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Introduction
The tibial nerve is one of the major branches of the sciatic nerve, which is formed from the ventral roots of segments L4, L5, S1, S2, and S3 of the spinal nerves [1-4]. This nerve contains sensory and motor nerve fibers that travel on the posterior leg and terminal in the foot area [4, 5]. At the posteromedial of the medial malleolus, the tibial nerve has many anatomical variations that correlate to clinical conditions. One of those common clinical conditions is foot neuropathy.
Foot neuropathy is one of the symptoms of foot pain resulting from tibial nerve injuries, presenting as burning, heat, impaired temperature perception, and tingling in the heels and sole [6-8]. Diabetics also tend to experience foot neuropathy due to alterations in the microcirculation of the feet [9]. This condition is known as diabetic polyneuropathy (DPN). Previous studies confirmed the presence of tibial nerve swelling in the tarsal tunnel and chronic compression of the tibial nerve in DPN patients [10].
Ultrasonography (US) is a widely used diagnostic tool to help visualize peripheral nerve conditions, including cross-sectional area (CSA) [11]. There are various reports of anatomical variations in the size of the tibial nerve CSA and their terminal branching point. Understanding the anatomical variations of this nerve is essential for clinical practice. In this review, we describe anatomical variations of the tibial nerve in clinical and cadaveric studies, focusing on variations in the terminal branching point and CSA of the tibial nerve at the level of the proximal tarsal tunnel of ankle area using the medial malleolus as an anatomical landmark.
Methods
Sources of research data were obtained from PubMed and Google Scholar databases. The article search was limited to the English language. We did not attempt to limit the time of publication of the articles. The keywords used in the search were tibial nerve, posterior tibial nerve, cross-sectional area, tarsal tunnel syndrome, neuropathy, and diabetic polyneuropathy.
Review
Type of anatomical variation of the tibial nerve terminal bifurcation point
In the ankle area, the tibial nerve, located on the posteromedial side of the medial malleolus, runs in the tarsal tunnel [12]. The tarsal tunnel is a narrow fibro-osseous space covered by the flexor retinaculum (laciniate ligament) [7, 13]. This tunnel includes multiple structures such as the tibialis posterior tendon, flexor digitorum longus tendon, posterior tibial artery, posterior tibial vein, tibial nerve, and flexor hallucis longus tendons [14]. Due to its narrow space and structures, this tunnel has been reported to be the most common site for entrapment of the tibial nerve [15].
The tibial nerve is divided into two terminal branches, the medial plantar nerve (MPN) and the lateral plantar nerve (LPN), which innervate the sole of the foot [13, 16]. While the MPN innervate the medial aspect of the sole, the LPN innervate the lateral aspect [4]. Studies have reported several different branching points of the tibial nerve in the ankle. Some researchers use the malleolar-calcaneal axis (MCA), extending from the medial malleolus to the medial calcaneal tuberosity, as a reference line to classify the branching points. Based on that axis, there are five types of tibial nerve branching points: type I (above the axis and within the tarsal tunnel); type II (as high as the axis); type III (distal end of the axis and within the tarsal tunnel); type IV (proximal axis and outside the tarsal tunnel); type V (distal axis and outside the tarsal tunnel) [17].
The difficulty of identifying the flexor retinaculum in cadaver specimens has led some researchers to use the 2 cm proximal and distal axis as a determinant of the area of the tarsal tunnel. Most researchers reported that the branching point of the tibial nerve at the ankle is found in the tarsal tunnel area and proximally of the MCA (Table 1) [13, 16-22]. Tibial nerve branches include not only the MPN and LPN but also the medial calcaneus nerve (MCN) and the inferior calcaneus nerve (ICN). MCN and ICN provide sensory innervation to the heel and the anterior aspect of the calcaneus area, respectively [16, 17]. Thus, Compression on the two nerves can result in pain in the two difference areas; while compression of MCN result in heel pain and compression of the ICN result in Baxter neuropathy [23]. The two nerves vary in origin and number (Table 2) [13, 16, 20].
