The ankle canal is a narrow fibrous bone channel in anatomy, including the tibial nerve and its branches, posterior tibial blood vessels, and deep flexor tendon of the calf. The nerve channels in the ankle canal are divided into four septa by the fascia, and branches of the tibial nerve travel at different intervals . These anatomical structures in the ankle canal are often related to many diseases, and the fine anatomy of the nerve in this area can be shown morphologically by MR, which is of great significance for the diagnosis and clinical treatment of related diseases [15,17].
Characteristics of tibial nerve and its branches on 3d-FIESTA-C sequenceCurrently, the MR research on peripheral nerve imaging mainly focuses on diffusion tensor imaging (DTI). However, DTI is susceptible to many factors of magnetic field and spatial and contrast resolution, and it lacks the accuracy for evaluating small branches ; hence, it is mainly applied to large nerves and branches. Further studies are needed to determine its role in daily clinical practice . There are few studies on the nerve morphology of the tibial nerve in the ankle canal with other MR sequences. In one study, Farooki et al.  preliminarily showed the inner and outer calcaneus nerves by using the orthogonal plane to conduct thin layer scan on the corpse, while Donovan proposed that the inner and outer calcaneus nerves were more obvious in the oblique coronal plane. As the 3D-FIESTA-C sequence is encoded in the 3D space, the inter-layer resolution of the 3D sequence is very high. A thin layer can ensure high-quality reformation images on any plane, and it can clearly show relatively small nerve branches. Hatipoglu applied this sequence to the study of posterior fossa nerve imaging. In the images of the 3D-FIESTA-C sequence, the nerve showed low signal, and between the muscle (slightly lower signal) and the tendon (lower signal), the peripheral fat showed high signal; the blood vessels showed high signal and the thicker blood vessels showed a low signal clipping sign. These are necessary conditions to show clear and distinguishable neural structures in multi-plane images. On the image of the transection of the tibial nerve, although the tibial nerve presents low signal on the whole, multiple low-signal nerve fiber bundles and slightly higher signal intervals between the fiber bundles can be seen inside, and fat high signal can be seen around the tibial nerve . The accompanying posterior tibial artery and vein on the medial side of the nerve should be noted. Because the tibial nerve and its branches are striped structures and a main longitudinal axis direction line along the human body, most of the nerves showed a wide range of morphology through oblique sagittal plane or oblique coronal plane. A few nerves that cannot be completely depicted in one plane can be reformatted to 2–3 planes to show their shape, and these planes will not affect the judgment of nervous (Figure 3a, 3b, 3c, and 3d).
The bifurcation position and branching pattern of the tibial nerve and branches at the ankle canal
The tibial nerve is generally cylindrical running behind and below the medial malleolus in the ankle canal, with two main branches: the medial plantar nerve and the lateral plantar nerve. Of course, Develi  reported a unique case of three branches, but this is very rare. The location of the distal branch of the tibial nerve is not constant. Bareither et al.  pointed out that the branch point can be within the range from 2.8 cm from the distal end of the medial malleolus tip to 14.3 cm from the proximal end. Dellon et al.  reported that the bifurcation point was within 2 cm of the malleolar-calcaneal axis. By analyzing 50 cases, Torres et al.  found that 88% of them were located in the ankle canal and 12% were located in the proximal end of the ankle canal. In this study, the medial plantar nerve and the lateral plantar nerve were divided into three types through positioning of the branch point, and 42.5% of the bifurcation points were located in the proximal part of the ankle canal (type I); 57.5% of the bifurcation points were located in the ankle canal (type Ⅱ), and no bifurcation point was found far from the distal end of the ankle cannal (type Ⅲ). These results were similar to those reported by Torres. In this study, the angle measured between the inner and outer plantar nerves ranged between 6° and 35°. The medial plantar nerve is one of the larger branches of the tibial nerve. It is located on the lateral side of the posterior tibial artery and in front of the medial plantar artery [17,25]. The lateral plantar nerve is a smaller branch that runs between the inner and lateral plantar arteries . The purpose of reformatting a single branch of the inner and outer nerves of the plantar sole and displaying them at the maximum display level is to clearly observe their morphology, their course, and whether there is compression on the pathway.
