# Pelvic symmetry studied for the treatment of acetabulum fractures in Traumatology

Stéphane Candoni, Virginie Taillebot, Marie Le Baron, Xavier Flecher

Martine Pithioux

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Stéphane Candoni, Virginie Taillebot, Marie Le Baron, Xavier Flecher, Martine Pithioux. Pelvic symmetry studied for the treatment of acetabulum fractures in Traumatology. Multidisciplinary Biomechanics Journal, 2025, 50th congress of the.., 10.46298/mbj.16139. hal-05191691v2

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# Pelvic symmetry studied for the treatment of acetabulum fractures in Traumatology

Stéphane Candoni<sup>a, b*</sup>, Virginie Taillebot<sup>a, b</sup>, Marie Le Baron<sup>c</sup>, Xavier Flecher<sup>d, e</sup>, Martine Pithioux<sup>a, b</sup>

<sup>a</sup> Aix Marseille Univ, CNRS, ISM, 13009 Marseille, France

<sup>b</sup> Aix Marseille Univ, APHM, CNRS, ISM, Sainte-Marguerite Hospital, Institute for Locomotion, Department of Orthopaedics and Traumatology, 13009 Marseille, France

<sup>c</sup> Assistance publique-Hôpitaux de Marseille, hôpital Nord, institut du mouvement et de l'appareil locomoteur, service de chirurgie orthopédique, pôle locomoteur, Marseille, France

<sup>d</sup> Institut du Mouvement et de l'appareil locomoteur, Aix Marseille Université - Assistance publique des Hôpitaux de Marseille, 270, boulevard Sainte-Marguerite, 13009 Marseille, France

<sup>e</sup> CHU de Marseille Nord, institut du mouvement et de l'appareil locomoteur, chemin des Bourrely, 13015 Marseille, France

* Corresponding author: stephane.candoni@univ-amu.fr

Received date: 06/04/2025

Accepted date: 27/06/2025

Publication date: 27/10/2025

**Keywords:** pelvis, 3D models, surgical planning, CT scan images, symmetry

© 2025 The Authors

Licence CC-BY 4.0

Published by Société de Biomécanique

## 1. Introduction

The reduction of acetabulum fractures is difficult due to the complex anatomy of the pelvis. The open reduction with internal fixation is the most common surgical treatment. However, it represents an invasive and time-consuming procedure. As a result, patients are exposed to increased risk of post-operative complications, morbidity and mortality. Especially for the elderly, the morbidity rate is almost 24% for people aged >70 years old (Albrektsson et al. 2023). In order to limit these complications, surgeons aim to reduce the fracture as close as possible to the healthy state, i.e. with less than 2 mm difference. This value represents the clinical threshold which considers an good reduction according to the Matta criteria (Durrani et al. 2024). However, the geometry of the pelvis and more particularly the acetabulum is complex, and in order to characterize this alignment, surgeons are mainly inspired by the healthy contralateral side.

The aim of this study is therefore to characterize the symmetry of healthy pelvises in order to determine to what extent the surgeon can use the contralateral side as a model when planning the operation.

## 2. Methods

### 2.1 Patients and scan images

The scans were chosen from the computed tomography (CT) scan image database of the Hôpital Nord de

Marseille. All procedure followed the Personal Data Protection Obligations and all patients have given their consent. These scans were TAP ones (Thoraco-abdomino-pelvic) and had to contain healthy pelvises with a slice thickness in the range [0.63-1.25] mm. The largest voxel size is 1,25mm×0,98mm×0.98 mm. The age limit that distinguishes medically young and old patients was defined as 60 years. Preliminary tests were made from a database of 28 patients. The average age is 55.5±20 years old with a range of 22-84 years old. In order to compare them, they are equally divided according to age and sex: 14 men vs 14 women, 14 young vs 14 old.

### 2.2 Construction of 3D models

Segmentation and 3D reconstruction followed a standardised semi-automatic process. The DICOM files were imported into Materialise Mimics (Materialise, Leuven, Belgium, version 24). The Mimics segmentation tools were used to create a mask to remove noise and isolate the pelvic bones from the soft tissue and other bones. The remaining holes were then filled manually if necessary. Finally, the two bones were reconstructed in 3D and smoothed. The resulting STL files were imported into 3-matics (Materialise, Leuven, Belgium, version 16.0) to improve mesh quality and exported in .txt format for data processing. The mesh
on each side was made uniform using triangular elements with dimensions of 1.3 mm. These steps result in a geometric error of 1.4 ± 0.34 %, calculated from the difference between the initial volume and the final volume of the pelvis.

