IntroductionLittle is known about the exact distribution of forces within healthy lumbar spine due to its complex geometry and physiology. Furthermore, defects by trauma or tumorous destruction may influence spinal biomechanics to a great extent. Surgery also changes the biomechanics of the spine, and the true way in which the spine behaves following, for instance, decompression and stabilisation can only be estimated.Loads applied onto the spine are shared among spinal components: this is called spinal load sharing. We know that lumbar spinal load sharing takes place, passing 80-96% of the applied load through the anterior part of the spine, and the remainder through the posterior ele- AbstractThe 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.

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✓ The purpose of this study was to generate a validated finite element (FE) model of the human cervical spine to be used to analyze new implants. Digitized data obtained from computerized tomography scanning of a human cervical spine were used to generate a three-dimensional, anisotropic, linear C5–6 FE model by using a software package (ANSYS 5.4). Based on the intact model (FE/Intact), a second was generated by simulating an anterior cervical fusion and plate (ACFP) C5–6 model in which monocortical screws (FE/ACFP) were used. Loading of each FE model was simulated using pure moments of ± 2.5 Nm in flexion/extension, axial left/right rotation, and left/right lateral bending. For validation of the models, their predicted C5–6 range of motion (ROM) was compared with the results of an earlier, corresponding in vitro study of six human spines, which were tested in the intact state and surgically altered at C5–6 with the same implants. The validated model was used to analyze the stabilizing effect of a new disc spacer, Cenius (Aesculap AG, Tuttlingen, Germany), as a stand-alone implant (FE/Cenius) and in combination with an anterior plate (FE/Cenius+ACFP). In addition, compression loads at the upper surface of the spacer were investigated using both models. As calculated by FE/Intact and FE/ACFP models, the ROM was within 1 standard deviation of the mean value of the corresponding in vitro measurements for each loading case. The FE/Cenius model predicted C5–6 ROM values of 5.5° in flexion/extension, 3.1° in axial rotation (left and right), and 2.9° in lateral bending (left and right). Addition of an anterior plate resulted in a further decrease of ROM in each loading case. The FE/Cenius model predicted an increase of compression load in flexion and a decrease in extension, whereas in the FE/Cenius+ACFP model an increase of graft compression in extension and unloading of the graft in flexion were predicted. The current FE model predicted ROM values comparable with those obtained in vitro in the intact state as well as after simulation of an ACFP model. It predicted a stabilizing potential for a new cage, alone and in combination with an anterior plate system, and predicted the influence of both loading modality and additional instrumentation on the behavior of the interbody graft.

IntroductionPosterior lumbar interbody fusion (PLIF) was first described by Briggs and Milligan in 1944 [4] and modified by Cloward in 1953 [6]. PLIF is a surgically sophisticated technique for arthrodesis and stabilisation within the intradiscal space of an unstable lumbar segment. It consists of a generous posterior decompression of the neural structures and then a discectomy and anterior interbody arthrodesis. The intradiscal work is performed between the exiting nerve root and the retracted transversal nerve root and thecal sac at the lumbar motion segment to be stabilised. Indications for this procedure include degenerative lumbar instability and spondylolisthesis [6,11,12,15]. This surgical approach has the advantage over posterior arthrodesis of obtaining a mature fusion in the anterior column, which bears 80% of the axial load and is under compression in an upright posture [20]. Cloward's technique [6] consisted of interbody fusion using autologous iliac crest bone grafts without additional internal stabilisation. The major disadvantage of this technique is the minimal initial internal stabilisation. Furthermore, the stiffness of these constructs decreases in the first few months during the graft absorption and initial integration phase. Graft-related complications with this technique have been reported to range from 3 to 18% [5,16,22]. To improve these initial results with PLIF operations, a variAbstract 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°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.

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.