Is remaining intervertebral disc tissue interfering with bone generation during fusion of two vertebrae?

STUDY DESIGN: laboratory research. BACKGROUND: Through the increasing number of minimally invasive procedures in spinal fusion surgery, the complete removal of intervertebral disc (IVD) tissue has become more a challenge. Remaining IVD may interfere with the biological process of bone formation. OBJECTIVE: In order to establish whether complete removal of IVD tissue will improve or inhibit the fusion process, the effects of different concentrations of extracts of inflamed disc tissue on the mitochondrial activity of mesenchymal stem cells (MSCs), and the capacity to mineralize their extracellular matrix by osteoblasts and differentiated MSCs were tested in vitro. METHODS: A MTT assay was conducted to measure the mitochondrial activity of MSCs, and an Alizarin Red S staining quantification assay to measure the deposition of calcium by osteoblasts and differentiated, bone marrow-derived MSCs. RESULTS: A significantly higher mitochondrial activity was shown in MSCs co-cultured with extracts of IVD tissue (10%, 50%, and 100%) compared with the control group after 48 hours of incubation, indicating that the IVD tissue extracts stimulated the mitochondrial activity of MSCs. This effect appeared to be inversely proportional to the concentration of IVD tissue extract. No significant differences in mineralization by human osteoblasts or differentiated MSCs were found between the samples incubated with IVD tissue extracts (3% and 33%) and the control samples. CONCLUSION: Our findings indicate that remaining IVD tissue has more of a stimulating than inhibiting effect on the activity of MSCs. Even if inflammatory cytokines are produced, these do not result in a net inhibition of cellular activity or osteogenic differentiation of MSCs.

Biomechanical evaluation of two minimal access interbody cage designs in a cadaveric model.

BACKGROUND: Different interbody grafts have been employed and evaluated for spinal fusion surgery. The Memory Metal Minimal Access Cage (MAC) is a hollow horseshoe shaped interbody fusion concept which provides a potentially major advantage with their small cage contact area and large graft space in comparison with other vertical cages. METHODS: This Biomechanical Cadaveric Study evaluates the primary stability and the amount of acute subsidence occurring in two new MAC cage designs; the Niti-l and Niti-s. Both cages were made of nitinol in the form of a wedge-shaped horseshoe with spikes on the edges. Differences were the higher weight and larger tranverse section area of the Niti-l due to his specific design with two different layers of thickness. Biomechanical axial compression tests were performed on ten fresh-frozen T11-L5 vertebral bodies. RESULTS: A direct relation between force at failure and BMD was found (p < 0.001). The displacements in the vertebral body at an axial force of 800 N were 1.91 mm and 1.88 mm for the NiTi-l and NiTi-s cage, respectively. The mean failure load for the NiTi-l cages was 2043 N, and 1866 N for de NiTi-s cages. No significant difference was established between the two cages. CONCLUSION: The biomechanical strength of both NiTi-l and NiTi-s cages is good and comparable to each other with a limited amount of short-term subsidence after the initial implantation of the cage spikes into the bone.

Altered head orientation patterns in children with idiopathic scoliosis in conditions with sensory conflict.

PURPOSE: Idiopathic scoliosis (IS) is the most common spinal deformity in adolescents. Defective postural equilibrium may be a contributing factor. The information of the three sensory systems combined enables the formation of a central representation of head position and body posture. Comparison of head angles of girls with and without scoliosis may result in a difference in head orientation. METHODS: 25 girls with IS and 16 girls without scoliosis (NS) between the age of 10-16 years stand in a special constructed box on a roll-tilting platform (tilt -14° to +14°). RESULTS: NS and IS subjects behave quite similarly if there is no sensory conflict, but if there is conflict, the differences between the two groups are greater, especially within the 13- to 14-year-old category. CONCLUSIONS: The differences between groups for different age categories suggest that the process of development of sensory integration for estimation of verticality appears to be different for girls with scoliosis.

The memory metal minimal access cage: a new concept in lumbar interbody fusion-a prospective, noncomparative study to evaluate the safety and performance.

