Bone Up on Lumbar Spinal Fusion
Part 1: Proper payment starts with understanding lumbar spinal fusion anatomy and procedures to better code them in the facility setting.
Spinal fusion (arthrodesis) procedures are performed every day around the world. Coding spinal fusion in an outpatient or ambulatory surgery center (ASC) setting with CPT® is an entirely different animal than in the inpatient setting with ICD-10-PCS. Understanding spinal anatomy is key to extrapolating core procedural steps from operative notes and converting that information into codes that represent a procedure properly and fully. Spinal anatomy can at first appear quite crowded and complex, and there are a great many variations of spine fusion procedures — making coding these procedures confusing for even seasoned coders. Because of the enormity of this specialty, we’ll focus on only one of the five regions of the spine — the lumbar region — and we’ll broach anatomy and the procedures this month and save coding for the October issue of Healthcare Business Monthly.
Lumbar Anatomy and Biomechanics
The spinal cord is protected by irregular bones of the vertebral column (see Figure 1). The spinal canal housing the cord passes through the vertebral foramen (Latin for opening) in the center of the vertebrae. The spinal cord is protected anteriorly by the cylindrical vertebral body and posteriorly by the posterior elements of the vertebrae, forming a bony ring surrounding the spinal canal. Projecting posteriorly from the vertebral body are the pedicles. They are two short, stout, cylindrical portions of bone that extend around the sides of the spinal canal to meet the broad, flat plates of the laminae, which form the posterior portion of the vertebral arch that encircles the spinal canal and protects it. The spinous process projects posteriorly from the point where these laminae connect and creates an attachment point for spinal muscles and ligaments. The transverse processes project out on each side at the point where the pedicles join the laminae, also creating an attachment point for spinal muscles and ligaments.
The motion segments of the spine facilitate flexion (bending forward), extension (bending backward), and twisting. The functional units of these motion segments are made up of two adjacent vertebrae, their shared intervertebral disc, two facet joints and capsules, and the connecting ligaments (see Figure 2). For each adjacent pair of vertebrae, there are four articular processes joining the two vertebral segments: two at the top (superior articular processes) and two at the bottom (inferior articular processes). These facet joints do most of the work of keeping the spine stable during movement and can be major players in back pain, whether due to trauma or degenerative conditions such as osteoarthritis.
On the anterior column of the spine are the large, round vertebral bodies. The vertebral body consists of an outer ring of very strong, very dense cortical bone, surrounding the softer, spongy cancellous (aka trabecular) bone in the center. Between each vertebral body lies a disc of tough, very strong tissue called fibrocartilage. These discs perform as shock absorbers for the spine when there is impact from activity. Much like the vertebral bodies, the intervertebral disc is made up of a strong, tough outer layer and a softer, spongy center. The outer layer of the disc is the annulus fibrosus, which is made up of lamellae — layers of collagen fibers with great tensile strength. The inner structure of the disc is the nucleus pulposus. The disc is often a source of pain due to degenerative disc disease (DDD). The bony spaces (i.e., the posterior elements of the spine and the interbody space) are the most often fused — usually after decompression of the spinal canal and/or exiting nerve roots — to eliminate painful motion and/or stabilize the spine.
The spinal canal is afforded additional protection in the non-bony intervertebral spaces by the posterior longitudinal ligament (PLL) — a flat, ribbon-like ligament, which runs along the anterior of the vertebral foramen and the ligamentum flavum, which is situated on the posterior of the vertebral foramen (see Figure 3). Hypertrophy (enlargement due to thickening) of the ligamentum flavum is a common cause of spinal canal stenosis (narrowing).
Not to be confused with the central vertebral foramen of the spinal column, nerve roots exit the spinal column via lateral spaces of each vertebral segment in the cervical, thoracic, and lumbar regions of the spine, and are called neural foramina or intervertebral foramina. Degenerative changes to the vertebrae and/or disc can cause these foramina to narrow and compress the nerves, leading to pain, numbness, and/or loss of function.
