When “TMJ” Isn’t TMJ: A Case Study in Coronoid Hyperplasia

By John E. Dinan, DMD MS

At New York TMJ & Orofacial Pain, we encounter numerous patients whose limited mouth opening has been attributed to temporomandibular disorders. While TMJ-related restrictions are indeed common, clinicians must remain vigilant for other potential causes. The following case study illustrates the importance of thorough diagnostic evaluation and highlights a rare but significant condition that can masquerade as conventional TMJ dysfunction.

Patient Presentation

Randy, a 31-year-old man, presented to our practice with a chief complaint of progressive difficulty opening his mouth. His history was particularly telling: over the preceding decade, he had experienced a gradual but relentless reduction in his mouth opening capacity. What struck me most was the absence of any precipitating trauma or specific event that might explain his symptoms.

Randy’s discomfort was minimal—he experienced some pain only when attempting to force his mouth open maximally. Otherwise, he was essentially pain-free, which immediately differentiated his presentation from typical TMJ disorders. His medical history was unremarkable except for seasonal allergies, and he appeared to be in excellent overall health.

For years, various practitioners had diagnosed Randy with “TMJ” and prescribed traditional therapies. He had been using a nightguard consistently, though without any noticeable improvement in his symptoms. This lack of response to conventional TMJ treatment was another red flag that suggested we were dealing with something beyond standard temporomandibular dysfunction.

Clinical Examination Findings

My clinical examination revealed several key findings that helped narrow the differential diagnosis. Randy’s maximum interincisal opening measured only 30mm, significantly below the normal range of 40mm or greater. More concerning was his complete inability to stretch beyond this limitation, regardless of effort or assistance.

Palpation revealed mild tenderness in the masseter region during maximum opening attempts. However, his temporomandibular joints themselves were non-tender, and other masticatory muscles showed no signs of dysfunction. This pattern—restricted opening with minimal joint involvement—suggested a mechanical rather than inflammatory cause.

Diagnostic Imaging

A panoramic radiograph from five years prior provided the crucial diagnostic clue. The images revealed dramatically enlarged coronoid processes that extended well beyond the normal anatomical boundaries. In Randy’s case, the coronoid processes actually extended higher than his mandibular condyles – a clear abnormality that had been previously overlooked.

To better understand the mechanical relationship, I ordered a cone-beam CT scan with Randy’s mouth in its maximum opening position. This imaging definitively demonstrated the coronoid processes impacting against the zygomatic arches, creating a mechanical block that prevented further mouth opening.

Understanding Coronoid Hyperplasia

Randy’s presentation was consistent with coronoid hyperplasia—a rare condition characterized by progressive overgrowth of the coronoid processes. This overgrowth eventually leads to “coronoid impedance,” where the enlarged processes physically contact the zygomatic arches during mouth opening, creating a mechanical limitation.

This condition predominantly affects young men and adolescent boys, making Randy’s age and gender typical for this diagnosis. While the exact etiology remains unclear, current theories suggest potential contributing factors, including genetic predisposition, previous trauma to the region, or chronic hyperactivity of the temporalis muscle, which has its primary insertion at the coronoid process.

Differential Diagnosis of Limited Mouth Opening

In my practice, restricted mouth opening represents one of the most common presenting complaints. The majority of cases stem from either protective muscle guarding or intra-articular TMJ pathology, such as disc displacement disorders or degenerative joint disease.

However, clinicians must consider less common etiologies, particularly when the presentation doesn’t fit typical TMJ patterns. These include:

• Myositis and other inflammatory muscle conditions

• Oromandibular dystonia with involuntary muscle contractions

• Tissue fibrosis secondary to radiation therapy or autoimmune conditions like scleroderma

• Neoplastic processes affecting the masticatory system

• Coronoid hyperplasia, as demonstrated in Randy’s case

Treatment Considerations

The definitive treatment for coronoid impedance involves surgical coronoidectomy—removal of the enlarged coronoid processes—followed by intensive physical therapy to restore normal range of motion. This procedure effectively eliminates the mechanical obstruction and typically results in significant improvement in mouth opening capacity.

However, treatment decisions must always consider the patient’s symptoms, functional limitations, and the impact on their quality of life. In Randy’s case, while his mouth opening was significantly restricted, the condition was causing minimal pain and only moderate functional impairment. After a thorough discussion of the surgical option and its risks and benefits, Randy elected to defer surgery and continue with conservative monitoring.

Clinical Implications

This case reinforces several important clinical principles for practitioners managing patients with limited mouth opening:

First, a thorough history and examination remain paramount. Randy’s painless, progressive restriction without trauma history suggested a non-inflammatory cause from the outset.

Second, imaging interpretation requires careful attention to anatomical landmarks. The enlarged coronoid processes were visible on his original panoramic radiograph but had not been previously recognized as clinically significant.

Third, when standard TMJ therapies fail to provide improvement, alternative diagnoses must be considered. Randy’s lack of response to nightguard therapy should have prompted earlier investigation of non-TMJ causes.

Finally, treatment recommendations must be individualized based on patient symptoms, functional impact, and personal preferences. While surgical intervention offers definitive treatment for coronoid hyperplasia, conservative management may be appropriate for patients with minimal symptoms.

Conclusion

Coronoid hyperplasia represents a rare but important cause of restricted mouth opening that can be easily missed if not specifically considered. The condition should be suspected in young men presenting with painless, progressive trismus, particularly when standard TMJ therapies prove ineffective.

For any patient presenting with reduced mouth opening of unknown etiology or those whose symptoms cause significant distress, referral to an orofacial pain specialist ensures comprehensive evaluation and appropriate management. Early recognition of conditions like coronoid hyperplasia allows for informed treatment decisions and optimal patient outcomes.