Comparative strength of three methods of fixation of transverse acetabular fractures.

With the advent of percutaneously placed lag screws for fixation of acetabular fractures, this study evaluated the strength of lag screw fixation compared with traditional fixation techniques of transverse acetabular fractures. Ten formalin-treated human, cadaveric pelvic specimens with bilateral, transtectal transverse acetabular fractures were used for this study. The right acetabular fractures were fixed with a five-hole plate and four screws with the central hole spanning the posterior fracture site. The left acetabular fractures were fixed with two lag screws, one each in the anterior and posterior columns, or with a screw and wire construct stabilizing both columns. The specimens were loaded to implant failure. Stiffness, yield strength, maximum load at failure, and site of failure was recorded. The plate and screw construct showed significantly greater yield and maximum strength when compared with the two lag screws. The stiffness of the lag screw method was 39% higher than that of the plating method, but this result was not statistically significant. In addition, the plate and screw method provided significantly greater maximum strength than the screw and wire technique. The quadrilateral plate seemed to be the weakest area of fixation because 83% of the implant failures occurred in this region. In patients in whom the risks of formal open reduction and internal fixation of acetabular fractures outweigh the possible benefits, such as in patients with burns or degloved skin, the advent of computer-assisted and fluoroscopically guided percutaneous surgical techniques have been instrumental. This study showed there is greater strength of fixation with a plate and screw construct, possibly secondary to supplementary fixation distal to the quadrilateral plate. However, lag screw fixation provided relatively greater stiffness, which may account for its clinical success. Percutaneous lag screw fixation of appropriate transverse acetabular fractures is a viable option.

A comparison of femoral neck fixation with the reconstruction nail versus cancellous screws in anatomic specimens.

Femoral neck fixation techniques were applied to five matched pairs of autopsy specimens to evaluate the fixation of the Russell-Taylor femoral nail in ipsilateral neck and shaft fractures of the femur. Reconstruction nail fixation of the femoral neck was compared with that of three parallel screws. The intact and postfixation femora were subjected to an applied bending moment in 0 degrees, 30 degrees, and 90 degrees of simulated hip flexion. The bending stiffness was determined from the load deformation data for each intact femur and then after the appropriate fixation. The fatigue response of the fixation, presence of osteopenia, degree of fracture reduction, and device alignment showed that the stiffness ratio (fixed to normal) of the nail was greater in most specimens. There was no statistical difference in retained stiffness after cyclic loading between the nail and cancellous screw fixations. The ultimate strength of the nail was 2.5 times the strength of the screw fixation of the femoral neck. Thus, the nail provided biomechanically sound fixation of the femoral neck.

Self-guiding interlocking intramedullary nail.

A critical step in closed interlocking intramedullary nailing is the insertion of the proximal and distal screws under fluoroscopic control. An intramedullary nailing system is described that does not require the use of direct fluoroscopic control for distal screw insertion. With the SG interlocking intramedullary nail, the location and fixation of the distal screw is achieved by keying in on a transversely placed guide pin, with imaging required only to confirm the alignment and final placement of the instrumentation.

Finite-element modelling of femoral shaft fracture fixation techniques post total hip arthroplasty.

The presence of a femoral prosthesis superior to a shaft fracture severely complicates fixation and treatment. This study uses two-dimensional, multithickness, plane stress finite-element models of a femur with prosthesis to investigate the stresses developed with the application of three popular fixation techniques: revision to a long stem prosthesis, lateral plating with a cortical bone allograft strut and cerclage wires, and custom plate application with proximal Parham band fixation with distal cortical screws (Ogden plate). The plate and bone contact as well as the fracture site contact were modelled by using orthotropic elements with custom-fit moduli so that only the normal stress to the interface was significant. A thermal analogy was used to model the cerclage and Parham band preloads so that representative preloads in the proximal fixation of the two types of plate treatments could be modelled. A parametric study was performed with the long-prosthesis model to show variations in stem lengths of one, two and three femoral diameters distal to the fracture site. The Ogden plate model showed a transfer of tensile stress near the proximal-most band, with the highest tensile stress being at the fracture site with evidence of stress shielding of the proximal lateral cortex. The cortical bone strut model showed a transfer of tensile stress to the bone strut but showed less shielding of the proximal cortex. The cerclage wires at the base of the bone strut showed the highest changes in load with the distalmost wire increasing to almost four times its original preload.(ABSTRACT TRUNCATED AT 250 WORDS)

Finite element analysis of interface geometry effects on the crestal bone surrounding a dental implant.