Most people have five lumbar vertebrae at the base of their spine, superior to the sacrum. Rarely, some may have an additional segment, called a transitional vertebra. The lumbar spine contains the largest vertebrae of the vertebral column to allow for their significant load-bearing responsibilities.
What Is Spinal Fusion?
Arthrodesis is the fusion of bones across a joint space to limit or eliminate painful movement and promote vertebral stability. Arthrodesis can occur spontaneously or as a result of a surgical procedure, such as fusion of the spine. Typically, in spinal fusion procedures instrumentation (plates, screws, rods, etc.) provides mechanical stability, while bone graft induces or supports biological bone formation over the next few months. Spinal fusion may be deemed medically necessary for patients suffering from DDD, spinal canal stenosis with instability, recurrent disc herniation, spondylolisthesis (slipping forward of a vertebral body on to the vertebrae underneath), or other painful, debilitating conditions.
Arthrodesis can be achieved with many different techniques or a combination of techniques, often after removal of arthritic bone and decompression of the spine and/or its exiting nerve roots. There are two general types of spinal arthrodesis procedures: interbody fusion and/or posterior fusion (non-interbody). Sometimes the two overarching spinal arthrodesis procedures may be combined. They also are broken down into several, specific surgical approaches:
Interbody Fusion – Fusion in the interbody space is achieved by bone graft placed in an intervertebral cage or with the use of an allograft spacer, which is placed between the bodies of two contiguous vertebrae after some or all of the disc is removed and after the vertebral endplates are prepared to facilitate fusion. Arthritic bone restricting and/or compressing the spinal column and nerves may be removed with laminectomy, laminotomy, foraminotomy, or facetectomy, or a combination of them (see Figure 4). Fusion is performed on the anterior column of the spine, via an anterior or posterior approach.
Posterior Fusion – The posterior structures of contiguous vertebrae are fused – either the laminae, facets, transverse processes, spinous processes, or a combination of them. Just like in interbody fusion, arthritic bone restricting and/or compressing the spinal column and nerves may be removed with laminectomy, laminotomy, foraminotomy, facetectomy, or a combination (see Figure 4). A posterior column fusion can be performed using a posterior, posterolateral, or lateral transverse technique. This technique differs from that of interbody fusion in that there is no interbody spacer or cage placed, and the posterior/lateral elements of the spine are fused (posterior column) but not the interbody space (anterior column).
For either type of fusion, surgeons often use hardware (e.g., plates and screws, rods and screws, interbody cages, allograft spacers, etc.) to stabilize the spine while the fusion takes place over approximately three to six months. If the bone graft does not work and there is no fusion (i.e., pseudoarthrosis), then the hardware could eventually break, so the two are often used together to ensure the best possible outcome. There are many types of bone graft that can be employed, but the two most common options are:
Autograft – The bone is harvested from the patient’s iliac crest or elsewhere. It’s removed during decompression of neural elements/spinal canal or bone marrow aspirate (auto- means self).
Allograft – donor tissue (allo- means other)
Posterior Approach vs. Posterior Column Fusion
Take care not to confuse a posterior approach with a posterior column fusion. Remember: Posterior approach refers to the anatomical access point to the spine (i.e., the site of the incision the surgeon makes). A posterior column fusion refers to the portion of the spinal column that is being fused (i.e., the posterior column of the spine (not anterior/interbody).
Spinal Fusion Surgical Approaches
Along with the different options for the fusion, the surgeon also has several approaches to choose from to access the spine. The surgeon chooses the approach most clinically appropriate for each patient and their individual circumstances and needs. There are generally two types of approaches with which the surgeon can access the patient’s spine: posterior or anterior (see Figure 5). All interbody fusions are performed on the anterior column of the spine, as the bone graft is placed in the interspace between two adjacent vertebrae. An anterior column (interbody) fusion can be achieved with either an anterior or posterior approach; however, a posterior column fusion cannot be achieved via an anterior approach. Within these two approach categories (anterior vs. posterior) are many variations, each designed for a specific purpose (e.g., an oblique lateral approach that avoids dissection or splitting of the psoas muscle and the lumbosacral plexus nerves that run through that muscle).
