IONM in Spinal Surgery

Introduction

IONM is used in a variety of spinal surgeries to assess spinal cord and nerve root function.

Relevant clinical symptoms

1. Foot drop. Foot drop is an abnormality in gait that makes it difficult to lift the foot. Injury to the deep peroneal nerve is the most common cause of foot drop. The peroneal nerve is a branch of the sciatic nerve that exits at nerve roots L4-S2 and innervates the anterior and lateral compartments of the leg, including the tibialis anterior and other muscles that allow us to raise our feet from the ankle (dorsiflexion). Foot drop can also tighten the muscles that allow us to point our feet downward (plantar flexion). The plantar flexor muscles, such as the gastrocnemius and soleus, are innervated by tibial nerve, another branch of the sciatic nerve.

2. Scoliosis. Scoliosis is an abnormal lateral curvature of the spine that includes the rotation of the vertebrae. The misalignment can be in the shape of a C or an S. Scoliosis is diagnosed when there is at least a 10 degree angle in the alignment of the vertebrae as viewed in the anterior-posterior plane. Scoliosis is broadly classified as congenital, neuromuscular, and idiopathic in origin. Physicians characterize the type of scoliosis using the Lenke classification system.

3. Kyphosis.Kyphosis is an abnormal outward curvature of the spine, giving a hunchback appearance. The normal curvature of the spine in the varies between 20-45 degrees when view from the side of the body. Kyphosis is diagnosed when the spinal curvature exceeds 50 degrees.

4. Lordosis. Lordosis is an abnormal inward curvature of the lower spine.

4. Muscle testing. Patients undergoing a corrective spinal surgery often exhibit weakness and a loss of muscle strength. Muscle testing can be used as a neurological and diagnostic tool to assess motor neuron function and a therapeutic tool to assess the patient outcome after the spinal surgery. The muscle testing scale ranges from 1-5, with 5 being a healthy patient who can maintain position against full applied resistance.

Spinal instrumentation

Different spinal surgeries require different combinations of spinal instrumentation.

Spinal fusions: screw, rods, and plates, etc. Spinal fusions are performed to relieve pressure on the spinal cord or stabilize the spine, which can cause symptoms like pain, numbness and weakness. A spinal fusion involves the connection (or fusion) of two or more vertebrae by inserting screws into the vertebral bone and other instrumentation. A fusion can be performed at any level of the spine, including the cervical, thoracic, lumbar, and sacral regions. Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, scoliosis, and kyphosis.

1. Screws. Surgeons will use different types and sizes of screws that depend on different factors, including the spinal level in which they are inserted, the size of the patient's vertebrae, etc. (1) Pedicle screws are inserted into the pedicle bone and are placed into spinal regions below the thoracic level. (2) In modern times, lateral mass screws are used almost universally for cervical level procedures and inserted into the lateral mass bone. There are different techniques that have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified variations), all of which use different entry points and trajectories.