The role of the anterior column of the acetabulum on pelvic stability: a biomechanical study

Digital Anatomy to Improve Screw Insertion Techniques for Plate-Screw Fixation of the Pubic Body

Objectives

This study aims to investigate screw insertion sites on the pubic body and the safe screw insertion parameters of a plate-screw fixation system based on the premise of avoiding damage to the inguinal canal and disruption of the rectus abdominis at the pubic symphysis and pubic crest.

Research Methods

Excluding cases with poor image quality, tumors, malformations, and fractures, the data of 80 healthy adults (40 males and 40 females aged from 21 to 83 years old, with an average age of 51.65 years) undergoing a computed tomography (CT) scan of the pelvis between January and June of 2017 were collected from Shandong Provincial Hospital. The CT scans were imported to Mimics® software to reconstruct three-dimensional pelvic models. A 3.5 mm pelvic reconstruction plate was placed starting at the outer edge of the pubic tubercle and along the pelvic brim. The two innermost screw insertion sites were marked. The safe range for the screw insertion sites was then determined. The screw insertion plane was selected to measure the safe screw insertion parameters. The length of the screw, the direction of insertion, and intersex differences were then explored via statistical analyses.

Results

The medial inclination angles (MIAs) of the screw insertion plane for males and females were 30.42±7.95° and 32.88±10.65°, respectively, with no statistically significant differences. For the medial screw, the maximum anterior inclination angle (MAIA), the maximum screw length, and the maximum posterior inclination angle (MPIA) were 46.51±4.01°, 12.40±9.53 mm, and 11.78±10.22° on average, respectively, with no significant differences by gender (P>0.05). For the lateral screw, the MAIA was 10.35±9.46° and showed no gender differences (P>0.05), but the male group had a greater MPIA (male 11.80±11.00° vs. female 6.23±7.91°, P<0.05) and maximum screw length (male 55.71±6.36 mm vs. female 48.68±8.65, P<0.001). For the tangential screw, the maximum screw length, MIA, and anterior/posterior inclination angle (APIA) were 52.19±8.33 mm, 31.65±9.42°, and 7.53±10.18°, respectively, with no significant differences in the angles by gender (both P>0.05), although the screw length in the male group was significantly longer than that in the female group.

Conclusions

Insertion of two screws into the pubic body through a plate from the lateral side of the pubic tubercle is safe and can maintain the origin of the rectus abdominis and the integrity of the inguinal canal compared to traditional plate-screw fixation. Considering that the pubic body is thinner on the lateral side, we suggest a more medial inclination angle for the lateral screw.

Segmental acetabular rim defects, bone loss, oversizing, and press fit cup in total hip arthroplasty evaluated with a probabilistic finite element analysis

PURPOSE

Management of segmental rim defects and bone mineral density (BMD) loss in the elderly prior to total hip replacement is unclear within classification systems for acetabular bone loss. In this study, our objectives were (1) to understand how a reduction in BMD in the elderly affects the oversizing of a press-fit cup for primary fixation and (2) to evaluate whether the location of the segmental defect affected cup fixation.

METHODS

A finite element (FE) model was used to simulate and evaluate cup insertion and fixation in the context of segmental rim defects. We focused on the distribution of patients over age 70 and used BMD (estimated from CT) as a proxy for aging's implications on THR and used probabilistic FE analysis to understand how BMD loss affects oversizing of a press-fit cup.

RESULTS

A cup oversized by 1.10 ± 0.28 mm provides sufficient fixation and lower stresses at the cup-bone interface for elderly patients. Defects in the anterior column and posterior column both required the same mean insertion force for cup seating of 84% (taken as an average of 2 anterior column and 2 posterior column defects) compared to the control configuration, which was 5% greater than the insertion force for a superior rim defect and 12% greater than the insertion force for an inferior rim defect.

CONCLUSIONS

A defect along the superior or inferior rim had a minimal effect on cup fixation, while a defect in the columns created cup instability and increased stress at the defect location.