Biomechanics after spinal decompression and posterior instrumentation

Effects of nucleotomy on segmental flexibility: A numerical analysis

Nucleotomy, a common treatment for disc herniations, aims to relieve pressure on spinal structures. While effective in alleviating symptoms, this intervention can compromise spinal stability. However, previous in vivo studies in sheep have demonstrated conflicting results with significant long-term stiffening of the spine following nucleotomy, with occasional spontaneous fusion of the affected motion segment. The objective of this study was to investigate the mechanical regulation of tissue adaptation processes post-nucleotomy using computational modeling. A parametric finite element model of the L4-L5 ovine spinal motion segment, developed previously, was modified to simulate surgical procedures that have been performed in prior in vivo studies. An iterative approach was used to simulate post-surgical tissue healing and adaptation processes. Two loading scenarios were simulated: one with combined axial compression and flexion moments, and the other incorporating axial rotation. An initial decrease in stability, with stiffness reduced by up to 50% due to disc decompression and nucleus removal, was followed by a gradual increase in stiffness over time as a consequence of bone healing and remodeling, with the most pronounced stiffening - up to 350% of the intact state - observed in axial rotation. The findings align with previous in vivo observations, suggesting that spontaneous fusion and increased rigidity may be natural consequences of mechano-biological adaptation. The results of this study highlight that healing processes accompanied by adaptive bone remodeling are directed towards restoration of spinal stability after nucleotomy. These findings align with previous in vivo observations, suggesting that spontaneous fusion and increased rigidity may be a natural consequence of post-nucleotomy mechano-biological adaptation. On the other hand, the results indicate a critical role of an appropriate loading regime on the outcome of these processes.

Biomechanical effects of the cephalad extent of laminotomy

PURPOSE

The superior aspect of the unilateral laminotomy for bilateral decompression (ULb) typically corresponds with the superior ligamentum flavum attachment. Unlike lateral expansion, cranial expansion is considered a viable option to ensure sufficient decompression. The aim of this study was to investigate how cranial expansion affects the biomechanical stability in the lumbar spine.

METHODS

Range of motion of eight fresh-frozen human cadaveric L1-L5 specimens was assessed in flexion-extension, lateral bending (LB), and axial rotation (AR). The workflow comprised testing in the intact state and after L3-4 ULb with sequential increase of the cephalad extent over 4 steps: (1)25% of the lamina height = insertion of Ligamentum Flavum, (2)50%, (3)75%, and (4)100%. Throughout all steps, constant mediolateral and caudal dimensions were maintained.

RESULTS

Even though within the tested sequences of the workflow an overall significant change was eminent for extension (p = 0.002), right LB (p = 0.030), and left AR (p = 0.009), no significant differences was detected when comparing their five different states pairwise. At L2-3, no overall significant changes were detected for all six motion directions (p ≥ 0.107).

CONCLUSION

ULb induced minor instabilities to the operated segment and no instability to the cranial adjacent segment. However, the absolute increase remained small under the tested conditions, suggesting that unilateral laminotomy is a safe technique at all cranial extents. With a bone-sparing laminotomy preferred, extending cranially appears to be a viable option to achieve sufficient decompression.

The importance of the posterior osteoligamentous complex of the lumbar spine: dogma changing biomechanical insights

BACKGROUND

During full flexion of the spine, the paraspinal muscles are largely inactive. This suggests that passive structures like the posterior osteoligamentous complex (POLC), consisting of interspinous and supraspinous ligaments and the spinous processes, play a key role in spinal stability and protection of the spinal column. The POLC, however, is often resected or damaged during spinal decompression surgeries, whereas the biomechanical implications of this resection or damage are not yet fully understood.

METHODS

A stepwise reduction study was performed on three fresh frozen cadaveric torsi (aged 30-78 years) using a custom setup which only allows sagittal plane motion. After preloading and locking in full flexion, the posterior lumbar structures were gradually resected in the following order: Skin, fascia, musculature, facet joints, ligamentum flavum, posterior ligamentous complex, and posterior longitudinal ligaments. Load cells measured force increase on the fixation frame after each resection step.

