Effects of fusion-bone stiffness on the mechanical behavior of the lumbar spine after vertebral body replacement

Comparison of anterior column reconstruction techniques after en bloc spondylectomy: a finite element study

Total en bloc spondylectomy (TES) effectively treats spinal tumors. The surgery requires a vertebral body replacement (VBR), for which several solutions were developed, whereas the biomechanical differences between these devices still need to be completely understood. This study aimed to compare a femur graft, a polyetheretherketone implant (PEEK-IMP-C), a titan mesh cage (MESH-C), and a polymethylmethacrylate replacement (PMMA-C) using a finite element model of the lumbar spine after a TES of L3. Several biomechanical parameters (rotational stiffness, segmental range of motion (ROM), and von Mises stress) were assessed to compare the VBRs. All models provided adequate initial stability by increasing the rotational stiffness and decreasing the ROM between L2 and L4. The PMMA-C had the highest stiffness for flexion-extension, lateral bending, and axial rotation (215%, 216%, and 170% of intact model), and it had the lowest segmental ROM in the instrumented segment (0.2°, 0.5°, and 0.7°, respectively). Maximum endplate stress was similar for PMMA-C and PEEK-IMP-C but lower for both compared to MESH-C across all loading directions. These results suggest that PMMA-C had similar or better primary spinal stability than other VBRs, which may be related to the larger contact surface and the potential to adapt to the patient's anatomy.

Effect of different fixation methods on biomechanical property of cervical vertebral body replacement and fusion

BACKGROUND

The main purpose of this study was to examine the effect of different fixation methods (anterior fixation, self-stabilizing fixation and anterior-posterior fixation) on biomechanical property of vertebral body replacement and fusion.

METHODS

Three finite element models of cervical vertebral body replacement and fusion were established. The implanted models included artificial vertebral body and fixation system, and the loads imposed on the models included 75 N compression load and 1 Nm moment load.

FINDINGS

For anterior-posterior fixation, the cervical load was mainly transmitted by the posterior pedicle screw and rod (more than 50%), and the stress shielding problem was the most significant than the self-stabilizing and anterior fixation. Self-stabilizing fixation was more helpful to the fusion of implant and vertebrae, but the higher risk of vertebral body collapse was worthy of attention if the cervical spine with osteoporosis. The stress of bone was mainly concentrated around the screw hole. The maximum stress (20.03 MPa) was lower than the yield stress of cortical bone and the possibility of fracture around the fixation device of cervical spine was low. The anterior fixation could meet the requirement of vertebral body replacement and fusion, and the addition of posterior pedicle screws and rods might obtain better treatment in cases of severe spine injury or osteoporosis.

INTERPRETATION

The findings of this study may provide guidance on clinical treatments for choosing more appropriate fixation methods for different patients.

Finite element modeling of temporal bone graft changes in XLIF: Quantifying biomechanical effects at adjacent levels

Extreme lateral interbody fusion allows for the insertion of a large-footprint interbody cage while maintaining the presence of natural stabilizing ligaments and the facets. It is unclear how the load-distribution mechanisms through these structures alter with temporal changes in the bone graft. The aim of this study was to examine the effects of temporal bone graft changes on load distribution among the cage, graft, and surrounding spinal structures using finite element analysis. Thoracolumbosacral spine computed tomography data from an asymptomatic male subject were segmented into anatomical regions of interest and digitally stitched to generate a surface mesh of the lumbar spine (L1-S1). The interbody cage was inserted into the L4-L5 region during surface meshing. A volumetric mesh was generated and imported into finite element software for pre-processing, running nonlinear static solves, and post-processing. Temporal stiffening was simulated in the graft region with unbonded (Soft Callus, Temporal Stages 1-3, Solid Graft) and bonded (Partial Fusion, Full Fusion) contact. In flexion and extension, cage stress reduced by 20% from the soft callus to solid graft state. Force on the graft was directly related to its stiffness, and load-share between the cage and graft improved with increasing graft stiffness, regardless of whether contact was fused with the endplates. Fused contact between the cage-graft complex and the adjacent endplates shifted load-distribution pathways from the ligaments and facets to the implant, however, these changes did not extend to adjacent levels. These results suggest that once complete fusion is achieved, the existing load paths are seemingly diminished.

