Removal of fixation construct could mitigate adjacent segment stress after lumbosacral fusion: A finite element analysis

Intraoperative capsule protection can reduce the potential risk of adjacent segment degeneration acceleration biomechanically: an in silico study

BACKGROUND

The capsule of the zygapophyseal joint plays an important role in motion segmental stability maintenance. Iatrogenic capsule injury is a common phenomenon in posterior approach lumbar interbody fusion operations, but whether this procedure will cause a higher risk of adjacent segment degeneration acceleration biomechanically has yet to be identified.

METHODS

Posterior lumbar interbody fusion (PLIF) with different grades of iatrogenic capsule injury was simulated in our calibrated and validated numerical model. By adjusting the cross-sectional area of the capsule, different grades of capsule injury were simulated. The stress distribution on the cranial motion segment was computed under different loading conditions to judge the potential risk of adjacent segment degeneration acceleration.

RESULTS

Compared to the PLIF model with an intact capsule, a stepwise increase in the stress value on the cranial motion segment can be observed with a step decrease in capsule cross-sectional areas. Moreover, compared to the difference between models with intact and slightly injured capsules, the difference in stress values was more evident between models with slight and severe iatrogenic capsule injury.

CONCLUSION

Intraoperative capsule protection can reduce the potential risk of adjacent segment degeneration acceleration biomechanically, and iatrogenic capsule damage on the cranial motion segment should be reduced to optimize patients' long-term prognosis.

A Rapid and Safe Minimally Invasive Procedure for Percutaneous Pedicle Screw Removal: A Case-Control Study and Technical Description

Purpose

Percutaneous pedicle screw fixation is a common minimally invasive treatment for traumatic thoracolumbar and lumbar fractures; however, research on hardware removal after successful healing is limited. We aimed to introduce a rapid, safe, minimally invasive, and cost-effective method for percutaneous pedicle screw removal.

Patients and Methods

We conducted a retrospective analysis of demographic (age, sex, body mass index, alcohol use, and current smoking), clinical (hypertension and diabetes mellitus), surgical (affected levels, number of screws, time of surgery, and blood loss), and treatment cost characteristics of 92 patients who had undergone percutaneous pedicle screw removal between May 2016 and February 2023. The first 57 patients underwent the conventional method, and the remaining 35 underwent the modified method. Independent-sample t-tests and chi-square tests were used to compare continuous and categorical variables, respectively, between the two groups.

Results

No significant differences were observed in the demographic parameters, complications, or affected levels between the groups. However, the average surgical time (P=0.000) was significantly shorter, and the average blood loss volume (P=0.002) and total cost (P=0.000) were significantly lower in the modified group than in the conventional group.

Conclusion

Compared with the conventional method, our modified method can shorten the surgical time, reduce blood loss, and reduce the total cost of treatment. It is a quick and safe minimally invasive method that does not require additional surgical instruments and is suitable for implementation in primary hospitals.

Risk factors for mechanical complications after fusion extension surgery for lumbar adjacent segment disease

PURPOSE

Adjacent segment disease (ASD) is a common complication in fusion surgery. In the event of solid segmental fusion, previous implants can be removed or preserved during fusion extension for ASD. To compare the surgical outcomes of patients with and without implants and analyzes the risk factors for postoperative mechanical complications.

METHODS

Patients who underwent fusion extension for lumbar ASD from 2011 to 2019 with a minimum 2 year follow-up were retrospectively reviewed. Spinopelvic parameters were measured preoperatively and postoperatively. Clinical outcomes and surgical complications were compared between groups with implants preserved and removed. Risk factors for mechanical complications, including clinical, surgical, and radiographic factors were analyzed.

RESULTS

Sixty-nine patients (mean age, 69.9 ± 6.9 years) were included. The mean numbers of initial and extended fused segments were 2.8 ± 0.7 and 2.7 ± 0.7, respectively. Previous implants were removed in 43 patients (R group) and preserved in 26 patients (P group). Both groups showed an improvement in clinical outcomes without between-group differences. The operation time was significantly longer in R group (260 vs 207 min, p < 0.001). Mechanical complications occurred in 13 patients (12 in R group and 1 in P group) and reoperation was needed in 3 patients (R group). Implant removal, index fusion surgery including L5-S1, and postoperative sagittal malalignment were risk factors for mechanical complications.

CONCLUSION

Implant removal was a risk factor for mechanical complications. Index fusion surgery including L5-S1 and postoperative sagittal malalignment were also risk factors for mechanical complications.

Annulus Calibration Increases the Computational Accuracy of the Lumbar Finite Element Model

STUDY DESIGN

Mechanical simulations.

OBJECTIVE

Inadequate calibration of annuli negatively affects the computational accuracy of finite element (FE) models. Specifically, the definition of annulus average radius (AR) does not have uniformity standards. Differences between the elastic moduli in the different layers and parts of the annulus were not fully calibrated when a linear elastic material is used to define its material properties. This study aims to optimize the computational accuracy of the FE model by calibrating the annulus.

METHODS

We calibrated the annulus AR and elastic modulus in our anterior-constructed lumbar model by eliminating the difference between the computed range of motion and that measured by in vitro studies under a flexion-extension loading condition. Multi-indicator validation was performed by comparing the computed indicators with those measured in in vitro studies. The computation time required for the different models has also been recorded to evaluate the computational efficiency.

RESULTS

The difference between computed and measured ROMs was less than 1% when the annulus AR and elastic modulus were calibrated. In the model validation process, all the indicators computed by the calibrated FE model were within ±1 standard deviation of the average values obtained from in vitro studies. The maximum difference between the computed and measured values was less than 10% under nearly all loading conditions. There is no apparent variation tendency for the computational time associated with different models.

CONCLUSION

The FE model with calibrated annulus AR and regional elastic modulus has higher computational accuracy and can be used in subsequent mechanical studies.

Fixation-induced surgical segment's high stiffness and the damage of posterior structures together trigger a higher risk of adjacent segment disease in patients with lumbar interbody fusion operations

BACKGROUND

Adjacent segment disease (ASD) is a commonly reported complication after lumbar interbody fusion (LIF); changes in the mechanical environment play an essential role in the generation of ASD. Traditionally, fixation-induced high stiffness in the surgical segment was the main reason for ASD. However, with more attention paid to the biomechanical significance of posterior bony and soft structures, surgeons hypothesize that this factor may also play an important role in ASD.

METHODS

Oblique and posterior LIF operations have been simulated in this study. The stand-alone OLIF and OLIF fixed by bilateral pedicle screw (BPS) system have been simulated. The spinal process (the attachment point of cranial ligamentum complex) was excised in the PLIF model; the BPS system has also been used in the PLIF model. Stress values related to ASD have been computed under physiological body positions, including flexion, extension, bending, and axial rotations.

RESULTS

Compared to the stand-alone OLIF model, the OLIF model with BPS fixation suffers higher stress values under extension body position. However, there are no apparent differences under other loading conditions. Moreover, significant increases in stress values can be recorded in flexion and extension loading conditions in the PLIF model with posterior structures damage.

CONCLUSIONS

Fixation-induced surgical segment's high stiffness and the damage of posterior soft tissues together trigger a higher risk of ASD in patients with LIF operations. Optimizing BPS fixation methods and pedicle screw designs and reducing the range of posterior structures excision may be an effective method to reduce the risk of ASD.

