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Geometric fit of the intramedullary nail to the bone cavity
Abstracts of the Fourth Meeting of the European Society of Biomechanics
544
fracture or osteotomy site to reduce the unsupported length and to increase the length over which the bone and plate act as a composite system. STRESS ANALYSIS
IN FRACTURE
FINITE
STABILISATION
ELEMENT
WITH BONE PLATES USING
THE
METHOD
I_. CLAES, E. PALME, G. KLING, G. RINKE and U. KIRSCHBAUM(UniversitBt Ulm, Abteilung Chirurgie, FH-Ulm, 79-Ulm, F.R.G.) To investigate the influence of different plate designs and materials with regard to the stress distribution on the bone and the strain in the fracture gap, the finite element method was used. Three dimensional finite element models were developed for the bone and the plate. The idealization of the diaphyseal part of the bone was performed using 750 triangular shell elements. One cross section consisted of at least twelve elements. Twenty subsections were made in the longitudinal direction of the bone. The inhomogeneous modulus of the elasticity of the bone, determined in previous investigations, was taken into consideration for the calculations. The six-hole plate structure was idealized by 6000 elements, allowing the simulation of complicated material structures such as fibre-reinforced materials. The stress distribution distinctly showed that a decrease of the stress underneath the plate occurs following the simulated application of plates to the bone model. This effect was more pronounced for the stiffer plates than for the more flexible plates. NEW LONG-TERM
RESULTS OF ANIMAL INVESTIGATIONS WITH A NEW ELASTIC FIXATION PLATE FOR OSTEOSYNT’HESIS
H. ZENKER, H.-H. SPRINGERand W. PLITZ (Ludwig-Maximilian
University, D-Munich, F.R.G.)
We tried to decrease bone loss by using an elastic fixation plate in animal investigations and to reduce refractures after removal of the plates. The first plate was a titanium elastic fixation plate. Using a second elastic plate made of glass fiber-reinforced epoxy resin, we succeeded in demonstrating in a long-term investigation of 1 yr with six dogs a normal bone structure. Fixation with a rigid plate clearly showed osteoporosis. We took the bending strength and the content of calcium salt as quantitative criteria, which were decreased by 41.5 % and 26.5 % respectively when a rigid plate was used instead of an elastic plate. Now in a new series we compared a new developed polyethylene glassfiber-plate with an AO-Plate. With five beagle dogs we performed the well-known clinically related long-term experiment. The results will be presented. BIOMECHANICAL
PROPERTIES AND FIRST CLINICAL RESULTS OF A NEWLY DESIGNED INTRAMEDULLARY FIXATION DEVICE
N. HAAS, C. KRETTEK and L. GOTZEN (Medizinische Hochschule Hannover, Unfallchirurgische D-3000 Hannover, F.R.G.)
Klinik,
To enlarge the applications of intramedullary fixation in the femur a new interlocking nailing device has been developed. On cadaver femora with osteotomies in the middle and distal third the new nail showed high torsional and bending stiffness with completely reversible deviation. The first clinical experiences give encouraging results in implantation, bone healing and nail extraction. GEOMETRIC
FIT OF THE INTRAMEDULLARY
NAIL TO THE BONE CAVITY
J. E. BECHTOLD,E. SCHNEIDER,P. VOCK and S. M. PERREN (Miiller-Institute of Bern, Bern, Switzerland)
for Biomechanics, University
Plastic deformation upon insertion and partial or complete fractures of the intramedullary nail have been reported as problems of an otherwise successful implant. Furthermore, the functional role of match or mismatch between the bone cavity and the implant has so far not been investigated. The current project therefore proposes a quantitative method of designing an improved implant which will conform as closely as possible to the inner geometry of a bone. A technique for the mathematical description of the surfaces of bone and implant has been developed, and the closeness of fit between bone cavity and implant has been evaluated. Parametric, hi-cubic B-splines were used to model the surface of the medullary cavity. A human femur from a 45 yr old male, scanned with a SOMATOM SF computer tomograph, provided sample geometric input data. The plane of major curvature was determined and two geometries of implants fitted to the medullary cavity: a circular arc and a second order polynomial. The nail surface was modeled by translating a B-spline representation of the clover leaf cross-section along the path of the curve defining the curvature of the bone. An index was used to describe the degree of configurational conformity between the two surfaces.