Investigation into factors affecting the mechanical behaviours of a patient-specific vertebral body replacement

Most vertebral body implants that are currently designed and produced in batches have difficulty meeting the patient-specific demands. Moreover, several complications, including a low fusion rate, subsidence occurrence, and rod displacement, are associated with these implants. This study aims to investigate the effects of patient-specific geometric and clinical parameters on the biomechanics of a vertebral body replacement. A three-dimensional patient-specific vertebral body replacement model was established as the basic model for parametric studies, including the anatomic design of the endplates, tilting angle, thickness, and dislocation of the vertebral body implant. A finite element analysis was applied to determine the stress distribution of the vertebral body implant when under various loading conditions. The model with an anatomical interfacing design generates 75% less stress concentration compared to a flat design; the peak stress of the model with a tilted angle closely matching the replaced vertebra segment is decreased by 30%; and the thickness close to the cortical bone can offer better bone growth capability and long-term stability. Patient-specific geometrical parameters were found to significantly affect the biomechanics of a vertebral body replacement, and therefore, a design customized especially for the endplates is necessary for better stability and long-term longevity of the prostheses. Regardless of such progress, how to balance the stability of a vertebral body implant and the safety of the peripheral nervous system remains a clinical challenge.

The Biomechanical Study of Extraforaminal Lumbar Interbody Fusion: A Three-Dimensional Finite-Element Analysis

Objective

Finite-element method was used to evaluate biomechanics stability of extraforaminal lumbar interbody fusion (ELIF) under different internal fixation.

Methods

The L3–L5 level finite-element model was established to simulate decompression and internal fixation at L4-L5 segment. The intact finite model was treated in accordance with the different internal fixation. The treatment groups were exerted 400 N load and 6 N·m additional force from motion to calculate the angular displacement of L4-L5.

Results

The ROMs were smaller in all internal fixation groups than those in the intact model. Furthermore, the ROMs were smaller in ELIF + UPS group than in TLIF + UPS group under all operating conditions, especially left lateral flexion and right rotation. The ROMs were higher in ELIF + UPS group than in TLIF + BPS group. The ROMs of ELIF + UPS + TLFS group were much smaller than those in ELIF + UPS group, and as compared with TLIF + BPS group, there was no significant difference in the range of experimental loading.

Discussion

The biomechanical stability of ELIF with unilateral pedicle screw fixation is superior to that of TLIF with unilateral pedicle screw fixation but lower than that of TLIF with bilateral pedicle screws fixation. The stability of ELIF with unilateral fixation can be further improved by supplementing a translaminar facet screw.

Improving the Process of Adjusting the Parameters of Finite Element Models of Healthy Human Intervertebral Discs by the Multi-Response Surface Method

The kinematic behavior of models that are based on the finite element method (FEM) for modeling the human body depends greatly on an accurate estimate of the parameters that define such models. This task is complex, and any small difference between the actual biomaterial model and the simulation model based on FEM can be amplified enormously in the presence of nonlinearities. The current paper attempts to demonstrate how a combination of the FEM and the MRS methods with desirability functions can be used to obtain the material parameters that are most appropriate for use in defining the behavior of Finite Element (FE) models of the healthy human lumbar intervertebral disc (IVD). The FE model parameters were adjusted on the basis of experimental data from selected standard tests (compression, flexion, extension, shear, lateral bending, and torsion) and were developed as follows: First, three-dimensional parameterized FE models were generated on the basis of the mentioned standard tests. Then, 11 parameters were selected to define the proposed parameterized FE models. For each of the standard tests, regression models were generated using MRS to model the six stiffness and nine bulges of the healthy IVD models that were created by changing the parameters of the FE models. The optimal combination of the 11 parameters was based on three different adjustment criteria. The latter, in turn, were based on the combination of stiffness and bulges that were obtained from the standard test FE simulations. The first adjustment criteria considered stiffness and bulges to be equally important in the adjustment of FE model parameters. The second adjustment criteria considered stiffness as most important, whereas the third considered the bulges to be most important. The proposed adjustment methods were applied to a medium-sized human IVD that corresponded to the L3–L4 lumbar level with standard dimensions of width = 50 mm, depth = 35 mm, and height = 10 mm. Agreement between the kinematic behavior that was obtained with the optimized parameters and that obtained from the literature demonstrated that the proposed method is a powerful tool with which to adjust healthy IVD FE models when there are many parameters, stiffnesses, and bulges to which the models must adjust.