Will the adjustment of insertional pedicle screw positions affect the risk of adjacent segment diseases biomechanically? An in-silico study

Background

The fixation-induced biomechanical deterioration will increase the risk of adjacent segment diseases (ASD) after lumbar interbody fusion with Bilateral pedicle screw (BPS) fixation. The accurate adjustment of insertional pedicle screw positions is possible, and published studies have reported its mechanical effects. However, no studies clarified that adjusting insertional screw positions would affect the postoperative biomechanical environment and the risk of ASD. The objective of this study was to identify this issue and provide theoretical references for the optimization of insertional pedicle screw position selections.

Methods

The oblique lumbar interbody fusion fixed by BPS with different insertional positions has been simulated in the L4-L5 segment of our previously constructed and validated lumbosacral model. Biomechanical indicators related to ASD have been computed and recorded under flexion, extension, bending, and axial rotation loading conditions.

Results

The change of screw insertional positions has more apparent biomechanical effects on the cranial than the caudal segment. Positive collections can be observed between the reduction of the fixation length and the alleviation of motility compensation and stress concentration on facet cartilages. By contrast, no pronounced tendency of stress distribution on the intervertebral discs can be observed with the change of screw positions.

Conclusions

Reducing the fixation stiffness by adjusting the insertional screw positions could alleviate the biomechanical deterioration and be an effective method to reduce the risk of ASD caused by BPS.

Recent advancement in finite element analysis of spinal interbody cages: A review

Finite element analysis (FEA) is a widely used tool in a variety of industries and research endeavors. With its application to spine biomechanics, FEA has contributed to a better understanding of the spine, its components, and its behavior in physiological and pathological conditions, as well as assisting in the design and application of spinal instrumentation, particularly spinal interbody cages (ICs). IC is a highly effective instrumentation for achieving spinal fusion that has been used to treat a variety of spinal disorders, including degenerative disc disease, trauma, tumor reconstruction, and scoliosis. The application of FEA lets new designs be thoroughly "tested" before a cage is even manufactured, allowing bio-mechanical responses and spinal fusion processes that cannot easily be experimented upon in vivo to be examined and "diagnosis" to be performed, which is an important addition to clinical and in vitro experimental studies. This paper reviews the recent progress of FEA in spinal ICs over the last six years. It demonstrates how modeling can aid in evaluating the biomechanical response of cage materials, cage design, and fixation devices, understanding bone formation mechanisms, comparing the benefits of various fusion techniques, and investigating the impact of pathological structures. It also summarizes the various limitations brought about by modeling simplification and looks forward to the significant advancement of spine FEA research as computing efficiency and software capabilities increase. In conclusion, in such a fast-paced field, the FEA is critical for spinal IC studies. It helps in quantitatively and visually demonstrating the cage characteristics after implanting, lowering surgeons' learning costs for new cage products, and probably assisting them in determining the best IC for patients.

The Biomechanical Effects of Different Bag-Carrying Styles on Lumbar Spine and Paraspinal Muscles: A Combined Musculoskeletal and Finite Element Study

OBJECTIVES

Bags such as handbags, shoulder bags, and backpacks are commonly used. However, it is difficult to assess the biomechanical effects of bag-carrying styles on the lumbar spine and paraspinal muscles using traditional methods. This study aimed to evaluate the biomechanical effects of bag-carrying styles on the lumbar spine.

METHODS

We developed a hybrid model that combined a finite element (FE) model of the lumbar spine and musculoskeletal models of three bag-carrying styles. The image data was collected from a 26-years-old, 176 cm and 70 kg volunteer. OpenSim and ABAQUS were used to do the musculoskeletal analysis and finite analysis. Paraspinal muscle force, intervertebral compressive force (ICF), and intervertebral shear force (ISF) on L1 were calculated and loaded into the FE model to assess the stress distribution on the lumbar spine.

RESULTS

Different paraspinal muscle activation occurred in the three bag-carrying models. The increase in the ICF generated by all three bags was greater than the bags' weights. The handbag produced greater muscle force, ICF, ISF, and peak stress on the nucleus pulposus than the backpack and shoulder bag of the same weight. Peak stress on the intervertebral discs in the backpack model and the L1-L4 segments of the shoulder bag model increased linearly with bag weight, and increased exponentially with bag weight in the handbag model.

CONCLUSION

Unbalanced bag-carrying styles (shoulder bags and handbags) led to greater muscle force, which generated greater ICF, ISF, and peak stress on the lumbar spine. The backpack produced the least burden on the lumbar spine and paraspinal muscles. Heavy handbags should be used carefully in daily life.

Effect of sagittal screw angle and distance of screw apex to superior endplate on adjacent segment disease after posterolateral lumbar fusion: a retrospective study

Background

Numerous complications of lumbar fusion surgery have been reported, with adjacent segment disease (ASD) being one of the most important. Few studies describe the effect of sagittal, horizontal screw angles and distance of pedicle screw apex to superior endplate on the incidence of ASD in lumbar spine. The purpose of this retrospective study is to evaluate the hypothesis that unsatisfactory pedicle screw insertion positions would increase the likelihood of ASD.

Methods

Outpatients with lumbar spinal stenosis underwent posterolateral lumbar fusion at L4-S1 with a least 2-year follow-up were studied. ASD at L3–L4 was defined as a condition in which intervertebral disk narrowing, posterior vertebral opening, and vertebral slippage progress at the last follow-up in comparison with the postoperative. Independent t test was performed to compare data between two groups; Spearman analysis was performed to analyze the relationship between two continuous variables. Multivariate binary logistic models were performed to identify the independent risk factors of ASD. The receiver operating characteristic (ROC) curve was performed to measure model discrimination and Hosmer–Lemeshow (H–L) test was used to measure calibration. ROC curve evaluated the discrimination ability of sagittal screw angle and distance in predicting incidence of ASD.

Results

Patients in ASD group exhibit higher incidence of osteoporosis, higher Visual analogue scale (VAS), Oswestry disability index (ODI), bigger sagittal screw angle, shorter distance of pedicle screw apex to superior endplate than those in non-ASD group (p < 0.05). VAS, ODI at the last follow-up were positively correlated with Pfirrmann grade of L3–4 disk and sagittal screw angle, while negatively correlated with distance of screw apex to superior endplate (p < 0.05). Multivariate binary logistic model indicated that follow-up time (odds ratio [OR] 1.637, 95% confidence interval [CI] 1.186–2.260), distance of screw apex to superior endplate (OR 0.150, 95% CI 0.067–0.336), sagittal screw angle (OR 2.404, 95% CI 1.608–3.594) were statistically significant. The models showed great discrimination and calibration. The area under the curve of ASD identified by sagittal angle and distance was 0.895 and the cut-off values were 5.500° and 6.250 mm, respectively.

Conclusion

Sagittal screw angle and distance of screw apex to superior endplate were significantly associated with the risk of ASD.

Influence of the facet joint angle on facet joint degeneration following pedicle screw fixation without fusion in thoracolumbar fractures

BACKGROUND

Posterior approach pedicle screw fixation without fusion is widely used in the treatment of neurologically intact type A3 thoracolumbar fractures.

