Update on Minimally Invasive Spinal Fusion Surgery

Introduction to minimally invasive fusion

A spine fusion is aimed at alleviating pain that is believed to originate from a motion segment in the spine. For some patients, it may be recommended to have a spine fusion surgery performed in both the front and back of the spine in an effort to provide extra stability for the painful motion segment. There is some new evidence about these types of procedures, and the use of a minimally invasive spinal fusion system may sometimes be considered.

At the time of this article, minimally invasive spine fusion technology is still in the early stages of development. This type of procedure uses a needle, which is inserted through small incisions in the back, and radiographic magnified images of the spine are projected onto television screens in the operating room. Narrow surgical instruments are then inserted into the small (1 to 1 ½ cm) incisions to complete the operation. See Figure 2, and Figure 3.

The goals of minimally invasive spine fusion surgery systems include:

• Reduced postoperative pain

• Diminished blood loss

• Faster recovery and reduced hospital stay

• Smaller scars

At the time of this article, minimally invasive spinal fusion technology is still new, and none of the above benefits have been definitively proven. Some spine surgeons have anecdotal evidence from their own practice indicating that patients have better outcomes with using a minimally invasive spine surgery system, and other surgeons have anecdotal evidence from their practice that patients have better outcomes with techniques associated with traditional (open) approaches to spine fusion surgery.

Use of a minimally invasive spine fusion surgery system is best suited for patients with degenerative disc disease or spondylolisthesis

. The technique does not allow the spine surgeon to do a central decompression. Foraminal decompression can be achieved by interbody distraction and grafting. If there is any central stenosis an open incision still needs to be made. Due to the limited visualization at the time of surgery, it is a difficult and possibly dangerous technique to use in patients with a severe deformity.

Spine fusion surgery background information

Initial spinal fusion surgery attempts were performed with the placement of posterior lateral (on the side) and posterior (in the back of the spine) bone grafts. With the advent of internal fixation, these spine fusions were performed with the addition of posterior instrumentation (pedicle screws), with fusion rates approaching 80 to 90%.

Surgeons sometimes perform an anterior lumbar interbody fusion (ALIF) in addition to the posterior lateral bone grafting and posterior instrumentation. An ALIF is considered effective due to the large surface area in the front of the spine, as well as studies reporting two thirds of the loads are transmitted through the anterior column (front of the spine). When both an ALIF and a posterior lateral bone grafting and posterior instrumentation are performed it is commonly referred to as a 360-degree spine fusion or a circumferential spine fusion.

Although associated with high spine fusion rates and a high level of patient satisfaction, follow-up radiographs of 360-degree fusions demonstrated inconsistent fusion of the posterior lateral bone mass, suggesting that the anterior lumbar interbody fusion (front of the spine) is the more structural component.

It was theorized that the placement of the anterior graft altered the biomechanics of the spine such that the posterior lateral bone graft did not consolidate. This raises the question: Is there a need for posterior lateral bone graft in the setting of an ALIF with posterior instrumentation?

This question was answered in a prospective randomized study by Schofferman et al. that compared 360-degree fusions versus an ALIF plus posterior instrumentation (270-degree fusions). Their results showed that both procedures were successful in alleviating pain with no clinical differences among the two; however the 270-degree fusions had less blood loss and shorter hospital stays.

Minimally invasive spine fusion surgery approach

Performing a 270-degree spine fusion by traditional surgical techniques requires an open procedure exposing the lamina, pars, facets, and transverse processes of the levels to be instrumented. This approach permits excellent visualization of the spine, and access to the portions of the spine to place the implants. Depending on the spine surgeon's technique, however, the muscle retraction and disruption of the posterior muscles and ligaments may lead to some degree of scar formation and damage. In turn, this is potentially associated with unwanted postoperative effects (sometimes referred to as "fusion disease").

Depending on the spine surgeon's technique and other factors, a traditional open spine fusion procedure may include a risk of:

• Disruption of the supraspinous and interspinous ligaments, which has been implicated in decreased flexion strength and delayed spinal instability.

• Sihoven et al. demonstrated local denervation atrophy of paraspinal muscles postoperatively from dissection and lateral muscle retraction. This was associated with a loss of support and stability potentially leading to continued pain after surgery (failed back surgery syndrome).

• Styf et al. also showed that retracting muscle during the surgical procedure can lead to regional ischemia (deprivation of oxygen), and EMG studies have demonstrated chronic denervation of paraspinal muscles following some types of open surgery.

Since previous studies demonstrated that posterior lateral bone grafting was not necessary for a successful spine fusion surgery, efforts have been made to perform a 270-degree fusion with anterior lumbar interbody fusion and minimally invasive, percutaneous posterior instrumentation. The hope is that this will allow for a 270-degree spine fusion without the complications potentially associated with open procedures.