Biomechanical Analysis of a New Type of Surface Total Hip Replacement

In this study we introduced a new type of surface replacement. The basic concept of the femoral component was to prevent stress shielding and avascular necrosis of the femoral head by minimizing the resurfaced area in comparison to the currently available surface replacement. A small peg was added to the center of the implant in order to provide the initial rigid fixation. In developing the new type of surface replacement, the fixation angle, covering angle, and effect of the femoral component peg on stress distribution, as well as the open angle of the acetabular component, were analyzed, using the rigid-body spring model and the finite element method. Two-dimensional finite element models and rigid-body spring models of the femoral head with the component were developed. The new and currently- used components were compared, and the effects of the fixation angle and the peg on stress distribution were analyzed. The optimal resurfaced area was also analyzed by changing the coverage angle from 120° to 180°, since an extremely small implant would lead to severely limited motion and failure of the fixation. For the acetabular component, the open angle should be decreased in order to prevent both socket-neck impingement and increase in the amount of resected bone in the acetabulum. However, the safety zone of the open angle must be also determined to avoid dislocation of the component. Rigid body spring models consisting of the acetabular and femoral components were developed with an anti-dislocation spring at the edge of the acetabular component. The direction of the resultant force that generates the force in the anti-dislocation spring was analyzed. Severe stress shielding was seen in the central portion of the femoral head with the current type of surface replacement, while the new type showed a similar pattern of stress distribution to that of the normal femoral head up to a coverage of 180°. Stress concentration was seen along the peg, while stress shielding occurred immediately below the implant with a long peg. Full coverage of the femoral head by the component and insertion of the implant parallel to the axis of the femoral neck could result in abnormal stress distribution, including stress shielding of the central portion of the femoral head and stress concentration at the medial side of the neck. A dislocation force was generated when the resultant force was applied within 30° of the edge of the acetabular component, which meant that there was a possibility of dislocation with an open angle of less than 120° under physiological loading conditions.