Using a two-dimensional axisymmetric finite element analysis technique, different geometrical configurations of implants, abutments, and interfaces have been investigated to alter the stress distribution in the crestal bone region. The crestal bone region is of particular interest due to observations of progressive bone resorption (saucerization). The ability of a prosthetic restoration-implant construct to transfer an appropriate stress at this region will, by definition of Wolff's law (bone's response to strain) and principles of bone remodeling, help to maintain the integrity of the surrounding bone via force transfer. The two geometries investigated involved a traditional flat mating surface and a slanted (oblique) mating surface. In both models a vertical load of 400 N (63 N/rad across 2 pi radians) was applied to the abutment apex. In the crestal bone region the oblique mating surface increased the transfer of horizontal stress 67 percent over the traditional flat mating surface design. The magnitude of stress transferred and the area which it was transferred across was increased in this region. Results indicate potentially more favorable mechanical conditions for bone maintenance surrounding an endosseous dental implant may be achieved if force is transferred preferentially via circumferential grooves and an oblique (dished) implant-abutment mating surface. These theoretical results are consistent with basic principles of stress transfer, stress shielding, and remodeling as well as clinical observations of bone maintenance and resorption.

Effect of isolated talocalcaneal fusion on contact in the ankle and talonavicular joints.

A cadaveric model was developed to establish the articular contact area and load distribution in the ankle joint, posterior facet of the talocalcaneal joint, and talonavicular joint using pressure sensitive film. Positions of dorsiflexion, neutral, and plantarflexion were evaluated. This model was further used to determine the effect of talocalcaneal fusion on the articular contact area in the talonavicular and ankle joints. Alteration of articular contact was most pronounced in the talonavicular joint. There, a statistically significant reduction in contact area postfusion was noted when the foot was in the plantarflexed position. Reductions in ankle joint articular contact area were observed in the dorsiflexed and plantarflexed positions in the majority of specimens. Lateral displacement of the region of articular contact was noted in some specimens. A pressure-weighted centroid calculation was performed to provide a quantitative measure of the shift of the contact region.

Finite element modelling of polymethylmethacrylate flow through cancellous bone.

A mathematical model based on the Finite Element Method is developed to simulate the non-linear flow of acrylic bone cement through cancellous bone. The cancellous bone bed is modelled as a bed of parallel capillaries filled with equal spaced toroidal trabeculae. By manipulating the relative size of the torus and the capillary, the flow within bone of varying porosity is simulated. An apparent permeability based on the volume weighted average viscosity and Darcy's law is developed to describe the flow of the acrylic through the cancellous bone bed. The model predicts a cancellous bone permeability of 5.6 x 10(-9)-8.3 x 10(-9) m2 for linear flow. The non-linear behavior of the acrylic cement results in an increase of apparent permeability when compared to the permeability computed for linear flow. Estimates of penetration are achieved by running the model in a quasi-steady state fashion with pressure applied over a fixed time increment. Close agreement is shown between model predictions of penetration depth and experimental results available in the literature.

Stress distribution surrounding endodontic posts.

This study compared the stress distribution during insertion and function of three prefabricated endodontic posts with different designs using the criteria of post length and diameter. Test blocks of photoelastic material were prepared with simulated endodontic canals. Three posts for each design, diameter, and depth were cemented. Each specimen was examined and photographed without load, with 135 Newton (N) compressive force, and with 90 N and/or 135 N oblique force applied at 26 degrees by use of a circular polariscope. Para-Post and Para-Post Plus posts produced similar, evenly distributed patterns of stress using the criteria of diameter, depth, and load. Flexi-Post posts produced asymmetric stress patterns with concentration of stress at each thread. During compressive loading and after cementation alone, Flexi-Post posts displayed significantly higher shoulder stresses and substantially greater stresses along the coronal surface of the post's length than Para-Post and Para-Post Plus posts. Apical stresses were similar for Flexi-Post, Para-Post, and Para-Post Plus posts during compressive loading.

Use of high-energy shock waves for bone cement removal.

The revision rate of total hip arthroplasty has increased dramatically over recent years, leading to different methods of extraction of the femoral cement mantle to reduce operative time and surgical risks. The use of high-energy shock waves produced by the Dornier HM.3 Lithotripter to interrupt the cement-bone interface and to reduce the material properties of the cement is investigated. Tests were conducted to measure the pull-out strength of cemented treated rods versus untreated rods, from the medullary canal of canine femurs. The treated femurs showed an average reduction in pull-out strength of 43%. An investigation involving the material properties of acrylic bone cement was also conducted. The properties tested were the compressive modulus of elasticity, the ultimate compressive strength, the ultimate tensile strength, and fracture toughness. The scanning electron microscope aided in determining whether microfractures in the cement resulted from the shock wave treatment. A theoretical study utilizing the finite element method was used to investigate areas of select shock wave treatment about the femoral prosthesis. Analysis of the results showed that the lithotripter treatment had no significant effect on the compressive properties but reduced the tensile properties and fracture toughness significantly. Scanning electron microscopy uncovered definite areas of induced microfractures not present in the control specimens. This study supports the concept of clinically noninvasive, preoperative shock wave treatment prior to total hip revision.

