Introduction

Calcaneal fractures are the most common tarsal fractures, accounting for about 2% of total body fractures. Approximately 60 to 75% of calcaneal fractures are intra-articular fractures [1,2,3]. Calcaneal fractures often occur in young physical laborers, and many patients have different degrees of physical activity disorders in the long term [4, 5]. At present, the treatment of calcaneal fractures is still controversial. Displaced intra-articular fractures are surgical indications for calcaneal fractures. One of the main objectives of surgery is to restore the posterior articular surface and reduce the occurrence of long-term subtalar arthritis. Timely and accurate identification of calcaneal fractures is essential for subsequent surgical plans.

There are a variety of classification systems for calcaneal fractures, each with a different emphasis. At present, the most common clinical classifications are Essex-Lopresti classification and Sanders classification. Essex-Lopresti divides intra-articular calcaneal fractures into two categories, tongue type fracture and joint-depression fracture, which described the morphology of the fracture fragment and could provide guidance for treatment of calcaneal fractures [6]. Sanders classified calcaneal fractures by the number and location of fracture fragments on the posterior articular surface of calcaneus on coronal and axial CT images, which could make predictions about patients’ outcomes [7]. Despite this classification has been widely used in the clinic, it still has limitations on concordance and inability to guide the surgical procedures [8,9,10].

In 1989, Carr et al., based on morphological study of cadaver bones, proposed that there were two primary fracture lines in sagittal and coronal positions for calcaneal fractures, which divided the calcaneus into different major fracture fragments [11]. Since the main fracture line involves the articular surface of the calcaneus, the morphology of the major fracture fragments determines the treatment and outcome of the patients.

Fracture mapping was first applied to scapular fractures in 2009 by Armitage et al. [12]. Fracture mapping technology, through three-dimensional reconstruction of CT data, superimposes fracture lines of multiple cases on the same standard template to obtain the distribution of the fracture lines, which further helps clinicians better understand the mechanism and morphology of fractures, and formulate treatment procedures. Currently, fracture mapping technology has been used in the proximal humerus, distal radius, ankle, spine, and other fractures of the whole body [13,14,15,16,17]. The aim of this study was to map primary fracture lines and incorporate these in to a combined classification system with specific treatment recommendations.

Materials and methods

Eligibility criteria

Patients with calcaneal fractures between March 2016 and January 2019 were reviewed. The inclusion criteria were as follows: (1) age ≥ 18 years; (2) intra-articular calcaneal fracture; (3) CT scanning layer thickness ≤ 3 mm. The exclusion criteria were as follows: (1) previous history of foot trauma or surgery on the affected side; (2) pathological fracture; (3) old calcaneal fracture; (4) difficulty in reconstructing the fragments.

Fracture mapping

CT images of all patients in digital imaging and communications in medicine (DICOM) format were imported into mimics 20.0 software (materials, Leuven, Belgium), and the fracture fragments were reconstructed, segmented, and reduced. All data were exported into 3-Matic 12.0 software (materials, Leuven, Belgium). The mirror, translation, and rotation functions were used to match the reduced calcaneal fracture model with the standard 3D calcaneal model (reconstructed by a healthy volunteer), and the line of fracture was delineated on the standard model. Finally, the primary fracture lines were imported into e-3D software (Central South University, Changsha, China) to create heat map of the fracture lines.

Data analysis

We characterized the fracture line heat maps from four directions: anterior, superior, medial, and lateral, with different color areas indicating different frequency of fracture line involvement within the region (gradually rising from blue to red). SPSS 18.0 software (IBM, Armonk, NY) was used for Chi-square analysis of patients with different Essex-Lopresti classification. Mann–Whitney test was used to analyze the proportion of fracture fragment area on articular surface of different fracture groups. A value of P < 0.05 was considered as significant difference.

Results

Patient characteristics

A total of 90 patients with CT data were included in this study. Sixty-nine patients were male and 21 were female. There were 42 cases of right foot and 48 cases of left foot. According to Essex-Lopresti classification, there were 35 tongue-type fractures and 55 joint-depression fractures. According to Sanders classification, there were 66 cases of Sanders type II, 19 cases of Sanders type III, and 5 cases of Sanders type IV. Patient characteristics are summarized in Table 1.

Table 1 Characteristics of patients
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Fracture mapping

We grouped 90 cases of calcaneal fractures according to the number and shape of posterior articular fracture fragments: (1) simple tongue-type fractures (26 cases, 28.9%); (2) simple joint-depression fracture (40 cases, 44.4%); (3) comminuted fracture (24 cases, 26.7%) (posterior articular fragments equal 2 refers to simple fracture, and fragments more than 3 refers to comminuted fracture). According to the above grouping, we made the distribution map of primary fracture lines and heat map, respectively (Figs. 1, 2, and 3).

