Investigation into Lumbosacral Vertebral Anatomy and Physeal Closure in Quarter Horses

Introduction: Spinal pathology in the lumbosacral region is a source of poor performance in horses. A previous study found lumbosacral anatomical variations and physeal closure abnormalities in Quarter Horses investigated for lumbar vertebral fracture. The objectives of this study were to characterize Quarter Horse lumbosacral anatomy and vertebral body physeal closure.

Materials and Methods: Lumbosacral specimens were collected from 16 Quarter Horses humanely euthanized for reasons other than lumbosacral disease or fracture (aged 0–22 years old). Specimens were assessed using computed tomographic, visual, and histological (H&E stained sections) examination.

Results: Overall, 15 of 16 (94%) cases had six lumbar vertebrae, with gross anatomical abnormalities present in 2 of 16 (12.5%) cases. Physes of the caudal lumbar and first sacral vertebrae were inactive by 2 years of age, with most becoming inactive by 15 months of age. Residual cartilage was present within physes until 5 to 8 years of age. As physes closed, an increase in cartilage disorganization and retention were present, with increased bone compaction frequently surrounding these regions. The majority of cases (12/16; 75%) had an indistinct, cartilage poor, proteoglycan rich central region within intervertebral disks, likely demonstrating a rudimentary nucleus pulposus.

Discussion/Conclusion: While physeal disorganization occurred frequently in cases with closing physes, increased bone compaction may indicate strain and pathology in these regions around the time of closure. The anatomical and histological findings in this study should help veterinarians perform more accurate diagnostics and make improved exercise recommendations in young animals.

Acknowledgment: Supported by an American Quarter Horse Foundation Young Investigators Grant.