OBJECTIVE

To analyze the influence of the facet joint (FJ) angle on FJ degeneration following posterior approach pedicle screw fixation without fusion in neurologically intact type A3 thoracolumbar fractures.

METHODS

Fifty-eight patients who underwent pedicle screw fixation via the traditional posterior approach (n= 28) or the Wiltse approach (n= 30) were enrolled. A CT scan was performed before fixation and before fixation removal (Within 1.5 to 2 years after fixation) to evaluate the FJs parameters, including FJ inclination (FJI), FJ tropism (FJT), FJ violation, and FJ degeneration grade (FJDG), of three fixed segments and the adjacent segment below the fixed segments.

RESULTS

There was no significant difference in FJ violation rate, FJDG deterioration, or FJ angle between the two groups (P> 0.05). FJDG deterioration showed a weak positive correlation with FJI and FJT before fixation, and the angular change in FJI (P< 0.05); and FJT before fixation and the angular change in FJI were risk factors for FJDG deterioration (P< 0.01).

CONCLUSION

The Wiltse approach did not increase the rate of FJDG deterioration and FJs angle changes. However, the FJT before fixation and the angular change in FJI were risk factors for FJDG deterioration.

Poor bone mineral density aggravates adjacent segment's motility compensation in patients with oblique lumbar interbody fusion with and without pedicle screw fixation: An in silico study

Objective

Motility compensation increases the risk of adjacent segment diseases (ASDs). Previous studies have demonstrated that patients with ASD have a poor bone mineral density (BMD), and changes in BMD affect the biomechanical environment of bones and tissues, possibly leading to an increase in ASD incidence. However, whether poor BMD increases the risk of ASD by aggravating the motility compensation of the adjacent segment remains unclear. The present study aimed to clarify this relationship in oblique lumbar interbody fusion (OLIF) models with different BMDs and additional fixation methods.

Methods

Stand-alone (S-A) OLIF and OLIF fixed with bilateral pedicle screws (BPS) were simulated in the L4–L5 segment of our well-validated lumbosacral model. Range of motions (ROMs) and stiffness in the surgical segment and at the cranial and caudal sides’ adjacent segments were computed under flexion, extension, and unilateral bending and axial rotation loading conditions.

Results

Under most loading conditions, the motility compensation of both cranial and caudal segments adjacent to the OLIF segment steeply aggravated with BMD reduction in S-A and BPS OLIF models. More severe motility compensation of the adjacent segment was observed in BPS models than in S-A models. Correspondingly, the surgical segment's stiffness of S-A models was apparently lower than that of BPS models (S-A models showed higher ROMs and lower stiffness in the surgical segment).

Conclusion

Poor BMD aggravates the motility compensation of adjacent segments after both S-A OLIF and OLIF with BPS fixation. This variation may cause a higher risk of ASD in OLIF patients with poor BMD. S-A OLIF cannot provide instant postoperative stability; therefore, the daily motions of patients with S-A OLIF should be restricted before ideal interbody fusion to avoid surgical segment complications.

Deterioration of the fixation segment's stress distribution and the strength reduction of screw holding position together cause screw loosening in ALSR fixed OLIF patients with poor BMD

The vertebral body's Hounsfield unit (HU) value can credibly reflect patients' bone mineral density (BMD). Given that poor bone-screw integration initially triggers screw loosening and regional differences in BMD and strength in the vertebral body exist, HU in screw holding planes should better predict screw loosening. According to the stress shielding effect, the stress distribution changes in the fixation segment with BMD reduction should be related to screw loosening, but this has not been identified. We retrospectively collected the radiographic and demographic data of 56 patients treated by single-level oblique lumbar interbody fusion (OLIF) with anterior lateral single rod (ALSR) screw fixation. BMD was identified by measuring HU values in vertebral bodies and screw holding planes. Regression analyses identified independent risk factors for cranial and caudal screw loosening separately. Meanwhile, OLIF with ALSR fixation was numerically simulated; the elastic modulus of bony structures was adjusted to simulate different grades of BMD reduction. Stress distribution changes were judged by computing stress distribution in screws, bone-screw interfaces, and cancellous bones in the fixation segment. The results showed that HU reduction in vertebral bodies and screw holding planes were independent risk factors for screw loosening. The predictive performance of screw holding plane HU is better than the mean HU of vertebral bodies. Cranial screws suffer a higher risk of screw loosening, but HU was not significantly different between cranial and caudal sides. The poor BMD led to stress concentrations on both the screw and bone-screw interfaces. Biomechanical deterioration was more severe in the cranial screws than in the caudal screws. Additionally, lower stress can also be observed in fixation segments' cancellous bone. Therefore, a higher proportion of ALSR load transmission triggers stress concentration on the screw and bone-screw interfaces in patients with poor BMD. This, together with decreased bony strength in the screw holding position, contributes to screw loosening in osteoporotic patients biomechanically. The trajectory optimization of ALSR screws based on preoperative HU measurement and regular anti-osteoporosis therapy may effectively reduce the risk of screw loosening.

Novel force-displacement control passive finite element models of the spine to simulate intact and pathological conditions; comparisons with traditional passive and detailed musculoskeletal models

Passive finite element (FE) models of the spine are commonly used to simulate intact and various pre- and postoperative pathological conditions. Being devoid of muscles, these traditional models are driven by simplistic loading scenarios, e.g., a constant moment and compressive follower load (FL) that do not properly mimic the complex in vivo loading condition under muscle exertions. We aim to develop novel passive FE models that are driven by more realistic yet simple loading scenarios, i.e., in vivo vertebral rotations and pathological-condition dependent FLs (estimated based on detailed musculoskeletal finite element (MS-FE) models). In these novel force-displacement control FE models, unlike the traditional passive FE models, FLs vary not only at different spine segments (T12-S1) but between intact, pre- and postoperative conditions. Intact, preoperative degenerated, and postoperative fused conditions at the L4-L5 segment for five static in vivo activities in upright and flexed postures were simulated by the traditional passive FE, novel force-displacement control FE, and gold-standard detailed MS-FE spine models. Our findings indicate that, when compared to the MS-FE models, the force-displacement control passive FE models could accurately predict the magnitude of disc compression force, intradiscal pressure, annulus maximal von Mises stress, and vector sum of all ligament forces at adjacent segments (L3-L4 and L5-S1) but failed to predict disc shear and facet joint forces. In this regard, the force-displacement control passive FE models were much more accurate than the traditional passive FE models. Clinical recommendations made based on traditional passive FE models should, therefore, be interpreted with caution.

