Biomechanical Rationale of Endoscopic Decompression for Lumbar Spondylolysis as an Effective Minimally Invasive Procedure - A Study Based on the Finite Element Analysis

Short-term lumbar disc and lumbar stability changes of one-hole split endoscope technique treatment of spinal stenosis

OBJECTIVE

Investigating the early biomechanical effects of the one-hole split endoscope (OSE) technique on lumbar spine after decompression surgery.

METHODS

A retrospective analysis was conducted on 66 patients with lumbar spinal stenosis (LSS) who underwent OSE technique surgery at the affiliated hospital of Binzhou Medical University from September 2021 to September 2022. The patients had complete postoperative follow-up records. The mean age was (51.73 ± 12.42) years, including 33 males and 33 females. The preoperative and postoperative imaging data were analyzed, including disc height (DH), foraminal height (FH), lumbar lordosis angle (LLA), changes in disc angle, anterior-posterior translation distance, and lumbar intervertebral disc Pfirrmann grading. The visual analogue scale (VAS) was applied to evaluate the severity of preoperative, postoperative day 1, postoperative 3 months, and final follow-up for back and leg pain. The Oswestry Disability Index (ODI) was applied to assess the functionality at all the listed time points. The modified MacNab criteria were applied to evaluate the clinical efficacy at the final follow-up.

RESULTS

In 66 patients, there were statistically significant differences (p < 0.05) in DH and FH at the affected segments compared to preoperative values, whereas no significant differences (p > 0.05) were found in DH and FH at the adjacent upper segments compared to preoperative values. There was no statistically significant difference in the LLA compared to preoperative values (p > 0.05). Both the affected segments and adjacent upper segments showed statistically significant differences in Pfirrmann grading compared to preoperative values (p < 0.05). There were no statistically significant differences in the changes in disc angle or anterior-posterior translation distance in the affected or adjacent segments compared to preoperative values (p > 0.05). The VAS scores for back and leg pain, as well as the ODI, significantly improved at all postoperative time points compared to preoperative values. Among the comparisons at different time points, the differences were statistically significant (p < 0.05). The clinical efficacy was evaluated at the final follow-up using the modified MacNab criteria, with 51 cases rated as excellent, 8 cases as good, and 7 cases as fair, resulting in an excellent-good rate of 89.39%.

CONCLUSIONS

The OSE technique, as a surgical option for decompression in the treatment of LSS, has no significant impact on lumbar spine stability in the early postoperative period. However, it does have some effects on the lumbar intervertebral discs, which may lead to a certain degree of degeneration.

Comparison of percutaneous transforaminal endoscopic decompression and short-segment fusion in the treatment of elderly degenerative lumbar scoliosis with spinal stenosis

Background

Degenerative lumbar scoliosis (DLS) combined with spinal stenosis is increasingly being diagnosed in the elderly. However, the appropriate surgical approach remains somewhat controversial. The aim of this study was to compare the results of percutaneous transforaminal endoscopic decompression (PTED) and short-segment fusion for the treatment of mild degenerative lumbar scoliosis combined with spinal stenosis in older adults over 60 years of age.

Methods

Of the 54 consecutive patients included, 30 were treated with PTED and 24 were treated with short-segment open fusion. All patients were followed up for at least 12 months (12–24 months). Patient demographics, and perioperative and clinical outcomes were recorded. Visual analog scale (VAS) scores, Oswestry disability index (ODI) scores, and modified Macnab criteria were used to assess clinical outcomes. At the same time, changes in disc height, segmental lordosis, coronal Cobb angle, and lumbar lordosis were compared.

Results

The mean age was 68.7 ± 6.5 years in the PTED group and 66.6 ± 5.1 years in the short-segment fusion group. At 1 year postoperatively, both groups showed significant improvement in VAS and ODI scores compared with preoperative scores (p < 0.05), with no statistically significant difference between groups. However, VAS-Back and ODI were lower in the PTED group at 1 week postoperatively (p < 0.05). According to the modified Macnab criteria, the excellent rates were 90.0 and 91.6% in the PTED and short-segment fusion groups, respectively. However, the PTED group had a significantly shorter operative time, blood loss, postoperative hospital stay, postoperative bed rest, and complication rate. There was no significant difference in radiological parameters between the two groups preoperatively. At the last follow-up, there were significant differences in disc height, segmental lordosis at the L4–5 and L5–S1 levels, and Cobb angle between the two groups.