The medial calcaneal nerve is one of the main branches of the tibial nerve, terminating in the skin of the heel and the weight-bearing surface, and providing sensory innervation to the inner posterior side of the heel [26,27]. The starting position and the number of medial calcaneal nerves vary greatly. Quantitatively, Dellon  found that 37% had one medial calcaneal nerve, 41% had two medial calcancal nerves, 19% had three medial calcancal nerves, and 3% had four medial calcancal nerves. Kim  and Yang  found that there were up to five medial calcaneal nerves. Govsa  and Kim  indicated that there were two common vessels on the medial surface of the calcaneus. In this study, up to three medial calcaneal nerves were reformatted, which may be related to the small number of samples. However, a maximum of two medial calcaneal nerves were reformatted in this study, which is consistent with the above-mentioned views. At the starting point, the medial calcaneal nerve may originate from the tibial nerve, the medial and lateral plantar nerve bifurcation points, and the lateral plantar nerve. Havel  and Dellon  found that the medial calcaneal nerve can also originate from the medial plantar nerve. These differences are mainly related to more than one calcaneal nerve in most cases, indicating the high rate of variation in the origin of the medial calcaneal nerve. Although the medial calcaneal nerve can originate from the tibial nerve to the lateral plantar nerve, segmental reformation showed that its terminal branches showed a consistent range of innervation, all of which went to the heel skin behind the calcaneal tuberosity, which was consistent with anatomic conclusions [10, 11, 29, 31].
The inferior calcaneal nerve is also known as the first lateral plantar nerve, the little toe abductor nerve, or the Baxter nerve [32–34]. Moroni , Oliva  and Govsa  found that the occurrence rate of this nerve was 100%. In our reconstructed image, the display rate was 95% and the anatomical data showed that the cross-sectional diameter at the beginning of the subcalcaneal nerve was 1.4±0.5 mm. It is possible that the cases in which the nerves were not clearly shown are related to the fine nerve, but this requires further study. The inferior calcaneal nerve always appears as a single branch, which was confirmed in our study, and no more than two medial calcaneal nerves were found. The origin position of the inferior calcaneal nerve is not constant. Arenson , Didia , and Govsa  et al. believed that the inferior calcaneal nerve could originate from the lateral plantar nerve, the medial plantar nerve, the lateral nerve bifurcation, and the tibial nerve, but our research results are consistent with those of Louisia  and Kim . In all images, most of the inferior calcaneal nerves originated from the lateral plantar nerve. In this study, we also found that the subcalcaneal nerve terminal shown by segmental reformation often disappeared in the muscular space or surrounding area in front of the tibial tubercle, and occasionally two branches could be reconstructed, with the anterior branch distributed in the area of the abductor muscle of the little toe and the posterior branch distributed in front of the calcaneal tubercle.
Clinical significance of positioning nerve bifurcation on body surface
Ankle canal lesions, especially ankle canal syndrome, often requires minimally invasive interventional therapy or even surgical treatment . During the surgical incision and route planning, positioning the nerve can avoid the nerve damage to the greatest extent, so that the patients can experience better treatment effect and less complications. When using the external fixation of the fracture, positioning the nerve can also minimize the nerve damage. During nerve block, the effective injection site of ankle canal for heel pain can be determined by positioning the nerve . In this study, we considered the horizontal line passing through the medial malleolus tip as the reference line, and by measuring the distance from the bifurcation point to the medial malleolus tip and the included angle with the reference line, we could conveniently mark the body surface registration point of each point on the medial malleolus. This method can also be used to localize the lesion in the body surface and measure its depth by the coronal position. These factors are of great significance for the invasive treatment of feet and ankles.