0198-0211/97/1801-0008$03.00/0 FOOT & ANKLE INTERNATIONAL Copyright © 1997 by the American Orthopaedic Foot and Ankle Society, Inc.

Mechanical Behavior of the Foot and Ankle After Plantar Fascia Release in the Unstable Foot Harold B. Kitaoka, MD.,* Zong Ping Luo, Ph.D.,t and Kai-Nan An, Ph.D.+ Rochester, Minnesota

INTRODUCTION

ABSTRACT The change in position of the bones of the foot was studied in three dimensions after plantar fascia release in intact and destabilized feet. Fifteen fresh-frozen human foot specimens were used. Physiologic loads of 445 newtons were applied axially to simulate standing at ease, and the three-dimensional position of tarsal bones was determined with a magnetic tracking device. The positions were presented in the form of screw axis displacements, quantitating rotation, and axis of rotation orientation. After fasciotomy in the six intact feet, significant differences in rotation were observed at the talotibial and calcaneotalar levels. After fasciotomy in the four unstable feet with three supporting elements sectioned, significant differences in position were observed at the talotibial joint and a significant decrease in arch height was observed. After fasciotomy in the five unstable feet with five supporting elements sectioned, significant differences in rotation were observed at the talotibial joint (mean, 5.5 ± 1.6°; P = 0.001), calcaneotalar joint (mean, 6.1 ± 2.1°; P = 0.003), and metatarsotalar level (mean, 9.3 ± 4.1°; P = 0.007). The average decrease in arch height was 7.4 ± 4.1 mm (P = 0.015). Displacement of all joints tested occurred after fasciotomy, with rotation about all three axes. These changes in displacement were more pronounced in unstable or destabilized feet. The data suggest that operations involving fasciotomy affect arch stability and should not be performed in patients with evidence of concomitant pes planus deformity, because of the likelihood of further deformation.

The plantar aponeurosis, or plantar fascia, spans the plantar aspect of the foot, extending from the calcaneus to the deep soft tissues of the forefoot to the proximal phalanges and superficially to the skin. 9 ,15 ,3o Its function is to assist in raising the arch when the toes are extended.!" There is also evidence that it provides stability to the arch of the toot."? Numerous reports recommend operations that feature plantar fascia release for conditions such as intractable plantar fasciitis. 2 ,4 - 8 ,10 - 14 ,16 ,17,2 1,22,24-27,29,31 ,33-36 This procedure is also recommended for cavus foot deformity. There has been renewed interest in fasciotomy, in part because of endoscopic operative techniques. 6 ,22 Most patients with plantar fasciitis are effectively treated without surgery. Anderson and Foster" reviewed reports of operative treatment for subcalcaneal pain, which were successful in about 91 % of cases. Limited information is available, however, on the occurrence of complications such as arch instability. In a long-term study." the results were successful in only 71 % of cases. In addition, a small but significant degree of flattening of the arch was observed radiologically that had not been reported in studies with a shorter follow-up period." Other evidence that suggests the importance of the fascia in supporting the arch is the observation that flattening of the arch occurs in patients who have spontaneous rupture of the plantar fascia." These observations indicate the need for determining the effects of plantar fascia release on foot mechanics. The extent to which sectioning the plantar fascia, the long and short plantar ligaments, and the spring ligament caused a decrease in overall arch height was reported in a previous investiqatlon,"? but that study did not define the specific joint or joints where displacement occurred or three-dimensional changes in bone and joint positions. The suggestion that fasciotomy affects foot stability to a greater de-

* Consultant, Department of Orthopedics, Mayo Clinic and Mayo Foundation; Associate Professor of Orthopedics, Mayo Medical School, Rochester, Minnesota. To whom requests for reprints should be addressed at Mayo Clinic, 200 First Street SW, Rochester, Minnesota 55905. t Associate Consultant, Division of Orthopedic Research, Mayo Clinic and Mayo Foundation; Assistant Professor of Bioengineering, Mayo Medical School, Rochester, Minnesota. :j: Chair, Division of Orthopedic Research, Mayo Clinic and Mayo Foundation; Professor of Bioengineering, Mayo Medical School, Rochester, Minnesota.

