Stephan M. Perren

A comparative study of the initial stability of cementless hip prostheses.

In the process of prosthetic loosening, the initial motion of the implant relative to the bone is a key element. In order to provide quantitative information, three straight and one curved cementless hip stems were submitted to in vitro tests in autopsy specimens of the femur under dynamic axial and torsional loads with the displacements measured in several directions. The results were compared to those of an additional group of cemented straight stem prostheses providing reference values. Subsidence of cementless implants on the average was significantly higher than the cemented reference, but single implants were comparable to the reference. Rotational motion was distinctly higher than the reference values, with considerable differences between designs. When comparing displacements perpendicular to the main load direction, the sagittal plane motion was found to be at least double the motion in the frontal plane. Micromotions at the prosthesis/bone interfaces under the load case selected were smallest for the curved and highest for one of the straight prostheses.

Effect of Plates on Cortical Bone Perfusion

Using disulphine blue, sheep tibial cortex perfusion was decreased by subperiosteal stripping but not by extraperiosteal exposure. Semitubular plates applied with either low (20 kp) force, sufficient for extraosseous vascular occlusion, or high (200 kp) force, similar to that used clinically, both produced significant ischemia beneath the plate. In contrast, the dynamic compression plate produced significantly greater ischemia only with the higher force.

Experimental method for the in vitro testing of the initial stability of cementless hip prostheses.

Micromotions at the interface between bone and prosthesis are believed to induce bone resorption and ultimately lead to loosening of the implant. Thus the initial stability achieved by a hip prosthesis is an important factor for the long-term function of the implant. Knowing the biological consequences of the mechanical conditions, it appears to be mandatory to measure the extent of these three-dimensional movements. An in vitro dynamic method for measurement of the micromotion of the femoral component of hip prostheses has been developed. Tests in cemented prostheses have confirmed that the use of cement reduces sinkage and rotation manyfold and have yielded reference values for stability. Comparison with two types of cementless prostheses has shown that certain cementless implants may achieve stability comparable to cemented ones in some load directions.

Stress analysis of compression plate fixation and its effects on long bone remodeling

A three-dimensional finite element model is generated for an intact plexiglass tube with an attached six-hole stainless steel compression plate. The results for a wide range of loads, including cyclic external loads and static tensile preloads in the plate and screws, are examined as specifically related to plate-induced osteopenia. The model demonstrates that disuse osteopenia, resulting from a reduction in magnitude of cyclic axial stress, should be limited to the central region between the inner screws. Also, the addition of a static preload negates any reduced axial stress levels in this region, thus raising questions on the relative importance of static and cyclic stresses for the internal remodeling of bone.

Deformation of the distal femur : a contribution towards the pathogenesis of osteochondrosis dissecans in the knee joint

Osteochondrosis dissecans (OD) is a process of subchondral bone necrosis occurring predominantly in young individuals at specific sites. The aetiology of this disease remains controversial with mechanical processes due to trauma and/or ischaemic factors being proposed. This study aims at explaining the aetiology of OD in the knee joint as a result of the particular deformation of the condyles. A finite element analysis of the distal third of the femur was performed. A three-dimensional model was developed based on computed tomography scans of a normal femur, consisting of cortical bone, cancellous bone and articular cartilage. This model was subjected to physiological loads at 0, 30, 60 and 90 degrees of knee flexion. A complex deformation was found within each condyle as well as between the two condyles. Both medial and lateral condyles are deformed in the medio-lateral direction and at the same time compressed between the patella and the tibia in the antero-posterior direction. This effect is highest at 60 degrees of knee flexion. In both planes, the medial condyle is distorted more than the lateral one. Strain concentration in the subchondral bone facing the patella varies with flexion, especially for angles exceeding 60 degrees. The deformation of the femur in the predominant locus of OD in the medial condyle exceeds that of the lateral condyle considerably. The analysis shows that repeated vigorous exercise including extreme knee flexion may produce rapidly changing strains which in turn could ultimately be responsible for local subchondral bone collapse.

