How much change in pelvic sagittal tilt can result in hip dislocation due to prosthetic impingement? A computer simulation study

Abstract

Developing spinal pathologies and spinal fusion after total hip arthroplasty (THA) can result in increased pelvic retroversion (e.g., flat back deformity) or increased anterior pelvic tilt (caused by spinal stenosis, spinal fusion or other pathologies) while bending forward. This change in sagittal pelvic tilt (SPT) can result in prosthetic impingement and dislocation. Our aim was to determine the magnitude of SPT change that could lead to prosthetic impingement. We hypothesized that the magnitude of SPT change that could lead to THA dislocation is less than 10° and it varies for different hip motions. Hip motion was simulated in standing, sitting, sit-to-stand, bending forward, squatting and pivoting in Matlab software. The implant orientations and SPT angle were modified by 1° increments. The risk of prosthetic impingement in pivoting caused by increased pelvic retroversion (reciever operating characteristic [ROC] threshold as low as 1-3°) is higher than the risk of prosthetic impingement with increased pelvic anteversion (ROC threshold as low as 16-18°). Larger femoral heads decrease the risk of prosthetic impingement (odds ratio {OR}: 0.08 [932 mm head]; OR: 0.01 [36 mm head]; OR: 0.002 [40 mm head]). Femoral stems with a higher neck-shaft angle decrease the prosthetic impingement due to SPT change in motions requiring hip flexion (OR: 1.16 [132° stem]; OR: 4.94 [135° stem]). Our results show that overall, the risk of prosthetic impingement due to SPT change is low. In particular, this risk is very low when a larger diameter head is used and femoral offset and length are recreated to prevent bone on bone impingement.

Keywords: computer modeling; hip arthroplasty; hip biomechanics.

Figure 1

This figure shows how the computer simulation model. (A) pelvis computed tomography (CT) scan. (B) Femur CT scan. (C) Acetabular cup computer aided design (CAD) model. (D) Acetabular liner CAD model. (E) Femoral stem and prosthetic head CAD model. (F) Computer simulation of sitting motion [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

(A) Polar axis (PA) represents the point where the line passing through the center of the prosthetic neck exits the head. (B) Motions of the (B, C) PA inside the cup (D) can be mapped and studied [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

This figure shows the area inside 28, 32, 36, and 40 mm liners. Red line represents a true prosthetic impingement and blue line represents the 90% distance between the center and the edge of the liner [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Anterior pelvic plane (APP) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Functional femoral neck anteversion was measured relative to the vertical plane in standing [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

This figure shows the motion simulation map. Each colored dot on the map shows the closest distance of the PA to the edge of the liner for each of the tested motions. For example, the colored dot which represents sit‐to‐stand, represents the closest position of PA to the edge of the liner during this motion when the pelvis is at this maximum anterior tilt, right before the patient gets up from the sitting position. PA, polar axis [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

This figure shows the sample ROC curve for sit‐to‐stand motion using a 28‐mm prosthetic head and a stem with 127° neck‐shaft angle. AUC, area under the ROC curve; ROC, rreceiver operating characteristic [Color figure can be viewed at wileyonlinelibrary.com]