[Stress levels in bones and bone cement in the thoracolumbar spine afer kyphoplasty. Finite element study]. / Knochen und Knochen-Zement-Belastungen in der thorakolumbalen Wirbelsäule nach Kyphoplastik. Eine Finite-Element-Studie.

The study quantified the stress levels in treated and untreated vertebral bodies following kyphoplasty. Three-dimensional FE models of treated and untreated T11, T12, L1, and L2 vertebral bodies were evaluated to examine the stress levels within the bone and bone cement. A validated T12-L1 model was used to investigate the effect of kyphoplasty treatment on adjacent vertebral stresses and strains. Using the single vertebral models, bone cement modulus changes had minimal effect on the stresses in the bone or the cement. The presence of bone cement had a minimal effect on the stress magnitudes or distribution in the adjacent T12 vertebra. This study provides quantification of the stress levels in bone cement and bone in vertebral bodies treated with kyphoplasty under in vivo-like loading conditions. The presence of bone cement immediately following kyphoplasty has only a slight effect on the stress magnitudes or distributions in adjacent vertebrae.

Anterior scleral canal geometry in pressurised (IOP 10) and non-pressurised (IOP 0) normal monkey eyes.

AIMS: To characterise lamina cribrosa and anterior scleral canal wall architecture in pressurised (IOP 10 mm Hg) and non-pressurised (IOP 0 mm Hg) normal monkey eyes. METHODS: Eight normal eyes from eight monkeys were enucleated before sacrifice and the optic nerve heads (ONH) trephined and immersion fixed in glutaraldehyde (IOP 0). Nine normal eyes from nine monkeys were perfusion fixed in situ with paraformaldehyde at IOP 10 mm Hg (IOP 10), and the ONHs trephined and stored in glutaraldehyde. Each ONH specimen was embedded in glycol methacrylate and cut into vertical or horizontal, 4 micro m thick, serial sections. Within digitised images of every sixth section, anterior laminar position and laminar thickness were measured at nine evenly spaced locations across the scleral canal opening. Additionally, scleral canal diameters at Bruch's membrane (SCD-B) and at the anterior laminar insertion (SCD-ALI) were measured within the 15 middle section images of each vertically sectioned ONH. RESULTS: Anterior laminar position was significantly more anterior (nearer Bruch's membrane) in the IOP 10 eyes, compared with the IOP 0 eyes (116 (+/-95% CI; 2) micro m v 184 (2) micro m, respectively). Also in the IOP 10 eyes, the lamina cribrosa was thinner (195 (2) micro m v 264 (2) micro m) and the scleral canal diameter was larger (SCD-B: 1751 (23) micro m v 1591 (19) micro m; SCD-ALI: 1961 (21) micro m v 1717 (17) micro m), compared with the IOP 0 eyes. CONCLUSION: The anterior scleral canal wall is expanded and the lamina cribrosa is thinned and more tautly stretched within pressurised (perfusion fixed at IOP 10) young monkey eyes, compared with non-pressurised (immersion fixed at IOP 0) young monkey eyes. The constricted scleral canal and the relaxed and thickened lamina in the non-pressurised eyes may represent phenomena that contribute to optic disc swelling in hypotonous eyes.

Posterior scleral thickness in perfusion-fixed normal and early-glaucoma monkey eyes.

PURPOSE: To characterize posterior scleral thickness in the normal monkey eye and to assess the effects of acute (15- to 80-minute) and short-term chronic (3- to 7-week) intraocular pressure (IOP) elevations. METHODS: Both eyes of four normal monkeys (both eyes normal) and four monkeys with early glaucoma (one eye normal and one eye with induced chronic elevation of IOP) were cannulated. In each monkey, IOP was set to 10 mm Hg in the normal eye and 30 or 45 mm Hg in the contralateral eye (normal or early glaucoma) for 15 to 80 minutes. All eight monkeys were perfusion fixed, yielding eight low IOP-normal eyes, four high IOP-normal eyes, and four high IOP-early glaucoma eyes. Posterior scleral thickness was measured histomorphometrically at 15 measurement points within each eye, and the data were grouped by region: foveal, midposterior, posterior-equatorial, and equatorial. RESULTS: Overall, posterior scleral thickness was significantly different in the various regions and among the treatment groups (P < 0.0001). In the low IOP-normal eyes, the posterior sclera was thickest in the foveal region (307 microm) and thinner in the midposterior (199 microm), posterior-equatorial (133 microm), and equatorial (179 microm) regions. In the high IOP-normal and high IOP-early glaucoma eyes, the posterior sclera was thinner both overall and within specific regions, compared with the low IOP-normal eyes. CONCLUSIONS: The posterior sclera in the perfusion-fixed normal monkey eye thins progressively from the fovea to the equator and is thinnest just posterior to the equator. Acute and short-term chronic IOP elevations cause regional thinning within the posterior sclera of some monkey eyes, which significantly increases stresses in the scleral wall.