Stress analysis of a simplified compression plate fixation system for fractured bones

Abstract A three-dimensional finite element model was generated of a plexiglass tube with an attached six-hole stainless steel compression plate to study the mechanics of internal fixation of fractured long bones. To demonstrate the importance of the plate-bone interface, this interface was represented three different ways in the finite element model. A plated tube with a uniform transverse osteotomy gap was also examined to study the mechanics of plated fractured bones. To validate the model, the results for the intact plated tube were compared to composite beam theory and strain gauge data from an instrumented physical model. Applications of the finite element model data included the prediction of screw failure modes, plate-induced osteopenia, and multi-axial strains in an interfragmentary region. The addition of sliding motion between the plate and tube resulted in a deviation from composite beam theory and improved correspondence with strain gage data when compared to a model having the plate and tube securely bonded. Sliding motion resulted in a much smaller region of bone subjected to reduced axial stress levels, which may decrease the extent of plate-induced osteopenia. The complex nature of induced strains in an osteotomy gap was also demonstrated, along with the tendency for failure of the screws nearest the fracture site.

Three-Dimensional Finite Element Analysis of a Simplified Compression Plate Fixation System

A three-dimensional, linear finite element model was generated for an intact plexiglass tube with an attached six-hole stainless steel compression plate. We examined external forces representing axial, off-center axial, and four-point bending, along with superimposed plate and screw pretension. Strain gage experiments were conducted to test model validity and the finite element results were contrasted to a composite beam theory solution. Excellent correspondence was observed between finite element and strain gage data for the most significant strain components. Composite beam theory tended to overestimate the neutral axis shift which results from plate application. The model also demonstrated fracture site distraction due to plate pretension, and the tendency for outer screw failure for the combination of bending-closed with a preload in the plate and screws.

Titanium as Implant Material for Osteosynthesis Applications

A bone fracture disables the function of the injured limb. Early and full restoration of function can be achieved by osteosynthesis, a method of treating the bone fracture by surgical means. The method consists of stabilizing the fragments of the fracture using implants [1]. The choice of the implant material depends, first of all, on the function to be achieved and also on the mode of application of the implant. The function of the implant consists not only in maintaining the proper shape of the bone under functional load to allow for prompt and reliable solid healing, but also, and more importantly, in restoring the full function of the limb and of the severely injured patient. Different principles of fracture treatment with different types of fracture healing result in different technical requirements. These requirements take into account both implant function and implant design and material. Therefore, the principles of fracture treatment are outlined first.

A locking hook spinal rod system for stabilization of fracture-dislocations and correction of deformities of the dorsolumbar spine. A biomechanic evaluation.

Existing internal fixation systems for the injured or deformed spine present problems with overdistraction and control of the contoured rod necessary for transverse forces. A locking hook spinal rod avoids these problems by using a locking cover to secure the lamina in the hook and meshing radial grooves to lock the contoured rod to both the upper and lower hooks in 6 degrees intervals of rotation. The 7-mm stainless-steel rod is 50% stronger than the 1/4-in Harrington rod and also avoids the weakening effect of the notches. Cadaver spine testing gives nearly a threefold increase in failure strength (125 +/- 17 Nm versus 44.1 +/- 2.1).

Anatomical Curvature of the Femoral Medullary Canal for Intramedullary Rodding

The shape of most of the intramedullary and interlocking nails commonly used has been determined empirically as a segment of a circle. The radius of curvature of the Kuntscher nail, e.g., has a length of 3500 mm, the Aesculap-Kuntscher 3500 mm, the Grosse-Kempf 3000 mm, the AO 4000 mm, and the Klemm-Schellmann 3500 mm. Despite the fact that all these implants do not match the geometry of the femoral cavity, the application of “physiologically” curved nails is reported by two groups (11,14,22). In clinical use, the exact location of the point of insertion, axial malalignement, additional comminution of the fractured area or perforation of the distal metaphysis are related to a misfit between the geometry of the femoral cavity and the shape of the implant (4,10,11,12,15,16,20,21). Although the anatomy of the femur has been studied extensively (5,8,9,13,17), limited data is available with respect to the geometrical properties pertinent for intramedullary nailing of this bone. Two groups (11,14,22 and Winquist personal communication) determined the radius of curvature of the femur in the antero-posterior plane by sagittal x-rays.