Conclusion

Both PTED and short-segment fusion for mild degenerative lumbar scoliosis combined with spinal stenosis have shown good clinical results. PTED under local anesthesia may be an effective supplement to conventional fusion surgery in elderly patients with DLS combined with spinal stenosis.

One-year clinical outcome after full-endoscopic interlaminar lumbar discectomy for isthmic lumbar spondylolisthesis: Two case reports

RATIONALE

For isthmic lumbar spondylolisthesis (ILS) associated with the removal of herniation, it remains challenging to perform less invasive and minimally disruptive procedures. Good results could potentially be obtained by further preserving the posterior elements in full-endoscopic lumbar discectomy (FESS), which is less invasive than microenscopic surgery (MES).

PATIENT CONCERNS

One patient complained of left leg pain, and another patient complained of right leg pain and low back pain.

DIAGNOSES

Two patients with ILS and Meyerding Grade 1 lumbar spondylolisthesis.

INTERVENTIONS

We performed a full-endoscopic lumbar discectomy via the interlaminar space (FESS-IL) for L5/S1 lumbar disc herniation (LDH) accompanied by isthmic lumbar spondylolisthesis. FESS-IL was performed in 2 patients with radiculopathy caused by different types of LDH using a full endoscopic system with a 4.1 mm working channel and 6.9 mm outer diameter. A 3.5-mm diameter high-speed drill was used in one patient for an upward-migrated LDH in the inner-rim of the infravertebral border. The other patient underwent minimal resection without bone resection.

OUTCOMES

The one-year clinical outcome included confirmation of pain relief and evacuation of migrated LDH on magnetic resonance imaging in all patients. There was no progression of slippage on radiography. The mean operative time was 82 min, and no complication was observed. The one-year clinical outcome demonstrated sufficient pain relief.

LESSONS THE Y

ear postoperative outcome showed improvement. We believe that FESS-IL is a viable alternative operative approach for LDH for ILS.

An Elite Triathlete with High-grade Isthmic Spondylolisthesis Treated by Lumbar Decompression Surgery without Fusion

The patient was a 48-year-old female recreational triathlete who had been experiencing mild low back pain since high school. She had recently developed right leg pain and had gradually worsening difficulty in running. She preferred to undergo spinal surgery without fusion so that she could return to triathlons as soon as possible, and she was referred to our hospital. Plain radiographs showed Meyerding grade 3 isthmic spondylolisthesis at L5 and a slipped L5 vertebral body. Selective nerve root block at L5 relieved the right leg pain temporarily. The final diagnosis was right L5 radiculopathy due to compression by the ragged edge of the L5 pars defect from the posterior side and by the upside-down foraminal stenosis at L5–S1. An L4–L5 partial laminectomy was performed with resection of the ragged edge and one-third of the caudal pedicle at L5. Adequate decompression was achieved by exposing the L5 spinal nerve root from the branch portion to the outside of the L5 pedicle. The right leg pain disappeared postoperatively and she returned to participating in triathlons. One year after surgery, there was slight radiographic progression of the slip in 5 mm; however, there had been no recurrence of the right leg pain. Several studies have reported excellent outcomes after decompression surgery in patients with isthmic spondylolisthesis. To our knowledge, this is the first report of successful lumbar decompression surgery without fusion for high-grade isthmic spondylolisthesis in a triathlete, although in short-term results.

Residual leg numbness after endoscopic discectomy treatment of lumbar disc herniation

Background

Transforaminal endoscopic discectomy was popular in the treatment of lumbar disc herniation. Previous study focuses on the leg pain of disc herniation, and little study concern the residual leg numbness after surgery. The purposes of this study were to evaluate the clinical outcomes of transforaminal endoscopic discectomy in the treatment of lumbar disc herniation with leg pain and numbness.

Methods

Patients with one level lumbar disc herniation who had transforaminal endoscopic lumbar discectomy from June 2016 to July 2019 were categorized into two groups according to the leg numbness. 293 patients initially fulfilled the study criteria, and 27 patients were lost to follow-up. Of the remaining 266 patients available for analysis, 81 cases with leg numbness and pain (A group), and 185 cases with leg pain (B). Endoscopic transforaminal lumbar discectomy was performed, and the clinical outcomes of blood loss, operation times, hospital stay days, pain (Visual Analog Scale, VAS-pain), numbness (VAS-numbness), functional disability (Oswestry Disability Index, ODI), and the disk height and intervertebral foramen height were recorded.

