Beat Gasser

Biomechanical evaluation of the less invasive stabilization system for the internal fixation of distal femur fractures.

Objective Comparison between a Less Invasive Stabilization System (LISS) using monocortical screws with angular stability and two conventional plate systems Condylar Buttress Plate (CBP) and Dynamic Condylar Screw (DCS) for the treatment of distal femoral fractures with respect to biomechanical properties. Design Biomechanical study using paired cadaver femurs. In Test Configuration 1 (distal test), a ten-millimeter gap at the diaphysis–metaphysis junction simulates a supracondylar femoral fracture. Test Configuration 2 (proximal test) has the same configuration, but the gap was cut in the isthmic region. Proximal and distal plate ends were fixed to corresponding cortical bone fragments in both tests. Optical displacement transducers served to quantify the systems ability to withstand a stepwise increased load. Reversible (deflection) and irreversible deformation (subsidence) of the bone–plate construct was investigated. Results In Test Configuration 1, LISS showed less irreversible deformation in 72 percent of the left–right comparisons. No correlation between bone mineral density, cross-section area of bones and the measured response of the construct under load was found between pairs. In Test Configuration 2, 83 percent of the left–right comparisons showed less permanent deformation but a higher elastic deformation for LISS. Conclusions These results suggest an enhanced ability to withstand high loads when using the monocortical screw fixation technique with angular stability. A higher elastic deformation of LISS compared with conventional plating systems in distal femoral fractures can be explained by the lower bending stiffness caused by different design and material properties.

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.

The effect of radial preload on the implant-bone interface: a cadaveric study

Summary The most common complication of external fixation is pin loosening. Preloading the implant-bone interface is believed to retard this process. Radial preload, in particular, may be useful, as it allows loading in more than one direction. To investigate the effect of varying degrees of radial preload on the pin-bone interface, 30 freshly thawed human cadaveric tibiae were sectioned into 4–cm segments. Uniform drill holes were produced in the anterior tibial ridge of all segments and custom experimental bolts, oversized in diameter by as much as 1 mm, were pressed into each specimen. Macroscopic surface fractures were noted at the time of bolt insertion for misfits >0.2 mm. Following histologic preparation, the implant-bone interface was evaluated microscopically based on the appearance of osteonal compression, lamellar distortion, and microfractures. Insertion of external fixator pins with misfits of greater than 0.4 mm resulted in significant microscopic structural damage to the bone surrounding the pin. High degrees of radial preload, exceeding the elastic limit of cortical bone, may be produced around pin holes by a small misfit. The use of oversized pins or screws must therefore be questioned.

Biometalle in der orthopädischen Chirurgie - Entwicklung, aktueller Stand und Perspektiven

Bei nahezu jeder zweiten Operation in der orthopadischen Chirurgie werden Biomaterialien eingebracht, bei etwa jeder dritten Biometalle. Von den kommerziell erhaltlichen Materialien sind zum momentanen Zeitpunkt lediglich metallische Werkstoffe in der Lage, der statischen Beanspruchung und der Dauerfestigkeit, die von hoch beanspruchten Werkstucken der internen Stabilisatoren bzw. des kunstlichen Gelenkersatzes gefordert werden, gerecht zu werden. Allgemein wird akzeptiert, dass es kaum gelingen wird, einen biologisch vollkommen inerten Werkstoff zu produzieren. Aufgrund der biologischen und mechanischen Anforderungen haben sich im Lauf der vergangenen dreisig Jahre Legierungen auf Basis der Metalle Eisen, Titan und Kobalt durchgesetzt. Inzwischen sind nahezu alle Implantate an ihrer Oberflache bearbeitet und behandelt. Dieses dient zur Verbesserung der Gewebevertraglichkeit und des Einwachsverhaltens sowie zur Reduzierung von Abrieb. In Zukunft durfte sich durch Mikrobeschichtungen mit nahezu inerten Materialien oder biologischen „Tarnkappen“ die Bioakzeptanz der metallischen Werkstoffe nochmals steigern lassen.

Comparative investigation on the biomechanical properties of the circular and other three-dimensional external fixators

External Fixation has developed during recent years to one of the standard methods of fracture treatment. A variety of fixators exists for a wide range of indications (4,5) as open fractures, pseudarthroses and limb lengthening. Numerous investigations (1,2,3) have been done comparing the properties of different types of external fixators among themselves. Although relevant and comparable parameters of clinical or mechanical nature are not trivial to find, stiffness of the fixator frame under different loading conditions has generally been accepted to be an important property. Fracture healing is believed to be accelerated when stiffness is reduced appropriately during the healing process.

The value of ‘coining’ on the improvement of endurance in slotted intramedullary nails

Cracking of intramedullary nails, starting at the proximal end of the partial slot and propagating in a circumferential direction, has been observed. These partial cracks are relatively frequent, although full failures are very rare and do not disturb fracture healing. Coining is a cold-working process used to improve the endurance of structures with residual compressive stresses generated by plastic deformation. The influence of coining was investigated to evaluate its practical value in controlling these fatigue cracks. Bending and torsional tests were performed on high frequency machines. Coined zones of different shapes and depths were examined, comparing the elapsed number of load cycles with crack initiation. The results showed that the endurance of coined nails was improved by factors of approximately 10 and 5 in bending and torsion, respectively. This increase in fatigue life corresponds to a more than 50% larger dynamic load after a million cycles. Variations in coining shape and depth did not yield any significant differences.