeck and Kriz 2001; Powell et al. 2010). The majority of reports have described small case series (Ross et al. 1988, n = 7; Lloyd et al.1988, n = 10; Dyson 1988, n = 13; Riggs 1994, n = 1; Pinchbeck and Kriz 2001, n = 2) in which Thoroughbred and Standardbred racehorses have predominated (Ross et al. 1988; Dyson 1988; Pleasant et al. 1992), although fractures have also been identified in sports horses and general purpose riding horses (Lloyd et al.1988; Dyson 1988; Riggs 1994; Pinchbeck and Kriz 2001). The objectives were to document the signalment of horses, clinical features, response to diagnostic analgesia, diagnostic imaging findings and follow-up data in a large group of horses with an incomplete longitudinal fracture or stress-related bone injury of the proximopalmar medial aspect of the McIII; and to compare the results between sports horses and racehorses.

Methods:
Horses were examined at the Centre for Equine Studies of the Animal Health Trust between 1985 and 2010. Horses were included in the study if pain was localised to the proximopalmar aspect of the metacarpal region, with radiological evidence of an incomplete longitudinal fracture or generalised increased radiopacity in the proximopalmar medial aspect of the McIII, or focal increased radiopharmaceutical uptake (IRU) in the proximopalmar aspect of the McIII. Horses with ultrasonographic abnormalities consistent with proximal suspensory desmitis or an avulsion fracture at the origin of the suspensory ligament were excluded. Age, breed, gender, height, bodyweight, work discipline, work history, duration of lameness, clinical signs and responses to diagnostic analgesia were recorded. Radiographic and scintigraphic images were assessed subjectively and objectively. Treatment included correction of foot imbalance, and a minimum of 3 months box rest with ascending amounts of controlled walking exercise.

Results:
There were 55 horses; 15 racehorses and 40 sports horses, including general purpose riding horses. The sports horses were aged 3 - 14 years (mean 7, median 6.5) and comprised Thoroughbreds (n = 13), Warmblood (n = 8), Thoroughbred cross (n = 4), other (n = 15).

Clinical signs:
Duration of clinical signs ranged from one week to 12 months. Most horses had no localising clinical signs. Twenty-eight percent of sports horses showed lameness at walk; at trot 92.5% had unilateral forelimb lameness and 7.5% had bilateral forelimb lameness. These findings were similar to racehorses. Sixty-five percent of sports horses had radiological abnormalities; 54% of sports horses with radiological abnormalities showed a characteristic pattern of lameness; the baseline lameness deteriorated the further the horse trotted in one direction, improved after changing direction and then progressively became more obvious again. Lameness was similar in straight lines (range0-5 on a scale of 0-8) and circles (range 1-5). The most frequent lameness grade was 4. Lameness severity was consistently worse with the lame limb on the outside of the circle in 54% of sports horses and was similar in degree on both reins in 26%. Lameness was similar in degree when lunged on soft and firm surfaces and was not affected by flexion tests.

Diagnostic imaging findings:
Sixty-five percent of lame limbs in sports horses had radiological abnormalities (94% of lame limbs in racehorses); 14% had increased radiopacity alone, 18.5% had a radiolucent line consistent with a fracture in the proximomedial aspect of the McIII, and 32.5% had a fracture and increased radiopacity.

In sports horses IRU in the lame limb ranged from mild to intense; 10% of non-lame limbs had mild IRU. Both radiological abnormalities and IRU in the proximopalmar aspect of the McIII were found in 45.5% of lame limbs, 54.5% had only IRU in the proximopalmar aspect of the McIII.

Follow-up:
Ninety-eight percent of all horses for which long term follow-up was available returned to full athletic function, resuming work
2.25-9 months after diagnosis.

Discussion/conclusion:
Clinical signs in sports horses and racehorses were similar. There was a spectrum of radiological abnormalities, reflecting a continuum of disease. Radiological abnormalities were more prevalent in racehorses than sports horses. Scintigraphy was essential in highlighting pathology which was not radiologically evident in sports horses.