David M. Nunamaker

Force measurements in screw fixation

Abstract An entire range of screw sizes and types from one manufacturer (Synthes, Switzerland) along with two experimental sizes were tested in bovine cancellous bone to measure torque of insertion, screwdriver pressure, axial compression generated by the screw, screw breaking strength and efficacy of various head types. The results showed a gradual increase in strength as diameter increased. Maximal axial compression peaked at 5.0 mm. The hexagonal shaped imbus head proved superior to the Phillips type.

Some geometric properties of the third metacarpal bone: A comparison between the thoroughbred and standardbred racehorse

Geometric properties of the third metacarpal bone were compared between the young and adult Standardbred and Thoroughbred racehorse. The change in shape during growth and superimposed training was dramatic in both breeds but the Thoroughbred showed the greatest difference in the minimum moment of inertia as the animal matured. Males had larger moments of inertia throughout the length of the diaphysis than did females. The differences in geometric properties of the third metacarpal bone between the Thoroughbred and Standardbred were related to the incidence of fatigue fractures which are common in the racing Thoroughbred but uncommon in the Standardbred racehorse.

Trabecular bone remodeling around smooth and porous implants in an equine patellar model

The objective of this investigation was to examine the stress-morphology relationships for trabecular bone around implants with different surface characteristics. Stainless steel spheres with either a polished surface or a sintered-bead porous coating were implanted unilaterally into equine patellae and maintained for a 6 month period. Stereological methods were used to quantify the trabecular bone morphology and finite element analyses were performed to predict the trabecular bone stresses. In general, the remodeling response around the smooth implants was greater than that around those porous implants that exhibited bone ingrowth. In accordance with these differences, the finite element models predicted greater changes in the stresses adjacent to the smooth implants due to the nonlinear boundary conditions. However, it did not appear that the trajectorial theory, in its simplest form, was applicable to the remodeling induced by the implants. A linear relationship between the change in bone areal density and the change in von Mises effective stress provides support for the hypothesis that the architecture of trabecular bone corresponds to an optimal structure. The results also demonstrated that, under certain circumstances, small changes in the stress state may result in large changes in the principal material orientation.

The horse-racetrack interface: a preliminary study on the effect of shoeing on impact trauma using a novel wireless data acquisition system

REASONS FOR PERFORMING STUDY There is a need to determine accelerations acting on the equine hoof under field conditions in order to better assess the risks for orthopaedic health associated with shoeing practices and/or surface conditions. OBJECTIVES To measure the acceleration profiles generated in Thoroughbred racehorses exercising at high speeds over dirt racetracks and specifically to evaluate the effect of a toe grab shoe compared to a flat racing plate, using a newly developed wireless data acquisition system (WDAS). METHODS Four Thoroughbred racehorses in training and racing were used. Based on previous trials, each horse served as its own control for speed trials, with shoe type as variable. Horses were evaluated at speeds ranging from 12.0-17.3 m/sec. Impact accelerations, acceleration on break over and take-off, and temporal stride parameters were calculated. Impact injury scores were also determined, using peak accelerations and the time over which they occurred. RESULTS Recorded accelerations for the resultant vector (all horses all speeds) calculated from triaxial accelerometers ranged 96.3-251.1 g, depending on the phase of the impact event. An association was observed between shoe type and change in acceleration in individual horses, with 2 horses having increased g on initial impact with toe grab shoes in place. In the final impact phase, one horse had an increase of 110 g while wearing toe grab shoes. Increased accelerations were also observed on break over in 2 horses while wearing toe grab shoes. CONCLUSIONS Shoe type may change impact accelerations significantly in an individual horse and could represent increased risk for injury. Further work is needed to determine if trends exist across a population. POTENTIAL RELEVANCE The WDAS could be used for performance evaluation in individual horses to evaluate any component of the horse-performance surface interface, with the goal of minimising risk and optimising performance.

A novel wireless data acquisition system for the measurement of hoof accelerations in the exercising horse.

