Purpose The dorsal bunion is a sagittal plane deformity of the first ray, characterized by dorsiflexion of the medial cuneiform and first metatarsal, plantar flexion of the hallux, and increased stiffness of the first metatarsal phalange joint (MTPJ). The purpose was to develop a cadaveric model of the dorsal bunion deformity using muscle imbalances and ligament attenuation.
Methods The feet were dissected to expose the extrinsic muscle tendons. The plantar medial cuneiform-first metatarsal and Lis Francs ligaments were attenuated. Pins were placed into the bones of the foot, and Polhemus FastrakTM sensors were attached to measure positional information. A PedarTM insole measured plantar forces. The feet were then secured to a loading frame capable of manipulating the extrinsic tendons. Physiologic norms of ankle joint and MTPJ angles and muscle forces were established for four phases in the gait cycle (30%, 48%, 52%, and 62%). Data from the balanced condition were compared with data from the following conditions: condition 1, overpull of FHL; condition 2, overpull of TA and underpull of PL; condition 3, overpull of FHL and TA and underpull of PL.
Results Condition 1 demonstrated increased plantar force under the hallux by 10.4 N (4.1 N), decreased plantar force under the head of the first metatarsal by −4.7 N (3.2 N), and a stiffness of 1.4 N/deg (1.9 N/deg). Condition 2 showed decreased force under the first metatarsal head by −7.1 N (3.7 N), plantar flexion of the hallux by 5° (4.5°). Condition 3 elicited increased force under the hallux by 6.3 N (5.5 N), decreased force under the first metatarsal head by −7.8 N (4.8 N), and an MTPJ stiffness of 1.2 N/deg (0.6 N/deg).
Conclusions The different imbalances demonstrated changes in the first ray characteristics that are consistent with the dorsal bunion deformity, especially in condition 3.
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