Posterior Approaches to the Lumbar Spine: Minimally Invasive Fusion

Midline Lumbar Interbody Fusion

The traditional approach for posterior lumbar interbody fusion (PLIF) requires a large dorsal midline exposure with manipulation of overlying ligamentous and muscular processes. When compared with other approaches to the lumbar spine, PLIF results in the highest complication rate, possibly owing to the broad exposure that is traditionally used. This complication rate makes the use of successful minimally invasive approaches particularly attractive. An increasing number of studies have examined minimally invasive surgery (MIS) PLIF, which has been shown to decrease blood loss, hospitalization time, and pain when compared with open PLIF.

The minimally invasive MidLIF technique is a variation on the open PLIF technique and was first described by Mizuno in 2014. This technique is performed using a midline lumbar incision, which entails less tissue disruption than PLIF. The new skill used is a variation on pedicle screw (PS) placement called cortical screw trajectory, described later, which allows a small midline incision to be used to perform the decompression and all of the instrumentation because the medial to lateral trajectory and anatomy of screw placement does not require as large of an exposure.

The procedure is performed with the patient in the standard prone position on a three-point bolster bed (e.g., Jackson table). The procedure should be performed with the spine in lordosis with the patient’s thighs extended.

The incision should be carefully localized over the target disc space using x-ray. This will allow targeted exposure of (1) the interlaminar space and index facet complex (for example, L4‒L5 interlaminar space and facet complex) and (2) the intersection of the rostral pars interarticularis and adjacent facet complex. This incision does not have to be much larger than a microdiscectomy incision. Exposure only has to be lateral enough to expose the pars. Use of the operating microscope significantly improves visualization, illumination, and ergonomic position of the neck, but the surgery can be done with loupes and a headlight.

Cortical screw placement is then performed using image guidance (fluoroscopy or computerized stereotactic navigation). The starting point of a cortical screw is the midpoint of the pars as it intersects with the rostral facet joint. A drill or combined drill tap is used to decorticate and then create a pilot hole in the pedicle that aims obliquely across the pedicle to engage the cortical bone. The distal target is the rostral junction of the pedicle and end plate in the lateral view and lateral pedicle wall in the anteroposterior (AP) view. The pilot hole will typically be shorter than a PS trajectory. A pedicle sound is used to verify that there is intact bone throughout the pilot hole. The pilot hole is then tapped. Typically, because of the quality of the cortical bone, the tap will be the same diameter as the planned screw and will be tapped the entire length. If a pars fracture occurs, then a separate transmuscular approach can be used to place a traditional PS. On a single-level instrumentation case the caudal screw can be placed either in a cortical screw trajectory as described above or in a PS trajectory, with the starting point targeting the caudal one-third of the facet joint. Using a PS trajectory allows the caudal screws to be placed through a smaller incision because the screw starting point is more rostral. The surgeon can use conventional PSs. The screws should not be placed right away because the screw heads will often be more medial and obstruct the decompression and interbody fusion.

Lumbar laminectomy, decompression, and interbody fusion are performed as is typical for a PLIF. Cage type (single or double) and graft selection can be as the surgeon desires.

The cortical purchase, because of the screw trajectory, increases pullout strength. Biomechanical studies evaluating cortical screw trajectory compared with traditional PS trajectory have demonstrated a 30% increase in uniaxial pullout load. In addition to increased pullout strength, cortical screws may have significant benefit in cases of severe osteoporosis, as cancellous bone purchase is less necessary.

Minimally Invasive Posterior Lumbar Interbody Fusion

One of the first and most illustrative reports of the use of MIS to limit collateral soft tissue damage during PLIF was presented in 2002 by Khoo and colleagues. This procedure requires learning to place percutaneous PSs and the use of a tubular retractor. Briefly, small incisions were made between 2 and 4 cm from the midline bilaterally at the level of interest. Under fluoroscopic guidance, sequentially larger dilators were then placed over a K-wire to ultimately place a tubular retractor that provided a working corridor 15 to 20 mm in diameter. ^, Decompression, hemilaminotomy, and discectomy were then performed endoscopically. During interbody graft placement, the authors used endoscopic visualization to ensure neural element retraction while working within the tubular system. Currently, it is more common to use microscopic visualization to perform these procedures.

Percutaneous PS-rod systems were used in concert to complete the instrumented arthrodesis. Generally, cannulation of the pedicles can be performed by using the same incisions that are made for the purpose of decompression after removal of the tubular retractor and endoscope. Fluoroscopy was used while a Jamshidi needle was advanced to the pedicle and subsequently swapped for a K-wire using the Seldinger technique. In lieu of traditional fluoroscopy, a fluoroscopic navigation system was used to track the needle in the sagittal, coronal, and axial planes. A tap was passed over the K-wire, and the multiaxial PSs (Sextant, Medtronic Sofamor Danek, Memphis, TN) were attached to screw extender sleeves and passed over the K-wire into the screw pathway. The remainder of the screw-rod assembly involved mating of the ipsilateral pedicle pairs using Sextant precontoured rods.