Posterior and Transforaminal Lumbar Interbody Fusion

Posterior approach interbody fusion techniques such as posterior lumbar interbody fusion and transforaminal interbody fusion are known as the workhorse procedures for lumbar spinal fusion. Over the years, advancements in procedural steps, technique, and technology have sought to improve patient outcomes. Within the last 2 decades, considerable emphasis has been placed upon minimally invasive techniques utilizing tubular retractors and conscious sedation. Innovation in materials engineering, visualization technology such as endoscopes, and enabling technologies such as augmented reality and robotics have served to enhance the procedures and their outcomes.

Key points

•

Over the last 2 decades, focus on minimally invasive techniques has led to increased popularity in tubular minimally invasive surgery-transforaminal interbody fusion (TLIF) and endoscopic TLIF approaches.
•

Minimally invasive techniques for posterior lumbar interbody fusion and TLIF have given rise to the “awake TLIF” performed under local anesthesia and conscious sedation.
•

Improvement in 3 dimensional printing and surface coating technology has facilitated innovation and optimization of interbody implant tribology.
•

Enabling technologies, including robotics and augmented reality, seek to improve patient outcomes, reduce complications, and improve workflow.

Introduction

Since the earliest description of spinal fusion, surgeons have continuously worked to improve the efficacy and safety of the techniques and technology available for lumbar arthrodesis. Early methods of fusion, which primarily focused on arthrodesis of the posterior arch, were fraught with failure. In the 1930s, Burns and Capener were credited with describing the first operations focused on vertebral body fusion via an anterior corridor. By the 1940s, spine surgeons had recognized that interbody fusion could be successfully achieved via a posterior approach; however, the exact origins of the posterior lumbar interbody fusion (PLIF) technique remain unclear. Within a few short years, multiple surgeons had described and attempted early versions of the PLIF technique including Cloward, Briggs, Milligan, Ovens, and Williams. The PLIF approach underwent several modifications over the following 40 years, most notably was the introduction of pedicle screws in the 1980s by Art Steffee. In 1982, Harms and Rolinger delineated a novel technique for fusing the intervertebral disc space via a transforaminal trajectory, utilizing a corridor now known as Kambin’s triangle. The technique further evolved in 1998 to be performed through an expanded transforaminal corridor facilitated by the removal of the ipsilateral facet joint, giving rise to the modern transforaminal lumbar interbody fusion (TLIF) approach. At the turn of the century, innovation in minimally invasive spinal surgery techniques led to an emphasis on smaller incisions, minimized muscle dissection, and improved patient recovery. ^, While the anatomic approaches for PLIF and TLIF surgery have remained consistent over the last few decades, significant innovation has focused on implant construction, retractor systems, visualization and navigation technology, and surgical robotics. In this review, we aim to highlight the latest advances in technique and technology relating to the PLIF and TLIF procedures.

Technique

Open Posterior and Transforaminal Lumbar Interbody Fusion

Introduction of the TLIF technique served as an improvement upon the existing PLIF approach, as it required less nerve root and dural retraction with superior disc space visualization. Since its modification to incorporate a total facetectomy, the surgical approach for a traditional “open” TLIF has remained largely unchanged. Multiple recent meta-analyses and systematic reviews have found that the TLIF procedure has numerous advantages over the PLIF approach, including reduced complication rate, blood loss, and operative duration. ^, Given the ease of access and excellent fusion rate, the open TLIF procedure has become the “workhorse” and “gold standard” for lumbar arthrodesis. Despite its numerous advantages, the TLIF requires relatively large incisions and extensive muscular dissection, which has been associated with prolonged postoperative recovery, particularly in elderly patients.

In 2014, the less invasive, midline lumbar interbody fusion (MIDLIF) technique was introduced as an alternative to both the PLIF and TLIF technique. The approach, which utilizes cortical bone trajectory screws, has since gained popularity as it requires reduced muscular dissection and can be performed through a smaller incision compared to the traditional TLIF technique. Cortical trajectory screws have been demonstrated to have an equivalent pull-out strength compared to traditional pedicle screws, with the added benefit of a more medial start point ( Fig. 1 ). ^, The MIDLIF procedure has been shown to have reduced operative time, hospital length of stay, blood loss, radiographic muscle wasting, and adjacent segment degeneration, while maintaining similar rates of fusion and patient-reported outcome measures (PROMs) when compared to the TLIF and PLIF procedures. This minimal-open procedure represents the latest advancement in technique for “open” posterior approach interbody fusion.

Minimally Invasive Spine Surgery

At the turn of the twenty-first century, the principles of minimally invasive surgery (MIS) were incorporated into multiple spinal procedures, focusing on tissue-sparing dissection, smaller incisions, and expedited patient recovery. In the early 2000s, Foley and Lefkowitz published their modification of the TLIF procedure to incorporate MIS principles. The approach, known as the MIS-TLIF, was modified to utilize bilateral 1-inch paramedian incisions and nonexpandable tubular retractors to surgically access the spine. The facetectomy, discectomy, and graft insertion were performed through the small tubular retractor while pedicle screws were placed using a percutaneous screw-rod system. This approach was postulated to reduce trauma and denervation of the paraspinal muscles when compared to the conventional open TLIF. Several recent studies have found that MIS-TLIF is associated with lower operative blood loss and shorter hospital stays while maintaining similar patient-reported outcome scores.

