The 3D mapping technique was described by Cole et al. and Mellema et al. [15, 16]. In the current study, we perform 3D mapping technology representing the distribution and frequency of fracture lines and comminuted areas of ICFs. The overall 3D mapping of ICFs showed that the fracture lines mainly concentrated at the critical angle of Gissane, extended along the laterally to posteriorly and affected the lateral wall, the anterior area of the posterior joint facet, developed along the posterior joint facet and calcaneus sulcus, and extended posteriorly, medially to affect the posterior aspect and cortical walls. The anterior of the rear facet joint was predisposed to plenty of fracture lines and comminution zones. Moreover, the research shows that ICFs have consistent fracture patterns and comminuted zones. These results reveal the calcaneal's internal structure characteristics, provided more detailed information than traditional X-ray or CT, indicating that the distribution of fracture lines of ICFs related to the internal anatomical structure.
Several studies have described the distribution and orientation of the calcaneus fracture lines [17, 18]. The primary fracture lines cut the calcaneal into two parts, medial and lateral [8, 9]. Carr et al.[19] reported that the fracture line typically separated the posterior facet and extended anteriorly to involve the anterior cuboid facet and could extend medially to affect the middle facet. Essex-Lopresti suggests that the anterolateral process of the talus generated the primary fracture lines to separate the middle facet. Warrick et al.[21] reported that the main fracture lines extend posterior medial of the sustentacular fragment, with differences in extension distance. Recently, Tsubone et al. predicted fracture lines of the calcaneal by a 3D finite element model. They showed that the fracture line always starts from the lateral of the posterior joint fragment and extends in the anteromedial and anterolateral directions. Ni et al.[14] showed that the fracture lines of ICFs were mainly concentrated in the calcaneus sulcus, extended medially, rear, anteriorly to affect the posterior facet surface and cortical walls. In addition, One study reported that the medial wall of the sustentacular and tuberosity fragments at the fracture site often had comminuted zones [22]. In the current study, the fracture lines and comminution zones were regularly distributed along the calcaneal surface. The majority of the fracture lines were located in the critical angle of Gissane and the area anterior to the posterior joint facet and extended along the posterior joint facet and calcaneus sulcus to the posteriorly of the tuberosity and was a typical characteristic of inverted “Y” pattern in the lateral wall. These findings have led to a better understanding of the pattern of calcaneal fractures, allowing for a better selection of surgical incisions and fixation methods.
The distribution of the fracture lines and comminution zones correlates well with the calcaneal's internal submicroscopic structure and biomechanics. The calcaneus is the most prominent tarsal bone, which provides elastic but forceful support for the body's weight, with a thin cortical shell surrounding cancellous bone [23, 24]. Athavale et al. [23] proved that the weaker zones of calcaneal and emphasize the primary influence of the internal architecture in predicting the fracture lines. Chen et al.[25] perform a finite element model to evaluate the biomechanical of locking plates, showed that the fragments at the posterior articular surface and the posterior tuberosity sustained more stress. Xu et al.[26] reported in another study the loading force was transmitted primarily by the anteroinferior portion, remarkably close to the bottom of the sinus tarsi, with the significant contact regions on the lateral, anterior and posterior sides. Wong et al.[27] evaluated the influence of foot impingement on the risk and location of calcaneal fracture by finite element model, and stresses were primarily in the angle of Gissane and posterior articular surface. One study reported that knowledge of the weak areas can improve the technique of internal fixation [23]. By 3D heat maps, the study to be consistent with previous studies. These more vulnerable zone’s location indicates that the area should be avoided when screw fixation is applied. Furthermore, the fracture lines and comminution areas of ICFs revealed in our study may improve the fixation concepts.
The calcaneus fractures rarely involve the sustentaculum tali. Several studies have proven that sustentaculum tali is a ‘‘constant fragment’’[22, 28, 29]. However, Heger et al. evaluated 25 patients with calcaneal fractures and reported the sustentacular fractures in eighteen [30]. Della Rocca et al. evaluated more than 300 cases of calcaneal fractures treated with surgery and found 19 cases of sustentacular fractures [31]. The present study found that 21 (25.9%) fracture lines passed into the sustentaculum tali. Our findings are consistent with the work of Heger et al. and Della Rocca et al. in that patient with ICFs involved the sustentacular fragment that challenges the notion of anatomic constancy as a ‘‘constant fragment’’. We also found that no comminution in the sustentaculum tali. Therefore, it provides an effective position for screw fixations. Although our study is a 3D reconstruction of superimposed all fracture lines in a standard template. Any comparison of the outcome should be cautions because of subtle methodologic differences. Moreover, the isolated lateral approach is based on the “constant” nature of the sustentacular fragments[32]. The idea that the fragment is “constant” invites an alternative surgical approach that should be improved. Berberian et al. said it seems reasonable to consider a medial approach or combined medial and lateral approaches when the sustentaculum tali is seen to be fractured on preoperative CT scans[32]. So, we think a CT scanning should be routinely performed when a suspected fracture is found in the sustentacular fractures.
