Use this link to view the PDF file: THE OS INTERPHALANGEUS SYNDROME: AN UNCOMMON AND UNDER-RECOGNIZED CAUSE OF GREAT TOE PAIN
Purpose: To demonstrate the normal anatomy and variations of the os interphalangeus, and the clinical and radiological entity of os interphalangeus syndrome.
Materials and Methods: Dorsoplantar, oblique and lateral radiographs of 100 feet in adult patients with great toe pain, ranging in age from 18-80 years, were reviewed. 45 feet were left sided, and 55 right sided. MRI's of 9 feet using a standard protocol of T1 and fat suppressed T2 weighted images in the short axis, long axis and sagittal planes (without contrast), were also reviewed.
Results: The os interphalangeus was present in 20 patients, 18 of which were located at the central plantar aspect of the first interphalangeal joint in the midline, and varied in size from 2-8mm. One was eccentrically located at the medial margin of the distal phalangeal base, and one at the superior margin. On one MRI with central plantar os interphalangeus, there was soft tissue and bone marrow edema at the location of the os interphalangeus, and this patient had the clinical features of Intractable Plantar Keratosis.
Conclusion: The os interphalangeus may be centrally or eccentrically located, and although originally believed to be a sesamoid bone in the flexor hallucis longus tendon, it is an ossicle located in the joint capsule of the interphalangeal joint and separated from the tendon by a bursa. When the ossicle is absent, the bursa is also absent and the tendon is attached to the joint capsule. Uncommonly, the location may be eccentric and reflect persistence of one of the ossification centers of the distal phalanx with an adjacent bony defect at the distal phalangeal base. Rarely, the os interphalangeus may be superior to the IP joint. The os interphalangeus is best appreciated on the AP radiograph, the lateral radiograph when the great toe is isolated, and on oblique radiographs for the eccentric variant. Pain is a result of altered mechanics and frictional effect, sometimes accompanied by a bursitis. There may be associated intractable plantar keratosis (IPK) in the adjacent skin. The ossicle may also displace into a dislocated interphalangeal joint, preventing reduction.