886 AJVR, Vol 69, No. 7, July 2008 L esions involving the musculoskeletal system of racehorses are some of the most common causes of financial loss in the racehorse industry worldwide. 1 These lesions can involve soft tissue and bony struc- tures and often result in days lost from training or catastrophic career-ending injuries. Many of the inju- ries develop without a specific traumatic event, yet are highly consistent in their morphology and frequently develop at the same locations within a bone. 2 In addi- tion, preexisting stress fractures are associated with the development of complete bone fractures. 3 Stress fractures have been described as damage to the bone that results from repetitive loading. Such Quantitative analysis of scintigraphic findings in tibial stress fractures in Thoroughbred racehorses Alejandro Valdés-Martínez, DVM; Gabriela Seiler, Dr Med Vet; Wilfried Mai, DVM, PhD; David M. Bolt, Dr Med Vet, MS; Margaret Mudge, VMD; Sarah A. Dukti, DVM; Jeremy D. Hubert, DVM, MS Objective—To develop a quantitative method of interpreting tibial scintigrams of Thorough- bred racehorses with tibial stress fractures that may facilitate diagnosis of fractures and to provide prognostic information regarding future performance of affected horses. Animals—35 Thoroughbred racehorses. Procedures—Static bone-phase scintigrams of tibial stress fractures were quantitatively analyzed by use of ratios of the mean radionuclide counts per pixel in a region of interest (ROI) drawn around the area of increased uptake of radiopharmaceutical to mean counts per pixel in a second ROI drawn around an apparently normal area of the tibial diaphysis. In horses with unilateral fractures, ratios for the contralateral tibia were determined by use of 2 ROIs drawn at the same positions as the ROIs in the fractured tibia. Ratios were com- pared between fractured versus apparently normal tibias, between horses that returned to racing versus those that did not, and among horses with various grades of lameness. The association between ratios for fractured tibias and intervals between diagnosis and return to racing was also assessed. Results—Mean ratio of ROIs in apparently normal tibias was 1.35 (95% confidence interval [CI], 1.21 to 1.50); that in tibias with stress fractures was 3.55 (95% CI, 2.50 to 4.60). These ratios were significantly different. None of the associations between ratios for fractured tibias and grades of lameness or performance outcomes were significant. Conclusions and Clinical Relevance—Tibial stress fracture scintigrams can be quantita- tively analyzed. A prospective study with a controlled rehabilitation period is necessary to evaluate the possible applications of this method. (Am J Vet Res 2008;69:886–890) fractures are commonly associated with microcracks in the bone. 4,5 Intense exercise, such as race training and racing, predisposes horses to this condition. Therefore, stress fractures are common in Thoroughbred racehors- es; however, all horses that undergo repetitive activities are susceptible. 4 In Thoroughbred racehorses, tibias are the bones most susceptible to stress fractures. 6 Nuclear scintigraphy is a highly sensitive method for identifying regions of bone in which remodeling ac- tivity is increased, compared with activity in apparently normal bone. This imaging modality is the method of choice for diagnosing stress fractures in athletic hors- es. 7 Currently, most diagnoses of tibial stress fractures via image analyses are performed subjectively. Subjec- tive grading systems for tibial stress fractures have been proposed in equine and human medicine. 8,9 A correla- tion between scintigraphic grades of tibial stress frac- tures and intervals between fracture and recovery in human athletes has been reported. 9 A repeatable and objective method of measuring the severity of a stress fracture may facilitate choice of treatment regimen and Received June 6, 2007. Accepted November 19, 2007. From the Department of Clinical Sciences, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104-6044 (Valdés-Martínez, Seiler, Mai); the Equine Referral Hospital, Royal Veterinary College, University of London, Hatfield, Hertfordshire, AL97TA, England (Bolt); the Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210 (Mudge); the Marion DuPont Scott Equine Medical Center, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech and University of Maryland, Leesburg, VA 24601-0442 (Dukti); and the Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803 (Hubert). Address correspondence to Dr. Valdés-Martínez. ABBREVIATIONS CI Confidence interval MRI Magnetic resonance imaging ROI Region of interest Unauthenticated | Downloaded 08/21/22 12:17 AM UTC