Original articlesBiomechanics of the first ray part V: The effect of equinus deformity: A 3-dimensional kinematic study on a cadaver model
Section snippets
Biomechanical influence of triceps surae
The triceps surae functions across the ankle and subtalar joints with the gastrocnemius component affecting the knee as well (8). The gastrocnemius muscle is the most consistently active muscle during static stance due to the center of gravity projecting anterior to the ankle (16, 17). The maximum forces attained by the gastrosoleal complex are: medial head of gastrocnemius 500 N, lateral head of gastrocnemius 700 N, and soleus 900 N. Approximately 450 N of tension is created by the Achilles
Equinus
Equinus deformity is defined as the inability to dorsiflex the ankle sufficiently enough to allow the heel to contact the supporting surface without subtalar or midtarsal joint pronation (19). Controversy exists regarding the magnitude of equinus that is clinically important. Nonetheless, there is a strong consensus that the ankle must dorsiflex past perpendicular for smooth ambulation. Schuster described the amount of ankle motion needed in walking as the “walking angle”(20). Because values
Compensation for equinus
The subtle pathomechanics of a shortened gastrocnemius aponeurosis has been known for over 100 years (37). Since these early descriptions, the various types of equinus compensations have become more clearly understood. Common modes of compensation for lack of ankle dorsiflexion include triplanar rearfoot motion (pronation), hypermobile flatfoot, an early heel-off (bouncy gait), and an abducted gait pattern (1, 19, 38). Proximal compensatory mechanisms for equinus have also been described,
Opposing forces
An antagonistic relationship exists between the triceps surae and the structures of arch retention. These structures include tibialis posterior, peroneus longus, plantar fascia, and the plantar ligaments. In open kinetic chain, Duchenne described Achilles and peroneus longus as having opposing roles (24). While in closed kinetic chain, investigators have observed the triceps as having an arch-flattening effect (26, 41). Thus, the analogy of a “tug-o-war” can be used to describe the triceps
First ray hypermobility
Although Duchenne described first ray mobility in the 1800s by stating, “The joints of the medial border of the forefoot have a certain amount of vertical motion,” (24) Morton advanced the concept of first ray hypermobility. Morton’s criteria for hypermobility of the first ray included clinically demonstrable “hyperextension” (dorsiflexion of the first ray), widening of the space between the first and second cuneiforms, and a thickened second metatarsal shaft (28, 45, 46, 47).
First ray motion
Materials and methods
This research design has been presented previously in more detail (54) and is summarized as follows.
The 7 fresh cadaver lower limb specimens used for this investigation were of mean age 84.1 years at time of death, ranging from 76 to 94 years (3 male, 4 female). Screening was performed by radiography and by visual inspection for abnormal joint space narrowing, significant alignment abnormalities of the medial column and rearfoot, and poor bone stock. Specimens without gross arch height
First metatarsal motion
First metatarsal rotational orientations of loaded foot specimens with varied Achilles tension were recorded in all three cardinal planes. As Achilles load increased, the position of the first metatarsal became significantly more inverted by 25.8% (P < .05).
Discussion
Using electrical stimulation techniques, Duchenne analyzed the function of the triceps surae (24). His most profound discovery involved the forcible plantarflexion of the lateral column while the medial column yielded to the slightest resistance. This finding was further reinforced by his determination that pure foot plantarflexion required combined stimulation of peroneus longus with triceps surae. He determined that there was an antagonistic relationship between triceps surae and peroneus
Conclusion
With increasing Achilles load, the influence of peroneus longus on the medial column is diminished. Equinus effects on an intact longitudinal arch seem to affect the distal components of the medial column, primarily in the frontal plane. A measurable arch flattening effect with plantarflexion of the talus and navicular and dorsiflexion of the first metatarsal and cuneiform occurs with increased Achilles pull.
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