Original articles
Biomechanics of the first ray part V: The effect of equinus deformity: A 3-dimensional kinematic study on a cadaver model

https://doi.org/10.1053/j.jfas.2005.01.003Get rights and content

The positional change of the medial column of the foot in closed kinetic chain with variable Achilles tendon tension was investigated in seven fresh frozen cadaver specimens using a 3-dimensional radio wave tracking system. The distal tibia and fibula and the intact ankle and foot and were mounted on a non-metallic loading frame. The frame allowed positioning of the foot to simulate midstance phase of gait while the tibia and fibula were axially loaded to 400 N. To record osseous motion, receiving transducers were attached to the first metatarsal, medial cuneiform, navicular, and talus. Movements of these bones in 3-dimensional space were measured as specimens were axially loaded and midstance motor function was simulated using pneumatic actuators. To simulate a progressive equinus influence, force was applied to the Achilles tendon at tensile loads of 0%, 30%, and 60% of predicted maximum strength during each test trial. Osseous positions and orientations were collected and analyzed in all three cardinal planes utilizing data recorded. As Achilles load increased, the position of the first metatarsal became significantly more inverted (P < .05). Although not statistically significant, remarkable trends of arch flattening motion were detected in the distal segments of the medial column with varied Achilles load. Increased Achilles load reduced the influence of peroneus longus on the medial column.

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.

References (60)

  • R.A. Brand et al.

    The sensitivity of muscle force predictions to changes in physiologic cross-sectional area

    J Biomech

    (1986)
  • R.I. Harris et al.

    Hypermobile flat-foot with short tendo achillis

    J Bone Joint Surg

    (1948)
  • C.W. DiGiovanni et al.

    Isolated gastrocnemius tightness

    J Bone Joint Surg Am

    (2002)
  • R.S. Hill

    Ankle equinus. Prevalence and linkage to common foot pathology

    J Am Podiatr Med Assoc

    (1995)
  • S.T. Hansen

    Functional Reconstruction of the Foot and Ankle

    (2000)
  • S.I. Subotnick

    Equinus deformity as it affects the forefoot

    J Am Podiatry Assoc

    (1971)
  • A.S. Banks et al.

    Charcot foot

    J Am Podiatr Med Assoc

    (1989)
  • M.S. Downey

    Ankle Equinus

  • T.E. Sgarlato et al.

    Tendo achillis lengthening and its effect on foot disorders

    J Am Podiatry Assoc

    (1975)
  • D.G. Armstrong et al.

    Lengthening of the Achilles tendon in diabetic patients who are at high risk for ulceration of the foot

    J Bone Joint Surg Am

    (1999)
  • L.A. Lavery et al.

    Ankle equinus deformity and its relationship to high plantar pressure in a large population with diabetes mellitus

    J Am Podiatr Med Assoc

    (2002)
  • D.R. Green et al.

    Correction of equinus-related forefoot deformitiesa case report

    J Am Podiatry Assoc

    (1976)
  • J.V. Basmajian
  • J. Perry

    Gait Analysis. Normal and pathological function

    (1992)
  • M.L. Root et al.

    Clinical Biomechanics. Volume II: Normal and abnormal function of the foot

    (1977)
  • O.F. Schuster

    Foot Orthopaedics

    (1927)
  • B.D. Baggett et al.

    Ankle joint dorsiflexion. Establishment of a normal range

    J Am Podiatr Med Assoc

    (1993)
  • R.W. Bohannon et al.

    Selected measures of ankle dorsiflexion range of motiondifferences and intercorrelations

    Foot Ankle

    (1989)
  • J.C. D’Amico

    Equinusidentification and clinical significance

    Arch Podiatr Med Foot Surg

    (1977)
  • G.B. Duchenne

    Physiologie des Mouvements

    (1867)
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