Tibial nerve cross-sectional area (CSA) variation in the ankle
Ultrasonography (US) is a common imaging technique used to depict the peripheral nerves, including the CSA of nerves [24]. Several studies have measured tibial nerve CSA using US with a frequency range of 5–18 MHz. It was found that the mean tibial nerve CSA in the ankle area ranged from 6.36 to 15.25 mm2 (Table 3) [11, 24-30]. This broad variation in mean values is likely due to the utilization of diverse research subjects. According to studies, the CSA of the tibial nerve at the level and 1 to 7 cm proximal of medial malleolus in Asian subject was between 11.1 and 12.7 mm2 [11, 24, 25]. Meanwhile, the mean CSA of tibial nerve in European varied between 6.36 and 13.7 mm2 [26-29]. Based on a study of 81 cadaveric lower limbs, the average CSA of the tibial nerve at the tip of medial malleolus in fresh and frozen cadaver lower limbs was 15.25±4.65 mm2 and 13.71±5.66 mm2, respectively [30].
Several studies have shown that the CSA of nerves increases with age [25, 27, 31]. In apart from age, the CSA value is related to body mass index (BMI) and weight. However, a meta-analysis reported no relationship between age, weight and tibial nerve CSA value [32]. Changes in tibial nerve’s form and size can be caused by the position of the ankle joint. Previous studies showed that CSA of tibial nerve in the plantarflexed was greater than that of dorsiflexed [33].
Variation of the cross-sectional area (CSA) of the tibial nerve in foot neuropathy
The increase of the tibial nerve CSA may indicate abnormality. Tarsal tunnel syndrome (TTS) and diabetic polyneuropathy (DPN) are conditions caused by prolonged tibial nerve compression, which is related to an increase in the CSA of this nerve [10]. DPN has several symptoms, including paranesthesia, loss of sensory sensation, and hyperesthesia [9]. Several previous studies was reported differences in the mean value of the tibial nerve CSA between healthy subjects and patients with symptomatic neuropathy (Table 4) [9, 10, 34-37].
According to a study conducted in France, patients with TTS had higher mean CSA tibial nerve than in healthy subject at the inside of tarsal tunnel. CSA tibial nerve in TTS patients was 20.1 mm2 and in healthy subjects was 10.3 mm2 [34]. The difference in the CSA tibial nerve between DPN and healthy subjects was also noted. A study in India reported that the mean value of CSA tibial nerve at 3 cm proximal medial malleolus in patient with DPN was 13.61 mm2 and in healthy subjects was 9.27 mm2 [35]. A study in Texas also reported a higher mean value of CSA tibial nerve in patients with DPN than in healthy subject, which was 24 mm2 and 12 mm2, respectively [10].
Another study comparing the right and left leg was also found the difference in the CSA tibial nerve value between DPN and healthy subjects. In patients with DPN, the mean tibial nerve CSA value was 33.83 mm2 on the right and 31.94 mm2 on the left leg, whereas in healthy subjects, the CSA tibial nerve was 18.67 mm2 on right leg and 18.43 mm2 on the left leg [9]. In diabetic subject with DPN and those without DPN, the CSA of tibial nerve had different mean values of 22.05 mm2 and 17.25 mm2, respectively [36]. A study conducted in Korea showed there was no significant difference between the mean CSA of the tibial nerve at the level of medial malleolus in DPN patients and healthy subjects (12.25 mm2 vs 12.36 mm2, respectively) [37].
Conclusion
In this review, the MCA was used as the baseline for determining the tibial nerve branching point and the tarsal tunnel was defined as the area 2 cm proximal and distal to the MCA. According to these baselines, the most common location of the tibial nerve branching point was type I. It was difficult to provide concise data since the studies used various baselines in determining the branching point of the tibial nerve in the ankle. Therefore, in the future, the agreement will be required to determine that baseline.
Another anatomical variation we have review was CSA of tibial nerve at the ankle. In healthy subject there was various size of CSA tibial nerve. Hence there was an increase CSA tibial nerve in neuropathy patient. On both sides of the leg, there was no significant difference in CSA of tibial nerve at the ankle, so that it can be used as an internal control in establishing the diagnosis.
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