Posterior Interbody Fusion Approaches
Posterior Lumbar Interbody Fusion (PLIF) – The patient is prone (lying on their stomach), and the incision is made in the back.
Transforaminal Lumbar Interbody Fusion (TLIF) – The patient is prone, and the incision is made in the back, unilaterally to one side of the spine, to access the vertebral body at an angle via an opening made by removing some of the facet joint. This may be repeated on the contralateral side if decompression is needed on both sides to properly free entrapped nerves and/or restore proper lordotic curvature of the vertebral column.
Both the PLIF and TLIF procedures allow for an interbody fusion through a posterior approach (with the patient positioned on their stomach (prone)). The interbody space is part of the anterior column of the spine. Both procedures can be combined with a posterior or posterolateral fusion of the posterior elements of the spine (i.e., posterior lumbar fusion). The combination is oftentimes referred to as a 360-degree fusion, which negates the need to reposition the patient to perform the second fusion on the posterior column, as they would if the interbody fusion was performed via an anterior approach (with the patient positioned face up (supine) or on their side (lateral)).
Anterior Interbody Fusion Approaches
Anterior Lumbar Interbody Fusion (ALIF) – The surgical approach to the interbody space is anterior, through the abdomen, and may require an access surgeon (e.g., vascular surgeon to safeguard the aorta and vena cava, which are directly anterior to the spine), with the patient placed in the supine position.
Direct Lateral Interbody Fusion (DLIF) – The surgical approach to the interbody space is anterior, on the side of the abdomen, with the patient placed in the lateral decubitus position (left or right side up).
Oblique Lateral Interbody Fusion (OLIF) – The surgical approach to the interbody space is anterior, in an oblique trajectory and away from the nerves in the psoas muscle, with the patient placed in the lateral decubitus position.
All three anterior procedures allow for an anterior interbody fusion through an anterior approach. The interbody space is part of the anterior column of the spine. These procedures can also be combined with a posterior or posterolateral fusion of the posterior elements of the spine.
Posterior or Posterolateral Fusion Approach
The patient is prone, and the incision is made overlying the vertebrae. Sometimes, this type of fusion procedure may be referred to as a “gutter” fusion or the placement of the bone graft referred to as “in the gutters.”
These procedures allow for posterior column fusion through a posterior, posterolateral, or lateral transverse approach. The posterior/lateral elements are part of the posterior column of the spine. An interbody device cannot be reported with a posterior column fusion, as interbody indicates an anterior column fusion; however, these procedures can be combined with an anterior interbody or posterior interbody fusion of the anterior column of the spine, sometimes without the need for repositioning the patient.
Medtronic’s Health Economics & Reimbursement Team have several spine coding and billing resources. You can find the information by scanning the QR code, searching for “Medtronic SpineLine” or emailing them at either: SpinalCodingHospital@medtronic.com (Support for Facilities) or SpinalCodingMD@medtronic.com (Support for Surgeons). The SpineLine® hotline is available at (877) 690-5353.
About the authors:
Anna Ward, CPC, is a coder with 22 years of healthcare experience, specializing in spinal coding. She is a coder and project coordinator for Medtronic Spinal & Biologics. Ward is a member of the Memphis, Tenn., local chapter.
Darren W. Nance, MS, CPC, is a senior reimbursement analyst for Medtronic Spinal & Biologics in Memphis, Tenn. He has 18 years of healthcare experience in case management and coding. Nance received his Bachelor of Science and Master of Science from the University of Tennessee. He is a member of Memphis, Tenn., local chapter.