As we illustrated in our last post there is a wide spectrum of indications for lumbar spinal fusion. As you move along this spectrum from unstable to more stable pathology the odds of a successful outcome decrease. At the far end of the spectrum of diagnoses, the end at which there is a lesser chance of a favorable outcome after fusion, is degenerative disc disease (DDD) and spondylosis (without instability) causing back pain. In my opinion this is softest indication for spinal fusion. I’m not saying that you should never perform a spinal fusion on a patient with only DDD, the patient just has to be properly vetted and they must understand that a good outcome isn’t guaranteed in these cases. On the opposite end of the spectrum is acute spinal instability caused by trauma or some other acutely destructive process such as tumor or infection. This is the clearest indication for a spinal fusion. NOTE: we’ve already discussed cervical spinal fusion (ACDF) here and here so this discussion will pertain primarily to the lumbar spine.
Classically, spinal stability is defined as the spine’s ability, under normal physiological loads (“normal” obviously varies widely depending on whether you’re a bank clerk or a mixed martial arts fighter), to a) protect the neural elements (i.e. nerve roots and spinal cord), and b) avoid painful deformity. Sounds complicated right? It may be easier to think about what happens when the spine becomes unstable: a) it may not be able to maintain proper alignment and thus may become deformed which causes severe pain; and b) it may not be able to properly protect the spinal cord within which could cause paralysis. So in a nutshell: a stable spine is one that is protecting you against pain and/or paralysis.
The concept of traumatic spinal fractures is a vast one that I won’t get into too much here. Generally, though, fractures are classified as stable or unstable (hopefully you’re starting to pick up on a theme here.) There are many complicated grading schemes that allow spine surgeons to look at a fracture on imaging and determine if it’s unstable or not. One classic scheme is that of Denis which divides the spine into three columns. Stable fractures typically only involve one column of the spine. Examples of stable fractures include fractures of the spinous process (a so-called clay shoveler’s fracture, see Figure 2), compression fractures and transverse process fractures. Stable fractures may be painful from the local trauma of the injury but they do not cause painful deformity nor do they threaten the spinal cord or nerve roots. Thus these types of fractures may be treated conservatively such as with bracing.
Figure 1: Illustration from Denis’ 1983 paper discussing his three spinal columns and their involvement in traumatic injuries.
Figure 2: Fracture of the C6 spinous process (clay shoveler’s fracture). Source: https://radiopaedia.org/images/3175670
Generally speaking if two or more of Denis’ columns are involved in a fracture it is considered unstable (again let me reiterate that Denis’ model is quite simplistic and analyzing a fracture isn’t always as easy as looking at the spine in only 3 columns.) When a fracture is determined to be unstable a spinal fusion may be indicated to restore stability. If an unstable fracture is left to heal without surgery it may heal poorly resulting in a painful deformity. Worse, if a patient with an unstable fracture is allowed to get up out of bed and loads their spine the fracture may shift resulting in injury to the spinal cord and paralysis.
Trauma isn’t the only cause of acute spinal instability. Indeed, aggressive tumors or infections can destroy the integrity of the spine thereby causing painful spinal deformity and perhaps paralysis. These lesions are treated in a similar manner as acute fractures depending on which part of the spinal column has been damaged. The case presentation below describes a case I had a few years ago of an elderly gentleman with severe damage to his spine caused by a staph infection.
Generally speaking when deciding which type of spinal fusion to perform for acute spinal instability, I’ll go to where the problem is: if the pathology primarily involves the vertebral body in front of the spine, for example, I’ll do a corpectomy to remove the fractured vertebral body. Once the body is removed I’ll reconstruct the spine with a spacer inserted where the damaged vertebral body was, and a combination of plating or screws to provide extra stability (we’ll talk about these devices in more detail in future posts.) The main goal of all of that surgery is to promote new bone growth across the damaged segment of the spine. It’s this new bone growth that restores spinal stability.
The patient is a 75yo male with methicillin-resistant staph aureus (MRSA) bacteremia (in his bloodstream) who presents with worsening mid-back pain. Imaging reveals T11-12 discitis. (Discitis is an infection of the intervertebral disc space that is probably the most painful condition that I see. You can usually make the diagnosis by very gently bumping the patient’s bed when you approach the bedside; if the patient screams out in pain it’s probably discitis. That’s how bad it is.) The medicine doctors tried a long course of antibiotics but unfortunately his pain didn’t improve. Repeat imaging revealed that the infection hadn’t been cleared and in fact had caused further destruction of the T11 and T12 vertebral bodies (see Figure 3.) This destruction resulted in spinal instability and kyphosis (a painful deformity in which the spine falls forward.)
Figure 3: CT scan illustrating T11-12 discitis resulting in severe bony destruction (red arrow) and resultant kyphotic deformity (blue arrow indicates top of spine falling forward).
When I met this patient he looked like he had given up and wanted to die. He’d been bedbound from his infection for weeks and now was quite debilitated. He agreed to undergo surgery and underwent a T11 and T12 corpectomy (via a lateral approach through the chest and behind the lung) followed by reconstruction of the spine with an expandable cage and percutaneous pedicle screws (see Figure 4.) By one month post-op the patient reported no pain and was walking without assistance. The last time I saw him about a year after his surgery he was living a normal life at home with his family. He looked like he’d been given a new chance at life.
Figure 4: Postoperative AP (left) and lateral (right) X-rays with expandable corpectomy spacer at T11-12 (red arrow) and percutaneous pedicle screws from T9-L2 (blue arrows).
I think I’ll spend the next post or two talking about the various forms of spinal implants that we use to achieve a spinal fusion. I had planned to do this later but I think that by presenting it first it will help you better understand the various spinal fusion procedures discussed in later posts.
Thanks for reading!
J. Alex Thomas, M.D.
Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine (Phila Pa 1976) 8:817–31, 1983.