A comprehensive finite element model of surgical treatment for cervical myelopathy

BackgroundCervical myelopathy is a common and debilitating chronic spinal cord dysfunction. Treatment includes anterior and/or posterior surgical intervention to decompress the spinal cord and stabilize the spine, but no consensus has been made as to the preferable surgical intervention. The objective of this study was to develop an finite element model of the healthy and myelopathic C2-T1 cervical spine and common anterior and posterior decompression techniques to determine how spinal cord stress and strain is altered in healthy and diseased states.MethodsA finite element model of the C2-T1 cervical spine, spinal cord, pia, dura, cerebral spinal fluid, and neural ligaments was developed and validated against in vivo human displacement data. To model cervical myelopathy, disc herniation and osteophytes were created at the C4-C6 levels. Three common surgical interventions were then incorporated at these levels.FindingsThe finite element model accurately predicted healthy and myelopathic spinal cord displacement compared to motions observed in vivo. Spinal cord strain increased during extension in the cervical myelopathy finite element model. All surgical techniques affected spinal cord stress and strain. Specifically, adjacent levels had increased stress and strain, especially in the anterior cervical discectomy and fusion case.InterpretationsThis model is the first biomechanically validated, finite element model of the healthy and myelopathic C2-T1 cervical spine and spinal cord which predicts spinal cord displacement, stress, and strain during physiologic motion. Our findings show surgical intervention can cause increased strain in the adjacent levels of the spinal cord which is particularly worse following anterior cervical discectomy and fusion.     

How does a novel knitted titanium nucleus prosthesis change the kinematics of a cervical spine segment? A biomechanical cadaveric study

BackgroundTotal disc replacement is a possible treatment alternative for patients with degenerative disc disease, especially in the cervical spine. The aim is to restore the physiological flexibility and biomechanical behavior. A new approach based on these requirements is the novel nucleus prosthesis made of knitted titanium wires.MethodsThe biomechanical functionalities of eight human cervical (C4-C7) spine segments were investigated. The range of motion was quantified using an ultra-sound based motion analysis system. Moreover, X-rays in full flexion and extension of the segment were taken to define the center of rotation before and after implantation of the nucleus prosthesis as well as during and after complex cyclic loading.FindingsThe mean range of motion of the index segment (C5/6) in flexion/extension showed a significant reduction of range of motion from 9.7° (SD 4.33) to 6.0° (SD 3.97) after implantation (P = 0.037). Lateral bending and axial rotation were not significantly reduced after implanting and during cyclic loading in our testing. During cyclic loading the mean range of motion for flexion/extension increased to 7.2° (SD 3.67). The center of rotation remained physiological in the ap-plane and moved cranially in the cc-plane (−27% to −5% in cc height) during the testing.InterpretationThe biomechanical behavior of the nucleus implant might lower the risk for adjacent joint disorders and restore native function of the index segment. Further in vivo research is needed for other factors, like long-term effects and patient's satisfaction.     

Upper and lower adjacent segment range of motion after fixation of different lumbar spine segments in the goat: an in vitro experiment

ObjectivesThe purpose of this study was to examine the biomechanical effects of fixation on range of motion (ROM) in the upper and lower adjacent segments of different lumbar spine segments in a goat spine model.MethodsFifteen goat spine specimens (vertebrae T12–S1) were randomly divided into three groups: A (single-segment fixation), B (double-segment fixation), and C (triple-segment fixation). Motion in different directions was tested using a spinal motion simulation test system with five external loading forces. Transverse, forward–backward, and vertical displacement of the upper and lower adjacent segments were measured.ResultsAs the external load increased, the upper and lower adjacent segment ROM increased. A significantly greater ROM in group C compared with group A was found when the applied external force was greater than 75 N. The upper adjacent segment showed a significantly greater ROM than the lower adjacent segment ROM within each group.ConclusionsAdjacent segment ROM increased with an increasing number of fixed lumbar segments. The upper adjacent segment ROM was greater than that of the lower adjacent segments. Adjacent segment stability after lumbar internal fixation worsened with an increasing number of fixed segments.     

