Michael J. Voor

Pin tract infection with contemporary external fixation: how much of a problem?

Objective To determine the incidence of pin tract infection. Design Retrospective chart review. Setting Level 1 trauma center in an urban community. Patients A total of 285 patients with 285 fractures over a 4-year period (1997–2001). Intervention External fixation. Main Outcome Measurement Incidence of pin tract infection. Results Of 285 fractures, 32 (11.2%) were complicated by infection. The incidence of infection according to montage was 3.9% (3/77) for ring fixators, which was significantly different (P < 0.04) from the 12.9% incidence (23/178) for unilateral fixators and the 20.0% incidence (6/30) for hybrid fixators (P = 0.004). The incidences of pin tract infection for the unilateral fixator group and the hybrid fixator group were not significantly different. Conclusions Patients with hybrid external fixators had a similar risk of pin tract infection as patients who had unilateral fixators. The infection rate in the ring fixator group was significantly lower than the hybrid external and unilateral fixator groups.

Second generation intramedullary nailing of subtrochanteric femur fractures: a biomechanical study of fracture site motion.

Objectives: To compare fracture site motion between different second-generation intramedullary nails used to fix subtrochanteric fractures of the proximal femur with and without femoral neck fractures. Design: Nondestructive mechanical testing of four types of femoral intramedullary nails was undertaken to evaluate fracture site motion using a model that simulated single-leg and double-leg stance. Methods: Three types of reconstruction nails (the Russell-Taylor Delta [Smith & Nephew, Memphis, TN], the Uniflex [Biomet, Warsaw, IN], Alta CFX [Howmedica-Osteonics, Rutherford, NJ]) and the Long Gamma nail (Howmedica-Osteonics, Rutherford, NJ), each measuring 11 × 380 mm, were inserted in fiberglass composite femurs. Four fracture patterns were studied (transverse subtrochanteric, subtrochanteric with posteromedial wedge comminution, subtrochanteric with one-centimeter gap, and a one-centimeter gap with a subcapital neck fracture). Single- and double-leg stance loading was simulated using a servohydraulic load frame (MTS, Eden Prairie, MN). Two-way analysis of variance and post hoc t tests were used to determine any statistically significant differences between groups. Results: In single-leg stance there were significant differences in coronal plane rotation, shear, and axial translation across the subtrochanteric fracture site between the different nail types and the different fracture patterns (p < 0.001). In double-leg stance there were significant differences in coronal plane rotation and femoral head vertical motion between the different nail types and the different fracture patterns (p < 0.001), and there were significant differences in shear and axial translation between the different fracture patterns (p < 0.001) but not the different nail types (p > 0.05). Conclusions: For simple, well-reduced fractures the choice of implant is not critical. As fracture severity increased (comminution, gap, and combined neck fracture), the choice of implant, particularly with reference to proximal nail dimensions and implant materials, was a significant factor in reducing fracture site motion. Therefore, our laboratory data suggest that when subtrochanteric fractures are unstable (e.g., comminution, segmental bone loss) and early weight bearing is desirable, the choice of implant is critical and should be restricted to implants that allow minimal fracture site motion (Long Gamma and Russell-Taylor).

Mechanical properties of compacted morselized cancellous bone graft using one-dimensional consolidation testing

Failures of orthopaedic procedures that use morselized cancellous bone (MCB) graft for load bearing are often due to gross displacement within the graft material. For this reason the mechanical behavior of MCB must be better understood. Our purpose is to present a detailed testing methodology for the mechanical characterization of MCB, and to illustrate how this methodology can be used to study the influence of water and fat content. Complete one-dimensional consolidation testing was performed on bovine cancellous bone processed to represent MCB typically used in surgery (52% water, 31% fat). The one-dimensional consolidation strain under a stress of 1.09MPa was 30.9% and the confined modulus was 8.0MPa. The coefficient of consolidation (rate of consolidation) was 2. 2x10(-5)cm(2)/s and the coefficient of secondary strain (steady-state creep rate) was 1.9%. While reducing the water content alone had some influence on properties, reducing the fat content improved both the static and dynamic behavior. A sample of MCB which had fat intentionally minimized and a lower overall moisture content (56% water, 5% fat) demonstrated 23.1% strain, a confined modulus of 9.6MPa, a coefficient of consolidation of 3.4x10(-3)cm(2)/s, and a coefficient of secondary strain of 0.9%. The test methods described in this technical note can be used to evaluate the influence of fluid content on the mechanical behavior of MCB.

