Temporohyoid osteoarthropathy: The challenge of modifying physiological forces

Temporohyoid osteoarthropathy (THO) is a debilitating and progressive disease affecting the stylohyoid bone and the petrous portion of the temporal bone (Hilton et al., 2009; Naylor et al., 2010). The clinical presentation of THO is highly variable; it ranges from mild signs to career-limiting and life-threatening neurological impairment. Horses may present with mild and nonspecific symptoms such as head shaking, ear shyness, resistance to the bit and masticatory disorders (Oliver & Hardy, 2015; Tanner et al., 2019). In more advanced disease, especially following fracture of the stylohyoid bone or petrous portion of the temporal bone, horses may exhibit neuronal insult to the facial (cranial nerve VII) and vestibulocochlear (cranial nerve VIII) nerves resulting in ipsilateral head tilt, varying degrees of ataxia, ear droop, ptosis, impaired palpebral function and muzzle deviation (Blythe et al., 1984). More severe cases can show severe neurological deficits, intracranial haemorrhage, pneumocephalus and sudden death (Divers et al., 2006; Walker et al., 2002). THO is characterised by progressive degenerative and proliferative changes to the temporohyoid articulation, often culminating in ankylosis of the joint (Hilton et al., 2009). While multiple pathophysiological pathways have been proposed, including chronic otitis media/interna, haematogenous septic arthritis and primary joint disease, the precise aetiology remains unclear (Divers et al., 2006; Inui et al., 2017). Several surgical techniques have been described and used over the years to treat and prevent the progression of THO. Hyoid releasing procedures encompass techniques that release the unaffected part of the hyoid apparatus from the ankylosed or ankylosing portion thus reducing the physiological forces transmitted through the hyoid apparatus from the tongue and larynx to the temporohyoid joint (THJ; Booth et al., 2023; Hall et al., 2021; Pease et al., 2004; Racine et al., 2019). The case report included in this issue by Kuhlmann et al. (2025) offers valuable insight on bone proliferation following a partial ceratohyoidectomy and recurrence of clinical signs 7 months after the original presentation. While THO is a recently described pathology, horses with clinical signs consistent with THO can be found in older literature. In 19th-century veterinary literature, a similar disease was described as ‘megrims’ or ‘vertigo’ due to the similarity to human ‘hemicranium’ pathologies. Notably, the term ‘megrims’ was integral to the etymological development and clinical characterisation of migraines in humans. During the 20th century, most of these horses were categorised as vestibular, and few surgical or medical therapies were described (Lardreau, 2012). The first description that categorises THO as a disease was a case report of three horses with enlargement of the THJ by Blythe et al. (1984). Soon after that, partial stylohyoid bone ostectomy was described as an attempt to interrupt the transmission of normal forces to the abnormal joint (Blythe et al., 1984). This procedure involves a deep dissection in an anatomically challenging area, leading to increased risk of complications. Significant bleeding from the lingual artery and vein, as they course parallel to the stylohyoid bone, was frequent; horses undergoing partial stylohyoid ostectomy were also at significant risk of iatrogenic damage to the hypoglossal nerve (Pease et al., 2004). Consequently, to avoid neural and vascular trauma, the stylohyoid bone periosteum is left in place (Blythe et al., 1994). It has been suggested that leaving the periosteum intact serves as a scaffold for bone remodelling and subsequent re-attachment of the ostectomy site, with re-growth of the stylohyoid bone being considered one of the major flaws of the technique. In 2004, Pease et al. described an alternative surgical procedure in which the entire ceratohyoid bone was disarticulated and removed. In the years to come, ceratohyoidectomy would become the treatment of choice for horses affected by THO (Pease et al., 2004). Compared to partial stylohyoid ostectomy, ceratohyoidectomy offers many advantages including a much easier dissection and the ability for the procedure to be performed standing. However, proximal dissection at the ceratohyoid-stylohyoid articulation still resulted in neurovascular trauma in a significant number of cases (Booth et al., 2023). Most notably, the lingual artery, hypoglossal and glossopharyngeal nerves run directly lateral to the ceratohyoid bone at its most proximal extent. At the ceratohyoid-stylohyoid articulation, the lingual artery courses medially towards the plane of dissection further increasing the risk for iatrogenic trauma to the vessel with manipulation of this region (Figure 1). In a recently described technique by Hall et al. (2021), the basihyoid–ceratohyoid joint is disarticulated and a substantial gap is created between the ceratohyoid and basihyoid bones (Hall et al., 2021). In the basihyoid–ceratohyoid disarticulation (BCD), deeper dissection is avoided, and the hyoid release occurs at the more superficial ceratohyoid–basihyoid articulation only. In comparison to the ceratohyoidectomy, in which laceration of the lingual artery is one of the most common complications, BCD eliminates the need for dissection of the