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Interaction of femur — Hip joint endoprosthesis with articulated stem
482
Abstracts
PBE-STBESSES GENERATED IN ACETABULAB BONE BY SCBEWED-CUP FIXATION IN THA M.Dalstra and RHuiskes Biomechanics Section, Institute of Orthopaedics, University of Nijmegen, the Netherlands Initial stability of screwed-cup fixation is based on high stresses in the surrounding acetabular bone, generated upon insertion of the cup. In this study, these pre-stresses were measured in-vitro in six post-mortem hemipelves, each applied with six rosette strain-gauges. Three of the hemipelves had a polyethylene (PE) cup inserted, the other three a titanium backed (MB) cup. After insertion, the cups were placed in a testing machine in which a hip joint force of 3,000 N was simulated in order to compare the pre-stresses to stresses due to normal loading. Although the torque, needed to insert the ME8 cup was twice as high as for the PE cup (the latter had no self-cutting thread and required pre-tapping), the magnitude of the pre-stresses in the bone for both types of cups were about the same, suggesting that the pre-stresses seem to depend more on the shape of the cup than on the cup’s flexibility and the insertion technique. Values varied between 40 MPa (tensile) and -50 MPa (compressive). During loading, stresses varied between 60 MPa and -50 MPa for the PE cup, whereas for the MB cup these values were only 25 MPa and -25 MPa. This shows that the order of magnitude of the pre-stresses is equal to that of the stresses due to normal loading. Superimposing of the two stress-states did not result in higher stresses everywhere, as tensile and compressive stresses cancelled each other out. These findings were qualitatively backed by results of a axisymmetric finite element study. Finally, it should be stated that large intra-specimen variations in bone quality cause the pre-stresses to be quantitatively unpredictable.
INTERACTIONOF FEMJR- HIP JOINT ENNX’ROSTHESIS WITHARTICLLATEO STEW J.JirovB, T.KauflerovB and J.Wca Institute of Theoretical and Applied Mechanics CSAS, Czechoslovakia The paper deals with the interaction between the POLO1endoprosthesis with an articulated stem and femur. The subject of the research was to gain basic knowledge about forceflow through the implant into the bone tissue and to find out stress distribution in the stem structure. The stem of this cementless endoprosthesis which consists of five dia. 4.5 mm bars 200 mn long is made of stainless steel. The upper ends of the bars are firmly connected with the collar, their lower ends are welded together. Primary fixation of the endoprosthesis works through steady wedging of the stem into the beforehand prepared bone cavity. Secondary fixation is ensured with bone ingrowth among the bars of the stem. An experimental analysis concerning the change of femur behaviour after the fitting of the endoprosthesis was carried out and a FiniteElement model was constructed. Implantation ofthe endoprosthesis with bar stem resulted primarily in a significant increase of stress values on the medial side below the collar in comparison with an intact bone. It can be stated that the secondary fixation resulted in more favourable biomechanical conditions is the bone, approaching the state of an intact femur. By means of the mathematical model it was possible also describe the behaviour of the stem structure inside the femur, and will enable the optimalization of the implants size and shape in their further development.
FEM ANALYSES OF HIP PROSTHESES: VALIDITY OF THE 2-D SIDE-PLATE MODEL AND THE EFFECTS OF TORSION. N. Verdonschot and R Huiskes Biomechanics Section, Institute of Orthopaedics, University of Nijmegen, The Netherlands The finite element method was used to analyse the stress-patterns in the femoral part of a bone/prosthesis system. For this study 2-D “side-plate”, symmetric 3-D, and “anatomic” 3-D models were applied. The objectives of the study were to compare the relative merits of the various models and to assess the effects of out-of-plane loading in the 3-D models. The results showed that the 2-D “side-plate” model which accurately reproduces the out-of-plane stiffness of the bone, gives reasonable estimates of stress patterns in the mid-frontal plane. Plane-strain formulation must be preferred to plain-stress, although the differences are only small. Symmetric 3-D models have the additional advantage of information about out-of-plane stress components, and global information about the effects of out-of-plane loading. However, when detailed, local information about stress patterns is required, the symmetric 3-D model is equally unsuitable as the 2-D model, and an “anatomic” model must be applied. Maximal interface stresses due to torsion have similar values as those due to a bending moment of the same magnitude in the mid-frontal plane. Since physiologically the bending moments produced by the hip-joint forces are much higher than the torsional moments it is likely that interface disruption is caused predominantly by the former loading mode.
