Traditional techniques for the treatment of isthmic spondylolisthesis pass a fibular dowel graft across the L5-S1 disc by using the anterior portion of the L5 body.
To introduce a technique for the treatment of isthmic spondylolisthesis in the setting of multilevel degenerative disc disease in adults. Our modified technique allows us to traverse the L5-S1 disc via the L4–5 disc space thereby treating the degenerated disc at L4–5 simultaneously.
A standard anterior discectomy was performed on L4–5. Using biplanar fluoroscopy, a Kirschner wire was placed beginning at the anterior third of the L5 superior end-plate and ending at S1. An anterior cruciate ligament reamer was used to make a channel for the fibular allograft. Then, a femoral ring allograft was placed in the disc space at L4–5, and standard anterior lumbar interbody fusions were performed at any additional cephalad level(s). Afterward, posterior instrumented fusion was performed to complement the anterior fusion procedure (except at L5), and wide decompression followed.
All patients presented with isthmic spondylolisthesis and all had multilevel fusions. The mean slip angle was 32.6 degrees (37.8 degrees preoperatively), and mean lumbar index was 67%. After the procedure, the average endplate-to-dowel angle was 107.1 degrees compared with 134 degrees. All patients had clinical and radiographic evidence of solid fusion without the need for revisions.
The proposed advantage of our modified technique is twofold. The graft is placed nearly perpendicular to the L5-S1 interface, as it will behave more efficiently with respect to interfragmental compression. Also, surgeons gain access to fuse L4–5 anteriorly and posteriorly.
The treatment of spondylolisthesis can be highly individualized. Severe spondylolisthesis has been treated with the use of fibular strut grafts for the fusion of L5 and S1.1–4Typically, this technique has been used in patients with Meyerding grade III or IV. This procedure can be coupled with in situ arthrodesis5–9 and/or anterior and posterior fusions.10–15 Its utility is to allow surgeons to bypass accessing the L5-S1 disc space to perform an anterior lumbar interbody fusion (ALIF) in the setting of high-grade spondylolisthesis. Fibular strut grafting has been successful in creating stable fusions and in minimizing time in the operation room.15 Furthermore, the need for extensive dissection is reduced, thereby reducing the risk of damage to the blood vessels and sympathetic plexus in the vicinity.15
Posterior fusion procedures alone can fail to inhibit the progression of the deformity due to an unfavorable biomechanical environment.15–17 Additionally, posterior strut grafting requires some retraction of the cauda equina to apply the reamer and strut graft, with some risk. Previous studies showed that an anterior approach is useful in increasing fusion rates, restoring sagittal balance, and indirectly decompressing nerve roots.18 Long-term issues may include cephalad adjacent segment disease, as evident on magnetic resonance imaging in as high as 73% of cases.19 This may be a predictor of poor outcome and continued pain.
Circumferential or 360-degree fusion procedures have been reported to produce high rates of clinical and radiographic fusion.19–21 Circumferential fusions can be performed via a posterior approach as in a posterior lumbar interbody fusion and transforaminal lumbar interbody fusion or as combined posterior and anterior approach, as used in ALIF.22 The downside includes increased surgical risk (including risks associated with anterior and posterior approaches), longer operating times, increased blood loss, higher initial costs, and longer hospital stays. However, reconstruction of the anterior column can reconstitute disc height, reduce the slip angle, reduce the anterolisthesis, and provide an optimal graft environment for fusion, whereas the combined posterior approach allows direct decompression, stabilization with pedicle screw fixation, and posterior lumbar fusion.18
Currently, patients presenting with high-grade spondylolisthesis are difficult to treat. This report illustrates our protocol and rationale for the modified fibular strut grafting technique combined with circumferential fusion. Our goal is to provide spine surgeons with an additional technique for a difficult clinical scenario.
Materials And Methods
Eight consecutive adult patients (5 men and 3 women) with high-grade isthmic spondylolithesis at L5-S1 were involved in the study. The operations were performed by the senior author between 2004 and 2007. Indications for surgery included patients with continued pain despite conservative treatments, progression of neurological symptoms, and progression of the slip. The average age of our cohort was 46.63 years, with an average of 27.88 months of follow-up. No patients were lost to follow-up. Plain radiographs were obtained and magnetic resonance imaging was performed. Radiographic parameters preoperatively and postoperatively and at follow-up included the Meyerding grade, the slip angle lumbar index, and the endplate dowel angle. All patients were treated with a modified technique in fibular dowel grafting. Figure 1 demonstrates L4–5 disc degeneration for the patient outlined in Figures 2 through 5.
