Metal/cement interface strength in cemented stem fixation

To characterize the strength of the interface between stem-type metal implants and bone cements, a fracture mechanics parameter was used. This parameter, the critical strain energy release rate (Gc), was determined from "push-out" tests of cylindrical specimens. The specimens, formed using molds of bone, were maintained and tested at body temperature. The strength of interfaces formed with cancellous bone surrounding the cement mantle was significantly less than the strength of those formed in apposition to cortical bone. A marked degradation of strength was found with saline immersion for SS316LVM/cement interfaces formed with Zimmer regular, Simplex-P, and Zimmer LVC cements. After 60 days of immersion the interface Gc was only 10-20% of the value for bulk cement. Interfaces formed with thin-film polymethylmethacrylate-precoated metals (SS316LVM, Co-Cr-Mo, and Ti-6A1-4V) yielded "dry" Gc values one order of magnitude greater than those measured with interfaces formed with uncoated metals. Moreover, the strength of precoated SS316LVM/cement interfaces formed with all three brands of cement did not change after saline immersion for 60 days.

Prolonged analgesia induced by cathinone. The role of stress and opioid and nonopioid mechanisms

Cathinone, the active principle of Catha edulis (khat), shows long-lasting analgesic effects when the tail-flick test is used in rats. The involvement of monoamines, endogenous opioids and stress in this analgesic effect was tested. Both early (30 min) and late (24 h) analgesic effects of cathinone were prevented by reserpine or p-chlorophenylalanine, which deplete catecholamines or serotonin, respectively, and by nomifensine, which prevents neuronal uptake of biogenic amines and amphetamines. The same inhibitory effect was obtained with a high dose (4 mg/kg) of naloxone. However, rats made tolerant to morphine retained both early and late analgesic response to cathinone. The increase in plasma ACTH induced by the tail-flick test at 30 min and 24 h was significantly enhanced by cathinone, in a naloxone-reversible way. However, the analgesic responses shown at these times were not prevented by either dexamethasone or adrenalectomy. We conclude that the prolonged analgesia induced by cathinone is primarily due to an amphetamine-like activation of monoaminergic pathways, but requires the integrity of non-mu-opioid mechanisms. The involvement of the adrenohypophyseal axis in this cathinone effect is less probable.

Tolerance develops to sympathetic effects of khat in humans

Khat chewing has been found to induce, like amphetamines, psychostimulation and increase in sympathetic tone. The possibility that tolerance develops to these effects has been studied by comparing the influence of the chewing of a standard dose of khat on some physiological parameters in chronic consumers and naive subjects. Systolic blood pressure appears to rise in both groups, but the increase in diastolic pressure was more marked in naive subjects and inversely related to the reduction of pulse rate. The increases of respiratory rate and body temperature were also higher in naive subjects. In both groups, all these effects disappeared after 18 h. These results are consistent with an amphetamine-like mechanism of action and suggest that tolerance develops to the sympathetic effects of khat.

A pressure distribution transducer for in-vitro static measurements in synovial joints

The basic features and the performance characteristics of a transducer to measure in-vitro static pressure distribution at the articular interfaces of intact synovial joints are described. Pressure distribution is interpreted from the micro-indentation pattern left on a thin plastic material, the indenter and the plastic material having been subjected to load between the articular surfaces. The effects of the finite thickness and compliance of the transducer and the effects of the time-dependent response properties of the articular cartilage on the accuracy of measurement have been estimated by means of specific experiments and analyses.

In-vitro measurement of static pressure distribution in synovial joints--Part I: Tibial surface of the knee

In this first part of a two-part paper, the results of measurement of static pressure distribution on the tibial surface of the knee are presented. Results with intact menisci have been obtained from 18 specimens. Eight of these specimens were the subject of further measurements following medial meniscectomy. The study has been carried out at various flexion angles of the knee with the joint subjected to a compressive force, with or without an initial passive relative displacement between the joint members. The results indicate that a significant fraction of the joint compressive load is transmitted through the menisci and that total meniscectomy causes a drastic alteration in the pressure distribution on the tibial surface. Clinical implications of these results, in terms of post-meniscectomy degenerative changes and mechanism of meniscal lesions, have been discussed.

In-vitro measurement of static pressure distribution in synovial joints--Part II: Retropatellar surface

This second part of a two-part paper is concerned with the measurement of static pressure distribution on the retropatellar surface. The study has been performed in a loading apparatus designed to simulate individually the lines of action and the magnitudes of the tensions in the components of the quadriceps femoris muscle group. Results have been obtained using 24 specimens in the knee flexion range 0 to 130 deg and employing a net quadriceps tension of 734 N. Particular emphasis has been placed on the evaluation of the sensitivity of the results to variations in the characteristics of the simulated quadriceps tension. The pressure distribution results have been interpreted in terms of variation of the normal force and the average contact stress on the retropatellar surface as a function of flexion angle. It has been shown that the "pulley" model of the patella consistently overestimates the actual patellofemoral joint reaction force throughout the range of flexion. Clinical implications of the results, in terms of etiology of degeneration of patellar cartilage, have been discussed.

