Discussion: Direct Lateral Transpsoas Approach
Interbody fusion provides a viable means of addressing degenerative changes to the anterior column of the lumbar spine. Spinal fusion surgery is traditionally thought of as painful. Minimally invasive techniques have been developed to reduce this morbidity. Muscle damage from the invasive "saber" type approaches anteriorly or midline posterior approaches has been associated with loss of strength and decreased mobility. In an attempt to reduce patient morbidity and preserve the concept of anterior column support, the traditional lateral retroperitoneal approach to the lumbar spine has been modified. Modifications include use of a tube assisted transpsoas approach for lateral lumbar interbody fusion as described above. Care provided to patients with mechanical low back pain has the potential to positively impact healthcare dollars spent in the United States (17). Reducing patient morbidity should have a constructive impact on functional improvement and reduction of healthcare expenditures.
In the presence of intact vertebral bodies direct lateral transpsoas lumbar interbody fusion has become the author's procedure of choice for anterior column support. Work by Heth et.al. has demonstrated potential advantages to lateral exposure versus direct anterior dissection (10). Prior experience with more invasive exposures has aided in the application of this technique. Concern regarding peritoneal penetration, neural and /or vascular injury has not materialized.
Injury to the lumbar plexus, particularly the genitofemoral nerve, was not encountered. Splitting of the psoas muscle as opposed to dissecting through the muscle is thought to be beneficial (18). While initial skin incision may measure one and a half inches in length (allowing direct visualization of the transversalis fascia in most cases), insertion of tube dilators obscure any deep visualization outside the confines of the tube. By manipulating the tube dilators, instrumentation of up to three levels through the same incision has been accomplished. The transition from "saber" to direct lateral approaches can be made safely with proper attention to detail. Patients with history of prior retroperitoneal exposure are not considered for this technique. Use of endoscopic balloon dilators have been used in the past (18) and were not found to be advantageous in this technique. Digital palpation and/or manipulation of the retroperitoneal space with the blunt probe are an important step in reducing risk to the peritoneal contents. Attention to initial placement of the guide wire on the lateral aspect of the vertebral body is important both for protection of vascular structures and potential posterior penetration of the spinal canal. Three-dimensional spatial considerations must be remembered. Loss of orientation while preparing the fusion bed within the disc space can lead to inadvertent penetration of the spinal canal or anterior vascular structures. Recent advances in image-guided technology have been touted to reduce episodes of disorientation.
Indirect decompression of the spinal canal or foramen via a laterally placed interbody device (bone, metal or PEEK) has eliminated radicular and claudicatory symptoms secondary to foraminal stenosis, central stenosis and spondylolisthesis in the authors experience. When reporting on their early experience with BAK application, Kuslich4 suggested that the one-year fusion rate for two-level procedures approached 78% versus greater than 90% for single level procedures. Prior use of allograft and autogenous bone has been replaced by rh-bmp 2 within the interbody device. Preliminary radiographs illustrate early incorporation of interbody devices with adjunctive rh-bmp. This experience has mimicked the results of other investigators utilizing rh-bmp as an adjunct to anterior lumbar spinal fusion (16,17). However, due to the preliminary follow-up, the data reported here is not intended to comment upon the ultimate fusion rate. These results are reported to demonstrate a useful adjunct of anterior column stabilization via a less destructive, less morbid approach.
The results of this experience are encouraging. Material presented illustrates an evolution in surgical technique from invasive "saber" type approaches to minimally invasive tube assisted direct lateral procedures. Causes of painful segmental degeneration of the lumbar spine are many. Therefore, the patient mix treated in this series speaks to the versatility of this procedure. However, technique modifications have produced a clear reduction in patient morbidity as demonstrated by estimated blood loss, operative time and hospital stay. Adverse events in the early postoperative period have not been an issue. Long-term follow-up will establish fusion rates. This initial experience with direct lateral transpsoas lumbar interbody fusion warrants additional use and investigation. Direct lateral transpsoas lumbar interbody fusion may prove to be an important improvement to our methods of handling degenerative conditions affecting the lumbar disc.