by: Winston T. Capel, M.D., MBA, FACS, FAANS
The definition of fusion means to unite; in spinal surgery, fusion is to arrest motion between vertebrae. The spine was designed to be mobile and allow motion while at the same time protect the neural elements: the spinal cord and spinal nerves.
Why would fusion be necessary?
Fusion is necessary when the stabilizing elements of the spinal column: bone, disc, ligaments can no longer provide the protection of the neural elements (spine is unstable). These stabilizing elements can be compromised by:
- Trauma: fractures, dislocations and ligament injury
- Tumor: primary tumor (originating in the vertebra) or metastatic (spreading from other organs)
- Infection: bacterial infection can weaken bone and cause neural compression
- Degeneration: degeneration (osteoarthritic) changes can weaken supporting structures and cause instability resulting in intractable pain and or neural compression requiring the removal of disc, ligament or bone. Decompression (the removal of bone, ligament and disc) is required if symptoms are intractable and there is failure of non-surgical treatments to relieve pain. If instability exists before decompression or results from decompression reconstruction then fusion is necessary to restore or maintain stability. Stability being grossly defined as the spine’s ability to move physiologically without jeopardy of the neural elements. Sometimes it is necessary to fuse the lumbar spine because normal motion produces intractable pain that has not responded to nonsurgical treatments such as: exercise, physical therapy and medications.
Examples of Some Common Fusion Scenarios
Cervical Disc Herniations
The cervical segment of the spinal column is very mobile and subject to excessive and early degenerative changes. These degenerative changes are accelerated by: genetics, wear and tear, repetitive trauma, heavy labor and nicotine exposure. When the disc breaks down in degenerative disc disease it can lose its mechanical integrity and start to migrate and fragment. This can lead to nerve and spinal cord compression. There is opportunity to manage the nerve compression nonsurgically with things such as: time, traction, neuromodulation drugs (Neurontin, Lyrica), and injections. Often, symptoms of nerve compression (radiculopathy) will improve and surgery can be avoided. If not, the problem is most commonly treated by an anterior approach where the disc is removed to decompress the symptomatic nerve. The removal of the disc has resulted in instability. There are 2 options: fusion and non-fusion. Fusion is to replace the disc with a polymer, autograft (from the patient) or allograft (bone bank) with the intent of growing bone where disc was. This bone growth requires bone forming cells (osteoblasts) to be protected from shear forces that interfere with bone growth. A metallic plate (titanium) is used to provide immediate stability while bone is growing over the next 3-4 months. If bone growth fails, the fusion fails and the plate will usually fail at the bone metal interface (loose screws) or less commonly metal fatigue (broken screws). Almost all fusions require or are greatly facilitated by the use of metallic instrumentation to optimize osteoblast activity.
It is not uncommon for more than one disc (level) to require this decompression, reconstruction and fusion. The non-fusion option involves the use of an artificial disc (Total Disc Replacement) where a combined metal plastic designed unit preserves motion and replaces the mechanical stability of the disc. Disc replacement surgery is limited to one level and insurance coverage is still variable.
Spondylolisthesis is the forward displacement of one vertebra on another. This is most often a degenerative process that occurs later in life but can be due to defect in the vertebrae at the pars interarticularis which results in premature disc degeneration allowing the slippage to occur. Spondylolisthesis is commonly associated with stenosis (narrowing) of the exiting foramen and or the spinal canal. The combined spondylolisthesis and stenosis can often be managed non-surgically with exercise, physical therapy, medications and injections. When these measures fail a decompression is required. Decompression tends to make the spondylolisthesis globally unstable resulting in progressive slip and reoccurrence of the stenosis. A fusion at the time of the decompression has been shown scientifically to result in a better clinical outcome than decompression alone. There are various techniques and surgeon preferences that determine the method of fusion. All of the techniques will require the application of bone or bone forming substances and most often instrumentation. This is usually done from a posterior approach but occasionally a combined anterior and posterior approach is required. If the degree (grade) of the spondylolisthesis is high, oftentimes a reduction (realigning) of the vertebra to maintain proper balance of the spine is required. Corrective forces are generated by the applied instrumentation.
Fusion for Chronic Low Back Pain from Degenerative Disc Disease (DDD)
The application of fusion to the treatment of low back pain (LBP) can be controversial. In general, the perceived failure of spine surgery can most often be attributed to this application of fusion. In my opinion, it is the proper selection of which patients receive this type of surgery that can be a great challenge. Often the failures to achieve the expected outcomes can be reduced to patient selection. It is known that patients with certain characteristics do not do as well clinically when fusion is applied for the treatment of chronic LBP secondary to DDD. The expected and desired outcome is: pain reduction, improved quality of life with increased functionality. This is less like to occur statistically in patients with these characteristics:
Worker’s compensation patients and third-party personal injury patients do not do as well generally as patients without this attribute. The spine literature in describing efficacy for fusion almost always will separate these patients out of patient populations to better understand clinical efficacy and outcomes. Litigation plays a significant role in perceived and or real outcomes for fusion in chronic LBP for DDD.
Psychology of the patient:
Patients with depression and anxiety do not do as well statistically. In the process of patient selection many spine surgeons will send patients to a pain psychologist to screen for the presence of psychopathology that can jeopardize a good outcome. Functional issues (including marital conflict) can factor into a poorer outcome if the chronic pain issue is interconnected to the social support system for a patient.
In all fusions, bone growth is the primary determinant of outcome. Nicotine antagonizes osteoblastic activity and is strongly associated with poorer clinical outcomes. Most spine surgeons will not do elective fusion procedures in smokers. In general, the fusion rate for a smoker is 30% lower than for a nonsmoker.
This is a complex issue especially given the current environment. The reality is that many patients with chronic LBP will be opioid dependent. An expected outcome by most is the reduction or elimination of the need for opioids after successful fusion surgery. The quantity used and the duration of chronic opioid maintenance therapy have predicted poorer outcomes in fusion surgery for chronic LBP secondary to DDD.
Obese patients have poorer outcomes due to a number of reasons. There is a lower fusion rate with morbid obesity with a higher rate of complications. Surgery on obese patients is more technically demanding for the spine surgeon.
In my opinion, a highly motivated patient, (without the above characteristics) with intractable pain can do well with lumbar fusion or TDR. Patient selection is a critical determinant of a good clinical outcome in elective fusion operations especially in the treatment of chronic LBP secondary to DDD.