Results

All patients with pain and numbness pre-operation in group A, complain of leg numbness during or just after walking or standing not diminished after surgery in group A, and no one complain numbness after surgery in group B. The pain index and ODI score were better than preoperational in all patients (P < 0.01), and no significant difference between two groups (P > 0.05). The postoperative disk and foramen height were no significant difference compare to preoperative in all patients (P > 0.05), and no significant difference between two groups (P > 0.05). The leg numbness symptoms last longer in central disc herniation patients (10.4 ± 2.2 months) than in paracentral (6.3 ± 2.1 months) and foraminal disc herniation patients (5.6 ± 2.3 months) after surgery (P < 0.01).

Conclusions

Based on the results of this study, transforaminal endoscopic lumbar discectomy was effective and safe procedures in the treatment of disc herniation with leg pain and numbness. The leg numbness symptoms last longer in central disc herniation patients than in paracentral and foraminal disc herniation patients after surgery.

Comparison of full-endoscopic and minimally invasive decompression for lumbar spinal stenosis in the setting of degenerative scoliosis and spondylolisthesis

OBJECTIVE

The management of lumbar spinal stenosis (LSS) with concurrent scoliosis and/or spondylolisthesis remains controversial. Full-endoscopic unilateral laminotomy for bilateral decompression (ULBD) facilitates neural decompression while preserving stabilizing osseoligamentous structures and may be uniquely suited for the treatment of LSS with concurrent mild to moderate degenerative deformity. The safety and efficacy of full-endoscopic versus minimally invasive surgery (MIS) ULBD in this patient population is studied here for the first time.

METHODS

A retrospective analysis of prospectively collected data was conducted on 45 consecutive LSS patients with concurrent scoliosis (≥ 10° coronal Cobb angle) and/or spondylolisthesis (≥ 3 mm). Patient demographics, operative details, complications, and imaging characteristics were reviewed. Outcomes were quantified using back and leg visual analog scale (VAS) scores and the Oswestry Disability Index (ODI) at 2 weeks, 3 months, and 1 year.

RESULTS

A total of 26 patients underwent full-endoscopic and 19 underwent MIS-ULBD with an average follow-up period of 12 months. The endoscopic cohort experienced a significantly shorter hospital length of stay (p = 0.014) and fewer adverse events (p = 0.010). Both cohorts experienced significant improvements in VAS and ODI scores at all time points (p < 0.001), but the endoscopic cohort demonstrated significantly better early ODI scores (p = 0.024).

CONCLUSIONS

Endoscopic and MIS-ULBD result in similar functional outcomes for LSS with mild to moderate deformity, while the endoscopic approach demonstrates a favorable rate of complications. Further studies are required to better delineate the characteristics of spinal deformities amenable to this approach and the durability of functional results.

Successful Endoscopic Surgery for L5 Radiculopathy Caused by Far-Lateral Disc Herniation at L5-S1 and L5 Isthmic Grade 2 Spondylolisthesis in a Professional Baseball Player

Background

We report the case of a professional baseball player who had severe leg pain due to lumbar lateral disc herniation at L4-5 and isthmic spondylolisthesis at L5 (double crash syndrome). For early recovery to competitive level, we performed minimally invasive endoscopic decompression surgery without fusion. There are few reports to discuss the usefulness of minimally invasive treatment for top athletes.

Methods

A 29-year-old professional baseball player who played catcher was referred to us with a complaint of right leg pain. The previous doctor diagnosed far-lateral disc herniation and Grade 2 isthmic spondylolisthesis and recommended arthrodesis at L5-S1 as treatment for both pathologies. Radiological imaging showed that the right L5 nerve root was impinged by the 2 lumbar disorders, namely, far-lateral disc herniation and a ragged edge around a pars defect. We had taken into account the patient's occupation and his wish to avoid a lengthy sick leave, and we had performed endoscopic decompression surgery during the offseason. The far-lateral disc herniation at L5-S1 was removed under local anesthesia by percutaneous endoscopic discectomy, after which the ragged edge at the pars defect was removed under general anesthesia using a microendoscopic discectomy system. Given that the patient did not have any low back pain, arthrodesis was not considered.

