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Biomedical subjects

T D Brown

Publications and source records attributed to T D Brown.

At least 235 records · Page 13Linked to original sources
PubMed1983

Pre-collapse stress redistributions in femoral head osteonecrosis--a three-dimensional finite element analysis.

Three-dimensional finite element analysis is used to explore the influence of several lesion characteristics upon mechanical stress distributions in segmentally necrotic human femoral heads. Variables studied parametrically included apparent modulus deficits within the lesion proper, as well as the depth, width, and location of the infarcted head regions. The detailed patterns of stress redistribution were complex and were found to be a strong function of the specific lesion characteristics. The salient phenomenon, however, was one of preferential load uptake by the stiffer bone surrounding the lesion. Since computed stress reductions within the infarctions were usually much smaller than experimentally observed strength reductions, the data suggest a strong tendency for an elevated incidence of trabecular fatigue fractures in the affected regions.

Adult↗
PubMed1983

Comparison of unrepaired, primarily repaired, and polyglactin mesh-reinforced Achilles tendon lacerations in rabbits.

The tensile and histologic characteristics of sharply transected Achilles tendons treated by cast immobilization, surgical repair, and surgical repair reinforced with polyglactin mesh were evaluated in 31 rabbits. All modes of tendon treatment resulted in tensile failure at significantly lower loads than in the contralateral control tendons. However, when the cross-sectional area and rupture stress of each pair of tendons were considered, the surgically repaired tendons more closely approximated the load tolerance of the control side than did the unrepaired, casted tendons. The mesh-reinforced tendons exhibited no mechanical superiority to the suture-repaired tendons. No microscopic difference in the quality of collagen deposition for the three types of repairs was apparent. Subcutaneous mesh did not evoke a significant cellular or fibrous reaction and was totally absorbed by the time of necropsy. A significant difference was found between the tolerated stress of the unrepaired and repaired tendons, but no functional benefit of repairing acutely transected Achilles tendons with synthetic mesh was observed.

Achilles Tendon↗
PubMed1982

A non-linear finite element analysis of some early collapse processes in femoral head osteonecrosis.

Some aspects of the early collapse biomechanics of the segmentally necrotic adult human femoral head were studied, using a small-deformation plane strain, elasto-plastic finite element model. The computational procedure used was based upon the initial stress technique, and permitted study of stress and strain fields and of the progression of failure regions as a function of incrementally applied joint loads. The results consistently demonstrated both subchondral and deep cancellous failure patterns similar to those seen clinically. There was a clear distinction, however, between these two failure regimes, dependent primarily upon the relative strength deficits input for the subchondral versus the deep cancellous regions. Usually, the failure zone was appreciable only at significantly supra-physiological loads, reflecting the likely importance of fatigue events in the clinical collapse process. Although subchondral failure was always limited to the entire base region of the infarction wedge, the zones of deep failure varied considerably with changes in lesion geometry, usually being concentrated within the infarct near the underlying necrotic/viable interface.

Elasticity↗
PubMed1981

Comparative assessment of the early mechanical integrity of repaired tendon Achillis ruptures in the rabbit.

The effectiveness of closed versus open management of Achilles tendon ruptures was studied in a series of 40 adult rabbits. Tensile test data of tendon repair specimens excised after 10 weeks of equinus cast immobilization revealed stiffness and strength deficits, relative to uninjured contralateral tendons, ranging from 70 to 88%. Although no statistically significant differences in these early tensile properties were noted among groups managed conservatively versus those treated openly, the closed management group healed with length increases more than three times those which resulted from suture repair. Apposition of a small, longitudinal polytetrafluorethylene (Gore & Assoc., Newark, DE) patch graft in order to stimulate fibroblastic proliferation at the repair site had no biomechanical advantage over simple Kleinert suturing. The tensile test data were found to be consistent with the histological appearance of the repair sites and with detailed rupture dynamics patterns recorded by high-speed cinematography.

Achilles Tendon↗
PubMed1981

Mechanical characteristics of bone in femoral capital aseptic necrosis.

