Iatrogenic Spinal Instability: Causes, Evaluation, Treatment, and Prevention





Introduction


Instability in the lumbar spine is a well-recognized entity for both degenerative and traumatic processes. Less well defined is instability following an index decompressive or fusion procedure. This complex issue can exist as iatrogenic destabilization at either the same or adjacent level or could even be unrecognized preoperative instability in the setting of a patient who does not show improvement after their index procedure. During this chapter, we will provide a framework for categorizing, identifying, and treating postoperative instability. Additionally, there will be a section outlining preventative strategies during index lumbar procedures.


Epidemiology


Postoperative instability can be viewed in two broad categories with subdivisions within those categories. First are the patients who undergo a decompressive procedure and exhibit instability at the same operative level postoperatively. Second are patients who develop evidence of adjacent segment instability adjacent to a fusion procedure. Both of these categories can have acute postoperative instability or a more gradual, insidious onset many years after the index procedure.


Postdecompression


In a case series and literature review, Ramhmdani et al. identified a 9.5% rate for postoperative instability requiring reoperation at the same level of a previous decompression. The average time to reoperation identified within their series was approximately 32 months. Additionally, they highlighted a number of studies with heterogeneous patient populations and found that the rate of reoperation varied widely (1%–32%) following decompression laminectomy. Within these series, postoperative instability was generally defined in the same terms as degenerative spondylolisthesis.


In one long-term series of patients after primary microdiscectomy with 12 years of follow-up, 16 of 69 patients developed segmental instability at the operative level. Interestingly, five patients showed segmental instability either above or below the operative level, but not at the operative level. The majority of postoperative segmental instability showed translation in excess of 5 mm.


Postfusion


Postfusion instability above or below an index construct is a poorly isolated phenomenon in the literature. Most cases of instability have been historically grouped into a larger segment of “adjacent segment disease,” which also includes discogenic failure, facet arthropathy, or foraminal/central stenosis. Fusions of the lumbar spine, as with any portion of the spine, transfer the previous motion forces to adjacent levels, accelerating degenerative changes of otherwise asymptomatic levels.


Aota et al. provided an early look at this patient population, finding that approximately 24.6% of their 61 patients undergoing decompression and fusion demonstrated instability. They also noted that the instability was more likely to occur above a fusion rather than below. The criteria used included greater than 3 mm of translation, greater than 15 degrees of rotatory hypermobility, or disc wedging greater than 5 degrees. In a later, 5-year long-term follow-up, Cheh and colleagues identified adjacent segment disease in nearly 43% of their 188 patients, but only 2.6% of the patients had “instability-type back pain” and most were stenosis-related symptoms.


Diagnosis


Clinical instability can be classically attributed to the failure of one of three key spinal kinematic domains. First, and most surgically relevant, is the intrinsic spinal column made up of the vertebrae, discoligamentous complex, and the posterior ligamentous complex (PLC). The second domain is the active stabilization of the spine comprising the paraspinal musculature and tendinous attachments. Finally, the neuromotor control of the coordination of spinal motion can result in a dyskinetic instability in a very small percentage of patients. For the purposes of postoperative and iatrogenic instability, attention should be focused on the passive, intrinsic spinal column.


Clinical


Instability can be detected either by soliciting feedback from the patient during the verbal portion of the encounter or by identifying subclinical instability on routine surveillance imaging. Pain is the primary presenting complaint for acute or delayed-onset postoperative instability, but unfortunately this is highly nonspecific. Questions can be centered around determining a patient’s tolerance for normal physiological loads, but the differential can also include adjacent segment disease without instability or recurrent/residual compression.


Radiographic


Imaging continues to be the mainstay of confirming the diagnosis of instability. Although there are no guidelines specific to defining postoperative instability relative to traditional degenerative or traumatic instability, the same concepts apply. The first image to consider is an upright lateral plain radiograph. The upright film implies that a standard physiological load is being applied and may be enough to highlight any changes from the preoperative imaging. Listhesis of 3 to 4 mm when comparing an upright image with supine imaging (plain or axial) can be a strong indicator of pathological instability. Comparative images can also include a lumbar flexion and extension series. Wood et al. posit that a lateral decubitus flexion position is the most reliable, as the active stabilizers of the spine can relax and provide a more complete image of the passive stabilizing mechanics. Their series revealed a significant increase in sensitivity by using that position. Of the 31 patients out of 50 in whom instability was present, 18 showed instability only in the lateral decubitus position. In another series of 56 patients, Chan and team found an absolute increase of segmental instability of 40% when comparing neutral standing films/supine magnetic resonance imaging (MRI) images with active flexion and extension films. These findings again highlight the impact of active spinal stabilizers when evaluating for the presence of dynamic segmental instability.


Plain radiography, despite being the gold standard, is not without its limitations. Nizard and coauthors noted that even small variations in directional imaging could result in 10% to 15% changes in perceived translation. He and his coauthors also noted the lack of standardization with respect to which anatomic landmarks should be included in a standard lateral view of the lumbar spine.