Biomechanical effects of uncinate process excision in cervical disc arthroplasty

BackgroundStudies on the role of uncinate process have been limited to responses of the intact spine and patient's outcomes, and procedures to perform the excision. The aim of this study was to determine the role of uncinate process on the biomechanical response at the index and adjacent levels in three artificial discs used in cervical disc arthroplasty.MethodsA validated finite element model of cervical spine was used. Flexion, extension, and lateral moments and follower load were applied to Bryan, Mobi-C, and Prestige LP artificial discs at C5-C6 level with and without uncinate process. Ranges of motion at index level and adjacent caudal and cranial segments, intradiscal pressures at adjacent segments, and facet loads at index level and adjacent segments were obtained. Data were normalized with respect to the preservation of uncinate process.FindingsUncinate process removal increased motions up to 27% at index and decreased up to 10% at adjacent levels, decreased disc pressures up to 14% at adjacent segments, decreased facet loads at adjacent segments up to 14%, while at index level, change in loads depended on mode and arthroplasty, with Mobi-C responding with up to 51% increase and Bryan disc up to 11% decrease, while Prestige LP increased loads by 17% in extension and decreased by 9%% in lateral bending.InterpretationAs surgical selection is based on morphology and surgeon's experience, the present computational findings provide quantitative information for an optimal choice of the device and procedure, while further studies (in vitro/clinical) would be required.     

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

BackgroundPrevious 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.MethodsA 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.FindingsFor 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).InterpretationAlthough 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.     

Kinematics of a selectively constrained radiolucent anterior lumbar disc: Comparisons to hybrid and circumferential fusion

Background

Despite encouraging clinical outcomes of one-level total disc replacements reported in literature, there is no compelling evidence regarding the stability following two-level disc replacement and hybrid constructs. The current study is aimed at evaluating the multidirectional kinematics of a two-level disc arthroplasty and hybrid construct with disc replacement adjacent to rigid circumferential fusion, compared to two-level fusion using a novel selectively constrained radiolucent anterior lumbar disc.

Methods

Nine osteoligamentous lumbosacral spines (L1–S1) were tested in the following sequence: 1) Intact; 2) One-level disc replacement; 3) Hybrid; 4) Two-level disc replacement; and 5) Two-level fusion. Range of motion (at both implanted and adjacent level), and center of rotation in sagittal plane were recorded and calculated.

Findings

At the level of implantation, motion was restored when one-level disc replacement was used but tended to decrease with two-level disc arthroplasty. The findings also revealed that both one-level and two-level disc replacement and hybrid constructs did not significantly change adjacent level kinematics compared to the intact condition, whereas the two-level fusion construct demonstrated a significant increase in flexibility at the adjacent level. The location of center of rotation in the sagittal plane at L4–L5 for the one-level disc replacement construct was similar to that of the intact condition.

Interpretation

The one-level disc arthroplasty tended to mimic a motion profile similar to the intact spine. However, the two-level disc replacement construct tended to reduce motion and clinical stability of a two-level disc arthroplasty requires additional investigation. Hybrid constructs may be used as a surgical alternative for treating two-level lumbar degenerative disc disease.     

A Comparison of Three Cervical Immobilization Devices

Objective. Prehospital cervical spinal cord injuries (SCIs) are rare but potentially catastrophic. Although spinal immobilization is resource-intensive, emergency medical services (EMS) personnel commonly immobilize trauma patients to prevent exacerbation of unrecognized SCI during transport. We compared the stabilization properties of a novel rigid, cervical immobilization collar (XCollar) with those of one-piece andtwo-piece rigid collars commonly used in the prehospital setting. Methods. This was a prospective laboratory study of healthy adult volunteers to determine total cervical motion in the horizontal, coronal, andsagittal planes in both seated andsupine positions. Goniometric techniques were used to measure head andneck movement after marking anatomic landmarks. Ranges of motion were compared with a one-way analysis of variance (ANOVA). A Bonferroni correction was applied for multiple comparisons, setting significance at p ≤ 0.004. Results. Twenty-five subjects (11 men; 14 women) completed the study. The subject pool represented a wide range of morphometrics. For most measurements, the XCollar permitted 10–15 millimeters of movement when applied without manual cervical stabilization. This was less than the movement permitted by both comparison collars. On average, the XCollar permitted less than 10 millimeters of movement in the sagittal andhorizontal planes when the subject was in the seated position. Conclusions. The XCollar provided superior cervical stabilization without augmentation by manual stabilization in healthy adult volunteers in both the seated andsupine positions when compared with other one-piece andtwo-piece rigid cervical collars. Although maximal stabilization was achieved only after the subjects were secured to a long spine board with a cervical immobilization device, the XCollar can provide an acceptable alternative to manual cervical stabilization in situations where the number of patients exceeds the number of EMS providers available to provide care.     

