The equine hyoid apparatus stabilizes the tongue, pharynx, and larynx. Disease of the hyoid bone can be manifested by clinical symptoms such as exercise intolerance, difficulty riding, headshaking, or anorexia (1–4). It is therefore important to raise awareness that hyoid bone diseases should be considered as a potential factor when these clinical symptoms arise.

The equine hyoid apparatus consists of the paired tympanohyoid, stylohyoid, epihyoid, ceratohyoid, and the unpaired basihyoid with the paired thyrohyoid processes. The tympanohyoid attaches the stylohyoid to the skull, the basihyoid is embedded in the tongue, and the thyrohyoid process articulates with the thyroid cartilage of the larynx. The various bones of the hyoid are connected via synovial joints, except for the temporohyoid joint (THJ), where the stylohyoid is connected to the temporal bone (TB) via the tympanohyoid, which consists of hyaline cartilage (5). The styloceratohyoid/epiceratohyoid joint (STCJ) connects either the stylohyoid, in case the epihyoid is fused with the stylohyoid, or the epihyoid with the ceratohyoid. The ceratobasihyoid joint (CBJ) is a connection between the ceratohyoid and the horizontal part of the basihyoid.

Imaging techniques used to assess the hyoid apparatus include radiography (2–4, 6), ultrasonography (4), endoscopy (1–4, 6), magnetic resonance (MR) imaging (6), and computed tomography (CT) (1–4, 6–9). In 20/24 horses, radiographs revealed bony changes of the THJ, tympanic bulla, or stylohyoid in horses with temporohyoid osteoarthropathy (THO) (6). In one case of hyoid malformation, a caudal deviation of the stylohyoid was visible (2). In both of these cases, the definitive diagnosis was achieved using CT. However, head radiographs were interpreted as normal in a case of lingual process (LP) fracture (3), and no bony trauma was detected in a case of basihyoid fracture (4). In both of these cases, the definitive diagnosis was achieved using CT. Ultrasonography can be used to assess the ventral aspects of the hyoid bone and was employed in a previous study to monitor a basihyoid fracture, for which the initial diagnosis was performed using CT (1, 4). Guttural pouch endoscopy offers a good overview of the THJ and the proximal stylohyoid, but the middle ear and the ventral part of the hyoid are not accessible (7, 10). MR imaging offers better visualization of soft tissues, enabling the identification of inflammatory fluid accumulation in the middle and inner ear; however, it is less accurate in assessing osseous structures (6), and general anesthesia is required for head MRs. CT is currently the most valuable imaging modality for evaluating the hyoid bone (8, 11, 12), as it can reveal abnormalities that cannot be detected with radiography (3, 4, 7) or endoscopy (1–4, 7). Furthermore, CT of the equine head can be performed standing, and recovering a neurologic horse from anesthesia presents an increased risk to the patient (13), a concern particularly relevant as neurological symptoms are frequently observed in horses with THO (6, 14).

Normal CT anatomy of the entire hyoid apparatus was recently described (5). Several CT signs have been described in the literature, but these are almost exclusively confined to the THO (6, 7, 9, 14, 15), with a few exceptions (16–18). CT signs of THO include: sclerosis (9), marginal exostosis and fracture of the TB (15), thickening (7), bridging and fusion of the THJ (15), as well as osteophytes at the proximal stylohyoid (18), and bony proliferations at the STCJ joint (7). Other published abnormalities of the hyoid apparatus include THJ subluxation (19), and stylohyoid (1, 20) or basihyoid fracture (4). Malformation of the hyoid apparatus (2, 21), arthropathy (17) of the STCJ, and LP fracture (3) have also been described.

The objective of this explorative study was: to (i) assess the frequency of pathological CT signs in individual articulations of the hyoid apparatus; (ii) to characterize the type of CT signs for each joint; (iii) to evaluate the effect of age, use, and weight on the occurrence and type of CT signs which appear, and (iv) to analyze the probability of concurrent occurrence of defined pathological CT signs in the hyoid apparatus, temporomandibular joint, and cheek teeth.

Documented cases of hyoid bone pathologies are described in the literature and have been known to result in exercise intolerance, equine riding problems, and dysphagia. The method of choice for examining the hyoid bone in horses is computed tomography. Therefore, in this study, we described and quantified the hyoid bone pathologies occurring in a subset of horses that, for various reasons, had undergone CT scans of the head. The prevalence of disease was found to be highest at the THJ, followed by the STCJ. In contrast, the CBJ was identified to be the joint with the lowest prevalence of disease. Significant associations were observed among CT signs in the examined joints of the hyoid apparatus. Age influenced the presence of THJ and STCJ abnormalities.

