The intersesamoidal ridge of the first metatarsal bone: anatomical basics and clinical considerations.

The first metatarsal shows a ridge at the plantar side of the head, aligned nearly longitudinally. On 100 macerated first metatarsals (53 right, 47 left) the existence and alignment of this ridge was examined. On the basis of a regression formula, the specimens were assigned to the genders; accordingly, 29 bones were female, the remaining 71 bones male. The intersesamoidal ridge existed in 95 cases (50 right, 45 left); all 29 female bones showed this ridge, male ones in 66 cases. The ridge deviated on average 7.99 degrees from the longitudinal axis of the first metatarsal towards distal-lateral (from -3 degrees to 19 degrees ). Significant differences ( p=0.001) were found for side (left: 6.04 degrees, right: 9.74 degrees ) but not for gender (female: 8.14 degrees, male: 7.92 degrees ). Noticeable correlations with the length of the first metatarsal and the size of the head could not be determined. The mean values of the alignment of the intersesamoidal ridge seem also not to correlate with the metatarsophalangeal angle. The ridge separates the joint surfaces for the sesamoid bones, which are built-in to the tendons of the two heads of the flexor hallucis brevis muscle. Thus, from the alignment of the ridge, the functional direction of this muscle can be concluded. Since the alignment deviates from the longitudinal axis of the first metatarsal, an adduction, in addition to the flexion, can be concluded. Operative techniques for the correction of hallux valgus often change the alignment of the ridge, and therefore also the biomechanical properties of the flexor hallucis brevis muscle. This could explain failures in several surgical therapies.