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Journal of the Anatomical Society of India

Anatomy of Tarsal Tunnel And Its Applied Significeanc

Author(s): Joshi S S; Joshi S D, Athavale S A

Vol. 55, No. 1 (2006-01 - 2006-07)

Joshi S S; Joshi S D and Athavale S A Professor Of Anatomy, Rural Medical College, RIMS Loni – 413736. (Dist. Ahmednagar)

Abstract:

Tarsal tunnel syndrome is a well known clinical entity resulting due to entrapment of tibial nerve or its branches deep to flexor retinaculum. Nerve blocks and surgical procedures like fixation of fractures with external nailing, medial displacement osteotomies and decompression in tarsal tunnel syndrome necessitate a proper understanding of detailed anatomy of tarsal tunnel.

With this aim in mind we have dissected medial aspect of ankle in 112 lower limbs (Rt: 56; Lt: 56) to study the tibial nerve and posterior tibial vessels within the Tarsal Tunnel. Division of tibial nerve and posterior tibial artery in relation to medio- malleo- calcaneal axis were noted. Neurovascular bundle occupied a separate compartment deep to flexor retinaculum. In the present study tibial nerve bifurcated proximal to the axis in 77% cases (type I), whereas in 70% the bifurcation of posterior tibial artery was at the level of this axis (type II). The distance between the bifurcations of tibial nerve and posterior tibial artery was found to be within 1-3 cm in 77% cases on right and 79% on left side. In approximately 98% cases the division of tibial nerve was proximal to that of posterior tibial artery (contrary to the description in some textbooks of Anatomy). In 58 % tibial nerve showed a distinct flattening before its bifurcation. In 35% posterior tibial artery was found to be tortuous before its bifurcation. In all the cases posterior tibial artery and its branches accompanied by venae comites formed a distinct superficial strata to the tibial nerve and proximal parts of medial and lateral plantar nerves. The distance of medial plantar nerve from the medial malleolus was 21.84 ± 5.22 mm on the right and 23.43 ± 5.2 mm on the left. Lateral plantar nerve was placed at a distance of 29.5 ± 6.18 mm from the medial tubercle of calcaneus on the right side and 28.29 ± 5.69 on the left side.

Key Words: Medial Calcaneal Nerve, Posterior Tibial Artery, Tarsal Tunnel, Tarsal Tunnel Syndrome, Tibial Nerve.

Introduction:

Tarsal tunnel is situated medial to the ankle lying deep to the flexor retinaculum (Standring et al. 2005). It is also termed as porta pedis (Datta A K 2004). It transmits the tendons of tibialis posterior, flexor digitorum longus and flexor hallucis longus along with posterior tibial vessels, tibial nerve and their bifurcations. The neurovascular bundle as well as other tendons lie in separate compartments deep to flexor retinaculum (Kohno et al. 2000). Compromise of space by any cause can lead to the development of ‘Tarsal Tunnel Syndrome’ (TTS).

This syndrome results from entrapment of tibial nerve in the tarsal tunnel (Mann 1983, Richardson 1998, Standring et al. 2005). Exact incidence of TTS for our population is not known, nor do we have a detailed description of tarsal tunnel and its contents amongst Indians. This prompted us to undertake a detailed study of Tarsal Tunnel in the cadavers available in this department. Detailed knowledge of anatomy of tarsal tunnel is not only important from the point of view of entrapment of tibial nerve and its branches but it may also serve to establish an anatomical guide for diagnosis and therapy of diseases of tarsal region including fractures requiring external nailing, medial displacement osteotomies and nerve blocks in pediatric medicine.

Material And Methods:

For the present study 112 lower limbs (Rt.- 56; Lt.- 56), available in the department were dissected over a period of two years. Each osseo-fibrous compartment deep to the flexor retinaculum was exposed. Tibial nerve, its point of bifurcation and number and origin of medial calcaneal branches were noted. Posterior tibial artery along with venae commitantes was traced through the tunnel up to and beyond its bifurcation. The distance between the bifurcations of the artery and nerve was measured with the help of divider and scale. Interrelationship between the nerve and blood vessels was noted. Tortuosity of vessels and flattening of nerves, when present, were also observed.