An optimization study of the screw orientation on the interfacial strength of the anterior lumbar plate system using neurogenetic algorithms and experimental validation

Anterior lumbar plate (ALP) systems have been widely used as an effective interbody fusion device for treating spinal cord compression. However, clinical complications, such as implant loosening and breakage, still occur. Past studies have investigated the effects of the screw orientation on the interfacial strength, but these studies were inconsistent. The purpose of this study was to identify an ALP system with excellent interfacial strength by varying the screw orientation. Three-dimensional finite element models of L4-L5 segments with an ALP system were first constructed. A neurogenetic algorithm, which combines artificial neural networks and genetic algorithms, was subsequently developed to discover the optimum plate design. Finally, biomechanical tests were conducted to validate the results of the finite element models and the engineering algorithm. The results indicated that the interfacial strength of the optimum plate design obtained using the neurogenetic algorithm was excellent compared with the other designs and that all of the locking screws should be inserted divergently. Both the numerical and experimental outcomes can provide clinical suggestions to surgeons and help them to understand the interfacial strength of ALP systems in terms of the screw orientation.

Relationship between biomechanical changes at adjacent segments and number of fused bone grafts in multilevel cervical fusions: a finite element investigation

OBJECT

Biomechanical studies have shown that anterior cervical fusion construct stiffness and arthrodesis rates vary with different reconstruction techniques; however, the behavior of the adjacent segments in the setting of different procedures is poorly understood. This study was designed to investigate the adjacent-segment biomechanics after 3 different anterior cervical decompression and fusion techniques, including 3-level discectomy and fusion, 2-level corpectomy and fusion, and a corpectomy-discectomy hybrid technique. The authors hypothesized that biomechanical changes at the segments immediately superior and inferior to the multilevel fusion would be inversely proportional to the number of fused bone grafts and that these changes would be related to the type of fusion technique.

METHODS

A previously validated 3D finite element model of an intact C3-T1 segment was used. Three C4-7 fusion models were built from this intact model by varying the number of bone grafts used to span the decompression: a 1-graft model (2-level corpectomy), a 2-graft model (C-5 corpectomy and C6-7 discectomy), and a 3-graft model (3-level discectomy). The corpectomy and discectomy models were also previously validated and compared well with the literature findings. Range of motion, disc stresses, and posterior facet loads at the segments superior (C3-4) and inferior (C7-T1) to the fusion construct were assessed.

RESULTS

Motion, disc stresses, and posterior facet loads generally increased at both of the adjacent segments in relation to the intact model. Greater biomechanical changes were noted in the superior C3-4 segment than in the inferior C7-T1 segment. Increasing the number of bone grafts from 1 to 2 and from 2 to 3 was associated with a lower magnitude of biomechanical changes at the adjacent segments.

CONCLUSIONS

At segments adjacent to the fusion level, biomechanical changes are not limited solely to the discs, but also propagate to the posterior facets. These changes in discs and posterior facets were found to be lower for discectomy than for corpectomy, thereby supporting the current study hypothesis of inverse relationship between the adjacent-segment variations and the number of fused bone grafts. Such changes may go on to influence the likelihood of adjacent-segment degeneration accordingly. Further studies are warranted to identify the causes and true impact of these observed changes.

Monitoring the load on a telemeterised vertebral body replacement for a period of up to 65 months

PURPOSE

To determine the postoperative temporal course of the forces acting on a vertebral body replacement (VBR) for two well reproducible activities.

METHODS

A telemeterised VBR was implanted in five patients. It allows the measurement of six load components. Implant loads were measured in up to 28 measuring sessions for different activities, including standing and walking.