## 2.3 The 3D deviation analysis

The search for a plane of symmetry and the calculation of deviation were carried out using the MATLAB programming language. An algorithm is developed to generate the best plane of symmetry between each side of the pelvis. It gave a first resulting mirror image. Then, a rigid alignment between each mirror image to its contralateral part was applied for a better concordance, giving a second mirror image. The algorithm used for the alignment was based on Coherent Point Drift (CPD) registration (Myronenko and Song 2010).

Finally, in both cases, the deviation was calculated between each node of a side in relation to the surface of its contralateral part.

The average deviation between each hemi-pelvis is given by the formula (Candoni S. 2024.):

$$ Deviation(X, Y) = \sqrt{\frac{1}{n+k} \left[ \sum_{i=1}^{n} d^2(x'_i, Y) + \sum_{j=1}^{k} d^2(y'_j, X) \right]} $$

* X is the right hemi-pelvis, Y the left hemi-pelvis, X' the mirror image of the right-hemi pelvis and Y' the mirror image of the left hemi-pelvis.

* d is the distance $d(y, y_{pro}) = ||y - y_{pro}||_2$ with $||.||_2$ the Euclidean norm and $y_{pro}$ the projection of y on the plane formed by the nearest triangle in the mesh surface of X.

Therefore, the symmetry of the pelvis was determined as the percentage of nodes that display less than 2 mm of deviation (figure 1), as the surgeon can use it as a reference to satisfy Matta's criteria:

$$ \tau_{sym} = \frac{\text{number of nodes with deviation} < 2\text{mm}}{\text{number total of nodes}} $$

Age and sex were compared using the Mann-Whitney U-test on Matlab (Candoni S. 2024.).

<figure><img src="image_2.png" /><figcaption>Figure 1. Map of deviation on a pelvis using Matlab.</figcaption></figure>

White area: with deviation ≥ 2mm; Orange area: with deviation < 2mm

## 3. Results and discussion

First of all, this study showed that according to a plane, the symmetry of the pelvis ($\tau_{sym} = 66.7 \pm 16.5\%$) was not sufficient for the surgeon. A higher degree of symmetry was obtained by applying a rigid alignment after obtaining the mirror image with regards to the plane ($\tau_{sym} = 91.5 \pm 3.7\%$).

In addition, a comparison by gender does not seem to reveal anything but the statistical study carried out between the two age groups seemed to show a difference ($p_{value} = 0,0081$). However, the values between the two groups are similar, with an average of 1.07 mm for young subjects and 1.24 mm for old ones. These results will be verified with a larger sample.

## 4. Conclusions

This study demonstrates the high degree of symmetry of the pelvis. It led to the development of tools and 3D visualisation that can be used to extend the study to local areas and help the surgeon to plan the treatment of acetabulum fractures digitally. For further studies, more patients will be taken into account in order to cross-reference comparisons by age and gender.

## Conflict of Interest Statement

None.

## References

Albrektsson, M., Möller, M., Wolf, O., Wennergren, D., & Sundfeldt, M. (2023). Acetabular fractures:
Epidemiology and mortality based on 2,132 fractures
from the Swedish Fracture Register. *Bone & joint open,*
*4*, 652-658. https://doi.org/10.1302/2633-1462.49.
BJO-2023-0085.R1

Candoni, S. (2024). *Pelvic symmetry studied with the segmentation software. Materialise for the treatment of acetabulum fractures in Traumatology*. University of Padova Department of Information Engineering.

Durrani, M. Y. K., Ali, U., Jamil, Z., Umer, M., Durrani, M. Y. K., Ali, U., Jamil, Z., & Umer, M. (2024). Radiological outcomes according to the matta score after the surgical fixation of acetabular fractures. *Cureus*, *16*(11), e74803. https://doi.org/10.7759/cureus.74803

Myronenko, A., & Song, X. (2010). Point set registration: Coherent point drift. *IEEE Transactions on Pattern Analysis and Machine Intelligence*, *32*(12):2262–2275. https://doi.org/10.1109/TPAMI.2010.46