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gree in feet with pre-existing arch instability (pes planus) has not been proven. To determine the response of the foot to loading, it is necessary to measure motions in three dimensions. Most in vivo studies of the foot have been restricted to motion in the sagittal plane measured on lateral radiographs of the foot while welqhtbearinq." Three-dimensional in vivo studies of the foot have used biplanar radioqraohy.i" Although the in vivo measurement techniques provide important information, they have shortcomings. The loads that are applied to the foot are unknown, the accuracy of the measurements is limited, and the implantation of metal markers in bones of human subjects is an invasive technique. The in vitro models allow for a more controlled environment for the study of the mechanical behavior of the foot. Techniques that are used to study foot and ankle mechanics should measure three-dimensional motion, allow for unconstrained physiologic foot and ankle movement, and simultaneously monitor the movement of multiple bones or joints. These testing techniques would be difficult or impossible to perform in human subjects. The present study was designed to meet these criteria and to address a clinically important issue. This study was designed to determine the threedimensional movement of the calcaneus relative to the talus (calcaneotalar), the talus relative to the tibia (talotibial), and the first metatarsal relative to the talus (metatarsotalar) under vertical load before and after plantar fasciotomy. These movements were also studied after transection of specific ligaments of the foot. We hypothesized that fasciotomy has a greater effect in unstable feet. MATERIALS AND METHODS

Fifteen fresh-frozen feet from seven male and five female cadavers were studied. None of the feet had a pre-existing deformity of ankle, hindfoot, or arch. The mean age at the time of death was 71 years (range, 20-89 years). The tibia and fibula were amputated at the junction of the middle and distal thirds of the leg. The skin, subcutaneous tissues, and muscle were dissected from the most proximal portion, and the tibia and fibula were then embedded in polymethylmethacrylate in an inverted position. An alignment apparatus was used to ensure consistent vertical orientation of the tibia during embedding. A specially designed acrylic plastic loading frame was constructed to apply an axial load of 445 newtons (N) (Fig. 1). Two plastic ball-bearing plates were inserted between the plantar foot and the loading platform to reduce the effects

PLANTAR FASCIA RELEASE IN UNSTABLE FOOT

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P Fig. 1. Foot testing apparatus. Note the ball-bearing plates between the foot specimen and loading platform to reduce the effects of shear force. (From Kitaoka, H.B., Lundberg, A., Luo, Z.P., and An, K.N.: Kinematics of the normal arch of the foot and ankle under physiologic loading. Foot Ankle Int., 16:492-499, 1995. With permission.)

caused by shear forces between the skin and the loading platform as foot deformation occurred. Three-dimensional movement of four bones (talus, navicular, calcaneus, and first metatarsal) relative to the fixed tibia was monitored with a magnetic tracking system (3Space Tracker System; Polhemus Navigational Sciences Division, Colchester, V1), consisting of one three-axis source, four three-axis sensors, and an electronic unit." The electronic unit interfaced with the host computer, an IBM 486 PC, and contained analog circuitry to generate and sense electromagnetic fields and to digitize the sensed analog signals. An 80286based processor contained in the electronics unit provided the drive signals that excited the source. This processor also performed all necessary computations

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Foot & Ankle Internationai/Vol. 18, No. 1/January 1997

KITAOKA ET AL.

DF~ MAYO ©,993

Fig. 3. Arch height defined as the distance between the dorsal navicular and a line between the medial calcaneal tubercle and the plantar first metatarsal head. C, calcaneus; MC, medial cuneiform; MT, first metatarsal; N, navicular; T, talus. (By permission of Mayo Foundation.)

Fig. 2. The x, y, and z coordinate system used to describe screw axis orientation and position as viewed from the anteromedial position. (By permission of Mayo Foundation.)