RESULTS

The postoperative temporal course of the resultant implant forces measured during standing and walking was similar in each patient, but the patterns varied strongly from patient to patient. In one patient, the forces decreased in the first year and then increased in the following 4 years. In another patient, the forces increased in the first few months and then decreased. In a third patient, the forces varied only slightly in the postoperative time. In two patients, there was a strong drop of the implant force in the first two postoperative months. The force was on average approximately 100 N or 71% higher for walking than for standing.

CONCLUSIONS

The strong force reduction in the first 2 months is most likely caused by implant subsidence, and the force reduction over a period of more than 6 months is most likely caused by fusion of the vertebrae adjacent to the VBR. The short-term force increase could be attributed to bone atrophy at the index level, and the long-term force increase could be attributed to an increase in the thoracic spine kyphosis angle.

Biomechanics of adjacent segments after a multilevel cervical corpectomy using anterior, posterior, and combined anterior-posterior instrumentation techniques: a finite element model study

BACKGROUND CONTEXT

Adjacent segment degeneration (ASD) after cervical fusion is a clinical concern. Despite previous studies documenting the biomechanical effects of multilevel cervical fusion on segments immediately superior and inferior to the operative segments, the pathogenesis of the initiation of degeneration progression in neighboring segments is still poorly understood.

PURPOSE

To test the hypothesis that changes in range of motion, disc stresses, and facet loads would be highest at the superior adjacent segment (C3-C4) after anterior C4-C7 corpectomy and fusion and that these changes would be the least in anterior fixation and the greatest in posterior or combined anterior-posterior instrumentation techniques.

STUDY DESIGN

A finite element (FE) analysis of adjacent vertebral segment biomechanics after a two-level corpectomy fusion with three different fixation techniques (anterior, posterior, and combined anterior-posterior).

METHODS

A previously validated three-dimensional FE model of an intact C3-T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques after a C4-C7 corpectomy and fusion. Motion patterns, disc stresses, and posterior facet loads at the levels cephalad and caudal to the fusion were assessed.

RESULTS

Range of motion, disc stresses, and posterior facet loads increased at the adjacent segments. Use of posterior fixation, whether alone or in combination with anterior fixation, infers higher changes in segmental motion, disc stresses, and posterior facet loads at adjacent segments compared with the use of anterior fixation alone. The superior C3-C4 motion was most affected during lateral bending and the inferior C7-T1 motion was most affected during flexion, whereas both superior C3-C4 and inferior C7-T1 motions were least affected during extension. However, disc stresses and facet loads were most affected during extension. Hence, it is speculated that the most remodeling changes in discs and facets might be related to the least changes in extension motion.

CONCLUSIONS

Biomechanical factors such as increased mechanical demand and motion that have been associated with the development of ASD progression are highest in the segment immediately superior to the fusion. These changes are even more pronounced when the fixation technique involves the addition of posterior instrumentation, thereby supporting the hypothesis of the present study. Increased degrees of stiffening of the fused segments not only may lead to degenerative changes in the disc but may also predispose the segments to premature facet degeneration. Over subsequent time period, any remaining construct micro-motion is further eliminated with fusion of the posterior facet joints and the remaining regions in the disc space also filled in with bone, which eventually results in a circumferential type of fusion. After a circumferential fusion, authors, however, speculate that the role of instrumentation in ASD progression might not be significant. In fact, sufficient evidence to support this speculation is still lacking in the literature.

Corpectomy versus discectomy for the treatment of multilevel cervical spine pathology: a finite element model analysis

BACKGROUND CONTEXT

After multilevel fusions, construct failure because of pseudoarthrosis and instrumentation complications is a well-recognized clinical problem. Little is known about the biomechanics governing the cervical spine after different anterior reconstruction techniques, specifically the number of bone grafts and screws used and whether discectomies versus corpectomies have been performed. A few research groups have compared the efficacy of corpectomy and discectomy procedures under common testing conditions; however, no quantitative stress measurements at graft-end plate and bone-screw interfaces have been reported to date.

PURPOSE

To test the hypothesis that increasing the number of bone grafts and screws would yield a more stable construct and decrease the stresses at the graft-end plate and bone-screw interfaces.