A reference line 1 cm. broad ‘Medio- Malleolar- Calcaneal axis’ (MMC axis) extending form the tip of medial malleolus to the medial tubercle of calcaneus (fig. 1) was used as a grid to classify the bifurcation points of nerve and artery into type I, II, and III. (Bilge et al. 2003). Type I, II and III signify bifurcations proximal to, deep to and distal to this axis respectively. Distance of medial plantar nerve from the tip of medial malleolus and lateral plantar nerve from medial tubercle of calcaneus were measured. Any other variations or abnormalities found during dissection were observed and noted.

OBSERVATIONS

Flexor retinaculum was not clearly demarcated at its superior and inferior borders and it imperceptibly merged with deep fascia of leg and foot. The neurovascular bundle was separated from tendons by fibrous septae, enclosed in a separate compartment (tunnel) where it was surrounded by a delicate fibrofatty cushion. The posterior tibial vessels were distinctly superficial to the tibial nerve and its branches within the tunnel (fig.1, 2). In many cases a well-defined grooving of tibial nerve was found at and near its bifurcation, forming a bed for posterior tibial vessels (fig. 2). Tibial nerve showed flattening just before bifurcation in 58.03% ( Rt: 57.14%; Lt.: 58.92%). Posterior tibial artery was tortuous in 34.82% (Rt: 33.92%; Lt.: 35.71%) (fig. 3). Table I shows the bifurcation points of tibial nerve and posterior tibial artery with reference to MMC axis.The distance between bifurcations of the nerve and artery was between 1 to 3 cm. in 76.78% on the right and 78.56% on the left .

There were variations seen in the number and origin of medial calcaneal branches. Only one medial calcaneal branch was found in 41.96%; 2 in 41.96%; 3 in 15.17% and 4 in 0.89% of cases. Medial calcaneal nerves were found to arise either from the trunk of tibial nerve or its lateral plantar branch.

Table II shows distances of medial and lateral plantar nerves ( at the level of MMC axis) from medial malleolus and medial tubercle of calcaneus respectively. In the majority ( 93%) the medial plantar nerve was lying at a distance ranging from 11 to 30 mm from the medial malleolus. Lateral plantar nerve was lying at a distance ranging from 21 to 40 mm from medial tubercle of calcaneus in 88.22%. The distance of medial plantar nerve from the medial malleolus was 21.84 ± 5.22 mm on the right and 23.43 ± 5.2 mm on the left. Lateral plantar nerve was placed at a distance of 29.5 ± 6.18 mm from the medial tubercle of calcaneus on the right side and 28.29 ± 5.69 mm on the left side.

neurovascular bundle... Tarsal Tunnel

Fig. 1: shows the neurovascular bundle in the Tarsal Tunnel. Medio- Malleolar- calcaneal axis (MMC axis) has been shown as 1 cm broad strip (two parallel lines) extending from the medial malleolus (upper left) to the medial tubercle of calcaneus (lower right).

tibial nerve bifurcation

Fig. 2: shows tibial nerve bifurcation (upper arrow). The flattening and grooving of the nerve at its bifurcation (occupied by posterior tibial artery) is distinctly seen. Bifurcation of posterior tibial artery (lower arrow) is also seen.

Fig. 3: (Unavailable) shows tortuous posterior tibial artery within the Tarsal Tunnel.

Table I:showing types of bifurcations of tibial nerve and posterior tibial artery with reference to MMC axis

Type Tibial Nerve Posterior Tibial Artery
Right Left Right Left
tubercleII 23.21 21.42 73.21 67.85
III 1.78 00 16.07 16.07
* All values are in percentages

Table II showing distance of medial plantar nerve (MPN) from medial malleolus and lateral plantar ( LPN) from medial tubercle of calcaneus

Distance
in mm.
MPN from MM LPN from medial tubercle
Right Left Right Left
0 – 10 1.78* 1.78 - -
11 – 20 42.85 33.92 10.71 8.92
21 – 30 53.57 57.14 48.21 58.92
31 – 40 1.78 7.14 37.5 30.35
41 -50 - - 3.57 1.78
* All values are in percentages

The average width of trunk of tibial nerve, just above its bifurcation, was found to be 8.23mm ( Rt: 8.27 ± 3.05 mm; Lt: 8.2 ± 2.77 mm). The width of medial plantar nerve was 5.32mm ( Rt: 5.28 ± 1.4 mm;Lt: 5.37 ± 1.49 mm) and that of lateral plantar was 4.61mm ( Rt: 4.52 ± 1.6 mm; Lt.: 4.72 ± 2.03 mm). In 2 limbs tendon of an accessory muscle Flexor digitorum accessories longus (FDAL) was seen to pass through the tarsal tunnel along with the neurovascular bundle but lying deep to it. In one case posterior tibial artery was absent in the leg. The peroneal artery passed superficial to the flexor hallucis longus, at the level of the ankle, to enter the tarsal tunnel dividing into medial and lateral plantar arteries, thereby replacing the posterior tibial artery deep to flexor retinaculum.