RESULTS

The load increased sequentially with each resection, demonstrating load transfer from the passive structures onto the fixation frame. The POLC, including the supraspinous and interspinous ligaments at L2-L5, accounted for 69 - 74% of the measured passive load resistance in full flexion, representing the largest contribution. Facet joints with their capsules contributed 10-18%, while muscular contributions were negligible (< 2%).

CONCLUSION

The experiment indicates that the POLC is the primary passive stabilizer of the fully flexed lumbar spine. Surgical resection of this structure can redistribute loads and increase stresses on remaining spinal tissues, potentially leading to spinal instability, accelerated degeneration, and poor clinical long-term outcomes.

Quantitative relationships between elastic modulus of rod and biomechanical properties of transforaminal lumbar interbody fusion: a finite element analysis

Background

Currently, some novel rods with lower elastic modulus have the potential as alternatives to traditional titanium alloy rods in lumbar fusion. However, how the elastic modulus of the rod (rod-E) influences the biomechanical performance of lumbar interbody fusion remains unclear. This study aimed to explore the quantitative relationships between rod-E and the biomechanical performance of transforaminal lumbar interbody fusion (TLIF).

Methods

The intact finite element model of L1-S1 was constructed and validated. Then 12 TLIF models with rods of different elastic moduli (ranging from 1 GPa to 110 GPa with an interval of 10 GPa) were developed. The range of motion (ROM) of the fixed segment, mean strain of the bone graft, and maximum von Mises stresses on the cage, endplate, and posterior fixation system models were calculated. Finally, regression analysis was performed to establish functional relationships between rod-E and these indexes.

Results

Increasing rod-E decreased ROM of the fixed segment, mean strain of the bone grafts, and peak stresses on the cage and endplate, while increasing peak stress on the screw-rod system. When rod-E increased from 1 GPa to 10 GPa, ROM decreased by 10.4%-39.4%. Further increasing rod-E from 10 GPa to 110 GPa resulted in a 9.3%-17.4% reduction in ROM. The peak stresses on the posterior fixation system showed a nonlinear increase as the rod-E increased from 1 GPa to 110 GPa under most loading conditions. The R 2 values for all fitting curves ranged from 0.76 to 1.00.

Conclusion

The functional relationships between rod-E and the biomechanical properties of TLIF were constructed comprehensively. When the rod-E exceeds 10 GPa, further increases may not significantly improve stability, however, it may increase the risk of fixation failure. Therefore, a rod with an elastic modulus of approximately 10 GPa may provide optimal biomechanical properties for TLIF.

Experimental ex vivo characterization of the biomechanical effects of laminectomy and posterior fixation of the lumbo-sacral spine

Laminectomy and posterior fixation are well-established surgical techniques to decompress nervous structures in case of lumbar spinal stenosis. While laminectomy is suspected to increase the instability of the spine, posterior fixation is associated with some complications such as adjacent segment degeneration. This study aimed to investigate how laminectomy and posterior fixation alter the biomechanics of the lumbar spine in terms of range of motion (ROM) and strains on the intervertebral discs. Twelve L2-S1 cadaveric spines were mechanically tested in flexion, extension, and lateral bending in the intact condition, after two-level laminectomy and after L4-S1 posterior fixation. The ROM of the spine segment was measured in each spine condition, and each loading configuration. The strain distribution on the surface of all the intervertebral discs was measured with Digital Image Correlation. Laminectomy significantly increased the ROM in flexion (p = 0.028) and lateral bending (p = 0.035). Posterior fixation decreased the ROM in all the loading configurations. Laminectomy did not significantly modify the strain distribution in the discs. Posterior fixation significantly increased the principal tensile and compressive strains in the disc adjacent the fixation both in flexion and in lateral bending. These findings can elucidate one of the clinical causes of the adjacent segment degeneration onset.

Minimally invasive robotic-assisted lumbar laminectomy

Aims

To report the development of the technique for minimally invasive lumbar decompression using robotic-assisted navigation.