Results

The leg pain resolved after surgery. The following year (2015), the patient resumed playing baseball from the beginning of the season and played in 41 games. In the 2016 season, he played in 71 games without any symptoms. No further slippage was observed at radiological follow up 1 year after the surgery.

Conclusions

Minimally invasive endoscopic surgery is an option for radiculopathy in very active patients who need an early return to their previous level of physical activity.

Bilateral three-level lumbar spondylolysis repaired by hook-screw technique

We report a case of bilateral three-level lumbar spondylolysis that was directly repaired by use of hook-screw technique. The patient complained of low back pain for 2 years that progressively worsened and was exacerbated with standing and walking. He also mentioned bilateral sciatalgia. The neurological examination was normal. Interestingly, we found bilateral lumbar spondylolysis in L3, L4, and L5 levels in imaging studies. After proving that spondylolysis was the source of the low back pain by local anesthetic agent injection, we used a direct technique for correction of spondylolysis by use of a hook-screw device plus decortications of lysis area and iliac crest autograft. We assessed the patient after surgery to evaluate pain recovery and fusion rate. The results were favorable and proved the efficacy of the hook-screw technique for treatment of symptomatic multilevel lumbar spondylolysis.

Newly occurred L4 spondylolysis in the lumbar spine with pre-existence L5 spondylolysis among sports players: case reports and biomechanical analysis

STUDY DESIGN

Case series and a biomechanical study using a finite element (FE) analysis.

OBJECTIVES

To report three cases with multi-level spondylolysis and to understand the mechanism biomechanically.

BACKGROUND

Multi-level spondylolysis is a very rare condition. There have been few reports in the literature on multi-level spondylolysis among sports players.

METHODS

We reviewed three cases of the condition, clinically. These patients were very active young sports players and had newly developed fresh L4 spondylolysis and pre-existing L5 terminal stage spondylolysis. Thus, we assumed that L5 spondylolysis may have increased the pars stress at the cranial adjacent levels, leading to newly developed spondylolysis at these levels. Biomechanically, we investigated pars stress at L4 with or without spondylolysis at L5 using the finite element technique.

RESULTS

L4 pars stress decreased in the presence of L5 spondylolysis, which does not support our first hypothesis.

CONCLUSIONS

It seems that multi-level spondylolysis may occur due to genetic and not biomechanical reasons.

Cervical spondylolysis in a judo player: a case report and biomechanical analysis

STUDY DESIGN

A case report and a biomechanical study using a finite element method.

OBJECTIVES

To report a case with the cervical spondylolysis and to understand the biomechanics of the cervical spine with spondylolysis at C6. Cervical spondylolysis, although not a common spinal disorder, can occur in athletes. Presently, the exact pathology, natural history and biomechanics are not known. Thus, treatment strategies for this disorder in athletes are in controversy. To treat and/or advise patients with cervical spondylolysis, the cervical spine biomechanics regarding this disorder should be understood.

METHODS

A case of a 12-year-old male judo player is presented. The patient presented with occipital and upper neck pain. Plain radiographs, reconstructed CT scan and MRIs of this patient were reviewed. Biomechanically, stress distributions were analyzed in response to 73.6 N axial compression and 1.5-Nm moment in flexion, extension, lateral bending, and axial rotation using a FE model of the intact ligamentous C3 to C7 segment. Bilateral spondylolysis was created in the model at C6. The stress results from the bilateral defect model were compared to the intact model predictions.

RESULTS

Plain radiographs showed bilateral C6 spondylolysis, and grade I spondylolisthesis. MRI showed mild disc degeneration at C6/7. With conservative treatment, the symptoms disappeared. In the spondylolysis model, the maximum Von Mises Stresses at C6/7 increased in all cervical spine motions, as compared to the intact case. Specifically, in axial rotation, the stress increase was 3.7-fold as compared to the intact model. The range of motion at C6/7 increased in the spondylolysis model as well. Again, during axial rotation, the increase in motion was 2.3-fold when compared to the intact model.

