Three-Dimensional CT Mapping and Anatomy for the Avulsion Fracture of the Fifth Metatarsal Base

Background: To clarify the injury mechanism of the avulsion fracture of the fth metatarsal combining 3-dimensional (3D) fracture mapping with anatomical measurements. Materials and Methods: 222 patients with the fth metatarsal base avulsion fractures, who were admitted to our hospital from August 2015 to August 2020. The 3D images of all mapped fracture lines for the fth metatarsal base avulsion fractures were compiled in an overall 3D image. The fth metatarsal base of 8 unpaired lower limbs of adult Asian frozen cadaveric specimens were also dissected to observe and measure the specic locations of the attachment points of the peroneus brevis, lateral band of the plantar fascia, and peroneus tertius to the fth metatarsal base. Results: Based on the type of fracture line produced and the specic locations of the attachment points of the tendons, the fth metatarsal base avulsion fractures can be classied into three types: 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 lateral band of the plantar fascia with one fracture line; type IIIB involves the joint action of the peroneus brevis and lateral band of the plantar fascia with two fracture lines. Conclusion: The lateral band of the plantar fascia and peroneus brevis play a major role in the fth metatarsal base avulsion fracture together or separately and proposed a novel classication based on the injury mechanism, which can serve as a reference for clinical treatment and diagnosis.

while avulsion fractures account for two-thirds of all fth metatarsal fractures. [1][2][3] Without treatment, it can lead to sequelae such as intractable pain and metatarsalgia. 4 The base of the fth metatarsal bone is attached to several tendons and ligaments, including the peroneus brevis, the peroneus tertius, and the lateral band of the plantar fascia. [5][6][7] When the foot is subjected to an abnormal force, resulting in hindfoot inversion or forefoot inversion and exion, the unique anatomical structure of the fth metatarsal bone directs the force to the proximal base of the bone, which is susceptible to avulsion fractures of the base.
Clinically, the fth metatarsal base avulsion fractures exhibit a diverse range of fracture morphologies, which have been beyond the description of existing classi cations, but research on their fracture line characteristics is limited. [8][9][10][11] And then it is di cult for clinicians to choose treatment or predict prognosis with existing classi cations for some special types. Therefore, in this study, we investigated the fracture three-dimensional distribution characteristics in the fth metatarsal base avulsion fractures and conducted an anatomical study on the attachment site locations for the lateral band of the plantar fascia, peroneus brevis, and the peroneus tertius on the fth metatarsal base. Through comparison and veri cation of the two, we describe the fracture line characteristics for the fth metatarsal base avulsion fractures and clarify the mechanisms of the ligaments and tendons in this fracture. We retrospectively analyzed the Computed Tomography (CT) scan data of 222 patients with the fth metatarsal base avulsion fractures, who met the selection criteria above and were admitted to our hospital from August 2015 to August 2020 . The   patients included 97 females and 125 males, aged 18-80 years, with 117 affected on the right side and 105 on the left side   (Table 1). This study was approved by the Institutional Ethics Committee of the authors' institution. Additionally, we selected 8 unpaired lower limbs of adult Asian frozen cadaveric specimens, with 4 on the left side and 4 on the right side. The specimens were of unknown age and gender and were randomly numbered from 1 to 8. Bone abnormalities such as osteoarticular degeneration, deformity, and tumor were excluded from the specimens using an X-ray. The articular surface area, tendon attachment area, and tendon diameter were measured with vernier calipers to an accuracy of 0.02 mm.

Anatomical dissection and measurements
A longitudinal incision was made on the skin of the lateral aspect of the fth metatarsal bone. The incision was rst extended proximally to identify the peroneus tertius and peroneus brevis, then distally to carefully separate the soft tissue, and free the peroneus tertius and peroneus brevis. The incision was then extended to the plantar aspect to identify and free the lateral band of the plantar fascia. Subsequently, the fth metatarsal base was further separated from the cuboid bone and the fourth metatarsal base, then the intact fth metatarsal bone, and the attached lateral band of the plantar fascia, peroneus brevis, and peroneus tertius were completely freed from the cadaveric specimen. The attachment location, shape, area, and tendon diameter of the fth metatarsal bone, peroneus brevis, lateral band of the plantar fascia and peroneus tertius to the fth metatarsal base were observed and measured with a vernier caliper. Tendon diameter measurements were taken 1 cm proximal to the tendon terminus at the fth metatarsal bone to indicate tendon strength.
Fracture line mapping and fracture surface heatmapping

Results
The fracture lines for the fth metatarsal base avulsion fractures were varied but mostly concentrated in an arc-shaped band on the dorsal view. The band begun at 1/4~4/5 from the posterior tangent, with the start of the fracture line immediately adjacent to the lateral tangent and the end almost immediately adjacent to the medial tangent. The area of fracture lines with the highest frequency begun 2/5 from the posterior tangent and ended 7/10 from the posterior tangent (Fig. 4).
Eight adult foot specimens were observed and measured (Fig. 5, Fig. 6). The coronal plane of the fth metatarsal base was triangular in shape, and the medial aspect was the articular surface of the fourth and fth metatarsals, which had a width of  (Table 2-Table 5). The anatomical measurements collected were analyzed using Excel (Microsoft, WA).    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 fth 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 studies [13][14][15] and conducted the 3D examination on the fracture line characteristics of 222 cases with the fth metatarsal base avulsion fractures. The fracture lines heat map on the dorsal view of fth 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 fth 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 fth metatarsal base. Since the attachment point of the peroneus tertius is farther away from the fth metatarsal base and the peroneus tertius has less strength than the other two tendons 16 , we inferred that the peroneus tertius does not play a major role in the avulsion fracture of the fth 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 fth metatarsal base avulsion fracture together or separately, which made fracture lines concentrate in three zones.
There are several clinical classi cations for the fth metatarsal base avulsion fractures. Ekrol et al. 17 further classi ed 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 fth metatarso-cuboid joint; type III: transverse fractures just passing through the junction between the fourth and fth metatarsal bases. Mehlhorn et al. 18 developed a classi cation system based on an increased risk for secondary displacement of fractures with a more medial joint entry of the fracture line at the fth metatarsal base. Type I, II, or III were de ned dependant on the joint entry of the fracture line at the fth 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 fth metatarsal base avulsion fractures can be found in clinical practice, for example, two fracture lines that intersect exist simultaneously. The classi cations above couldn't provide an adequate description for these special types of fracture lines and are insu ciently three-dimensional and a more comprehensive and rational classi cation, 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 fth metatarsal base avulsion fractures and thus better describe such avulsion fractures by the injury mechanism. Finally, we developed a new classi cation for the fth 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 classi cation is based on injury mechanism, it can describe almost all types of the fth metatarsal base avulsion fractures more accurately than existing classi cations.
The clinical treatments for the fth metatarsal base avulsion fractures are also diverse [19][20][21] . Although some study showed the vast majority of the fth 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 signi cantly 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 classi ed 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 classi cation.

Conclusion
In conclusion, the fth metatarsal base avulsion fractures produce various fracture lines. By examining their 3D distribution and anatomical features, this study clari ed the lateral band of the plantar fascia and peroneus brevis play a major role in the fth metatarsal base avulsion fracture together or separately and proposed a novel classi cation for such fractures, which can serve as a reference for clinical treatment and diagnosis.