to determine the spatial location of the sensors. The sensor triad measured the field strength, and the electronic unit software solved for the relative source sensor position and orientation. According to the manufacturer's specifications, the system has, within a 76-cm range from the source, a translational accuracy of 2.54 mm root mean square and an angular accuracy of 0.5 root mean square. The translational resolution is 0.001 per millimeter range, and the angular resolution is 0.1 Field testing demonstrated that it was possible to achieve greater accuracy and resolution if the sourceto-sensor distance was limited. We performed a series of experiments to test the accuracy of the system in the measurement of joint kinematics. On the basis of our calibration, when the sensors were located within a range of 10 to 25 em from the source, the translational accuracy was 0.2 mm root mean square. In the present study, the sensors were placed within this range. The magnetic source was fixed to the loading frame to which the tibia was secured. Sensors were fixed to the dorsal talus, posterolateral calcaneus, dorsomedial navicular, and dorsal first metatarsal. Because of the potential interference with electromagnetic fields, use of large metal objects was avoided. A global coordinate system was used, with the right foot as a reference (Fig. 2), fixed on the tibia with the origin at the ankle center. The x axis was along the tibial shaft through the ankle center. The z axis was 0

0

•

parallel to the projection of a line connecting the center of the heel and the second metatarsal on a plane perpendicular to the x axis, and the y axis was the product of the x axis and z axis, following the righthand rule. Three axes (x, y, and z) constructed three mutually perpendicular planes: coronal (x-y plane), sagittal (x-z plane), and transverse (y-z plane). The specimens were loaded axially at 445 N. Specimens were preloaded for 10 seconds for each loading condition, and then data were collected in real time at 60 Hz for 3 seconds. The initial resting position of the foot against the ball-bearing plates with no load applied was considered the neutral, or unloaded, position. The position of each tarsal bone (talus, calcaneus, navicular, and first metatarsal) relative to the tibia and the relative motion between each of the two bones constituting a joint (calcaneotalar, naviculotalar, metatarsonavicular, and talotibial) were expressed in terms of screw axes with eight variables. 3 ,2 0 .2 8 Three components of directional cosine defined the orientation of the axis, three coordinates defined the position of the axis, one component defined the angle of rotation about the axis, and one component defined the distance of translation along the axis. For the purpose of this study, we focused attention on the four rotation variables. In addition, the longitudinal arch of the foot was represented as a series of anatomic landmarks on each of the bones constituting the arch; these were digitized at the beginning of the experiment using one of the magnetic sensors. Arch height was defined as the distance from a point on the dorsal navicular, equidistant from the anterior and posterior margins of the navicular, to a line connecting the plantar central first metatarsal head (the crista of the metatarsal head) and the calcaneus at the medial calcaneal tubercle in the sagittal plane (Fig. 3). Because complete fasciotomy was the standard

Foot & Ankle InternationallVol. 18, No. 1/January 1997

PLANTAR FASCIA RELEASE IN UNSTABLE FOOT

technique in 21 clinical studies (1963-1995) in 838 feee,5,6,1 0-14,16,17,21,22,24,25,27,29,31,33-36 and only two surgeons advocated partial fasciotomy,7,8,26 we sectioned the plantar fascia completely. Plantar fasciotomy was performed through a curvilinear plantar incision just anterior to the heel pad. The plantar fascia was divided transversely and completely near its attachment to the medial calcaneal tubercle. This procedure was performed alone in six feet (group 1). In preliminary tests of fresh-frozen cadaver feet, we created an unstable foot (flatfoot, pes planus) by sectioning supporting ligamentous structures and applying an axial load. To assess the effect of fasciotomy in unstable feet, the fascia was sectioned after the talocalcaneal interosseous ligament, medial talocalcaneal ligament, and tibionavicular portion of the deltoid ligament were sectioned in four feet (group 2). We also assessed the effect of fasciotomy in five feet (group 3) that were made even more unstable by sectioning the spring ligament and the long/short plantar ligaments after sectioning the three structures in group 2. Each specimen was tested before and after fasciotomy, and screw axis rotation after fasciotomy was determined. The 0 to 445 N position after fasciotomy was compared with the 0 to 445 N position before fasciotomy for each group. The results were expressed as the mean ± standard deviation in each group. The difference between paired groups was analyzed with a paired two-tailed r-test at a significance level of 0.05. RESULTS

As the foot was loaded, obvious deformation of the arch occurred with apparent depression of the arch and rotation of the forefoot as a whole in relation to the hindfoot. In groups 1 and 2, severe deformation of the arch did not occur after plantar fasciotomy, but marked deformation occurred after plantar fasciotomy in group 3.

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Before fasciotomy II1II After fasciotomy ~

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