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 bilateral screws into the pedicle bone or lateral mass and connecting them with instrumentation, such as rods or a plate. 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 depending on different considerations, including the spinal level on which they are working, the size and morphology of the patient's vertebral bones, etc. (1) Pedicle screws are inserted into the pedicle bone and placed into the thoracic, lumbar and sacral spinal levels. (2) In the modern era, lateral mass screws are used almost universally for cervical level procedures and inserted into the lateral mass region. Different techniques have been developed for the insertion and fixation of lateral mass screws (i.e., Roy-Camille, Magerl, and modified techniques), all of which use slightly different entry points and trajectories. In the Roy-Camille method, for example, the screws are inserted at a 90 degree angle to the posterior spine and angled laterally at a 10 degree angle relative to the lateral mass, whereas the Magerl method starts at a 45 degree angle to the posterior spine and angles laterally at a 25 degree angle.

Insertion of pedicle screw in the thoracic spine remains technically challenging due to the smaller size and more complex morphology of the thoracic pedicle bone. The Roy-Camille method is the most commonly used technique for inserting pedicle screws into the thoracic spine, but there remains a high incidence of pedicle bone breach. Screw placement with a partial laminectomy may reduce the incidence of pedicle bone breach [Spine 1998;23(9):1065-8].

2. Rods and plates. The fixation of screws into the spinal column always requires the use of rods or a plate to join them together. The decision to use rods or plates depends on the surgical approach: anterior vs. posterior. Plates are used for anterior approaches because the anterior side of the spine contains the vertebral body, which is more flat in morphology and can be fused by a simple plate with screws. For posterior spinal procedures, rods are preferred. The rods come in different sizes and curvatures, which the surgeon chooses based on factors such as the length of the fusion and the region and curvature of the spine. A single rod is used to connect all the screws on each side of the spine column. Therefore, there are two sets of rods, one for each side of the spine.