Anterior Surgical Technique
Patients were placed supine on a Jackson spinal table (Mizuho OSI, Union City, California). A standard left-sided paramedian retroperitoneal approach was used with exposure of the L4–5 disc as well as additional superior vertebrae as each case dictated. L4–5 was identified, and a standard anterior discectomy was performed. The disc material and cartilaginous endplates were removed to the posterior longitudinal ligament. The disc space was sized, and this process was repeated for any additional diseased levels.
Using biplanar fluoroscopy, a Kirschner wire (K-wire) was placed beginning at the anterior third of the L5 endplate. The K-wire was advanced into the L5 body, across the L5-S1 disc, and into the body of S1 (Figure 2). An effort was made to have the K-wire traverse the S1 endplate in the most perpendicular position that the L4–5 access space would allow. A depth measurement was taken to determine the length of the fibular graft. A 12-mm anterior cruciate ligament reamer was used to overdrill the K-wire and make a channel for the fibular allograft (Figure 3). The fibular graft was prepared to the appropriate length using a burr. The cortex was contoured so that it was able to freely pass through a 13-mm guide. We then packed allograft bone into the fibular canal. Once the fibular dowel was adequately prepared, the K-wire was removed and the fibula was tapped into the channel using a mallet and impactor. Any remaining fibular graft that was prominent in the L4–5 disc space was burred down so that it was flush with the end-plate of L5. Subsequently, the L4–5 disc space was treated with a standard ALIF procedure with a fem oral ring allograft. (Figure 4). Any other cephalad levels in need of fusion were subsequently addressed with ALIFs as well.
Posterior Surgical Technique
Once the anterior portion of the case was completed, the patient was turned prone so that posterior decompression and fusion could be performed. Pos terior exposure was obtained to fit the number of levels that required fusion, always ending with S1. Pedicle and/or sacral alar screws were then placed at each level except L5 and then wide de-compression followed (Figure 5).
table 1 details the pertinent demographics and surgical information. Our cohort’s mean age was 46.63 years old with a range of 35 to 62 years. Five patients underwent 2-level fusions, 2 had 3-level fusions, and 1 patient had a 4-level fusion. The average operating room time was 408 minutes (range 320–530 minutes). Mean total estimated blood loss was 887 mL. All patients presenting with isthmic spondylolisthesis (Meyerding grade III) and multilevel degenerative disc disease were treated with the same procedure. The Meyerding grades were unchanged postoperatively. However, this technique allowed surgeons to reduce the mean slip angle by an average of 5.2 degrees. At final follow-up, the mean slip angle was 32.6 degrees (37.8 degrees preoperatively). The index remained consistent at 67%. The average endplate-to-dowel angle was 107.1 degrees. table 2 includes relevant radiographic measures and complications.
Studies of posterolateral fusion in treating isthmic spondylolis-thesis show a range of failed fusions.10,11,13 In situ fusions have been linked with pseudarthrosis and have been shown to be less successful with spondylolisthesis slip angles exceeding 50%.7,13 The concern about pseudarthrosis is especially valid with posterior decompression alone because of further destabilization to the posterior column. From a biomechanical perspective, posterolateral grafting is at risk of failure by tension and shearing forces; therefore, it is less likely to produce a solid fusion without instrumentation.23 The anterior column, in contrast, is subjected to a compressive force, and the graft material at this site is more likely to incorporate into fusion.2 Anterior instrumented fusions can be problematic because of the high risk of damage to surrounding neurovasculature as a result of the extensive dissection required. It is because of limitations of techniques, mixed operative results, and stability problems that various newer procedures have been developed.
In a recent study by Sasso et al,15 a group of patients (mean age 29.8 years) presenting with high-grade isthmic spondylolisthesis at L5-S1 were treated effectively by decompression and transver-tebral, transsacral strut grafting with fibular allograft. The technique of fibular dowel grafting is not new, and there are reports of graft breakage in some cases.2 This tends to occur when there is no instrumentation and the shearing forces exceed the strength of the strut graft. We also hypothesize that suboptimal placement of the graft may be a variable in graft breakage. The endplate-to-graft angle may be an important factor in leading up to graft failure because the graft is offered protection from shearing forces when it is placed more perpendicular to the S1 endplate. As the angle of the dowel becomes more obtuse, it is subjected to increased shearing forces and undue stress, which may lead to failure. Our endplate-dowel angle outcomes are consistently close to 90 degrees, which allows more efficient interfragmentary compression and superior protection from shearing forces.24 The key to establishing a more perpendicular angle lies in the ability to access the L4–5 disc space as the starting point. Historically, the starting point was at the anterior aspect of the L5 vertebral body.23 Also, with a posterior and anterior approach, surgeons have the benefit of correcting the sagittal alignment and obtaining large interbody grafts with substantial surface area for fusion. The point of failure of fusion tends to be at the L5-S1 junction, which further emphasizes the importance of using a perpendicular strut.2
An important consideration when performing this procedure is blood loss. The vertebral bodies can lose a fair amount of blood after reaming. The fibular graft may act as a tamponade for some bleeding, but the blood may still exit via the canal of the fibula. Packing the dowel with allograft before its insertion into the patient helps to minimize blood loss. Also, when the ALIF is performed at L4–5, the bleeding tends to resolve.
