The effect of posterior instrumentation following PLIF with BAK cages is most pronounced in weak bone.

BACKGROUND: The use of BAK-cages for lumbar fusion has become very popular but complications such as cage subsidence and settling occurred. To treat these complications posterior instrumentation was used to improve segmental stability. It is, however, poorly understood, why some patients require additional posterior instrumentation, whereas the majority do not. The objectives of the study presented were first to determine the influence of bone mineral density (BMD) to the initial compressive stiffness of a segment that underwent posterior lumbar interbody fusion (PLIF) with two BAK-cages. Second, to estimate the importance of additional posterior instrumentation for compressive stiffness with respect to bone mineral density. METHODS: A validated finite element model (FEM) including posterior decompression and stabilisation by two BAK-cages (BAK_FEM) was used to predict the initial compression stiffness in axial loading of 600 N. This model was used to predict the influence of various grades of BMD on compression stiffness. A second FEM was generated in which additional posterior screw-rod instrumentation was simulated (BAK+PI_FEM) and this model used to predict the influence of BMD in axial loading. FINDINGS: The responses of all FEM suggested that initial compressive stiffness will increase if there is an increase of BMD. The stiffness as predicted by BAK+PI_FEM was always superior to FEM_BAK. This difference was most pronounced for weak bone quality. INTERPRETATION: Compression stiffness following PLIF with BAK-cages depends on BMD. Additional posterior instrumentation results in an additional increase of compression stiffness. This effect is most pronounced in simulated soft bone quality. These results may help to select patients for combined stabilisation.

[A validated finite element model of the human spine--description of the model and initial application]. / Ein validiertes Finite-Elemente-Modell der menschlichen Halswirbelsäule--Modellbeschreibung und erste Anwendung.

AIM: To generate a finite-element model of the human cervical spine and evaluate the first application of the model to the analysis of new c-spine implants. METHODS: CT-data were used to generate a three-dimensional, anisotrophic, linear model of the human C4-C7 motion segments using the software ANSYS 5.4. As a next step, anterior cervical fusion and plate fixation using mono- and bicortical screws was simulated in the model. Loading of the finite-element models was simulated using pure moments of +/- 2.5 Nm in flexion/extension, axial left/right rotation, and left/right lateral bending. The range of motion was calculated. The results were compared to the results of an in vitro study using human cadaveric c-spine segments C4-C7, with the same implants and moments on both the intact and surgically treated specimens. RESULTS: The results obtained by the finite-element model were always within one standard deviation of the results of the in vitro study. CONCLUSION: Keeping in mind the simplifications of such a mathematical model, it may be used for a first analysis of the shape of new c-spine implants or to predict the initial stability of a new device.

The influence of cancellous bone density on load sharing in human lumbar spine: a comparison between an intact and a surgically altered motion segment.

The aim of the current study is twofold: first, to compare load sharing in compression between an intact and a surgically repaired lumbar spine motion segment L3/4 using a biomechanically validated finite element approach; second, to analyse the influence of bone mineral density on load sharing. Six cadaveric human lumbar spine segments (three segments L2/3 and three segments L4/5) were taken from fresh human cadavers. The intact segments were tested under axial compression of 600 N, first without preload and then following instrumented stabilisation. These results were compared to a finite element model simulating the effect of identical force on the intact segments and the segments with constructs. The predictions of both the intact and the surgically altered finite element model were always within one standard deviation of the mean stiffness as analysed by the biomechanical study. Thus, the finite element model was used to analyse load sharing under compression in an intact and a surgically repaired human lumbar spine segment model, using a variety of E moduli for cancellous bone of the vertebral bodies. In both the intact and the surgically altered model, 89% of the applied load passed through the vertebral bodies and the disc if an E modulus of 25 MPa was used for cancellous bone density. Using 10 MPa--representing soft, osteoporotic bone--this percentage decreased, but it increased using 100 MPa in both the intact and the altered segment. Thus, it is concluded that reconstruction of both the disc and the posterior elements with the implants used in the study recreates the ability of the spine to act as a load-sharing construction in compression. The similarity in load sharing between normal and instrumented spines appears to depend on assumed bone density, and it may also depend on applied load and loading history.

[Effect of spongiosa density on load bearing of the lumbar spine.A finite element analysis]. / Der Einfluss der Spongiosadichte auf das Load-Sharing der Lendenwirbelsäule. Eine Finite-Elemente-Analyse.

OBJECTIVE: Different parts of the human spine have to accomplish different functions. But little is known about the exact distribution of forces within the spine and whether this is influenced by bone quality. The purpose of this study was to predict fields and extent of greatest load in compression in a human lumbar spine motion segment using a finite element model. METHODS: A three dimensional isotrophic finite element model was generated using the software ANSYS 5.4. Spinal loading was performed in axial compression (600 N). The model was validated by biomechanical analysis using 12 human spinal segments that were loaded with the same forces. Prediction was done with different E-modulus for cancellous bone, representing a wide range of bone quality between osteoporotic and strong bone quality. RESULTS: Load-sharing was influenced by bone quality: the weaker bone quality is, the higher is the extent of load that is passed through the posterior part of the spine. CONCLUSION: This finite element model predicts that load-sharing in a lumbar spine segment with decreased bone mineral density is different from that in healthy segments. A decrease of bone mineral density is resulting in an increase of load that is passed through the posterior part of the lumbar spine. Keeping in mind the simplifications of this model, the results may influence surgical treatment of patients suffering from osteoporosis or osteolytic destructions of the lumbar spine.

