Biomechanics of the Diabetic Foot
The Road to Foot Ulceration C. H. M. van Schie, MS c , PhD and A. J. M. Boulton, MD , FRCP
FOOT FUNCTION
One of the principal functions of the foot is its shock-absorbing capability during heel strike and its adaptation to the uneven surface of the ground during gait. In this function the subtalar joint plays a basic role. The subtalar joint allows motion three planes and is described as pronation (a combination of eversion, abduction, and dorsi- flexion) and supination (a combination of inversion, adduction, and plantar flexion) (1,2).
The ankle joint is the major point for controlling sagittal plane movements of the leg relative to the foot, which is essential for bipedal ambulation over flat or uneven terrain (3).
The midtarsal joint, represents the functional articulation between the hindfoot and midfoot. The inter-relationship of the subtalar and midtarsal joint provides full pronation and supination motions throughout the foot. The first metatarsophalangeal joint (MTPJ) incorporates the first metatarsal head (MTH), the base of the proximal phalanx, and the superior surfaces of the medial and lateral sesamoid bones within a single joint capsule.
The main motion of the first MTPJ and the lesser MTPJs is in the sagittal plane (dorsiflexion and plantar flexion). During propulsion the body weight is moving forward over the hallux creating relative dorsiflexion of the first MTPJ. This occurs with the hallux planted firmly on the ground and with the heel lifting for propulsion. The force acting across the first MTPJ approximates body weight, whereas the force across other MTPJs is considerably less (4). Maximum loading of the first MTH and hallux is practically at the same time during stance in normal gait, highlighting the importance of the load- bearing function of both the hallux and first MTH.
During gait, the foot is required to be unstable at first for shock absorption and to adapt to the terrain, whereas during the propulsive phase the foot has to be stable to function as a lever. Foot flexibility and rigidity are mainly controlled with pronation and supination of the subtalar and midtarsal joints. As subtalar joint pronation after heel strike is a major shock-absorbing mechanism, limited joint mobility (LJM) or structural abnormality could compromise flexibility and shock absorption, thereby placing increased stress on the plantar skin surface (5,6). In addition, limited ankle dorsiflexion could result in increased pressure on the forefoot, particularly during the late stance phase of gait, caused by an early heel rise or compensatory pronation (5,7,8).
From: The Diabetic Foot, Second Edition
Edited by: A. Veves, J. M. Giurini, and F. W. LoGerfo © Humana Press Inc., Totowa, NJ