Tensile strength of bovine trabecular bone

Data on the tensile and compressive properties of trabecular bone are needed to define input parameters and failure criteria for modeling total joint replacements. To help resolve differences in reports comparing tensile and compressive properties of trabecular bone, we have developed new methods, based on porous foam technology, for tensile testing of fresh/frozen trabecular bone specimens. Using bovine trabecular bone from an isotropic region from the proximal humerus as a model material, we measured ultimate strengths in tension and compression for two groups of 24 specimens each. The average ultimate strength in tension was 7.6 +/- 2.2 (95% C.I.) MPa and in compression was 12.4 +/- 3.2 MPa. This difference was statistically significant (p = 0.013) and was not related to density differences between the test groups (p = 0.28). Strength was related by a power-law function of the local apparent density, but, even accounting for density influences, isotropic bovine trabecular bone exhibits significantly lower strengths in tension than in compression.     

Tensile strength of bone fixation of hydroxyapatite coated Schanz screws of the Heidelberg External Fixation System (HEFS)--comparative torque measurements in clinical use and in cadaver tibia]

It is claimed in the literature that hydroxyapatite(HA)-coated screws of external fixators have superior fixation strength in bone, which is postulated to lead to a substantial decrease in loosening and infection rates. We report on a study of the maximum torque values developed while inserting and removing 30 HA-coated Schanz screws of 8 Heidelberg external fixation systems applied to the tibia to correct leg length differences and axial deformities. The infection rate was determined in accordance with defined criteria, and was found to be about 20% for the HA-coated screws. Screws without infection showed an extraction torque above insertion torque, screws with infection an extraction torque below. A significant correlation (p = 0.05) was seen between infection and decrease in fixation strength (quotient: loosening torque/tightening torque). To exclude the impact of such biological processes as osteointegration and bone remodelling, the clinical results were compared with the torques measured for coated and uncoated Schanz screws in a human cadaveric tibia. A significantly higher fixation strength in bone was found for HA-coated screws in comparison with uncoated screws (p = 0.002). These data warrant a clinical study directly comparing HA-coated and uncoated Schanz screws.     

椎弓根螺钉抗拔出强度的生物力学测试

目的:分析骨密度(bone mineral density,BMD)和植入角度对椎弓根螺钉拔出强度的影响.方法:6例新鲜尸体脊柱T10-L2骨,分解为单个椎体30个,共60个椎弓根.根据骨密度检查结果分组,每个标本脊椎左侧使螺钉在椎弓根平行植入,在脊柱右侧使螺打在与椎弓根成7°角植入.将60个椎弓根分别进行拔出测试(5mm/min的速度垂直方向拔出).结果:螺打平行植入与椎弓根成7°角植入,后者其拔出力大20%(P<0.05).骨密度时拔出力的影响,正常组与骨质疏松组相差57%(P<0.05).结论:选择与椎弓根成角度植入螺钉有助于提高螺固定相对稳定性;对于骨质疏松的病例,应注意抗拔出力不能满足生理要求的危险性.     

Tensile strength of cell walls of living cells

A gas decompression technique was used to determine the breaking strength of cell walls of single cells. Breaking strengths of the bacterium Salmonella typhimurium and the unicellular green alga Chlamydomonas eugametos were 100 and 95 atmospheres, respectively, while those of sporophytes of the water mold Blastocladiella emersonii were 65 atmospheres, and those of suspension cultured cells of carrot were only 30 atmospheres. Estimation of wall tensile stress based on breaking pressures, cell radii, and estimation of wall thickness, indicates that microfibrillar walls are not necessarily stronger than walls of primitive organisms. Hence, alternative hypotheses for their evolution must be considered.     

Tensile bond strength of orthodontic resins on the human tooth surface

Composite resin has been used in the field of orthodontics for more than twenty years. Although there have been many studies and discussions regarding the bond strength of orthodontic brackets to the human tooth surface, a definitive conclusion has not yet been established. In this study, sixty bicuspids extracted from teenagers were employed for testing, in conjunction with 6 different brands of orthodontic resins: Concise, Unitek, Ormco, American, Mono-lok and Right-on. On the buccal surface of the crown, the tensile bond strength for the various resins were tested and recorded. The bond strength was 0.69 kg/mm2 for Concise, 0.64 kg/mm2 for Unitek, 0.58 Kg/mm2 for Ormco, 0.55 kg/mm2 for American, 0.54 kg/mm2 for Mono-log and 0.45 kg/mm2 for Right-on, respectively. The broken surfaces were examined by scanning electron microscopy, as well as energy dispersive x-ray spectrometry. The broken surfaces were either at interface between the resin and bracket base, at the resin itself, interface between tooth surface and resin or in a combination of them. On the broken surface of the bracket base, components of broken tooth fragments were also detected. The percentage for the frequency of samples of each type of orthodontic resin of broken tooth fragments found was 30% for Concise, 10% for Unitek, 50% for Ormco, 40% for American, 50% for Mono-lok and 80% for Right-on.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bone strength in hypokinesia]

Experiments (20- and 100-day) on rats showed that decrease of the animal motor activity with preserved static function of the extremities led to a marked thinning of the cortical layer of the femoral bone. However, the ability of the whole bone to bear mechanical load decreased but slightly. An increase in the firmness of the bone tissue as a result of its mineralization compensated for the cortical layer thinning.     