Abstracts
PBE-STBESSES GENERATED IN ACETABULAB BONE BY SCBEWED-CUP FIXATION IN THA M.Dalstra and RHuiskes Biomechanics Section, Institute of Orthopaedics, University of Nijmegen, the Netherlands Initial stability of screwed-cup fixation is based on high stresses in the surrounding acetabular bone, generated upon insertion of the cup. In this study, these pre-stresses were measured in-vitro in six post-mortem hemipelves, each applied with six rosette strain-gauges. Three of the hemipelves had a polyethylene (PE) cup inserted, the other three a titanium backed (MB) cup. After insertion, the cups were placed in a testing machine in which a hip joint force of 3,000 N was simulated in order to compare the pre-stresses to stresses due to normal loading. Although the torque, needed to insert the ME8 cup was twice as high as for the PE cup (the latter had no self-cutting thread and required pre-tapping), the magnitude of the pre-stresses in the bone for both types of cups were about the same, suggesting that the pre-stresses seem to depend more on the shape of the cup than on the cup’s flexibility and the insertion technique. Values varied between 40 MPa (tensile) and -50 MPa (compressive). During loading, stresses varied between 60 MPa and -50 MPa for the PE cup, whereas for the MB cup these values were only 25 MPa and -25 MPa. This shows that the order of magnitude of the pre-stresses is equal to that of the stresses due to normal loading. Superimposing of the two stress-states did not result in higher stresses everywhere, as tensile and compressive stresses cancelled each other out. These findings were qualitatively backed by results of a axisymmetric finite element study. Finally, it should be stated that large intra-specimen variations in bone quality cause the pre-stresses to be quantitatively unpredictable.
INTERACTIONOF FEMJR- HIP JOINT ENNX’ROSTHESIS WITHARTICLLATEO STEW J.JirovB, T.KauflerovB and J.Wca Institute of Theoretical and Applied Mechanics CSAS, Czechoslovakia The paper deals with the interaction between the POLO1endoprosthesis with an articulated stem and femur. The subject of the research was to gain basic knowledge about forceflow through the implant into the bone tissue and to find out stress distribution in the stem structure. The stem of this cementless endoprosthesis which consists of five dia. 4.5 mm bars 200 mn long is made of stainless steel. The upper ends of the bars are firmly connected with the collar, their lower ends are welded together. Primary fixation of the endoprosthesis works through steady wedging of the stem into the beforehand prepared bone cavity. Secondary fixation is ensured with bone ingrowth among the bars of the stem. An experimental analysis concerning the change of femur behaviour after the fitting of the endoprosthesis was carried out and a FiniteElement model was constructed. Implantation ofthe endoprosthesis with bar stem resulted primarily in a significant increase of stress values on the medial side below the collar in comparison with an intact bone. It can be stated that the secondary fixation resulted in more favourable biomechanical conditions is the bone, approaching the state of an intact femur. By means of the mathematical model it was possible also describe the behaviour of the stem structure inside the femur, and will enable the optimalization of the implants size and shape in their further development.
FEM ANALYSES OF HIP PROSTHESES: VALIDITY OF THE 2-D SIDE-PLATE MODEL AND THE EFFECTS OF TORSION. N. Verdonschot and R Huiskes Biomechanics Section, Institute of Orthopaedics, University of Nijmegen, The Netherlands The finite element method was used to analyse the stress-patterns in the femoral part of a bone/prosthesis system. For this study 2-D “side-plate”, symmetric 3-D, and “anatomic” 3-D models were applied. The objectives of the study were to compare the relative merits of the various models and to assess the effects of out-of-plane loading in the 3-D models. The results showed that the 2-D “side-plate” model which accurately reproduces the out-of-plane stiffness of the bone, gives reasonable estimates of stress patterns in the mid-frontal plane. Plane-strain formulation must be preferred to plain-stress, although the differences are only small. Symmetric 3-D models have the additional advantage of information about out-of-plane stress components, and global information about the effects of out-of-plane loading. However, when detailed, local information about stress patterns is required, the symmetric 3-D model is equally unsuitable as the 2-D model, and an “anatomic” model must be applied. Maximal interface stresses due to torsion have similar values as those due to a bending moment of the same magnitude in the mid-frontal plane. Since physiologically the bending moments produced by the hip-joint forces are much higher than the torsional moments it is likely that interface disruption is caused predominantly by the former loading mode.