Effect of Achilles tendon loading on plantar fascia tension in the standing foot
Introduction
The plantar aponeurosis (fascia), which originates from the medial tubercle of the calcaneus and inserts into the phalanges through a complex network of fibrous tissue (Hicks, 1954) is one of the major stabilising structures of the longitudinal arch of the foot. Cadaveric studies revealed that release of the plantar fascia decreased arch height, confirming the arch-supporting function of the plantar fascia (Kitaoka et al., 1997, Murphy et al., 1998).
Insertional plantar fasciitis, which is common in athletes as well as the general population (Cornwall and McPoil, 1999, Warren, 1990), usually associates with a chronic painful syndrome at the inferior heel region. Excessive stretching, repetitive and abnormal stress induced in the plantar fascia and its calcaneal insertion may cause inflammation and injury, which are thought to be the major causes of plantar fasciitis (Cornwall and McPoil, 1999, Warren, 1990). Conditions which predispose the plantar fascia to increased tension during weight-bearing such as excessive pronation, flat or high-arched foot structures and tight Achilles tendon are often suggested as implicating factors of plantar fasciitis (Warren, 1990).
Conservative treatment such as anti-inflammatory medication, stretching and strengthening exercise and foot orthoses has been used effectively to alleviate the painful syndrome of plantar fasciitis (Barry et al., 2002, Pfeffer et al., 1999, Probe et al., 1999). Stretching exercise and dorsiflexion night splints are often prescribed to help relieve the Achilles tendon tension with an attempt to reduce arch deformation, excessive pronation, rearfoot valgus and the tension of the plantar fascia. The dorsiflexion night splint was first described for use in the treatment of plantar fasciitis by Wapner and Sharkey (1991) and has been proven to be beneficial by several prospective and randomised control studies (Barry et al., 2002, Probe et al., 1999).
Despite the success of Achilles tendon stress relief for treatment of plantar fasciitis, the biomechanical relationship between Achilles tendon loading and tension on the plantar fascia has not been well documented. A larger number of cadaveric studies (Crary et al., 2003, Donahue and Sharkey, 1999, Kitaoka et al., 1997, Thordarson et al., 1995) and computational analyses (Cheung et al., 2004, Gefen, 2002, Giddings et al., 2000, Kim and Voloshin, 1995) have focused on the biomechanical response of the plantar fascia under different loading and supporting conditions and the biomechanical consequences of fasciotomy. Because of the difficulties and invasive nature of in vivo measurements of Achilles tendon tension (Finni et al., 1998), the biomechanical effect of varying Achilles tendon loading on plantar fascia has only been investigated by cadaveric studies. Thordarson et al. (1995) documented the arch-deforming effect of the Achilles tendon loading and the arch-supporting mechanism of the plantar fascia with toe extension via a 3D movement analysis on cadavers. Carlson et al. (2000) measured an increased plantar fascia strain with increasing loading on the Achilles tendon and with toe extension under static loading conditions of the foot. Erdemir et al. (2004) found a positive correlation between plantar fascia tension and Achilles tendon force during simulations of the stance phase of gait in a cadaver model.
Although cadaveric studies (Carlson et al., 2000, Erdemir et al., 2004) have been done to investigate the effect of Achilles tendon loading on the loading response of the plantar fascia, these studies were subjected to certain limitations. For instance, physiological loading condition was not simulated by Carlson et al. (2000) because the body weight on the foot was not considered. Erdemir et al. (2004) measured the tension of the plantar fascia during simulated normal walking using a dynamic cadaveric model without a strict control on the effects of other extrinsic muscle forces, ground reaction forces and ankle–foot position. Therefore, their cadaveric simulation cannot provide a sensitivity analysis on the effects of Achilles tendon tension on the loading response of the plantar fascia. In this study, a 3D finite element (FE) foot model was employed to quantify the biomechanical effect of varying Achilles tendon loading on the plantar fascia, longitudinal arch deformation and plantar pressure distribution of the standing foot.
Section snippets
Methods
A geometrically accurate FE model of the human foot and ankle was developed from 3D reconstruction of coronal Magnetic Resonance (MR) images of 2 mm intervals from the right foot of a normal male subject of age 26, height 174 cm and weight 70 kg in the neutral position (Cheung et al., 2005). The neutral foot position (Wu et al., 2002) of the supine lying subject was maintained by a custom ankle–foot orthosis during the MR scanning. The undeformed arch height of the subject was 55 mm during upright
Finite element predictions and cadaveric foot measurements under pure compression
The load–deformation curve of the six foot specimens exhibited an increasingly stiffening response under vertical compression (Fig. 3(a)). The FE predictions also characterised a nonlinear load–deformation response, but with a larger magnitude of vertical deformation. The average vertical deformation of the six specimens under a compression force of 700 N was 5.6 mm while the FE model predicted a value of 10.6 mm.
The plantar fascia experienced increasing strains with the increased vertical
Finite element predictions and cadaveric foot measurements under pure compression
Under pure compression, the FE model predicted a similar profile but a larger magnitude of vertical foot deformation than the cadaveric foot measurements. The discrepancy might probably be the results of neglecting the joint capsules and the stabilising effects from the structural interactions between the joints and the ligamentous and muscular tissues, which reduced the joint stiffness of the ankle–foot structures. Besides, the effect of tissue desiccation and the process of freezing and
Conclusions
A 3D geometrically accurate FE model of the human foot and ankle was employed to quantify the biomechanical effects of Achilles tendon tension on the loading response of the plantar fascia in the standing foot. The plantar fascia was found to be an important arch-supporting structure, which sustained high tensions during weight-bearing. An increase in Achilles tendon load resulted in reductions of arch height and increases in plantar fascia tensions. The Achilles tendon load was found to have
Acknowledgement
This work was supported by the Hong Kong Jockey Club endowment, the research studentship from The Hong Kong Polytechnic University and a grant from the Research Grant Council of Hong Kong (PolyU 5249/04E). The support on cadaveric study from the Institute of Clinical Anatomy, Southern Medical University, Guangzhou, China is acknowledged.
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