Pedicle screw-based posterior dynamic stabilisation of the lumbar spine: in vitro cadaver investigation and a finite element study.

Pedicle screw-based dynamic constructs either benefit from a dynamic (flexible) interconnecting rod or a dynamic (hinged) screw. Both types of systems have been reported in the literature. However, reports where the dynamic system is composed of two dynamic components, i.e. a dynamic (hinged) screw and a dynamic rod, are sparse. In this study, the biomechanical characteristics of a novel pedicle screw-based dynamic stabilisation system were investigated and compared with equivalent rigid and semi-rigid systems using in vitro testing and finite element modelling analysis. All stabilisation systems restored stability after decompression. A significant decrease in the range of motion was observed for the rigid system in all loadings. In the semi-rigid construct the range of motion was significantly less than the intact in extension, lateral bending and axial rotation loadings. There were no significant differences in motion between the intact spine and the spine treated with the dynamic system (P>0.05). The peak stress in screws was decreased when the stabilisation construct was equipped with dynamic rod and/or dynamic screws.

Comparison of eight published static finite element models of the intact lumbar spine: predictive power of models improves when combined together.

Finite element (FE) model studies have made important contributions to our understanding of functional biomechanics of the lumbar spine. However, if a model is used to answer clinical and biomechanical questions over a certain population, their inherently large inter-subject variability has to be considered. Current FE model studies, however, generally account only for a single distinct spinal geometry with one set of material properties. This raises questions concerning their predictive power, their range of results and on their agreement with in vitro and in vivo values. Eight well-established FE models of the lumbar spine (L1-5) of different research centers around the globe were subjected to pure and combined loading modes and compared to in vitro and in vivo measurements for intervertebral rotations, disc pressures and facet joint forces. Under pure moment loading, the predicted L1-5 rotations of almost all models fell within the reported in vitro ranges, and their median values differed on average by only 2° for flexion-extension, 1° for lateral bending and 5° for axial rotation. Predicted median facet joint forces and disc pressures were also in good agreement with published median in vitro values. However, the ranges of predictions were larger and exceeded those reported in vitro, especially for the facet joint forces. For all combined loading modes, except for flexion, predicted median segmental intervertebral rotations and disc pressures were in good agreement with measured in vivo values. In light of high inter-subject variability, the generalization of results of a single model to a population remains a concern. This study demonstrated that the pooled median of individual model results, similar to a probabilistic approach, can be used as an improved predictive tool in order to estimate the response of the lumbar spine.

Biomechanics of posterior dynamic stabilization systems.

Spinal rigid instrumentations have been used to fuse and stabilize spinal segments as a surgical treatment for various spinal disorders to date. This technology provides immediate stability after surgery until the natural fusion mass develops. At present, rigid fixation is the current gold standard in surgical treatment of chronic back pain spinal disorders. However, such systems have several drawbacks such as higher mechanical stress on the adjacent segment, leading to long-term degenerative changes and hypermobility that often necessitate additional fusion surgery. Dynamic stabilization systems have been suggested to address adjacent segment degeneration, which is considered to be a fusion-associated phenomenon. Dynamic stabilization systems are designed to preserve segmental stability, to keep the treated segment mobile, and to reduce or eliminate degenerative effects on adjacent segments. This paper aimed to describe the biomechanical aspect of dynamic stabilization systems as an alternative treatment to fusion for certain patients.

Biomechanics of disc degeneration.

Disc degeneration and associated disorders are among the most debated topics in the orthopedic literature over the past few decades. These may be attributed to interrelated mechanical, biochemical, and environmental factors. The treatment options vary from conservative approaches to surgery, depending on the severity of degeneration and response to conservative therapies. Spinal fusion is considered to be the "gold standard" in surgical methods till date. However, the association of adjacent level degeneration has led to the evolution of motion preservation technologies like spinal arthroplasty and posterior dynamic stabilization systems. These new technologies are aimed to address pain and preserve motion while maintaining a proper load sharing among various spinal elements. This paper provides an elaborative biomechanical review of the technologies aimed to address the disc degeneration and reiterates the point that biomechanical efficacy followed by long-term clinical success will allow these nonfusion technologies as alternatives to fusion, at least in certain patient population.

