Biomedical subjects

Andrew J Rapoff

Publications and source records attributed to Andrew J Rapoff.

10 recordsLinked to original sources

PubMed2006

Application of an image-based weighted measure of skeletal bending stiffness to great ape mandibles.

Traditional measures of structural stiffness in the primate skeleton do not consider the heterogeneous material stiffness distribution of bone. This assumption of homogeneity introduces an unknown degree of error in estimating stiffness in skeletal elements. Measures of weighted stiffness can be developed by including heterogeneous grayscale variations evident in computed tomographic (CT) images. Since gray scale correlates with material stiffness, the distribution of bone quality and quantity can be simultaneously considered. We developed weighted measures of bending resistance and applied these to CT images at three locations along the mandibular corpus in the hominoids Gorilla, Pongo, and Pan. We calculated the traditional (unweighted) moment of inertia for comparison to our weighted measure, which weighs each pixel by its gray-scale value. This weighing results in assignment of reduced moment of inertia values to sections of reduced density. Our weighted and unweighted moments differ by up to 22%. These differences are not consistent among sections, however, such that they cannot be calculated by simple correction of unweighted moments. The effect of this result is that the rank ordering of individual sections within species changes if weighted moments are considered. These results suggest that the use of weighted moments may spur different interpretations of comparative data sets that rely on stiffness measures as estimates of biomechanical competence.

Animals↗

PubMed2006

Orthotropic index for bone.

An orthotropic index (OI) is proposed to indicate the existence of a preferred material direction in each of the symmetry planes of an orthotropic material such as bone. Currently, this function is performed by the anisotropy ratio (AR) of any two Young's moduli or compressive (A(c)) and shear (A(s)) anisotropy factors comprised of complicated functions of the elastic constants. The OI incorporates the four independent engineering constants (the shear modulus and Poisson's ratio in addition to the two Young's moduli) in each symmetry plane into a single index. The OI thus improves upon the AR by reflecting orthotropy in a more holistic sense and upon the AR, A(c) and A(s) by taking on a unique value (zero) only when the material is in fact isotropic.

Animals↗

PubMed2006

Change in tibial plateau angle after tibial plateau leveling osteotomy in dogs.

OBJECTIVE: To determine the change in tibial plateau angle (TPA) during healing after tibial plateau leveling osteotomy (TPLO) performed for cranial cruciate ligament insufficiency in dogs and to examine factors that may be associated with the change. STUDY DESIGN: Retrospective study. STUDY POPULATION: One hundred and forty-nine canine stifles after TPLO procedure. METHODS: Records of dogs that had TPLO were reviewed. Patient age, weight, sex, breed, pre- and postoperative TPA, recheck TPA, time to recheck, type of implant used, and radiographic evidence of healing were analyzed. RESULTS: Mean time to recheck evaluation was 46 days (range, 28-65 days). Mean difference between immediate postoperative and recheck TPA measurements was 1.5 degrees (range, -3 to 9 degrees). Recheck TPA was a significantly greater (numerically higher) than immediate postoperative TPA (P<.0001). There was no significant effect of patient weight, type of plate used, or healing status of the osteotomy at the time of recheck. No correlation between pre- or postoperative TPA angles and change in TPA angle was detected. CONCLUSIONS: TPA changes during osteotomy healing after TPLO, but factors influencing this change were not identified. CLINICAL RELEVANCE: TPA may increase during healing after TPLO despite apparently adequate osteotomy fixation. The clinical relevance of this increase is unknown but is likely minimal.

Animals↗

PubMed2005

Finite-element modeling of the anthropoid mandible: the effects of altered boundary conditions.

Finite-element modeling provides a full-field method for describing the stress environment of the skull. The utility of finite-element models, however, remains uncertain given our ignorance of whether such models validly portray states of stress and strain. For example, the effects of boundary conditions that are chosen to represent the mechanical environment in vivo are largely unknown. We conducted an in vitro strain gauge experiment on a fresh, fully dentate adult mandible of Macaca fascicularis to model a simplified loading regime by finite-element analysis for purposes of model validation. Under various conditions of material and structural complexity, we constructed dentate and edentulous models to measure the effects of changing boundary conditions (force orientation and nodal constraints) on strain values predicted at the gauge location. Our results offer a prospective assessment of the difficulties encountered when attempting to validate finite-element models from in vivo strain data. Small errors in the direction of load application produce significant changes in predicted strains. An isotropic model, although convenient, shows poor agreement with experimental strains, while a heterogeneous orthotropic model predicts strains that are more congruent with these data. Most significantly, we find that an edentulous model performs better than a dentate one in recreating the experimental strains. While this result is undoubtedly tied to our failure to model the periodontal ligament, we interpret the finding to mean that in the absence of occlusal loads, teeth within alveoli do not contribute significantly to the structural stiffness of the mandible.