Effect of a cement-bone composite layer and prosthesis geometry on stresses in a prosthetically resurfaced tibia

A comparative study of stress distributions in the component materials of a number of models of a prosthetically resurfaced tibia is presented. Although the geometry is idealized to be axisymmetric, the loadings for which the finite element analyses are performed are considered to be nonaxisymmetric, simulating more realistically the loading conditions in vivo. The different models are chosen with the view of determining the influence of changes in the prosthesis design on the induced stress distribution in the component materials. The changes considered are in the thickness of the cement and the cement-bone composite layers, and in the shape of the prosthesis. Experimentally measured values of strains are compared with the analytically predicted values to check the validity of the assumptions used in the finite element modeling. The comparison of induced stresses in the different materials reveals the desirability, from a mechanical behavior point of view, of introducing a cement-bone composite layer and using a prosthesis with domed subsurface in the fixation system. It is shown that for a model incorporating these features, considerable reduction of stresses in the cement, in its bulk and at its interface with the prosthesis plate, is achieved. The reduced stresses can be expected to have beneficial effects on the long-term behavior of the cement and its interfaces in the fixation system.

Some static mechanical properties of the lumbar intervertebral joint, intact and injured

The in-vitro static load-displacement properties of the intact and the injured human lumbar intervertebral joint have been investigated in a loading apparatus which allows entirely unconstrained relative displacement between the joint members. The spatial relative displacement produced by a given load, alone or in combination with another load (preload), were measured using 14 specimens and emphasizing strict control of the secondary variables which are known to affect the results. An attempt has been made to interpret the results in terms of the relative load-bearing roles of the disk, the facets and the posterior ligaments as a function of the type of load. The results indicate that the disk is the major load-bearing element in lateral and anterior (with respect to fixed superior vertebra) shears, axial compression and flexion, while the facets play a major role in posterior shear and axial torque.

Transient and residual stresses and displacements in self-curing bone cement - Part II: thermoelastic analysis of the stem fixation system

In this second part of a two-part report, an idealized model of the stem fixation system is analyzed to determine the adverse effects of the thermal stresses and displacements of bone cement during its curing process. The Shaffer-Levitsky stress-rate strain-rate law for chemically hardening material has been used. The results show that if the cement is surrounded by cancellous bone, as opposed to cortical bone, then transient tensile circumferential stresses in the cement and similar radial stresses at the stem/cement interface are generated. The former may cause flaws and voids within the still cement, while the latter may cause gaps at the interface.

Transient and residual stresses and displacements in self-curing bone cement - Part I: characterization of relevant volumetric behavior of bone cement

In this first part of a two-part report, some aspects of the volumetric behavior of bone cement during its curing process are examined as a prelude to an analysis for the transient and residual stresses and displacements in stem fixation systems. Experiments show that stress generation in the cement is associated with its temperature while curing and that during the cooling phase, the stresses are mainly due to thermal as opposed to bulk shrinkage. The appropriate coefficient of thermal expansion of bone cement has been evaluated from measurements in a simulated fixation system in conjuction with a thermoelastic analysis.

The role of secondary variables in the measurement of the mechanical properties of the lumbar intervertebral joint

A general method is proposed for the testing of the structural properties of lumbar intervertebral joints. The apparatus designed includes a loading jig capable of applying any wrench resultant load while allowing unconstrained motion of the specimen and a three-dimensional displacement transducer for measurement of principal and coupled displacements. Four secondary independent variables, load axis position, nucleus pressure, the number of initial cycles and the load duration are shown to significantly affect the dependent variable, displacement. When performing this type of experiment, these secondary variables must be standardized and controlled.

The demonstration of two pregnancy-associated proteins with SP1 determinants in placental extracts

The SP1 content of the placenta has been examined. It was found that, as in pregnancy serum, two variants of the protein existed in the placenta. The smaller molecule, with a beta 1 electrophoretic mobility, is readily extractable with phosphate buffered saline. The second larger molecule is more tightly bound in the placenta and requires Triton X-100 to be solubilized. The hypothesis is put forward that the larger alpha variant is the precursor form which is related more irregularly and in a smaller amount from the placenta into the maternal circulation.

The blood-nerve barrier and reconstitution of the perineurium following nerve grafting

Sural nerve autografts were performed on intact rat sural nerves and on sural nerves excised proximal to the site of grafting. The effect of the presence or absence of regenerating axons upon reconstitution of the perineurium at the graft junctions and upon re-establishment of the blood-nerve barrier to horseradish peroxidase (HRP) were studied over the succeeding 3--24 weeks. Compartmentation of the nerve fascicle occurred at the graft junctions where the perineurium was damaged. Each compartment contained Schwann cells with or without axons and was surrounded by elongated fibroblast-like cells which resembled perineurial cells in the longer surviving animals. It was concluded that, (a) compartments form in a nerve at the site of perineurial damage even in the absence of axons; (b) although compartmentation may be a mechanism for perineurial regeneration and reconstitution of the blood-nerve barrier, blood vessels and the cell layers forming compartments at graft junctions remain permeable to HRP for at least 6 months; and (c) the intact perineurium around the distal stump of a denervated nerve is permeable to HRP but the endoneurial blood vessels are not.