Influence of mono-axis random vibration on reading activity.

Recent studies on train passengers' activities found that many passengers were engaged in some form of work, e.g., reading and writing, while traveling by train. A majority of the passengers reported that their activities were disturbed by vibrations or motions during traveling. A laboratory study was therefore set up to study how low-frequency random vibrations influence the difficulty to read. The study involved 18 healthy male subjects of 23 to 32 yr of age group. Random vibrations were applied in the frequency range (1-10 Hz) at 0.5, 1.0 and 1.5 m/s(2) rms amplitude along three directions (longitudinal, lateral and vertical). The effect of vibration on reading activity was investigated by giving a word chain in two different font types (Times New Roman and Arial) and three different sizes (10, 12 and 14 points) of font for each type. Subjects performed reading tasks under two sitting positions (with backrest support and leaning over a table). The judgments of perceived difficulty to read were rated using 7-point discomfort judging scale. The result shows that reading difficulty increases with increasing vibration magnitudes and found to be maximum in longitudinal direction, but with leaning over a table position. In comparison with Times New Roman type and sizes of font, subjects perceived less difficulty with Arial type for all font sizes under all vibration magnitude.

Spondylolysis originates in the ventral aspect of the pars interarticularis: a clinical and biomechanical study.

Lumbar spondylolysis is a stress fracture of the pars interarticularis. We have evaluated the site of origin of the fracture clinically and biomechanically. Ten adolescents with incomplete stress fractures of the pars (four bilateral) were included in our study. There were seven boys and three girls aged between 11 and 17 years. The site of the fracture was confirmed by axial and sagittal reconstructed CT. The maximum principal tensile stresses and their locations in the L5 pars during lumbar movement were calculated using a three-dimensional finite-element model of the L3-S1 segment. In all ten patients the fracture line was seen only at the caudal-ventral aspect of the pars and did not spread completely to the craniodorsal aspect. According to the finite-element analysis, the higher stresses were found at the caudal-ventral aspect in all loading modes. In extension, the stress was twofold higher in the ventral than in the dorsal aspect. Our radiological and biomechanical results were in agreement with our clinical observations.

Melanocytic nevus-like hyperplasia and melanoma in transgenic BRAFV600E mice.

BRAF, a cellular oncogene and effector of RAS-mediated signaling, is activated by mutation in approximately 60% of melanomas. Most of these mutations consist of a V600E substitution resulting in constitutive kinase activation. Mutant BRAF thus represents an important therapeutic target in melanoma. In an effort to produce a pre-clinical model of mutant BRAF function in melanoma, we have generated a mouse expressing BRAF V600E targeted to melanocytes. We show that in these transgenic mice, widespread benign melanocytic hyperplasia with histological features of nevi occurs, with biochemical evidence of senescence. Melanocytic hyperplasia progresses to overt melanoma with an incidence dependent on BRAF expression levels. Melanomas show CDKN2A loss, and genetic disruption of the CDKN2A locus greatly enhances melanoma formation, consistent with collaboration between BRAF activation and CDKN2A loss suggested from studies of human melanoma. The development of melanoma also involves activation of the Mapk and Akt signaling pathways and loss of senescence, findings that faithfully recapitulate those seen in human melanomas. This murine model of mutant BRAF-induced melanoma formation thus provides an important tool for identifying further genetic alterations that cooperates with BRAF and that may be useful in enhancing susceptibility to BRAF-targeted therapeutics in melanoma.

Minimally invasive decompression for lumbar spinal canal stenosis in younger age patients could lead to higher stresses in the remaining neural arch -- a finite element investigation.