Methods

Robotic planning software was used to map out bone removal for a laminar decompression after registration of CT scan images of one cadaveric specimen. A specialized acorn-shaped bone removal robotic drill was used to complete a robotic lumbar laminectomy. Post-procedure advanced imaging was obtained to compare actual bony decompression to the surgical plan. After confirming accuracy of the technique, a minimally invasive robotic-assisted laminectomy was performed on one 72-year-old female patient with lumbar spinal stenosis. Postoperative advanced imaging was obtained to confirm the decompression.

Results

A workflow for robotic-assisted lumbar laminectomy was successfully developed in a human cadaveric specimen, as excellent decompression was confirmed by postoperative CT imaging. Subsequently, the workflow was applied clinically in a patient with severe spinal stenosis. Excellent decompression was achieved intraoperatively and preservation of the dorsal midline structures was confirmed on postoperative MRI. The patient experienced improvement in symptoms postoperatively and was discharged within 24 hours.

Conclusion

Minimally invasive robotic-assisted lumbar decompression utilizing a specialized robotic bone removal instrument was shown to be accurate and effective both in vitro and in vivo. The robotic bone removal technique has the potential for less invasive removal of laminar bone for spinal decompression, all the while preserving the spinous process and the posterior ligamentous complex. Spinal robotic surgery has previously been limited to the insertion of screws and, more recently, cages; however, recent innovations have expanded robotic capabilities to decompression of neurological structures.

One-hole split endoscope versus unilateral biportal endoscopy for lumbar spinal stenosis: a retrospective propensity score study

BACKGROUND

The one-hole split endoscopy (OSE) was first proposed and clinically applied in China in 2019. The aim of this study was to compare the clinical efficacy of one-hole split endoscopy (OSE) and unilateral biportal endoscopy (UBE) for treating lumbar spinal stenosis (LSS).

METHODS

One hundred sixty patients with LSS who met the inclusion from November 2020 to August 2022 were analyzed and divided into OSE and UBE groups. The propensity score matching (PSM) method was used to adjust the imbalanced confounding variables between the two groups. After matching, surgical outcomes were recorded, and clinical data, including functional scores and imaging findings, were compared. Functional scores included the visual analog scale of leg pain (VAS-LP) and back pain (VAS-BP), the Japanese Orthopedic Association score (JOA), and the Oswestry Disability Index (ODI). Imaging data included dural sac cross-sectional area (DCSA), lumbar range of motion (ROM), and sagittal translation (ST).

RESULTS

After PSM, 104 LSS patients were included in the study, and all covariates were well-balanced between the two groups. Among the matched patients, the OSE showed advantages over the UBE regarding operative time (62.42 ± 4.86 vs. 68.96 ± 4.56) and incision length (2.30 ± 0.14 vs. 2.70 ± 0.15) (P < 0.001). However, differences between the two groups in intraoperative blood loss, hospital length of stay, and complication rates were not statistically significant (P > 0.05). There was no statistically significant difference regarding VAS-BP, VAS-LP, JOA, and ODI between the two groups (P > 0.05). However, all clinical and functional scores significantly improved postoperatively (P < 0.05). Postoperative DCSA of both groups was significantly found to be improved (P < 0.05), ROM and ST remained within the normal range, and no cases of lumbar instability were recorded. According to the modified MacNab criteria, the excellent and good rates in the OSE and UBE groups were 94.23% and 90.38%, respectively, with no statistically significant difference (P = 0.713).

CONCLUSION

OSE is an alternative technique to UBE for the treatment of LSS, with similar satisfactory clinical outcomes, shorter operative time, and smaller incision length. Further studies are needed for long-term efficacy.

Basivertebral nerve ablation with concurrent lumbar laminotomy

Lumbar radiculopathy due to impingement of nerve roots from facet hypertrophy and/or disc herniation can often coincide with vertebrogenic low back pain. This is demonstrated on MRI with foraminal stenosis and Modic changes. We examine the potential of using a combination of basivertebral nerve ablation (BVNA) and lumbar laminotomy as an alternative to traditional spinal fusion in specific patient populations. This unique combination of surgical techniques has not been previously reported in the medical literature. We report a man in his late 30s with chronic low back pain and lumbar radiculopathy, treated with BVNA and concurrent laminotomy. The patient reported progressive improvements in his mobility and pain over the next 2 years. We discuss the advantages of using this technique for lumbar radiculopathy and Modic changes compared with conventional surgical modalities.