Metallic Implants Used in Lumbar Interbody Fusion

Over the last decade, pedicle fixation systems have evolved and modifications in spinal fusion techniques have been developed to increase fusion rates and improve clinical outcomes after lumbar interbody fusion (LIF). Regarding materials used for screw and rod manufacturing, metals, especially titanium alloys, are the most popular resources. In the case of pedicle screws, that biomaterial can be also doped with hydroxyapatite, CaP, ECM, or tantalum. Other materials used for rod fabrication include cobalt-chromium alloys and nitinol (nickel-titanium alloy). In terms of mechanical properties, the ideal implant used in LIF should have high tensile and fatigue strength, Young's modulus similar to that of the bone, and should be 100% resistant to corrosion to avoid mechanical failures. On the other hand, a comprehensive understanding of cellular and molecular pathways is essential to identify preferable characteristics of implanted biomaterial to obtain fusion and avoid implant loosening. Implanted material elicits a biological response driven by immune cells at the site of insertion. These reactions are subdivided into innate (primary cellular response with no previous exposure) and adaptive (a specific type of reaction induced after earlier exposure to the antigen) and are responsible for wound healing, fusion, and also adverse reactions, i.e., hypersensitivity. The main purposes of this literature review are to summarize the physical and mechanical properties of metal alloys used for spinal instrumentation in LIF which include fatigue strength, Young's modulus, and corrosion resistance. Moreover, we also focused on describing biological response after their implantation into the human body. Our review paper is mainly focused on titanium, cobalt-chromium, nickel-titanium (nitinol), and stainless steel alloys.

Coordination Between Trunk Muscles, Thoracolumbar Fascia, and Intra-Abdominal Pressure Toward Static Spine Stability

STUDY DESIGN

Numerical in-silico human spine stability finite element analysis.

OBJECTIVE

The purpose of this study was to investigate the contribution of major torso tissues toward static spine stability, mainly the thoracolumbar fascia (TLF), abdominal wall with its intra-abdominal pressure (IAP), and spinal muscles inclusive of their intramuscular pressure.

SUMMARY OF BACKGROUND DATA

Given the numerous redundancies involved in the spine, current methodologies for assessing static spinal stability are limited to specific tissues and could lead to inconclusive results. A three-dimensional finite element model of the spine, with structured analysis of major torso tissues, allows for objective investigation of static spine stability.

METHODS

A novel previously fully validated spine model was employed. Major torso tissues, mainly the muscles, TLF, and IAP were individually, and in combinations, activated under a 350N external spine perturbation. The stability contribution exerted by these tissues, or their ability to restore the spine to the unperturbed position, was assessed in different case-scenarios.

RESULTS

Individual activations recorded significantly different stability contributions, with the highest being the TLF at 75%. Combined or synergistic activations showed an increase of up to 93% stability contribution when all tissues were simultaneously activated with a corresponding decrease in the tensile load exerted by the tissues themselves.

CONCLUSION

This investigation demonstrated torso tissues exhibiting different roles toward static spine stability. The TLF appeared able to dissipate and absorb excessive loads, the muscles acted as antagonistic to external perturbations, and the IAP played a role limiting movement. Furthermore, the different combinations explored suggested an optimized engagement and coordination between different tissues to achieve a specific task, while minimizing individual work.Level of Evidence: N/A.

The Mismatch Between Bony Endplates and Grafted Bone Increases Screw Loosening Risk for OLIF Patients With ALSR Fixation Biomechanically

The mismatch between bony endplates (BEPs) and grafted bone (GB) triggers several complications biomechanically. However, no published study has identified whether this factor increases the risk of screw loosening by deteriorating the local stress levels. This study aimed to illustrate the biomechanical effects of the mismatch between BEP and GB and the related risk of screw loosening. In this study, radiographic and demographic data of 56 patients treated by single segment oblique lumbar interbody fusion (OLIF) with anterior lateral single rod (ALSR) fixation were collected retrospectively, and the match sufficiency between BEP and GB was measured and presented as the grafted bony occupancy rate (GBOR). Data in patients with and without screw loosening were compared; regression analyses identified independent risk factors. OLIF with different GBORs was simulated in a previously constructed and validated lumbosacral model, and biomechanical indicators related to screw loosening were computed in surgical models. The radiographic review and numerical simulations showed that the coronal plane’s GBOR was significantly lower in screw loosening patients both in the cranial and caudal vertebral bodies; the decrease in the coronal plane’s GBOR has been proven to be an independent risk factor for screw loosening. In addition, numerical mechanical simulations showed that the poor match between BEP and GB will lead to stress concentration on both screws and bone-screw interfaces. Therefore, we can conclude that the mismatch between the BEP and GB will increase the risk of screw loosening by deteriorating local stress levels, and the increase in the GBOR by modifying the OLIF cage’s design may be an effective method to optimize the patient’s prognosis.

Biomechanical Effects of Pedicle Screw Positioning on the Surgical Segment in Models After Oblique Lumbar Interbody Fusion: An in-silico Study

Background

Bilateral pedicle screw (BPS) is the “gold standard” of fixation methods for patients with lumbar interbody fusion. Biomechanical deterioration initially triggers complications in the surgical segment. Studies proved that BPS positions and trajectory changes affect the local biomechanical environment. However, no study illustrates the biomechanical effect of insertional screw positions’ change on the surgical segment.

Methods

Oblique lumbar interbody fusion (OLIF) with different BPS insertional positions has been simulated in a well-validated lumbo-sacral model. Fixation stability and stress responses on the surgical segment were evaluated under identical loading conditions.

Results

There is no clear variation tendency for the risk of BPS failure and the change of strain energy density of the grafted bone. However, shifting the insertional screw position close to the surgical segment will increase the range of motions (ROM) in the surgical segment and lead to stress concentration of bony structures, especially in the caudal side of the surgical segment.

Conclusion

Adjusting the insertional position of BPS close to the surgical segment in OLIF models will lead to stress concentration of bony structures and surgical segmental instability. Therefore, reducing BPS’s fixation length was not recommended, which may increase the risk of segmental instability, non-union, and cage subsidence.

A Dynamic Interbody Cage Improves Bone Formation in Anterior Cervical Surgery: A Porcine Biomechanical Study

BACKGROUND

Anterior cervical discectomy and fusion (ACDF) with a rigid interbody spacer is commonly used in the treatment of cervical degenerative disc disease. Although ACDF relieves clinical symptoms, it is associated with several complications such as pseudoarthrosis and adjacent segment degeneration. The concept of dynamic fusion has been proposed to enhance fusion and reduce implant subsidence rate and post-fusion stiffness; this pilot preclinical animal study was conducted to begin to compare rigid and dynamic fusion in ACDF.

QUESTIONS/PURPOSES

Using a pig model, we asked, is there (1) decreased subsidence, (2) reduced axial stiffness in compression, and (3) improved likelihood of bone growth with a dynamic interbody cage compared with a rigid interbody cage in ACDF?

METHODS

ACDF was performed at two levels, C3/4 and C5/6, in 10 pigs weighing 48 to 55 kg at the age of 14 to 18 months (the pigs were skeletally mature). One level was implanted with a conventional rigid interbody cage, and the other level was implanted with a dynamic interbody cage. The conventional rigid interbody cage was implanted in the upper level in the first five pigs and in the lower level in the next five pigs. Both types of interbody cages were implanted with artificial hydroxyapatite and tricalcium phosphate bone grafts. To assess subsidence, we took radiographs at 0, 7, and 14 weeks postoperatively. Subsidence less than 10% of the disc height was considered as no radiologic abnormality. The animals were euthanized at 14 weeks, and each operated-on motion segment was harvested. Five specimens from each group were biomechanically tested under axial compression loading to determine stiffness. The other five specimens from each group were used for microCT evaluation of bone ingrowth and ongrowth and histologic investigation of bone formation. Sample size was determined based on 80% power and an α of 0.05 to detect a between-group difference of successful bone formation of 15%.

