Isolated tibial nerve injury: a rare presentation

Introduction

The tibial nerve lies between the superficial and the deep muscles of the posterior compartment of the leg. It is well-protected from direct trauma due to this thick cover of muscles. In the lowermost part of the leg and ankle, the nerve is relatively superficial but is guarded anteriorly by the posterior surface of the medial malleolus, superficially by the flexor retinaculum and posteriorly by the Achilles's tendon. This protected location makes isolated injury to the tibial nerve is very uncommon. Even in cases of open fractures and associated vascular injuries of the lower extremity complete transaction of the tibial nerve is rare. [1] Injuries to the sciatic and common peroneal nerves are more common due to their vulnerable position. [2],[3] Most of the available literature on peripheral nerve injuries in the lower extremity has documented the results and the treatment options for peroneal and sciatic nerve injuries.Other common causes of lower limb neuropathy are diabetic neuropathy [4] and compression neuropathies. [5] Tibial nerve involvement is more common in these chronic conditions. The common end result of the tibial nerve injury or neuropathy around the ankle is loss of sensations of the plantar foot, vasomotor changes due to lack of auto-regulation, subsequently leading to callosities and recurrent ulcerations and paralysis of the intrinsic muscles of foot leading to toe deformities.We report a very unusual presentation of isolated posterior tibial nerve injury following a road traffic accident. Our search failed to reveal any similar case reported in the English literature.Case report

A written consent was obtained from the patient and her attendants. A 15-year-old girl sustained injury to her left leg following road traffic accident. Though the exact mechanism of injury could not be elicited, she remembered falling down from her two-wheeler after it collided with a car. She sustained a small puncture wound over the lower posterior leg and was referred to our hospital after the first aid at a local hospital. On examination, she had a penetrating wound over the Achilles tendon with some soft tissue mass avulsed through the tear in the Achilles tendon [Figure 1]. The avulsed soft tissue was tender on touch. She had a lack of sensation over the plantar foot, and the foot was warm. The skin texture and turgor were found to be normal. With a provisional diagnosis of tibial nerve injury, the wound was explored. The soft tissue avulsed and protruding through the tendon was the tibial nerve [Figure 2a]. While the posterior tibial vessels and the flexor tendons were intact.

 Discussion

Well recognized and documented examples of nerve injuries in the lower limb are the sciatic nerve injury at the hip during posterior fracture dislocations, iatrogenic injuries during injections and peroneal nerve injury following fracture of the neck of the fibula. [2],[3],[6] Injury to the posterior tibial nerve is, fortunately, very rare. [1],[7],[8] Though the mechanism of injury is not exactly known in the presented case, some sharp object must have pierced the Achilles tendon to reach the nerve and due to change of the direction as the patient fell down it must have got entangled around the nerve, ultimately avulsing it.The actual site of nerve injury was much higher than perceived site of injury, possibly a relatively fixed point like a muscular branch and is difficult to predict. [9] This can be considered analogues to brachial plexus injuries where the forces involved usually avulse the nerves from a relatively fixed point. This may also help in explaining the fact that the upper roots are either avulsed or ruptured (as they have few branches in the neck), but the lower roots are almost always avulsed in a global brachial plexus injury. Nerves are much tougher structures and coniderable force is required to avulse a nerve completely. In the presented case the nerve was completely avulsed, indicating the force involved.Nerve repair or reconstruction should be carried out as early as possible after the injury. Children have better potential for nerve recovery and primary repair should be attempted as and when possible. For grossly contaminated wounds, injuries with extensive crushing and cases where it is difficult to know the exact extent of injury, delayed primary repair is recommended. [10] Radical debridement up to vital axons and nerve grafting was the only chance for recovery. In our case the avulsion of the nerve and the amount of damage observed on table warranted the waiting period of 3 weeks before definitive reconstruction.After diagnosing the injury, there were two possible ways of reconstruction of the defect/loss. One option was a primary nerve transfer [11],[12] and the other was that of nerve reconstruction with nerve grafts. [13] The number of axons in the donors locally available (superficial peroneal and sural) are limited, and also they have only sensory fibers. On the contrary, the tibial nerve has sensory and motor fibers for the intrinsic muscles of the foot. Nerve grafting in children has better success rate than in adults. [14] We harvested nerve grafts from contra-lateral sural nerve to keep the option of nerve transfer open if required in the future. She recovered sensations completely in around one and half years after the reconstruction, and her age was the most important factor in her recovery.Injury to the tibial nerve in the lower leg leads to the loss of sensation at the plantar foot. Though the function of the leg muscles was preserved in this case, the insensate foot can be equally disabling due to loss of position sense and predisposition to injuries to the plantar foot. Atrophy and vasomotor changes complicate the injury. Furthermore, the paralysis of the intrinsic muscles of the foot leads to deformities over a period.These patients need to protect their feet from injuries till they regain the protective sensations. Importance of the care of the insensate part has to be stressed during each follow-up visit. At the initial visit, the exploration of the wound for debridement and assessment of injury and the middle third of the leg for assessing the status of the proximal stump were necessary. We feel that these could have been possible through two separate incisions to decrease scarring. The nerve reconstruction also would have been possible at a later date through the same scars by tunneling the nerve grafts subcutaneously. In this case as the initial scar was present we went through the same scar for reconstruction.In a selected and cooperative patient, nerve grafting in lower extremity can result in rewarding results.

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