Preliminary Report: Biomechanics of Vertebral Artery Segments C1-C6 During Cervical Spinal Manipulation

ObjectiveThe purpose of this study was to measure strains in the human vertebral artery (VA) within the cervical transverse foramina and report the first results on the mechanical loading of segments of the VA during spinal manipulation of the cervical spine.MethodsEight piezoelectric ultrasound crystals of 0.5-mm diameter were sutured into the lumen of the left and right VA of one cadaver. Four hundred–nanosecond ultrasound pulses were sent between the crystals to measure the instantaneous lengths of the VA segments (total segments n = 14) at a frequency of 200 Hz. Vertebral artery engineering strains were then calculated from the instantaneous lengths during cervical spinal range of motion testing, chiropractic cervical spinal manipulation adjustments, and vertebrobasilar insufficiency testing.ResultsThe results of this study suggest complex and nonintuitive strain patterns of the VA within the cervical transverse foramina. Consistent (for 2 chiropractors) and repeatable (for 3 repeat measurements for each chiropractor) elongation and shortening of adjacent VA segments were observed simultaneously and could not be explained with a simple model of neck movement. We hypothesized that they were caused by variations in the location and stiffness of the VA fascial attachments to the vertebral foramina and by coupled movements of the cervical vertebrae. However, in agreement with previous work on VA strains proximal and distal to the cervical transverse foramina, strains for cervical spinal manipulations were consistently lower than those obtained for cervical rotation.ConclusionsAlthough general conclusions should not be drawn from these preliminary results, the findings of this study suggest that textbook mechanics of the VA may not hold, that VA strains may not be predictable from neck movements alone, and that fascial connections within the transverse foramina and coupled vertebra movements may play a crucial role in VA mechanics during neck manipulation. Furthermore, the engineering strains during cervical spinal manipulations were lower than those obtained during range of motion testing, suggesting that neck manipulations impart stretches on the VA that are well within the normal physiologic range of neck motion.     

Effects of Restrained Cervical Mobility on Involuntary Eye Movements

Abstract

The presumed connection between cervicogenic dizziness, cervical evoked involuntary eye movements and intervertebral joint blocks of the high cervical spine serves as a starting point for the manual therapist in treating patients suffering from neck pain, headache and dizziness. Cervical evoked involuntary eye movements are of diagnostic importance and proprioceptive cervical positional nystagmus is seen as pathognomic for high cervical intervertebral joint blocks. In a period of two years, 157 patients with neck pain, headache and dizziness were referred to and examined in our department. Thirty eight patients were diagnosed as having functional vertebrobasilar insufficiency and 17 patients presented with benign paroxysmal positional vertigo. These 55 patients were excluded from the study. The remaining 102 patients were included in the study. Passive functional tests of cervical motion segments OCC-C1, C1 through C4 according to Van der EI and Dvorák and Dvorák and the test for cervical evoked involuntary eye movements according to Oosterveld were carried out on the last group. In 84 of the 102 patients (82%) the passive functional tests of OCC-C1, C1 through C4 were evaluated as positive for one or more cervical motion segments in one rotational direction; in 18 of the 102 patients (18%) as positive in two rotational directions. In 13 of 102 patients (13%) the cervical evoked involuntary eye movements were evaluated as present for proprioceptive cervical positional nystagmus (mean latency: 4 sec.; frequency: decreasing); in 89 patients (90%) as absent. In 12 of the 13 patients (92.3%) intervertebral joint blocks were present at four levels of the high cervical spine OCC-C1, C1 through C4. There was a significant correlation between the number and level of the blocked cervical motion segments and the presence of the proprioceptive cervical positional nystagmus (Kendall's tau-b = 0.59; p < 0.05). The results of this study are not comparable with those reported in other studies because of differences in the methods and judgement of the cervical evoked involuntary eye movements (ENG versus visual observation using Frenzel's spectacles) and the function of the high cervical spine (no specific information versus manual diagnostic information per cervical motion segment). The findings suggest the need for further research.     