BACKGROUND AND PURPOSE: A young patient group with the symptoms of acquired spinal stenosis has been identified recently in the literature. The patients between 25-50 years of age were found to have signs of lumbar spinal stenosis because of degenerative spinal changes. Some of them were operated on using the same limited decompression approaches as the older patients. However, this group differs from the geriatric population due to the scarcity of remodeling degenerative signs at the spine. Therefore, the possible ligamentous laxity, facet joint degeneration or only the removal of some spinal structures could lead to the increased stresses in the remaining spinal arch and could have an unfavorable course of events after the procedure. A biomechanical study has been done using an experimentally validated finite element model (FEM) of the intact L3-S1 lumbar spine to elucidate the influence of the limited decompression on range of motion (ROM) and stress distribution on the neural arch in this patient group. METHODS: We simulated unilateral laminotomy L4 and medial facetectomy L4-5, medial facetectomy L4-5 and lateral fenestration of L5 pars interarticularis, combined transarticular lateral and medial approach with partial facetectomy L4-5, "port-hole" decompression at the L4 level, and hemilaminectomy L4 with medial facetectomy L4-5. The ROM and maximum von Mises stresses were analyzed in flexion, extension, lateral bending, and axial rotation in response to a 10.6 Nm moment with 400 N axial compression. The data were compared with the intact spine and hemilaminectomy L4 with medial facetectomy L4-5 models. RESULTS AND CONCLUSION: The investigation revealed almost the same ROM after simulation but a considerable increase in stresses at both the pars interarticularis and the inferior facet after limited decompressions, especially in extension and rotation to the contralateral side. Stresses at the contralateral L4 pedicle were highest after L4 hemilaminectomy and medial facetectomy L4-5. Due to the observed increases in stresses, the surgeon should be aware of the possibilities of stress-fractures in this patient group.

Biomechanical comparison of lumbar spine with or without spina bifida occulta. A finite element analysis.

STUDY DESIGN: Biomechanical study using finite element model (FEM) of lumbar spine. OBJECTIVES: Very high coincidence of spina bifida occulta (SBO) has been reported more than in 60% of lumbar spondylolysis. The altered biomechanics due to SBO is one considerable factor for this coincidence. Thus, in this study, the biomechanical changes in the lumbar spine due to the presence of SBO were evaluated. SETTING: United States of America (USA). METHODS: An experimentally validated three-dimensional nonlinear FEM of the intact ligamentous L3-S1 segment was used and modified to simulate two kinds of SBO at L5. One model had SBO with no change in the length of the spinous process and the other had a small dysplastic spinous process. Von Mises stresses at pars interarticularis were analyzed in the six degrees of lumbar motion with 400 N axial compression, which simulates the standing position. The range of motion at L4/5 and L5/S1 were also calculated. RESULTS: It was observed that the stresses in all the models were similar, and there was no change in the highest stress value when compared to the intact model. The range of motion was also similar in all the models. The lumbar kinematics of SBO was thus shown to be similar to the intact model. CONCLUSION: SBO does not alter lumbar biomechanics with respect to stress and range of motion. The high coincidence of spondylolysis in spines with SBO may not be due to the mechanical factors.

Effects of branched beta-carbon dehydro-residues on peptide conformations: syntheses, crystal structures and molecular conformations of two tetrapeptides: (a) N-(benzyloxycarbonyl)-DeltaVal-Leu-DeltaPhe-Leu-OCH3 and (b) N-(benzyloxycarbonyl)-DeltaIle-Ala-DeltaPhe-Ala-OCH3.