proximal portion of the ceratohyoid bone, offering significant advantages (Figures 2 and 3). In 2023, a case series of 10 horses undergoing partial ceratohyoidectomy was described by Booth et al. (2023). This technique, like BCD, involves less dissection of the ceratohyoid bone, especially towards the stylohyoid bone, which should result in less intraoperative risk. In the case report by Kuhlmann et al. (2025), proliferation of the transected ceratohyoid bone and pseudoankylosis of the joint is thought to have an association with recurrence of clinical signs of THO. Nonetheless, these became evident after manipulation of the jaw for a dental procedure (Kuhlmann et al., 2025). Evidence of bony proliferation following partial ceratohyoidectomy is not a new finding; repeat head computed tomography showed that 3 of 10 horses developed significant proliferation of either the ceratohyoid or basihyoid bones 3 months after the procedure, as reported by Booth et al. While these three horses retained a gap between the basihyoid and ceratohyoid bones, and none developed clinical signs of recurrence, it is unknown whether this bony proliferation would become significant in future months or years. Experimentally, bilateral local anaesthesia of the hypoglossal nerve at the level of the ceratohyoid bone induces dorsal displacement of the soft palate in horses undergoing exercise (Divers et al., 2006). Inadvertent damage to the hypoglossal nerve during ceratohyoidectomy has the potential to worsen the prognosis for athletic use in horses undergoing surgery for THO. Laceration of the linguofacial vein, lingual artery and/or facial artery will result in significant haemorrhage. If any of the regional vessels are transected during dissection, it is the author's opinion that direct ligation of the vessels or application of vascular clamps is preferential to achieve haemostasis and packing alone should be avoided. Packing the incision site to stop blood loss is discouraged, as pressure buildup between the ramus of the mandibles leads to dorsal displacement of the floor of the oropharynx; this can induce respiratory distress or complete pharyngeal collapse. If the exact location of haemorrhage cannot be identified and packing is pursued, a temporary tracheostomy is indicated until swelling has resolved (Hall et al., 2021). Based on the authors' clinical experience, disarticulation of the ceratohyoid-basihyoid joint alone is enough to improve clinical signs and produce long lasting favourable outcomes for treated patients. Most notably, horses tolerate bilateral disarticulation, performed as a standing surgery, through either one sagittal or two parasagittal incisions when both THJs show endoscopic or computed tomography enlargement. The disarticulation of the ceratohyoid-basihyoid joint, ceratohyoidectomy and partial ceratohyoidectomy reduces forces from the basihyoid bone to the THJ enough to decrease inflammation and, in many cases, improve clinical signs. It does not, however, eliminate all forces travelling to the joint. The styloglossus and hyoglossus muscles remain attached to the stylohyoid bones. These muscles are responsible for the retraction of the tongue and change the angulation of the stylohyoid bone as the tongue moves. During dental exams, it is common for horses to undergo significant traction of the tongue. In response, normal sedated horses tend to fight manipulation to varying degrees. Excessive manipulation or lack of awareness of the horse's medical history could therefore result in iatrogenic damage and irritation to the THJ. Veterinarians should be cognisant that any manipulation of the hyoid apparatus, during dental exams or nasogastric intubation can potentially irritate or induce fractures of the THJ. Horses with subclinical THO can become acutely neurological following any of these procedures. It is recognised that dental examinations are an integral part of preventative medicine. Care should be taken to avoid excessive tongue manipulation, and clients should be aware of the risk especially in horses that have undergone ceratohyoidectomy, partial ceratohyoidectomy and basihyoid–ceratohyoid disarticulation. At present, our understanding of the physiological force transmission from the tongue to the THJ is limited. Nonetheless, the release of the basihyoid–ceratohyoid joint, plus or minus the partial or complete removal of the ceratohyoid bone, is associated with improvement of clinical signs, and owners report high satisfaction with the procedures (Booth et al., 2023; Hall et al., 2021; Racine et al., 2019). The case report in this issue establishes a precedent for a previously unknown complication. However, bony proliferation following a partial ceratohyoidectomy is expected, as bone remodelling is a normal consequence of bone trauma. Even though recurrence of clinical signs was evident in this horse, the benefit of a minimally invasive standing procedure such as a BCD or a partial ceratohyoidectomy is not outweighed by the minimal risk of pseudoankylosis and potential recurrence of signs that necessitate surgical revision. S. Larriva: Writing – original draft; writing – review and editing. A. Brien: Writing – original draft; writing – review and editing. N. P. Hall: Writing – original draft; writing – review and editing. There are no funders to report for this submission. No conflicts of interest have been declared.