CONCLUSIONS

Cervical spondylolysis can cause biomechanical alterations, especially in axial rotation, leading to increased disc stresses and range of motion. The increased stresses in the disc and the hypermobility would be a dangerous condition for athletes participating in contact sports such as judo. Thus, we recommended that judo players with cervical spondylolysis should change to non-contact sports, such as jogging.

Lumbar fusion leads to increases in angular motion and stress across sacroiliac joint: a finite element study

STUDY DESIGN

The assessment of sacrum angular motions and stress across sacroiliac joint (SIJ) articular surfaces using finite element lumbar spine-pelvis model and simulated posterior fusion surgical procedures.

OBJECTIVE

To quantify the increase in sacrum angular motions and stress across SIJ as a function of fused lumbar spine using finite element lumbar spine-pelvis model.

SUMMARY OF BACKGROUND DATA

A review of the literature suggests that for 20% to 30% of spine surgery patients, failed back surgery syndrome as a possible complication. The SIJ might be a contributing factor in failed back surgery syndrome in 29% to 40% of cases. The exact pathomechanism which leads to SIJ pain generation is not well understood. We hypothesized that lumbar spine fusion leads to increased motion or stresses at the SIJ; this alone could be a trigger of the pain syndrome.

METHODS

A finite element model of the lumbar spine-pelvis was used to simulate the posterior fusion at L4-L5, L4-S1, and L5-S1 levels. The magnitude of the sacrum angular motion and average of stresses across SIJ articular surfaces were compared with intact model in flexion, extension, lateral bending, and axial rotation motions.

RESULTS

The computed sacrum angular motions in intact spine, after L4-L5, L5-S1, and L4-S1 fusion gradually increased with maximum value in L4-S1 fusion model. Also, the average stress on SIJ articular surfaces progressively increased from minimum in L4-L5 to maximum in L4-S1 fusion models.

CONCLUSION

The fusion at the lumbar spine level increased motion and stresses at the SIJ. This could be a probable reason for low back pain in patients after lumbar spine fusion procedures.

Minimally invasive direct repair of lumbar spondylolysis with a pedicle screw and hook construct

Object

Lumbar spondylolysis occurs in approximately 6% of the population and presents with localized mechanical back pain, often in young athletes. Surgical treatment may involve decompression, lumbar intersegmental fusion, or direct repair of pars defects. Although such open procedures may effectively resolve symptoms, minimal-access approaches may additionally decrease collateral damage to soft tissues, allowing young, active patients to resume athletic activities sooner. In this study, the authors review their experience repairing bilateral lumbar spondylolyses with screw and hook constructs placed via a minimal-access approach.

Methods

Five consecutive pediatric patients with bilateral L-5 spondylolysis were treated. Bilateral incisions (2.5 cm) were made over L-5. Exposure was maintained with bilateral expandable tubular retractor systems. Pedicle screws were placed in the L-5 pedicles and attached to hooks under the L-5 laminae. A direct repair was performed at the pars defect. Clinical characteristics, operative variables, and postoperative outcomes were collected.

Results

All 5 patients underwent surgery; 4 were male (80%) and 1 was female (20%), and the mean age was 15.8 years (range 15–17 years). The mean estimated blood loss and duration of surgery were 37 ml (range 15–75 ml) and 1.94 hours (range 1–3 hours), respectively. Postoperative hospital stays ranged from 1 to 3 days (mean 1.8 days). The only complication occurred in 1 patient who experienced minor superficial wound breakdown. All patients have experienced resolution of symptoms at this preliminary stage, which has continued over an 8-month follow-up period.

Conclusions

Lumbar spondylolysis can be adequately and safely treated via minimal-access surgical repair of the pars interarticularis by using pedicle screws and rod-hook constructs. This approach may decrease the collateral soft tissue damage common to open dissections, and may be ideal for young, active surgical candidates.

Biomechanical rationale of ossification of the secondary ossification center on apophyseal bony ring fracture: a biomechanical study

BACKGROUND

Apophyseal ring fracture is one of the important pathologies causing low back pain in children and adolescents. Most of the patients are reported to be in the ossification stage of the ring during growth period rather than early cartilaginous ring stage. There is no previous study clarifying the mechanism of the high prevalence of this disorder in the ossification stage. Thus, in this study, we investigated the effects of ossification of the ring on lumbar spine biomechanics.