RESULTS

With the numbers available, there was no difference in subsidence between the two groups. Seven of 10 operated-on levels with rigid cages had subsidence on a follow-up radiograph at 14 weeks, and subsidence occurred in two of 10 operated-on levels with dynamic cages (Fisher exact test; p = 0.07). The stiffness of the unimplanted rigid interbody cages was higher than the unimplanted dynamic interbody cages. After harvesting, the median (range) stiffness of the motion segments fused with dynamic interbody cages (531 N/mm [372 to 802]) was less than that of motion segments fused with rigid interbody cages (1042 N/mm [905 to 1249]; p = 0.002). Via microCT, we observed bone trabecular formation in both groups. The median (range) proportions of specimens showing bone ongrowth (88% [85% to 92%]) and bone volume fraction (87% [72% to 100%]) were higher in the dynamic interbody cage group than bone ongrowth (79% [71% to 81%]; p < 0.001) and bone volume fraction (66% [51% to 78%]; p < 0.001) in the rigid interbody cage group. The percentage of the cage with bone ingrowth was higher in the dynamic interbody cage group (74% [64% to 90%]) than in the rigid interbody cage group (56% [32% to 63%]; p < 0.001), and the residual bone graft percentage was lower (6% [5% to 8%] versus 13% [10% to 20%]; p < 0.001). In the dynamic interbody cage group, more bone formation was qualitatively observed inside the cages than in the rigid interbody cage group, with a smaller area of fibrotic tissue under histologic investigation.

CONCLUSION

The dynamic interbody cage provided satisfactory stabilization and percentage of bone ongrowth in this in vivo model of ACDF in pigs, with lower stiffness after bone ongrowth and no difference in subsidence.

CLINICAL RELEVANCE

The dynamic interbody cage appears to be worthy of further investigation. An animal study with larger numbers, with longer observation time, with multilevel surgery, and perhaps in the lumbar spine should be considered.

Negative biomechanical effects of large grade nuclectomy in the transforaminal endoscopic discectomy increased the risk of adjacent segment diseases: A finite element study

PURPOSE

The protection of articular processes (AP) in the transforaminal endoscopic discectomy (TED) was proven to optimise post-operative biomechanical environments. Published studies reported a large grade of nuclectomy leading to poor prognosis, but the underlying biomechanical mechanism was unclearly illustrated. This study aimed to investigate the changes of biomechanical environments after an in-out TED with intact AP and a large grade of nuclectomy and an out-in TED with limited foraminoplasty and a smaller grade of nuclectomy.

METHODS

A previously constructed and validated lumbo-sacral model was used in this study, and in-out TED with intact AP and out-in TED with limited foraminoplasty, a smaller grade of nuclectomy was simulated. Biomechanical changes in the L5-S1 segment related to the degeneration acceleration were computed under different directional loading conditions.

RESULTS

Post-operative biomechanical changes after the out-in TED with limited foraminoplasty were slight, except for the facet contact pressure under the extension position. By contrast, significant biomechanical deterioration, both in the adjacent disc and zygapophyseal joints, is observed under extension in the model after the in-out TED with large nuclectomy.

CONCLUSION

A large grade of nuclectomy is regarded as an independent risk factor of adjacent segment disease in the caudal functional spinal unit after the in-out TED.

Disc measurement and nucleus calibration in a smoothened lumbar model increases the accuracy and efficiency of in-silico study

Backgrounds

Finite element analysis (FEA) is an important tool during the spinal biomechanical study. Irregular surfaces in FEA models directly reconstructed based on imaging data may increase the computational burden and decrease the computational credibility. Definitions of the relative nucleus position and its cross-sectional area ratio do not conform to a uniform standard in FEA.

Methods

To increase the accuracy and efficiency of FEA, nucleus position and cross-sectional area ratio were measured from imaging data. A FEA model with smoothened surfaces was constructed using measured values. Nucleus position was calibrated by estimating the differences in the range of motion (RoM) between the FEA model and that of an in-vitro study. Then, the differences were re-estimated by comparing the RoM, the intradiscal pressure, the facet contact force, and the disc compression to validate the measured and calibrated indicators. The computational time in different models was also recorded to evaluate the efficiency.

Results

Computational results indicated that 99% of accuracy was attained when measured and calibrated indicators were set in the FEA model, with a model validation of greater than 90% attained under almost all of the loading conditions. Computational time decreased by around 70% in the fitted model with smoothened surfaces compared with that of the reconstructed model.

Conclusions

The computational accuracy and efficiency of in-silico study can be improved in the lumbar FEA model constructed using smoothened surfaces with measured and calibrated relative nucleus position and its cross-sectional area ratio.

TELD with limited foraminoplasty has potential biomechanical advantages over TELD with large annuloplasty: an in-silico study

Background

Facetectomy, an important procedure in the in–out and out–in techniques of transforaminal endoscopic lumbar discectomy (TELD), is related to the deterioration of the postoperative biomechanical environment and poor prognosis. Facetectomy may be avoided in TELD with large annuloplasty, but iatrogenic injury of the annulus and a high grade of nucleotomy have been reported as risk factors influencing poor prognosis. These risk factors may be alleviated in TELD with limited foraminoplasty, and the grade of facetectomy in this surgery can be reduced by using an endoscopic dynamic drill.

Methods

An intact lumbo-sacral finite element (FE) model and the corresponding model with adjacent segment degeneration were constructed and validated to evaluate the risk of biomechanical deterioration and related postoperative complications of TELD with large annuloplasty and TELD with limited foraminoplasty. Changes in various biomechanical indicators were then computed to evaluate the risk of postoperative complications in the surgical segment.

Results

Compared with the intact FE models, the model of TELD with limited foraminoplasty demonstrated slight biomechanical deterioration, whereas the model of TELD with large annuloplasty revealed obvious biomechanical deterioration. Degenerative changes in adjacent segments magnified, rather than altered, the overall trends of biomechanical change.

Conclusions

TELD with limited foraminoplasty presents potential biomechanical advantages over TELD with large annuloplasty. Iatrogenic injury of the annulus and a high grade of nucleotomy are risk factors for postoperative biomechanical deterioration and complications of the surgical segment.

Stress distribution of different lumbar posterior pedicle screw insertion techniques: a combination study of finite element analysis and biomechanical test

At present, the pedicle screw is the most commonly used internal fixation device. However, there are many kinds of common posterior pedicle screw insertion techniques performed to reconstruct the lumbar stability. Therefore, spinal surgeons often face a difficult choice. The stress distribution of internal fixation system is an important index for evaluating safety. Unfortunately, little had been known about the difference of stress distribution of screw-rod systems that established by Roy-Camille, Magerl and Krag insertion techniques. Here, combination of finite element analysis and model measurement research was adopted to evaluate the difference of stress. Following different pedicle screw insertion techniques, three lumbar posterior surgery models were established after modeling and validation of the L1–S1 vertebrae finite element model. By analyzing the data, we found that stress concentration phenomenon was in all the postoperative models. Roy-Camille and Magerl insertion techniques led to the great stress on screw-rod systems. Then, fresh frozen calf spines were selected as a model for subsequent measurements. Fitted with a specially designed test pedicle screw, L5–L6 vertebrae were selected to repeat and verify the results of the finite element analysis. With the aid of universal testing machine and digital torque wrench, models simulated flexion, extension, lateral bending and rotation. Finally, the strain value was captured by the strain gauge and was then calculated as the stress value. Krag and Magerl were found to be the safer choice for pedicle screw insertion. Overall, our combination method obtained the reliable result that Krag insertion technique was the safer approach for pedicle screw implantation due to its relatively dispersive stress. Therefore, without the consideration of screw size, pedicle fill, bone density, and bone structures, we recommend the Krag insertion technique as the first choice to reconstruction of lumbar stability. Additionally, the combination method of finite element analysis and strain gauge measurement can provide a feasible way to study the stress distribution of spinal internal fixation.

