Sonographic assessment of the tarsal tunnel compared to cadaveric findings: a pictorial study

The study protocols were approved by the National Medical Ethics Committee of the Republic of Slovenia (approval number: 0120459/2018/3). Anatomical dissection was performed on a defrosted adult human cadaver ankle-foot specimen with no signs of trauma, surgery or any major deformity.

An incision was made posterior to the medial malleolus at the medial border of the Achilles tendon. Skin flaps were raised from 10 cm proximal to the medial malleolus up to the plantar surface of the foot. Soft tissues were cleared over the medial malleolus to expose the flexor retinaculum. The flexor retinaculum was then removed to expose TT contents.

The contents of the TT were dissected to reveal the relevant anatomical structures: PTT, FDLT, FHLT and the neurovascular bundle. The sheath of the neurovascular bundle was then incised to separate and demonstrate the various components, namely, the tibial nerve (TN) and its branches (medial plantar nerve (MPN), lateral plantar nerve (LPN) and medial calcaneal nerve (MCN)), the posterior tibial artery (PTA), and the posterior tibial vein (PTV) and their branches. Adipose and connective tissues were carefully removed, and the vascular bundle was cut proximal to the TT and rotated to expose the deeper-lying TN and its branches.

The overlying abductor hallucis (AH) muscle was cut from its calcaneal origin and rotated towards the forefoot for better visualization of the terminal neural branches; a part of the deep fascia of the AH muscle was preserved. In our specimen, we showed the bifurcation of TN into MPN and LPN, and identified two MCN branches ramifying from TN and continuing superficially into the subcutaneous tissue. In addition, BN was identified as the first branch of LPN. Lastly, we have coloured the structures of the neurovascular bundle for easier differentiation; neural structures were coloured green, arteries were coloured red, and veins were coloured blue.

From anterior to posterior, the proximal TT contains the following structures: PTT, FDLT, PTA, PTV, TN, and FHLT (Fig. 1). Each tendon lies in its own fibro-osseous compartment^(7,8,10,21). PTT, which has a typical tendinous fibrillar ultrasound appearance and lies in a shallow groove in the medial malleolus, is the most anteriorly located structure on an axial view of the proximal TT⁽¹⁶⁾. Posterior to PTT lies FDLT, which is usually about half the size of PTT⁽¹⁶⁾. Posterior to FDLT lies the neurovascular bundle with PTA and accompanying PTV and TN. The accurate identification of vascular structures can be aided by doppler ultrasound. Generally, nerves can be visualized under the vessels⁽⁴⁾. TN can be visualized as superficial and anterior to FHLT and presents with a fascicular or beehive-like echo pattern⁽¹⁶⁾. The flexor retinaculum, which appears as a hyperechoic fibrillar band on an ultrasound, extends over the TT from the medial malleolus to the medial calcaneus (Fig. 1)⁽⁸⁾.

From anterior to posterior, the distal TT contains PTT, FDLT, FHLT, and terminal branches of the neurovascular structures. PTT is the most anteriorly located structure on an axial view of the distal TT. Next to it lies the FDLT, which in its distal course gradually runs posteriorly towards the sustentaculum tali on an axial view of TT. Posterior to the two tendons, the terminal branches of TN can be observed: MPN running anteromedially and LPN running postero-laterally, both with accompanying terminal branches of PTA and PTV (Fig. 2)^(3,11). BN can be visualized posterior to LPN. FHLT lies below MPN and runs posteriorly to the sustentaculum tali⁽⁸⁾.

At the level of the distal TT, the medial septum separates the medial tunnel containing MPN and the lateral tunnel containing LPN^{(4,5,7,9,11,17)}. The medial septum presents a possible nerve entrapment site^(2,4). It can be identified on ultrasound as a hyperechoic fibrillar structure between MPN and LPN in the distal TT (Fig. 3) ^(4,8,17). At the level of distal TT, the QP muscle can be seen at the base of TT⁽³⁾.

TN has three terminal branches: MPN, LPN and MCN^{(1–3,5,7,13,16)}. The fourth branch, BN, usually branches from LPN. MPN and LPN are mixed nerves that provide motor innervation for the intrinsic muscles of the foot. In addition, they provide sensory innervation for the medial and lateral aspects of the plantar surface of the foot. Sensory branches of MPN innervate the first three and a half of the fourth toe, while LPN innervates the remaining portion of the fourth and the fifth toe⁽⁷⁾. TN usually divides into MPN and LPN inside the TT, less commonly proximally to the TT, and rarely below the level of the TT (Fig. 4)^{(1,7,9,11,12,20)}. TN branching site can be identified by scanning the TT by elevator technique and following the distal path of the TN (Fig. 4).

