George A. Arangio

A biomechanical model of the foot

The foot is modeled as a statically indeterminate structure supporting its load at the heads of the five metatarsals and the tuberosity of the calcaneous. The distribution of support is determined through an analysis of the deformations caused in the structure as a result of the forces at these locations. The analysis includes the effect of the plantar aponeurosis and takes into account the deformation of the metatarsals and bending of the joints. A parametric study is presented to illustrate the behavior of the solution under a broad range of conditions.

A biomechanical analysis of posterior tibial tendon dysfunction, medial displacement calcaneal osteotomy and flexor digitorum longus transfer in adult acquired flat foot

BACKGROUND Biomechanical models have been used to study stress in the metatarsals, subtalar motion, lateral column lengthening and subtalar arthroereisis. Posterior tibial tendon dysfunction has been associated with increased loads in the arch of the acquired flat foot. We examine whether a 10 millimeter (mm) medial displacement calcaneal osteotomy and flexor digitorum longus transfer to the navicular reduces these increased loads in the flat foot. METHODS The response of a normal foot, a foot with posterior tibial tendon dysfunction, and a flat foot to an applied load of 683Newton was analyzed using a multi-segment biomechanical model. The distribution of load on the metatarsals, the moment about each joint, the force on each of the plantar ligaments and the muscle forces were computed. FINDINGS Posterior tibial tendon dysfunction results in increased load on the medial arch, which may cause the foot to flatten. A 10mm medial displacement calcaneal osteotomy substantially decreases the load on the first metatarsal and the moment at the talo-navicular joint and increases the load on the fifth metatarsal and the calcaneal-cuboid joint. Adding the flexor digitorum longus transfer to the medial displacement calcaneal osteotomy has only a small effect on the flattened foot. INTERPRETATION Our biomechanical analysis illustrates that when the foot becomes flat, the force on the talo-navicular joint increases substantially from its value for the normal foot, and that medial displacement calcaneal osteotomy can reduce this increased force back toward the value occurring in the normal foot. This study provides a biomechanical rationale for medial displacement calcaneal osteotomy treatments for posterior tibial tendon dysfunction.

A Biomechanical Model of the Foot: The Role of Muscles, Tendons, and Ligaments

A biomechanical model of the foot is developed and analyzed to determine the distribution of support under the metatarsal heads, the tension in the plantar aponeurosis, and the bending moment at each of the joints of the foot. This model is an extension of our earlier work to include the role of muscles, tendons, and ligaments. Two cases are presented: in the first the center of gravity of the body is over the mid foot, and in the second, the center of gravity is anterior, over the metatarsals, and no support is provided by the heel. The model shows the extent to which the muscles reduce the force in the supporting ligaments at each of the joints and decrease the tension in the plantar aponeurosis, and that this effect is more pronounced when the center of gravity of the body is moved forward.

Biomechanical study of stress in the fifth metatarsal

OBJECTIVE: The stress throughout the fifth metatarsal was determined under various loading conditions, in order to better understand the causes of fractures to this bone. DESIGN: A mathematical approach was taken, in which the stresses were analysed using the methods of beam theory. BACKGROUND: Finite element analysis has frequently been used to determine the stress in bones. Beam theory provides an easier method for determining the force and moment resultant in any cross-section. The distribution of stress throughout the cross-section can then be found by solving certain partial differential equations defined on the cross-sections. METHODS: Cross-sections of the bone were obtained by slicing a mould, into which the bone was placed, at numerous intervals along its length. Analytic expressions describing each cross-section were obtained by fitting a Fourier series to a sequence of points along the boundary. RESULTS: The maximum stress found in the fifth metatarsal resulted from an oblique load, and had a magnitude less than would occur in a subject during normal walking. CONCLUSIONS: Since the magnitude of the stress is submaximal, this study lends theoretical support to the clinical observation that the diaphyseal fracture is indeed a stress fracture. RELEVANCE: Our analysis adds a biomechanical rationale to the pathomechanics of diaphyseal stress fractures of the fifth metatarsal. It suggests that inversion during repetitive activities may predispose the foot to fractures at a predictable location.

Radiographic comparison of standing medial cuneiform arch height in adults with and without acquired flatfoot deformity.

Background: Adult acquired flatfoot (AAF) is characterized by decreased arch height, talar depression, medial arch depression and elongation, and forefoot abduction. We have measured standing arch height in AAF patients and in a control group of patients using the standing lateral medial cuneiform arch height radiographic measurement. Methods: Fifteen (25 feet) patients were selected with the clinical diagnosis of symptomatic AAF with no secondary diagnoses. A control group consisted of 36 (72 feet) patients with no foot deformities or prior foot surgeries. Arch height was measured in millimeters using the standing medial cuneiform height on the lateral radiographic view. Results: The mean standing medial cuneiform arch height in the control group was 18.38 mm. The mean arch height in the AAF group was 11.04 mm (p < 0.001). There were no differences between right and left feet in the control group or symptomatic and contralateral feet in the AAF group. Body mass index (BMI) in the control group was 26.17 and in the AAF 33.74. (p = 0.007). Conclusion: These data provide a control value for the arch height using the medial cuneiform as reference. The decrease in arch height is a strong indicator of AAF. A study with larger numbers of patients is necessary.