More recently, additional technique variations have been proposed to further modify the originally described MIS-TLIF approach while still maintaining the principles of MIS. This has cumulated in numerous operations being defined as a minimally invasive TLIF; thus, no singular technique is being solely associated with the term MIS-TLIF. ^, To best characterize and understand the gradient of invasiveness in TLIF technique, Lener and colleagues proposed a 5 tier grading system with the (1) traditional midline open TLIF being the most invasive, followed by surgeries utilizing (2) expandable nontubular retractors, (3) expandable tubular retractors, (4) nonexpendable tubular retractors, and finally, least invasive, and (5) a percutaneous endoscopic approach. Procedures within tier 3 and 4 are typically categorized as MIS-TLIF while procedures within tier 5 are referred to as endoscopic TLIF (E-TLIF). Although many variations of the MIS-TLIF workflow have been described, the techniques observe a few central tenants: use of a tubular retractor, magnification/visualization system, and paramedian approach. Reports of new variations in MIS-TLIF technique often involve the use of a novel retractor system, unique workflow of exposure and instrumentation, and advanced applications of stereotactic navigation. ^, Innovation within retractor systems have focused on integrated light sources, cameras, and neuromonitoring systems. Additional modifications such as pedicle-based retractor systems have improved surgeon’s ability to distract the intervertebral space during MIS-TLIF.

Endoscopic spine surgery, first introduced in the 1980s by Kambin and colleagues, has undergone considerable change in technique secondary to improved optical engineering. ^, From the early procedures of percutaneous discectomy, endoscopic spine surgery has evolved to accommodate larger instruments and create larger working channels. In an effort to further reduce the surgical footprint of the MIS-TLIF, Zhou and colleagues described the first endoscopic-assisted TLIF in 2008, which was performed using a tubular reactor system. Since then, surgeons have adopted an endoscope-only approach removing the need for tubular retractors. Current endoscopic techniques include water-based endoscopic environments with uniportal or biportal access. There are 2 primary surgical approaches that have been described for E-TLIF: trans-Kambin and posterolateral. The trans-Kambin approach is typically performed with a uniportal system while the posterolateral approach, which can be performed via either system, is typically performed with a biportal system. The trans-Kambin approach utilizes a similar surgical approach to Kambin’s triangle as the previously described endoscopic discectomy, thus performance of central decompression is limited ( Fig. 2 ). ^, ^, The superior articulating process can be removed during the trans-Kambin approach to facilitate foraminal decompression and access to the disc space for discectomy and cage insertion. The posterolateral biportal approach is similar to currently described MIS-TLIF tubular-based techniques and requires partial or complete facetectomy to access the disc space. The biportal approach is typically performed unilaterally with one channel for the endoscope and another working channel. ^, In both the trans-Kambin and posterolateral techniques, surgeons utilize fluoroscopy or navigation for percutaneous insertion of pedicle screws and rods. Fully E-TLIF and endoscope-assisted interbody fusions have gained popularity over the last decade as they have demonstrated reduced postoperative pain, shorter hospital length of stay, reduced operative blood loss, and similar long-term PROMs when compared to more invasive techniques. ^, ^, However, the steep learning curve and lack of difference in long-term PROMs have led many surgeons to favor more familiar approaches.

Awake Spinal Fusion

In addition to minimizing the invasiveness of PLIF and TLIF procedures, surgeons have worked closely with their anesthesia colleagues to develop alternatives to general anesthesia (GA) for select spinal procedures. The progressive innovation in minimally invasive and endoscopic techniques have allowed surgeons to facilitate an “awake TLIF” under conscious sedation and/or spinal anesthesia (SA). ^, Inspired by the reduced operative time, opioid consumption, and blood loss associated with the use of SA for lumbar laminectomy and discectomy, surgeons sought to apply this technique to the TLIF procedure. In 2016, Wang and Grossman reported the first series of 10 patients that underwent awake lumbar fusion utilizing an endoscopic trans-Kambin approach. Since then, early adopters of this technique have modified the anesthesia protocol to suit a MIS-TLIF approach using tubular retractors. More recently, robotic navigation has been utilized in conjunction with SA to perform a robotic-assisted awake TLIF. Multiple retrospective studies have compared outcomes between GA and SA for MIS-TLIF and found reduced postoperative pain and operative time as well as faster mobilization with SA. ^, Despite early positive results, surgeons worldwide have been reluctant to adopt SA for either spinal decompression or fusion citing a lack of proven benefit in 2023 survey results.

