Mervyn Evans

Induction of bone formation in rat tail vertebrae by mechanical loading.

We have developed an experimental model in which pins, inserted into the seventh and ninth caudal vertebrae of 13-week-old rats, are used to load the eighth caudal vertebra in compression. Four groups of animals were used in the study: unpinned; animals with pins inserted, but non-loaded; animals loaded once, for 360 cycles at 0.5 Hz; and animals subjected to daily loading for 36 cycles at 0.5 Hz. Pins were immobilised by clamps when not undergoing loading. The animals were killed 9 days after pinning, and the eighth caudal vertebra was subjected to histomorphometric and histodynamic analysis. We found that vertebrae subjected to 36 daily loading cycles showed a 30-fold increase in bone formation compared to non-loaded controls. A single loading regime of 360 cycles was sufficient to increase bone formation 4-fold. Bone formation on trabecular surfaces was of lamellar rather than woven bone and was accompanied by a decrease in indices of bone resorption. Loaded vertebrae also showed substantial periosteal woven bone formation, although a minor degree of periosteal woven bone formation was also seen in one non-loaded pinned control vertebra. Our results suggest that in the rat, as in avian species, short loading regimes are capable of inducing bone formation. The model may assist an analysis of the interactions between bone resorption, bone formation and mechanical stimuli, and may enable identification of the molecular signals that mediate induction of lamellar bone formation on trabecular surfaces.

The design of anthropomorphic prosthetic hands: A study of the Southampton Hand

The design of prosthetic hands is constrained by a series of strict conditions. Despite this, many different design strategies have been explored. One particular form is the Southampton Hand system. This is a hierarchically controlled, electrically driven hand, with multiple axes, in an anthropomorphic form. This paper details the range of mechanical solutions adopted to address the conditions. It also compares them with other solutions.

An Intelligent Anthropomorphic Hand, with Automatic Grasp

The current designs of commercial artificial hands have a low level of innovation. As feedback to the user is difficult to achieve reliably, most devices are simple in design and operation, and limited in functional range. If information on the state of the hand, the forces and any slippage that is occurring is fed back to a microcontroller then more than one degree of freedom can be controlled and a greater and more natural functional range is possible. This paper describes the development of such a device. It outlines the design requirements, the methods of detection of the signals and the training required to operate the hand.

Dynamic interfragmentary motion in fractures during routine patient activity

Natural interfragmentary motion was measured in tibial fractures during normal patient activity, and the results were interpreted using correlations from the literature to examine the influence of natural motion on healing. Ten patients were selected with reduced, diaphyseal tibial fractures stabilized with Orthofix external fixators. Three-dimensional motion was monitored with an instrumented spatial linkage during walking, standing, and muscle activities at 2 and 4 weeks postfixation. Fracture motion arising from dorsal to plantar flexion while supine produced peak cyclic displacements of the same order of magnitude as that seen during weightbearing activity. Thus, therapeutic exercise may be used to provide a stimulus to osteogenic repair processes in patients who are unable to bear weight. In 3 patients, maximum amplitudes of axial motion during walking were 1 mm or greater. This implied regular gap closure and high tissue strains within the 1 mm ± 0.5 mm gaps. In 3 patients, axial motion was less than 0.25 mm. These 2 extremes may indicate a range of displacement relative to gap size that embraces inhibitive and stimulative influences on healing. Transverse shear displacements also varied greatly from between 0.6 and 0.75 mm in 3 patients to less than 0.2 mm in 5 patients.

Measurement of impact force, simulation of fall and hip fracture.

It has been shown that the incidence of hip fracture in the elderly may be influenced by the type of floor covering commonly used in homes for the elderly. This study describes the development of a method for modelling a fall during a hip fracture event, to examine the influence of different floors on impact force. An impact transducer is dropped in free fall through a smooth plastic tube. The impactor nose of the transducer models the curvature of the greater trochanter, and a steel spring is used to simulate the compliance of the skeletal structure. A weight, which corresponds to one-sixteenth of average body mass, compresses the spring and applies force to the impactor nose on striking the floor. The temporal variation in the force of impact with the floor is measured by the transducer to within 0.41 percent (SD = 0.63%, n = 10). Five common floor coverings were tested over a concrete floor slab (vinyl, loop carpet and pile carpet--both with and without underpad). ANOVA analysis showed that the differences between mean forces for each floor covering were highly significant (p > 0.001), with the thicker coverings producing 7 percent lower forces. The transducer may be used to examine the correlation between impact force and fracture incidence for a variety of different floors in homes for the elderly.