Uniaxial compression loading of small cube-shaped samples in an Instron machine was used to study material property deficits of middle- and late-stage necrotic cancellous bone taken from adult human femoral heads. Although the experimental data should not be extrapolated to phenomena of viable trabecular response in a functioning osteonecrotic hip, they provide insight into the salient passive mechanical response characteristics of the pathologic tissue. Individual differences in the revascularization and/or collapse patterns occurring within femoral heads lead to substantial variability in the strength and stiffness of test samples taken from the involved regions. As an overall average, however, samples extracted from major infarct regions exhibit substantially reduced (52%) yield strength, severely reduced (72%) elastic modulus, and modestly increased (29%) strain-to-failure, relative to samples from normal femoral heads. However, statistically significant changes are not observed in the Poisson ratio, the post-yield modulus, or in the incidence of pre-yield load/deformation discontinuities.

Biomechanical Phenomena↗
PubMed1980

Transcription of Friend virus proviral sequences in isolated nuclei.

Transcription studies using isolated Friend nuclei and Escherichia coli polymerase are presented. Combination of the techniques of thiol-Sepharose chromatography and cDNA-Sepharose hybridisation has resulted in a system in which the transcription of the Friend virus proviral sequences with endogenous and E. coli polymerase can be examined. The results show that the percentage of Friend viral-specific sequences in RNA transcribed by E. coli RNA polymerase and by endogenous RNA polymerase in isolated nuclei are similar. The percentage of viral-specific sequences synthesized in isolated nuclei is similar to that found in Friend cell nuclear RNA.

Animals↗
PubMed1980

Load transmission through the proximal femur of the growing child: a finite element analysis.

Growth-related variations in the distribution of mechanical stress in the normal juvenile proximal femur are explored using a computer-based finite element stress analysis technique. Input data for the model are obtained from serial patient roentgenograms and from laboratory compression tests of fresh autopsy material. The results for the limiting case of epiphyseal closure (18-year-old male) are in substantive agreement with previous experimental and mathematical studies of the adult proximal femur. The computational results for the juvenile hip show that in spite of the complex morphological alterations occurring during growth, the basic mechanisms of load transmission at first ambulation are still operational at skeletal maturity. Foremost among these are the distally-concentrated weight bearing compressive stresses in the primary trabeculation system, the abductor-induced tensile stresses in the greater trochanter, and the large longitudinal stresses within the proximal femoral cortices. As growth advances, however, the importance of transverse compressive stresses in the proximal lateral metaphysis progressively diminishes, while shearing stresses in the plane of the epiphyseal plate increase markedly. These two major growth-related changes results largely from decreases in the neck-shaft angle, and if excessive, may be implicated in development coxa vara or slipped capital femoral epiphysis.

Adolescent↗
PubMed1980

Ankle arthrodesis: a comparative clinical and biomechanical evaluation.

Arthrodesis of the ankle joint is difficult to achieve when there are associated problems with bony deformity or abnormalities of the surrounding soft tissues. In a series of 50 ankle arthrodeses, in which 68% had either damaged soft tissues or fixed ankle deformity, the pseudarthrosis rate was 30%. This high failure rate led us to devise biomechanical studies to evaluate the inherent stability of the Charnley-type compression arthrodesis, the R.A.F. fusion, and a new dynamic compression T-plate. The stability of the Charnley apparatus was directly related to the tension upon the wingnuts. However, in spite of the massive compression generated by the Charnley apparatus, it was relatively unable to resist rotatory forces. The T-plate had excellent rotatory and flexion-extension stability and developed approximately 25 pounds/inch compression across the ankle fusion. The T-plates's clinical application is demonstrated in eight successful arthrodeses. The importance of an intraoperative roentgenogram to confirm satisfactory ankle alignment cannot be overemphasized.

Adult↗
PubMed1979

The effects of mercury-substitution on the hybridisation characteristics of nucleic acids.

The effect of different levels of mercury substitution on the rate and extent of hybridisation of globin mRNA with a complementary DNA (cDNA) copy has been investigated. It was found that mercuration significantly reduces both the rate of hybridisation and the extent of the reaction, but that these effects are abolished when at least stoichiometric amounts of 2-mercaptoethanol are included in the hybridisation medium. As a preliminary to using this technique to isolate specific groups of sequences after long-term hybridisations, we have investigated both the rate of demercuration of RNA and its retention on thiol-sepharose columns after extended incubation under commonly employed hybridisation conditions at 43 degrees or 60 degrees. Retention was essentially quantitative even after incubation times of 300 hours at 43 degrees, but decreased significantly after 48 hours at 60 degrees. It is concluded that thiol-sepharose chromatography offers considerable advantages over hydroxyapatite chromatography for the recovery of hybridised sequences, particularly with regard to the lower levels of non-specific binding obtained and its ability to distinguish directly between DNA-DNA and DNA-Hg RNA hybrids.

