Anterior lumbar interbody fusion L5/S1 for lytic spondylolisthesis (transverse incision) using Globus MIS Independence
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Lumbar spondylolysis is a common condition with a prevalence of 5% in the general population. The spondylolysis (pars defect) is thought to most commonly occur at some point during childhood and may be related to repetitive extension activities. Over time, some of these people may develop a lytic (isthmic) spondylolisthesis. Most people will not experience any symptoms whilst others may develop symptoms during their adult life. They may complain of back pain related to the spondylolysis. The other common symptom is a radiculopathy due to a progressive foraminal stenosis that compresses the exiting nerve at that level of the spine. Symptoms often come on with standing or walking and improve with leaning forwards or sitting down. In some instances, patients may also experience a radiculopathy while at rest.
The most common level of the spine to develop spondylolysis is L5 and this can result in an L5/S1 spondylolisthesis. This is most commonly a low grade (0-50%) slip but in rare circumstances can progress to a high grade slip (50-100%) or spondyloptosis (>100% slip). The Meyerding classification describes the magnitude of the slip with grade 1 <25%, grade 2 25-50%, grade 3 50-75% and grade 4 75-100%.
As patients become older, the natural ageing process of the spine causes the disc height to be reduced and this can lead to reduction in height of the foramen. In a combination with the spondylolisthesis, this compresses the exiting nerve root and causes radicular pain or claudication.
It often presents insidiously and manifests itself as paraesthesia or aching, progressing to pain and weakness. It can affect one or both lower limbs and often starts when patients have been standing in one position for more than a few minutes, or if they walk for more than 5-10 minutes. Patients will often complain symptoms are worse if they have to walk slowly around shops. However, leaning forward on a stick or shopping trolley often alleviates their symptoms since flexion makes the lumbar foramina wider. Patients also describe that they need to sit down or lean forwards to allow their symptoms to resolve.
The vast majority of patients can be managed non-operatively and should be encouraged to stay as active as possible, with modification of activities that bring on their symptoms. However, for those who have exhausted non-operative measures, surgery may be beneficial.
Surgical management options require a fusion procedure to stabilise the segment and allow decompression of the exiting nerve root. This cannot be achieved by performing a decompression procedure alone since you will not be able to resect enough bone to decompress the foramen without destabilising the segment. There are a number of ways to perform the fusion required and, at L5/S1, these include posterolateral instrumented fusion, transforaminal lumbar interbody fusion, posterior lumbar interbody fusion, or anterior lumbar interbody fusion. The first three options are posterior approaches which require muscle stripping, pedicle screw insertion, direct neural decompression, and potentially insertion of an interbody cage.
An anterior approach is less invasive since it splits fascial planes rather than mobilising the paravertebral muscles. It allows insertion of a larger cage which provides a larger surface area for fusion and also allows greater release so that lordosis and disc height can be restored to more normal values for that patient; this allows an indirect neural decompression by increasing the disc height and thus the foraminal height to take the pressure of the nerve. Therefore, there is lower risk of neural injury and inadvertent durotomy since the nerves are not directly manipulated; however, there is a higher risk of major blood vessel injury and injury to parasympathetic presacral nerves that can occasionally cause retrograde ejaculation in men.
Some surgeons perform an L5/S1 anterior lumbar interbody fusion via a longitudinal incision. I prefer a transverse incision since this allows wound healing along Langer’s line whilst providing good access to the spine and allowing exposure of the required blood vessels and other structures.
There are a variety of anterior lumbar interbody cages available on the market. When selecting which to use, you should consider how much lordosis and disc height you are aiming to achieve. Ensure the AP and lateral dimensions of the cage will allow it to fit in the disc space. There is also the option of inserting a cage by itself, cage with integral screws or blades (for additional stability), cage with an anterior plate, or anterior cage with supplemental posterior fixation. For this case, I aimed to use an anterior cage with 20 or 25 degrees of lordosis, without any posterior fixation. To ensure that I have good stability of the implant, I have used one with integral blades. Due to the implant design, these are fairly easy to insert without needing to guide screws in. However, they are not easy to remove so, if this may be required during your case, it may be best to use another implant.
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Author:Mr Stephen Morris FRCS(Tr & Orth)
Institution: Avon Orthopaedic Centre, Southmead Hospital, Bristol