Andrew J. Rapoff

Cervical interbody fusion cages. An animal model with and without bone morphogenetic protein

Study Design. The Alpine goat model for multilevel anterior cervical discectomy and fusion was used to analyze the use of an intervertebral fusion device to promote an arthrodesis after anterior cervical discectomy. Comparisons were drawn with biomechanical, histologic, and radiographic data. Objectives. To analyze the use of an intervertebral fusion device, with and without a bone graft substitute, to promote an arthrodesis after anterior cervical discectomy. Summary of Background Data. In previous studies, the goat cervical spine has proven to be an excellent model for examining the healing of fusions using bone grafts, instrumentation, or bone substitutes. Methods. Three‐level anterior cervical discectomies were performed on 21 mature Alpine goats. Three treatment groups of seven goats each were used. Group I used a standard titanium cervical BAK device filled with autogenous bone graft. Group II used a hydroxyapatite‐coated BAK device filled with autogenous bone graft. Group III used a BAK device filled with recombinant human bone morphogenetic protein‐2. Results. Radiographically, no cages became displaced. Lucencies were seen around 3 of the 21 cages in Group I, 4 cages in Group II, and none in Group III. Fluorochrome analysis revealed that the recombinant human bone morphogenetic protein‐2‐filled cages had an accelerated rate of bone growth around and through each cage‐vertebral body interface at 3 weeks. A successful arthrodesis was also more likely with a recombinant human bone morphogenetic protein‐2‐filled cage (95%) than the hydroxyapatite‐coated (62%) or the standard (48%) cage. Biomechanical stiffness testing did not reveal any statistically significant differences between the three groups. There was a tendency for successfully arthrodesed interspaces to be stiffer than those that were not. Conclusions. The use of a threaded intervertebral fusion cage, with or without hydroxyapatite coating, filled with autogenous bone graft provides a fusion rate that is slightly better than those previously reported using autogenous interbody bone grafts with or without plate stabilization. Recombinant human bone morphogenetic protein‐2‐filled cages resulted in a much higher arthrodesis rate and accelerated bone formation compared with either autogenous bone‐filled BAK devices, or autogenous interbody bone grafts with or without plate stabilization.

Biomechanical comparison of posterior lumbar interbody fusion cages.

Study Design. Cadaveric human and bovine lumbar spine models simulating the acute postoperative period were used to compare the biomechanical properties of two designs of intervertebral body threaded fusion cages. The instrumented spines were compared with intact spines and with spines with resected posterior elements, representing a revision case. Objective. To determine the relative biomechanical performance of these competing devices. Summary of Background Data. These cages are currently under clinical investigation, and basic biomechanical data are needed. Methods. Insertion torques and maximum pushout loads were measured for each cage. Intact spines, posteriorly instrumented spines (posterior lumbar interbody fusion), and spines with resected posterior elements were loaded in axial compression, flexion and extension bending, and axial torsion. Stiffness comparisons were made between the different configurations. Results. Insertion torques and pushout loads were similar for the cages. Both cages significantly increased stiffnesses above those of the intact spines and the resected spines. The BAK‐instrumented spines were more stiff in axial compression, while the Threaded Interbody Fusion Device spines were more stiff in extension. Conclusions. This study revealed the two cages to have similar biomechanical characteristics immediately after posterior insertion and warrant further clinical studies.

Biomechanical comparison of spondylolysis fixation techniques

STUDY DESIGN A load-controlled biomechanical analysis of flexion, extension, and torsional stiffness in instrumented calf spines. OBJECTIVES To compare biomechanically the performance of various fixation techniques for the repair of spondylolytic defects in the pars interarticularis. SUMMARY OF BACKGROUND DATA Several techniques have been developed to stabilize a spondylolytic defect in the lumbar spine. There are, however, no comprehensive biomechanical studies in which these techniques are compared. METHODS Nine fresh-frozen and thawed calf cadaveric lumbar L2-L6 spines were used for mechanical testing. Scotts technique, Bucks technique (screw fixation in the lamina across the defects), modified Scotts technique (wire loops around cortical screws placed into both pedicles and tightened under the spinous process), and screw-rod-hook fixation were applied on the calf lumbar spines in which bilateral spondylolytic defects were created in the L4 vertebra. Motion across the defects for each direction of loading in flexion, extension, and rotation was measured using extensometers. The intervertebral rotations and the strain at the site of the spondylolytic defect were computed from the acquired load-displacement data. RESULTS Each fixation technique significantly increased stiffness and returned the intervertebral rotational stiffness to nearly intact levels. Displacement across the defect under flexion loading was significantly suppressed by each instrumentation technique, but the least motion (P < 0.05) was allowed with the screw-rod-hook fixation or Bucks technique. CONCLUSIONS All four fixation techniques restored the intervertebral rotational displacements under flexion and torsional loading to the intact condition. The screw-rod-hook fixation allowed the least amount of motion across the defect during flexion.