Study Design/Objective. A single-centre, prospective, non-comparative study of 25 patients to evaluate the performance and safety of the Memory Metal Minimal Access Cage (MAC) in Lumbar Interbody Fusion. Summary of Background Data. Interbody fusion cages in general are designed to withstand high axial loads and in the meantime to allow ingrowth of new bone for bony fusion. In many cages the contact area with the endplate is rather large leaving a relatively small contact area for the bone graft with the adjacent host bone. MAC is constructed from the memory metal Nitinol and builds on the concept of sufficient axial support in combination with a large contact area of the graft facilitating bony ingrowth and ease in minimal access implantation due to its high deformability. Methods. Twenty five subjects with a primary diagnosis of disabling back and radicular leg pain from a single level degenerative lumbar disc underwent an interbody fusion using MAC and pedicle screws. Clinical performance was evaluated prospectively over 2 years using the Oswestry Disability Index (ODI), Short Form 36 questionnaire (SF-36) and pain visual analogue scale (VAS) scores. The interbody fusion status was assessed using conventional radiographs and CT scan. Safety of the device was studied by registration of intra- and post-operative adverse effects. Results. Clinical performance improved significantly (P < .0018), CT scan confirmed solid fusion in all 25 patients at two year follow-up. In two patients migration of the cage occurred, which was resolved uneventfully by placing a larger size at the subsequent revision. Conclusions. We conclude that the Memory Metal Minimal Access Cage (MAC) resulted in 100% solid fusions in 2 years and proved to be safe, although two patients required revision surgery in order to achieve solid fusion.

The effect of three-dimensional geometrical changes during adolescent growth on the biomechanics of a spinal motion segment.

During adolescent growth, vertebrae and intervertebral discs undergo various geometrical changes. Although such changes in geometry are well known, their effects on spinal stiffness remains poorly understood. However, this understanding is essential in the treatment of spinal abnormalities during growth, such as scoliosis. A finite element model of an L3-L4 motion segment was developed, validated and applied to study the quantitative effects of changing geometry during adolescent growth on spinal stiffness in flexion, extension, lateral bending and axial rotation. Height, width and depth of the vertebrae and intervertebral disc were varied, as were the width of the transverse processes, the length of the spinous process, the size of the nucleus, facet joint areas and ligament size. These variations were based on average growth data for girls, as reported in literature. Overall, adolescent growth increases the stiffness with 36% (lateral bending and extension) to 44% (flexion). Two thirds of this increase occurs between 10 and 14 years of age and the last third between 14 years of age and maturity. Although the height is the largest geometrical change during adolescent growth, its effect on the biomechanics is small. The depth increase of the disc and vertebrae significantly affects the stiffness in all directions, while the width increase mainly affects the lateral bending stiffness. Hence, when analysing the biomechanics of the growing adolescent spine (for instance in scoliosis research), the inclusion of depth and width changes, in addition to the usually implemented height change, is essential.

Geometry of the intervertebral volume and vertebral endplates of the human spine.

Replacement of a degenerated vertebral disc with an artificial intervertebral disc (AID) is currently possible, but poses problems, mainly in the force distribution through the vertebral column. Data on the intervertebral disc space geometry will provide a better fit of the prosthesis to the vertebrae, but current literature on vertebral disc geometry is very scarce or not suitable. In this study, existing CT-scans of 77 patients were analyzed to measure the intervertebral disc and vertebral endplate geometry of the lumbar spine. Ten adjacent points on both sides of the vertebrae (S1-superior to T12-inferior) and sagittal and transverse diameters were measured to describe the shape of the caudal and cranial vertebral planes of the vertebrae. It was found that the largest endplate depth is located in the middle or posterior regions of the vertebra, that there is a linear relationship between all inferior endplate depths and the endplate location (p < 0.0001) within the spinal column, and that the superior endplate depth increases with age by about 0.01 mm per year (p < 0.02). The wedge angle increases from T12-L1 to L5-S1. The results allow for improvement of the fit of intervertebral disc-prostheses to the vertebrae and optimized force transmission through the vertebral column.

[Kyphoplasty as treatment for osteoporotic vertebral compression fractures: relatively safe, but still no evidence of functional improvement; a review of the literature]. / Kyfoplastiek als behandeling van een osteoporotische wervelcompressiefractuur: relatiefveilig, maar nog zonder evidence voor verbeterd functioneren; literatuuroverzicht.