The roles of branched beta-carbon dehydro-residues in the design of peptide conformations have not been systematically explored so far. In order to determine the effects of branched beta-carbon dehydro-residues on the peptide conformations, two N-protected tetrapeptides containing new combinations of DeltaVal and DeltaPhe in (a) N-(benzyloxycarbonyl)-DeltaVal-Leu-DeltaPhe-Leu-OCH(3) and DeltaIle and DeltaPhe in (b) N-(benzyloxycarbonyl)-DeltaIle-Ala-DeltaPhe-Ala-OCH(3) were synthesized by solution procedure. The crystal structures of these peptides were determined by X-ray diffraction methods. Single crystals of both peptides were grown by slow evaporation method from their solutions in acetone-water mixtures (80 : 20) at 25 degrees C. The crystals of these peptides belong to the orthorhombic space group P2(1)2(1)2(1) with cell dimensions of a = 12.342(1) A, b = 15.659(1) A, c = 18.970(1) A for peptide (a) and a = 8.093(1) A, b = 15.791(1) A, c = 23.816(1) A for peptide (b) having Z = 4 in the unit cells of both peptides. The structures were refined by full-matrix least-squares procedure to R-factors of 0.076 and 0.052 respectively. Both peptides adopt the right-handed 3(10)-helical conformations stabilized by two intramolecular (i + 3-->i) hydrogen bonds between the CO of N-terminal benzyloxycarbonyl (Cbz) group and the NH of residue at position 3, and between the CO of residue at position 1 and NH of the residue at position 4. The two consecutive 10-membered rings formed by the hydrogen bonds have dihedral angles corresponding to the standard values for type III beta-turns. DeltaVal and DeltaIle in peptides (a) and (b) respectively are located at the (i + 1) position of the first beta-turn while DeltaPhe is located at the (i + 2) position of the second beta-turn. In the crystals, the molecules are linked head to tail by intermolecular hydrogen bonds to form long helical chains. The axes of helices are parallel to the b-axes while the neighbouring helices run in the opposite directions. The crystal packings are further stabilized by van der Waals forces between the columns of molecular packings.

Biomechanical rationale of endoscopic decompression for lumbar spondylolysis as an effective minimally invasive procedure - a study based on the finite element analysis.

We evaluated the biomechanical behavior of the endoscopic decompression for lumbar spondylolysis using the finite element technique. An experimentally validated, 3-dimensional, non-linear finite element model of the intact L3 - 5 segment was modified to create the L4 bilateral spondylolysis and left-sided endoscopic decompression. The model of Gill's laminectomy (conventional decompression surgery of the spondylolysis) was also created. The stress distributions in the disc and endplate regions were analyzed in response to 400 N compression and 10.6 Nm moment in clinically relevant modes. The results were compared among three models. During the flexion motion, the pressure in the L4/5 nucleus pulposus was 0.09, 0.09 and 0.16 (MPa) for spondylolysis, endoscopic decompression and Gill's procedure, respectively. The corresponding stresses in the annulus fibrosus were 0.65, 0.65 and 1.25 (MPa), respectively. The stress at the adjoining endplates showed an about 2-fold increase in the Gill's procedure compared to the other two models. The stress values for the endoscopic and spondylolysis models were of similar magnitudes. In the other motions, i. e., extension, lateral bending, or axial rotation, the results were similar among all of the models. These results indicate that the Gill's procedure may lead to an increase in intradiscal pressure (IDP) and other biomechanical parameters after the surgery during flexion, whereas the endoscopic decompression did not change the segment mechanics after the surgery, as compared to the spondylolysis alone case. In conclusion, endoscopic decompression of the spondylolysis, as a minimally invasive surgery, does not alert mechanical stability by itself.

Role of mechanical factors in the evaluation of pedicle screw type spinal fixation devices.

Prior to implantation, spinal implants are subjected to rigorous testing to ensure safety and efficacy. A full battery of tests for the devices may include many steps ranging from biocompatibility tests to in vivo animal studies. This paper describes some of the essential tests from a mechanical engineering perspective (e.g., motion, load sharing, bench type tests, and finite element model analyses). These protocols reflect the research experience of the past decade or so.

A finite element investigation of upper cervical instrumentation.

