Commentary on “Comparison of Adjacent Segment Degeneration after Nonrigid Fixation System and Posterior Lumbar Inter-Body Fusion for Single-Level Lumbar Disc Herniation: A New Method of MRI Analysis of Lumbar Nucleuspulposus Volumen”

This article refers to:

Comparison of Adjacent Segment Degeneration After Nonrigid Fixation System and Posterior Lumbar Interbody Fusion for Single-Level Lumbar Disc Herniation: A New Method of MRI Analysis of Lumbar Nucleus Pulposus Volume

The idea of damaging functional vertebral units due to increase in loads after fusing a segment is an old idea and keeps on being one of the most controversial issues in spine surgery, especially in degenerative spine patients. There are several key factors at play, such as the natural history of disc degeneration, and biomechanical stresses caused by the fusion and disruption of anatomy during index surgery [1]. Based on such hypothetical concentration of biomechanical stresses after fusing a segmenty, there is some recent high-level review in literature supporting the use of dynamic systems for protecting adjacent levels [2, 3] but some other works could not demonstrate such protective effect [4].

Spinal alignment and balance have also been shown to be relevant factors [5, 6], together with injury to adjacent levels of facets and obesity [7].

In this paper [8], the issue is studied in the setting of a retrospective study on lumbar surgery. The series comprises patients with disc herniation, assumedly candidates for a fixation procedure. There are clear inclusion and exclusion procedures, where instability is an inclusion procedure, but to this author, dynamic fixation in unstable segment remains something to be discussed because if instability is the part of problem, a stabilizing fusion should be superior to a motion-preserving system that does not stabilize the segment. A fusion technique, Posterior Lumbar Interbody Fusion (PLIF), versus a dynamic fixator (ITIF) is compared. There might be issues on which patients were assigned for fusion and fixation techniques, but as both groups are homogeneous for the variable studied (volume), which is measured and compared, it is difficult to perceive this as a possible explanation for the results.

Pre- and postoperatively, an MRI-based study of the nucleuspulposus of the cranially adjacent disc was performed. A volumetric study is carried out, together with a split study of each dimension: anteroposterior and transverse radii – hemi-diameter, and also height, with the results that in patients with fused segment there seems to be decremental changes in volume, with a loss in height and an increase in depth (anteroposterior dimension) after two years follow-up.

The volume change difference at two years was just 1 mm³, which means 20–25%; and linear changes in height were around 0.1–0.2 mm (10). We must assume that the slice cut was exactly at the same level. These changes happened mostly in the PLIF group between six months and one year after index surgery, while changes in the anteroposterior diameter happen afterwards between one year and two years of follow-up. The ITIF group did not show significant changes [8].

All these small but significant changes are supported by a rigorous statistical work. On the other hand, when fixing without fusion, there is always the concern about implant loosening, especially in osteoporotic patients [9] or implant breakage, and this cannot be excluded after two years of follow-up.

There is no clinical correlation to these changes in volume, but again two years is too short a time to ascertain anything from this information.

The biomechanics of the spine with an ITIF is not defined and not all dynamic fixations allow the same amount and kind of movement, but clearly it is different from a fusion.

In summary, probably there is definitely an increment in loads at adjacent segments, especially cranially. In fact, some literature suggest that it is not the immediate but the second disc that suffers most, and this might bring about an adjacent segment syndrome, but mostly if previously injured or degenerated. A key point differentiates adjacent segment degeneration (which might be, and usually, asymptomatic) from adjacent segment disease, which is, in fact, the actual clinical problem. That is, it seems that a “healthy” disc can cope with this “overload”; conversely, a degenerative spine is the one with ongoing degeneration, and that any other disc that degenerates to a remarkable point might just signal the progression of the disease.

Having said, this paper kindles again the discussion on whether a fusion may predispose an adjacent disc to a faster degenerative process if not on clinical grounds, at least fostering a more vulnerable disc scenario for the future to be.

Declaration of interest: The author reports no conflict of interest. The authors alone are responsible for the content and writing of the article.