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Biomedical subjects

A D Johnston

Publications and source records attributed to A D Johnston.

At least 19 recordsLinked to original sources
PubMed1991

The effect of catalyst structure on the synthesis of a dental restorative monomer.

The addition product of 2-hydroxyethyl methacrylate (HEMA) and pyromellitic dianhydride (PMDA), known as PMDM, is a mixture of two structural isomers. The para PMDM isomer--currently used in mediating adhesive bonding of restorative materials to hard tooth tissues--is a crystalline solid. The meta isomer is a liquid. In the synthesis of PMDM, the para isomer, which can be purified by crystallization, is usually present to the extent of only 50% of the product mixture. The effect of the amine catalyst structure was studied relative to its role in increasing the yield of the para isomer, either by a reduction in the amount of the meta isomer or by an increase in the extent of overall reaction. The chemical structure of the amine catalyst had an important role in the synthesis of PMDM and influenced the ratio of the isomers. Among aliphatic amines, especially noteworthy as catalysts that gave excellent yields of the para isomer in high purity were N,N-di-isopropyl-ethylamine and hexamethylenetetramine.

Adhesives↗
PubMed1991

Dentin-bonding molar efficiency using N-phenylglycine, N-phenyl-beta-alanine, or N-methyl-N-phenylglycine.

Three structurally related substituted amino acids (N-compounds) were studied in a three-step dentin-bonding protocol. The first step of an acidic ferric oxalate solution and the third step of a surface-active comonomer were held constant throughout the study. In the second step, the amount of the N-compound--either N-phenylglycine (NPG), N-methyl-N-phenylglycine (NMNPG), or N-phenyl-beta-alanine (NPBA)--was varied in acetone from 0 mol/L through 5 x 10(-1) mol/L in seven steps. At 1 x 10(-3) mol/L for NPG and NMNPG, average bond strength values were 7.4 +/- 2.2 and 10.5 +/- 2.7 MPa. The highest bond strength value for NMNPG was at 1 x 10(-2) mol/L, with 13.2 +/- 4.0 MPa. The highest value for NPG was at 1 x 10(-1) mol/L, with a value of 11.8 +/- 2.5 MPa. The average bond strength for NPBA did not differ from zero across the entire range of concentrations. Molar efficiency was defined as the bond strength per mole of these applied N-compounds. For the two N-compounds that did provide adhesion to dentin, NPG and NMNPG, the average bond strengths rose, peaked, and fell as the amounts of applied N-compound were increased. The molar efficiency dropped off as the concentration of applied N-compound rose. The least operator-sensitive and most efficient N-compound, NMNPG, delivered a bond strength equivalent to that of NPG, with 10% of the applied NPG amount.

Adhesives↗
PubMed1991

Protective coatings for tooth crowns.

Transfer of technology to the dental office could provide thin polymeric coatings over tooth crowns and accessible root surfaces. These coatings can be prepared from relatively inexpensive, commercially available materials in one-step reactions. When the monomer technique becomes available, it will provide caries protection for all of your patients.

Benzoates↗
PubMed1989

Mechanically-induced generation of radicals in tooth enamel.

Mechanical instrumentation of enamel leads to the formation of long-lived free radicals that can be conveniently measured by electron paramagnetic resonance (EPR) spectroscopy. Powdered enamel tissue exhibited EPR signals remarkably similar to the radicals formed by ionizing radiation. The observations described below lead to the conclusion that physical stress will induce a free-radical formation in dental tissues. These observations have significance for other areas of study such as dosimetry and archeological dating.

Dental Enamel↗
PubMed1989

Substitutes for N-phenylglycine in adhesive bonding to dentin.

Using bond strength measurements, we investigated a number of related compounds in order to elucidate the role of the surface-active ingredient, N-phenylglycine (NPG), in experimental two-step and three-step bonding protocols resulting in adhesive bonding to dentin. All active compounds identified for the two-step or the three-step protocol were N-aryl-alpha-amino acids, and the results delineate some of the key features of the NPG molecule for bonding. For the three-step protocol, there was a requirement for a secondary or tertiary aromatic amino group, a carboxylic acid group, and a single (secondary or tertiary) methylene unit between those two functional groups of the amino acid. For the two-step protocol, additional substitutions at the para position of the phenyl ring on the amine improved the bond strength. In both protocols, para-methyl- and para-chloro-substituted NPG analogues ranked higher than NPG. A "catalytic" effect of the aromatic tertiary amino group on the polymerization of the adhering resin in both procedures could not be ruled out.