544
fracture or osteotomy site to reduce the unsupported length and to increase the length over which the bone and plate act as a composite system. STRESS ANALYSIS
IN FRACTURE
FINITE
STABILISATION
ELEMENT
WITH BONE PLATES USING
THE
METHOD
I_. CLAES, E. PALME, G. KLING, G. RINKE and U. KIRSCHBAUM(UniversitBt Ulm, Abteilung Chirurgie, FH-Ulm, 79-Ulm, F.R.G.) To investigate the influence of different plate designs and materials with regard to the stress distribution on the bone and the strain in the fracture gap, the finite element method was used. Three dimensional finite element models were developed for the bone and the plate. The idealization of the diaphyseal part of the bone was performed using 750 triangular shell elements. One cross section consisted of at least twelve elements. Twenty subsections were made in the longitudinal direction of the bone. The inhomogeneous modulus of the elasticity of the bone, determined in previous investigations, was taken into consideration for the calculations. The six-hole plate structure was idealized by 6000 elements, allowing the simulation of complicated material structures such as fibre-reinforced materials. The stress distribution distinctly showed that a decrease of the stress underneath the plate occurs following the simulated application of plates to the bone model. This effect was more pronounced for the stiffer plates than for the more flexible plates. NEW LONG-TERM
RESULTS OF ANIMAL INVESTIGATIONS WITH A NEW ELASTIC FIXATION PLATE FOR OSTEOSYNT’HESIS
H. ZENKER, H.-H. SPRINGERand W. PLITZ (Ludwig-Maximilian
University, D-Munich, F.R.G.)
We tried to decrease bone loss by using an elastic fixation plate in animal investigations and to reduce refractures after removal of the plates. The first plate was a titanium elastic fixation plate. Using a second elastic plate made of glass fiber-reinforced epoxy resin, we succeeded in demonstrating in a long-term investigation of 1 yr with six dogs a normal bone structure. Fixation with a rigid plate clearly showed osteoporosis. We took the bending strength and the content of calcium salt as quantitative criteria, which were decreased by 41.5 % and 26.5 % respectively when a rigid plate was used instead of an elastic plate. Now in a new series we compared a new developed polyethylene glassfiber-plate with an AO-Plate. With five beagle dogs we performed the well-known clinically related long-term experiment. The results will be presented. BIOMECHANICAL
PROPERTIES AND FIRST CLINICAL RESULTS OF A NEWLY DESIGNED INTRAMEDULLARY FIXATION DEVICE
N. HAAS, C. KRETTEK and L. GOTZEN (Medizinische Hochschule Hannover, Unfallchirurgische D-3000 Hannover, F.R.G.)
Klinik,
To enlarge the applications of intramedullary fixation in the femur a new interlocking nailing device has been developed. On cadaver femora with osteotomies in the middle and distal third the new nail showed high torsional and bending stiffness with completely reversible deviation. The first clinical experiences give encouraging results in implantation, bone healing and nail extraction. GEOMETRIC
FIT OF THE INTRAMEDULLARY
NAIL TO THE BONE CAVITY
J. E. BECHTOLD,E. SCHNEIDER,P. VOCK and S. M. PERREN (Miiller-Institute of Bern, Bern, Switzerland)
for Biomechanics, University
Plastic deformation upon insertion and partial or complete fractures of the intramedullary nail have been reported as problems of an otherwise successful implant. Furthermore, the functional role of match or mismatch between the bone cavity and the implant has so far not been investigated. The current project therefore proposes a quantitative method of designing an improved implant which will conform as closely as possible to the inner geometry of a bone. A technique for the mathematical description of the surfaces of bone and implant has been developed, and the closeness of fit between bone cavity and implant has been evaluated. Parametric, hi-cubic B-splines were used to model the surface of the medullary cavity. A human femur from a 45 yr old male, scanned with a SOMATOM SF computer tomograph, provided sample geometric input data. The plane of major curvature was determined and two geometries of implants fitted to the medullary cavity: a circular arc and a second order polynomial. The nail surface was modeled by translating a B-spline representation of the clover leaf cross-section along the path of the curve defining the curvature of the bone. An index was used to describe the degree of configurational conformity between the two surfaces.