STUDY DESIGN: The finite element technique was used to predict changes in biomechanics that accompany the application of a novel instrumentation system designed for use in the upper cervical spine. OBJECTIVE: To determine alterations in joint loading, kinematics, and instrumentation stresses in the craniovertebral junction after application of a novel instrumentation system. Specifically, this design was used to assess the changes in these parameters brought about by two different cervical anchor types: C2 pedicle versus C2-C1 transarticular screws, and unilateral versus bilateral instrumentation. SUMMARY OF BACKGROUND DATA: Arthrodesis procedures can be difficult to obtain in the highly mobile craniovertebral junction. Solid fusion is most likely achieved when motion is eliminated. Biomechanical studies have shown that C1-C2 transarticular screws provide good stability in craniovertebral constructs; however, implantation of these screws is accompanied by risk of vertebral artery injury. A novel instrumentation system that can be used with transarticular screws or with C2 pedicle screws has been developed. This design also allows for unilateral or bilateral implantation. However, the authors are unaware of any reports to date on the changes in joint loading or instrumentation stresses that are associated with the choice of C2 anchor or unilateral/bilateral use. METHODS: A ligamentous, nonlinear, sliding contact, three-dimensional finite element model of the C0-C1-C2 complex and a novel instrumentation system was developed. Validation of the model has been previously reported. Finite element models representing combinations of cervical anchor type (C1-C2 transarticular screws vs. C2 pedicle screws) and unilateral versus bilateral instrumentation were evaluated. All models were subjected to compression with pure moments in either flexion, extension, or lateral bending. Kinematic reductions with respect to the intact (uninjured and without instrumentation) case caused by instrumentation use were reported. Changes in loading profiles through the right and left C0-C1 and C1-C2 facets, transverse ligament-dens, and dens-anterior ring of C1 articulations were calculated by the finite element model. Maximum von Mises stresses within the instrumentation were predicted for each model variant and loading scenario. RESULTS: Bilateral instrumentation provided greater motion reductions than the unilateral instrumentation. When used bilaterally, C2 pedicle screws approximate the kinematic reductions and instrumentation stresses (except in lateral bending) that are seen with C1-C2 transarticular screws. The finite element model predicted that the maximum stress was always in the region in which the plate transformed into the rod. CONCLUSIONS: To the best of the authors' knowledge, this is the first report of predicting changes in loading in the upper cervical spine caused by instrumentation. The most significant conclusion that can be drawn from the finite element model predictions is that C2 pedicle screw fixation provides the same relative stability and instrumentation stresses as C1-C2 transarticular screw use. C2 pedicle screws can be a good alternative to C2-C1 transarticular screws when bilateral instrumentation is applied.

Slippage mechanism of pediatric spondylolysis: biomechanical study using immature calf spines.

STUDY DESIGN: This study analyzed the skeletal-age-dependent strength of the lumbar growth plate to resist anterior shearing forces using the MTS system in the immature calf spine with pars defects. OBJECTIVE: To clarify the pathomechanism of the skeletal-age-dependent incidence of slippage in pediatric patients with pars defects by comparing the strength of the lumbar growth plate among three skeletal age groups. SUMMARY OF BACKGROUND DATA: Isthmic spondylolisthesis occurs and progresses more frequently during the growth period, whereas it is rare afterward. However, little evidence has been demonstrated to elucidate the etiology. METHODS: For this study, 15 lumbar functional spine units were divided into three groups according to their skeletal ages. Five were from neonates (Group 1), five from calves approximately 2 months old (Group 2), and five from calves about 24 months old (Group 3). An anterior shearing force was applied to each specimen until failure, after bilateral pars defects were created. Failure load (newtons) and displacement at failure (millimeters) were calculated from the load-displacement curve. The site of failure was confirmed by plain radiograph. RESULTS: All 15 functional spine units failed at the growth plate. The failure load was 242.79 +/- 46.05 N in Group 1, 986.40 +/- 124.16 N in Group 2, and 2024.54 +/- 245.53 N in Group 3. Statistically significant differences were found among the three groups (P < 0.05). The displacement at failure was 7.52 +/- 1.84 mm in Group 1, 11.10 +/- 2.30 mm in Group 2, and 8.15 +/- 2.66 mm in Group 3. There were no significant differences among the groups. CONCLUSIONS: The results indicate that the strength of the growth plate, the weakest link in this model, against anterior shearing forces depends on the skeletal maturity, and that the biomechanical weakness of the growth plate plays an important role in the slippage mechanism.

