A 43-year-old man sustains a fracture of the tibia. There are no neurologic or muscular lesions noted on careful examination. An above-knee cast is applied. After 6 weeks, the plaster is removed. It is noted that he has a foot drop and is unable to extend his ankle because of pressure injury to which of the following?
A 43-year-old man sustains a fracture of the tibia. There are no neurologic or muscular lesions noted on careful examination. An above-knee cast is applied. After 6 weeks, the plaster is removed. It is noted that he has a foot drop and is unable to extend his ankle because of pressure injury to which of the following?
💡 Explanation
## **Core Concept**
The question tests knowledge of the anatomy and function of the nerves in the lower limb, particularly those susceptible to injury from prolonged pressure, such as during immobilization in a cast. The **common peroneal nerve** (also known as the common fibular nerve) is a key structure here, responsible for controlling muscles involved in ankle extension and foot movements.
## **Why the Correct Answer is Right**
The correct answer, **common peroneal nerve**, is right because this nerve is particularly vulnerable to injury from pressure, especially around the fibular neck. The common peroneal nerve wraps around the lateral aspect of the fibular neck, making it susceptible to compression injuries, such as those sustained from a tight cast. This nerve injury leads to **foot drop** (inability to dorsiflex the foot) and weakness in ankle extension due to the involvement of the muscles innervated by this nerve, notably the tibialis anterior.
## **Why Each Wrong Option is Incorrect**
- **Option A:** The **tibial nerve** primarily controls muscles involved in plantarflexion of the foot and innervates the intrinsic muscles of the foot. Injury to this nerve would not typically cause foot drop or weakness in ankle extension.
- **Option B:** The **femoral nerve** innervates muscles involved in hip flexion and knee extension. Damage to this nerve would not result in the inability to extend the ankle or cause foot drop.
- **Option D:** The **sciatic nerve** is the parent nerve to both the common peroneal and tibial nerves. While injury to the sciatic nerve could potentially cause a range of deficits, the specific pattern of foot drop and inability to extend the ankle due to pressure injury would more directly implicate the common peroneal nerve.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the common peroneal nerve is at high risk of injury at the fibular neck due to its superficial location. Clinicians must be vigilant about monitoring for signs of nerve compression when applying casts or splints, especially around this area. A classic presentation of common peroneal nerve palsy is the inability to dorsiflex the foot, leading to a "foot drop" gait.