Does the postoperative cervical lordosis angle affect the cervical rotational range of motion after cervicothoracic multilevel fusion?

BackgroundLaminectomy and multilevel fusion in patients with degenerative cervical myelopathy lead to severe restriction in cervical spine mobility. Since fusions from C₂ to the thoracic spine result in a permanently stiff subaxial cervical spine, it seems obvious to restore physiological cervical lordosis, especially with regard to sagittal balance. However, there are reports that a fusion in a more lordotic position leads to a reduction of rotational cervical range of motion in the still mobile segments C₀-C₂. This study investigates the relationship between postoperative cervical lordosis and the objective rotational range of motion and subjective restriction.MethodsIn this single-center, retrospective cohort study, patients with degenerative cervical myelopathy operated via laminectomy and fusion from C₂ to the thoracic spine were included. X-ray imaging was evaluated for common lordosis parameters. The patient-reported rotational restriction of cervical spine mobility was acquired by a five-step score. Objective rotational range of motion was measured. The radiological parameters for cervical lordosis (C₂-C₇ lordotic angle, C₂-C₇ Cobb angle) were correlated with the measurements and the patient-reported subjective scores.FindingsWe found a significant, medium negative correlation between the measurements for rotation and the C₂-C₇ lordotic angle and a significant, large negative correlation to the C₂-C₇ Cobb angle. For subjective restriction, no or only small correlation was observed.InterpretationWe found significant negative correlations between radiological cervical lordosis and objective measurements for rotation. These results indicate that for this particular patient population, a stronger postoperative cervical lordosis does not seem favorable under the aspect of rotational range of motion.     

Recurrent neck pain patients exhibit altered joint motion pattern during cervical flexion and extension movements

BackgroundImpaired sensorimotor ability has been demonstrated in recurrent neck pain patients. It is however not clear if cervical joint motion and pressure pain sensitivity in recurrent neck pain patients are different from asymptomatic controls.MethodsCervical flexion and extension motions were examined by video-fluoroscopy and pressure pain thresholds were assessed bilaterally over C2/C3, C5/C6 facet joints and right tibialis anterior in eighteen recurrent neck pain patients and eighteen healthy subjects. Individual joint motion was analyzed by dividing fluoroscopic videos into 10 epochs. The motion opposite to the primary direction (anti-directional motion) and motion along with the primary direction (pro-directional motion) of each joint were extracted across epochs. Total joint motion was the sum of anti-directional and pro-directional motions. Joint motion variability was represented by the variance of joint motions across epochs.FindingsCompared to controls, recurrent neck pain patients showed: 1) decreased anti-directional motion at C2/C3 and C3/C4 (P < 0.05) and increased anti-directional motion at C5/C6 and C6/C7 (P < 0.05) during extension motion. 2) Increased overall anti-direction motion during flexion motion (P < 0.05). 3) Lower joint motion variability at C3/C4 during extension motion (P < 0.05).InterpretationRecurrent neck pain patients showed a redistribution of anti-directional motion between the middle cervical spine and the lower cervical spine during cervical extension and increased overall anti-directional motion during cervical flexion compared with healthy controls. The anti-directional motion was more sensitive to neck pain compared to other cervical joint motion parameters in the present study.     

Impact of anchor type on porcine lumbar biomechanics: Finite element modelling and in-vitro validation

BackgroundRigid posterior implants used for spinal stabilization can be anchored to the vertebrae using pedicle screws or screws combined with transverse process hooks. In the present study, a finite element model of a porcine lumbar spine instrumented with screws and hooks is presented and validated.MethodsThe porcine lumbar spine model was validated using in-vitro measurements on six porcine specimens. Validation metrics included intervertebral rotations (L1 to L6) and nucleus pressure in the topmost cranial instrumented disc. The model was used to compare the biomechanical effect of anchor types.FindingsGood agreement was observed between the model and validation experiments. For upper transverse hooks construct, intervertebral rotations increased at the upper instrumented vertebra and decreased at the adjacent level. Additionally, nucleus pressures and stress on the annulus decreased in the adjacent disc and increased in the upper instrumented disc. The pull-out forces predicted for both anchor configurations were significantly lower than the pull-out strength found in the literature.InterpretationThese numerical observations suggest that upper transverse process hooks constructs reduce the mobility gradient and cause less stress in the adjacent disc, which could potentially reduce adjacent segment disease and proximal junction kyphosis incidence without increasing the risk of fixation failure. Future work needs to assess the long-term effect of such constructs on clinical and functional outcomes.     

