Craig A. Bourgeault

Recombinant Human Osteogenic Protein-1 Induces Bone Formation in a Chronically Infected, Internally Stabilized Segmental Defect in the Rat Femur

BACKGROUND Recombinant human osteogenic protein-1 (rhOP-1), combined with a collagen carrier, has been shown to induce new-bone formation in a variety of animal models. The purpose of the present investigation was to test the hypotheses that rhOP-1 would accelerate bone formation in an internally stabilized, chronically infected, critical-size defect in the rat femur and that this effect would be enhanced by the administration of systemic antibiotic. METHODS A 6-mm segmental defect was created surgically, stabilized with a polyacetyl plate and six Kirschner wires, and contaminated with 10(4) colony-forming units of Staphylococcus aureus in one femur in each of 168 Sprague-Dawley rats. After two weeks, these infected defects were débrided surgically and were assigned to one of six treatment groups. The defects in the thirty animals in the first group received lyophilized collagen carrier mixed with 200 microg of rhOP-1 dissolved in buffer, the defects in the thirty animals in the second group received carrier with 20 microg of rhOP-1 in buffer, and the defects in the twenty-four control animals in the third group received carrier mixed with buffer without rhOP-1. The last three groups were treated identically to the first three groups, except that the animals also received the antibiotic ceftriaxone for twenty-eight days after débridement. The animals were killed at two, four, eight, or twelve weeks after débridement. Newly mineralized callus within the defect, and adjacent to and bridging the outside of the defect, was assessed with use of quantitative high-resolution radiography, microcomputed tomography, torsional failure testing, and histological analysis of undecalcified sections. RESULTS Bacterial cultures confirmed the presence of a chronic infection during the study period in all animals. At the later time-points, significantly more newly mineralized callus was present within and adjacent to the débrided defects that had been treated with 200 microg of rhOP-1, whereas minimal amounts of callus were present within and adjacent to the defects that had been treated without rhOP-1 and with 20 microg of rhOP-1. At eight and twelve weeks after débridement, there was significantly more newly mineralized callus in the group that had been treated with 200 microg of rhOP-1 with antibiotic than in the group that had been treated with 200 microg of rhOP-1 without antibiotic (p < 0.05). At twelve weeks, the values for torque, energy to failure, and linear stiffness for femora that had been treated with 200 microg of rhOP-1 with antibiotic were not significantly different from the values for intact, contralateral control femora, whereas the values for femora that had been treated with 200 microg of rhOP-1 without antibiotic remained significantly lower than those for the intact, contralateral controls (p < 0.05). CONCLUSIONS Recombinant human osteogenic protein-1 maintained its osteoinductive capability in the presence of chronic infection, and this property was enhanced by antibiotic therapy. No substantial callus formed in the infected defects without a sufficiently high dose of rhOP-1. CLINICAL RELEVANCE The treatment of an infection at the site of a fracture often necessitates removal of internal fixation. However, internal fixation is needed for fracture stability. This study presents an intervention that may accelerate fracture-healing in the presence of infection and colonized hardware, thereby permitting earlier removal of the hardware and more timely and effective treatment of the infection.

Screw Position Affects Dynamic Compression Plate Strain in an In Vitro Fracture Model

Objective This investigation considers the effect of a variety of screw positions on plate strain in three fracture models. Design Dynamic compression plate fixation of in vitro fracture models. Methods To model a fracture, a plastic pipe was cut transversely and a twenty-hole dynamic compression plate was attached by screws. Eighteen stacked, rectangular, rosette strain gauges were installed on the plate to evaluate strain. Three models were evaluated: two constructs in which there was no contact between the cut ends of the pipe under the fixation plate (small-and large-gap models) and a construct in which there was direct apposition of the cut ends (no-gap model). The pattern and magnitude of strains were assessed as a function of varying combinations of screw position for each model. Results Maximal plate strain in the gap models was lowest with screws placed closest to the gap, compared with screws placed away from the gap or spaced apart. The no-gap model showed significantly lower strains when screws were placed further from the osteotomy site than when screws were positioned close together or spaced apart. In all cases, maximal plate strain occurred adjacent to the most central screw holes and rapidly dissipated along the length of the plate. Conclusion In a model simulating a comminuted fracture (gap), this study found that screws should be placed as close to the fracture site as possible to minimize plate strain. In an anatomically reduced two-part fracture model (no gap), widely spaced screws or those placed away from the fracture resulted in lower strains.

In vitro stability of cemented and cementless femoral stems with compaction.

