THE EFFECT OF FOOT TYPE ON PLANTAR PRESSURE William R. Ledoux1,2,3, Eric S. Rohr1, Charles Harp1,3, Randal P. Ching1,3, and Bruce J. Sangeorzan1,2 1
RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Health Care System, Seattle, WA Departments of 2Orthopaedics and Sports Medicine and 3Mechanical Engineering, University of Washington, Seattle, WA E-mail:
[email protected] Web: www.seattlerehabresearch.org
INTRODUCTION It has long been held that foot structure can affect foot function. Foot type is a means of structurally describing the foot. Feet can be classified as pes cavus (PC, high arch), neutrally aligned (NA, normal arch), and pes planus (PP, low arch). Pes planus feet may be further subdivided into asymptomatic (PPA) and symptomatic (PPS) groups. Plantar pressure is considered a measure of foot function. It has been retrospectively and prospectively associated with ulceration (Boulton et al. 1983; Veves et al. 1992) and it has been correlated to measures of foot shape (Morag and Cavanagh 1999). The purpose of this study was to explore how foot type (structure) affects plantar pressure (function). METHODS A total of 40 subjects were enrolled with ten in each of four foot type groups: PC, NA, PPA, and PPS. Each subject contributed one foot to this analysis. Foot type was determined via clinical examination by an orthopaedic surgeon. Each foot was Xrayed (AP and lateral) and a weight-bearing CT scan was performed. The X-ray and the CT measurements of foot shape were both used to support the determination of foot type (i.e., there were significant differences between foot types, data not shown). Age, weight, height and gender were recorded.
Ten trials of barefoot plantar pressure data were collected with an EMED-SF system. Velocity was recorded with two infrared emitter-reflector-detector systems placed a fixed distance apart on either side of the pressure plate. Stance phase was divided into contact phase (heel strike to foot-flat), mid-stance phase (foot-flat to heel-off) and propulsion phase (heel-off to toe-off). Peak pressure and the pressure-time integral (PTI, i.e., a measure of high pressure dosage) was determined over the entire foot as well as 10 subdivisions: the hallux, lesser toes, first through fifth metatarsal heads, medial midfoot, lateral midfoot and heel. These subdivisions were obtained by overlaying the AP X-ray on top of an actual-size composite peak plantar pressure print out. The Center of Pressure Excursion Index (CPEI), a measure of dynamic foot function, was also determined, as was foot angle (the angle of the foot bisection with the sagittal plane). Adjusting for age, body mass index, and velocity, a linear mixed effects model was used to explore differences between foot types. RESULTS There were no significant differences in age, BMI or gender between the four groups, but PC subjects walked significantly slower than all other foot types (Table 1, p