Discussion:

Nagaoka (1990) stated that flexor retinaculum was not clearly demarcated at its superior and inferior borders. Standring et al. (2005) have also stated that there is no clear demarcation between its borders and deep fascia of leg. Our observations about flexor retinaculum are in agreement with these workers. Richardson (1998) has described formation of separate compartments deep to flexor retinaculum by means of fibrous septae extending from the deep surface of the retinaculum; the neuro-vascular bundle being lodged in an independent compartment within the tarsal tunnel. Findings during our dissections confirm the observations of Richardson (1998), and are also in agreement with the description of Nagaoka (1990) and Standring et al. (2005) .

A distinct superficial – deep relationship was observed between vessels (superficial) and nerve (deep) within the tarsal tunnel. During any surgical exploration of tarsal tunnel the posterior tibial vessels form a distinct superficial strata which partially hides the terminal part of tibial nerve and the beginnings of medial and lateral plantar nerves from view. It is suggested that in place of the usual text book description ( Standring et al 2005, Romanes 1978) wherein vessels are described medial and nerve lateral; it would be more appropriate to describe them as superficial and deep.

In majority (98%) of cases tibial nerve bifurcated very much proximal to the bifurcation of posterior tibial artery (Fig. 2). Bilge et al. (2003) have also described a higher level of bifurcation of tibial nerve; whereas, Mc Minn (1990) states that the nerve ends at a slightly lower level than the artery. Similar description is given by Datta (2004). Thus our findings contradict the statements of these two authors.

In 34.82% (Rt: 33.92%; Lt.: 35.71%) posterior tibial artery, before its bifurcation, showed tortuosity (fig. 3) proximal and deep to the retinaculum. The tortuous course of the artery can compress the nerve in the tunnel and may be a predisposing factor in the causation of TTS.

Dellon and Mackinson (1984) have described the bifurcation of tibial nerve within 1 cm. of MMC axis in 90% cases. Havel et al. (1988) have observed the bifurcation of the nerve within the tunnel in 93% cases and in 7% it was proximal to the tunnel. Davis and Schon (1995) have stated that bifurcation of nerve was within 2 cm of MMC axis in 90% of cases studied. Recently Nidaye et al. (2003) observed the bifurcation within 1 cm of the axis in 90% cases and proximal to the axis in 10% cases. These workers, although, have utilized MMC line as a reference zone but have not recorded the observations into three types as done by us. Hence it will not be very appropriate to compare our findings with their observations.

Bilge et al. (2003) in a detailed study of tarsal tunnel have shown the bifurcation of tibial nerve to be of type I in 84%, type II in 12% and type III in 4%. In the present series type I was seen in 85.2%, type II in 14.7% and type III in 0.89%, which corroborate well with their observations.

The tibial nerve at its bifurcation showed a distinct flattening (fig. 2) in 58.03% ( Rt: 57.14%; Lt.:58.92%). Kohno et al. (2000) found nerve flattening in all the cases of TTS suggesting that this could be due to nerve compression by adjacent AV complex. Although some cases in the present series showed an association of tortuous vessels with the flattening of the nerve but it was not a constant feature.

The arterial bifurcation was always distal to the bifurcation of the nerve except in 3 cases where they were at the same level. Utilizing Bilge et al. (2003) classification we found arterial bifurcation to be of type I in 16.17%, type II in 72.04% and type III in 11.76%. The corresponding figures in their series are: type I – 10%, type II – 44% and type III – 46%. The reason for this discrepancy between the two series cannot be explained.