When the standard fibular dowel grafting and posterior instrumented fusion is performed, the posterior instrumentation uses the pedicles at L4 and S1. L5 is bypassed because of the difficulty of instrumenting at that level. The previously described fibular grafting with entry into the L5 vertebral body leaves the L4–5 anterior disc intact. L4–5 is often degenerated secondary to altered biomechanics in the listhesed spine and can lead to continued axial back pain even after a successful fusion at L5-S1. In the traditionally described procedure, the L4–5 disc is included in the posterior fusion via instrumentation from L4 to S1. In our modified procedure, we are able to increase the chance of fusion and a good clinical outcome with an ALIF at L4–5 in addition to the posterior instrumentation. Figure 6 shows lateral and anteropos-terior radiographs of a patient’s lumbar spine fixed with a modified fibular strut graft, demonstrating a stable 2-level fusion at 27 months postoperatively.
This modified technique of fibular strut grafting at the L5-S1 level in the presence of isthmic spondylolisthesis proved to be safe and effective for adult patients. Accessing the L4–5 disc space to place the fibular dowel graft will allow surgeons to create a more reliable construct for the treatment of L5-S1 spondylolisthesis in the setting of multilevel degenerative disc disease. Future studies need to be performed to determine the superiority of one technique over another in a given situation.
Eskander et al describe a modification of a currently utilized technique for treatment of high-grade spondylolisthesis at L5-S1 with multilevel disc degeneration. They review their experience with eight adult patients treated with this approach over a 3-year period. The main thrust of the article was to describe the surgical technique, which the authors did in a clear and detailed fashion. Other variables reported included the change in mean slip angle and the average endplate-to-dowel angle.
Commonly in the setting of high-grade spondylolisthesis at L5-S1, the fusion is extended to include the L4–5 spinal unit because of the decreased surface area available for graft at L5-S1 and the high shear forces applied to that level secondary to the deformity that hinder the fusion capability (Heary RF and Bono CM. Circumferential fusion for spondylolisthesis in the lumbar spine. Neurosurg Focus. 2002;13(1):E3). The use of this modified technique allows for the addition of an interbody graft at L4–5 thereby increasing the likelihood of fusion. As well, the near perpendicular endplate-to-dowel angle theoretically protects the graft more adequately from shear force than the standard dowel technique. For the above stated reasons, this technique may decrease the rate of pseudarthrosis, thereby benefitting the patient in the long term.
However, with this approach, a patient is subjected to both an anterior and posterior approach, increasing the immediate morbidity. Posterior-only approaches, including trans-sacral fibular dowel graft (Sasso RC, Shively KD, and Reilly TM. Transvertebral Transsacral strut grafting for high grade isthmic spondylolisthesis with fibular allograft. J Spinal Disord Tech; 2008;21(5):328–333.) may be combined with an L4–5 TLIF/PLIF for circumferential arthrodesis from L4-S1 without exposing the patient to the risk of an anterior approach as well. Still, the argument of optimizing the endplate to graft angle potentially decreasing the risk of pseudarthrosis specifically at the L5-S1 level is appealing. Futures comparative study of circumferential fusion from a posterior approach and this modified technique will help us better define the risk/benefit ratio.
Overall, however, I do believe it is beneficial for spine surgeons to have varied options for fusion techniques as each individual patient’s anatomy may not be receptive to a standard technique. This article by Eskander et al clearly adds to our armamentarium of surgical techniques.
Erica F. Bisson
Salt Lake City, Utah
anterior lumbar interbody fusion
- diagnostic radiologic examination
- anterior cruciate ligament
- bone plates
- complement system proteins
- spinal fusion
- surgical procedures, operative
- tissue transplants
- isthmic spondylolisthesis
- degenerative disc disease
- femoral ring
- occipital nerve stimulation
- kirschner wires