Spinal Fusion Explained

Understanding the Most Common Procedure in Spine Surgery

Introduction:

The term fusion is often scary for patients. “Will my back stop moving? Will it help cure my pain? Will I be worse off than I started?” Those are just a sampling of the questions I often get. Patients have heard horror stories from friends and family, and thus carry a healthy amount of fear about lumbar fusion. Although lumbar fusion is a major spine operation and should be undertaken only with careful thought, it has the power to dramatically transform your life and lead to improved function. In this article, we will answer the patient questions listed above, explain what spinal fusion is used for, and identify the different types of lumbar fusion. We will also discuss the long-term efficacy of the procedure.

What is a lumbar fusion?

A lumbar fusion is a surgical procedure performed to get at least two adjacent lumbar vertebrae to grow together (fuse) to become one bone. Spinal fusions are similar in concept to healing a broken bone. When you have a broken bone, your body tries to fuse the shattered ends together to make the bone whole again. In a spinal fusion, you are creating a similar biological process to get two vertebrae to fuse together via a bony bridge without causing a break.

Will lumbar fusion reduce the motion across your back? The short answer is yes, but then why would so many patients benefit from lumbar fusion?  The longer answer is that most candidates for lumbar fusion are so degenerated already they have little if any motion remaining. Lumbar fusion is incredibly well tolerated in these individuals because they have already “paid” the “cost” of the procedure (the loss of motion). Spinal disease can become so crippling that a lumbar fusion in the right candidate can dramatically increase functional capacity, including ability to walk and perform daily activities.

What are the basics concepts of a lumbar fusion?

How do you get the two bones to grow together? The answer is by implanting bone graft between the vertebrae. Bone grafts are used to heal lumbar fusions. Bone grafts can either come from your own body (autografts) or from the body of cadavers (allografts). Autografts typically consist of bone removed during a laminectomy (local bone) which otherwise would be thrown away. This is the best source of bone graft because, for the procedure to be successful, bone must be removed anyway, and such removal does not require a separate incision or added invasiveness. Autografts can also be removed from your pelvis, so-called iliac crest bone grafts, which are excellent at promoting fusion but usually require a separate incision. Bone graft accelerants—compounds that speed and strengthen bone healing—may also be used, such as bone morphogenic protein (BMP).

Cadaver bone is also commonly used in lumbar fusion, which may sound frightening. However, cadaver bone is rigorously processed (sterilized, tested for disease, made biologically inert) before it is implanted. Cadaver bone can be thought of more as a mineral implant that your body will populate with your own cells, making it no longer a cadaver.

So, what is the purpose of the hardware in a spinal fusion? The hardware immobilizes the vertebrae so they cannot move. Imaging trying to heal a broken bone without a cast. Every time you would move your arm, you would rebreak it, and it would never heal. Although spinal fusions can be performed without the use of hardware, hardware greatly increases the chances of a successful fusion because it acts as the cast to hold the bones together. Once the bone is healed, the hardware theoretically is no longer needed.

Fifty years ago, spinal hardware was rudimentary, consisting of wires and hooks. Now the science of lumbar fusion has advanced significantly to include specialized screws and cages which provide excellent stability to a broken-down spine. The screws used are called pedicle screws and are inserted into the bone. The orthopedic hardware holds the two vertebrae together and acts as a scaffolding to allow the bones to knit together as they heal. We now have not only improved hardware but also improved ways of implanting these devices to minimize soft-tissue trauma and speed recovery.

What conditions are lumbar fusions used for?

Even through spinal instability has been documented since ancient Egypt, lumbar fusion treatments did not develop until the nineteenth century, when anesthesia, antisepsis, and medical imaging requirements were high enough to allow a safe approach to the spine (1). Lumbar fusions are used for many different spinal conditions, but particularly to restore destroyed lumbar anatomy, decompress pinched nerves, or stabilize unstable vertebrae in the spine. Lumbar fusions are most commonly used for is lumbar spondylolisthesis, a condition in which one vertebra slips forward on another. Spondylolisthesis means the vertebrae are no longer linked closely together and there is abnormal increased motion between them. This can be stabilized with a lumbar fusion to dramatically relieve pain and has been well studied in randomized controlled trials (2). Lumbar fusions can also be used to treat degenerative disc disease, lumbar stenosis, and any lumbar deformity such as scoliosis. Thus, lumbar fusions are versatile and can be used to treat a number of low-back conditions that cause back pain and sciatica. Please see my article on causes of low-back pain for a detailed description of how these conditions evolve.

Who should not have a lumbar fusion?