Animals↗

PubMed2004

Effect of various distal ring-block configurations on the biomechanical properties of circular external skeletal fixators for use in dogs and cats.

OBJECTIVE: To evaluate mediolateral, axial, torsional, and craniocaudal bending behavior of 6 distal ring-block configurations commonly used to stabilize short juxta-articular bone segments in small animals. SAMPLE POPULATION: 8 circular external skeletal fixator constructs of each of 6 distal ring-block configurations. The distal ring-block configurations were composed of combinations of complete rings, incomplete rings, and drop wires. PROCEDURE: Constructs were nondestructively loaded in axial compression, craniocaudal bending, mediolateral bending, and torsional loading by use of a materials testing machine. Gap stiffness was determined by use of the resultant load displacement curve. RESULTS: Circular external skeletal fixator configurations and constructs significantly affected gap stiffness in all testing modes. Within each loading mode, gap stiffness was significantly different among most configurations. In general, complete ring configurations were significantly stiffer than similar incomplete ring configurations, and addition of a drop wire to a configuration significantly increased stiffness of that configuration. CONCLUSIONS AND CLINICAL RELEVANCE: When regional anatomic structures permit, the use of complete ring configurations is preferred over incomplete ring configurations. When incomplete ring configurations are used, the addition of a drop wire is recommended.

Analysis of Variance↗

PubMed2003

Load sharing in Premier and Zephir anterior cervical plates.

STUDY DESIGN: An in vitro biomechanical study using a simulated anterior cervical discectomy and interbody fusion model to compare the load sharing properties of two semiconstrained cervical (Premier and Zephir) plates. OBJECTIVES: To determine the percent load transmission through these plates and grafts under simple axial compression. SUMMARY OF BACKGROUND DATA: No published data exist as to the load transmission through these semiconstrained plates. METHODS: Cadaveric calf spines were subjected to axial compression loading while instrumented with an interbody graft and with the graft plus one of the plates. Load transmission was computed through an analysis of the load-displacement data. RESULTS: A mean load transmission of 23% was shared by the Premier plate. The Zephir, a more constrained plate but still semiconstrained, shared a mean of 32% of the load. CONCLUSIONS: The semiconstrained plates tested allow more graft loading than some previously tested constrained plates. However, there are differences between the research methods used in these studies that provide a less than satisfactory comparison.

Animals↗

PubMed2003

Interbody allograft in a skeletally immature spine model.

The objective of this cadaveric biomechanical study was to establish further bovine spines as models for evaluating lumbar interbody allografts and to provide guidance for their use in pediatric humans. It is unknown whether interbody allografts can be used in the pediatric spine without failure of the host vertebral bone. Allografts were placed in cow and calf spines and loaded in compression. The cow spines were much stronger and stiffer than the calf, but moderate in vivo activities were estimated to result in loads on the allograft constructs that would result in host bone failure. Bovine spines were established as suitable models for the compressive behavior of interbody allografts in the human spine, when bone density is considered. Interbody allografts should continue to be used with adjunctive instrumentation so as to preclude host bone failure.

Adult↗

PubMed2003

Understanding stress concentration about a nutrient foramen.

We investigated the microstructural basis of a reduced stress concentration around the primary nutrient foramen of the equine third metacarpus. We quantified the spatial variations of compositional parameters (mineral content, volume fraction, histological architecture, and osteonal trajectories) from microradiographs and polarizing microscopic images of thin sections. These variations in composition and organization in turn cause variations in mechanical properties of cortical bone. We modeled the spatially inhomogeneous anisotropic elastic properties based on the measured compositional parameters and used the properties as inputs to a finite element model of the bone containing the foramen. This model, spatially constructed solely from the microscopic images, was subsequently validated by our mechanical test results. We found that: (1) a primary mechanism for stress concentration reduction appears to be due to an increased compliance near the foramen: the sharp discontinuity represented by the hole is softened by embedding it in a compliant region; (2) a reinforcing ring of increased stiffness exists at some distance from the foramen; and (3) a ring of lamellar bone exists along the foramen inside edge, which might serve to reduce the chance of cracks forming there. Our work is allowing us to design biomimetic structures with holes by mimicking the microstructure near the nutrient foramen.