Animals↗
PubMed1979

Relief of patellofemoral contact stress by anterior displacement of the tibial tubercle.

Elevation of the patellar tendon by means of displacement of the tibial tubercle has been advocated by Maquet, Ficat and Hungerford, and Murray as an alternative to patellectomy for the treatment of patellofemoral arthralgias. Although tubercle elevation certainly reduces the over-all patellofemoral contact force, its effects on the complex local patterns of contact stress are of more direct significance. In a laboratory series using fresh amputation material, arrays of six miniature contact stress sensors were embedded in the retropatellar cartilage of knees subjected to isometric quadriceps-extension forces. The experimental data revealed that elevation of the patellar tendon generally afforded relief of local contact stress regardless of the joint configuration (zero degrees, 45 degrees, or 90 degrees of flexion), but that its effects were most pronounced at 90 degrees of flexion. Progressive increase in the tendon elevation caused progressive reduction in the contact stress. Most of the contact stress relief was achieved, however, with the first one-half inch of tendon elevation; further elevations to one and one and one-half inches were only marginally useful. In view of the increased superior patellar pole contact associated with distal pole flotation, the results indicate that under most circumstances the optimum amount of elevation of the tibial tubercle is about one-half inch.

Cartilage, Articular↗
PubMed1978

Studies of Epstein-Barr virus (EBV)-associated nuclear antigen: solubilization from Raji cell chromatin with 5 M-urea-2 M-nacl and fractionation on hydroxyapatite.

EBNA-containing chromatin from Raji cells was solubilized by treatment with high concentrations of urea and salt and fractionated by hydroxyapatite chromatography. Fractions eluting at different phosphate concentrations were analysed for the presence of EBNA by means of an anti-EBNA-specific 125I-labelled IgG absorption assay. The antigen was found in fractions containing non-histone chromatin proteins.

Antigens, Viral↗
PubMed1978

The development of a computational stress analysis of the femoral head. Mapping tensile, compressive, and shear stress for the varus and valgus positions.

Using a computer-based, two-dimensional finite-element analysis which has wide application to problems involving the hip joint, alterations in the distribution of stress in the femoral head consequent to varus and valgus osteotomy were studied. The mathematical model used in this analysis incorporated experimentally measured spatial variations in the stiffness of the bone of the femoral head and neck. These variations led to patterns of load transmission that were strikingly different from those in a homogeneous material. Because of their lower stiffness, the central region of the head and the medullary region of the neck make very little contribution to weight-bearing, regardless of the orientation of the femoral head. In the neutral configuration (normal neck-shaft angle), the lateral cortex of the neck is in slight tension, while the medial cortex is under strong compression and provides the support for the over-all load on the joint. With increasing valgus angulation, the bending component of the joint load disappears progressively, and when the valgus angulation is 30 degrees in excess of normal essentially equal compressive stresses prevail in both cortices. Varus osteotomy exaggerates the bending component relative to the compressive component of the load. The computed stress patterns in the femoral head and neck for the normal neck-shaft angle show elevations of shear stress where the lateral epiphyseal artery enters and branches within the femoral head. This finding may be significant since this region is the area at risk of infarction in both Legg-Perthes disease in children and idiopathic aseptic necrosis in adults. Since a 30-degree varus angulation induces tensile stresses in the lateral cortex of the neck that are increased fourfold above those for the neutral configuration, it is postulated that force transmitted through the femur when the hip is in abduction could produce shear fractures of the bone in the region of the central branch of the lateral epiphyseal artery and thus occlude this vessel and initiate aseptic necrosis of the femoral head.

Adult↗
PubMed1978

Delayed surgical repair of ruptured ligaments: a comparative biomechanical and histological study.

The importance of prompt surgical repair of complete ligament ruptures was quantitated using 38 rabbits with standardized transection injuries ofthe medial collateral ligament of one knee. Tensile testing of the mechanical properties, coupled with histological evaluation allowed assessment of the integrity of suture repairs performed immediately after ligament injury, versus those with 3 and 10 day delay times as well as those with non-operative management. Surgical repair performed within the first 3 days is beneficial in terms of final repair strength. Irreversible ligament strength and stiffness deficits progressively accumulate during the 10 days immediately following injury. Surgical repair after more than 10 days postinjury is contraindicated since the postoperative mechanical properties of the healed ligament do not differ from those for non-operative injury management.

Animals↗