Lumbar fusion is not able to cure all types of low-back pain. In fact, if done in the wrong person the procedure can actually exacerbate low-back pain. If your low-back pain is from muscular strain, you should not have a lumbar fusion. Lumbar fusion also has mixed results in treating garden-variety disc degeneration without nerve compression, so talk carefully with your surgeon about whether lumbar fusion is right for you. If you have very poor bone quality (osteoporosis) your body may not be able to support a lumbar fusion. Luckily there are new medicines and techniques that can be used to treat the osteoporotic spine. 

You should also undergo metal allergy testing prior to a lumbar fusion if you have a history of metal sensitivity. 

What are the different types of lumbar fusion?

There are many ways to perform a lumbar fusion. Lumbar fusions may be performed with bone graft alone, or with bone grafts and orthopedic hardware (cages and screws). If just bone graft is used it is called an “insitu” fusion, meaning the bones are fused in their place. Insitu fusions are an older technique but still used occasionally if the patient cannot tolerate hardware. If orthopaedic hardware is used in the procedure, it is called an instrumented fusion.

Lumbar fusions are used to restore collapsed and deformed disc spaces from degenerative disc disease and other conditions of the spine that may be pinching off nerves. This is typically accomplished by removing the spent disc (discectomy) and replacing it with a spacer “interbody” graft that reconstructs the normal alignment of the disc space. Hence the term “lumbar interbody fusion” or LIF, which means a graft is being inserted into the disc space. There are many different ways to perform lumbar fusion, and each type has its specific uses.

Interbody grafts are small wedge-shaped blocks of material that are strong and rigid and are designed to replicate the normal anatomy of the disc. Interbody grafts can be used to prop up the disc space.  The interbody graft is a cube of material, either metal, plastic, or bone. These days metal (titanium), is the most common graft material that I use in surgery. This is because titanium is particularly compatible with bone. Even more impressive are that the interbody grafts can be three dimensionally printed to look like bone in both a gross structural and microscopic senses. This makes the titanium graft the best interbody device (3).

Interbody grafts are also usually perforated or contain receptacles for bone graft or bone morphogenic protein (a bone-healing accelerant) that can be packed inside. Bone grafts stimulate the bony healing necessary for the fusion to heal correctly, as described above.

Once the interbody graft is placed into the disc space, pedicle screws will be inserted into the bone. The screws are called pedicle screws because they are put into the cylindrical tubes on either side of the vertebra known as the pedicles. The screws are locked together with vertical rods, two on either side. Finally, bone graft is placed along the sides of the spine and in the back of the spine, known as a posterior lateral fusion. This completes the spinal fusion process.

What are the LIFs?

It’s easy to get lost in the world of LIFs as a patient because of the terminology. The acronym LIF is generally preceded by a letter (TLIF, ALIF, LLIF, PLIF). The letter denotes which direction the graft is placed in your body. To simplify things, imagine that there are three main directions a graft can be placed in the body. From the front (anterior), which is known as ALIF; from the side (lateral), which is known as LLIF; or from the back (posterior/transforaminal), labeled PLIF/TLIF. The direction from which the graft is placed is known as the approach.

Anterior approaches (ALIF):

Anterior approaches are performed through the abdomen. Sounds scary, but this approach is actually very powerful because it affords direct access to the disc space and allows the insertion of a wide graft. In an anterior approach your surgeon will need to move the abdominal contents aside by accessing a window known as the retroperitoneal space.  The anterior approach is also favorable because it does not require splitting of muscles. The disadvantages are that the visceral organs and great blood vessels are at risk during an anterior approach. In addition, not everyone has the right anatomy for a safe anterior approach.

If you are planning on undergoing an ALIF procedure it is important to let your surgeon know if you have had prior abdominal, kidney surgery, or spinal infection. These conditions produce significant scarring that makes the anterior approach to the spine dangerous. Furthermore, if you have only one kidney or a history of significant vascular surgery you should let your surgeon know, as these can increase the risk of major complication from an anterior approach.

Lateral approaches (LLIF):

The lateral approach or LLIF, can also be called XLIF, which stands for extreme lateral. LLIFs are performed through your flank, as there is a safe space in this area through which a surgeon can access the spine. Lateral approaches are performed by creating a partition in the Psoas muscle without cutting it. LLIFs are powerful because, like ALIFS, they can allow surgeons to put in large grafts. The downsides are that XLIFs cannot be used at all levels, and they may result in hip pain or quad weakness after the procedure from nerve or psoas muscle irritation. The great blood vessels and abdominal contents are also at risk in lateral approaches.

One of the major weaknesses of the LLIF approach is that it cannot be used to treat the L5–L1 level. Surgeons may work around this by combining the LLIF with an ALIF or TLIF.

As with all approaches performed through the abdomen, you should let your surgeon know if you have had prior abdominal surgery.