Prediction of complications and fusion outcomes of fused lumbar spine with or without fixation system under whole-body vibration

Lumbar fixator has been widely used, which can stabilize the lumbar spine and improve the fusion outcomes, but also lead to many complications. The effects of the internal fixator on biomechanical properties of the fused lumbar spine have been widely concerned for many years. However, most studies only considered the static loads and did not consider the effect of the fixator on the properties of the human lumbar spine under whole-body vibration (WBV). The purpose of this study is to investigate how the fixation system affects the biomechanical characteristics of the lumbar spine, fusion outcomes, and complications under WBV based on the finite element analysis. A three-dimensional nonlinear osteoligamentous finite element model of the intact L1-sacrum spine with muscles was established. A 5-Hz, 40-N sinusoidal vertical load supplemented with a 400-N preload was applied at L1 to simulate the vibration of the human body. For the adjacent segments, the fixation system may increase the risk of the adjacent segment disease under WBV. For the fused segments, the fixation system may decrease the risk of subsidence and cage failure including fatigue failure under WBV. The fixation system may provide a more stable and suitable environment for vertebral cell growth under WBV and lead to better fusion outcomes. This study reveals insights into the effect of the fixation system on the vibration characteristics of the lumbar and provides new information on the fixation system, fusion outcomes, complications, clinical evaluation, and selection of fixation system.

Prediction of the influence of vertical whole-body vibration on biomechanics of spinal segments after lumbar interbody fusion surgery

BACKGROUND

Previous studies have shown that for healthy spine, cyclic loading encountered due to whole-body vibration exposure generated higher responses in spinal tissues than static loading. However, how whole-body vibration affects spine biomechanics after interbody fusion surgery is poorly understood. This study aimed at comparing the effects of vibration loading on spinal segments between postsurgical and healthy lumbar spines.

METHODS

A validated finite element model of healthy human lumbosacral spine was modified to simulate interbody fusion at L4-L5 level considering the statuses immediately after surgery (before bony fusion) and after bony fusion. Biomechanical responses at its adjacent levels for the healthy and fusion models to a sinusoidal axial vibration load of ±40 N and the corresponding static axal loads (-40 N and 40 N) were computed using transient dynamic and static analyses, respectively.

FINDINGS

For both healthy and fusion models, vibration amplitudes of the predicted responses were significantly higher than the corresponding changing amplitudes under static loads. Specifically, the increasing effect of vibration load in disc bulge, disc stress and intradiscal pressure at L3-L4 level reached 255.9%, 215.0% and 224.4% for the healthy model, 157.4%, 177.8% and 171.8% for the fusion model (before bony fusion), 141.9%, 152.6% and 160.1% for the fusion model (after bony fusion).

INTERPRETATION

Although whole-body vibration is still more dangerous for the lumbar spine after interbody fusion surgery than static loading, the sensitivity of adjacent segment in postsurgical spine to vibration loading is decreased compared with healthy spine, especially when reaching to bony fusion.

The biomechanical effects of Ti versus PEEK used in the PLIF surgery on lumbar spine: a finite element analysis

Titanium (Ti) and polyetheretherketone (PEEK) are commonly used in posterior lumbar interbody fusion (PLIF). The study investigated biomechanical effects of Ti versus PEEK used as materials of cage and rods on the lumbar spine. Four different configurations of PLIF were constituted. Stiff Ti rods provided satisfactory initial stability but increased the stress on rods significantly under simulated physiological load conditions. Ti cage increased the stress on bone endplates significantly. Materials of cage and rods had insignificant effects on the nucleus pressure and facet joint force of non-instrumented segments. Further clinical studies and follow-up observations are essential for corroborating these findings.

The protection of superior articular process in percutaneous transforaminal endoscopic discectomy should decreases the risk of adjacent segment diseases biomechanically

PURPOSE

Facetectomy is a useful procedure in percutaneous transforaminal endoscopic discectomy (PTED) for the enlargement of surgical field and operative space and for the decompression of existing nerve roots for patients who suffer foraminal stenosis. Biomechanical deterioration can initially trigger the adjacent segment disease (ASD), and our previous literature proved that a large grade of facetectomy can increase the risk of biomechanical deterioration and resulting low back pain. However, no study has discussed whether different grades of facetectomy influence the risk of ASD.

METHODS

A validated osteoligamentous lumbosacral finite element model and corresponding PTED models with quarter and half facetectomy were constructed in our previous study. Biomechanical indicators were computed and recorded to evaluate the risk of ASD.

RESULTS

Obvious differences between the intact model and the quarter facetectomy model had no basis. Nevertheless, in most body positions, most of the above indicators deteriorated in the half facetectomy model.

CONCLUSION

On the basis of achieving the surgical purpose in PTED, the superior articular process should be protected to decrease the risk of ASD biomechanically.

Biomechanical evaluation of anterior and posterior lumbar surgical approaches on the adjacent segment: a finite element analysis

The purpose of this study was to use models of spine to compare range of motion and intradiscal pressure of adjacent segments performing anterior and/or posterior lumbar surgical approaches and predict potential risk of adjacent segment degeneration. A previously validated finite element model of the intact L1-S1 segments was used. Three different anterior and one posterior surgical fixation approaches for tuberculosis were performed in L3-L5. Three different anterior surgical models were constructed according to the anterior approaches involving debridement, bone graft with or without titanium mesh, and internal fixation with different number of screws and rods. The posterior surgical approach involved transforaminal lumbar interbody debridement, bone graft, and internal fixation. Range of motion and intradiscal pressure of segments adjacent to the fusion were assessed, and biomechanical influences were compared. Intradiscal pressure and range of motion of the adjacent L2/3 and L5/S1 increased during different physiological movements after anterior and/or posterior surgical approaches as compared to baseline values. Comparison between the biomechanical values assessed after different anterior surgical approaches yielded no significant difference. After anterior and posterior surgical approaches were performed on the same model, there were no significant differences in intradiscal pressure and range of motion of the adjacent L2/3 and L5/S1. Anterior and/or posterior lumbar surgical approaches increased range of motion and intradiscal pressure in L2/3 and L5/S1, suggesting each lumbar surgical approach assessed has the potential risk of adjacent segment degeneration. However, there were no significant differences between the biomechanical measurements across the different surgical approaches evaluated.

