Abstract
117
Objectives: The large size of equine athletes and the training programs, built over months to target specific competitions, make the horse a unique model for sports injury encountered in humans. Stress-related injuries of bone are common in racehorses, ranging from subchondral stress remodeling to catastrophic breakdown. Planar bone scintigraphy with 99mTc-MDP is commonly used to image equine athletes, but the value of this technique remains limited due to the low spatial resolution and the 2-dimensional data. The aim of this study was to assess the value of 18F-Sodium Fluoride (18F-NaF) PET for detection of bone injuries in racehorses. Methods: Nine Thoroughbred racehorses, with thoracic limb lameness, were included in the study. PET imaging was performed under general anesthesia two hours after intravenous injection of 740 MBq of 18F-NaF using a portable PET scanner with 22 cm bore and 2 mm spatial resolution. Data from 20 metacarpo/metatarsophalangeal joints (MC/MTPJ) were available. These joints were also imaged with planar bone scintigraphy and CT. Low-field weight-bearing MRI was performed on five metacarpophalangeal joints. One horse was euthanized and micro-CT and histology were performed. All PET and scintigraphic images were reviewed independently by three board-certified veterinary radiologists and graded at specific anatomical locations for 18F-NaF. A three-grade scale (normal, mild increased uptake, moderate to severe uptake) was used. The kappa weighted statistics was used to assess agreement between radiologists. The median of the three scores was used for further analysis. CT and MRI were assessed by consensus between two radiologists.
Results: Seventeen out of twenty MC/MTPJ had abnormal uptake on PET in at least one area, whereas only 11/20 MC/MTPJ appeared abnormal on scintigraphy. The interobserver agreement was higher with PET (Kappa weighted for the three pairs of radiologists: 0.4371; 0.3553 and 0.3962) than with scintigraphy (Kappa weighted: 0.2105; 0.2500 and 0.1935). The most common site of increased uptake were the lateral proximal sesamoid bones (11/20), the palmar/plantarlateral (8/20) and the palmar/plantarmedial (8/20) metacarpal/tarsal condyles, the medial proximal sesamoid bone (6/20) and the dorsoproximal aspect of the proximal phalanx (6/20). The most common sites of increased uptake on scintigraphy were the metacarpal and the metatarsal condyles (6/20) and the lateral proximal sesamoid bone (6/20). Medial proximal sesamoid bone uptake was identified on scintigraphy in only 1 of 20 MC/MTPJ. MRI and CT showed abnormalities in most metacarpal condyles with increased 18F-NaF uptake but failed to identify lesions in the proximal sesamoid bones. Micro-CT and histology confirmed the presence of active bone remodeling in areas of increased 18F-NaF uptake. Conclusions: 18F-NaF PET is extremely useful to assess osseous stress remodeling in the equine athlete. PET has a better interobserver agreement and detects more lesions than scintigraphy. The areas of 18F-NaF uptake observed with PET in this study correspond to known areas of stress remodeling in the equine athlete. 18F-NaF PET can detect stress related bone injuries that are not apparent on other imaging modalities.