Characterization of the properties of differentially modified titanium surfaces for dental implantology. 2: In vitro biocompatibility studies]

The aim of the present study was to determine whether specific surface modifications are capable of improving the biocompatibility of a titanium surface, and whether there is a correlation between the physico-chemical properties of the implant material and its biocompatibility. To this end, the properties of titanium surfaces were modified using various methods or the latter were coated with various materials. Plasma treatments under different atmospheric conditions (N2-plasma, SO2-plasma, acetylene plasma) as well as plasma polymerization were used to affect the biological response. Characterization of the physico-chemical surface properties by means of X-ray photoelectron spectroscopy (XPS), contact angle measurements and the calculation of surface tensions or surface energy provided important information on the interactions at the interface between the implant material and the aqueous environment. The influence of the respective surface modification on cell proliferation, cell viability and the activity of mitochondrial dehydrogenases was evaluated in specific in vitro tests with human gingiva fibroblasts. It was show that different modifications of the titanium samples induce different biological responses of the gingiva fibroblasts. The results confirm the existence of correlations between thermodynamic surface properties and cellular reactions under in vitro conditions.     

Physiological loading of the mandibular osteosynthesis: development of an improved mandibular load simulator]

Loading conditions physiologically approximating those acting on the normal masticatory system were incorporated into a new mandibular load simulator. Separate tension wires attached to each ramus of the mandible simulated the resultant force vectors of the masticatory musculature. The muscle insertion points were chosen in accordance with the anatomical situation, and the maximum in vivo forces acting on the joint. In a first application, the stability of a 2.4 mm LC-WDCP was compared with that of a 2.7 mm EDCP in plastic mandible models. It was found that under largely physiological loading, the 2.4 mm LC-EDCP exerted a stabilizing effect similar to that of a 2.7 mm EDCP. Although of smaller dimensions, the 2.4 mm LC-EDCP appears to enable an osteosynthesis of similar stability in the treatment of fractures of the mandibular angle.     

Biomechanical investigations of different osteosynthesis procedures in the proximal tibia-introduction of a new measuring system]

In this article a new measuring system for biomechanical studies of various osteosynthesis procedures on the tibia is described. The paired tibias are osteotomized and embedded in PMMA in accordance with a randomization protocol. Testing is then carried out under nonaxial loading at 350 N, 600 N and 900 N, uniaxial bending applying a force of 12 Nm, and torsion at a force of 5 Nm in a universal pneumatic testing machine. Ultrasonic sensors pick up movement at the fracture gap under external loading. Testing of the measuring setup has been successful, and relevant results were obtained.     

A study of some factors which affect the strength of screws and their insertion and holding power in bone

The mechanical properties of certain bone screws have been examined. The torque developed during insertion of 4 mm (5/32″) non-self-tapping and self-tapping screws and their holding power in bone have been measured. Screws should be inserted into tapped holes using a torque limiting device if failures of screws at insertion are to be prevented. The construction of a surgical torque limiting screwdriver is given.     

Injectable calcium phosphate cement for augmentation around cancellous bone screws. In vivo biomechanical studies

In lower cancellous apparent bone density, it can be difficult to achieve adequate screw fixation and hence stable fracture fixation. Different strategies have been proposed, one of them is through augmentation using calcium phosphate cement in the region at or close to the screw thread itself. To support the hypothesis of an improved screw fixation technique by augmentation of the bone surrounding the implanted screw, in vivo biomechanical and densitometric studies are performed on rabbit specimen where normal and simulated weak bone quality are considered. In particular, the evolution of screw stability till 12 weeks following the implantation is quantified. A statistical significance in the pull out force for augmented versus non-augmented screws was found for the shorter time periods tested of ≤ 5 days whilst the pull out force was found to increase with time for both augmented and non-augmented screws during the 12 week course of the study. The results of the study demonstrate that the use of an injectable calcium phosphate cement which sets in vivo can significantly improve screw pull out strength at and after implantation for normal and simulated weak bone quality.     

Neutron scattering studies of nucleosome structure at low ionic strength

Ionic strength studies using homogeneous preparations of chicken erythrocyte nucleosomes containing either 146 or 175 base pairs of DNA show a single unfolding transition at about 1.5 mM ionic strength as determined by small-angle neutron scattering. The transition seen by some investigators at between 2.9 and 7.5 mM ionic strength is not observed by small-angle neutron scattering in either type of nucleosome particle. The two contrasts measured (H2O and D2O) indicate that only small conformational changes occur in the protein core, but the DNA is partially unfolded below the transition point. Patterson inversion of the data and analysis of models indicate that the DNA in both types of particle is unwinding from the ends, leaving about one turn of supercoiled DNA bound to the histone core in approximately its normal (compact) conformation. The mechanism of unfolding appears to be similar for both types of particles and in both cases occurs at the same ionic strength. The unfolding observed for nucleosomes in this study is in definite disagreement with extended superhelical models for the DNA and also disagrees with models incorporating an unfolded histone core.     

Electrophoretic study of muscle extracts of low ionic strength]

The method of Na-dodecylsulfate electrophoresis in polyacrylamide gel demonstrated that the so-called tonoactomyosin of smooth muscles extracted from the muscle homogenate with salt media of low ionic power represented a complicated protein system into whose composition there was included a heavy myosin chain with the mol wt of 210000, premyosin subunit with the mol wt of 230000, actin, and, possibly, a number of other proteins. The extracts of low ionic power possessed Mg2+ and Ca2+ activated by ATP-ase activity. The premyosin subunit was also revealed in the extracts of low ionic power from the skeletal muscle homogenates. It is supposed that premyosin subunit was included into the enzymatic system responsible for the ATP-asic properties of the extracts of low ionic power from the homogenates of different types of muscles.