# Quantitative 3D assessment of marginal impaction volume in posterior wall acetabular fractures: a pilot study

## TL;DR

This pilot study introduces a new 3D CT method to measure marginal impaction in hip fractures, showing it can predict poor outcomes better than current qualitative methods.

## Contribution

The first volumetric quantification of marginal impaction in acetabular fractures using 3D CT analysis.

## Key findings

- An impaction volume threshold of 2000 mm3 strongly predicts poor functional outcomes.
- Patients with higher impaction volumes had significantly worse hip scores and higher osteoarthritis rates.
- Impaction volume showed strong negative correlations with long-term functional scores.

## Abstract

Marginal impaction in acetabular posterior wall fractures significantly influences long-term prognosis, yet current assessment methods remain qualitative. This study introduces quantitative volumetric analysis using three-dimensional computed tomography to evaluate the prognostic significance of impaction volume—representing the first volumetric quantification of marginal impaction in the literature.

Twenty-two patients with acetabular posterior wall fractures and marginal impaction treated between May 2021 and October 2023 at a tertiary trauma center were retrospectively analyzed. Preoperative computed tomography scans were processed using 3D Slicer-5.8.1 software to measure impacted fragment volumes and impaction volume-to-acetabular volume ratios. Functional outcomes were assessed using Harris Hip Score (HHS) and Modified Merle d'Aubigné-Postel Score (MMAS) at 6 months and 2 years. Statistical analysis included correlation analysis, receiver operating characteristic curves, and multivariate regression to identify prognostic factors.

Mean patient age was 45.3 ± 16.8 years (range 23–74) with mean body mass index of 26.4 ± 3.8 kg/m2. Mean impaction volume was 1,847.3 ± 1,124.6 mm3 (range 89–3,842 mm3). Receiver operating characteristic curve analysis identified an exploratory threshold of 2000 mm3 for predicting poor functional outcomes, with area under the curve 0.91 (95% CI: 0.78–1.00, p < 0.001). Patients with impaction volumes > 2000 mm3 (n = 8, 36.4%) demonstrated significantly worse functional outcomes at 2 years compared to those with ≤ 2000 mm3 (HHS: 68.3 ± 10.8 vs 88.5 ± 7.2, p < 0.001; MMAS: 13.4 ± 2.3 vs 17.6 ± 1.2, p < 0.001). Post-traumatic osteoarthritis developed in 9 patients (40.9%), with significantly higher rates in the high-volume group (75.0% vs 21.4%, p = 0.012). Impaction volume showed strong negative correlation with functional scores at 2 years (HHS: r = -0.782, p < 0.001; MMAS: r = -0.758, p < 0.001). The impaction volume-to-acetabular volume ratio averaged 5.12 ± 3.15% and demonstrated similar prognostic value.

This study presents the first quantitative volumetric measurement of marginal impaction in acetabular fractures, which may offer improved prognostic discrimination compared to qualitative assessment in this cohort. An exploratory threshold of 2000 mm3, derived from receiver operating characteristic analysis, appears to stratify patients into different risk categories in this cohort for poor functional outcomes. This objective measurement tool may enhance surgical decision-making and patient counseling in acetabular fracture management. Future multicenter studies are needed to validate this threshold and establish standardized volumetric protocols.