Cross-Sectional Geometry of the Sacral Ala for Safe Insertion of Iliosacral Lag Screws : A Computed Tomography Model

OBJECTIVE To measure the dimensions of the narrowest portion of the sacral ala for safe insertion of iliosacral lag screws. DESIGN Computed tomography (CT) model. SETTING Level One trauma center. PATIENTS Thirteen adult patients underwent pelvic CT imaging. MAIN OUTCOME MEASURE Axial CT scans of intact pelves were reformatted in the sagittal plane at three-millimeter intervals from the first sacral body (S1 body) to the sacroiliac (SI) joint. Computer analysis and measurements of sacral geometry were used to determine the narrowest portion of the bony sacral ala. The maximum height, maximum width, and slope of the sacral ala through its geometric center in cross-section were measured. RESULTS The narrowest portion of the sacral ala in all patients was consistently located at the junction between the sacral body and the alar wings, termed the sacral pedicle, directly cephalad to the first sacral foramen. The average slope of the sacral ala at the sacral pedicle was 45.08 degrees (range 25 to 65 degrees). The average maximum height at the geometric center in cross-section was 27.76 millimeters, and the average width was 28.05 millimeters. However, outside the geometric center there was a sharp decrease in height and width of the sacral ala that was in large part determined by its relative slope. CONCLUSION Although the cross-sectional geometry of the sacral ala is highly variable among patients, there is ample space for iliosacral screws. To ensure safe insertion, iliosacral lag screws must be positioned in the geometric center of the sacral ala to avoid extraosseous placement.

The role of homocysteine in bone remodeling

Abstract Bone remodeling is a very complex process. Homocysteine (Hcy) is known to modulate this process via several known mechanisms such as increase in osteoclast activity, decrease in osteoblast activity and direct action of Hcy on bone matrix. Evidence from previous studies further support a detrimental effect on bone via decrease in bone blood flow and an increase in matrix metalloproteinases (MMPs) that degrade extracellular bone matrix. Hcy binds directly to extracellular matrix and reduces bone strength. There are several bone markers that can be used as parameters to determine how high levels of plasma Hcy (hyperhomocysteinemia, HHcy) affect bone such as: hydroxyproline, N-terminal collagen 1 telopeptides. Mitochondrion serves an important role in generating reactive oxygen species (ROS). Mitochondrial abnormalities have been identified during HHcy. The mechanism of Hcy-induced bone remodeling via the mitochondrial pathway is largely unknown. Therefore, we propose a mitochondrial mechanism by which Hcy can contribute to alter bone properties. This may occur both through generations of ROS that activate MMPs and could be extruded into matrix to degrade bone matrix. However, there are contrasting reports on whether Hcy affects bone density, with some reports in favour and others not. Earlier studies also found an alteration in bone biomechanical properties with deficiencies of vitamin B12, folate and HHcy conditions. Moreover, existing data opens speculation that folate and vitamin therapy act not only via Hcy-dependent pathways but also via Hcy-independent pathways. However, more studies are needed to clarify the mechanistic role of Hcy during bone diseases.

Optimal entry point for retrograde femoral nailing.

Objective The purpose of this study is to identify the optimum entry point for retrograde femoral nailing, defined as that point which will provide adequate fracture alignment while minimizing soft-tissue and articular cartilage injury. Design Cadaveric study. Setting Biomechanics laboratory. Main Outcome Measure Anatomic relationships and fracture reduction. Methods Eleven cadaveric femori with attached knee joints underwent retrograde femoral nailing with a Synthes femoral nail (Synthes, Paoli, PA, U.S.A.). After placement of the nail, the specimens underwent an osteotomy 3 inches proximal to the articular surface. Multiple entry points were tested to determine fracture alignment and extent of articular cartilage injury. Medial–lateral and anterior–posterior displacements, in addition to any soft-tissue or articular surface trauma, were recorded for these various points of entry. Results An entry point of 1.2 cm anterior to the femoral origin of the posterior cruciate ligament resulted in the least anterior–posterior displacement of the femoral shaft following fracture. In the coronal plane, an entry point at the midpoint of the intercondylar sulcus was identified as minimizing the displacement following fracture. This ideal position allows for proper seating of the nail within the intercondylar sulcus, resulting in minimal damage to the articular cartilage and posterior cruciate ligament and minimal disruption of the patella femoral joint. Conclusion Retrograde femoral nailing should be used cautiously in select patients, when conventional antegrade nailing cannot be used, due to the unavoidable injury to the knee articular surface associated with this technique. The optimum entry point of 1.2 cm anterior to the femoral posterior cruciate ligament origin and centered in the intercondylar sulcus provides the optimal balance of fracture reduction and knee joint sparing. It may be difficult to target this site with a percutaneous technique and may require direct visualization of the intercondylar sulcus for ideal nail placement.

Mechanical study of the safe distance between distal femoral fracture site and distal locking screws in antegrade intramedullary nailing.

