Midline Lumbar Interbody Fusion With Cortical Bone Trajectory Pedicle Screw Placement

Over the last few decades, a variety of surgical techniques for spinal arthrodesis have been developed for the treatment of spinal pathology requiring mechanical stabilization. Interbody grafts are combined with segmental stabilization to achieve fusion, maintain spinal stability, and preserve neurological function. Transforaminal lumbar interbody fusion (TLIF) and posterior lumbar interbody fusion (PLIF) supplemented with pedicle screw fixation are among the most popular and standard surgical techniques. Alternative approaches such as anterior lumbar interbody fusion, direct lateral interbody fusion, and oblique lumbar interbody fusion techniques attempt to overcome the perceived disadvantages of the traditional posterior techniques. However, posterior approaches have continued to evolve. In addition to percutaneous techniques for PLIF and pedicle screw fixation, a novel, less invasive midline lumbar interbody fusion (MIDLF) technique has also been developed that allows direct decompression and lumbar interbody fusion with simultaneous stabilization using a cortical bone trajectory (CBT) pedicle screw placement.

The CBT uses a more medial insertion of a cortical transpedicle screw that requires bone exposure only up to the lateral edge of the pars for a medial-to-lateral trajectory. This requires less soft tissue dissection and retraction as compared with traditional open pedicle screw placement, which needs a wider exposure to allow for proper medialization of screw placement through the cancellous portion of the pedicle. A self-retaining retractor system with diverging blades is used to provide the necessary surgical corridor. Compared with traditional posterior approaches using pedicle screw fixation, use of the CBT allows for a smaller operative corridor that reduces muscle dissection and retraction, with potential decreased blood loss, less postoperative pain leading to quicker mobilization and decreased hospital stay, and shortened operative time once the initial learning curve has been accomplished. When compared with percutaneous approaches and tubular retractor systems, this technique is unique in facilitating an MIS midline alternative with an enhanced learning curve by using the familiar midline posterior approach and anatomical surface landmarks for placing spinal instrumentation. This approach also allows direct, bilateral decompression of central neural elements and placement of unilateral or bilateral interbody grafts.

The entry point of the CBT is a unique and consistent surface landmark, because it is preserved even in the setting of advanced degenerative disease. The insertional pathway of the cortical screw runs medial to lateral and caudocephalad through the higher density bone formed by the union of the inferolateral border of the pedicle and the lateral edge of the superior articulating facet. The screw is aimed into the posterior aspect of the superior apophyseal ring of the vertebral body.

In 2009, Santoni et al. first reported this technique as a strategy to enhance spinal fixation over standard pedicle screws, especially in osteoporotic patients. Clinical and cadaveric studies have confirmed the biomechanical stability of CBT placement as compared with traditional screw placement. Cortical screws are generally smaller (usually 5.0–5.5 mm in diameter and 35–40 mm in length). The trajectory allows the majority of the screw to engage the dense cortical bone, as compared with 20% cortical bone purchase of a traditional pedicle screw ( Fig. 122.1 ). This potentiates greater pullout strength when compared with traditional pedicle screws, independent of trabecular bone mineral density, which may be advantageous in osteoporotic patients requiring stabilization. ^{, ,} This technique may have a lower risk of injury to neural elements because of the medial-lateral trajectory in the axial plane away from the spinal canal and foramen.

Open full size image

The cortical bone trajectory approach achieves a much larger area of cortical bone purchase ( A ) than traditional pedicle screw placement ( B ).

From Medtronic Sofamor-Danek, Memphis, TN.

A systematic review published in 2017 analyzed 42 anatomical, biomechanical, and clinical studies that compared CBT placement with traditional screw placement. This review reported that CBT provides greater stiffness with cephalocaudal and mediolateral loading and resistance to flexion/extension, as compared with a standard pedicle trajectory based on biomechanical studies; however, it is inferior in lateral bending and axial rotation. This study also reported better perioperative results by minimizing intraoperative blood loss, improving clinical postoperative pain scores, and decreasing duration of surgery and hospital stay among patients with similar rates of comorbidities.

The MIDLF approach can be modified to allow for unilateral or bilateral decompression and interbody placement with central and lateral decompression or lateral decompression only. This approach is also advantageous for treating symptomatic adjacent segment disease after prior lumbar fusion by allowing cortical screw placement in a previously instrumented pedicle, obviating the need for removal of existing hardware. The medial orientation of the cortical screws can also allow connection to existing hardware with side-to-side rod connectors.