Calcaneal fractures caused by axial load is the most common [2]. However, the mean angle of fracture lines concerning the LCA was 29.1 (range, -71.45° to73.99°) and 19.2 (range, -71.45° to73.99°) in the lateral wall and medial wall. The vertical fracture line is rare. We also demonstrated that fracture lines distribution in the anterior process of the calcaneus was relatively rare, and the CCJ was involved with the fracture lines consistent with Ni et al.[14]. In addition, we found that the comminution zones tend to involve the inferior one-third aspect of the medial of the CCJ. Several studies have confirmed the probability of CCJ involvement in calcaneal fractures ranges from 33–76% [33–35]. Previous studies have shown that poor reduction of CCJ can lead to impingement symptoms or lateral peritalar subluxation [35]. Many studies relied on x-rays only and cannot routinely perform CT scans[36]. In the 3D heat maps, the comminution zones were located on the inferior one-third aspect of the medial of the CCJ surface. These might not be apparent in traditional radiology, so CT scanning should be routinely performed for patients with calcaneal fractures.
3D mapping can help develop a more comprehensive classification system. Earlier classification systems for calcaneal fractures were based on traditional X-rays; the Essex-Lopresti system is the best known [2]. This study provided a good description of the mechanism of injury and the orientation of the fracture line and helped identify extra-articular injuries and intra-articular injuries. However, to our knowledge, visualization of the calcaneal anatomy and specific comminution zones at conventional X-ray is limited. The involvement of the subtalar articular surface and medial wall cannot reflect by Essex Lopresti classification. Moreover, the interobserver reliability among radiologists was poor for the Essex-Lopresti classification (kappa = 0.26) [37]. In this study, we found that the fracture lines directions are continuous variables. Designating the location of fracture lines as dichotomous variables in the form of the classification systems will never result in an entirely consistent result. We believe that the subjective classification of fracture patterns into two types described by Essex Lopresti et al. may not obtain satisfactory interobserver agreement. Therefore, the calcaneus classification based on X-ray findings is obsolete. In 1993, based on coronal and axis, CT images of the Sanders classification are the most used system for classifying ICFs [38], subdividing into four types, depending on the number of fractures and the position of fracture lines at the posterior calcaneal facet[5]. Despite its being widely used, the value of this classification is always disputed due to its limited reliability and validity [39]. This system does not consider fracture displacement in the sagittal or axial plane relative to the widest undersurface of the posterior talar articular surface and the pathological changes of calcaneal fractures. Therefore, a new classification that can reflect morphological changes and damage to the subtalar articular surface should be seriously considered. In the current study, the areas with the highest concentration of fracture lines and comminuted zones were described by 3D mapping. The orientation of the fracture lines and the location of the comminuted zones, which can be accurately reflected. The fracture mapping can provide clearer, more accurate information as well as enhancing our understanding [40]. Compared with previous anatomical and radiological reports, the present 3D mapping provides more detailed information and may prove helpful in facilitating improved comminuted zones and morphology understanding of classification concepts to manage complex ICFs injuries better.
Improved understanding of ICFs morphology and fracture lines by 3D heat maps may facilitate preoperative planning and development of fixation concepts. Several biomechanical studies comparisons the advantage of indifferent fixed ways [25, 41, 42]. To adapt to the anatomical and biomechanical characteristics of the subtalar and CCJ, plate fixation may be a good option [43]. Plate fixation using a sinus tarsi approach, which is currently popular, directly reduction the articular surface through the incision [44]. This approach was the most popular minimally approach for treatment of calcaneal fractures. However, in the sagittal plane, the fracture line always points to the critical angle of Gissane and extends posteriorly the calcaneal tuberosity of the lateral wall. The most concentration of fracture lines was slightly below the tarsal sinus approach. This fracture line’s location indicates that when fixation from the lateral wall, the screws should be positioned to avoid this area as much as possible and potentially suggests that a lower preoperative incision approach would be better.
Although there are important discoveries by these studies, there are also limitations to be considered. First, patients with insufficient CT data were excluded. The exclusion of these patients resulted in a statistical error in fracture incidence. Second, the number of included patients was relatively small, and the more accurate the results might be if the larger the number of cases. Third, the methods and results were descriptive, and one may argue that the interpretation of 3D mapping is subjective. Fourth, because of the limitations of 3D mapping technology, some reconstructed models cannot well match the 3D calcaneal model. The fracture lines and comminution zones superposition on the calcaneus model might be subtly different. Finally, due to the virtual reduction procedure, the existing 3D heat maps technique can only show the distribution of the fracture line and comminution zones on the calcaneal surface rather than the displacement and compression of the fragments. However, our study also had important strengths. To our knowledge, we are the first to apply the 3D mapping technique[11] to describe the correlation between the common comminution zones and fracture lines in ICFs.