Posterior approaches (PLIF and TLIF):

The posterior approaches include the PLIF (posterior lumbar interbody fusion) and the TLIF (transforaminal interbody fusion). The difference between the PLIF and the TLIF is essentially the angle at which the graft is inserted into the spine from the posterior direction, and thus the graft spacers are shaped slightly differently. A PLIF uses a more inline straight approach, whereas the TLIF requires a more angled approach through the neural canal. Both are powerful and can be great for generating fusions. The great benefit of the TLIF and PLIF compared to XLIF and ALIF is that they can be done through the same approach used to insert pedicle screws, so only one incision is needed. TLIF and PLIF approaches also have inherently less risk to major abdominal structures such as the great blood vessels because the abdomen is not exposed. The downside is that the grafts are slightly smaller. Both the TLIF and the PLIF require nerve root retraction, which can be painful or cause spinal fluid leaks.

Which approach is best?

The best approach is constantly debated by surgeons at academic meetings. The truth is, different approaches have different uses, and ultimately it is up to your surgeon to decide the best approach for you. Fusion and complication rates are similar between the approaches, and the main differences surround the types of complications (4). For example, anterior and lateral approaches put the abdominal contents at risk, while posterior approaches have a higher risk for spinal fluid leaks (5).

What are the risks of lumbar fusion?

Lumbar fusion surgeries are major operations that may require months of recovery. The risks include but are not limited to infection, damage to nerves and blood vessels, and hardware complications. Spinal fluid leaks can occur at a rate of around 4% of cases and usually need to be repaired (6). There is also the risk for medical complications such as heart attack, stroke, embolism, etc.

Although lumbar fusion can effectively treat a diseased segment of the spine, it is still possible for the adjacent non-fused discs to continue to experience degeneration. Thus, it is possible for the discs above and below a spine fusion to degenerate, resulting in what is known as adjacent segment degeneration (ASD). Fifteen percent of patients will require a revision surgery for ASD at 5 years post-op and 30% will require it within 10 years post-op (7).

Another problem with lumbar fusions occurs when they fail to heal or fuse together, called non-union or pseudoarthrosis. The overall rate of fusion is approximately 88–90% and is comparable between anterior and posterior procedures (6). The overall risk for

What is the recovery process after a lumbar fusion?

Lumbar fusion works by getting at least two lumbar vertebrae to grow together, which means new bone must grow between them. The process of bony healing takes approximately one year to consolidate, and the hardware holds things in place until this is accomplished. However, most of the healing will be done within three months (8). During those months, it is important to try to reduce the amount of strain across the fusion site by limiting, bending, twisting, and lifting using the back. I usually tell my patients not to lift anything heavier than a gallon of milk. You are encouraged to walk as much as you can tolerate without pain during your convalescence.

Even though there have been tremendous improvements to surgical techniques, speeding recovery and making the surgery less painful, lumbar fusions are still major operations. I tell my patients to expect major back pain for at least 2–6 weeks following the procedure. For this reason, outpatient physical therapy is not initiated immediately after a lumbar fusion but instead is begun after the bones have had time to grow together.

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2.         Fischgrund JS, Mackay M, Herkowitz HN, Brower R, Montgomery DM, Kurz LT. 1997 Volvo Award winner in clinical studies. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective, randomized study comparing decompressive laminectomy and arthrodesis with and without spinal instrumentation. Spine. 1997 Dec 15;22(24):2807–12.

3.         Olivares-Navarrete R, Hyzy SL, Slosar PJ, Schneider JM, Schwartz Z, Boyan BD. Implant Materials Generate Different Peri-implant Inflammatory Factors: Poly-ether-ether-ketone Promotes Fibrosis and Microtextured Titanium Promotes Osteogenic Factors. Spine. 2015 Mar;40(6):399–404.

4.         Sebastian AS, Wagner SC, Kaye ID, Kepler CK. Achieving Lumbar Fusion: An Evidence-Based Approach to Selecting Technique, Implants, and Biologics. Instr Course Lect. 2022 Jan;71: 427–38.

5.         Phan K, Lackey A, Chang N, Ho YT, Abi-Hanna D, Kerferd J, et al. Anterior lumbar interbody fusion (ALIF) as an option for recurrent disc herniations: a systematic review and meta-analysis. Journal of Spine Surgery. 2017 Dec;3(4):587–95.

6.         Phan K, Thayaparan GK, Mobbs RJ. Anterior lumbar interbody fusion versus transforaminal lumbar interbody fusion—systematic review and meta-analysis. Br J Neurosurg. 2015 Nov;29(5):705–11.

7.         Ghiselli G, Wang JC, Bhatia NN, Hsu WK, Dawson EG. Adjacent segment degeneration in the lumbar spine. J Bone Joint Surg Am. 2004 Jul;86(7):1497–503.

8.         Katsuura Y. The Spine Encyclopedia: Everything You’ve Wanted to Know About Back and Neck Pain but Were Too Afraid to Ask. Subtitulas Press, 2022.