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Biomedical subjects

T M Wright

Publications and source records attributed to T M Wright.

At least 91 records · Page 5Linked to original sources
PubMed1984

Fatigue crack propagation behavior of ultrahigh molecular weight polyethylene.

The relative fatigue crack propagation resistance of plain and carbon fiber-reinforced ultrahigh molecular weight polyethylene (UHMWPE) was determined from cyclic loading tests performed on compact tension specimens machined from the tibial components of total knee prostheses. Both materials were characterized by dynamic mechanical spectroscopy, X-ray diffraction, and differential scanning calorimetry. The cyclic tests used loading in laboratory air at 5 Hz using a sinusoidal wave form. Dynamic mechanical spectroscopy showed that the reinforced UHMWPE had a higher elastic storage modulus than the plain UHMWPE, whereas X-ray diffraction and differential scanning calorimetry showed that the percent crystallinity and degree of order in the crystalline regions were similar for the two materials. Fatigue crack propagation in both materials proved to be very sensitive to small changes in the applied cyclic stress intensity range. A 10% increase in stress intensity resulted in approximately an order of magnitude increase in fatigue crack growth rate. The fatigue crack propagation resistance of the reinforced UHMWPE was found to be significantly worse than that of the plain UHMWPE. This result was attributed to poor bonding between the carbon fibers and the UHMWPE matrix and the ductile nature of the matrix itself.

Carbon↗
PubMed1984

The effect of antibiotic additions on the fracture properties of bone cements.

Two commercially developed cements (Palacos and Zimmer) were tested for fracture toughness with and without gentamicin additions. Compact tension specimens were molded under standardized conditions and divided into four groups. Each group contained specimens of both plain and both antibiotic cements. One group was tested as zero-time controls. The remaining three groups were radiation sterilized. One group was tested as zero-time sterilization controls. Another group was tested after 2 months immersion in Ringer's lactate to elute gentamicin. The last group was tested after being implanted for 2 months subcutaneously in dogs. Comparison of the fracture toughness of the two zero-time groups showed no effect of radiation sterilization on any of the four types of cement. The results from both the group immersed in Ringer's solution and the group implanted in dogs showed no significant effect on fracture toughness with gentamicin additions. Both these groups, however, did have greater toughness values than the zero-time groups, probably caused by the more complete polymerization with time. Furthermore, the Palacos cement exhibited a greater toughness than the Zimmer cement. The results of this study demonstrate that the addition of gentamicin to bone cement is not deleterious to the fracture properties.

Animals↗
PubMed1983

Retrieval analysis of total knee prostheses: a method and its application to 48 total condylar prostheses.

A technique for the classification and quantification of damage in retrieved total knee prostheses is presented and applied to the examination of 48 removed total condylar-type knee replacements. The technique involves inspection of all metallic and polyethylene components for evidence of gross deformation, fracture, and damage to articulating surfaces. A grading system was developed to quantitate surface damage on polyethylene components. Results of the examinations are combined with patient variables (weight, activity level, radiographic findings, time of implantation, and results of histology performed on surrounding tissue) to determine correlations between clinical variables and the mechanical damage experienced by the prostheses. For the 48 total condylar-type prostheses, significant positive correlations were found for the surface damage correlated with the patient's weight and the time the prosthesis was implanted.

Biomechanical Phenomena↗
PubMed1983

Soft tissue attenuation of acoustic emission pulses.

The soft tissue attenuation of acoustic emission signals was measured by transmitting pulses through volunteers and measuring the decay of the waveform characteristics of the pulse as a function of the thickness of the interposed tissue. Waveform characteristics of the received signal (signal duration, number of counts, peak amplitude, energy, and rise time) demonstrated an exponential decrease with increasing tissue thickness. The decrease appeared insensitive to the frequency of the pulse within the range of 50 to 600 KHz.

Acoustics↗
PubMed1981

Mechanical properties of poly(methyl methacrylate) bone cements.

Samples of low viscosity poly(methyl methacrylate) (PMMA), graphite reinforced PMMA, and graphite reinforced low viscosity PMMA were evaluated for their compression strength and fracture toughness. These results were compared with two currently used plain PMMA bone cements. There were no statistically significant differences in compression strength between the five cements. Graphite reinforcement of plain cement produced a 32% increase in fracture toughness over plain cement. Graphite reinforcement of low viscosity cement also produced a significant increase in toughness (31%) over low viscosity cement with fiber reinforcement. However, low viscosity cement demonstrated significantly less fracture toughness than plain PMMA.