Various anesthesia regimens have been established to successfully complete an awake fusion procedure including conscious sedation and SA. Although both epidural and intrathecal medications have been previously utilized in the setting of awake spine surgery, surgeons have preferred the use of intrathecal injection for the awake TLIF procedure. ^, De Biase and colleagues described a multimodal analgesic and anesthetic regimen for MIS-TLIF, achieving anesthesia with isobaric bupivacaine intrathecally and sedation with dexmedetomidine and propofol intravenously. Chan and colleagues achieved anesthesia with intrathecal bupivacaine and fentanyl and sedation with intravenous propofol and ketamine for their MIS-TLIF procedure. Pedicle screw tracts were infiltrated with liposomal bupivacaine for prolonged pain control. To maximize intraoperative patient feedback during awake E-TLIF, Wang and colleagues ^, utilized intravenous propofol and ketamine to maintain patients under light sedation while avoiding any additional spinal, epidural, or local anesthesia during initial discectomy and cage insertion. Liposomal bupivacaine was subsequently injected prior to pedicle screw insertion. While some surgeons have restricted utilization of SA and conscious sedation to patients with single level pathology, recent report by Chan and colleagues describes a novel “in-parallel” technique to address 3 segment disease. Overall, non-GA protocols for awake TLIF are still in early development and undergoing rapid evolution; thus, there is significant variety in anesthesia and surgical techniques reported across the literature. ^, ^,

Implants

Advancements in Materials

Since the introduction of interbody fusion in 1933, the materials and devices used to promote fusion, maintain structure, and provide alignment correction have expanded exponentially in both material and design. ^, The earliest techniques reported the use of autograft to achieve interbody fusion. Since then, titanium, introduced in the 1980s, and polyetheretherketone (PEEK), introduced in the 1990s, have become the dominant material in spinal interbody implants. ^, More recently, innovation in surface coating technology and 3 dimensional (3D) printing have given way to numerous modified implant materials including porous titanium, porous PEEK, titanium-coated PEEK, tantalum-coated PEEK, carbon-fiber PEEK, and hydroxyapatite-coated PEEK. Modification of the original titanium and PEEK implants have allowed for reduction in complications and improvements in bone on-growth and in-growth. 3D printing has allowed manufacturers to control the roughness and precise size of implant porosity, influencing both its Young’s Modulus and ability for osseous in-growth. ^, Furthermore, 3D printing has allowed for the production of personalized implants to optimize various tribological properties of the implant including contact surface area. Plasma processing has allowed for surface modification of implants with materials such as titanium, hydroxyapatite, and carbon-fiber to modify the material properties, enhance osseointegration, and improve biocompatibility. Aside from modification of titanium and PEEK implants, innovation within spinal implants has focused on the identification of new materials that may optimize spinal fusion and minimize complications. Materials such as silicon nitride, tantalum, and nitinol have recently been under investigation for interbody implants due to their favorable properties. ^,

Advancements in Design

In addition to innovation in materials, TLIF and PLIF implants have significantly changed in geometry and functionality since their introduction. The earliest lumbar interbody cage, known as the Bagby and Kuslich implant, was designed as a static, cylindrical, hollow, titanium device. ^, Although static cages and implants remained the gold standard in the 1990s, innovation in MIS spine surgery placed an emphasis on smaller implants that were still able to restore intervertebral height and provide lordotic correction. ^, Expandable cages, introduced in the early 2000s, met this need by minimizing the cage insertion size while maximizing the expanded height and surface contact. Various expansion mechanisms have been employed over the years to optimize bone graft delivery, height restoration, and ease of implantation. ^, Due to a concern with graft subsidence in expandable cages, recent innovation had led to the development of implants that expand both vertically and horizontally to maximize surface contact. ^, These multidirectionally expanding cages are particularly common in E-TLIF procedures that utilize the smallest surgical access among TLIF techniques. ^, ^,

Advances in static cage design include the development of steerable “banana” cages that maximize endplate surface contact. This technology has been combined with expandable mechanisms to provide further improved height restoration and lower subsidence rates when compared to straight expandable cages.

Technology

Robotics

One of the most recent advances in navigation and technology within spine surgery has been the introduction of robotic-assisted surgery. Introduced in 2004, the Mazor SpineAssist (Mazor Robotics Ltd., Caesarea, Israel) was the first robotic system to achieve Food and Drug Administration (FDA) approval for use in spine surgery. Currently, there are multiple FDA-approved robotic systems including MazorX Stealth Edition (Medtronic, Minneapolis, MN), ROSA ONE Spine (Zimmer Biomet Robotics, Montpelier, France), CirQ (BrainLAB AG, Mucich, Germany), CUVIS-spine (Curexo Inc., Seoul, Korea), and ExcelsiusGPS (Globus Medical, Inc., Audubon, Pennsylvania). ^,

The most recent generation of spinal robotics feature notable improvements upon earlier designs including robot self-detection to facilitate collision avoidance, identification of patient movement, incorporation of preoperative and intraoperative imaging, real-time visualization of instrumentation, and advanced sensors that can detect unexpected drill movement. However, despite continued innovation, the primary role of robotic assistance within spine surgery has been focused on placement of instrumentation ( Fig. 3 A–C ). Most recently, robotic systems have sought to incorporate additional capabilities such as facet decortication and bone graft delivery into their platform.