Fig. 1
figure 1

Primary fracture lines of simple tongue type fractures on medial, lateral, anterior, and superior views. A Distribution map, B Heat map

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Fig. 2
figure 2

Primary fracture lines of simple joint-depression fractures on medial, lateral, anterior, and superior views. A Distribution map, B Heat map

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Fig. 3
figure 3

Primary fracture lines of comminuted fractures on medial, lateral, anterior, and superior views. A Distribution map, B Heat map

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The primary fracture lines of calcaneal fractures are concentrated at the Gissane angle and extend posteriorly across the posterior articular surface to the calcaneal tuberosity, the fracture lines of the lateral wall extend from the Gissane angle, and the fracture lines of the medial wall run anteriorly and plantar around the sustentaculum tali process posteriorly in the tuberosity.

The fracture lines of a simple joint-depression fracture on the lateral wall are more sparse than those of the tongue-type fracture and the fracture lines of the medial wall are closer to the anterior. Compared to the simple fractures, comminuted fractures have a concentrated area of fracture lines on the lateral side of the anterior process extending forward to the calcaneocuboid joint.

Calcaneocuboid joint involvement

Different types of calcaneal fractures have different rates of calcaneocuboid joint involvement. There were 43 cases of fracture involving calcaneocuboid joint, including 32 cases (74.4%) of joint-depression fracture and 11 cases (25.6%) of tongue-type fracture. Chi-square test showed statistical differences between them (P < 0 0.05), as shown in Table 2.

Table 2 Calcaneocuboid joint involvement
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Posterior articular surface fragment size

The distribution trend of primary fracture lines of simple tongue type fracture is similar to that of simple joint-depression fracture. On the posterior articular surface, the fracture line of the tongue-type fracture is more medial than that of the joint-depression fracture. We measured the ratio of the area of the lateral fracture fragment to the area of the posterior articular surface. Mann–Whitney test showed that there was a statistical difference between the two groups (P < 0 0.05), as shown in Table 3.

Table 3 Area ratio of lateral fragment of simple fracture
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Discussion

Calcaneal fractures are more common in high-energy injuries with complex fracture morphology, which is not conducive to the study of mechanism and treatment [4].

In 2021, Ni et al. first described the fracture line distribution of calcaneal fractures by fracture mapping technology. Through the analysis of 62 cases of complex intra-articular calcaneal fracture lines, it was found that the calcaneal fractures mainly involved the anterior border of the posterior articular surface, and the fractures rarely involved the sustentaculum tali, posterior calcaneal tuberosity, and anterior process [18]. In the same year, Guo et al. analyzed Sanders II joint-depression fracture and tongue-type fracture respectively through fracture mapping technology. In addition to describing the overall calcaneal fracture lines, they described in detail the fracture lines of calcaneal anterior process, and proposed different fixation recommendations for different subtypes of fractures [19, 20]. In 2022, Yu et al. reconstructed 226 intra-articular calcaneal fractures and drew a two-dimensional fracture map from six sections. This study was the first to associate the distribution of calcaneal fracture lines with the internal structure of the calcaneus, suggesting that the anatomical morphology of the talus and calcaneus and the internal structure of the calcaneus play a crucial role in calcaneus fracture [21].

The lateral and medial wall and plantar side of the calcaneus are often comminuted with multiple irregular fracture fragments. The previous fracture mapping studies of calcaneal fractures superimposed all the fracture lines on the standard template, and finally obtained the fracture line distribution map and heat map [18,19,20,21]. However, the treatment core of calcaneal fracture is the reduction of articular surface, and anatomical reduction is not required for the comminuted fragments on the side wall and plantar side. It is of little significance to represent these parts of fracture line in the heat map for the selection of clinical treatment methods and understanding of injury mechanism. Therefore, it is very important to simplify calcaneal fractures.

The primary fracture lines mentioned by Carr in 1989 divided the calcaneus into different major fracture fragments [11]. Through the study of the primary fracture line, we could Integrate the existing classification and proposed treatment recommendations. Sanders classification proposed in 1993 is actually the classification of calcaneal fractures through the position of the primary fracture line in the sagittal position [7]. In our study, we hid the secondary fracture lines of the medial walls, lateral walls, and the plantar side of the calcaneus, and created the distribution map and heat map of the primary fracture lines, which can help us better understand the morphology and treatment of calcaneal fractures.

