Fracture Line Morphology and a Novel Classification of Pilon Fractures

Abstract

Objective: Currently, there is no research that includes a comprehensive three-dimensional fracture mapping encompassing all types of Pilon fractures. Moreover, the existing classification systems for Pilon fractures exhibit only moderate to fair consistency and reproducibility. Additionally, some of these classification systems fail to accurately depict the morphological characteristics of the fractures. This study aimed to create a fracture map encompassing all types of Pilon fractures by three-dimensional fracture mapping. In addition, this study conducted a finite element analysis of the normal ankle joint, and based on the distribution of fracture lines and the stress distribution at the distal tibia, proposed a new classification for Pilon fractures.

Methods: A retrospective analysis of Pilon fractures in our hospital from January 2018 to January 2024 was performed. A total of two hundred forty-four Pilon fractures were included, and their fracture lines were transcribed onto the tibia and fibula templates, and fracture maps and heat maps were created. A nonhomogeneous model of the ankle joint was constructed and verified, and the stress distribution on the distal tibia articular surface was measured and analyzed in three models (neutral, dorsiflexed, and plantarflexed model). Based on the fracture map and stress distribution, a five-column classification system for Pilon fractures was proposed, and the intraobserver and interobserver reliability was calculated using Cohen and Fleiss k statistics.

Result: The fracture line on the distal tibia articular surface showed a V-shaped distribution. One branch extended from the junction of the medial malleolar articular surface and the inferior tibial articular surface toward the medial malleolus. The other branch extended from the middle of the fibular notch to the posterior part of the medial ankle, toward the tibial shaft. The fibula fracture line mainly extended from the anterior and lower part of the lateral malleolus to the posterior and upper part. As evidenced by the neutral, dorsiflexed, and plantar flexion models, the stress on the posterolateral articular surface (posterolateral column) was low, while the majority of the stress was concentrated in the center. Three-column fractures were the most common, followed by two-column fractures. Using the five-column classification, the K-weighted values of interobserver and intraobserver analysis were 0.653 (p < 0.001) and 0.708 (p < 0.001), respectively.

Conclusions: In this study, the fracture line and morphological characteristics of Pilon fractures were analyzed in detail by three-dimensional mapping. In addition, this study conducted a finite element analysis of the stress distribution on the distal tibial joint surface of the normal ankle joint. Moreover, a novel classification system was proposed to reflect these findings. The new classification not only exhibits greater consistency, facilitating accurate communication of fracture characteristics among surgeons, but also aids in understanding the mechanisms of injury and formulating surgical strategies.

Keywords: classification; computed tomography; finite element analysis; fracture mapping; pilon fractures.

FIGURE 1

Representative images of steps in the method used for 3D mapping of Pilon fractures. In this example of a Pilon fracture, each fragment was reconstructed (A), segmented (B), and virtually reduced (C). The fracture was then mirrored (if left) and matched to the model (C) of one‐third of the tibia. The contour of every fracture fragment was marked with smooth curves to delineate the fracture lines (D).

FIGURE 2

Overview of finite element analysis workflow. (A) First, acquire the three‐dimensional CT data of the volunteers; (B) reconstruct the bone using Mimics; (C) further refine the model in Geomagic; (D) perform ligament reconstruction in SolidWorks; (E) mesh the model in Hypermesh; (F) import the mesh model into Mimics for material assignment; (G) import the model into Ansys for solving; and (H) conduct post‐processing of the results in Workbench.

FIGURE 3

Material assignment for tibia and fibula. (A) Bone density (ρ) and elastic modulus (E) of each material. (B) Number of elements of every material. (C) External material properties of the tibia. (D) Internal material properties of the tibia.

FIGURE 4

Five‐column classification of Pilon fracture and definition of posterolateral column. (A) Schematic layout of five‐column classification of Pilon fractures. (B) The definition of posterolateral column (L1:L2 = 6:1).

FIGURE 5

The maps of the hot zones of 3D fracture lines of all Pilon fractures. (A–E) Representative views of the distal tibia and fibula. (F–J) 3D heat mapping superimposed with all Pilon fracture lines, including the axial, front, lateral, posterior, and medial views. Red represents a higher frequency of fracture line density.

FIGURE 6

Distribution of von Mises stress in the articular surface of the distal tibia. Von Mises stress distribution of the distal tibial articular surface in neutral, dorsiflexion and plantar flexion.

FIGURE 7

Number of fractures per subtype. The bar graph shows the number of each subtype. Me: Medial column; An: Anterior column; Mi: Middle column; Po: Posterolateral column; and La: Lateral column.

FIGURE 8

The intervals for the standard surgical approaches for Pilon fractures. The intervals for the standard surgical approaches for Pilon fractures including anteromedial (A), anterior (B), anterolateral (C), medial (D), posteromedial (E), posterolateral fibula (F), and posterolateral tibia (G).