STUDY DESIGN

Stability of fusion constructs with three different multilevel reconstruction techniques.

METHODS

A previously validated C3-T1 intact finite element model was modified to evaluate three different anterior C4-C7 fusion models: a two-level corpectomy alone (one graft and four screws), a corpectomy-discectomy (two grafts and six screws), and a three-level discectomy alone (three grafts and eight screws). Two unicortical screws were placed parallel to the corresponding end plates inside the vertebral bodies-C4 and C7 for the corpectomy alone; C4, C6, and C7 for the corpectomy-discectomy; and C4, C5, C6, and C7 for the discectomy alone. Range of motion, graft stresses, end plate stresses, and bone-screw stresses were evaluated.

RESULTS

Although total construct motion decreased with an increasing number of bone grafts and screws, this was not significantly different between reconstruction techniques. Stresses in the bone grafts, end plates, and bone near screws decreased as a result of increasing the number of bone grafts and screws, thereby confirming the present study hypothesis.

CONCLUSIONS

Although the chances of pseudarthrosis have been shown to be lower after multilevel cervical corpectomy versus discectomy, because of fewer bone-graft interfaces required for healing, this benefit should be weighed against the higher bone-screw stresses, operating time, blood loss, and costs associated with corpectomy. Future biomechanical studies focusing on corpectomy and discectomy procedures in similar testing protocols are warranted to compare the findings presented here.

Which postures are most suitable in assessing spinal fusion using radiostereometric analysis?

BACKGROUND

Up to now, plain radiographs are not well suited to assess spinal fusion. Radiostereometric analysis performed for two postures may deliver more reliable results. However, it is unknown, which postures are most suitable for this procedure.

METHODS

In a finite element study, spinal fusion at the level L4-5 was simulated assuming a posterior approach and the implantation of two cages and a spinal fixation device. The change of the distance between markers in vertebrae adjacent to the cages was calculated for moving from one of the following postures standing, flexion, extension, axial rotation, lying, and extension in a lying position to another. The changes of marker distances were calculated for the intact model, as well as for the situations: directly after surgery before fusion started, in the early-fusion-phase and in the late-fusion-phase. Differences in the marker motion between two postoperative situations were also calculated.

FINDINGS

The most anteriorly placed markers showed the greatest motion between two postures. The greatest differences in marker motions between the two situations before-fusion and early-fusion-phase (0.54 mm) as well as between early-fusion-phase and late-fusion-phase (0.34 mm) were found for the two postures flexion while standing and extension in a lying position.

INTERPRETATION

Pairs of X-rays taken while standing with maximum flexed upper body and while lying with maximum extended trunk are most suited for the assessment of spinal fusion when using radiostereometric analysis.

Biomechanical effects of anterior, posterior, and combined anterior-posterior instrumentation techniques on the stability of a multilevel cervical corpectomy construct: a finite element model analysis

BACKGROUND CONTEXT

Multilevel corpectomy, with or without anterior instrumentation, has been associated with both graft and anterior screw-plate complications. The addition of posterior instrumentation after anterior fixation has been shown to increase the overall stiffness of fused segments and decrease the likelihood of instrumentation failure. Little biomechanical information exists for providing guidance in the selection of an appropriate instrumentation technique after a multilevel cervical corpectomy. Clinical studies have also been inconclusive in choosing an optimum fixation strategy.

PURPOSE

To test the hypothesis that combined anterior-posterior fixation would lower the stresses on the bone-screw interfaces observed after an isolated anterior fixation and on the graft-end plate interfaces observed after an isolated posterior fixation.

STUDY DESIGN

A finite element (FE) analysis of a C4-C7 corpectomy fusion with three different fixation techniques: anterior, posterior, and combined anterior-posterior.

METHODS

A previously validated three-dimensional FE model of an intact C3-T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques following a C4-C7 corpectomy fusion. Construct stability at the cephalad and caudal levels of the corpectomy was assessed.

