Ultrasonography of the Metacarpal/Tarsal-Phalangeal Joints in Healthy Racehorses: Normal Appearance, Breed-Related and Age-Related Features

SIMPLE SUMMARY: The ultrasound appearance of cartilage thickness and subchondral bone changes according to the age of the horse and the health status of the joints. Specific ultrasound features have been described in different joint diseases in horses. Knowledge of the normal features is thus essential during clinical investigations. The present research evaluates the ultrasound features of metacarpal/tarsal-phalangeal joints in healthy racehorses, according to the age and breed. Twenty-eight racehorses (standardbreds and thoroughbreds) were enrolled and grouped according to age (<5 and ≥5 years old). Fetlock cartilage appearance, thickness and subchondral bone appearance were ultrasonographically assessed. In younger compared to older standardbreds, the cartilage appeared thinner and with a normal ultrasound appearance, which suggests the use of specific references for the ultrasound features of the fetlock. ABSTRACT: In adult horses, specific ultrasound (US) features and reference values have been reported for the appearance of the joint cartilage and thickness according to the type of joint, such as femoropatellar and tarsocrural. The US appearance of the fetlock has been described in several diseases. The present research evaluates the US features of the metacarpal/tarsal-phalangeal joints in healthy racehorses according to age and breed, since no information is available in the literature. Seventy-one fetlocks in 28 healthy horses (15/28 thoroughbreds and 13/28 standardbreds) were assessed. The horses were grouped as follows: group A < 5 years old vs. group B ≥ 5. A portable ultrasound machine and a linear transducer (5–7.5 MHz) were used. Dorsal metacarpal/tarsal-phalangeal joints were scanned. The US images were reviewed offline in terms of articular cartilage appearance, thickness, and subchondral bone appearance by an experienced observer. Data were reported as the median, minimum, and maximum for cartilage thickness values, and differences between groups were evaluated. Cartilage thickness values were statistically lower in group A than B in the standardbreds, except for the lateral thickness in longitudinal view. No differences were detected in the thoroughbreds within age groups. All of the young standardbreds showed a normal cartilage and subchondral appearance. No statistical differences were found between breeds. Our results highlight the characteristics of the US appearance of metacarpal/tarsal-phalangeal joints specifically in racehorses, with some variations according to age. Since the cartilage can change according to joint growth, age and training activity, the present findings suggest the use of specific references for US features, which are key to correctly evaluating the health of the fetlock.