Several studies on THO describe CT signs of disease in the THJ, such as proliferation of the TB and stylohyoid, and fractures of these bones (6, 7, 26). Stylohyoid proliferation was found in 16/16 horses (7) and 2/2 horses (6), TB fractures in 4/16 (7) and 16/39 horses (15) and stylohyoid fractures in 4/16 horses with THO (7). Osteophytes at the joint margins (16, 18) and remodeling of the proximal stylohyoid (16, 18) have been seen in horses with age-related degeneration of the THJ. In the present study, the most common CT signs of disease of the hyoid apparatus occurred at the THJ: 52% of the sides examined showed a marginal exostosis of the TB, and 44% showed osteophytes of the proximal stylohyoid. Remodeling of the proximal stylohyoid, as seen in THO, occurred in only 4% (13/322) of the sites examined in our population. The relatively high percentage of 52% of the sides showing marginal exostosis of the TB could be explained by age-related primary osteoarthritis, as described by Naylor et al. (18) as lipping of the TB. Additionally, the styloid process sheath, a bony sheath of the TB surrounding the styloid process; the tympanohyoid, and the caudodorsal part of the stylohyoid (27, 28), were interpreted as marginal exostosis of the TB in this study. A significant association with age has also been described for the styloid process sheath size (29). In summary, the findings of this study revealed that age-related degenerative disease at the THJ predominated in the mixed study population.

The STCJ is the synovial joint between the stylohyoid and ceratohyoid, or, if the epihyoid is not fused to the stylohyoid, the joint between the epihyoid and ceratohyoid (5). A separate epihyoid was found in 33% (66/200) (5) of horses, and 38.6% (112/290) (17) of horses in other studies, and was presumed to be an ossification center of the stylohyoid (5). Thickening of the ceratohyoid and a bony proliferation of the joint have been reported in horses with THO (7, 19). In this study, a separate epihyoid was present in 38.7% (127/328) of the examined sides. In addition, marginal exostosis of the distal stylohyoid was found in 29% (95/328) of the sides examined, and osteoproliferative changes of the proximal ceratohyoid in 16.5% (54/328) of the sides examined. Age influenced the frequency of the latter two parameters, which enabled us to visualize that, in addition to abnormalities of the STCJ associated with THO, degenerative disease of this joint also occurs regularly.

The CBJ has a spacious joint space with a smaller craniomedial and a larger caudolateral recess (5). Disease of this joint is not reported in the literature. In one case of hyoid malformation, the joint was reported to be widened on the side affected by the malformation (2). In a recent report on hyoid malformation, the joint appeared to be fused (21). In another case report, a fracture of the horizontal part of the basihyoid was described, but without involvement of the CBJ (4). The most common disease observed in this joint, as reported in this study, was a marginal exostosis of the basihyoid in 2.7% (9/328) of the examined sides. The other two CT signs of this joint were rare, with 1.5% (5/328) for the basihyoid OCLL and 1.2% (4/328) for the distal ceratohyoid OCLL. CT signs of the CBJ were associated with those of the STCJ, and the CT findings in this joint were related to the CT signs of the THJ. It is therefore proposed that CBJ abnormalities, despite their low prevalence in the CBJ and the fact that they represent the least common joint disease of the hyoid apparatus, may, in fact, also be observed in cases of THJ disease. Given its accessibility via ultrasonography, more frequent evaluation of the CBJ in horses with ridden performance issues may facilitate earlier detection and increased reporting of joint disease.

The basihyoid consists of: the LP, which is embedded in the body of the tongue; the corpus or horizontal part, which lies in the root of the tongue; and the two thyrohyoid processes, which connect the hyoid apparatus to the thyroid cartilage of the larynx. The CT appearance of the LP varies with age (5): Under 1.5 years of age, it is about half the size of an adult horse (5). Between 1.5 and 3 years, a separate ossification center is visible for the tip of the LP (5). We did not examine developmental forms in this study and therefore excluded all horses under 3 years of age. There are reports in the literature of an LP fracture with refusal to take the bit and collect the neck when ridden, as well as headshaking (3); as well as a report of a basihyoid fracture with an inability to eat and drink, and showing resentment toward any manipulation of the tongue (4). In this study, we found an LP fracture in 1/160 (0.6%) of the horses and an LP asymmetry in 9/152 (5.9%) of the horses. Although diseases of the LP are quite rare, when they do occur, they can affect the ability to eat and drink, as well as their ridden performance. In this study, CT scans were performed on horses with LP disease to detect more serious problems, such as mandibular fractures or space-occupying masses in the head region. This leads us to conclude that both LP asymmetry and fracture can occur without severe clinical symptoms; however, as described in the literature, they need to be considered when other pathologies cannot be detected.