Biomechanical analysis on of anterior transpedicular screw-fixation after two-level cervical corpectomy using finite element method

BackgroundAnterior cervical trans-pedicle screw fixation was introduced to overcome some of the disadvantages associated with anterior cervical corpectomy and fusion. In vitro biomechanical studies on the trans-pedicle screw fixation have shown excellent pull-out strength and favorable stability. Comprehensive biomechanical performance studies on the trans-pedicle screw fixation, however, are lacking.MethodsThe control computed tomography images (C2-T2) were obtained from a 22-year-old male volunteer. A three dimensional computational model of lower cervical spine (C3-T1) was developed using computed tomography scans from a 22 year old human subject. The models of intact C3-T1 (intact group), anterior cervical trans-pedicle screw fixation (trans-pedicle group), and anterior cervical corpectomy and fusion (traditional group) were analyzed with using a finite element software. A moment of 1 N·m and a compressive load of 73.6 N were loaded on the upper surface and upper facet joint surfaces of C3. Under six conditions, four parameters such as the range of motion, titanium mesh plant stress, end-plate stress, and bone-screw stress were measured and compared on two treatment groups.FindingsCompared with the intact model, the range of motions for treatment groups were decreased. Compared with cervical corpectomy and fusion, the titanium plant, C4 upper end-plate and C7 lower end-plate stresses in trans-pedicle group were reduced. No significant difference was discovered on bone-screw stress between the two groups for lateral flexion and rotation, but bone-screw stress is smaller in trans-pedicle group when compared with traditional group. With exception of individual difference, trans-pedicle group had better biomechanical results than traditional group in range of motions, titanium mesh plant stress, end-plate stress and bone-screw stress.InterpretationThe trans-pedicle method has better biomechanical properties than the anterior cervical corpectomy and fusion making it a viable alternative for cervical fixations.     

腰椎人工髓核植入对上位邻近节段椎体椎间盘影响的生物力学研究

目的探讨腰椎人工髓核植入对邻近节段腰椎稳定性的生物力学影响。方法新鲜小牛腰椎脊柱标本8具，分别制备成正常组、腰椎髓核摘除组、人工髓核植入组模型，自体对照，分别记录髓核摘除后、人工髓核植入与正常组对腰椎邻近节段的应变及应力改变，以此评估其对腰椎生物力学稳定性的影响。结果单纯髓核摘除术后病变节段上方相邻节段椎体的应变明显下降（P〈0．05）；人工髓核植入后其相应部位的应变与正常相比仅有数值上的增加，无统计学意义（P〉0．05）；髓核摘除后上位节段邻椎的椎体、椎间盘的应力均有增加，髓核植入后其应力基本恢复正常。结论人工髓核植入能有效维持上位相邻腰椎椎间盘、椎体的正常应力以及应变水平，维持脊柱正常的生物力学，延缓上位邻近椎体、椎间盘的退变。     

In vivo intersegmental motion of the cervical spine using an inverse kinematics procedure

Background

The main functions of the cervical spine are the stabilization and the orientation of the head. Pathologies are complex and difficult to diagnose. The first sign of the dysfunction is an abnormal intervertebral motion. It is the purpose of this feasibility study to determine the intersegmental motions and loading conditions of the cervical spine in vivo with standard clinical investigation methods.

Methods

We propose a new approach which merges full flexion–extension X-ray images, and continuous motion of the whole cervical spine obtained with a tracking motion system. These data were used as input for a subject-specific rigid body model of the cervical spine computed with the software MSC.Adams. This model simulates the cervical spine extension/flexion, the intervertebral motions are deduced using an inverse kinematics procedure.

