To our knowledge, this is the first study to elucidate the fifth metatarsal base avulsion fractures by means of 3D fracture mapping techniques in combination with anatomical research. Several anatomical and radiographic studies have been conducted on the fifth metatarsal base. Nurcan et al.5 used magnetic resonance imaging and anatomical dissection to study differences in the attachment site of the peroneus brevis to the fifth metatarsal base and have categorized it into six attachment types and mentioned that a narrowly inserted tendon may apply more stress since the internal force applied on per unit area will increase when compared with a wider insertion area. Increased stress may eventually lead a higher tension and may result in an increased risk of fracture. DeVries et al.8 performed anatomical studies on 10 frozen cadaveric specimens, where the attachment sites of the peroneus brevis and the lateral band of the plantar fascia at the fifth metatarsal base was defined as the boundary to divide the tuberosity of the fifth metatarsal into three zones. Parisa et al.12 performed anatomical and biomechanical study of the effect of peroneus brevis on the stability of the base fracture of the fifth metatarsal bone showed that mechanical instability secondary to the deforming force of the peroneus brevis may play a contributory role in delayed union and nonunion of these fractures. Conversely, by spanning the fracture site of avulsion fractures, the peroneus brevis insertion may act to stabilize avulsion injuries in a tension band manner. Existing studies fail to properly describe the fracture characteristics in the fifth metatarsal base avulsion fractures and explain how they were produced because each study was started only from a single aspect, radiographic or anatomical, and failed to elucidate the mechanism of the fracture line formation. Therefore, the present study improved the 3D heat mapping of fracture lines by previous studies13–15 and conducted the 3D examination on the fracture line characteristics of 222 cases with the fifth metatarsal base avulsion fractures. The fracture lines heat map on the dorsal view of fifth metatarsal base could be divided into three zones (bounded by red bands). The distal and proximal fracture lines rarely pass through the tarsometatarsal articular surface, while the middle fracture line (the most common) often passes through the tarsometatarsal articular surface and sometimes through both cortices without involving the articular surface. But the fracture lines rarely involve the fourth and fifth metatarsal articular surfaces. And for the anatomical study, the lateral band of the plantar fascia and peroneus brevis are attached to the dorsolateral aspect of the fifth metatarsal base. Since the attachment point of the peroneus tertius is farther away from the fifth metatarsal base and the peroneus tertius has less strength than the other two tendons16, we inferred that the peroneus tertius does not play a major role in the avulsion fracture of the fifth metatarsal base. Then separate heat maps were generated for the fracture lines in the three zones that appeared most frequently on the overall heat map. Combined with the anatomical study, we found that the avulsion fragment was essentially coincident with the area containing the lateral band of the plantar fascia and peroneus brevis (Fig. 7). Therefore, we inferred that the lateral band of the plantar fascia and peroneus brevis play a major role in the fifth metatarsal base avulsion fracture together or separately, which made fracture lines concentrate in three zones.
There are several clinical classifications for the fifth metatarsal base avulsion fractures. Ekrol et al.17 further classified Lawrence zone I fractures into three types: type I: avulsion fractures of the tip of the tuberosity; type II: oblique fracture lines from the tuberosity to the fifth metatarso-cuboid joint; type III: transverse fractures just passing through the junction between the fourth and fifth metatarsal bases. Mehlhorn et al.18 developed a classification system based on an increased risk for secondary displacement of fractures with a more medial joint entry of the fracture line at the fifth metatarsal base. Type I, II, or III were defined dependant on the joint entry of the fracture line at the fifth metatarsal base (lateral one-third, middle one-third, and medial one-third). Fractures without displacement were summarized as A-type (I-IIIA) and with a fracture-step-off >2 mm as B-type (I-IIIB). However, in our study, we found some cases with two fracture lines or across two zones, and even some fracture lines that were curved or folded. Various types of the fifth metatarsal base avulsion fractures can be found in clinical practice, for example, two fracture lines that intersect exist simultaneously. The classifications above couldn’t provide an adequate description for these special types of fracture lines and are insufficiently three-dimensional and a more comprehensive and rational classification, which can help clinicians in academic communication, preoperative conversation and treatment plans, is needed. Hence, we can use the imaging presentations of the patient’s fracture lines to clinically distinguish which tendons played a dominant or joint role in the fracture line formation in various the fifth metatarsal base avulsion fractures and thus better describe such avulsion fractures by the injury mechanism. Finally, we developed a new classification for the fifth metatarsal base avulsion fractures (Fig. 8): type I predominantly involves the action of the lateral band of the plantar fascia; type II predominantly involves the action of the peroneus brevis; type IIIA involves the joint action of the peroneus brevis and the lateral band of the plantar fascia, with only one fracture line; and type IIIB involves the joint action of the peroneus brevis muscle and the lateral band of the plantar fascia, with two fracture lines. As this classification is based on injury mechanism, it can describe almost all types of the fifth metatarsal base avulsion fractures more accurately than existing classifications.
The clinical treatments for the fifth metatarsal base avulsion fractures are also diverse19–21. Although some study showed the vast majority of the fifth metatarsal base avulsion fractures are very successfully treated non-operatively, we followed up 30 conserved treated patients for 3 months from January 2021 to September 2021 and 5 patients (all of them were type III) experienced fracture redisplacement. For the fractures of type III with significantly displaced (≥2mm) we still prefer surgical treatment. There are two main reasons. Firstly, the forces produced by both tendons may lead to an increased risk of redisplacement and non-healing. Also, articular surface injury occurred in almost all the fractures of type III in this study and conservative treatment may lead to an intractable pain caused by traumatic arthritis. What's important is that, in this study, we found 10 patients classified type I who actively requested surgical treatment were combined peroneus brevis rupture at the tip of the lateral malleolus. This may be due to the fact that these patients are actually type III but the force from peroneus brevis was buffered by its self-rupture. So for patients with type I fractures, doctors should consciously check the integrity of peroneus brevis using Magnetic Resonance Imaging (MRI) or ultrasound.
The limitations of this study include the small sample size and that the 3D heat map was a descriptive investigation with subjective results. Further clinical application and biomechanical study are required to demonstrate the reliability of this classification.