A preliminary study of patient-specific mechanical properties of diabetic and healthy plantar soft tissue from gated magnetic resonance imaging.

Foot loading rate, load magnitude, and the presence of diseases such as diabetes can all affect the mechanical properties of the plantar soft tissues of the human foot. The hydraulic plantar soft tissue reducer instrument was designed to gain insight into which variables are the most significant in determining these properties. It was used with gated magnetic resonance imaging to capture three-dimensional images of feet under dynamic loading conditions. Custom electronics controlled by LabVIEW software simultaneously recorded system pressure, which was then translated to applied force values based on calibration curves. Data were collected for two subjects, one without diabetes (Subject A) and one with diabetes (Subject B). For a 0.2-Hz loading rate, and strains 0.16, 0.18, 0.20, and 0.22, Subject A's average tangential heel pad stiffness was 10 N/mm and Subject B's was 24 N/mm. Maximum test loads were approximately 200 N. Loading rate and load magnitude limitations (both were lower than physiologic values) will continue to be addressed in the next version of the instrument. However, the current hydraulic plantar soft tissue reducer did produce a data set for healthy versus diabetic tissue stiffness that agrees with previous trends. These data are also being used to improve finite element analysis models of the foot as part of a related project.

The design and validation of a magnetic resonance imaging-compatible device for obtaining mechanical properties of plantar soft tissue via gated acquisition.

Changes in the mechanical properties of the plantar soft tissue in people with diabetes may contribute to the formation of plantar ulcers. Such ulcers have been shown to be in the causal pathway for lower extremity amputation. The hydraulic plantar soft tissue reducer (HyPSTER) was designed to measure in vivo, rate-dependent plantar soft tissue compressive force and three-dimensional deformations to help understand, predict, and prevent ulcer formation. These patient-specific values can then be used in an inverse finite element analysis to determine tissue moduli, and subsequently used in a foot model to show regions of high stress under a wide variety of loading conditions. The HyPSTER uses an actuator to drive a magnetic resonance imaging-compatible hydraulic loading platform. Pressure and actuator position were synchronized with gated magnetic resonance imaging acquisition. Achievable loading rates were slower than those found in normal walking because of a water-hammer effect (pressure wave ringing) in the hydraulic system when the actuator direction was changed rapidly. The subsequent verification tests were, therefore, performed at 0.2 Hz. The unloaded displacement accuracy of the system was within 0.31%. Compliance, presumably in the system's plastic components, caused a displacement loss of 5.7 mm during a 20-mm actuator test at 1354 N. This was accounted for with a target to actual calibration curve. The positional accuracy of the HyPSTER during loaded displacement verification tests from 3 to 9 mm against a silicone backstop was 95.9% with a precision of 98.7%. The HyPSTER generated minimal artifact in the magnetic resonance imaging scanner. Careful analysis of the synchronization of the HyPSTER and the magnetic resonance imaging scanner was performed. With some limitations, the HyPSTER provided key functionality in measuring dynamic, patient-specific plantar soft tissue mechanical properties.

Talonavicular joint coverage and bone morphology between different foot types.

This study explored three dimensional (3D) talonavicular joint (TNJ) coverage/orientation and bone morphology to reveal parameters that could classify and identify predispositions to cavus and planus feet. 3D models of 65 feet from 40 subjects were generated from computed tomography images classified as pes cavus, neutrally aligned, or asymptomatic/symptomatic pes planus. We calculated the talar and navicular overlap (TNJ coverage). We also measured orientation of the navicular, morphological parameters of the talus and navicular, and angular position of the talar head to body. Pes cavus showed significantly less talonavicular coverage (58 ± 2% talus and 86 ± 2% navicular) compared to asymptomatic pes planus (63 ± 2% and 95 ± 2%) and neutrally aligned feet (98 ± 2% navicular), and significantly more navicular dorsiflexion and adduction relative to the talus (p < 0.0083). The talar head in cavus feet was inverted relative to the body compared to planus feet (p < 0.0083). For symptomatic pes planus, significant abduction was measured for the navicular relative to the talus and the talar head was plantar flexed relative to the body (p < 0.0083). The talar head in planus feet was everted relative to the body compared to neutrally aligned feet. Both intrinsic (bone morphology) and extrinsic (bone position) differences exist in groups of feet described as cavus and planus.

Marker-based validation of a biplane fluoroscopy system for quantifying foot kinematics.