[Initial stability of two PLIF-techniques. A biomechanical comparison using a finite element model]. / Die Primärstabilität von zwei Plif-Techniken.Ein biomechanischer Vergleich mit der Finite-Elemente-Methode.

The purpose of this study was to compare the initial stiffness of two techniques for posterior interbody lumbar fusion (PLIF) by a finite element approach. Thus a finite element model of a human L3/4 spinal segment was generated. Stiffness of the intact model was tested under compression (600 N), torsion (25 Nm) and shearing forces (250 N) without preload. The results were compared to the stiffness following simulation of PLIF with two BAK-Cages and PLIF with two Harms-Cages and additional posterior screw-rod-osteosynthesis. PLIF with two BAK-Cages resulted in a loss of stiffness in compression, torsion and shearing. PLIF with two Harms-Cages and posterior osteosynthesis resulted in an increase of stiffness in compression, torsion and shearing.

[Is the prediction of initial stability of cervical spinal osteo-synthesis possible using a finite element model?]. / Ist die Beurteilung der Primärstabilität zervikaler Osteosynthesen durch ein Finite-Elemente Modell möglich?

The study is dealing with a three segmental (C4-C7) finite element model of the intact human cervical spine. Additionally, anterior cervical fusion and plating (ACFP) with Caspar-plate and bicortical screws in C5/6 was simulated. The models were loaded using pure moments of 2.5 Nm in flexion-extension, axial rotation and lateral bending. The range of motion in C5/6 was calculated and compared to the results of a biomechanical in vitro study, that used six cadaveric human spinal segments C4-C7 for analysing range of motion C5/6 in the intact state and following ACFP. The predictions of the finite element models were always within one standard deviation of the results of the in vitro study. Thus, the current model could be used for first analysis on new C-spine implants. However, the results should be interpreted as a trend and the limitations of these models should be kept in mind.

Motion of threaded cages in posterior lumbar interbody fusion.

A high rate of pseudarthrosis and a high overall rate of implant migration requiring surgical revision has been reported following posterior lumbar interbody fusion using BAK threaded cages. The high rate of both pseudarthrosis and implant migration may be due to poor fixation of the implant. The purpose of this study was to analyse the motion of threaded cages in posterior lumbar interbody fusion. Six cadaveric human lumbar spine segments (three L2/3 and three L4/5 segments) were prepared for biomechanical testing. The segments were tested, without preload, under forces of axial compression (600 N), torsion (25 Nm) and shearing force (250 N). The tests were performed first with the segments in an intact state, and subsequently following instrumented stabilisation with two BAK cages via a posterior approach. These results were compared with those of a finite element model simulating the effects of identical forces on the segments with constructs. As the results were comparable, the finite element model was used for analysing the motion of BAK cages within the disc space. Motion of the implants was not seen in compression. In torsion, a rolling motion was noted, with a range of motion of 10.6 degrees around the central axis of the implant when left/right torsion (25 Nm) was applied. The way the implants move within the segment may be due to their special shape: the thread of the implants can not prevent the BAK cages rolling within the disc space.

[Primary stability of 2 PLIF (posterior lumbar interbody fusion)--a biomechanical in vitro comparison]. / Die Primärstabilität zweier PLIF-Techniken--Ein biomechanischer In-vitro-Vergleich.

OBJECTIVE: The purpose of this biomechanical in-vitro-study was to compare two different PLIF-techniques with two types of implants on human lumbar spine: PLIF with threaded cages, (Bagby and Kuslich, Spinetech, Minneapolis, USA) and PLIF with the Moss-Miami-implants, (DePuy International Limited, Leeds, Great Britain). METHODS: Six cadaveric human lumbar spine segments L2-5 were explanted, frozen at -20 degrees C and thawed before preparation. They were cut in two parts by discectomie and arthrotomie L3/4, so six specimen L2/3 and six specimen L4/5 were obtained and used in a crossover-trial. Analysis included testing in a tension-torsion-machine under axial compression with 600 N, rotation (left-right) with 25 Nm and shearing forces with 250 N without preload. This was first done in the intact and then in the fused specimen. RESULTS: Stiffness before treatment was comparable in both groups irrespective of location. Posttreatment stiffness was higher with MOSS-MIAMI-implants as compared to PLIF with BAK-cages. Average relative superiority (and 95%-confidence intervall) were 1.98 (1.01-3.69) for compression, 2.30 (0.85-6.24) for rotation and 1.73 (0.78-3.84) for shearing. Statistical comparison of log posttreatment stiffness was significant for compression but not for rotation and shearing (2-sided independent crossover t-test). CONCLUSION: This biomechanical in-vitro-study demonstrates the higher initial stability of PLIF with titanium surgical mesh and posterior instrumentation when compared to PLIF with threaded cages alone.

[Primary stability of 2 PLIF (posterior lumbar interbody fusion) techniques--a biomechanical and finite element analysis]. / Die Primärstabilität von zwei PLIF-Techniken--Eine biomechanische und Finite Elemente Analyse.

The purpose of this study was to compare the initial stiffness of two techniques for posterior interbody lumbar fusion by biomechanical and finite element analysis. Initial stiffness was tested under compression, torsion and shearing forces. The effect of an increasing initial stability by additional posterior instrumentation is proven by the biomechanical analysis and the finite element method.