Trevor Noel Gardner

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.

Mechanical influences on tibial fracture healing

Selected studies are summarized that measure interfragmentary fracture displacement in 6° of freedom at intervals throughout healing in groups of patients with tibial diaphyseal fractures treated by external skeletal fixation. The results are compared with those obtained from experimental studies in which the ideal mechanical conditions for fracture healing were predicted. A finite element analysis model of the healing tibial fracture also was developed. Measured data were used for the analysis, and stress and strain patterns were defined for different stages of healing. Interfragmentary movement measured in the first 6 weeks after injury usually is a magnitude smaller in patients treated by external fixation than in patients treated with cast immobilization. This movement can be much smaller than that predicted to be optimal by experimental studies. A greater amplitude can be achieved, even in stable fractures, by ensuring patient activity. The interfragmentary movement is elastic during loading activity and is generally sinusoidal during steady walking. At the time of dynamization (the unlocking of the frame), a permanent set occurs at the fracture site in all planes. The cyclical movement range in each plane often decreases immediately after unlocking. The model analysis study of fracture healing predicts that tissue damage may occur in the later (hard callus) phase of healing, even while the fixation device is in place, because of abnormally high stresses and strains. This study indicates that fracture mechanics should be controlled more rigorously to provide amplitudes of movement in the first 4 to 6 weeks after fracture. The rigidity of fixation should be increased in the subsequent weeks until the fracture has healed and the frame is removed.

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.

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.

Rapid application fracture fixators - an evaluation of mechanical performance.

OBJECTIVE This study evaluates the mechanical performance of the Pinless and Centrafix fixators for rapid application to tibial fractures in a disaster or battlefield scenario. DESIGN Comparative study based on measurements made in the laboratory. BACKGROUND The Pinless and Centrafix fixators may be considered for rapid application to stabilise fractures in emergency conditions without the aid of electrical equipment such as power drills for bone screw insertion or image intensifiers to facilitate bone alignment. METHODS Stiffnesses, maximum service loads and fatigue strengths of the fixators were measured in the orientations of loading that correspond to walking and stretcher-bearing. These properties were compared with measurements on three conventional fixators, the AO, Shearer and Triax. RESULTS The Centrafix stiffnesses were 31 N/mm (axial), 1 N/ degrees (torsional shear), 0.4 N/ degrees (coronal plane bending), 4 N/ degrees (sagittal plane bending) and 11 N/mm (transverse shear) and strengths were 95 N (axial) and 1.9 Nm (bending). Corresponding Pinless stiffnesses were 43 N/mm, 0.7 N/ degrees, 0.3 N/ degrees, 8 N/ degrees and 50 N/mm, and strength was 55 N (axial). CONCLUSIONS The stiffness and strength of both rapid application fixators in simulated walking was judged to be low, and additionally the stiffness and strength of the Centrafix in simulated stretcher-bearing was judged to be low. RELEVANCE The Pinless is not recommended for weight-bearing subjects with unstable fractures. The Centrafix is not recommended for stretcher-bearing or weight-bearing with unstable fractures.

Prevention of deformity during limb lengthening

Deformity occurs frequently at the site of distraction during leg lengthening and can add to disability. The elastic and nonelastic displacements have been measured in a model that simulates leg lengthening in the laboratory. Measurements have been made for different fixator systems. The angulation in the vertical plane that occurs during leg lengthening is minimized if the distance between the bone and the fixator bar is kept as small as possible, if three screws are inserted in the proximal and distal bone fragments, and if the peak loads on the fixator are reduced by adjusting the rhythm of distraction. However, even if these precautions are taken, the results show that some fixators designed for leg lengthening will fail and lead to deformity at the osteotomy site. This may occur under the repeated cycles of high loads associated with the rises in soft tissue tension that are known to occur in certain groups of patients. This study suggests that deformity can be prevented by the proper selection of a suitable frame and the adjustment of its configuration to meet the loading requirements.