Animals↗

PubMedDate not supplied

A comparison of the accuracy and safety of vertebral body pin placement using a fluoroscopically guided versus an open surgical approach: an in vitro study.

OBJECTIVE: To compare the safety and accuracy of Steinmann pin placement in vertebral bodies T10 through L7 using either an open or closed fluoroscopic method. STUDY DESIGN: In vitro radiographic and anatomic study. ANIMALS: Ten medium-sized canine cadavers. METHODS: Cadavers were randomly assigned to 2 groups: open and closed. Steinmann pins were placed in vertebral bodies through a standard dorsal incision in the open group and percutaneously with the aid of fluoroscopy in the closed group. Pins were placed bilaterally in vertebral bodies T10 through L7 at approximately 30 degrees from horizontal and driven to a uniform depth. Necropsies were performed to examine potential pulmonary, vascular, or neurological trauma as a result of pin placement. Spines were cross-sectioned through intervertebral disc spaces, and radiographs were performed to evaluate accuracy of pin placement. Descriptive statistics were determined for pin angle, percentage of bone purchase, and penetration length. Means of interest between groups were compared using a Student t test. Complication incidence was compared using Chi;(2) analysis. Significance was P <.05. RESULTS: Mean pin insertion angle was significantly different than 30 degrees for the open group in thoracic and lumbar vertebrae and for the closed group in thoracic vertebrae. Mean pin insertion angle for all vertebrae was significantly greater than 30 degrees for the open group. Mean pin penetration distance in each vertebra was significantly different between groups with the closed group having less penetration and lower variance. Both groups were significantly different from the ideal penetration distance. The mean percentage of bone purchase was greater in the closed group for all vertebrae except T10 and T11. The complication incidence was significantly greater in the open group for thoracic vertebrae. CONCLUSION AND CLINICAL RELEVANCE: The results of this study suggest that a closed technique for placement of Steinmann pins in lumbar vertebrae for use in external skeletal fixation is a reasonable and safer alternative to the traditional open technique. Use of either technique in thoracic vertebrae should be avoided.

Animals↗

PubMedDate not supplied

Biomechanical comparison of a circular external skeletal fixator construct to pin and tension band wire fixation for the stabilization of olecranon osteotomies in dogs: a cadaveric study.

OBJECTIVE: To compare anatomic reduction and the biomechanical properties of a circular external skeletal fixator (CESF) construct to pin and tension band wire (PTBW) fixation for the stabilization of olecranon osteotomies in dogs. STUDY DESIGN: Cadaveric study. ANIMALS: Forelimbs from 12 skeletally mature mixed-breed dogs, weighing 23 to 28 kg. METHODS: An olecranon osteotomy was stabilized with either a CESF construct or PTBW fixation. A single distractive load to failure was applied to each specimen through the triceps tendon. Osteotomy reduction and biomechanical properties were compared between fixation groups. RESULTS: Reduction was not significantly different (gap: P =.171; malalignment: P =.558) between fixation groups. Osteotomies stabilized with the CESF had greater stiffness (P <.0001) and maximum load sustained (P <.0001) compared to PTBW fixation. There was no significant difference for yield load (P =.318) or for load at 1 mm of axial displacement (P =.997) between fixation groups. Failure of fixation occurred by bending of the intramedullary Steinmann pin and the fixation wires in the CESF specimens and by untwisting of the tension band wire knot with pullout and bending of the Kirschner wires in the PTBW specimens. CONCLUSIONS: Specimens stabilized with the CESF construct had similar reduction and yield load, greater stiffness and maximum load sustained, and less elastic deformation than specimens stabilized with PTBW fixation. CLINICAL RELEVANCE: The CESF construct may provide a biomechanically favorable alternative to PTBW fixation for stabilization of olecranon osteotomies in dogs, and its application warrants clinical investigation.

Animals↗