REASONS FOR PERFORMING STUDY A device is needed to safely and wirelessly evaluate accelerations experienced by the horse hoof under a variety of surface conditions with the horse exercising at training or racing speeds. OBJECTIVES To develop a miniaturised wireless data acquisition system (WDAS) which reliably records hoof accelerations and the times over which they occur in a minimally invasive manner in the exercising Thoroughbred. METHODS The following criteria were set for device development: production of a lightweight and minimally invasive system, which provides an adequate acceleration range, appropriate frequency response to capture high speed events, and compatibility with a low power wireless telemetry system. Following device development, the WDAS was calibrated, and tested in 6 Thoroughbred horses over a variety of surfaces. RESULTS Collection of acceleration in seven trials using 6 horses over a variety of surfaces resulted in repeatable acceleration data with respect to the overall characteristic shape of the impact profile. Impact accelerations varied with surface, ranging 34.8-191.7 g. Accelerations on take off were in a similar range, although higher in some trials. Peak impact accelerations tended to larger over the grass paddock surface, than either the indoor arena or the dirt track. During dirt track trials, accelerations on take-off were often comparably larger than those observed on impact within the same footfall. CONCLUSIONS This study reports the development of a wireless system that successfully measures hoof acceleration in a minimally invasive manner over a variety of surface and exercise conditions. POTENTIAL RELEVANCE The WDAS will be used in further studies to evaluate various components of the horse-racetrack interface, in an attempt to identify risk factors for musculoskeletal injury in the Thoroughbred racehorse.

A Tapered-Sleeve Transcortical Pin External Skeletal Fixation Device for Use in Horses: Development, Application, and Experience

OBJECTIVE To report development, configuration, application, and results of a tapered-sleeve transcortical pin external skeletal fixation device (TSP ESFD) for use on fractures of the distal aspect of the equine limb. STUDY DESIGN Optimization analysis of a TSP ESFD was carried out with mechanical testing and review of clinical case outcomes. ANIMALS Cadaveric adult third metacarpal bones (MC3) for mechanical testing; horses (n=7) with severely comminuted proximal (6; P1) or second (1) phalanx fractures. METHODS Mechanical testing of methods for attachment of TSP to the sidebars were tested as well as optimization of pin diameters. Outcome of clinical cases managed with the TSP ESFD were compared with outcomes of horses treated with previous ESFD. RESULTS A TSP ESFD using 7.94 mm diameter pins was used. Survival rate was 71%. One horse with an infected P1 fracture and contralateral laminitis subsequently fractured MC3 through the distal pin hole while wearing the TSP ESFD. One mare with contralateral laminitis was euthanatized. Three horses had open fractures. CONCLUSIONS Design improvements incorporated into the TSP ESFD should increase treatment success. CLINICAL RELEVANCE External skeletal fixation continues to be a viable treatment alternative for severe injuries of the distal aspect of the equine limb. TSP ESFD should improve on the success of treatment by decreasing complications of bone fracture at the pin interface.

In vitro comparison of a novel external fixator and traditional full-limb transfixation pin cast in horses.

OBJECTIVE To compare the mechanical properties and failure modes of a standardized short oblique distal radial metaphyseal osteotomy stabilized using either a transfixation pin cast (TPC), a modular-sidebar external skeletal fixator (ESF), or a solid-sidebar ESF (modular- or solid-ESF, respectively) using static or cyclic axial loading to failure. STUDY DESIGN In vitro study. ANIMALS Equine cadaver forelimbs. METHODS A 30 degrees oblique distal radial osteotomy was created and stabilized using 1 of the 3 fixation methods: (1) TPC, (2) modular-ESF, or (3) solid-ESF. Limbs were tested using static (TPC, modular-ESF, and solid-ESF) or cyclic (TPC and solid-ESF) axial loading to failure. The stiffness, yield load, yield displacement, failure load, and failure displacement for static loading and the cycles to failure for cyclic loading at 75% failure load were obtained. Data were analyzed using a Kruskal-Wallis test. Level of significance was P<.05. RESULTS The solid-ESF had a greater stiffness, higher yield and failure load and a lower yield and failure displacement than the TPC (P=.01) and the modular-ESF (P=.02). TPC had a higher yield load, failure load, and yield displacement than the modular-ESF (P=.01). Mean cycles to failure for TPC was 2996+/-657 at a load of 16,000 N and for solid-ESF 6560+/-90 cycles at a load of 25,000 N. CONCLUSIONS The solid-ESF was stiffer and stronger than the TPC and modular-ESF and failed at a greater number of cycles in axial loading compared with the TPC. CLINICAL RELEVANCE This study is an initial step in evaluating the solid-ESF. Further testing needs to be performed, but this fixation may offer a viable alternative to the traditional TPC for stabilization of long bone fractures in adult horses.