Morphological characteristics of the flexor hallucis longus groove and tendon quality in patients with hallux rigidus: A CT-based study.

Background: The etiology of hallux rigidus is still unknown and elucidating targeted factors to prevent hallux rigidus or improve treatment outcomes is essential. An association between flexor hallucis longus (FHL) tendon tightness and hallux rigidus development has been suspected; however, the characteristics of the FHL tendon in the hallux rigidus remain unclear. This study aimed to explore the quality of the FHL tendon and the morphological characteristics of the FHL tendon groove at the talus and sustentaculum tali in the pathogenesis of hallux rigidus.

Methods: Twenty-one feet with hallux rigidus and 15 feet without hallux rigidus were retrospectively reviewed. The ratio of the FHL tendon groove depth to the FHL tendon anteroposterior diameter (groove/tendon ratio), Hounsfield unit (HU) values of the cancellous bone below the FHL tendon groove to those of the navicular (groove HU ratio) were measured using computed tomography at the talus and sustentaculum tali. The ratio of HU values inside the FHL tendon to those inside the tibialis anterior tendon (tendon HU ratio) were measured at the FHL tendon groove of the talus, that of the sustentaculum tali, and below the sesamoids.

Results: The depth of the FHL tendon groove, groove/tendon ratio, and groove HU ratio were significantly larger in the hallux rigidus group; the FHL tendons of the hallux rigidus group were deeply fitted into the FHL groove than those of the control group. The tendon HU ratio was significantly smaller in the hallux rigidus group.

Conclusions: The FHL tendon in the hallux rigidus can be more constrained at the FHL groove of the talus and sustentaculum tali, and its quality can change compared to that in healthy feet. Tightness of the FHL tendon should be considered when treating hallux rigidus.

Level of evidence: Ⅲ.