Rhesus monkey intervertebral disk viscoelastic response to shear stress.

A viscoelastic model of the shear response of the Rhesus monkey intervertebral disk, represented by a series chain of four Kelvin units is presented. Two types of investigations are carried out to assess the validity of the model: 1) determination of material properties by comparison with experimental strain creep behavior of the disk; and 2) validation of this viscoelastic model by accurately predicting the experimental results of stress relaxation tests. The use of the series Kelvin units approach provides the first analytical mechanical model capable of predicting the creep and relaxation functions for the intervertebral disk in shear.

Response of the intervertebral disk of the rhesus monkey to P-A shear stress.

The relative functions of spinal elements in resisting P-A shear stress are determined from measurements of force and displacement. The response of intact, enucleate and degenerate disks are presented, as well as stress-strain characteristics of the intervertebral disk and the roles of the articular facets and the nucleus pulposus.

Analytical model of the fatigue characteristics of bone.

The existing data for the bending fatigue strength of bone are analyzed to determine the effects of various test procedures for human and bovine bone. Additional experiments, using bovine bone, are presented to evaluate the effects of stress frequency and stress reversal on fatigue life. An analytical model which relates fatigue life to ultimate strength, stress amplitude, stress frequency, and stress reversal is developed.

Fatigue characteristics of posterior elements of vertebrae.

The fatigue strength of vertebral posterior elements was determined by applying repetitive forces to the inferior joints between the articular processes of human rhesus-monkey vertebrae. The results indicate that in vivo stresses resulting from unusual activities for sustained periods can induce fatigue fracture of the lumbar vertebrae. Comparison of these data with those of other investigators suggests the use of ultimate strength as a normalizing factor to correlate fatigue-strength data obtained under different test conditions.

A stereological analysis of venous nicking observed at retinal arteriovenous crossings.

The accepted hypotheses for the explanation of retinal venous nicking are critically reviewed and an alternative hypothesis based on a stereological analysis is presented. The ophthalmoscopic appearance of retinal venous nicking is best explained by partial collapse of the vein with the major axis of the venous cross section at right angles to the internal limiting a membrane of the venous blood flow nor of the likelihood of tributary vein occlusion. A partially collapsed vein may be very sensitive to small changes in intraocular pressure. Patients with impending tributary vein occlusion may benefit from reducing intraocular pressure.

The skiing sequelae of tibial torsion.

The purpose of this communication is to support the following hypotheses: 1. That variations in tibial torsion are functionally important in parallel skiing. 2. That external tibial torsion in particular is a "deformity" that can interfere markedly with parallel skiing. 3. That even normal individuals have greater external torsion of the right tibia than the left, and that this normal difference may correlate with the preference of many skiers for traversing and maneuvering primarily in one direction. 4. That a relatively small difference in the anatomic alignment of the lower extremities may entail substantial alterations in balance and in distribution of forces in the skier and that these differences should be searched out by centers preparing to equip skiers with bindings and skiis in an attempt to further refine the skier-ski linkage. These hypotheses derive from a blend of the personal experience of the principal author and a detailed search of the English and French literature of the past century relating to rotational alignment, motion, and control of the lower extremity in general, and to tibial torsion in particular.