Rigidity of initial fixation is a key factor contributing to the longevity of cemented and cementless femoral components in total hip arthroplasty. The objective of this study was to measure the initial stability of primary cemented and cementless femoral components under load when 15 pairs of cadaveric femurs were prepared by outward compaction of femoral cancellous bone in situ or by conventional extraction broaching. Three-dimensional micromotion was measured at proximal and distal locations on the femoral components using a device with spherical targets and linear variable differential transformers. External loads simulating the stance phases of level walking and stair ascent were applied to the femoral components by a materials test machine. Bone preparation method significantly affected each of the translation and rotation components of micromotion with cemented and cementless fixation. Micromotion with broaching was consistently greater than with compaction. Compared with compaction, the magnitude of the micromotion translation vector for broaching was an average of 3.9 (standard deviation, 3.1) times greater with cemented fixation, and an average of 2.3 (standard deviation, 1.4) times greater with cementless fixation. The results of this study showed the effectiveness of compaction of femoral cancellous bone in improving the initial stability of cemented and cementless femoral components in primary total hip arthroplasty.

Femoral fracture risk in hip arthroplasty: smooth versus toothed instruments.

Compaction of cancellous bone with smooth tamps in total hip arthroplasty has been shown to improve initial implant fixation. It is not known, however, whether this improved fixation occurs at the expense of an increased risk of intraoperative femoral fracture. The current authors explore this issue by comparing the risk of fracture in 10 pairs of femurs prepared with either smooth tamps or conventional toothed broaches. Using one pass for each size, smooth tamps were advanced incrementally into one femur of each pair and toothed broaches were advanced incrementally into the contralateral femur. A controlled impulse, representative of a typical impact during surgery, was applied to the instruments by a drop tower (mean starting force, 3017 N). When the instruments no longer advanced distally, the applied force was increased incrementally. Instrument sizes were increased until a femoral fracture was observed or the impact exceeded 8000 N without causing a femoral fracture. At preoperative templated size, significantly more femurs that had tamps had fractured (eight of 10), compared with femurs that had broaches (two of 10). Smooth tamps therefore increased the risk of intraoperative femoral fracture in vitro in this particular implant design developed for cemented fixation of the femoral component.

Mechanical Evaluation of Various External Skeletal Fixator-Intramedullary Pin Tie-in Configurations Using a Tubular Plastic Bone Model

Abstract Use of external skeletal fixator–intramedullary pin tie-in (ESF-IM pin tie-in) fixators is an adjustable and effective method of fracture fixation in birds. The objective of this study was to evaluate the elements of the ESF-IM tie-in configuration used in birds. Ten variations of constructs were applied to a plastic bone model with a standard gap. Variants included non-tied and tie-in configurations, use of a 6- or 10-mm acrylic bar or a thermoplastic connecting bar, variation in the placement of the proximal fixation pin, use of 1.1-mm (0.045-in) or 1.6-mm (0.062-in) fixation pins, and configurations of 2, 3, or 4 fixation pins. The various constructs were loaded in bending, torque, and compression, and response variables were determined from resulting load-displacement curves (stiffness, load at 1-mm displacement). Results showed that, by using the tie-in configuration, increasing the diameter of the acrylic connecting bar, and increasing the diameter or number of fixation pins, each significantly increased the stiffness in all assessments. Placing the fixation pin distally in the proximal bone model segment increased the stiffness in bending, and adding a fixation pin to the distal bone model segment increased the stiffness in torque and bending. These results quantified the relative importance of specific parameters that effect stiffness and safe load of the ESF-IM tie-in construct as applied to a plastic bone fracture model.

Mechanical Evaluation of External Skeletal Fixator- Intramedullary Pin Tie-in Configurations Applied to Cadaveral Humeri From Red-tailed Hawks (Buteo jamaicensis)

Abstract Use of external skeletal fixator-intramedullary pin (ESF-IM) tie-in fixators is an adjustable and effective method of fracture fixation in birds. The objective of this study was to determine the contribution of each of the following parameters to the compressive and torsional rigidity of an ESF-IM pin tie-in applied to avian bones with an osteotomy gap: 1) varying the fixation pin position in the proximal bone segment and 2) increasing the number of fixation pins in one or both bone segments. ESF-IM pin tie-in constructs were applied to humeri harvested from red-tailed hawks (Buteo jamaicensis) (n  =  24) that had been euthanatized for clinical reasons. Constructs with a variation in the placement of the proximal fixation pin and with 2, 3, or 4 fixation pins applied to avian bone with an osteotomy gap were loaded to a defined displacement in torque and axial compression. Response variables were determined from resulting load-displacement curves (construct stiffness, load at 1-mm displacement). Increasing the number of fixation pins from 1 to 2 per bone segment significantly increased the stiffness in torque (110%) and compression (60%), and the safe load in torque (107%) and compression (50%). Adding a fixation pin to the distal bone segment to form a 3-pin fixator significantly increased the stiffness (27%) and safe load (20%) in torque but not in axial compression. In the configuration with 2 fixation pins, placing the proximal pin distally in the proximal bone segment significantly increased the stiffness in torque (28%), and the safe load in torque (23%) and in axial compression (32%). Results quantified the relative importance of specific parameters affecting the rigidity of ESF-IM pin tie-in constructs as applied to unstable bone fracture models in birds.