PubMed · 9584713
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R Riley. 1995. Basic biomechanical considerations for implant case planning.. https://pubmed.ncbi.nlm.nih.gov/9584713/
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Single-molecule observation and manipulation have come of age. With the advent of optical tweezers and other methods for probing and imaging single molecules, investigators have circumvented the model-dependent extrapolation from ensemble assays that has been the hallmark of classical biochemistry and biophysics. In recent years, there have been important advances in the understanding of how motor proteins work. The range of these technologies has also started to expand into areas such as DNA transcription and protein folding. Here, recent experiments with rotary motors, linear motors, RNA polymerase, and titin are described.
STUDY DESIGN: A load-controlled biomechanical analysis of flexion, extension, and torsional stiffness in instrumented calf spines. OBJECTIVES: To compare biomechanically the performance of various fixation techniques for the repair of spondylolytic defects in the pars interarticularis. SUMMARY OF BACKGROUND DATA: Several techniques have been developed to stabilize a spondylolytic defect in the lumbar spine. There are, however, no comprehensive biomechanical studies in which these techniques are compared. METHODS: Nine fresh-frozen and thawed calf cadaveric lumbar L2-L6 spines were used for mechanical testing. Scott's technique, Buck's technique (screw fixation in the lamina across the defects), modified Scott's technique (wire loops around cortical screws placed into both pedicles and tightened under the spinous process), and screw-rod-hook fixation were applied on the calf lumbar spines in which bilateral spondylolytic defects were created in the L4 vertebra. Motion across the defects for each direction of loading in flexion, extension, and rotation was measured using extensometers. The intervertebral rotations and the strain at the site of the spondylolytic defect were computed from the acquired load-displacement data. RESULTS: Each fixation technique significantly increased stiffness and returned the intervertebral rotational stiffness to nearly intact levels. Displacement across the defect under flexion loading was significantly suppressed by each instrumentation technique, but the least motion (P < 0.05) was allowed with the screw-rod-hook fixation or Buck's technique. CONCLUSIONS: All four fixation techniques restored the intervertebral rotational displacements under flexion and torsional loading to the intact condition. The screw-rod-hook fixation allowed the least amount of motion across the defect during flexion.
The assembly of synthetic, controllable molecular mechanical systems is one of the goals of nanotechnology. Protein-based molecular machines, often driven by an energy source such as ATP, are abundant in biology. It has been shown previously that branched motifs of DNA can provide components for the assembly of nanoscale objects, links and arrays. Here we show that such structures can also provide the basis for dynamic assemblies: switchable molecular machines. We have constructed a supramolecular device consisting of two rigid DNA 'double-crossover' (DX) molecules connected by 4.5 double-helical turns. One domain of each DX molecule is attached to the connecting helix. To effect switchable motion in this assembly, we use the transition between the B and Z forms of DNA. In conditions that favour B-DNA, the two unconnected domains of the DX molecules lie on the same side of the central helix. In Z-DNA-promoting conditions, however, these domains switch to opposite sides of the helix. This relative repositioning is detected by means of fluorescence resonance energy transfer spectroscopy, which measures the relative proximity of two dye molecules attached to the free ends of the DX molecules. The switching event induces atomic displacements of 20-60 A.