Optic disc surface compliance testing using confocal scanning laser tomography in the normal monkey eye.

PURPOSE: To determine the effect of acute, experimentally increased intraocular pressure on deformation of the surface of the optic nerve head (optic nerve head surface compliance testing) in normal monkey eyes using confocal scanning laser tomography. METHODS: A total of 156 compliance tests were performed on 48 normal eyes of 30 monkeys in three separate studies. Compliance testing involved obtaining confocal scanning laser tomographic images using a 10 degrees and/or 15 degrees and/or 20 degrees scan angle at various times after intraocular pressure was raised from 10 to 30 or 45 mm Hg. At each point, six images were analyzed to provide a value for a parameter, called mean position of the disc, which was used to express the amount of deformation the surface of the optic nerve head had undergone at that point. Statistical analysis (ANOVA) was performed to evaluate differences in the amounts of deformation in individual eyes at different intraocular pressures and at different compliance testing sessions (studies 1 and 2) and in the two eyes of individual monkeys under the same conditions (study 3). RESULTS: The majority of eyes showed posterior deformation of the surface of the optic nerve head ranging from 15 to 86 microm as early as 10 minutes after intraocular pressure was increased from 10 to 30 mm Hg. When pressure was increased from 30 to 45 mm Hg in a subset of these eyes, most showed additional deformation. Of the 12 eyes for which both 15 degrees and 20 degrees images were obtained at the same compliance test, 7 showed larger amounts of deformation in the 20 degrees images. Of the 18 monkeys tested in both eyes, 12 showed some differences and 4 showed substantial differences between the two eyes. CONCLUSIONS: In the normal monkey eye, the surface of the optic nerve head deforms rapidly (in as few as 10 minutes) in response to increased intraocular pressure. The amount of deformation varies between subjects and even within the two eyes of individual monkeys. Increasing the scan angle from 15 degrees to 20 degrees frequently increases the amount of deformation detected, suggesting that the peripapillary sclera and the optic nerve head may be involved in the deformation in some eyes.

The optic nerve head as a biomechanical structure: initial finite element modeling.

PURPOSE: To study the relationship between intraocular pressure (IOP) and the IOP-related stress (force/cross-sectional area) it generates within the load-bearing connective tissues of the optic nerve head. METHODS: Thirteen digital, three-dimensional geometries were created representing the posterior scleral shell of 13 idealized human eyes. Each three-dimensional geometry was then discretized into a finite element model consisting of 900 constituent finite elements. In five models, the scleral canal was circular (diameters of 0.50, 1.50, 1.75, 2.00, and 2.56 mm), with scleral wall thickness (0.8 mm) and inner radius (12.0 mm) held constant. In three models, the canal was elliptical (vertical-to-horizontal ratios of 2:1 [2.50 x 1.25 mm], 1.5:1 [2.1 x 1.4 mm], and 1.15:1 [1.92 x 1.67 mm]), with the same constant scleral wall thickness and inner radius. In five additional models, scleral canal size was held constant (1.92 x 1.67 mm), and either scleral wall thickness (three models, 0.5, 1.0, and 1.5 mm) or inner radius (two models, 13.0 and 14.0 mm) was varied. In all models, each finite element was assigned a single isotropic material property, either scleral (modulus of elasticity, 5500 kPa) or axonal (modulus of elasticity, 55 kPa). Maximum stresses within specific regions were calculated at an IOP of 15 mm Hg (2000 Pa). RESULTS: Larger scleral canal diameter, elongation of the canal, and thinning of the sclera increased IOP-related stress for a given level of IOP. For all models, maximum IOP-related stress ranged from 6 x IOP (posterior sclera) to 122 x IOP (laminar trabeculae). For each model, maximum IOP-related stress was highest within the laminar trabecular region and decreased progressively through the laminar insertion, peripapillary scleral, and posterior scleral regions. Varying the inner radius had little effect on the maximum IOP-related stress within the scleral canal. CONCLUSIONS: Initial finite element models show that IOP-related stress within the load-bearing connective tissues of the optic nerve head is substantial even at low levels of IOP. Although the data suggest that scleral canal size and shape and scleral thickness are principal determinants of the magnitude of IOP-related stress within the optic nerve head, models that incorporate physiologic scleral canal and laminar geometries, a more refined finite element model meshwork, and nonisotropic material properties will be required to confirm these results.