Findings

Subject-specific rigid body models were computed from data of two subjects. The intersegmental motion and loading conditions were calculated. We found that the loading amplitudes depended on the intervertebral level, and that subject specific patterns were highlighted. We noticed an unsymmetrical behavior in flexion and extension. Furthermore intervertebral rotations were correlated with the global motion of the cervical spine.

Interpretation

A subject-specific rigid body model merged data from classical flexion–extension radiographs and noninvasive external motion capture. Our approach is based on inverse kinematics allowing the estimation of the intervertebral motion and mechanical behavior of the cervical spine in vivo, which gives valuable information concerning biomechanics of the cervical spine in vivo for cervical spine clinical investigation.     

Biomechanical evaluation of anterior lumbar interbody fusion with various fixation options: Finite element analysis of static and vibration conditions

BackgroundAnterior lumbar interbody fusion combined with supplementary fixation has been widely used to treat lumbar diseases. However, few studies have investigated the influence of fixation options on facet joint force and cage subsidence. The aim of this study was to explore the biomechanical performance of anterior lumbar interbody fusion with various fixation options under both static and vertical vibration loading conditions.MethodsA previously validated finite element model of the intact L1–5 lumbar spine was employed to compare five conditions: (1) Intact; (2) Fusion alone; (3) Fusion combined with anterior lumbar plate; (4) Fusion combined with Coflex-F fixation; (5) Fusion combined with bilateral pedicle screw fixation. The models were analyzed under static and vertical vibration loading conditions respectively.FindingsBilateral pedicle screws provided highest stability at surgical level. Applying supplementary fixation diminished the dynamic responses of lumbar spine. Compared with anterior lumbar plate and Coflex-F device, bilateral pedicle screws decreased the stress responses of the endplates and cage under both static and vibration conditions, while increased the facet joint force at adjacent levels. As for comparison between Coflex-F device and anterior lumbar plate, results showed a similarity in biomechanical performance under static loading, and a slightly higher dynamic response of the latter under vertical vibration.InterpretationThe biomechanical performance of lumbar spine was significantly influenced by the variation of fixations under both static and vibration conditions. Bilateral pedicle screws showed advantages in stabilizing surgical segment and relieving cage subsidence, but may increase the facet joint force at adjacent levels.     

Posterior cervical spine crisscross fixation: Biomechanical evaluation

BackgroundBiomechanical/anatomic limitations may limit the successful implantation, maintenance, and risk acceptance of posterior cervical plate/rod fixation for one stage decompression-fusion. A method of posterior fixation (crisscross) that resolves biomechanical deficiencies of previous facet wiring techniques and not reliant upon screw implantation has been devised. The biomechanical performance of the new method of facet fixation was compared to the traditional lateral mass plate/screw fixation method.MethodsThirteen human cadaver spine segments (C2-T1) were tested under flexion-compression loading and four were evaluated additionally under pure-moment load. Preparations were evaluated in a sequence of surgical alterations with intact, laminectomy, lateral mass plate/screw fixation, and crisscross facet fixation using forces, displacements and kinematics.FindingsCombined loading demonstrated significantly lower bending stiffness (p < 0.05) between laminectomy compared to crisscross and lateral mass plate/screw preparations. Crisscross fixation showed a comparative tendency for increased stiffness. The increased overall motion induced by laminectomy was resolved by both fixation techniques, with crisscross fixation demonstrating a comparatively more uniform change in segmental motions.InterpretationThe crisscross technique of facet fixation offers immediate mechanical stability with resolution of increased flexural rotations induced by multi-level laminectomy. Many of the anatomic limitations and potentially deleterious variables that may be associated with multi-level screw fixation are not associated with facet wire passage, and the subsequent fixation using a pattern of wire connection crossing each facet joint exhibits a comparatively more uniform load distribution. Crisscross wire fixation is a valuable addition to the surgical armamentarium for extensive posterior cervical single-stage decompression-fixation.     