OBJECTIVE: To determine, on the basis of the available literature, whether kyphoplasty is a safe technique for the treatment of osteoporotic vertebral compression fractures that leads to functional improvement in the patient's activities of daily living. DESIGN: Review of the literature. METHOD. Publications on the results of kyphoplasty were retrieved from Medline, Embase and the Cochrane Central Register of Controlled Trials (CENTRAL). Studies were included on the treatment of osteoporotic vertebral compression fractures with kyphoplasty involving more than 15 patients and with a follow-up of at least 6 months. The quality of the studies was assessed using the Newcastle Ottawa quality assessment scale. RESULTS: 12 studies met the inclusion criteria. No randomised controlled trials were retrieved. Severe complications such as pulmonary embolism, spinal stenosis, radiculopathy, and epidural haematoma occurred in 13 of the 737 patients. Leakage of cement occurred in 133 out of 1205 treated vertebrae. In 2 studies in which the functional outcome was evaluated, there was a significant improvement in comparison with the control group after six months. CONCLUSION: Kyphoplasty for osteoporotic vertebral compression fractures has a low complication risk. However, the quality of the retrieved studies was not sufficient to answer the question whether kyphoplasty leads to improved function of the patient.

Neurenteric cyst--a case report of this rare disorder.

Neurenteric cysts are rare congenital lesions that are believed to be the result of the split notochord syndrome. We report the clinical case of a 5-year-old boy presenting with vague gastrointestinal symptoms and fatigue, who had undergone resection of a small intestine duplication cyst as a newborn. Computed tomography revealed a mediastinal neurenteric cyst with partial destruction of several thoracic vertebrae. Resection of the tumor proved effective. Recognition of this disorder is important: because of its benign nature, the prognosis after surgical resection can be good. If the diagnosis is made in an early stage, unnecessary progressive destruction of surrounding structures may be prevented.

Numerical simulation of asymmetrically altered growth as initiation mechanism of scoliosis.

The causes of idiopathic scoliosis are still uncertain; buckling is mentioned often, but never proven. The authors hypothesize another option: unilateral postponement of growth of MM Rotatores or of ligamentum flavum and intertransverse ligament. In this paper, both buckling and the two new theories of scoliotic initiation are studied using a new finite element model that simulates the mechanical behavior of the human spine. This model was validated by the stiffness data of Panjabi et al. (J. Biomech. 9:185-192, 1976). After a small correction of the prestrain of some ligaments and the MM Rotatores the model appeared to be valid. The postponement in growth was translated in the numerical model in an asymmetrical stiffness. The spine was loaded axially and the resulting deformation was analyzed for the presence of the coupling of lateral deviation and axial rotation that is characteristic for scoliosis. Only unilateral postponement of growth of ligamentum flavum and intertransverse ligament appeared to initiate scoliosis. Buckling did not initiate scoliosis.

A new orthotic device in the non-operative treatment of idiopathic scoliosis.

A transverse force system, consisting of an anterior progression force counteracted by a posterior force and torque, acts on the vertebrae of a scoliotic spine. The aim of the newly introduced TriaC brace is to reverse this transverse force pattern by externally applied and constantly present orthotic forces. In the frontal plane the force system in the TriaC brace is in accordance with the force system of the conventional braces. However, in the sagittal plane the force system acts only in the thoracic region. As a result, there is no pelvic tilt, and it provides flexibility without affecting the correction forces during body motion. In the current preliminary study it is demonstrated that the brace prevents further progression of the Cobb angle and axial rotation in idiopathic scoliosis. The new brace has the added advantage of comfort for the wearer, and it offers a better cosmetic appearance, as well as, potentially, a better compliance.

Improving the fixation of an artificial intervertebral disc.

The fixation of an artificial intervertebral disc has been studied especially with respect to the dimensions, the convexity of the endplates and the size of the fixation elements. From literature and cadaveric vertebrae, the dimensions and shape of the lumbar vertebral endplates were determined and the dimensions of fixation ribs for the artificial intervertebral disc were calculated. To withstand shear forces and prevent dislocation, two sagittal ribs, each 3.5 mm in height and at least 20 mm in length and four transversal ribs, each 1.5 mm in height and with a total length of 60 mm are sufficient. A range of five different sagittal diameters was selected as sufficient for all patients. At least 72.6 % of the endplate of the vertebrae is covered. A convexity with a radius of 140 mm limits the gap to 0.62 mm.