Biomechanical Assessment of Vertebroplasty Combined with Cement-Augmented Screw Fixation for Lumbar Burst Fractures: A Finite Element Analysis

A hybrid fixation method, using a combination of vertebroplasty and cement-augmented screws, has been demonstrated as a useful technique for securing osteoporotic burst fractures. The purpose of this study was to assess changes in the range of motion (ROM) and stress in the spine after treating a lumbar burst fracture with this hybrid method. Five finite element models were developed: (a) intact lumbar spine (INT), (b) INT with vertebroplasty at L3 (AwC), (c) two-segment fixation of AwC (AwC-TSF), (d) AwC-TSF model with cement-augmented screws (AwC-TSF-S), and (e) INT with an L3 burst fracture treated with two-segment fixation (TSF). After loading, the models were evaluated in terms of the ROM of each motion segment, stiffness of fusion segments, and stresses on the endplates and screws. The results showed that the TSF model has a larger ROM at the instrumented segments than both the AwC-TSF and AwC-TSF-S models. The stiffness at L2–L4 under extension and lateral bending in AwC-TSF, AwC-TSF-S and TSF was approximately nine times greater than the INT model. In conclusion, the hybrid fixation method (AwC-TSF-S) results in a stiffer construct and lower ROM at instrumented segments, which may also reduce the risk of fracture of adjacent vertebrae.

Biomechanical analysis of single-level interbody fusion with different internal fixation rod materials: a finite element analysis

Background

Lumbar spinal fusion with rigid spinal fixators as one of the high risk factors related to adjacent-segment failure. The purpose of this study is to investigate how the material properties of spinal fixation rods influence the biomechanical behavior at the instrumented and adjacent levels through the use of the finite element method.

Methods

Five finite element models were constructed in our study to simulate the human spine pre- and post-surgery. For the four post-surgical models, the spines were implanted with rods made of three different materials: (i) titanium rod, (ii) PEEK rod with interbody PEEK cage, (iii) Biodegradable rod with interbody PEEK cage, and (iv) PEEK cage without pedicle screw fixation (no rods).

Results

Fusion of the lumbar spine using PEEK or biodegradable rods allowed a similar ROM at both the fusion and adjacent levels under all conditions. The models with PEEK and biodegradable rods also showed a similar increase in contact forces at adjacent facet joints, but both were less than the model with a titanium rod.

Conclusions

Flexible rods or cages with non-instrumented fusion can mitigate the increased contact forces on adjacent facet joints typically found following spinal fixation, and could also reduce the level of stress shielding at the bone graft.

Lumbar Disc Herniation Within Solid Fused Segments After Removal of Pedicle Screws: A Case Report

CASE

A 73-year-old man with instrumented fusion of L3-4-5 underwent the removal of previous pedicle screws and posterior instrumented fusion of L1-2-3. The solid dorsal fusion mass of L3-5 was identified intraoperatively and preoperatively. At 3 years after the second surgery, he presented with abrupt radiating pain in the left leg. Plain radiographs showed a collapse of the intervertebral disc space, and magnetic resonance imaging showed disc herniation of L3-4.

CONCLUSIONS

Disc herniation even in solid fused segments may occur. Removal of pedicle screws and cranial extension of interbody fusion may increase the intradiscal stress associated with physiologic cantilever motion of the disc.

Effect of lordosis on adjacent levels after lumbar interbody fusion, before and after removal of the spinal fixator: a finite element analysis

BACKGROUND

Literature indicates that adjacent-segment diseases after posterior lumbar interbody fusion with pedicle screw fixation accelerate degenerative changes at unfused adjacent segments due to the increased motion and intervertebral stress. Sagittal alignment of the spine is an important consideration as achieving proper lordosis could improve the outcome of spinal fusion and avoid the risk of adjacent segment diseases. Therefore, restoration of adequate lumbar lordosis is considered as a major factor in the long-term success of lumbar fusion. This study hypothesized that the removal of internal fixation devices in segments that have already fused together could reduce stress at the disc at adjacent segments, particularly in patients with inadequate lordosis. The purpose of this study was to analyze the biomechanical characteristics of a single fusion model (posterior lumbar interbody fusion with internal fixation) with different lordosis angles before and after removal of the internal fixation device.

METHODS

Five finite element models were constructed for analysis; 1) Intact lumbar spine without any implants (INT), 2) Lumbar spine implanted with a spinal fixator and lordotic intervertebral cage at L4-L5 (FUS-f-5c), 3) Lumbar spine after removal of the spinal fixator (FUS-5c), 4) Lumbar spine implanted with a spinal fixator and non-lordotic intervertebral cage at L4-L5 (FUS-f-0c), and 5) Lumbar spine after removal of the spinal fixator from the FUS-f-0c model (FUS-0c).

RESULTS

The ROM of adjacent segments in the FUS-f-0c model was found to be greater than in the FUS-f-5c model. After removing the fixator, the adjacent segments in the FUS-5c and FUS-0c models had a ROM that was similar to the intact spine under all loading conditions. Removing the fixator also reduced the contact forces on adjacent facet joints and reduced the peak stresses on the discs at adjacent levels. The greatest increase in stress on the discs was found in the FUS-f-0c model (at both L2/L3 and L3/L4), with intervertebral stress at L3/L4 increasing by 83% when placed in flexion.

CONCLUSIONS

This study demonstrated how removing the spinal fixation construct after bone fusion could reduce intradiscal pressure and facet contact forces at adjacent segments, while retaining a suitable level of lumbar lordosis.

Reducing the extent of facetectomy may decrease morbidity in failed back surgery syndrome

Background

Percutaneous transforaminal endoscopic discectomy (PTED) is widely used for the treatment of lumbar disc herniation. Facetectomy in PTED is necessary for accessing the intraspinal region and for decompressing the exiting nerve roots in patients who suffer from hypertrophy of the facet joints. However, this may increase morbidity in failed back surgery syndrome (FBSS) and has not been clearly elucidated.

Methods

A three-dimensional lumbosacral model was reconstructed and validated. And corresponding models after PTED with one-quarter and one-half excisions of the superior articular process were reconstructed. The maximum shear stress on the annulus in L5, von Mises stress of the facet cartilage, maximum principle capsular strain and deformation of the lumbosacral model were calculated using finite element methods.

Results

Calculated results show no significant differences in the complete model and the model with one-quarter excision of the superior articular process, but all biomechanical indexes have been deteriorated under most of the loading conditions tested in the model with one-half excision of the superior articular process.

Conclusions

Less facetectomy is better because it may reduce the risk of biomechanical deterioration and consequently, that of FBSS.

[Effect of pre-existing adjacent segment degeneration on short-term effectiveness after lumbar fusion surgery]

Objective

To analyze the prospective effect of pre-existing spinal stenosis of adjacent segment on the short-term effectiveness after lumbar fusion surgery.

Methods

A prospective comparative study was conducted to divide 183 patients with L ₄-S ₁ lumbar spinal stenosis who met the selection criteria between July 2015 and December 2017 into two groups according to the status of adjacent segment degeneration (ASD) judged by preoperative disc degeneration and spinal stenosis. There were 98 patients in group A (no degeneration of adjacent segments before operation) and 85 patients in group B (adjacent segments degenerated before operation). There was no significant difference in gender, American Society of Anesthesiologists (ASA) grade, body mass index (BMI), combined spondylolisthesis, and preoperative visual analogue scale (VAS) score of low back pain and leg pain, Japanese Orthopaedic Association (JOA) score, and Oswestry disability index (ODI) score between the two groups ( P>0.05); the age of group A was significantly younger than that of group B ( t=-3.560, P=0.000). The operation time, intraoperative blood loss, hospitalization stay, and perioperative complications were recorded and compared. The VAS score of low back pain and leg pain, JOA score, and ODI score at last follow-up were used to evaluate the effectiveness. The incidence of ASD after operation was compared between the two groups, and logistic regression was used to analyze the independent risk factors affecting the occurrence of ASD after operation.