Objective: To determine the safe distance for distal femoral fractures relative to the distal locking screws in antegrade intramedullary femoral nailing using a currently available titanium alloy nail design. Design: Cyclic (fatigue) mechanical testing study. Setting: Biomechanics laboratory. Intervention: Intramedullary nailing of left synthetic fiberglass composite femora with type 32/33-C fractures at 1, 2, 3, and 4 cm from the more proximal of the distal locking screws. Main Outcome Measurement: The number of loading cycles to failure of the nail. Results: A load level of 700 N through the femoral mechanical axis was validated as adequate to cause fatigue failure within 200,000 cycles in slotted stainless- steel nails. In the nonslotted titanium alloy nails, this load level caused failure in only 1 of 3 nails with a fracture at 2 cm from the more proximal of the 2 distal locking screws and in 2 of 3 nails with a fracture at 1 cm from the more proximal of the 2 distal locking screws. All of the other nails did not fail >1 million cycles. Conclusions: Under laboratory conditions, it is safe to assume that an antegrade titanium alloy nail will survive 1 million compression/bending cycles when the fracture is ≥3 cm from the more proximal of the 2 distal locking screws.

Hybrid external fixation of the proximal tibia: strategies to improve frame stability.

Objective To determine the specific frame construction strategies that can increase the stability of hybrid (ring with tensioned wires proximally connected by bars to half-pins distally) external fixation of proximal tibia fractures. Design Repeated measures biomechanical testing. Setting Laboratory. Specimens Composite fiberglass tibias. Methods Using the Heidelberg and Ilizarov systems, external fixators were tested on composite fiberglass tibias with a 1-cm proximal osteotomy (OTA fracture classification 41-A3.3) in seven frame configurations: unilateral frames with 5-mm diameter half-pins and 6-mm diameter half-pins; hybrid (as described above), with and without a 6-mm anterior proximal half-pin; a “box” hybrid (additional ring group distal to the fracture connected by symmetrically spaced bars to the proximal rings) with and without an anterior, proximal half-pin; and a full, four-ring configuration. Each configuration was loaded in four positions (central, medial, posterior, and posteromedial). Main Outcome Measurements Displacement at point of loading of proximal fragment. Results The “box” hybrid was stiffer than the standard hybrid for all loading positions. The addition of an anterior half-pin stiffened the standard hybrid and the “box” hybrid. Conclusions The most dramatic improvements in the stability of hybrid frames used for proximal tibial fractures result from addition of an anterior, proximal half-pin.

Bone Blood Flow and Vascular Reactivity

Blood flow is essential for normal bone growth and bone repair. Like other organs, the regulation of blood flow to bone is complex and involves numerous physiologic mechanisms including the sympathetic nervous system, circulating hormones, and local metabolic factors. Our studies addressed the following questions: (1) Which endogenous vasoconstrictor agents regulate in vivo blood flow to bone? (2) Does a decrease in bone vascular reactivity to vasoconstrictor hormones account for the increase in blood flow during bone healing? (3) Does the endothelium influence bone arteriolar function? An intact bone model was developed in the rat to assess hormonal regulation of in vivo bone blood flow and in vivo bone vascular reactivity. An isolated, perfused bone arteriole preparation was employed to characterize the responsiveness of small resistance-size arterioles (diameter < 100 µm) to vasoconstrictor hormones and to evaluate the role of the vascular endothelium to modulate vascular smooth muscle reactivity. Our results indicate that: (1) though exogenous endothelin is a potent constrictor of the in vivo bone vasculature, endogenous endothelin does not actively regulate in vivo blood flow; (2) the increase in blood flow to a bone injury site is not due to a decrease in bone vascular sensitivity to norepinephrine, and (3) isolated bone arterioles of young rats are very sensitive to vasoconstrictor hormones but exhibit only modest endothelium-mediated vasodilation.

Anterolateral and Medial Locking Plate Stiffness in Distal Tibial Fracture Model

Background: The purpose of this study was to compare the axial and torsional stiffness between anterolateral and medial distal tibial locking plates in a pilon fracture model. Materials and Methods: The biomechanical stiffness of anterolateral or medial plated pilon fracture models was evaluated. Six Sawbones Composite Tibiae with a simulated pilon fracture representing varus or valgus comminution (OTA 43-A2.2) were plated with a Synthes 3.5-mm contoured LCP anterolateral or medial locking distal tibia plate. Load as a function of axial displacement and torque as a function of angular displacement were recorded. Each tibia was tested with a fracture wedge in place and removed with a medial and then anterolateral plate. Results: Loading the tibial plateau medial to the central axis, no significant difference in mean stiffness between the anteroateral and medial plates was demonstrated with the fracture wedge in place. A significant difference was demonstrated with the wedge removed. Loading the plateau posterior to the central axis, no significant difference in mean stiffness between plates was demonstrated with the wedge in place or removed. With the wedge in place, there was a significant difference in mean torsional stiffness for clockwise rotation, but not counterclockwise rotation. With the wedge removed, no significant difference appeared in mean stiffness for clockwise and counterclockwise rotation. Conclusion: Distal tibia extra-articular fractures stabilized with anterolateral or medial locking plate constructs demonstrated no statistically significant difference in biomechanical stiffness in compression and torsion testing. Clinical Relevance: We believe this study indicates the primary concern when treating a pilon fracture may be soft-tissue considerations. Further clinical studies are required before definitive changes can be recommended regarding pilon fracture fixation.