Duane A. Robinson

In vitro antibacterial properties of magnesium metal against Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus

Bacterial infections are a costly sequela in any wound. The corrosion properties of 0.15, 0.30, 0.45 and 0.60 g of Mg metal were determined in Mueller-Hinton broth by serially measuring the Mg(2+) concentrations and pH over 72 h. In addition, the effect of Mg metal, increased Mg(2+) concentration and alkaline pH on the in vitro growth of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus were evaluated in three separate experiments. The primary outcome measure for culture studies was colony-forming units/ml compared to appropriate positive and/or negative controls. Regardless of the mass of Mg added, there was a predictable increase in pH and Mg(2+) concentration. The addition of Mg and an increase of pH resulted in antibacterial effects similar to the fluoroquinolone antibiotic; however, a simple increase in Mg(2+) concentration alone had no effect. The results demonstrate an antibacterial effect of Mg on three common aerobic bacterial organisms, the mechanism of which appears to be an alkaline pH.

Relationship between objective and subjective assessment of limb function in normal dogs with an experimentally induced lameness

OBJECTIVE To evaluate the relationship between previously used subjective and objective measures of limb function in normal dogs that had an induced lameness. STUDY DESIGN Prospective, blinded, and induced animal model trial. ANIMALS Normal, adult, and mixed-breed dogs (n=24) weighing 25-35 kg. METHODS Force platform gait analysis was collected in all dogs before and after induction of lameness. All gait trials were videotaped; 60 video trials were evaluated by 3 surgeons with practice limited to small animal orthopedics and 3 first year veterinary students in an effort to establish the relationship between subjective and objective measures of lameness. Evaluators were unaware of the force platform data. RESULTS Concordance coefficients were low for all observers and were similar between students and surgeons. These values were further decreased when normal and non-weight bearing trials were removed. Agreement with the force platform data was low even when observers only had to be within +/-10% of the ground reaction forces. When repeat trials were evaluated surgeons had a much higher repeatability compared with students. CONCLUSIONS Subjective evaluation of the lameness in this study varied greatly between observers and agreed poorly with objective measures of limb function. CLINICAL RELEVANCE Subjective evaluation of gait should be interpreted cautiously as an outcome measure whether performed from a single or from multiple observers.

Hip Joint Contact Force in the Emu (Dromaius novaehollandiae) during Normal Level Walking

The emu is a large, (bipedal) flightless bird that potentially can be used to study various orthopaedic disorders in which load protection of the experimental limb is a limitation of quadrupedal models. An anatomy-based analysis of normal emu walking gait was undertaken to determine hip contact forces for comparison with human data. Kinematic and kinetic data captured for two laboratory-habituated emus were used to drive the model. Muscle attachment data were obtained by dissection, and bony geometries were obtained by CT scan. Inverse dynamics calculations at all major lower-limb joints were used in conjunction with optimization of muscle forces to determine hip contact forces. Like human walking gait, emu ground reaction forces showed a bimodal distribution over the course of the stance phase. Two-bird averaged maximum hip contact force was approximately 5.5 times body weight, directed nominally axially along the femur. This value is only modestly larger than optimization-based hip contact forces reported in literature for humans. The interspecies similarity in hip contact forces makes the emu a biomechanically attractive animal in which to model loading-dependent human orthopaedic hip disorders.

The apparent critical isotherm for cryoinsult-induced osteonecrotic lesions in emu femoral heads.

Cryoinsult-induced osteonecrosis (ON) in the emu femoral head provides a unique opportunity to systematically explore the pathogenesis of ON in an animal model that progresses to human-like femoral head collapse. Among the various characteristics of cryoinsult, the maximally cold temperature attained is one plausible determinant of tissue necrosis. To identify the critical isotherm required to induce development of ON in the cancellous bone of the emu femoral head, a thermal finite element (FE) model of intraoperative cryoinsults was developed. Thermal material property values of emu cancellous bone were estimated from FE simulations of cryoinsult to emu cadaver femora, by varying model properties until the FE-generated temperatures matched corresponding thermocouple measurements. The resulting FE model, with emu bone-specific thermal properties augmented to include blood flow effects, was then used to study intraoperatively performed in vivo cryoinsults. Comparisons of minimum temperatures attained at FE nodes corresponding to the three-dimensional histologically apparent boundary of the region of ON were made for six experimental cryoinsults. Series-wide, a critical isotherm of 3.5 degrees C best corresponded to the boundary of the osteonecrotic lesions.

Development of a fracture osteomyelitis model in the rat femur.