METHODS

Two three-dimensional finite element pediatric lumbar models were created and analyzed. One model had ossified apophyseal rings and the other one had cartilaginous apophyseal rings. To simulate standing posture, 341N axial compression was applied. Then, 10Nm moment was applied to the model in the six directions of lumbar motion: flexion, extension, lateral bending and axial rotation. Maximum Von Mises stresses in the apophyseal ring were calculated and compared between the two models.

FINDINGS

The maximum stresses were always higher in the bony ring in all lumbar motion at all lumbar levels compared to the cartilaginous ring. The stresses at L4 caudal apophyseal ring in extension were 2.60 and 0.68 (MPa) for bony and cartilaginous rings respectively. In flexion, stresses were 3.95 and 1.49 (MPa), in lateral bending, stresses were 6.75 and 2.66 (MPa), and in axial rotation, stresses were reported to be 3.15 and 1.72 (MPa). Thus, the bony ring was stressed by at least 2-fold more than the cartilaginous ring.

INTERPRETATION

Apophyseal ring has at least two times more stresses in the ossified stage when compared to the cartilaginous stage resulting in frequent fractures at the interface of bone and cartilage.

The effect of removing the lateral part of the pars interarticularis on stress distribution at the neural arch in lumbar foraminal microdecompression at L3-L4 and L4-L5: anatomic and finite element investigations

STUDY DESIGN

The assessment of L3 and L4 pars interarticularis thickness and finite element analysis of stress distribution across L3 and L4 pars interarticularis.

OBJECTIVE

To quantify the morphology of the region of the L3 and L4 pars interarticularis and to assess the stress increase as a function of access size using the finite element lumbar spine model.

SUMMARY OF BACKGROUND DATA

Inadequate decompression and traction of the nerve structures are several causes of the unsatisfactory outcomes in patients after foraminal stenosis decompression and far lateral disc herniation removal by extraforaminal exposure. Enlarging the access of the foraminal exposure by the removal of the lateral aspect of the pars interarticularis may be able to diminish the inadequate decompression and traction of the nerve structures; however, it may lead to increase stress and fracture of the neural arch.

METHODS

We used 15 human cadaver L3 and L4 lumbar vertebrae for measuring the thickness of the pars interarticularis. The ventral and dorsal surfaces were subdivided into 4 equal parts, and the thickness of each part was measured using a digital caliper. An experimentally validated 3-dimensional nonlinear finite element model of the intact L3-S1 segment was used to simulate the lateral removal of one fourth and one half of the L3 and L4 pars interarticularis.

RESULTS

The mean thicknesses of the pars interarticularis showed a gradual increase toward the lateral edge. Finite element model analyses predicted stresses increased to 35% and 40% after removal of one half of the lateral part of the L3 and L4 pars interarticularis, respectively, and were much closer to the intact spine after removal of one fourth of the lateral part of the pars interarticularis.

CONCLUSION

The removal of one fourth of the lateral aspect of the pars interarticularis has minimal influence on the stresses in the remaining L3 and L4 neural arches. The lateral half of the pars has the largest thickness, and its removal leads to considerable stress increases.

Spondylolysis

Low back pain, specifically "spondylo" conditions, has traditionally been misunderstood and often times ill-treated. A thorough understanding of the function of pars interarticularis and its relationship to the entire vertebral unit and low back health are essential for successful treatment and rehabilitation outcomes. Lifestyle awareness and controlled progression through the inflammation, stabilization, strength, and functional rehabilitation phases provide primary guidance for patients. In addition, a broad spectrum of pharmacological, psychological, therapeutic modality, and newer surgical techniques must be considered in the overall treatment plan. Having a strong understanding of the anatomy, biomechanics, treatment, and rehabilitation of this condition, will help the allied healthcare provider better meet the individualized needs of spondylolysis patients.

Residual sagittal motion after lumbar fusion: a finite element analysis with implications on radiographic flexion-extension criteria

STUDY DESIGN

Finite element analysis of a lumbar fusion model.

OBJECTIVES

To quantify residual sagittal angular motion following various types and levels of completeness of lumbar fusion in order to understand better the validity of current recommendations for interpreting flexion-extension radiographs to assess fusion.

