Keywords: pelvic bone, acetabulum, cement layer, finite element model, contact stress analysis.

Hip joint replacement is the only way for millions of patients to recover their mobility. Although new designs of replacements are developing every year, manufacturing technologies are advancing, new materials are emerging and surgical methods are also improving, the optimal hip joint replacement has not yet been designed.

The three-dimensional geometrical model of the left pelvic bone was generated from a sequence of 240 CT slices using segmentation procedures. Tissue segmentation was followed by surface reconstruction and geometric model development either volume-based or voxel-based methods.

Modelling and all contact simulations were carried out using the ANSYS finite element (FE) package. For both analyses, boundary conditions at the sacro-illiac joint were specified, where the nodes are fixed in all directions and at the contralateral side of the pubic symphysis, the nodes were fixed in both the x and y directions. For analysis of the cemented acetabular component, quasistatic joint contact forces were defined, representative of the stance phase of gait [1]. The loading was applied to the centre of the ceramic head. In the case of hemiarthroplasty the factors that could influence the migration were considered as parametric inputs. The main geometrical factors, which may influence the migration of the implant are the centre edge (CE) angle and the direction of the loading force. The loading of the acetabulum was modeled by assigning acceleration to the mass steel replacement head. The loading resultant intersected the center of the replacement head. The frontal plane inclination from the plane of interest of the model was set to 19°. Model was solved in various loading directions.

For the cemented acetabular component the stress distribution depends on the quality of the cement layer. Stress distribution in subchondral bone tissue depends on the imperfection in the cement layer. Computational modelling has shown that the subchondral bone at the contact with the cement layer is even unphysiologically loaded by tension stresses during the routine activities of the man. The FE model is appropriate for the solving contact stress analysis of the interaction between the pelvic bone and the cemented acetabular component and will be used in the next analysis. In the case of hemiarthroplasty the stress concentrations and maximal displacement show that primary migration of the replacement head follows the direction of loading. Both the decrease of the CE angle and the increase of the loading direction angle concentrate the stress to acetabular lumbrum area. This very unfavourable situation probably initiates the migration, therefore the natal predispositions of acetabular lumbrum have high influence on the migration. When the prosthesis is fitted higher in comparison with the intact head, the muscles on the lateral side are prestressed and the resultant force is deflected from the vertical direction laterally. The head of the hip joint replacement is fixed correctly, if its centre is 1-3 mm under the greater trochanter [2].

1

Bergmann G., Deuretzbacher G., Heller M., Graichen F., Rohlmann A., Strauss J., Duda G., "Hip contact forces and gait patterns from routine activities", Journal of Biomechanics, 34(7):859-871, 2001. doi:10.1016/S0021-9290(01)00040-9

2

Bartonicek J., "After-surgery exploration", Clinics of orthopaedy, 3, LF UK, 2003.

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