Pamela Gerrard, Clare Kelly, Lisa Molloy
Discussion
Upon dissection of the sole of the foot the superficial skin and fascia was found to be thick and fibrous. This is consistent with the findings in the literature, which describes increased thickness and rigid anchoring of fascia to underlying tissues necessary to allow absorption of large compression and shearing forces during gait (Bressler & Brusler 1991). A well developed, thick fat pad was also observed at the heel which was shown to be the main shock absorber at heel strike (Donatelli 1985).
In concurrence with the current literature, the plantar aponeurosis in the present specimen originates from the medial process of the calcaneal tuberosity and splits into five bands that are attached to each metatarsophalangeal joint, merging with the fascia of the foot (Mitchell et al 1991, Pontious et al 1996). Mitchell et al (1991) described the plantar aponeurosis as a narrow thick proximal attachment becoming broader and thinner as it progresses distally. The aponeurosis was observed as a tough, fibrous layer that is reported to histologically consist of collagen and elastic fibers (Hedrick 1996).
In the present specimen, the plantar fascia was observed as thin extensions of the aponeurosis covering the abductor hallicus medially and abductor digiti minimi laterally, which is contradictory to the findings of Hedrick (1996) where the lateral component of the plantar fascia was described as being well developed. Snow et al (1995) suggest a continuation between the plantar fascia and achilles tendon in young adults. However, our specimen showed an insertion of fibers from both structures to the calcaneus with no continuation.
In the literature, the plantar fascia has been functionally described as a secondary restraint in the foot that maintains and stabilizes the medial longitudinal arch by tensing like a bowstring (Batt and Tanji 1995, Kitaoka et al 1993, Pontious et al 1996). The short and long plantar ligaments and spring ligament have shown to assist in maintaining the integrity of the arch (Kim and Volashin 1995). As described by the ‘windlass mechanism’, during the push-off phase of gait, extension of the proximal phalanges tightens the plantar fascia that draws the calcaneus forward, leading to supination of the subtalar joint and external tibial rotation. This mechanism heightens the longitudinal arch, locks the mid-tarsal joints, and thus converts the foot into a rigid lever for propulsion during walking and running (Donatelli 1985, Pontious et al 1996). During weight-bearing, the flattened arch has shown to generate increased tension in the plantar aponeurosis which unwinds the windlass and transmits a plantar flexion force to the phalanges (Pontious et al 1996). Thus, it is clinically conceivable that abnormal biomechanical factors such as pes cavus and excessive pronation may influence loading on the plantar fascia and thus contribute to the Heel Pain Triad (Kim and Voloshin 1995, Labib et al 2002).
The flexor retinaculum was identified and found to be continuous with the fascia enveloping the muscles of the medial aspect of the leg, with no distinct demarcation from this fascia. The fibers of the flexor retinaculum converged anteriorly to the medial malleolus, where it continued deep to the extensor reticulum. Fibers were seen to be thicker immediately posterior to the medial malleous, becoming thinner as they radiate outward towards the calcaneous and achilles tendon. The flexor retinaculum was firmly attached to the medial malleolus superiorly and loosely attached inferiorly.
In current literature, the tibialis posterior muscle is stated to arise from the interosseous membrane and the adjacent tibia and fibula in the proximal third of the leg. The musculotendinous junction occurs in the distal third of the leg superior to the medial malleolus (Mosier et al 1999). The proximal attachments of this muscle were not explored in this study.
The tendon courses within its sheath, posterior to the medial malleolus, beneath the flexor retinaculum. From there, it passes posterior to the axis of the ankle joint and medial to the subtalar joint, which is consistent with findings in this specimen.
In the literature, its attachments are described as extensive; primarily onto the navicular tuberosity. It fans out on the plantar aspect of the foot and laterally, forming multiple divisions attaching to the cuneiforms and bases of the 2nd, 3rd and 4th metatarsals. These multiple insertions blend with the ligamentous architecture of the medial arch (Mosier et al 1999, Conti 1999). The tendon has been observed to connect to the inferior extensor retinaculum and the talonavicular ligament dorsally. Medially, it has been shown to blend with the plantar fascia, calcaneonavicular and the tibionavicular ligaments. On the plantar surface, it has been shown to attach to the deep fascia, peroneus longus tendon, the tarsometatarsal ligaments, and the long and short plantar ligaments (Mosier et al 1999).