Through the description of fracture lines of 90 patients with calcaneal fracture, we found that the primary fracture lines of calcaneus were distributed in two rings on the surface of calcaneus, and the calcaneus was divided into two main fracture fragments by two fracture rings. The lateral fracture ring originates from the Gissane angle, extends posteriorly to the calcaneal tuberosity through the posterior articular surface, then turns laterally to the lateral wall and finally returns to the Gissane angle. This fracture ring forms lateral calcaneal fracture fragment on the calcaneus. The medial fracture ring partially overlaps with the lateral fracture ring at the posterior articular surface and calcaneal tuberosity. It turns around the sustentaculum tali to the medial wall in the middle and rear of the calcaneal tuberosity, and then returns to the anterior process from the front of the middle joint to finally form the medial fracture ring, and divides the medial fracture fragment, which is attached to the triangular ligament complex and is in a relatively stable position in the calcaneal fracture (Fig. 4). Several of the fractures present an anterolateral fracture ring, which originates from Gissane angle, travels anteriorly from the anterior process, and involves the calcaneocuboid joint, eventually turning below the calcaneus to the lateral wall and returning to the Gissane angle, forming an anterolateral fracture fragment involving the calcaneocuboid joint. In our opinion, the treatment of calcaneal fractures is to match the lateral fracture ring with the medial fracture ring.

Fig. 4
figure 4

The fracture rings (red: lateral ring; blue: medial ring)

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In contrast to Carr et al.’s study, we used fracture mapping technology to depict the primary fracture lines of the calcaneus, obtaining a more three-dimensional view of the primary fracture lines and the morphology of the main fracture fragment. According to the morphology of the main fracture fragment, the displaced calcaneal intra articular fractures were summarized into three types: type I: simple tongue type fracture; type II: simple joint-depression fracture; type III: comminuted fracture. Interestingly, the heat maps of the primary fracture lines for these three types of fractures are remarkably similar, which suggests that different types of intra-articular calcaneal fractures may have similar injury mechanisms. At the same time, combined with Athavale’s research on calcaneal trabecular bone, we speculated that the position of the foot and the magnitude of the force can cause the trabecular bone to break in different areas and produce different types of fracture [22].

Because joint-depression fractures are more common to involve the calcaneocuboid joint, we should evaluate type II fractures more carefully before operation and expand the incision to expose the calcaneocuboid joint if necessary. In the process of CT reconstruction and reduction of patients, we found that the shape of posterior articular surface fragments ≥ 3 (type III comminuted fracture) is not a simple 3-column or 4-column as described by Sanders et al. in the diagram [7]. We found that there were transverse fracture lines or small triangular bone blocks on the articular facet (Fig. 5), which could not be classified simply by Sanders classification. This type of fracture has different shapes of fracture fragments on the posterior articular surface and the heat map shows that the calcaneocuboid joint is involved in a high proportion, so more adequate exposure is needed during treatment [23].

Fig. 5
figure 5

Special fracture (A there is a triangular fracture fragment on the posterior articular surface; B there is a transverse fracture line on the posterior articular surface)

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Compared with previous fracture mapping studies of calcaneal fractures, we hide the secondary fracture lines on the surface of calcaneus for the first time, and depict the primary fracture lines of calcaneus and create the heat map [18,19,20,21]. Through the fracture map of the primary fracture lines of the calcaneus, we summarized the fracture lines on the calcaneus surface into two fracture rings for the first time, which could help us have a further understanding of the morphology and treatment of calcaneal fractures.

Our study has some limitations. Firstly, our study included a small number of cases and did not include non-displaced intra-articular calcaneal fractures, which may bias the results to some extent. Secondly, some complex fractures were excluded from this study due to the inability of effective reconstruction and reduction of some comminuted fractures. Finally, because the calcaneal morphology of each patient is not completely matched with the standard template, there will also be some bias in drawing the fracture line.

Conclusion

In this study, the morphology of calcaneus fractures was further understood by describing the main fracture lines of calcaneus. The main conclusions are as follows:

  1. 1.

    Despite differing levels of comminution and displacement, the primary fracture lines of the calcaneus, which could be summarized into two rings, are remarkably similar between classes of well-established clinical classification systems.

  2. 2.

    The calcaneocuboid joint is more common involved in joint-depression fractures.

  3. 3.

    The area of posterior articular surface involved in the lateral fracture fragment of tongue type fracture is larger.

Based on the distribution of primary fracture rings, we integrated calcaneal fractures and proposed some treatment recommendations.