Scoliosis correction with shape-memory metal: results of an experimental study.

The biocompatibility and functionality of a new scoliosis correction device, based on the properties of the shape-memory metal nickel-titanium alloy, were studied. With this device, the shape recovery forces of a shape-memory metal rod are used to achieve a gradual three-dimensional scoliosis correction. In the experimental study the action of the new device was inverted: the device was used to induce a scoliotic curve instead of correcting one. Surgical procedures were performed in six pigs. An originally curved squared rod, in the cold condition, was straightened and fixed to the spine with pedicle screws. Peroperatively, the memory effect of the rod was activated by heating the rod to 50 degrees C by a low-voltage, high-frequency current. After 3 and after 6 months the animals were sacrificed. The first radiographs, obtained immediately after surgery, showed in all animals an induced curve of about 40 degrees Cobb angle - the original curve of the rod. This curve remained constant during the follow-up. The postoperative serum nickel measurements were around the detection limit, and were not significantly higher compared to the preoperative nickel concentration. Macroscopic inspection after 3 and 6 months showed that the device was almost overgrown with newly formed bone. Corrosion and fretting processes were not observed. Histologic examination of the sections of the surrounding tissues and sections of the lung, liver, spleen and kidney showed no evidence of a foreign body response. In view of the initiation of the scoliotic deformation, it is expected that the shape-memory metal based scoliosis correction device also has the capacity to correct a scoliotic curve. Moreover, it is expected that the new device will show good biocompatibility in clinical application. Extensive fatigue testing of the whole system should be performed before clinical trials are initiated.

On the determination of the angular orientation of a vertebra.

In this paper we consider the spatial orientation of vertebrae. We take the view that, in determining their rotation angles from X-rays, the procedure applied by Drerup yields the most reliable empirical results, viz. the three angles through which a vertebra rotates about its own symmetry axes in a specific sequence. With a view to the further use of this information to analyze deformations or the motion of a spine we recommend that the Drerup angles be converted into the well-known Eulerian angles. How this can be done is the subject of this report.

Requirements for an artificial intervertebral disc.

Intervertebral disc degeneration is an important social and economic problem. Presently available artificial intervertebral discs (AIDs) are insufficient and the main surgical intervention is still spinal fusion. The objective of the present study is to present a list of requirements for the development of an AID which could replace the human lumbar intervertebral disc and restore its function. The list addresses geometry, stiffness, range of motion, strength, facet joint function, center of rotation, fixation, failsafety and implantation technique. Date are obtained from the literature, quantified where possible and checked for consistency. Existing AIDs are evaluated according to the presented list of requirements. Endplate size is a weak point in existing AIDs. These should be large and fit vertebral bodies to prevent migration. Disc height and wedge angle should be restored, unless this would overstretch ligaments. Finally, stiffness and range of motion in all directions should equal those of the healthy disc, except for the axial rotation to relieve the facet joints.

The geometry of the human paraspinal muscles with the aid of three-dimensional computed tomography scans and 3-Space Isotrak.

STUDY DESIGN: Accurate determination of the three-dimensional coordinates of paraspinal muscles is presented. OBJECTIVES: To determine the precise position and the size of the paraspinal muscles. SUMMARY OF BACKGROUND DATA: The accurate measurements of the muscle moment arms are important in the evaluation of computer models for spinal movement. It has been reported that a change in the modelling of the erector spinae muscles can alter the compressive forces in the spinal column by 20% and can increase the offsetting shear forces. Classic studies used measurements from cross-sectional anatomic diagrams of human cadavers, scaled to the width and depth of the trunk of the subject, to calculate the moment that produces the joint torque. METHODS: Computed tomography scans of two male cadavers provided data on the bony elements. Three-dimensional coordinates of the origins and insertions of the muscle-fascicles of the paraspinal muscles were obtained with the use of 3-Space Isotrak equipment. The bony and muscle coordinates were combined in the ANSYS (version 5.4; Swanson Analysis Systems, Canonsberg, PA) program. RESULTS: Results of this combination and the three-dimensional coordinates of the paraspinal muscles as acquired by the 3-Space Isotrak are given. CONCLUSION: The position and the moment arms of paraspinal muscles were accurately determined. This is important for further evaluation of mathematical models.