The influence of external fixators on fracture motion during simulated walking

This experimental study examines the relative influence of five unilateral external fixators on tibial fracture stability during simulated walking. Stability during routine patient activity is important, because cyclic inter fragmentary motion, or strain, has been shown to affect fracture healing. In model stable fractures simulating early healing (six weeks), it was found that fixators do little to constrain against axial inter fragmentary strains as great as 100% at only nominal weight-bearing (6.0 kg). These strains may occur repeatably at peak amplitudes of motion during walking. Similarly, peak angular movements may lead to additional axial strains of up to 25% at the external cortex and shear movements may lead to shear strains of up to 100%. Such strains are great enough to yield and possibly refracture the intra gap fracture tissue that may be composed of a combination of granulation tissue, fibrous cartilage, cartilage and bone. It was also shown that the procedure of releasing the fixator column to telescope (dynamize) has little influence on peak cyclic axial motion and on loading at the fracture, although increases occurred in peak transverse and torsional shear strains of up to 100%. Since permanent inter fragmentary translation also arises from the consequent compaction of the intra gap tissue, it may be permanent displacement rather than any change in the amplitude of motion that is responsible for the beneficial effect on healing claimed for the dynamizing procedure. In unstable fractures that are unable to support tibial load at the fracture, the peak amplitudes of cyclic movement were as great as those reported for fractures stabilized by plaster casts, and were approximately twice the movement of the stable fractures simulating early healing. Therefore, patients with unstable fractures supported by external fixators, may be expected to have similar patterns of healing to plaster-casted patients with similar fractures.

Temporal changes in dynamic inter fragmentary motion and callus formation in fractures

This study examined whether callus proliferation at long bone fractures is triggered by cyclical inter-fragmentary displacement which arises from routine activity. It also examined whether a growing callus increases the stability of a fracture, thereby reducing displacement amplitude during relative motion. Seven tibial fractures stabilised with external fixators were monitored up to and beyond fixator removal. An instrumented spatial linkage was developed which was attached to the bone screws to measure inter-fragmentary displacement at the fracture in all six degrees of freedom during routine walking. Callus index (final bone width/initial bone width) was measured at the posterior and lateral cortical surfaces from orthogonal radiographs. In all seven subjects, callus growth was initiated subsequent to a peak in displacement which occurred within the first 42 days; at nine of the 14 surfaces occurred callus initiation occurred within 14 days of the peak displacement. With the exception of two lateral surfaces, maximum callus size, subsequent to fixator removal (at up to 119 days after removal). Displacement reduced during callus growth in five out of six subjects. Since the reduction in displacement did not arise from reduced weight-bearing, increasing callus size must correlate with progressive mechanical union. This was confirmed by end point stiffness tests. Therefore, peak cyclical displacement appears to be the stimulus for callus growth, the effect of which is to reduce displacement and strain which allows the following stages of bone formation and remodelling to unite the fracture.

Effect of Surface Replacement Arthroplasty on Stability of the Ankle

Loosening after total ankle arthroplasty may result from absorption of tensile and shear forces by relatively small bone-cement interfaces. A surface replacement arthroplasty similar in principle to the Oxford knee arthroplasty offers theoretical advantages over other designs. The ability of such a prosthesis to provide a stable articulation was tested by examination of ankle laxity before and after replacement of the tibiotalar articular surfaces. The motion response characteristics of four specimens were measured for internal-external rotation moment (±3 nm), inversion-eversion moment (±3 nm), and anteroposterior displacement (±50 N). The prosthesis restored normal internal-external and inversion-eversion rotatory stability, but the mean anteroposterior laxity was increased from 5.5 to 9.7 mm (P < 0.05). Anteroposterior stability could not be restored by increasing the thickness of the bearing interposed between the prosthetic surfaces. The findings can be explained by consideration of the anatomy of the ligaments in relation to the contour of the normal and prosthetic articular surfaces. The normal tibial articular surface, which is concave in the sagittal plane, provides restraint against anteroposterior motion which is lost when the surface is replaced by the flat prosthetic surface. The findings question the suitability of this type of prosthesis for the ankle.

Relative stiffness, transverse displacement and dynamization in comparable external fixators

Several unilateral external fixators were laboratory tested under applied axial, bending, and torsional loads. In certain cases the shear movement at the fracture site was substantial, up to 4 mm, and in all cases it was as significant as axial displacement. Also, under average full weight bearing, all the fixator/bone screw frameworks will allow too much axial movement at the fracture. This could inhibit healing and cause peak bone and screw stresses to approach yield.

Temporal variation of applied inter fragmentary displacement at a bone fracture in harmony with maturation of the fracture callus

The amplitude of inter fragmentary displacement in long bone fractures greatly influences the pattern and speed of healing. Unfortunately, the amplitude of natural cyclical displacement arising from patient activity is random because of the inherent flexibility of fixation devices under natural loading. Although fixators may be designed to control the amplitude of this displacement, the amplitudes most beneficial to healing have not been determined. Furthermore, the appropriate amplitude must vary during healing as the reparative tissue (callus) progresses histologically and stiffens during maturation. In this study on an experimental fracture, the amplitude of applied cyclical displacement is varied during healing to correspond with the inverse of the callus stiffness versus time curve. In vivo mechanical stiffness tests on the callus indicate that the end point of the fixation period is achieved more rapidly than with a constant level of applied displacement.