Masayoshi Abo

Effect of acetabular component anteversion on dislocation mechanisms in total hip arthroplasty

Quantifying soft-tissue tension around the hip joint during total hip arthroplasty remains difficult. In this study, a three-dimensional computer-aided design model was developed to clarify how component position in total hip arthroplasty contributes to the primary cause of posterior dislocation in cases of flexion, adduction and internal rotation. To better understand the influences of anteversion angle of the acetabular component, its effects on the primary causes of dislocations and the range of motion were investigated. Three different primary dislocation mechanisms were noted: impingement of the prosthetic femoral neck on the cup liner; impingement of the osseous femur on the osseous pelvis; and spontaneous dislocation caused by soft-tissue traction without impingement. Spontaneous dislocation could be detected by calculating hip forces at any thigh position using the computer-aided design model developed. In computer analysis, a transition from prosthetic impingement rate to osseous impingement rate occurred with increasing anteversion angle of the acetabular component. Spontaneous dislocation was detected at angles > 10° of anteversion of the acetabular component when flexion occurred with extreme adduction and internal rotation. This study demonstrated the possibility of spontaneous dislocation that results not from prosthetic or bony impingement but from muscle traction with increased range of motion.

Effects of Adduction, Internal Rotation, and Flexion Angles on Dislocation for Total Hip Arthroplasty

Passive muscle tension around the hip joint following total hip arthroplasty (THA) plays an important role in post-surgery dislocation mechanisms, especially posterior dislocation. To analyze dislocation objectively and to clarify the distinction between implant-to-implant impingement and dislocation, three-dimensional finite element (FE) models of hybrid THA components were generated. An acetabular component was implanted into the acetabulum in 20 degrees of anteversion and 45 degrees of inclination. The bearing surface had 26 mm hemispherical plus 1 mm flat depth and a chamfered edge. In this study, posterior dislocation-prone movements such as flexion, adduction, internal rotation, and their combinations were analyzed, starting with the femoral component oriented in a manner corresponding to the hip being flexed to impingement with 0°, 10°, 20°, and 30° of internal rotation and 0°, 10°, 20°, and 30° of adduction. The nonlinear explicit FE simulations were driven by inputting a series of incremental flexion moments and hip joint forces concurrently. The angles of internal rotation and adduction affected both impingement and dislocation angles of flexion. The flexion angles both at impingement and dislocation decreased by increasing the internal rotation angles. Although the peak flexion moment to make the hip joint dislocate increased by increasing the internal rotation angle, it didn’t always increase with an increase in the adduction angle. The highest value of the peak flexion moment to dislocation was observed at 30° of internal rotation. Conversely, the lowest value of the peak flexion moment was observed at 10° of adduction. This lowest value means that the hip joint is easy to dislocate at this adduction angle.