However, in the present specimen, the navicular attachment was observed, with variation in the extensions from the tibialis posterior tendon. Fibrous bands were seen to extend from the navicular tuberosity towards the cuneiform, attaching between the 2nd and 3rd metatarsals, 3rd and 4th metatarsals and one band attaching to the 5th metatarsal. A fibrous band was also noted extending towards the cuboid.
The literature suggests the primary function of the tibialis posterior muscle is to invert and plantarflex the foot (Conti 1999). In addition, it functions to adduct the forefoot at the midtarsal joint acting as an antagonist to peroneus brevis (Mosier et al 1999).
In the stance phase of gait, the tibialis posterior has been shown to function as a shock absorber for the subtalar joint, limiting hindfoot eversion by eccentric contraction. At mid-stance, it has been shown to induce subtalar inversion, locking the calcaneocuboid and talonavicular joints, which forms a rigid lever for forward propulsion of the foot over the metatarsal heads. Aiding in the propulsive phase, it has been shown to accelerate subtalar joint supination and thus, assisting in heel lift.
The lateral shift followed by medial shift of the centre of body mass through the longitudinal axis of the foot is achieved by the balanced activity of the tibialis posterior and peroneal muscles. The tibialis posterior becomes inactive shortly after heel lift, but aids in acceleration during swing phase (Mosier et al 1999, Conti 1999).
Compression neuropathies of the tibial nerve and its branches cause pain, hypoesthesia, and paraesthesia in the heel and sole of the foot (Davis and Schon 1995). The tibial nerve bifurcates into the medial and lateral plantar nerves at approximately the level of the medial malleolus. Dellon and Mackinnon (2002) found that in 90% of ankles the bifurcation was within 1cm of this axis, with 55% at the level, 16% 1cm distal, and 19% 1cm proximal (Sarrafian 1993). Davis and Schon (1995) found a similar result, in which 18 of 20 ankles showed bifurcation of the tibial nerve within 2cm of this axis. Our specimen displayed an unusually high bifurcation, 2.5cm above the MMC axis.
There exists some controversy in the literature regarding which nerve is implicated in medial heel pain. McCrory et al (2002) has reported that the medial plantar nerve may become entrapped where it passes under the fibrous arch of the abductor hallucis origin and through the fibro-osseus space formed by the attachment of the flexor hallucis brevis to the calcaneus. In our specimen the medial plantar nerve passed under the muscle belly of the abductor hallucis, not near its attachment at the calcaneus. Thus, in this specimen, this does not seem like a likely area of entrapment.
Compression neuropathies of the first branch (FB) of the lateral plantar nerve (LPN) have been well documented in the literature (Davis and Schon 1995, Sarrafian 1993). This nerve has also been named the nerve to abductor digiti minimi and the inferior calcaneal nerve. The FB supplies motor innervation to the abductor digiti minimi and sensory innervation to the medial calcaneal periosteum. The point of entrapment of this nerve is located between the thick deep fascia of the abductor hallucis medially and the medial border of the quadratus plantae laterally (Davis and Schon 1995). However, in the present specimen, the lst branch of the LPN was not observed. The undersurface of the abductor hallucis was observed to be thick and fibrous, thus providing a possible location of entrapment.
Didia and Horsefall (1990) have discussed the importance of the medial calcaneal nerve (MCN) in relation to heel pain. The MCN is a sensory nerve that supplies the skin of the heel and its weight-bearing surface and transmits pain arising from superficial structures. The origin of the MCN is variable. The MCN has been shown to originate from the tibial nerve proximal to its bifurcation in 81% of ankles, with 19% of MCN originated from the LPN (Didia and Horsefall 1990). Dellon et al (2002) has reported a high variability in the origin of the MCN, with 64% of ankles having more than one MCN. The MCN has shown to originate from the tibial nerve (56%), LPN (66%), and MPN (46%) (Dellon et al 2002). From its point of origin, the MCN has been shown to course postero-inferior, perforating the flexor retinaculum to lie superior to the aponeurosis, abductor hallucis, and flexor digitorum brevis (Didia and Horsefall 1990). In the present specimen, the MCN was not observed.
