Pelvic Bone Stress and Fatigue Fractures in Endurance Horses

Headshot of an Arabian endurance horse, which might be at risk for a pelvic fracture.
Endurance horses in the UAE have to train in deep sand, which can contribute to pelvic stress fractures, especially if the horses originate from other countries that train on different ground surfaces. iStock

Pelvic fractures in horses are not common. They most frequently occur in horses withstanding a repetitive load in training and competition. Accumulated microdamage in bone can interfere with remodeling enough to weaken bone and result in a fracture. While the bulk of athletic-induced pelvic fractures occur in racehorses, there have been reports of this in endurance horses. Because deep sand might be a significant influencer on the risk of pelvic fractures, a study evaluated medical records for pelvic fatigue fractures from Jan 2012 – March 2020 in endurance-trained and competed horses in the UAE [Pucetti, M.; Beccai, F.; Denoix, JM. Bone stress injuries and fatigue fractures of the pelvis in endurance horses. Equine Veterinary Journal, vol. 54, issue 6, pp. 1065-1075;].

Study of 60 Endurance Horses

Physical and dynamic examinations of 60 endurance horses included inspection, palpation, walking on a circle and straight line, and when possible, trotting on a straight line. Lameness values were assigned according to the AAEP scale of 0-5. The pelvis was assessed using diagnostic ultrasound—both transcutaneous and transrectal approaches—to identify bone stress and fatigue fractures. These abnormalities were then classified according to the anatomical location of the injury.

Horse age ranged from 5-16 years, with 57% Anglo-Arab and 43% pure Arabian. Elite individuals numbered 40% with the others competing at novice or intermediate levels. Musculoskeletal injuries had occurred in 52% at a different anatomical location. However, previous injuries did not affect the type of pelvic injury, according to the study. Bone stress injury or fatigue fracture was identified in 58% of the cases following a training session, and 42% were identified during or just after a competition. Two-thirds of the cases developed pelvic injuries in their first year of training and competition. The authors noted that endurance horses in the UAE might originate from other countries that train on different ground surfaces. Introduction to deep sand significantly modifies musculoskeletal efforts.

Common Complaints Associated With Pelvic Fractures

Horses were examined for a variety of complaints: a) consistent lameness; b) poor engagement and propulsion; c) intermittent lameness; d) trot and canter on three tracks: e) reluctance to walk or trot; f) weight loss; g) strange canter pattern; or, h) elimination from competition due to irregular gait and/or metabolic disorder. Asymmetry of pelvic bony landmarks was identified in 40%. Gluteal muscle atrophy was evident in 30%. Pressure over the tuber sacrale elicited a pain reaction in 25%. Gait abnormalities were instructive, as well: 40% were lame at a walk with half showing reduced cranial phase of the stride; 30% had a choppy gait; and 32% trotted on three tracks. Median AAEP lameness grade was 3 with a range of 0-4.

Injury Locations

Of the 60 horses, 103 pelvic bone stress injuries and/or fatigue fractures were identified with ultrasound. In 45%, injury occurred at the same site or at different sites but were bilateral. Injury locations were identified:

  • Iliac wing (45%) with 25% bilateral and 17% unilateral.
  • Tuber ischiadicum (tuberosity) incurred unilateral injury in 15%.
  • Unilateral injury of the pubis in 10%.
  • Iliac shaft in 0.03% (two individuals) with one bilateral and one unilateral.
  • Bilateral injury to the ischium occurred in a single horse.
  • Of the other 17 cases, a variety of pelvic locations were identified with stress fractures, particularly of the iliac wing and/or ischium.

For every increase in grade of lameness, the risk of bone-stress injury and/or fatigue fractures of the pelvic floor increased by 208%. Asymmetry of boney landmarks was associated with injury to the tuber ischiadicum. Horses with bone stress and/or fatigue fracture of the pubis and ramus or table of the ischium were 8.62 times likely to trot on three tracks. These cases did develop bone asymmetry.

The most common location of isolated fatigue fracture was the iliac wing at the level of the sacroiliac joint. Half of these injuries occurred bilaterally, a similar finding to Thoroughbred racehorses. Anglo-Arabs were 4.4 times more likely to develop an isolated iliac fracture than pure Arabians. The next common location affected the pubis; this is not common in Thoroughbreds. The authors noted that ultrasound is not able to assess changes such as bone marrow edema that occur in early stages of stress fractures. They added that nuclear scintigraphy is more highly sensitive for diagnosis of pelvic stress fractures.

Prognosis of Pelvic Fractures in Endurance Horses

After rest and rehabilitation, 62% of cases returned to successful competition, 20% retired for unrelated reasons, three horses retired as a consequence of their fracture, and two were euthanized due to lesion severity such as displaced iliac wings or sacroiliac joint dislocation. Six horses (10%) were still undergoing rehabilitation at the time of the study.

The study has brought to light that endurance horses training and competing in deep sand are at risk of pelvic fractures or bone stress injuries. Of all musculoskeletal injuries identified in 997 UAE endurance horses, 7.3% experienced bone stress or fatigue fracture of the pelvis at sites similar to findings in Thoroughbred racehorses. Repetitive microdamage that leads to more significant injury is considered an overuse injury. Endurance horses must undergo consistent repetitive loading efforts to not only condition the musculoskeletal system but also the cardiovascular and metabolic adaptations necessary for long-distance competition. 

Risk Factors: Conditioning and Footing

While a study of 235 endurance horses trained in Italy and competed in Europe over 10 years did not identify pelvic bone injury or stress fractures, it is relevant that training of endurance horses in the UAE involves different techniques. They are trained at the canter for 45 minutes up to 4 hours a day at 11 – 12.5 mph, at least several days a week, usually in deep sand at least 4 inches deep. Dry sand can increase heart rate at a lower speed for training benefits to the cardio-respiratory system, but it is accompanied by an increase in muscular effort. While sand decreases impact forces and loading rate on the limbs, the poor damping effect of deep sand amplifies concentric muscle activation of propulsive muscles. Muscles adapt more quickly than bone such that increased muscle activity may result in bone fatigue fracture, particularly if this is accompanied by an increase in training effort or change to the surface.

The authors said that it is important for trainers to implement and allow time for training adaptations to sand to better balance bone and muscle responses. Microdamage occurs naturally in the bone remodeling process, but without time to adapt, damage develops before the tissue heals. An endurance horse that works in deep sand should be evaluated for pelvic injury if there is lameness, poor performance, and/or unexplainable development of irregular gait patterns.

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