RESULTS

Biomechanical comparisons between these instrumentation techniques show the least amount of construct motion in the combined anterior-posterior instrumentation model. The use of both anterior and posterior fixation shields the graft-end plate and screw-bone interfaces from peak stresses as compared with an isolated anterior or an isolated posterior fixation, thereby supporting the hypothesis of this study.

CONCLUSIONS

A combined fixation technique should be balanced against increased operating room time and surgery costs because of dual anterior and posterior fixation and the increased risk of long anterior plating, such as dysphasia, plate or screw dislodgement, or migration. Our study suggests that the use of posterior fixation, whether alone or in combination with anterior fixation, infers comparable stability. Further studies are warranted to identify whether the current findings are consistent with other biomechanical studies.

Different arm positions and the shape of the thoracic spine can explain contradictory results in the literature about spinal loads for sitting and standing

STUDY DESIGN

Loads acting in vivo on a vertebral body replacement (VBR) and the shape of the back were measured.

OBJECTIVE

To find an explanation for the contradictory results in literature regarding spinal loads for sitting and standing.

SUMMARY OF BACKGROUND DATA

In several in vivo studies, the intradiscal pressure was shown to be higher for sitting than for standing. However, stadiometric measurements, load measurements on internal spinal fixators, and 1 study on intradiscal pressure have shown contradictory results. It therefore remains unknown whether sitting or standing causes greater loading on the spine.

METHODS

Telemeterized VBR was implanted into 5 patients. Implant loads were measured in several sessions during standing and during relaxed sitting on a stool. In the sitting position, the subjects' arms were either hanging at their sides or placed on their thighs. The shape of the back during sitting and standing was additionally determined by rasterstereography.

RESULTS

When sitting with their arms hanging, the loads for the 5 patients ranged from 107% to 228% of the values for standing. A relationship was found between this sit-to-stand load ratio and both the kyphosis angles, which increased from 41° to 67°, and the distance between vertebra prominens and lordosis apex related to the body height, which increased from 0.21 to 0.26. By placing the arms on the thighs, the force on the VBR was reduced by an average of 13% (2%-41%), when compared to sitting with the arms hanging at the sides.

CONCLUSION

The spinal load differences between sitting and standing depend on several key factors, most notably arm position and individual spinal shape. These 2 parameters, however, varied between studies in the literature and may therefore account for the continued contradictory discussion. Patients can reduce their spinal load during sitting by supporting the upper body by the arms.

Loads on a spinal implant measured in vivo during whole-body vibration

After spinal surgery, patients often want to know whether driving a car or using public transportation can be dangerous for their spine. In order to answer this question, a clinically proven vertebral body replacement (VBR) has been modified. Six load sensors and a telemetry unit were integrated into the inductively powered implant. The modified implant allows the measurement of six load components. Telemeterized devices were implanted in five patients; four of them agreed to exposure themselves to whole-body vibration. During the measurements, the patients sat on a driver seat fixed to a hexapod. They were exposed to random single-axis vibrations in X, Y, and Z directions as well as in multi-axis XYZ directions with frequencies between 0.3 and 30 Hz. Three intensity levels (unweighted root mean square values of 0.25, 0.5 and 1.0 m/s(2)) were applied. Three postures were studied: sitting freely, using a vertical backrest, and a backrest declined by an angle of 25 degrees . The patients held their hands on their thighs. As expected, the maximum force on the VBR increased with increasing intensity and the number of axes. For the highest intensity level and multi-axis vibration, the maximum forces increased by 89% compared to sitting relaxed. Leaning at the backrest as well as lower intensity levels markedly decreased the implant loads. Driving a car or using public transportation systems-when the patient leans towards the backrest-leads to lower implant loads than walking, and can therefore be allowed already shortly after surgery.

Screw angulation affects bone-screw stresses and bone graft load sharing in anterior cervical corpectomy fusion with a rigid screw-plate construct: a finite element model study

BACKGROUND CONTEXT

Anterior corpectomy and reconstruction with bone graft and a rigid screw-plate construct is an established procedure for treatment of cervical neural compression. Despite its reliability in relieving symptoms, there is a high rate of construct failure, especially in multilevel cases.

PURPOSE

There has been no study evaluating the biomechanical effects of screw angulation on construct stability; this study investigates the C4-C7 construct stability and load-sharing properties among varying screw angulations in a rigid plate-screw construct.