One proposed aetiopathogenesis of THO is progressive otitis media, partly as an extension of otitis externa, which appears to spread ventrally and which causes osteitis of the tympanic bulla, of the TB, and of the stylohyoid (30, 31). A recent study found no association between imaging evidence of otitis media or externa and THO (16). Postmortem CT scans were excluded from the assessment of signs of disease of the external and/or middle ear in this study because there was a greater amount of fluid in the tympanic bulla and external ear canal on postmortem CT scans compared to CT scans of live animals. Signs of external/middle ear disease were found in 6/273 sides of horses examined in the present study. Of these, 4 (67%) showed no radiological signs of THO, and 2 (33%) had radiological signs of THO. In this study, a small number of horses exhibited signs of external/middle ear disease, while many horses had THJ disease. Nevertheless, the present study did not specifically examine CT signs of THO per se; consequently, it was not possible to evaluate a correlation between THO CT signs and signs of external or middle ear disease.

OCLLs are radiolucent areas of bone, with a sclerotic rim next to the weight-bearing part of the articular surface (32–35). Possible underlying pathomechanisms of OCLLs include: developmental disorders, trauma, or focal sepsis (36). They may be associated with clinical problems or represent an incidental radiographic finding (36). In this study, the presence of OCLLs at all joints of the hyoid apparatus was investigated. They were present in all joints and bones with varying frequencies. As with all CT signs, OCLLs were most commonly found at the THJ, with 44/323 (14%) of the sides examined showing OCLLs at the proximal stylohyoid, and 34/313 (11%) of the sides examined showing OCLLs at the TB. At the STCJ, OCLLs were detected on 24/328 (10%) of the sides examined on the distal stylohyoid and 25/328 (8%) of the sides examined on the proximal ceratohyoid. In 4/328 (1%) of the CBJ, OCLLs appeared on the distal ceratohyoid, and in 5/328 (2%) of the sides examined, OCLLs appeared on the basihyoid. 10/25 (40%) OCLLs of the proximal ceratohyoid occurred together with diseases of the mandibular cheek teeth, on the same side. The statistical analysis revealed a significant association between these two parameters. Of the sides where diseases co-occurred, there were mandibular fractures on two sides and a squamous cell carcinoma on two other sides. This would explain the observed association between these two parameters. In the case of mandibular fractures, the OCLLs of the proximal ceratohyoid could have been caused by the trauma itself. In the case of the squamous cell carcinoma of the mandible, the OCLLs could have been caused by focal sepsis.

The study by Naylor et al. (18) reported on the age-related degeneration of the THJ. Subsequent studies have identified associations between age-related CT signs in the THJ (23, 29) and alterations in the remaining hyoid apparatus (23). However, no correlation was found between clinical symptoms and THJ grading (16), nor was there an association between CT findings of the THJ and headshaking (17). Additionally, Thoroughbreds and Arabians have been suggested to exhibit a predisposition to more pronounced THJ remodeling, whereas the Cob breed may possess a protective effect (16). In the present study, age was found to significantly influence six parameters of the THJ and two parameters of the STCJ. The effect of breed was not evaluated due to multicollinearity with other variables. However, the potential influence of use and weight was analyzed, and no significant effects of these factors on the evaluated CT signs were identified. These findings further support the role of age in the development of THO and reveal degenerative disease in the STCJ. Conversely, use and weight appear to have no significant impact on the progression of changes to the hyoid apparatus.

In conclusion, this study presents a detailed collection of the computed tomographic CT signs of disease in all joints of the hyoid apparatus. The most common diseases were seen in the THJ. Disease in the CBJ was associated with those in the STCJ, which, in turn, were associated with disease in the THJ. Age had an effect on six THJ and two STCJ CT signs. Use and weight, however, did not have an effect on the development of CT signs of disease of the hyoid. Given the frequencies of specific CT signs, particularly in the STCJ and the CBJ, these findings may assist clinicians distinguish between normal age-related degeneration and potentially clinically significant CT diseases. In future studies, the clinical relevance of the hyoid joint disease should be analyzed, possibly with the refinement or establishment of tests to examine the different joints of the hyoid bone. Additionally, the effect of the hyoid joints on ridden performance should be evaluated.