Monoclonal antibodies against the iron regulated outer membrane Proteins of Acinetobacter baumannii are bactericidal.

BACKGROUND: Iron is an important nutrient required by all forms of life.In the case of human hosts,the free iron availability is 10(-18) M,which is far less than what is needed for the survival of the invading bacterial pathogen. To survive in such conditions, bacteria express new proteins in their outer membrane and also secrete iron chelators called siderophores. RESULTS/ DISCUSSION: Acinetobacter baumannii ATCC 19606, a nosocomial pathogen which grows under iron restricted conditions, expresses four new outer membrane proteins,with molecular weight ranging from 77 kDa to 88 kDa, that are called Iron Regulated Outer Membrane Proteins (IROMPs). We studied the functional and immunological properties of IROMPs expressed by A.baumanii ATCC 19606. The bands corresponding to IROMPs were eluted from SDS-PAGE and were used to immunize BALB/c mice for the production of monoclonal antibodies. Hybridomas secreting specific antibodies against these IROMPs were selected after screening by ELISA and their reactivity was confirmed by Western Blot. The antibodies then generated belonged to IgM isotype and showed bactericidical and opsonising activities against A.baumanii in vitro. These antibodies also blocked siderophore mediated iron uptake via IROMPs in bacteria. CONCLUSION: This proves that iron uptake via IROMPs,which is mediated through siderophores,may have an important role in the survival of A.baumanii inside the host,and helps establishing the infection.

Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical investigation.

Cervical spine disorders such as spondylotic radiculopathy and myelopathy are often related to osteophyte formation. Bone remodeling experimental-analytical studies have correlated biomechanical responses such as stress and strain energy density to the formation of bony outgrowth. Using these responses of the spinal components, the present study was conducted to investigate the basis for the occurrence of disc-related pathological conditions. An anatomically accurate and validated intact finite element model of the C4-C5-C6 cervical spine was used to simulate progressive disc degeneration at the C5-C6 level. Slight degeneration included an alteration of material properties of the nucleus pulposus representing the dehydration process. Moderate degeneration included an alteration of fiber content and material properties of the anulus fibrosus representing the disintegrated nature of the anulus in addition to dehydrated nucleus. Severe degeneration included decrease in the intervertebral disc height with dehydrated nucleus and disintegrated anulus. The intact and three degenerated models were exercised under compression, and the overall force-displacement response, local segmental stiffness, anulus fiber strain, disc bulge, anulus stress, load shared by the disc and facet joints, pressure in the disc, facet and uncovertebral joints, and strain energy density and stress in the vertebral cortex were determined. The overall stiffness (C4-C6) increased with the severity of degeneration. The segmental stiffness at the degenerated level (C5-C6) increased with the severity of degeneration. Intervertebral disc bulge and anulus stress and strain decreased at the degenerated level. The strain energy density and stress in vertebral cortex increased adjacent to the degenerated disc. Specifically, the anterior region of the cortex responded with a higher increase in these responses. The increased strain energy density and stress in the vertebral cortex over time may induce the remodeling process according to Wolff's law, leading to the formation of osteophytes.

Experimental validation of a finite element model of the temporomandibular joint.

PURPOSE: A 2-dimensional finite model of the temporomandibular joint (TMJ) was previously developed to provide a way of studying the specific roles of individual components as well as the overall dynamics of joint motion. This study was undertaken to show that the previously reported finite element model provides results that are consistent with the experimentally obtained results. MATERIALS AND METHODS: The upper compartment of a TMJ of a fresh cadaver specimen was exposed to allow the insertion of a small strip of pressure-sensitive film. Measured loads were applied to the chin and angle of the mandible, pressing the condyle into the glenoid fossa. The resulting stresses in the joint stained the film, providing a way to determine their magnitude. Similar loads were applied to the finite element model and the stresses in the TMJ were mathematically calculated. RESULTS: Experimental results were successfully obtained in 4 separate attempts, recording maximum stresses of 5.6, 8.6, 6.4, and 9.9 MPa (megapascals), respectively. The corresponding finite element model predictions were 7.3, 6.9, 6.4, and 8.2 MPa, respectively. CONCLUSIONS: This study indicates that the results of the previously reported finite element model of the TMJ provide a reasonable approximation of the actual physical situation.