STUDY DESIGN

A finite element model of a two-level cervical corpectomy with static anterior cervical plate.

METHODS

A three-dimensional finite element (FE) model of an intact C3-T1 segment was developed and validated. From this intact model, a fusion model (two-level [C5, C6] anterior corpectomy) was developed and validated. After corpectomy, allograft interbody fusion with a rigid anterior screw-plate construct was created from C4 to C7. Five additional FE models were developed from the fusion model corresponding to five different combinations of screw angulations within the vertebral bodies (C4, C7): (0 degrees, 0 degrees), (5 degrees, 5 degrees), (10 degrees, 10 degrees), (15 degrees, 15 degrees), and (15 degrees, 0 degrees). The fifth fusion model was termed as a hybrid fusion model.

RESULTS

The stability of a two-level corpectomy reconstruction is not dependent on the position of the screws. Despite the locked screw-plate interface, some degree of load sharing is transmitted to the graft. The load seen by the graft and the shear stress at the bone-screw junction is dependent on the angle of the screws with respect to the end plate. Higher stresses are seen at more divergent angles, particularly at the lower level of the construct.

CONCLUSION

This study suggests that screw divergence from the end plates not only increases load transmission to the graft but also predisposes the screws to higher shear forces after corpectomy reconstruction. In particular, the inferior screw demonstrated larger stress than the upper-level screws. In the proposed hybrid fusion model, lower stresses on the bone graft, end plates, and bone-screw interface were recorded, inferring lower construct failure (end-plate fractures and screw pullout) potential at the inferior construct end.

Load- and displacement-controlled finite element analyses on fusion and non-fusion spinal implants

This study used finite element (FE) analysis with the load-controlled method (LCM) and the displacement-controlled method (DCM) to examine motion differences at the implant level and adjacent levels between fusion and non-fusion implants. A validated three-dimensional intact (INT) L1—L5 FE model was used. At the L3—L4 level, the INT model was modified to surgery models, including the artificial disc replacement (ADR) of ProDisc II, and the anterior lumbar interbody fusion (ALIF) cage with pedicle screw fixation. The LCM imposed 10 N m moments of four physiological motions and a 150 N preload at the top of L1. The DCM process was in accordance with the hybrid testing protocol. The average percentage changes in the range of motion (ROM) for whole non-operated levels were used to predict adjacent level effects (ALE%). At the implant level, the ALIF model showed similar stability with both control methods. The ADR model using the LCM had a higher ROM than the model using the DCM, especially in extension and torsion. At the adjacent levels, the ALIF model increased ALE% (at least 17 per cent) using the DCM compared with the LCM. The ADR model had an ALE% close to that of the INT model, using the LCM (average within 6 per cent), while the ALE% decreased when using the DCM. The study suggests that both control methods can be adopted to predict the fusion model at the implant level, and similar stabilization characteristics can be found. The LCM will emphasize the effects of the non-fusion implants. The DCM was more clinically relevant in evaluating the fusion model at the adjacent levels. In conclusion, both the LCM and the DCM should be considered in numerical simulations to obtain more realistic data in spinal implant biomechanics.

Finite element application in implant research for treatment of lumbar degenerative disc disease

Surgical treatment for disc degeneration can be roughly grouped as fusion, disc replacement and dynamic stabilization. The clinical efficacy and biomechanical features of the implants used for disc degenerations can be evaluated through short- or long-term follow up observation, in vitro and in vivo experiments and computational simulations. Finite element models are already making an important contribution to our understanding of the spine and its components. Models are being used to reveal the biomechanical function of the spine and its behavior when healthy, diseased or damaged. They are also providing support in the design and application of spinal instrumentation. The article reviewed the most recent studies in the application of FE models that address the issue of implant research for treatment of low back pain. The published studies were grouped and reviewed thoroughly based on the function of implants investigated. The considerations of the finite element analysis in these studies were further discussed.