The third TN terminal branch, the MCN, has variable anatomy and most commonly originates from TN and less frequently from LPN^{(1,4,7,9,11,12,21)}. A few cases of MCN originating from MPN have been reported (9,11). MCN is a sensory branch innervating the surface of the heel^(5,7,12,21). The site of MCN branching has been described to occur proximally to or inside the TT^{(4,9,12,15,20)}. MCN is reported to branch off proximally to BN^(4,6,12). Up to 5 branches of MCN have been reported^(9,11). Occasionally, the first branch of MCN can originate high above the flexor retinaculum; a distance of more than 150 mm above DMA has been recorded⁽⁹⁾. When MCN bifurcates above the level of the flexor retinaculum, it can run superficially and does not necessarily enter the TT^(9,15). Due to its small size, MCN is difficult to visualize on ultrasound^(3,16). We were unable to locate MCN during our ultrasound examination. The anatomic dissection of TN with two MCN branches arising from TN can be observed in Fig. 7.

Fig. 7.

BN, also called Inferior Calcaneal Nerve or First Branch of LPN, most commonly originates from LPN or, less frequently, as a branch of TN^{(3,4,6,8,9,11,12)}. BN innervates the abductor digiti minimi muscle and provides sensory innervation for the calcaneus^(4–6,8). The site of BN branching can be either proximal, distal to or be inside the TT (Fig. 5)^(9,11). Up to two branches of BN have been reported, with one branch being the most common variation^(11,12). BN perforates the medial septum and runs between the AH and the QP muscle^(3–6,8).During an ultrasound examination, LPN should be followed by the elevator technique to find the branching point of BN from its posterior border (Fig. 5)^(4,6). If no branch originating from LPN can be visualized, TN should be examined by the same technique⁽⁶⁾. When BN originates from TN, one has to be careful not to mistake it for MCN. BN branches off distally to MCN; thus, the more proximally bifurcating TN branch usually represents MCN and the second, more distally originating branch, represents BN^(6,9,12). However, it is also important to trace the distal course of each nerve branch as both MCN and BN can originate from either TN or LPN, and there can be multiple branches of MCN. Both continue posteriorly, but MCN terminates subcutaneously, whereas BN runs between the AH and QP muscle before continuing to the plantar surface (Fig. 6)⁽⁶⁾.

The vascular structures have branching patterns similar to those of the nervous structures and also present with high anatomical variability^(11). PTA generally bifurcates distally to TN into the medial plantar artery (MPA), lateral plantar artery (LPA), and medial calcaneal artery (Fig. 8)⁽¹¹⁾.

Fig. 8.

We decided to perform ultrasound examination on a healthy volunteer because it allowed us to image TT vascular structures. This would not be possible on a cadaver specimen. However, there are anatomical differences between the two samples. Firstly, in our cadaver specimen, we identified two MCN nerves ramifying from the posterior part of TN. We were unable to identify MCN during our ultrasound examination. Secondly, there are differences between the anatomic relations of the contents of the neurovascular bundle, most notably at the level of the distal TT above PTA bifurcation (Fig. 2) and at the level of TN division (Fig. 4). At the level of the distal TT above PTA bifurcation (Fig. 2), MPN and LPN run anterior to PTA in our cadaver specimen (Fig. 2 A), whereas MPN runs anterior to PTA and LPN runs posterior to PTA in our healthy volunteer (Fig. 2 B). At the level of TN division (Fig. 4), MPN and LPN run anterior to PTA in our cadaver specimen (Fig. 4 A), whereas MPN and LPN run posterior to PTA in our healthy volunteer (Fig. 4 B). Additionally, a difference between the Baxter’s nerve and LPA and LPV anatomic relation can be observed on the image showing the distal tarsal tunnel below the level of posterior tibial artery bifurcation (Fig. 3) with the Baxter’s nerve lying posteroinferior to LPA and LPV in our cadaver specimen (Fig. 3 A) and anterosuperior to LPA and LPV in our healthy volunteer (Fig. 3 B).