Strain behavior of malaligned cervical spine implanted with metal-on-polyethylene,metal-on-metal,and elastomeric artificial disc prostheses – A finite element analysis

BackgroundPostoperative alterations in cervical spine curvature (i.e. loss of lordotic angle) are frequently observed following total disc replacement surgery. However, it remains unclear whether such changes in lordotic angle are due to preoperative spinal deformities and/or prostheses design limitations. The objective of the study is to investigate strain and segmental biomechanics of the malaligned cervical spine following total disc replacement.MethodsThree disc prostheses were chosen, namely a metal-on-polyethylene, a metal-on-metal, and an elastomeric prosthesis, which feature different geometrical and material design characteristics. All discs were modelled and implanted into multi-segmental cervical spine finite element model (C3-C7) with normal, straight and kyphotic alignments. Comparative analyses were performed by using a hybrid protocol.FindingsThe results indicated that as the spine loses lordotic alignment, the prosthesis with elastomeric core tends to produce significantly larger flexion range of motion (difference up to 6.1°) than metal-on-polyethylene and metal-on-metal prostheses. In contrast, when the treated spine had normal lordotic alignment, the range of motion behaviors of different prostheses are rather similar (difference within 1.9°). Large localized strains up to 84.8% were found with the elastomeric prosthesis, causing a collapsed anterior disc space under flexion loads.InterpretationChanges in cervical spinal alignments could significantly affect the surgical-level range of motion behaviors following disc arthroplasty; the in situ performance was largely dependent on the designs of the artificial disc devices in particular to the material properties.     

Stabilizing effect of the rib cage on adjacent segment motion following thoracolumbar posterior fixation of the human thoracic cadaveric spine: A biomechanical study

BackgroundAlthough the rib cage provides substantial stability to the thoracic spine, few biomechanical studies have incorporated it into their testing model, and no studies have determined the influence of the rib cage on adjacent segment motion of long fusion constructs. The present biomechanical study aimed to determine the mechanical contribution of the intact rib cage during the testing of instrumented specimens.MethodsA cyclic loading (CL) protocol with instrumentation (T4–L2 pedicle screw-rod fixation) was conducted on five thoracic spines (C7–L2) with intact rib cages. Range of motion (±5 Nm pure moment) in flexion-extension, lateral bending, and axial rotation was captured for intact ribs, partial ribs, and no ribs conditions. Comparisons at the supra-adjacent (T2–T3), adjacent (T3–T4), first instrumented (T4–T5), and second instrumented (T5–T6) levels were made between conditions (P ≤ 0.05).FindingsA trend of increased motion at the adjacent level was seen for partial ribs and no ribs in all 3 bending modes. This trend was also observed at the supra-adjacent level for both conditions. No significant changes in motion compared to the intact ribs condition were seen at the first and second instrumented levels (P > 0.05).InterpretationThe segment adjacent to long fusion constructs, which may appear more grossly unstable when tested in the disarticulated spine, is reinforced by the rib cage. In order to avoid overestimating adjacent level motion, when testing the effectiveness of surgical techniques of the thoracic spine, inclusion of the rib cage may be warranted to better reflect clinical circumstances.     

Comparison of effects of four interbody fusion approaches on the fused and adjacent segments under vibration

BackgroundWhich lumbar fusion approaches having fewer impacts on the lumbar spine, reducing the risk of complications and the most conducive to bone fusion under whole-body vibration is urgent to know.ObjectivesThis study researched the best approach under vibration by comparing the effects of four different approaches on the spine, especially regarding some significant indexes related to complications and outcomes.MethodsThe L1-L5 finite element model was modified to simulate anterior, posterior, trans-foraminal and direct lateral lumbar interbody fusion approaches with bilateral pedicle screw fixation at L4-L5 level.FindingsAnterior lumbar interbody fusion decreased the corresponding vibration amplitude of the dynamic response at adjacent segments compared with the other three approaches. Direct lateral lumbar interbody fusion decreased the maximum stress in the cage, the endplates at the fused level, and the maximum compressive stress at the interface between the cage and endplates. The maximum disc height and segmental lordosis of Direct lateral lumbar interbody fusion model were the highest among these fusion approaches.InterpretationAnterior lumbar interbody fusion may provide a more stable environment for the adjacent segments under vibration. Direct lateral lumbar interbody fusion may reduce the risk of subsidence, cage failure, and adjacent segment disease. Direct lateral lumbar interbody fusion may provide a more stable and suitable environment for vertebral cell growth and lead to better fusion outcomes. The findings may help us understand the effect of various fusion approaches on lumbar and provide some references for choosing a fusion approach.