A biomechanical comparison between anterior and transverse interbody fusion cages.

STUDY DESIGN: Human cadaveric lumbar spines underwent placement of threaded fusion cages (TFCs) in either an anterior or transverse orientation. Spines underwent load testing and angular rotation measurement in the intact state, after diskectomy, after cage placement, and after fatiguing. Angular rotations were compared between cage orientations and interventions. OBJECTIVE: To determine which cage orientation resulted in greater immediate stability. SUMMARY OF BACKGROUND DATA: There has been extensive biomechanical study of interbody fusion cages. The lateral orientation has been increasingly used for intervertebral fusion, but a direct biomechanical comparison between cages implanted either anteriorly or transversely in human cadaveric spines has not been performed. METHODS: Fourteen spines were randomized into the anterior group (anterior diskectomy and dual anterior cage placement) and the lateral group (lateral diskectomy and single transverse cage placement). Pure bending moments of 1.5, 3.0, 4.5, and 6.0 Nm were applied in flexion, extension, lateral bending, and axial rotation. Load testing was performed while intact, after diskectomy, after cage placement, and after fatiguing. Angular rotation was compared between anterior and lateral groups and, within each group, among the different interventions. RESULTS: Segmental ranges of motion were similar between spines undergoing either anterior or lateral cage implantation. CONCLUSIONS: These results demonstrate few differences between angular rotation after either anterior or lateral TFC implantation. These findings add to data that find few differences between orientation of implanted TFCs. Combined with a decreased risk of adjacent structure injury through a lateral approach, these data support a lateral approach for lumbar interbody fusion.

Load-sharing between anterior and posterior elements in a lumbar motion segment implanted with an artificial disc.

STUDY DESIGN: A nonlinear three-dimensional finite element model of the osteoligamentous L3-L4 motion segment was used to predict changes in posterior element loads as a function of disc implantation and associated surgical procedures. OBJECTIVES: To evaluate the effects of disc implantation on the biomechanics of the posterior spinal elements (including the facet joints, pedicles, and lamina) and on the vertebral bodies. SUMMARY OF BACKGROUND DATA: Although several artificial disc designs have been used clinically, biomechanical data-particularly the change in loads in the posterior elements after disc implantation-are sparse. METHODS: A previously validated intact finite element model was implanted with a ball-and-cup-type artificial disc model via an anterior approach. The implanted model predictions were compared with in vitro data. To study surgical variables, small and large windows were cut into the anulus, and the implant was placed anteriorly and posteriorly within the disc space. The anterior longitudinal ligament was also restored. Models were subjected to either 800 N axial compression force alone or to a combination of 10 N-m flexion-extension moment and 400 N axial preload. Implanted model predictions were compared with those of the intact model. RESULTS: Facet loads were more sensitive to the anteroposterior location of the artificial disc than to the amount of anulus removed. Under 800 N axial compression, implanted models with an anteriorly placed artificial disc exhibited facet loads 2.5 times greater than loads observed with the intact model, whereas posteriorly implanted models predicted no facet loads in compression. Implanted models with a posteriorly placed disc exhibited greater flexibility than the intact and implanted models with anteriorly placed discs. Restoration of the anterior longitudinal ligament reduced pedicle stresses, facet loads, and extension rotation to nearly intact levels. CONCLUSIONS: The models suggest that, by altering placement of the artificial disc in the anteroposterior direction, a surgeon can modulate motion-segment flexuralstiffness and posterior load-sharing, even though the specific disc replacement design has no inherent rotational stiffness.