Curve progression and spinal growth in brace treated idiopathic scoliosis.

The risk of progression of idiopathic scoliosis is correlated primarily to factors that predict potential remaining skeletal growth. The aim of the current study was to evaluate spinal growth, measured as the length of the scoliotic spine on serial longitudinal radiographs, and its relationship to progression of the scoliotic curve. The retrospective study was based on measurements made on standing anteroposterior radiographs of 60 patients with adolescent idiopathic scoliosis. In all patients, a Boston brace was prescribed during the followup period. Despite brace treatment, a significantly greater average progression rate of the scoliotic curve was seen in periods of rapid to moderate growth (> or = 10 mm per year) compared with periods of small or no growth (< 10 mm per year). The difference in progression rates concerned the increase of the Cobb angle and the increase of lateral deviation and axial rotation. These findings indicate the length of the spine measured on subsequent radiographs is an excellent parameter to determine spinal growth and thus an excellent predictor of scoliosis progression. With the presented growth charts, which were derived from the measured individual growth velocity values of the patients in the study, it is possible to predict future spinal growth at different chronologic ages.

The aetiology of idiopathic scoliosis: biomechanical and neuromuscular factors.

The aetiology of idiopathic scoliosis: biomechanical and neuromuscular factors small curve develops due to a small defect in the neuromuscular control system and a second stage during adolescent growth in which the scoliotic curve is exacerbated by biomechanical factors.

A biomechanical analysis of the vertebral and rib deformities in structural scoliosis.

Although the structural changes occurring in the scoliotic spine have been reported as early as the 19th century, the descriptions and biomechanical explanations have not always been complete and consistent. In this study, three-dimensionally rendered CT images of two human skeletons with a scoliotic deformity and two patients with serious scoliosis were used to describe the intrinsic vertebral and rib deformities. The pattern of structural deformities was found to be consistent. Apart from the wedge deformation of the apical vertebrae, a rotation deformity was found in the transversal plane between the vertebral body and the posterior complex: the vertebral body was maximally rotated towards the convexity of the scoliotic curve, whereas the tip of the spinous process was pointed to posterior. The rib deformities at the convex side of the scoliotic curve showed an increased angulation of the rib at the posterior angle, whereas the rib curve on the concave side was flattened. The observed vertebral deformities suggest that these are caused by bone remodelling processes due to forces in the anterior spinal column, which drive the apical vertebral body out of the midline, whereas forces of the musculo-ligamentous structures at the posterior side of the spinal column attempt to minimize the deviations and rotations of the vertebrae. The demonstrated rib deformities suggest an adaptation to forces imposed by the scoliotic spine.

Electrochemical and surface characterization of a nickel-titanium alloy.

For clinical implantation purposes of shape memory metals the nearly equiatomic nickel-titanium (NiTi) alloy is generally used. In this study, the corrosion properties and surface characteristics of this alloy were investigated and compared with two reference controls, AISI 316 LVM stainless steel and Ti6A14V. The anodic polarization curves, performed in Hanks' solution at 37 degrees C, demonstrated a passive behaviour for the NiTi alloy. A more pronounced difference between the corrosion and breakdown potential, i.e. a better resistance to chemical breakdown of passivity was found for the NiTi alloy compared to AISI 316 LVM. X-ray electron spectroscopy (XPS) and scanning electron microscopy (SEM) were undertaken to study the elemental composition and structure of the surface films prior to, and after immersion in Hanks' solution. The passive film on the NiTi alloy consists of a mainly TiO2-based oxide with minimal amounts of nickel in the outermost surface layers. After immersion in Hanks' solution the growth of a calcium-phosphate layer was observed. The passive diffusion of nickel from the NiTi alloy, measured by atomic absorption spectrophotometry reduced significantly in time from an initial release rate of 14.5 x 10(-7) microg cm(-2) s(-1) to a nickel release that could not detect anymore after 10 days. It is suggested that the good corrosion properties of the NiTi alloy and the related promising biological response, as reported in literature, may be ascribed to the presence of mainly a TiO2-based surface layer and its specific properties, including the formation of a calcium-phosphate layer after exposure to a bioenvironment.