IONM in Spinal Surgery

Introduction

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

1. 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.

Screws. Surgeons will use different types and sizes of screws depending on different factors, 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 directed at a 90 degree angle to the lateral mass and then angled laterally at a 10 degree angle, whereas the Magerl method starts at a 45 degree angle to the lateral mass and then angled laterally at a 25 degree angle. The goal is to avoid hitting the vertebral artery and the exiting nerve roots.

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 compared to the lumbar 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].

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 surface of the vertebral body is exposed, 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 spinal column. Therefore, there are two sets of rods, one for each side of the spine.

2. Interbody cages and bone grafts. For spinal surgeries that require a discectomy, the removal of part of all of the intervertebral disc, it is necessary to fill the empty disc space with either a bone graft (e.g., autograft, allograft) or an interbody cage to restore the height of the spine. These devices are cylindrical or square-shaped and often threaded for increased stability. The interbody cage or bone graft is inserted by distracting the space between the discs. Some interbody cages are expandable, which allows for a more optimal fit.

Scoliosis surgery

The instrumentation for surgical treatment of scoliosis is similar to that of other posterior fusion procedures but includes more anchors to connect the rod and the spine, which improves the correction of the spine. Modern techniques often utilize segmented pedicle screw constructs that allow the rods to be interconnected or hybrid constructs made of pedicle screws, hooks, and wires.

Lumbosacral fusion

Lumbosacral fusions are performed to relieve pressure on the nerve roots or to stabilize the spine, which can cause symptoms like pain, numbness and weakness in the legs. A lumbar fusion involves the connection (or fusion) of two or more vertebrae by inserting screws into the pedicle bones, bilaterally, and connecting them with rod instrumentation. A fusion can be performed at any level of the lumbosacral spine. Spinal problems that require a fusion include degenerative disc disease, disc herniation, spondylolisthesis, spondylosis, vertebral fractures, spinal tumors, and scoliosis.

1. Screws. Surgeons will use different types and sizes of screws depending on different factors, 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) I

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 compared to the lumbar 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].

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 surface of the vertebral body is exposed, 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 spinal column. Therefore, there are two sets of rods, one for each side of the spine.

Others

1. Ondontoid (dens) fracture There are three different types of odontoid fractures, which are classified by the anatomical location of the fracture (Anderson and D’Alonzo classification). Type II fractures are the most common Type I: avulsion fracture of the apex. Type II: fracture through the base of the dens, at the junction of the odontoid base and the body of C2. Type III: fracture extends into the body of the axis.

The C1 and C2 vertebrae are atypical because of their structure and lack of intervertebral discs. The C1 is known as the atlas, and the C2 is known as the axis. The axis shows a peg-like process called the odontoid bone, which projects superiorly from the body. The odontoid process lies anterior to the spinal cord and acts as an axis or pivot for the rotation of the head. The C1 rotates on the ondontoid process. The craniovertebral joint between the atlas and the axis is called, the atlanto-axial joint. The craniovertebral joints differ from the others vertebral joints because they do not have intervertebral discs. This allows them a greater range of motion than the other vertebrae.