Comparison of Applied Anatomy at L4-L5 and L5-S1 in Context of Tubular Decompression for Lumbar Canal Stenosis

Discussion

LCS most commonly affects the L4-L5 level and correspondingly minimally invasive surgeons are most experienced and equipped to decompress the L4-L5 segment using tubular retractors. The tenets of tubular decompression and operative steps are imbibed during the training period by various means such as assisting in surgeries and attending workshops. Tubular OTT decompression is one of the popular and prevalent techniques of minimally invasive decompression.10 The operative principles involve unilateral laminotomy, tilting of the tube, microscope, and the operating table to the contralateral side, and performing central and contralateral decompression. This is followed by ipsilateral tilting of the operating table, tube, and the microscope, making it convenient for ipsilateral decompression. Hence, for a successful global (central canal and bilateral lateral recesses) decompression, a lot depends upon the ability to incline the operating table, the tubular retractor, and the microscope. The inclination is necessary especially to reach the contralateral lateral recess and foramen while keeping the vision of the operative field as straight and vertical as possible for the operating surgeon. This is necessary from the point of ergonomics as well as feasibility.

The factors that determine the degree of inclination of the above 3 components (table, tube, and microscope) necessary are the depth of the contralateral recess and the angle made by the contralateral lamina (collinear with the tube) with a vertical line. It is safe to state that the more acute the angle of the lamina with the vertical, the lesser the inclination that is necessary (Figure 3). Again, the deeper the contralateral recess, the more deep is the length of excursion of instruments like Kerrison rongeur to decompress the opposite nerve root. These are some important factors that come into play in consideration of tubular over-the-top decompression at L5-S1 in comparison to the more frequently operated L4-L5 level. The current study analyzes these factors in an objective way to put forth an argument that tubular microdecompression at L5-S1 is laid with some unique complexities.

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Figure 3

Figure demonstrating table tilting and tube tilting for contralateral over-the-top decompression at (A) L4-L5 and (B) L5-S1 level.

First, the authors found that the interlaminar angle is significantly higher at L5-S1 in comparison to L4-L5 and this makes the working angle steeper for the operating surgeon in the axial plane. This challenge basically demands a higher degree of tilt for the 3 components discussed above. This is also reflected in the angle made by the lamina with the vertical which is much higher at the L5-S1 level, meaning that a greater effort has to be made by the operating surgeon to obtain a more convenient access to the contralateral lateral recess. Again, the difficulty is compounded by the lordosis at L5-S1 which demands an oblique disposition of the surgeon’s head toward the headend of the operating table. This can be circumvented to a certain extent by raising the headend of the table in order to reduce the plane at L5-S1 to horizontal.

Second, the first challenge is complicated by a longer stride to reach the corners of the contralateral recess at L5-S1 in comparison to L4-L5 because of the natural widening of the interpedicular distance as we move down in the caudal direction. This is objectively noted in the current study by measuring the laminar length from the midline to the lateral recess, which appears to be significantly higher at L5 compared to L4, almost close to 1.5 times. This feature assumes importance since it makes the access to contralateral decompression deeper and farther.

Third, thickness of the lamina is important in the context of execution of decompression of the contralateral side. A thick lamina will provide sufficient latitude to perform an internal decompression using the drill and Kerrison rongeurs without fracturing. On the contrary, a thin lamina, especially at the site of the isthmus (thinnest part), has the potential to break while drilling or decompressing, leading to inadvertent entry into the soft tissues on the opposite side. This can lead to disorientation of the anatomy and bleeding from the contralateral soft tissues that is difficult to control. To a certain extent, it also works against the philosophy of the OTT concept of decompression that advocates unilateral laminotomy and global decompression without exposure of the contralateral side. The current study reveals the challenges that L5-S1 decompression would pose in comparison to L4-L5 in this context, demonstrating a much thinner laminar isthmus at L5 in contrast to L4.

The limitation of the current study relates to the nonstenotic MRI images that were considered and evaluated. However, the authors comprehend that this will not influence the revelations and impact of the study.