Mechanical properties of BIS-GMA resin short glass fiber composites.

The use of short glass fibers as a filler for dental restorations or cement resins have not been examined extensively. The mechanical properties and untreated glass fibers (5 microns dia x 25 microns) in Bis-phenol A glycidyl methacrylate (BIS-GMA) diluted with triethylene-glycol dimethacrylate (TEGDMA) resin were investigated for possible use as a restorative dental composite or bone cement. Compression, uniaxial tension and fracture toughness tests were conducted for each filler composite mixtures of 40, 50, 60 and 70%. Set time and maximum temperature of polymerization were determined. The results show that the elastic modulus, tensile strength and compressive strength are dependent on the percent of filler content. Elastic modulus and compressive yield (0.2%) strength of silane treated glass fibers filled composite increased from 2.26 to 4.59 GPa and 43.3 to 66.6 MPa, respectively, wtih increasing the filler content while the tensile strength decreased from 26.7 to 18.6 MPa. The elastic modulus of the untreated composite was less than that of the silane treated fiber composite. The tensile strength and compressive strengths were 20 to 50% lower than those of silane treated composites. The fracture toughness of the silane treated glass fiber additions were not significantly different from the untreated additions. The highest fracture toughness was obtained at 50% filler content with 1.65 MPa m.5. Set time increased from 3.5 to 7.7 minutes with increased filler content and peak temperature dropped from 68.3 to 34 degrees C. The results of this study indicate that the addition of silane coated glass fiber to BIS-GMA resin increased the elastic modulus, tensile and compressive strengths compared with non-treated fibers. The addition of either treated or non-treated fibers increased the set time of the material and decreased the maximum temperature.

Mechanical properties and material characteristics of orthopaedic casting material.

In this study, two types of orthopaedic casting materials were evaluated: the Johnson & Johnson Specialist plaster bandage and the 3M Scotch-cast Plus fiber glass bandage. The materials were evaluated using tensile tests to determine the elastic modulus, yield strength, and ultimate tensile strength. To determine the structural characteristics and stiffness of a cylindrical cast, each material was formed around a foam cylinder core and tested in a four-point bend jig. A computer-based model using the finite element method (FEM) was developed for a cylindrical cast of both types and compared with the experimental findings. A second FEM model with loads applied at the periphery was performed to simulate the clinical observations of plaster bandage breakdown at the ends of a cast. It is with these tests that the two bandage materials were compared and evaluated. It was concluded that the plaster bandage, while initially stiffer than the fiber glass bandage, had much lower yield stress. This implies that the plaster cast may break down under loads that would leave the fiber glass cast intact. It was also determined that the plaster bandage load displacement curve is bilinear. The bilinear characteristic of the plaster bandage explains its breakdown at the ends of a cast.

Fatigue testing of acrylic bone cements: statistical concepts and proposed test methodology.

In order to determine the fatigue behavior of a material, a standard procedure and methodology must be proposed and validated. This article describes a standard test method and statistical analysis for describing the fatigue properties of acrylic bone cement. It is proposed that bone cement fatigue data can be subjected to probability of failure (P) analysis and the establishment of a distribution function describing the data. It is also proposed that an Olgive type curve can be used to describe the stress (S) vs. cycles to failure (N) data. These two data sets can then be combined to determine the P-S-N relationship which fully describes the fatigue characteristics of the material.

Orthogonal bone cutting: saw design and operating characteristics.

The cutting process of orthopaedic bone saws was considered as orthogonal (two-dimensional) cutting for determination of the horizontal and vertical force components of single edge cutting tools with rake angles of 0 to -30 degrees. The Merchant analysis for orthogonal cutting was used to determine the resultant force and other force and work relationships. The effect of an imposed lateral vibration on the cutting tool was also investigated. The results of the tests indicated a strong interaction between the measured and derived forces with the rake angle and feed velocity. It was concluded that to reduce the cutting forces and work expenditure, a negative rake angle between 0 and -10 degrees, high feed velocity, and an imposed lateral vibration provided the greatest reduction in force and energy expenditure.