Axial imaging is an extremely common step taken before revision surgery. It can provide useful indicators of subtle instability, in addition to providing a detailed look at compression and degeneration. MRI, in particular, can assist the physician by demonstrating the increased fluid content in the facet capsules. Fluid levels in excess of 1.5 mm ( Fig. 24.1 ) have been shown to correlate strongly with early instability even in the absence of clear sagittal displacement, whereas fluid levels that measured less than 0.5 mm showed a greater than 90% probability of no instability at that level. Finally, as most axial imaging is obtained with the patient supine on the table, it can be a useful adjunct to show the reduction of translation seen in upright or flexion radiographs.




Fig. 24.1


T2-weighted axial image of the lumbar spine demonstrating fluid in the facet joints at a level demonstrating both clinical and radiographic instability. The arrow indicates the fluid between the superior and inferior articular process.


Interestingly, in a series of microendoscopic decompressions, postoperative MRIs were obtained on all 165 patients to evaluate for postdecompression facet effusion. Twenty-eight (17%) of the patients who had undergone the above decompression were found to have this facet effusion. Only one of the 28 patients with facet effusion ultimately required revision with fusion, and three others required a revision decompression. Although this series identified similar levels of “low back pain” between those with and without the facet effusions, “mechanical” low back pain was seen only in those with the effusion seen on MRI.


Treatment


Once the treating surgeon has confirmed the diagnosis of postsurgical instability, the treatment pathway nearly always funnels toward a revision procedure to stabilize the offending level. Same-level and adjacent-level instability can typically be treated with a single vertebral segment fusion with instrumentation. Treating surgeons should exercise routine clinical judgment when finalizing fusion plans. Care should be taken to avoid violating adjacent structures as outlined in the Prevention section further in this chapter. Emphasis on rigid fixation modalities and appropriate biological modifications should be followed with the intent of avoiding further revision procedures on the patient.


Prevention


Our understanding of the passive, intrinsic spinal column and its contribution to the overall stability of the spine has been well outlined, starting with early descriptions by Denis in 1976 and 1984. His work in defining the three-column model and the relative contribution of the three columns to the mechanics of the spine in trauma was further expanded by basic science demonstrations of progressive instability with ordered sectioning of specific components of the lumbar spine.


In 1990, Abumi and colleagues performed graded facetectomies in the cadaver lumbar spine to determine the progressive instability with flexion, extension, and rotational forces. The broad categories of progressive excision were as follows. First, the posterior ligaments and interspinous ligaments were sectioned. This was followed in a stepwise fashion by a left unilateral medial facetectomy, bilateral medial facetectomies, a left total facetectomy with right medial facetectomy, and finally, bilateral total facetectomies. The results demonstrated profound increases in instability with flexion and extension forces (as high as 70% more than the unsectioned spine) when bilateral facetectomies were performed, as well as rotational forces (nearly 100% more). This understanding of bony anatomy allows surgeons to be cognizant of potential intraoperative destabilizers of the spine during decompressive procedures.


More recent studies have targeted and highlighted the combined effect of the PLC and its role in stabilization. A finite element analysis by Wu et al. in 2018 demonstrated a theoretical increase in the segmental range of motion of the T12–L1 junction of up to 6 degrees in flexion, extension, and lateral bending with the removal of the interspinous or supraspinous ligaments, or the facet capsular ligament. In an in vitro biomechanical investigation, Li and colleagues performed a stepwise resection of the PLC similarly to Abumi’s method of sequential bony resection. They drew the conclusion that the ligamentous complex could be considered intact if the supraspinous ligament and ligamentum flavum were both intact, and once either of these components was compromised, there would be a more substantial relative instability. Absolute increases in motion were level-dependent and highest at the T12–L1 junction. In a 2017 series in Korea of 100 consecutive patients, Kim et al. performed a decompressive lumbar laminoplasty procedure instead of a complete laminectomy. None of the 100 patients had postoperative instability at a follow-up of just over 3 years. The authors posited some of their success with maintenance and repair of the ligamentous complexes.


In another retrospective series, Lai et al. evaluated the characteristics of the decompressive laminectomy at the fused level. Some 101 patients were grouped based on the integrity of the PLC at adjacent articulating levels. Only 2 of 31 (6.5%) patients with intact posterior complex demonstrated adjacent level instability, whereas 17 of 70 (24.3%) patients without preservation of the complex demonstrated this instability.


Patients with more sagittally oriented facet complexes may be at an increased risk of postoperative instability ( Fig. 24.2 ). As the joint begins to rotate, it becomes increasingly technically difficult to resect just enough to visualize the lateral recess pathology without removing too much of the inferior articular process and thus destabilizing the spine. Evaluating this detail preoperatively may highlight patients who could be better candidates for a unilateral approach with bilateral decompression, as opposed to a wide, complete laminectomy.


May 5, 2021 | Posted by in NEUROSURGERY | Comments Off on Iatrogenic Spinal Instability: Causes, Evaluation, Treatment, and Prevention

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