Analysis of Variance↗
PubMed1981

The effect of carbon fiber reinforcement on contact area, contact pressure, and time-dependent deformation in polyethylene tibial components.

To study the mechanical behavior of a commercially available UHMWPE reinforced with carbon fibers, measurements of contact area, contact pressure and time-dependent deformation were made on tibial components from a contemporary total knee replacement loaded to physiological levels. For comparison, similar measurements were performed on identical components manufactured from plain UHMWPE. Contact area and pressure results reflected the increased stiffness of the carbon-UHMWPE material, with smaller contact areas and generally higher contact pressures versus the plain UHMWPE at the same load. Stresses in both materials under the contact area approached or exceeded the yield stresses for the materials when the physiological load was high or the radius of the femoral component indenter decreased (similar to flexion of the knee). Time-dependent deformation over a 24 h period was reduced significantly in the carbon-UHMWPE components for a high physiological load.

Arthroplasty, Replacement, Knee↗
PubMed1980

Biomechanical analysis of the sliding characteristics of compression hip screws.

UNLABELLED: We examined the effect of screw-plate angle on the sliding characteristics and jamming potential of four popularly used stainless-steel and cobalt-chromium-molybdenum compression hip screws. The actual coefficient of sliding friction for these alloys was measured in each device. The force on the screw required to overcome the static frictional force also was determined, by varying the lengths of screw engaged in the barrel under conditions of static equilibrium representing 130-degree and 150-degree screw-plate angles. For the 130-degree loading configuration, this force was significantly (p < 0.001) higher than that required for the 150-degree loading configuration for all four screw types. The actual coefficient of friction was relatively constant for each material, although slight variations due to differences in design between screw types were found. A positive correlation (p < 0.01) was seen between the apparent coefficient of friction (the ratio of sliding force to normal force) and the length of the screw extending from the barrel. All stainless-steel screws jammed in the 130-degree tests when not completely engaged in the barrel. None of the 150-degree tests produced jamming and none of the cobalt-chromium-molybdenum screws jammed in either the 130-degree or the 150-degree test. Examination of jammed devices by scanning electron microscopy showed galling on the superior surface of both the screw and the barrel. CLINICAL RELEVANCE: Understanding the conditions that facilitate sliding of hip screws aids in ensuring their proper use. The higher the nail-plate angle, the easier it is to impact the hip-fixation device and thus allow bone impaction and stability at the fracture site. The potential for jamming a sliding hip screw is decreased by maximum engagement of the screw in the barrel. Differences in the material and design of sliding hip-fixation devices have relatively little effect on the sliding characteristics compared with the nail-plate angle and the engagement of the screw in the barrel.

Alloys↗
PubMed1979

A method for the postmortem evaluation of an in situ total hip replacement.

A method was developed for the post-mortem evaluation of a total hip replacement retrieved in situ. The hip replacement had been implanted for six years. The evaluation procedure employed existing techniques in a logical sequence such that earlier tests would not compromise results from subsequent ones. These techniques included the measurement of range of motion, aspiration of the joint for analysis of debris, radionuclide arthrograms for determination of looseness of the components, gross and histological inspection of the capsule and synovial tissue, serial sectioning of the femoral component, determination of some mechanical properties of the bone and femoral stem, and scanning electron microscopy of the articulating surfaces of the prosthetic components. Positive findings in the specimen studied were: excessive wear of the articulating surface of the acetabular component, and associated polyethylene debris in the aspirate and surrounding synovial and granulation tissue.

Acetabulum↗
PubMed1976

The fracture mechanics of fatigue crack propagation in compact bone.

The purpose of this investigation was to apply the techniques of fracture mechanics to a study of fatigue crack propagation in compact bone. Small cracks parallel to the long axis of the bone were initiated in standardized specimens of bovine bone. Crack growth was achieved by cyclically loading these specimens. The rate of crack growth was determined from measurements of crack length versus cycles of loading. The stress intensity factor at the tip of the crack was calculated from knowledge of the applied load, the crack length, and the specimen geometry. A strong correlation was found between the experimentally determined crack growth rate and the applied stress intensity. The relationship takes the form of a power law similar to that for other materials. Visual observation and scanning electron microscopy revealed that crack propagation occurred by initiation of subcritical cracks ahead of the main crack.

Animals↗