Biomechanical evaluation of occipitocervical fixation devices.

Human cadaveric occipitocervical specimens were implanted with three types of instrumentation. The devices were tested biomechanically under three modes of loading to determine the stiffness of spinal constructs and the failure mechanisms of the constructs under extreme flexion. The devices tested were the AXIS Fixation System (with custom plate), the Y-Plate, and the Luque rectangle. No significant differences in stiffness among the devices were found under compression and flexion. The stiffnesses of the plate systems were statistically higher than the Luque rectangle in extension and torsion. In extreme flexion, the plate systems failed by fracture of the C2 pedicles. Modern plate systems, for occipitocervical fixation, provide more stiffness and stability than traditional wiring techniques. This study provides surgeons with information on the relative merits of modern plate and screw systems compared with traditional rod and wire constructs.

Anterior cervical graft and plate load sharing.

Anterior discectomy and fusion with an interbody bone graft and anterior plate is a common procedure in cervical spine surgical management. However, the graft may be shielded from some mechanical loading by the plate. Mechanical testing was performed on six cadaveric calf spines that were subjected to a simulated anterior cervical discectomy and fusion with an interbody bone graft alone and with an anterior plate to determine the amount of load sharing between the graft and plate. The load-displacement data were used to compute the amount of load sharing between the graft and the plate as a continuous function of the applied axial compression load. Although the percent load transmitted through the graft decreased (53 to 41%) as the axial load increased (45 to 90 N), the magnitude of load transmitted through the graft increased (24 to 37 N), with corresponding intervertebral strains <6%. In a single-level procedure, an anterior cervical plate serves as a load-sharing device rather than a load-shielding device, enabling graft consolidation as observed in clinical studies.

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.

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.

Material property variation of mandibular symphyseal bone in colobine monkeys

The anterior mandibular corpus of anthropoid primates is routinely subjected to masticatory loads that result in relatively high local levels of stress and strain. While structural morphological responses to these loads have been extensively explored, relatively little is known about material property variation in mandibular bone of nonhuman primates. Consequently, the role of regional and local variation in bone stiffness in conditioning stress and strain gradients is poorly understood. We sampled elastic modulus variation in the bone of the anterior mandibular corpus in two species (N = 3 each) of sympatric colobine monkeys, Procolobus badius and Colobus polykomos. These monkeys were chosen for comparison owing to their distinctive dietary regimens, as P. badius rarely includes hard objects in its diet while C. polykomos habitually processes obdurate items during feeding. Elastic modulus is determined through bone hardness data obtained via microindentation, which enables the description of stiffness variation on sub‐millimeter scales. Labial bone stiffness exceeds that of lingual bone in the sample overall. Female mandibular bone is generally stiffer than that found in males, and overall Procolobus mandibular bone is stiffer than that in Colobus. These results, interpreted collectively, suggest that the material response to elevated masticatory stress is increased compliance of the affected bone. J. Morphol., 2009.

Reduced stiffness of alveolar bone in the colobine mandible

Alveolar bone has several mechanical functions, including tooth support and accommodation of occlusal and other masticatory forces. Its unique functional-mechanical environment is reflected by its structural characteristics, but whether alveolar bone is materially distinct from bone elsewhere in the primate facial skeleton is uncertain. This uncertainty is attributable not only to a limited amount of data but also to conflicting findings among these data. We evaluated elastic modulus variation in the mandibular corpus of eight adult specimens of the monkeys Procolobus badius and Colobus polykomos via microindentation to evaluate whether alveolar bone is more compliant than basal bone and to quantify patterns of variation between sexes and species. We sampled Vickers hardness from six serial transverse sections and one coronal section from both the alveolar process and the basal corpus. Hardness values were converted to elastic modulus via regressions specific for bone tissue. Analysis of variance reveals that a plurality of variation is found on a regional scale; i.e., alveolar bone is more compliant than adjacent basal bone. Species affiliation and sex are not significant sources of variation. These findings support a hypothesis that compliance of alveolar bone represents a material solution for avoiding large stress concentrations arising from occlusal loads. Other comparative data suggest important differences between colobine and cercopithecine mandibles in terms of bone stiffness, both overall and in terms of relative stiffness of alveolar and basal cortical bone.

Avulsion Fracture of the Great Toe: A Case Report

A case involving an isolated bony avulsion fracture of the extensor insertion on the distal phalanx of the great toe is described. The fracture was displaced 2 mm with 30° dorsal angulation; the joint was congruent and not subluxed. The patient was treated nonsurgically with a rigid-soled sandal. Bony healing at the fracture site was clearly evident at 16 weeks postinjury. The patient began using a normal shoe at 10 weeks and resumed his running activities without pain at 16 weeks. The nonsurgical treatment of this injury, similar to that of a mallet finger, was successful.