SUMMARY OF BACKGROUND DATA

Recommended threshold criteria for solid fusion using flexion-extension radiographs have varied from 0 degrees to 5 degrees of angular motion between vertebrae. Notwithstanding this wide variation and lack of uniform consensus, the validity of these criteria has not been previously biomechanically assessed to the authors' knowledge. To investigate this issue, the authors sought to test various types of simulated healed, noninstrumented lumbar fusions using finite element modeling to determine the amount of residual angular motion under physiologic stresses.

METHODS

A validated 3-dimensional, nonlinear finite element model of an intact adult human L3-L4 motion segment was developed. Four fusion types were simulated using this model, including anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), intertransverse process fusion, and interspinous process fusion. Variations of completeness of fusion were also represented. For ALIF and PLIF, this included tests of solid bridging bone within the posterior or anterior 75%, 50%, or 25% disc space. In addition, PLIF was also tested with either a unilateral or bilateral facetectomy to simulate commonly used surgical techniques. Variations of intertransverse process fusion included unilateral or bilateral bridging bone with or without medial fusion to the pars interarticularis. Only 1 scenario of a healed, solid interspinous process fusion was tested. The intact model and all fusion models were stressed with 10.6-Nm flexion and extension moments. The angular deflections were recorded in degrees.

RESULTS

A wide range of sagittal angular motion was recorded. For ALIF, this ranged from 0.8 degrees (complete, 100% fusion) to 3.3 degrees (solid fusion of the posterior 25% disc space). For PLIF, the numbers were more varied, ranging from 0.7 degrees (complete, 100% fusion) to 6.9 degrees (solid fusion of posterior 25% disc space with bilateral facetectomy). For intertransverse process fusion, the least motion was with a solid bilateral fusion, with medial healing to the pars (2.0 degrees); the greatest motion was found with a solid unilateral fusion without medial healing (6.0 degrees). Interspinous process fusion allowed only 1.9 degrees of motion.

CONCLUSIONS

The amount of residual flexion-extension motion with simulated lumbar fusions (presumably allowed by the bone's inherent elasticity) under physiologically comparable moments varies with fusion type and, more substantially, with varying amounts of completeness. The current study documents a range of sagittal angular motion after several types of simulated lumbar fusion that appear to have considerable overlap with previously purported radiographic criteria for solid fusion using flexion-extension radiographs. However, it also suggests the possibility that some scenarios of solid, yet incomplete, fusion may allow motion that is substantially greater than 5 degrees, which is beyond the most liberal of previously published threshold criteria.

Buck's direct repair of lumbar spondylolysis restores disc stresses at the involved and adjacent levels

BACKGROUND

Lumbar spondylolysis was reported to cause disc degeneration at both caudal and cranial adjacent level. However, basic disc biomechanics in the spondylolytic spine is not fully understood. Purpose of this study was to analyze the disc stresses at cranial and caudal adjacent level of lumbar spondylolysis. Also, the biomechanical effects of Buck's technique on disc stresses at these two segments were evaluated.

METHODS

An experimentally validated three-dimensional non-linear finite element model of the intact ligamentous L3-S1 segment was used. Bilateral lumbar spondylolysis was simulated by creating bilateral pars defects with 1.0 mm gap at L5. Buck's direct repair model was simulated with 4.0 mm cannulated Titanium screws, placed bilaterally across the defect. Von Mises stresses in the annulus fibrosus and nucleus pulposus at L4/5 (cranial adjacent) and L5/S (caudal adjacent) disc levels were analyzed in flexion, extension, lateral bending and axial rotation in response to 400 N of axial compression and 10.6 Nm moment. The highest values were compared among the three models, intact, spondylolysis and Buck's technique.

FINDINGS

After spondylolysis occurred at L5, annulus fibrosus and nucleus pulposus stresses at L4/5 increased to 111% and 120%, respectively. After the Buck's technique it recovered to 102% and 105%, correspondingly. On the other hand, at L5/S, annulus fibrosus stress increased to 168%, and nucleus pulposus, 155%, which was much higher when compared to the stresses at L4/5. After the Buck's technique the stresses were decreased to 125% and 120%, correspondingly. During rotation motion, especially, the operation normalized the disc stress completely.

INTERPRETATION

Spondylolysis increases disc stresses at the affected as well as cranial adjacent level, and it may lead to disc degeneration at both levels. However, the increase in stresses is higher at the affected caudal level, when compared to the cranial level. Buck's technique may restore the disc stresses back to normal at both disc levels. Thus, this technique may be beneficial from a biomechanical perspective as well.

