Biomechanics of the first ray. Part II: Metatarsus primus varus as a cause of hypermobility. A three-dimensional kinematic analysis in a cadaver model.

Variation in functional stability of the first metatarsocuneiform joint was analyzed between transverse plane deviated (adducted) and corrected first metatarsal positions in a closed kinetic chain model. Six fresh frozen cadaver specimens with intact ankles and feet were fitted with a custom fabricated titanium metatarsal jig, which allowed for manipulation of the first metatarsal in the transverse plane. Specimens were mounted into a custom-made acrylic load frame and axially loaded to 400 N. Radiowave three-dimensional tracking transducers were attached to the following osseous segments: first metatarsal head and base, medial cuneiform, and second metatarsal. A dorsally directed load was applied to the first metatarsal segment and resultant movements were measured. Repeated testing was performed on a transverse deviated and corrected first metatarsal positions with the hallux plantargrade and maximally dorsiflexed to engage the windlass mechanism. With the windlass mechanism engaged and first metatarsal corrected, a 26% increase in first ray plantarflexion occurred from a deviated to a corrected first metatarsal position (p < or = .05). This suggests that the windlass mechanism is more efficient when the first metatarsal, sesamoid apparatus, and hallux position are properly aligned with the orientation of the plantar aponeurosis. Clinically, this may explain the correlation of first ray hypermobility with the progression of bunion severity. Our study validates the earlier work of Hicks and adds additional insight into the functional stability in the medial column of the foot.