Three-Dimensional Mapping of Chaput Tubercle Fractures: Evaluation of Morphologic Characteristics and Anterior Inferior Tibiofibular Ligament Involvement

Abstract

Category: Ankle; Trauma Introduction/Purpose: Chaput tubercle fractures, which are thought to represent tibial-sided avulsions of the anterior inferior tibiofibular ligament (AITFL), are prevalent in up to 30% of trimalleolar ankle fractures. The optimal treatment of small Chaput avulsions is debatable; direct fixation with suture anchor devices and indirect (syndesmotic) fixation are considered as viable options, with proponents on both sides. Moreover, recent literature highlights the potential anatomical alterations to the incisura tibialis resulting from malreduction of large Chaput fragments, furthering the case for direct fixation. Hence, we performed a CT- based three-dimensional fracture mapping study to identify the morphological characteristics of these fractures, and to determine whether they consistently involve the AITFL, tibial plafond and incisura tibialis. Methods: This study included adult patients who had an ankle fracture with a Chaput component; the scheme described by Rammelt et al. was used to classify these fractures. CT scans were obtained, and 3D models were generated. The models were superimposed over a statistical shape model of the right tibia which served as a template and fracture lines were marked. The footprints of proximal and main bands of the AITFL and Basset’s ligament were also marked on the template tibia. The tibial template along with the fracture lines was then imported into MATLAB, and an automated script was used to determine the fragment size (length, breadth, and height), fracture surface area, involvement of the tibial plafond, tibial incisura, and the anterior inferior tibiofibular ligament (AITFL) and Basset’s ligament. Fracture maps and heat maps were generated. Agglomerative cluster analysis using Ward’s linkage was used to identify discrete fracture categories. Results: 76 patients, 21 males and 55 females were included in this study. Cluster analysis identified two distinct groups of fractures, each with two unique subgroups. We present this as a modification of the existing classification system. The first group, corresponded to Rammelt Type 1 fractures (sub centimetric extra-articular avulsion fractures, n=47). Of these, 19% (n=9) did not involve the AITFL, which we termed as Type 1a, and 91% (n=48) involved the AITFL, which we termed as Type 1b. The second group consisted of large intra-articular fractures that corresponded to Rammelt Type 2 injuries. Of these 23% (n=6) involved only the incisura, which we termed as Type 2a; 77 % involved both the incisura and the tibial plafond and were termed as Type 2b. Conclusion: We propose a modification of the existing classification of Chaput fractures on the basis of quantitative fracture mapping. This study provides new insights into the morphological characteristics of Chaput fractures. 19% of small Chaput fractures do not involve the AITFL and may not require direct fixation. Conversely, all large-sized fragments involve the incisura and necessitate anatomical reduction to achieve accurate syndesmotic reduction. Our proposed modification can aid in surgical decision-making, particularly in choosing between direct and indirect syndesmotic fixation.