Pathomechanism of ligamentum flavum hypertrophy: a multidisciplinary investigation based on clinical, biomechanical, histologic, and biologic assessments

STUDY DESIGN

A multidisciplinary study involving clinical, histologic, biomechanical, biologic, and immunohistologic approaches. OBJECTIVE.: To clarify the pathomechanism of hypertrophy of the ligamentum flavum.

SUMMARY OF BACKGROUND DATA

The most common spinal disorder in elderly patients is lumbar spinal canal stenosis, causing low back and leg pain, and paresis. Canal narrowing, in part, results from hypertrophy of the ligamentum flavum. Although histologic and biologic literature on this topic is available, the pathomechanism of ligamentum flavum hypertrophy is still unknown.

METHODS

The thickness of 308 ligamenta flava at L2/3, L3/4, L4/5, and L5/S1 levels of 77 patients was measured using magnetic resonance imaging. The relationships between thickness, age, and level were evaluated. Histologic evaluation was performed on 20 ligamentum flavum samples, which were collected during surgery. Trichrome and Verhoeff-van Gieson elastic stains were performed for each ligamentum flavum to understand the degree of fibrosis and elastic fiber status, respectively. To understand the mechanical stresses in various layers of ligamentum flavum, a 3-dimensional finite element model was used. Von Mises stresses were computed, and values between dural and dorsal layers were compared. There were 10 ligamenta flava collected for biologic assessment. Using real-time reverse transcriptase polymerase chain reaction, transforming growth factor (TGF)-beta messenger ribonucleic acid expression was quantitatively measured. The cellular location of TGF-beta was also confirmed from 18 ligamenta flava using immunohistologic techniques.

RESULTS

The ligamentum flavum thickness increased with age, however, the increment at L4/5 and L3/4 levels was larger than at L2/3 and L5/S1 levels. Histology showed that as the ligamentum flavum thickness increased, fibrosis increased and elastic fibers decreased. This tendency was more predominant along the dorsal side. Von Misses stresses revealed that the dorsal fibers of ligamentum flavum were subjected to higher stress than the dural fibers. This was most remarkably observed at L4/5. The largest increase in ratio observed between the dorsal and dural layer was approximately 5-fold in flexion at L4/5 in flexion. Expression of TGF-beta was observed in all ligamenta flava, however, the expression decreased as the ligamentum flavum thickness increased. Immunohistochemistry showed that TGF-beta was released by the endothelial cells, not by fibroblasts.

CONCLUSIONS

Fibrosis is the main cause of ligamentum flavum hypertrophy, and fibrosis is caused by the accumulation of mechanical stress with the aging process, especially along the dorsal aspect of the ligamentum flavum. TGF-beta released by the endothelial cells may stimulate fibrosis, especially during the early phase of hypertrophy.

Biomechanical comparison of lumbar spine with or without spina bifida occulta. A finite element analysis

STUDY DESIGN

Biomechanical study using finite element model (FEM) of lumbar spine.

OBJECTIVES

Very high coincidence of spina bifida occulta (SBO) has been reported more than in 60% of lumbar spondylolysis. The altered biomechanics due to SBO is one considerable factor for this coincidence. Thus, in this study, the biomechanical changes in the lumbar spine due to the presence of SBO were evaluated.

SETTING

United States of America (USA).

METHODS

An experimentally validated three-dimensional nonlinear FEM of the intact ligamentous L3-S1 segment was used and modified to simulate two kinds of SBO at L5. One model had SBO with no change in the length of the spinous process and the other had a small dysplastic spinous process. Von Mises stresses at pars interarticularis were analyzed in the six degrees of lumbar motion with 400 N axial compression, which simulates the standing position. The range of motion at L4/5 and L5/S1 were also calculated.

RESULTS

It was observed that the stresses in all the models were similar, and there was no change in the highest stress value when compared to the intact model. The range of motion was also similar in all the models. The lumbar kinematics of SBO was thus shown to be similar to the intact model.

CONCLUSION

SBO does not alter lumbar biomechanics with respect to stress and range of motion. The high coincidence of spondylolysis in spines with SBO may not be due to the mechanical factors.