Subtalar Pronation — Relationship to the Medial Longitudinal Arch Loading in the Normal Foot

A three-dimensional biomechanical model was used to calculate the mechanical response of the foot to a load of 683 Newtons with the subtalar joint in the neutral position, at five degrees of pronation, and at five degrees of supination. Pronation causes the forefoot to evert, increasing the load borne by the first metatarsal. This results in a 47% increase in the moment about the talonavicular joint and a 58% increase in the moment about the navicular-medial cuneiform joint. Subtalar joint supination causes the forefoot to invert and results in a 55% increase in the moment about the calcaneal-cuboid joint.

Medial displacement calcaneal osteotomy reduces the excess forces in the medial longitudinal arch of the flat foot.

OBJECTIVE The hypothesis tested was that the increased load on the medial arch of the flat foot can be reduced through a medial displacement calcaneal osteotomy. DESIGN A three-dimensional, biomechanical, multisegment model was used in conjunction with experimental data from the literature. BACKGROUND Biomechanical models have been used to study the plantar fascia, medial arch height, subtalar motion and distribution of forces in the foot. METHODS Responses of a normal foot, a flat foot and a flat foot with a medial displacement calcaneal osteotomy to an applied load of 683 Newtons were analyzed, and the distribution of support among the metatarsal heads and moment about various joints were computed. RESULTS Compared to the normal foot, our flat foot model shifts the distribution of support from the lateral to the medial side, decreasing support provided by the fifth metatarsal from 11% to 1% of the total load, increasing support provided by the first metatarsal from 12% to 22% and increasing the moment about the talo-navicular joint from 20 to 28 Newton-meters. A ten millimeter medial displacement calcaneal osteotomy shifts support back toward the lateral side, with 11% provided by the fifth metatarsal and 13% by the first metatarsal. The moment at the talo-navicular joint decreases to eighteen Newton-meters. CONCLUSION Our analysis indicates that a ten millimeter medial displacement calcaneal osteotomy in a flat foot model decreases the load on the medial arch.

THE FOOT AS A SHOCK ABSORBER

A mathematical analysis of the deformation of the foot is developed to determine the role that stretch of ligaments and tendons plays in absorbing shock following impact. Our analysis is based on an anatomical biomechanical model that includes each of the bones of the foot. We calculate the time course of the deflection of the joints and the elongation of the ligaments and tendons and determine the ground reaction force acting on the heel. Quasi-linear viscoelastic theory is used for soft tissue constitutive relationships. With biomechanical data selected from the literature, we obtain a vertical force impact peak of 8000 N, occurring at 16 ms following heel strike. This is of higher magnitude and shorter duration than is found experimentally, as is to be expected, since we did not include the heel pad in our model and we assumed that the impact surface was ideally rigid.

Hindfoot alignment valgus moment arm increases in adult flatfoot with Achilles tendon contracture.

Background: Adult acquired flatfoot is often associated with Achilles tendon contracture and may be associated with isolated spring ligament insufficiency without Achilles tendon contracture. We have studied the hypothesis that standing valgus hindfoot alignment moment arm is increased in adult acquired flatfoot with Achilles tendon contracture when compared to adult acquired flatfoot without Achilles tendon contracture. Materials and Methods: The standing hindfoot alignment, standing lateral tibial-calcaneal angle, lateral talo-first metatarsal angle, lateral medial cuneiform arch height, and anteroposterior talonavicular coverage angle were measured in 22 patients with a clinical diagnosis of adult acquired flatfoot with one foot with clinical Achilles tendon contracture and one without that diagnosis. We compared the adult acquired flatfoot group to a control group of 15 patients with no foot or ankle deformities or previous foot or ankle surgeries. Results: In patients with flatfoot and Achilles tendon contracture, there was a significantly increased valgus hindfoot alignment, talo-first metatarsal angle, talonavicular coverage angle, tibiocalcaneal angle and a decreased arch height when compared to the control group. In all flatfeet, we found an increased tibiocalcaneal angle. In both flatfoot groups, an increasing tibiocalcaneal angle and an increasing talo-first metatarsal angle was correlated to a decreasing arch height. In adult acquired flatfoot without Achilles tendon contracture diagnosed by clinical exam, an increasing talonavicular coverage angle was correlated to an increasing talo-first metatarsal angle and a decreasing arch height. Conclusion: Adults with flatfoot and Achilles tendon contracture have a significantly increased standing hindfoot valgus alignment moment arm and other associated deformities. Level of Evidence: III, Comparative Series

Intraarticular pressures in a gravity-fed arthroscopy fluid delivery system.

Seven consecutive patients undergoing arthroscopic surgery in a gravity-fed fluid delivery system were studied. The average minimum adequate intraarticular pressure (MAIP) was 55 mm Hg. The height of a saline bag above the knee necessary to achieve this average MAIP was 75 cm. There was a positive correlation between diastolic blood pressure and the minimum adequate intraarticular pressure in this study. There was no positive correlation between systolic blood pressure and the MAIP.