Novel production method and in-vitro cell compatibility of porous Ti-6Al-4V alloy disk for hard tissue engineering

Porous metals are attractive due to its unique physical, mechanical, and new bone tissue ingrowth properties. In the present study, the production of highly porous Ti-6Al-4V parts by powder metallurgical technology and subsequently it's uses in in vitro bone tissue engineering is described. A space-holder method using carbamide with different particle size to produce parts with porosities between 35 and 70% were applied. The compressive strength and Young's modulus of porous Ti-6Al-4V were determined. Results indicated that stress and Young's modulus decrease with increasing porosity and pore size. The porous parts are characterized by scanning electron microscopy. Furthermore, study was to investigate the effects of three different porosities of porous Ti-6Al-4V (35, 50, and 70%) on proliferation, differentiation, and cell-matrix interaction of mouse osteoblast-like cells, MC-3T3. Results showed that the cell proliferation was significantly (p < 0.05) higher on 70% porous Ti-6Al-4V. However, synthesis of different types of extra cellular matrix proteins was also more abundant on 70% porous Ti-6Al-4V than 35 and 50% porous Ti-6Al-4V disk except some specific proteins. An increase in alkaline phosphate activity was significantly (p < 0.05) higher on 70 and 50% porous Ti-6Al-4V disk after 12 days of MC-3T3 cells incubation. Above all, results indicated that porosity (nearly 70%) of porous Ti-6Al-4V topography affects proliferation and differentiation of osteoblast-like MC-3T3 cells. The results showed that this novel process is a promise to fabricate porous biomaterials for bone implants.

Effect of position and height of a mobile core type artificial disc on the biomechanical behaviour of the lumbar spine

The extent of natural disc removal and the implant position and height of an artificial disc with a mobile core were studied for their effects on intersegmental rotation, intradiscal pressure, and facet joint force. A validated finite element model of the lumbar spine was used. The model was loaded with the upper body weight, a follower load, and muscle forces to simulate standing, flexion, extension, lateral bending, and axial rotation. The implant position was varied up to 2 mm in an anterior and posterior direction and up to 3 mm in a lateral direction. Three different implant heights were simulated. The effect of removing the lateral parts of the annulus was also studied. The implant position and height markedly affect intersegmental rotation and facet joint forces but have hardly any influence on intradiscal pressure in the adjacent discs.

Removing the lateral parts of the annulus increases intersegmental rotation and facet joint force mainly for lateral bending and axial rotation. The calculated translation of the mobile implant core is about 1 mm at most, and thus its effect is often overestimated. Great care should be taken to choose the optimal implant height and to insert the implant in the best position for each individual patient.

A preliminary biomechanical evaluation in a simulated spinal fusion model

Object

A preliminary in vitro biomechanical study was conducted to determine if the pressure at a bone graft–mortise interface and the load transmitted along a ventral cervical plate could be used as parameters to assess fusion status.

Methods

An interbody bone graft and a ventral plate were placed at the C3–4 motion segment in six fresh cadaveric goat spines. Polymethylmethacrylate (PMMA) was used to simulate early bone fusion at the bone graft site. The loads along the plate and the simultaneous pressures induced at the graft–endplate interfaces were monitored during simulated stages of bone healing. Each specimen was nondestructively tested in compression loading while the pressures and loads at the graft site were recorded continuously. Each specimen was tested under five conditions (Disc, Graft, Plate, PMMA, and Removal).

Results

The pressure at the interface of the bone graft and vertebral endplate did not change significantly with the addition of the ventral plate. The interface pressure and segmental stiffness did increase following PMMA augmentation of the bone graft (simulating an intermediate phase of bone fusion). The load transmitted along the ventral plate in compression increased after the addition of the bone graft, but decreased after PMMA augmentation. Thus, there was an increase in pressure at the graft–endplate interface and a decrease in load transferred along the ventral plate after the simulation of bone fusion. Upon removal of the ventral plate, the simulated fusion bore most of the axial load, thus explaining a further increase in graft site pressure.

Conclusions

These observations support the notions of load sharing and the redistribution of loads occurring during and after bone graft incorporation. In the clinical setting, these parameters may be useful in the assessment of fusion after spine surgery. Although feasibility has been demonstrated in this preliminary study, further research is needed.