A distinct but thin layer of fibro-fatty cushion was seen to surround the neuro-vascular bundle within the tunnel. In cases of TTS Kohno et al. (2000) have advocated use of a graft of a similar nature to be placed in the tunnel after decompression to act as a cushion. Nidaye et al. (2003) found the presence of ‘one’ Medial Calcaneal Nerve (MCN) in 80% cases, whereas Dellon et al. (2002) reported it in 37% and in our series its incidence was found to be 41.96%. We found 2 MCN in 41.96%, which is very close to that reported by Dellon et al.(2002) i.e. 41%. Nidaye et al. (2003) in their series reported it to be only 20%. We found 3 MCN in 15.17% and 4 MCN in 0.89%. Dellon et al.(2002) have reported this as 19% and 3% repectively. Dellon and Mackinnon (1984) described great variability in the number of MCN. Havel et al. (1988) described nine different branching patterns of MCN and attributed this factor as an explanation for heel sparing in cases of TTS. Davis and Schon (1995) have also shown multiple MCN branches. In our series MCN branches were mostly derived from trunk of the tibial nerve. The additional branches arose from lateral plantar nerve. Although Dellon (2002) showed MCN branches from medial plantar nerve in 46%, in our series we did not get even a single MCN arising from medial plantar nerve. It seems logical that as the lateral plantar nerve is close to the heel, the MCN should be derived from lateral plantar rather than from medial plantar nerve. Bilge et al (2003) have measured the diameters of tibial nerve and its branches. They have found the left tibial nerve thicker than the right (Rt.: 5.8± 0.8 mm.; Lt.: 6.03± 0.91 mm.). In our series we have found that in the majority it is difficult to measure the thickness of the nerve, it being flat and hence there is a greater variability of width of tibial nerve and its branches in our series. In the majority the medial plantar nerve was lying at a distance ranging from 11 to 30 mm. from medial malleolus. Lateral plantar nerve was lying at a distance ranging from 21 to 40 mm from medial tubercle of calcaneus in 88.22%; which was equal on both sides. As no data of this nature is available in literature, these observations cannot be compared with other workers in this field. The importance of these measurements lies in the fact that we should not blindly perform an external nailing in fractures of calcaneus from the medial aspect.

The findings of the present work bring forth very succinctly the following anatomical facts which have direct relevance in clinical practice:

  1. Deep to flexor retinaculum there is an independent compartment ( tarsal tunnel) which lodges the neurovascular bundle.
  2. Vascular plane is superficial to the plane of nerves.
  3. Bifurcation of tibial nerve is at a higher level (proximal) to that of posterior tibial artery.
  4. In majority of cases the terminal part of tibial nerve shows flattening and grooving which lodges the artery.
  5. In 1/3 of cases the posterior tibial artery is tortous proximal and deep to the retinaculum.
  6. There is no safe zone on the medial aspect of calcaneus along the MMC axis for blind external fixation of calcaneal fractures.

References:

  1. Bilge O, Ozer M A, Govsa F. Neurovascular branching in the tarsal tunnel. Neuroanatomy, 2003, Volume 2, Pages 39-41.
  2. Datta A K. (2004). Posterior crural region . In: Essentials of Human Anatomy. Part III, 3rd Ed. Kolkata: Current Books International, pp 204.
  3. Davis T J, Schon L C. Branches of the tibial nerve: anatomic variations Foot Ankle Int. 1995 Jan; 16 (1): 21-9.
  4. Dellon A L, Kim J, Spaulding C M.. Variations in the origin of the medial calcaneal nerve. J Am Podiatr Med Assoc.2002 Feb;92 (2):97-101
  5. Dellon A L, Mackinnon S E. Tibial nerve branching in the tarsal tunnel. Arch Neurol.1984 Jun; 41 (6): 645-6.
  6. Havel P E, Ebraheim N A, Clark SE, Jackson W T, DiDio L. Tibial nerve branching in the tarsal tunnel. Foot Ankle. 1988 Dec; 9 (3): 117-9.
  7. Hollinshead W H. 1982.The back and limbs: knee, leg, ankle and foot. In: Anatomy for surgeons. vol 3, 3rd Ed. London: Harper and Row publishers, pp 797.
  8. Mc Minn R M H. 1990. In: Last’s Anatomy: Regional and Applied. 8th Ed.. Singapore: ELBS Churchill Livingstone, pp 196.
  9. Nagaoka M. An anatomical study of tarsal tunnel. Nippon Seikeigeka Gakkai Zasshi.1990 Apr; 64(4): 208-16.
  10. Romanes G J. 1978. Cunningham’s text book of Anatomy. 11thEd London: Oxford university press, pp 377-78.
  11. Standring S, Ellis H, Healy J C, Jhonson D, William A et al. 2005 . In: Gray’s Anatomy –The Anatomical Basis of Clinical Practice. 39th Ed. London: Churchill Livingstone, pp 1502-1504.
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