Minimally Invasive Lumbar Interbody Fusion: The Direct Lateral Transpsoas Approach

Medical technology continues to evolve. Reduction in patient morbidity and improvement in overall function are primary goals in patient care. Caring for patients with complaints of chronic low back pain is particularly difficult. Less invasive techniques are categorized as minimally invasive techniques and are replacing or at least modifying traditional approaches to surgical spine care. Minimally invasive procedures including laparoscopic, endoscopic, thoracoscopic, or mini-open procedures have dominated technique development over the past two decades (1, 2). Despite the procedure of choice (discectomy, arthroplasty, spinal fusion) patient selection remains paramount when anticipating surgical outcome (3,4,5,6).

Biomechanically, lumbar motion segments have undergone extensive study (7,8,9,10,11,12). The anterior aspect of the lumbar motion segment clearly withstands the majority of axial load. Nachemson's (12) classic article suggests that the anterior disc and vertebral body experience approximately 80% of the axial load versus 20% for the facet joints.

All approaches to the lumbar intervertebral disc space are associated with advantages and disadvantages. Posterior approaches to the intervertebral disc space (PLIF) have been associated with increased risk of neural injuries from nerve root manipulation and the potential for significant segmental instability with excessive bone resection (7,13,14). PLIF when combined with a posterolateral fusion does provide circumferential stabilization of the motion segment and the ability to directly decompress neural structures.

Anterior lumbar interbody fusion (ALIF) also produces direct stabilization of the involved disc space. Excessive anterior soft tissue resection without proper tensioning of the remaining structures does produce the potential for segmental instability. Partial or extensive resection of the anterior longitudinal ligament can destabilize the motion segment in extension (9,10). Anterior column support via the anterior or lateral approach reduces cantilever stress on posterior instrumentation. However, the direct anterior approach is associated with an increased risk of vascular injury, particularly at the L4-5 level. Retrograde ejaculation following disruption of parasympathetic tissues and visceral injuries during exposure of lower lumbar interspaces has also been reported (15,16). Lateral exposure of the anterior column may provide advantages over the standard anterior approach. Risk to vascular structures, retrograde ejaculation and destabilization of the anterior soft tissues is reduced (10). However, approaching L4-5 by means of the direct lateral approach is difficult and the iliac crest blocks the L5-S1 disc space.

A classic lateral retroperitoneal "Saber" approach may be performed through a long oblique flank incision applied below the rib cage from the posterior axillary line to the lateral aspect of the rectus abdominis muscle anteriorly. Upon close inspection this incision has been compared to the wound inflected by slashing with a saber. Lateral "saber" style exposures for vertebral body corpectomy have given way to tube-assisted percutaneous techniques. The direct lateral lumbar interbody fusion (DLIF) technique described in this report is an extension of the experience gained from application of the more invasive "saber" incision. The focus of this technique is reduction of patient morbidity via limited soft tissue disruption to muscle and anterior longitudinal ligament, as well as, protection of the lumbar plexus within the psoas muscle via blunt dilation versus mobilization from the lateral aspect of the vertebral body and disc space (Figure 1).

Figure 1: Intraoperative photograph of deep exposure to lumbar spine via "saber" incision. The genitofemoral nerve on the surface of the psoas muscle is illustrated, as well as the peritoneum.

Experience gained from the more traditional "saber" approach is critical to understanding the fine points of minimally invasive surgery. Topographic contours of the lateral aspect of the lumbar vertebra provide tactile input regarding the location of neurovascular structures. Segmental vessels at each vertebral level are located at the waist or valley of the lateral contour (Figure 2), while the disc space or "safe" zone is represented by the peak or prominence of the lateral contour. Following the valley or waist of each vertebra posteriorly directs the surgeon into the neural foramen (Figure 3).

Figure 2: Intraoperative photograph illustrating the isolation of a segment vessel during exposure of the lateral vertebral body.

Figure 3: Intraoperative photograph Illustrating the lateral wall of vertebral body, vertebral foramen and retracted psoas muscle.

Major vascular structures, aorta and vena cava, lie anterior to the valley or peak of each motion segment. While injury to either of these vascular structures could be catastrophic, approaching from the left side places the more delicate vein at slightly less risk. In the course of this limited exposure, protection of the intra-abdominal structures is aided by patient positioning, initial use of blunt probes, and an understanding of the three dimensional anatomy.

DLIF has limitations based on the anatomic restriction imposed by the rib cage superiorly and iliac crest inferiorly (Figure 4).

Figure 4: Preoperative radiograph illustrating presence of iliac crest above level of L4-L5 disc space, making access to this interspace more difficult.

Present indications for DLIF include degenerative pathology involving the anterior or middle column of mid-lumbar motion segments excluding L5-S1.