INTRODUCTION: Radiostereometric analysis has demonstrated its capacity to track precise motion of the bones within a subject during motion. Existing devices for imaging the body in two planes are often custom built systems; we present here the design and marker-based validation of a system that has been optimized to image the foot during gait. METHODS: Mechanical modifications were made to paired BV Pulsera C-arms (Philips Medical Systems) to allow unfettered gait through the imaging area. Image quality improvements were obtained with high speed cameras and the correction of image distorting artifacts. To assess the system's accuracy, we placed beads at known locations throughout the imaging field, and used post processing software to calculate their apparent locations. RESULTS: Distortion correction reduced overall RMS error from 6.56 mm to 0.17 mm. When tracking beads in static images a translational accuracy of 0.094 ± 0.081 mm and rotational accuracy of 0.083 ± 0.068° was determined. In dynamic trials simulating speeds seen during walking, accuracy was 0.126 ± 0.122 mm. DISCUSSION: The accuracies and precisions found are within the reported ranges from other such systems. With the completion of marker-based validation, we look to model-based validation of the foot during gait.

The comparative morphology of idiopathic ankle osteoarthritis.

BACKGROUND: Osteoarthritis is the most common joint disease and the leading cause of chronic disability in the U.S. However, symptomatic osteoarthritis at the ankle occurs nine times less frequently than at the knee and hip, even though the ankle experiences greater pressure and is the most commonly injured joint in the human body. This study sought to quantify the shape and coverage of the talar and tibial articular surfaces by comparing the three-dimensional morphology of the ankle in patients with ankle osteoarthritis and in those without arthritis, including a subset of different foot shapes. METHODS: We created three-dimensional models of the joint surfaces of ankles with and without arthritis. We fit cylinders to the joint surfaces, and measured the radius of the tibial and talar articular surfaces, the tibial coverage angle of the talus, and the degree of joint skew. We hypothesized that these measurements would be different between those with and without ankle osteoarthritis and among foot types. We evaluated a total of 108 limbs. RESULTS: The mean tibial and talar radii were significantly higher and the mean coverage angle was significantly lower in feet with ankle osteoarthritis than in all other foot categories. The mean coronal skew in limbs with ankle osteoarthritis was significantly higher than in the neutral and flatfoot groups. The high arched feet had several significantly different skew angles from other foot types. No significant differences in joint morphology measures between neutrally aligned feet and flatfeet were found. CONCLUSIONS: Ankles with osteoarthritis had larger tibial and talar radii, a smaller coverage angle, and larger skew angles than ankles without osteoarthritis. Together, these findings suggest a flatter ankle joint with less stability, depth, and containment and reduced articular constraint and support.

Evaluating foot kinematics using magnetic resonance imaging: from maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation.

The foot consists of many small bones with complicated joints that guide and limit motion. A variety of invasive and noninvasive means [mechanical, X-ray stereophotogrammetry, electromagnetic sensors, retro-reflective motion analysis, computer tomography (CT), and magnetic resonance imaging (MRI)] have been used to quantify foot bone motion. In the current study we used a foot plate with an electromagnetic sensor to determine an individual subject's foot end range of motion (ROM) from maximum plantar flexion, internal rotation, and inversion to maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation. We then used a custom built MRI-compatible device to hold each subject's foot during scanning in eight unique positions determined from the end ROM data. The scan data were processed using software that allowed the bones to be segmented with the foot in the neutral position and the bones in the other seven positions to be registered to their base positions with minimal user intervention. Bone to bone motion was quantified using finite helical axes (FHA). FHA for the talocrural, talocalcaneal, and talonavicular joints compared well to published studies, which used a variety of technologies and input motions. This study describes a method for quantifying foot bone motion from maximum plantar flexion, inversion, and internal rotation to maximum dorsiflexion, eversion, and external rotation with relatively little user processing time.

Cadaveric simulation of a pes cavus foot.

BACKGROUND: The pes cavus deformity has been well described in the literature; relative bony positions have been determined and specific muscle imbalances have been summarized. However, we are unaware of a cadaveric model that has been used to generate this foot pathology. The purpose of this study was to create such a model for future work on surgical and conservative treatment simulation. MATERIALS AND METHODS: We used a custom designed, pneumatically actuated loading frame to apply forces to otherwise normal cadaveric feet while measuring bony motion as well as force beneath the foot. The dorsal tarsometatarsal and the dorsal intercuneiform ligaments were attenuated and three muscle imbalances, each similar to imbalances believed to cause the pes cavus deformity, were applied while bony motion and plantar forces were measured. RESULTS: Only one of the muscle imbalances (overpull of the Achilles tendon, tibialis anterior, tibialis posterior, flexor hallucis longus and flexor digitorum longus) was successful at consistently generating the changes seen in pes cavus feet. This imbalance led to statistically significant changes including hindfoot inversion, talar dorsiflexion, medial midfoot plantar flexion and inversion, forefoot plantar flexion and adduction and an increase in force on the lateral mid- and forefoot. CONCLUSION: We have created a cadaveric model that approximates the general changes of the pes cavus deformity compared to normal feet. These changes mirror the general patterns of deformity produced by several disease mechanisms. CLINICAL RELEVANCE: Future work will entail increasing the severity of the model and exploring various pes cavus treatment strategies.