Osteomyelitis contributes significantly to fracture morbidity. Our objective was to develop a model of induced implant‐associated osteomyelitis following fracture repair by modifying an existing rat femur fracture model. Thirty male Sprague–Dawley rats were divided into three groups (Control, Staphylococcus aureus, S. aureus + ceftriaxone). The closed femur fracture model (right femur), stabilized with an intramedullary pin, was combined with inoculation of 104 colony‐forming units (CFU) of S. aureus. Radiographs were obtained immediately after surgery and at weeks 1, 2, and 3 and were evaluated by individuals blinded to treatment group. At necropsy the CFU of S. aureus per femur and pin were determined and synovial tissue and blood were cultured. The fractured femur from two rats in each group was evaluated histologically. A statistically significant difference in the CFU/femur and CFU/pin was found across treatment groups, with the highest CFU in the S. aureus group and the lowest in the Control group. Cultures of synovial tissue were positive in 11/19 of inoculated limbs. Osteomyelitis was present both radiographically and histopathologically in both S. aureus groups but not in the controls. No rats were systemically ill or had positive blood cultures at the study endpoint. This model will be useful for the evaluation of treatments or prophylactics designed for use in implant‐associated osteomyelitis.

STEROID-INDUCED VERSUS CRYOINSULT-INDUCED FEMORAL HEAD OSTEONECROSIS: STATISTICAL MEASUREMENT OF HISTOLOGIC ABNORMALITY FOCALIZATION

Systematic studies of treatment options for femoral head osteonecrosis have been hindered by the lack of an animal model that mimics the human event of femoral head collapse. The (bipedal) emu model of osteonecrosis has proven to progress to collapse following an external insult. Since osteonecrosis clinically has multiple etiologies, various alternatives are seemingly appropriate to initiate lesions for research purposes. This study compared osteonecrosis initiated by a local cryoinsult, to that initiated by a large systemic dose of steroid (methylprednisolone acetate), and by a combination of steroid and cryoinsult. One year post-insult, the animals were sacrificed and their femoral heads harvested. A custom-written computer program was used to expedite analysis of the global distribution of histologically apparent osteonecrosis, as determined by the Ficat scale. Following whole-head analysis, grade distributions were calculated, and a measure of spatial clustering of histologic abnormality was calculated using Morans I statistic. Normal femoral heads showed very small amounts of histological abnormality, and no significant spatial clustering. Femoral heads receiving isolated cryoinsult had a greater incidence of high-grade abnormality, as well as statistically significant spatial clustering of that abnormality. Steroid-only animals had a higher percentage of diffuse, low-grade abnormality. The group receiving both steroid and cryoinsult showed the highest overall abnormality, with statistically significant clustering of that high-grade abnormality. Cryoinsult, both in isolation and in combination with a steroid regimen, appeared necessary to mimic the clinically typical focal lesions of osteonecrosis.

Cryoinsult parameter effects on the histologically apparent volume of experimentally induced osteonecrotic lesions

Investigation of femoral head osteonecrosis would benefit from an animal model whose natural history includes progression to bony collapse of a segmental necrotic lesion. The bipedal emu holds attraction for systematic organ‐level study of collapse mechanopathology. One established method of experimentally inducing segmental lesions is liquid nitrogen cryoinsult. Four cryoinsult parameters—hold temperature, freeze duration, freeze/thaw repetition, and thaw duration—were investigated to determine their individual and combined effects on resulting necrotic lesion morphology. 3D distributions of histologically apparent osteocyte necrosis from 24 emus receiving varying cryoinsults were used to develop univariate and multivariate linear regression models relating resulting necrotic lesion morphology to particular cryoinsult input parameters. These models were then applied to predict lesion size in four additional emus receiving differing input cryoinsults. The best multivariate regression model predicted lesion volumes that were accurate to better than 8% of overall emu femoral head volume. The hold temperature during cryoinsult was by far the most influential cryoinsult input parameter. The utility of this information is to enhance the consistency and predictability of cryoinsult‐induced segmental lesion size for the purposes of systematic laboratory studies at the whole‐organ level.

Torsional and axial compressive properties of tibiotarsal bones of red-tailed hawks (Buteo jamaicensis)

OBJECTIVE To describe the torsional and axial compressive properties of tibiotarsal bones of red-tailed hawks (Buteo jamaicensis). SAMPLE 16 cadaveric tibiotarsal bones from 8 red-tailed hawks. PROCEDURES 1 tibiotarsal bone from each bird was randomly assigned to be tested in torsion, and the contralateral bone was tested in axial compression. Intact bones were monotonically loaded in either torsion (n = 8) or axial compression (8) to failure. Mechanical variables were derived from load-deformation curves. Fracture configurations were described. Effects of sex, limb side, and bone dimensions on mechanical properties were assessed with a mixed-model ANOVA. Correlations between equivalent torsional and compressive properties were determined. RESULTS Limb side and bone dimensions were not associated with any mechanical property. During compression tests, mean ultimate cumulative energy and postyield energy for female bones were significantly greater than those for male bones. All 8 bones developed a spiral diaphyseal fracture and a metaphyseal fissure or fracture during torsional tests. During compression tests, all bones developed a crushed metaphysis and a fissure or comminuted fracture of the diaphysis. Positive correlations were apparent between most yield and ultimate torsional and compressive properties. CONCLUSIONS AND CLINICAL RELEVANCE The torsional and axial compressive properties of tibiotarsal bones described in this study can be used as a reference for investigations into fixation methods for tibiotarsal fractures in red-tailed hawks. Although the comminuted and spiral diaphyseal fractures induced in this study were consistent with those observed in clinical practice, the metaphyseal disruption observed was not and warrants further research.