Introduction/Purpose: The hyoid apparatus of small animals is an arrangement of small bones connected by fibrocartilage. In adult humans, the apparatus is fused into a single hyoid bone and fracture is rare, accounting for 0.002% of all fractures. The cause is usually direct trauma (e.g.; strangulation or hanging), and the greater cornua of the hyoid bone is most commonly fractured. Hyoid bone fracture has been reported in shepherd dogs trained using choke chain collars; however, the prevalence is unknown. The objective of this study is to describe the prevalence of hyoid bone injury (fracture or luxation) in computed tomographic (CT) studies of dogs and cats. Our hypotheses are that hyoid apparatus injury will be more common in large breed dogs than in small breed dogs or cats, and the most commonly fractured bone will be the thyrohyoid (somewhat analogous to the greater cornua).
Methods: A medical records search was performed to identify CT studies of dogs and cats over a three-year period (May 2013 to May 2016), and inclusion required imaging of the entirety of the hyoid apparatus. Cases were excluded if the medical record did not list age, weight, breed, sex, and species. For statistical analysis, the outcome of interest was presence or absence of hyoid injury. Potential risk factors included age, weight, sex, breed, and species. Association between presence of hyoid injury and each risk factor was tested using the 2-sample t-test (age and weight) and Fisher’s exact test (breed, sex, and species). Statistical significance was set to p < 0.05. All analyses were performed using SAS version 9.4 (Cary, NC, USA).
Results: Of 886 CT studies, a total of 239 dogs and 78 cats were included. Hyoid fracture or luxation was present in 8 dogs, 75% (6/8) of which had no history of trauma. The most commonly fractured bone was the epihyoid (75% (6/8) fractures), and the only two ceratohyoid fractures were in dogs with known or presumed trauma. Fracture of the stylohyoid, basihyoid or thyrohyoid bone was not detected in any case. While no cats were affected, the power of the study raises the possibility of a type II error. The proportion of hyoid injury was greater among shepherd-type dogs (20.0% [2/10]) than among all the other breeds combined (1.7% [4/229]) (p=0.0215). Age, weight, and sex were not associated with presence of hyoid injury (p>0.05).
Discussion/Conclusion: Hyoid bone injuries are rare in dogs and fracture usually occurs in the epihyoid bone. Hyoid injury may be more common in shepherd-type dogs. Interestingly, the majority of affected dogs had no history of trauma, dysphagia, or dyspnea. A prospective study could help elucidate the cause. In summary, hyoid bone injury, particularly epihyoid bone fracture, may be an incidental finding in dogs.