Using nonlinear finite element models to analyse stress distribution during subluxation and torque required for dislocation of newly developed total hip structure after prosthetic impingement.

Dislocation is a serious potential complication of total hip replacement. Previous studies have proposed a newly developed total hip structure that meets the required oscillation angle of 120°, for which the chamfer on the acetabular liner rim was designed to enable the neck to impinge on the chamfer over a large area after impingement occurs. This study adopted the finite element method to further analyse the torque limits leading to dislocation and the contact stresses at the impingement and egress sites of the liner during subluxation. The compressive stress-strain curve for ultra-high molecular weight polyethylene is nonlinear. The results reveal that an adequate chamfer angle of the acetabular cup liner can significantly increase dislocation torque and decrease contact stress on the liner rim. By means of the new design, when the head-neck ratio (HNR) is 2.5 or 3.0, the maximum torque value that a 36-mm head can withstand is 1.38 (8.7 Nm/6.3 Nm) or 1.47 (8.4 Nm/5.7 Nm) times that of a 22-mm head, while the maximum stress of a 36-mm head is 0.41 (14.58 MPa/35.73 MPa) or 0.70 (33.71 MPa/47.90 MPa) times that of a 22-mm head. When the head diameters are identical, the dislocation torque of the HNR = 2.5 structure is slightly greater than that of the HNR = 3.0 structure (3.3-10.5%); thus, the newly developed structure can disperse contact stress, and the structure of a large head with a low HNR exhibits a higher dislocation torque value and lower stress.