The objective of this study
To evaluate the risk biomechanical deterioration and resulting postoperative complications in the surgical segment caused by the optimized in-out technique with intact SAP and TESSYS TED with a small grade of facetectomy, intact lumbo-sacral models with and without disc degeneration and corresponding models after these operations have been constructed and biomechanical indicators shortly related to lumbar degenerative diseases (LDD) have been computed and evaluated. Considering the importance of biomechanical environment for postoperative clinical outcomes has been repeatedly demonstrated [23, 50, 52], investigations for the biomechanical change caused by these two different surgical techniques in TED was of great significance for the reference of surgical strategy selection.
Notable points in the model construction process
Adjacent segments, rather than the surgical segment were selected for the construction of DD models. This model construction strategy was based on our clinical practice. As is mentioned above, DD was very common in TED patients, This nature degenerative change may not lead to serious clinical symptoms, but will adversely affect the biomechanical environment in adjacent segments [23, 48, 49]. Hence, the simulation of disc degeneration was meaningful for the evaluation of real postoperative biomechanical environment. Disc collapse during the process of DD lead to a reduction in the cross-sectional area in Kambin triangle, and the risk of exiting nerve root injury in a degenerated disc will increase during the insertion of working cannula without facetectomy [67, 68]. As a result, LDH with narrow disc space can be seen as a contraindication for the application of modified in-out TED and the surgical segment has been excluded from the construction of degenerative change models.
Besides, although ZJ degeneration was shortly related to DD [43, 45, 58, 69], and some FE studies construct ZJ degenerative models by reducing the facet gap [48, 57], we still give up the construction of ZJ degeneration. The gap thickness of ZJ should be seen as a reflection of the cartilage wear, sclerosis and hyperplasia of subchondral bone [60, 70, 71], and this pathological process was difficult to simulate in the model construction. Specifically, the decrease of the facet gap by thickening the facet cartilage was completely contrary to the pathological change of ZJ degeneration. Besides, if the gap was reduced by thickening the bone tissue of articular process, the definition of material properties for sclerosis subchondral bone structures was also inaccurate for which was obviously differ from normal bone tissues [15, 72, 73], and the casual definition of material properties without reliable data will decrease the credibility of this study. Hence, we chose to construct DD models without the change of facet gap [49, 57].
Additionally, the grade of facetectomy in TESSYS TED models and discectomy in in-out TED models were set as one-third, this grade was consistent with the maximum one in our clinical practice. This modeling strategy was selected for facetectomy and nucleotomy were assumed to be main reasons for poor clinical outcomes after these operations respectively. Therefore, larger grades of these two procedures should lead to more pronounced biomechanical deterioration and which could provide us a clear reference for the evaluation of these two techniques.
Clinical significance of biomechanical indicators
Disc collapse and DD acceleration in the surgical segment and resulting several kind of secondary pathological changes were most significant causes for the poor clinical outcome for patients after non-fusion lumbar surgery [33, 74, 75]. As is reported by Adam et. al, the injury of endplates and annulus can be seen as two different separately pathways in the DD process . The maximum von-Mises stress and the strain energy of endplates were recorded to evaluate the risk of DD caused by the endplate lesion and ossification. Specifically, endplates play a key role in the pressure distribution, post operative abnormal stress concentration on endplates increases the risk of endplate lesions [21, 76, 77], resulting inflammatory response, autoimmune reaction and disc innervation can be seen as significant triggers for DD acceleration and increase risk of low back pain (LBP) [51, 78-80].
Besides, IVD was an avascular structure, and the most important pathway for its metabolism was the trans-endplate diffusion [81, 82]. According to the Wolff’s law, the concentration of strain energy, a kind of compensatory reaction for the endplate stress concentration can be seen as a predicted factor for its ossification [83, 84], and resulting occlusion of trans-endplate diffusion pathway will lead to DD acceleration [43, 80, 85, 86]. Moreover, endplate injury caused by the abnormal stress concentration was closely associated with the disruption of the annulus and can also be reflected by the deterioration of biomechanical indicators on the annulus, especially in the post and post lateral part of annulus [21, 29, 82]. The concentration of shear stress and compressive stress have been proved to be related to different kinds of annulus tear and resulting DD, more significantly, to resulting discogenic LBP and RLDH [22, 23, 85]. Hence, we can speculate that above biomechanical indicators can be seen as credible predictors for the assessment of postoperative prognosis.
Meanwhile, the foramen stenosis was another vital reason for the clinical outcome deterioration, and special attention should be taken for models after in-out TED with intact SAP for the risk of foramen stenosis will get worse with disc collapse caused by a larger grade of discectomy in the surgical segment without foraminoplasty [74, 75, 87]. Noteworthy, the increase of FCF can not only be seen as a risk factor for the cartilage wear and resulting degenerative osteoarthritis of ZJ [15, 44, 73], a trigger of LBP [22, 25, 45], but also for the foramen stenosis considering a larger load will promote the osteogenic activity [43, 83, 88]. More importantly, disc collapse and degeneration in the surgical segment will lead to pathological stress concentration on ZJ cartilages and resulting further degenerative osteoarthritis and osteophytes formation in which [15, 45]. Besides, lumbar instability was also an important cause of prognosis deterioration after non-fusion surgery, and which have been proven to be related to LBP and further DD [86, 89, 90]. Therefore, ROM can be used not only as an indicator for model validation, but also for the assessment of postoperative complications. In a word, there was close interactions between different biomechanical indicators, and biomechanical deterioration will lead to a series of clinical symptoms and deterioration of prognosis.
Firstly, ligaments were constructed by cable elements, and the simulation for the LF excision was accomplished by reducing its CSA. Cable elements can not stimulate the fold, the hypertrophy and the calcification of ligaments, and there pathological changes were reported to be vital risk factors for the spinal stenosis and nerve compression.
Meanwhile, as a common issue of FE studies, the proliferation of scar on the annulus and its biomechanical effects can not be evaluated. Considering that the size of annulus breakage was an important variable in this study, biomechanical changes caused by the formation of annulus scar tissue (although its strength proved not strong enough to stop RLDH) may also have its potential biomechanical impact on patients prognosis. Hence, current computational results should be recognized and discussed on the basic of the awareness of this defect and follow-up clinical study is still necessary for more definitive conclusions.