Biomechanical response of decompression alone in lower grade lumbar degenerative spondylolisthesis--A finite element analysis

BACKGROUND

Previous studies have demonstrated the clinical efficacy of decompression alone in lower-grade spondylolisthesis. A higher rate of surgical revision and a lower rate of back pain relief was also observed. However, there is a lack of relevant biomechanical evidence after decompression alone for lower-grade spondylolisthesis.

PURPOSE

Evaluating the biomechanical characteristics of total laminectomy, hemilaminectomy, and facetectomy for lower-grade spondylolisthesis by analyzing the range of motion (ROM), intradiscal pressure (IDP), annulus fibrosus stress (AFS), facet joints contact force (FJCF), and isthmus stress (IS).

METHODS

Firstly, we utilized finite element tools to develop a normal lumbar model and subsequently constructed a spondylolisthesis model based on the normal model. We then performed total laminectomy, hemilaminectomy, and one-third facetectomy in the normal model and spondylolisthesis model, respectively. Finally, we analyzed parameters, such as ROM, IDP, AFS, FJCF, and IS, for all the models under the same concentrate force and moment.

RESULTS

The intact spondylolisthesis model showed a significant increase in the relative parameters, including ROM, AFS, FJCF, and IS, compared to the intact normal lumbar model. Hemilaminectomy and one-third facetectomy in both spondylolisthesis and normal lumbar models did not result in an obvious change in ROM, IDP, AFS, FJCF, and IS compared to the pre-operative state. Moreover, there was no significant difference in the degree of parameter changes between the spondylolisthesis and normal lumbar models after undergoing the same surgical procedures. However, total laminectomy significantly increased ROM, AFS, and IS and decreased the FJCF in both normal lumbar models and spondylolisthesis models.

CONCLUSION

Hemilaminectomy and one-third facetectomy did not have a significant impact on the segment stability of lower-grade spondylolisthesis; however, patients with LDS undergoing hemilaminectomy and one-third facetectomy may experience higher isthmus stress on the surgical side during rotation. In addition, total laminectomy changes the biomechanics in both normal lumbar models and spondylolisthesis models.

Determining a relative total lumbar range of motion to alleviate adjacent segment degeneration after transforaminal lumbar interbody fusion: a finite element analysis

BACKGROUND

A reduction in total lumbar range of motion (ROM) after lumbar fusion may offset the increase in intradiscal pressure (IDP) and facet joint force (FJF) caused by the abnormally increased ROM at adjacent segments. This study aimed to determine a relative total lumbar ROM rather than an ideal adjacent segment ROM to guide postoperative waist activities and further delay adjacent segment degeneration (ASD).

METHODS

An intact L1-S1 finite element model was constructed and validated. Based on this, a surgical model was created to allow the simulation of L4/5 transforaminal lumbar interbody fusion (TLIF). Under the maximum total L1-S1 ROM, the ROM, IDP, and FJF of each adjacent segment between the intact and TLIF models were compared to explore the biomechanical influence of lumbar fusion on adjacent segments. Subsequently, the functional relationship between total L1-S1 ROM and IDP or total L1-S1 ROM and FJF was fitted in the TLIF model to calculate the relative total L1-S1 ROMs without an increase in IDP and FJF.

RESULTS

Compared with those of the intact model, the ROM, IDP, and FJF of the adjacent segments in the TLIF model increased by 12.6-28.9%, 0.1-6.8%, and 0-134.2%, respectively. As the total L1-S1 ROM increased, the IDP and FJF of each adjacent segment increased by varying degrees. The relative total L1-S1 ROMs in the TLIF model were 11.03°, 12.50°, 12.14°, and 9.82° in flexion, extension, lateral bending, and axial rotation, respectively.

CONCLUSIONS

The relative total L1-S1 ROMs after TLIF were determined, which decreased by 19.6-29.3% compared to the preoperative ones. Guiding the patients to perform postoperative waist activities within these specific ROMs, an increase in the IDP and FJF of adjacent segments may be effectively offset, thereby alleviating ASD.