There is a fear of instability following decompression adjacent to a fused segment. The need to stabilize consequent levels, including the decompressed stable level adjacent to an instrumented unstable level stems from the treasure of existent literature [1-4]. There is a significant body of evidence condemning stand-alone decompression adjacent to a level that is simultaneously undergoing fusion [5,6]. The authors challenge this understanding in the wake of the opportunities provided by tubular retractor-based minimally invasive approach. Unlike in a conventional laminectomy procedure, the posterior ligamentous complex (PLC) is not sacrificed while performing a tube- assisted minimally invasive transforaminal lumbar interbody fusion (TLIF) or tubular decompression. Integrity of PLC is extremely critical in maintaining spinal stability, especially in the context of adjacent segment disease of proximal junctional kyphosis. The authors venture to exploit the PLC-saving property of tubular approach and justify the concept of hybrid minimally invasive surgery (MIS; different MIS techniques at adjacent levels) using a few illustrations and biomechanical evidence.

Written informed consent was obtained from the respective patients for the publication purpose. These techniques involve MIS-TLIF using Medtronic matrix tubular retractors for decompression, interbody fusion and percutaneous pedicle screws for stabilization at an unstable or deformed stenotic level and stand-alone tubular decompression using a paramedian approach incorporating an ‘over-the-top’ approach at the adjacent stable level. This enables surgeons to perform both spinal fusion and direct decompression of the spinal nerves, while preserving the stability that is needed to minimize the extension of fusion.

The authors would like to discuss the cases represented in the background of the dictum of simultaneously fusing the decompressed level adjacent to a fused level to avoid potential instability in the future [7-10]. In the case illustration 1 (Figure 1), an unstable L4–5 degenerative spondylolisthesis with stenosis (that qualifies for decompression and instrumented fusion) adjacent to which there is stable stenosis at L5–S1. Conventional teaching would suggest inclusion of L5–S1 in the fusion construct to minimize the risk of adjacent level stress rendering L5–S1 unstable later in life. The other option is to perform a calculated decompression at L5–S1 keeping the posterior tension band inclusive of interspinous and supraspinous between L5 and S1 intact. Striking a balance in such situation between adequate decompression and maintaining stability is a challenge and needs skill. A compromise between the two is a possibility and this was a point of discussion in a scientific article published previously [11]. The tubular minimal access approach probably answers this dilemma (Figure 4). The tubular approach provides an option of sliding both the ways of fusion as well as decompression unlike an open surgery where the decompression would involve a significant violation of the PLC in a rather stable level, the tubular approach solves this dilemma of decompression without the need for instrumentation (Figure 5). Similarly in the case scenarios 2 (Figure 2) and 3 (Figure 3), 3-consecutive-level stenosis was addressed with instrumentation at an unstable level followed by decompression of the adjacent 2 levels through unilateral laminotomy for bilateral decompression (Figure 6). In either of these case scenarios, the segments which have reached a state of auto-stabilization, there was only a need to address the stenosis (Figure 7). On the contrary a conventional midline approach for the same would have demanded probably fusion at all the levels. The hybrid approach revolves around the integrity of posterior ligament complex (PLC) that coincidently is untouched and preserved in tubular approaches.

PLC comprising of supraspinous ligament, interspinous ligament, ligamentum flavum, and facet joint capsules play a pivotal role in the overall stability and the integrity of a spinal segment. PLC integrity has been the centrepiece of discussions in reducing the development of the adjacent segment degeneration since it plays a primary role in maintaining the structural stability of the spine by preventing excessive flexion, rotation, and translation between vertebral segments. Due to this, PLC is a critical component in assessing the severity and stability of thoracolumbar spinal injuries and is a part of the thoracolumbar injury classification and severity score, where it has been assigned the maximum scoring in the scale, that is 3 [12]. A biomechanical study by Panjabi et al. [13] described the functional spinal unit to be the least stable in a scenario where all the posterior supporting ligaments were sacrificed with a flexion load. Gillespie and Dickey [14] adopted sequential removal of constituents of the PLC from outside to inside. They found that the supraspinous ligament-interspinous ligament complex contributed 35.9% to resistance of the peak flexion moment.

Unlike open surgeries where conventional laminectomy is performed in the process of decompression, tubular surgeries are known to preserve PLC in view of its adoption of “over-the-top” technique.

The philosophy of hybrid MIS finds resonance in basic biomechanical studies also. In one-of-a-kind load bearing flexibility study done on cadaveric models by Grunert et al. [11], different types of spine surgeries like single level MIS instrumentation with consecutive level minimally invasive (MI) laminotomy over-the-top decompression, MI decompression with bilateral partial facetectomy, open laminectomy with bilateral partial facetectomy and their effect on the spinal biomechanics were imitated. Instrumentation significantly decreased mobility at the fused level by 90% during flexion-extension, 69% during axial rotation, and 82% during lateral bending. In the consecutive level where MI over-the-top decompression was performed using laminotomy, there was an increase in the segmental range of movement (ROM) during flexion-extension by 20% and 26% during axial rotation from the baseline. In the scenario where the consecutive level underwent MI decompression with bilateral facetectomy, the axial rotation increased by 41% from the baseline while flexion-extension and lateral bending was not significant. In the fourth scenario where the consecutive level underwent laminectomy with bilateral facetectomy, the instability peaked with an increase in the flexion-extension by 38% and axial rotation by 58% from the baseline. They reported minimally invasive over-the-top decompression through laminotomy is biomechanically superior to open laminectomy and may allow decompression of the level adjacent the fused segment without any additional extension of the fusion. Their data shows that laminectomy leads to a drastic increase in segmental ROM (up to 53% from baseline) adjacent to instrumented segments, which is significantly more than minimally invasive decompression (up to 26%).

Again, multilevel spinal stenosis with or without instability or deformity is an off shoot of degenerative cascade and there is a fear of instability adjacent to the fused segment following decompression in conventional open techniques [15,16]. Open techniques often follow an extensive stabilization plan that works like one size that fits all. On the contrast, there are various MIS options available for treating spinal conditions, each tailored to address specific pathologies and patient needs. Since it is not uncommon to face patients of different age groups with different clinical situations demanding variable treatment plans. These options include alternating treatment plans of fusion at one level and stand-alone decompression/ discectomy at the adjacent level, the concept of the entire plan going against the dictum of fusing the adjacent level next to a fused level [10,17]. By combining these techniques, hybrid tubular MIS allows for a customized treatment plan that can effectively manage pain, maintain spinal function, and reduce the risk of complications associated with more invasive surgeries. The rationale behind these varied MIS techniques is grounded in the Kirkaldy-Willis and Burton’s theory of the degenerative cascade [18]. This theory narrates the natural history of a spinal segment due to aging and degeneration which goes from degeneration to instability to stability. It is not necessary that all the lumbar levels run through these changes simultaneously. In many cases of multilevel stenosis, the involved levels are usually in various phases of this cascade and will be requiring a custom approach while operating.

Hybrid tubular surgery is a new paradigm in the wake of evolving spine surgeries. These techniques have a place in the spinal science since the benefits seem attractive in terms of maintaining motion, minimizing morbidity and reducing the overall expenditure while achieving the needed objectives. While the technique shows promise, the authors emphasize the need for further clinical research, especially muticentric-based to validate the long-term benefits of hybrid MIS.