Results

There was no significant difference in operation time, intraoperative blood loss, and hospitalization stay between the two groups ( P>0.05). The incidence of perioperative complications in groups A and B was 13.3% and 20.0%, respectively, with no significant difference ( χ ²=1.506, P=0.220). Two groups of patients were followed up, the follow-up time of groups A and B was (24.9±8.8) months and (24.8±7.8) months, respectively, there was no significant difference ( t=0.050, P=0.960). At last follow-up, no adjacent segment disease was found in either group. There was no significant difference in Pfirrmann grade between the two groups at last follow-up ( P>0.05), and there was significant difference in Pfirrmann grade between the two groups before operation and at last follow-up ( P<0.001). At last follow-up, 21 cases (21.4%) in group A and 53 cases (62.4%) in group B had ASD, with significant difference ( χ ²=31.652, P=0.000). The main cause of ASD was the severity of adjacent spinal canal stenosis. The clinical scores of the two groups at last follow-up were significantly improved when compared with those before operation ( P<0.05). The JOA score of group A was significantly higher than that of group B at last follow-up ( P<0.05). In group B, the VAS score of low back pain and ODI score in patients with ASD after operation at last follow-up were significantly higher than those in patients without ASD ( P<0.05). logistic regression analysis showed that preoperative pre-existing degeneration and BMI were independent risk factors for ASD after operation ( P<0.05).

Conclusion

Pre-existing mild spinal stenosis in adjacent segment can significantly affect the effectiveness, and can significantly increase the risk of ASD early after operation. The main pathological type of ASD was the severity of adjacent segment spinal stenosis. For preoperative assessment of pre-existing degeneration, we should evaluate the overall degeneration of the adjacent segment of the spinal canal, rather than simply evaluating the degeneration of the adjacent disc and facet joints.

Biomechanical study of proximal adjacent segment degeneration after posterior lumbar interbody fusion and fixation: a finite element analysis

Purpose

To investigate the biomechanical changes in the proximal adjacent segment with different grades of degeneration after posterior lumbar interbody fusion (PLIF).

Methods

We created three finite element models of the L3–5 with different grades of degeneration (healthy, mild, and moderate) at the L3–4 that were developed by changing the disc height and material properties of the nucleus pulposus. The L4–5 were operated by interbody fusion cages and pedicle screws. All models were loaded with a compressive pre-load of 400 N and a bending moment of 10 N a in three planes to recreate flexion, extension, lateral bending, and axial rotation. The range of motion (ROM), nucleus pressure, and annulus fibrosus pressure of the L3–4 were evaluated.

Results

The ROM, nucleus pressure, and annulus fibrosus pressure increased at the L3–4 after PLIF. As the degeneration progressed in the L3–4, the ROM of the L3–4 decreased while the nucleus pressure and annulus fibrosus pressure increased.

Conclusions

Adjacent segment degeneration (ASD) may be related to the ROM and intradiscal pressure after PLIF. However, as the degeneration of the proximal adjacent segment increases, the ROM in the proximal adjacent segment gradually decreases, but the pressure on the nucleus pulposus and annulus fibrosus gradually increases. Degeneration of the proximal adjacent segment before operation is a risk factor for the ASD after PLIF.

Ti-24Nb-4Zr-8Sn Alloy Pedicle Screw Improves Internal Vertebral Fixation by Reducing Stress-Shielding Effects in a Porcine Model

To ensure the biomechanical properties of Ti-24Nb-4Zr-8Sn, stress-shielding effects were compared between Ti-24Nb-4Zr-8Sn and Ti-6Al-4V fixation by using a porcine model. Twelve thoracolumbar spines (T12–L5) of 12-month-old male pigs were randomly divided into two groups: Ti-24Nb-4Zr-8Sn (EG, n = 6) and Ti-6Al-4V (RG, n = 6) fixation. Pedicle screw was fixed at the outer edge of L4-5 vertebral holes. Fourteen measuring points were selected on the front of transverse process and middle and posterior of L4-5 vertebra. Electronic universal testing machine was used to measure the strain resistance of measuring points after forward and backward flexion loading of 150 N. Meanwhile, stress resistance was compared between both groups. The strain and stress resistance of measurement points 1, 2, 5, 6, 9, and 10–14 in Ti-24Nb-4Zr-8Sn fixation was lower than that of Ti-6Al-4V fixation after forward and backward flexion loading (P < 0.05). The strain and stress resistance of measurement points 3, 4, 7, and 8 was higher in Ti-24Nb-4Zr-8Sn fixation than that of Ti-6Al-4V fixation (P < 0.05). Stress-shielding effects of Ti-24Nb-4Zr-8Sn internal fixation were less than that of Ti-6Al-4V internal fixation. These results suggest that Ti-24Nb-4Zr-8Sn elastic fixation has more biomechanical goals than conventional Ti-6Al-4V internal fixation by reducing stress-shielding effects.

Assessment of the suitability of biodegradable rods for use in posterior lumbar fusion: An in-vitro biomechanical evaluation and finite element analysis

Interbody fusion with posterior instrumentation is a common method for treating lumbar degenerative disc diseases. However, the high rigidity of the fusion construct may produce abnormal stresses at the adjacent segment and lead to adjacent segment degeneration (ASD). As such, biodegradable implants are becoming more popular for use in orthopaedic surgery. These implants offer sufficient stability for fusion but at a reduced stiffness. Tailored to degrade over a specific timeframe, biodegradable implants could potentially mitigate the drawbacks of conventional stiff constructs and reduce the loading on adjacent segments. Six finite element models were developed in this study to simulate a spine with and without fixators. The spinal fixators used both titanium rods and biodegradable rods. The models were subjected to axial loading and pure moments. The range of motion (ROM), disc stresses, and contact forces of facet joints at adjacent segments were recorded. A 3-point bending test was performed on the biodegradable rods and a dynamic bending test was performed on the spinal fixators according to ASTM F1717-11a. The finite element simulation showed that lumbar spinal fusion using biodegradable implants had a similar ROM at the fusion level as at adjacent levels. As the rods degraded over time, this produced a decrease in the contact force at adjacent facet joints, less stress in the adjacent disc and greater loading on the anterior bone graft region. The mechanical tests showed the initial average fatigue strength of the biodegradable rods was 145 N, but this decreased to 115N and 55N after 6 months and 12 months of soaking in solution. Also, both the spinal fixator with biodegradable rods and with titanium rods was strong enough to withstand 5,000,000 dynamic compression cycles under a 145 N axial load. The results of this study demonstrated that biodegradable rods may present more favourable clinical outcomes for lumbar fusion. These polymer rods could not only provide sufficient initial stability, but the loss in rigidity of the fixation construct over time gradually transfers loading to adjacent segments.