Adhesiveness↗
PubMed1987

Adsorption of N-phenylglycine on hydroxyapatite: role in the bonding procedure of a restorative resin to dentin.

The adsorption of N-phenylglycine (NPG) onto synthetic hydroxyapatite from ethanol solutions was studied to elucidate the role of surface-active moieties in determining the character of adsorption, and to explore the role of NPG in a bonding procedure of restorative resin to dentin. The adsorption isotherm of N-phenylglycine is reversible (and Langmuirian) from ethanol (99.8%). At maximum adsorption the phenyl rings of the adsorbed molecules lie flat on the surface and the carboxylate oxygens and amino nitrogens are anchored to the surface. The N-phenylglycine serves as an amine accelerator since the peroxide containing monomer polymerizes with the adsorbate-covered apatite. The diametral tensile strength of this composite is approximately equal to the composite filled with untreated apatite. The role of various factors contributing to adhesive strength in the bonding procedure is also discussed.

Adsorption↗
PubMed1987

Thermal effects of polymerization of methyl-methacrylate on small tubular bones.

This study was designed to investigate the thermal effects of polymerization on small tubular bones. The results demonstrated only a slight increase in temperature during cement-curing in the medullary canals of these bones. The maximum rise (11.7 degrees C) was within the levels of tolerance for living bone cells. It was demonstrated that the small volumes of methyl-methacrylate cool more rapidly, hence further decreasing the deleterious effects of heat. The histological studies showed no recognisable adverse reaction to the thermal effects of methyl-methacrylate polymerization on small tubular bones or on the overlying tissue.

Animals↗
PubMed1987

Aneurysmal bone cyst of the hand.

Diagnosis of aneurysmal bone cyst in the hands is confused by rarity (2%) and overlapping characteristics of other lesions, especially benign osteoblastoma and giant-cell tumor. Pseudosinusoidal vascular channels create interfaces between liquid blood (maintained by fibrolysin) and connective tissue cells. The role of trauma is inexplicable.

Bone Cysts↗
PubMed1986

Isolated blastomycotic osteomyelitis: a case simulating a malignant tumor of the distal radius.

Blastomycosis osteomyelitis frequently complicates systemic blastomycosis, but it is unusual as an isolated osteolytic lesion in the asymptomatic patient. An isolated blastomycotic osteomyelitis of the distal radius was the initial manifestation of blastomycosis in a healthy young man. The lesion was originally interpreted as a sarcoma. Fungal osteomyelitis must be differentiated from radiologically similar bone neoplasms.

Adult↗
PubMed1985

Prosthetic synovitis.

The term "prosthetic synovitis" is applied to reactive changes resulting from a synovial-like membrane formed between a failed prosthesis (noninfected) and the bone interface. This report is the result of light-microscopic and clinical examination of more than 100 specimens obtained at surgery of failed previous hip replacements. The morphology and cell distribution of those tissues removed at surgery in 51 noninfected cemented total hip operations allowed a quantitative estimate of surface cell population by a "touch imprint" technique; qualitative and quantitative estimate (scale, 1 to 4+) of cell population and foreign body materials by light microscopy; and electron microscopy and biochemical analysis of selected samples. Histologic examination included the following cell population, in decreasing order of frequency: acidophilic histiocytes (95%); giant cells (80%); fibronoid material (80%); lymphocyte and plasma cells (26%); and neutrophils (8%). Microscopic examination showed that the largest particles of acrylic cement and shards of high-density polyethylene appeared to be walled off by connective tissue capsules. The majority of smaller particles were incorporated into the histiocyte/macrophage or giant cell population. Histochemistry indicated that these particles elicited "foci" of cellular activity within the synovial-like membrane. This increased activity included the appearance of increased endogenous peroxidase activity in those macrophages within the "foci"; increased betagalactosidae activity among these histiocytes; and a localization of acid phosphates activity within giant cells along the borders of inclusions within the cell cytoplasm. We conclude that wear products resulting from total hip arthroplasty, including the bone cement, can induce increased lysosomal and proteolytic activity within the histiocyte and giant cell populations. It may be important to emphasize that there were "reactive foci" within the membrane and that the entire membrane, even though infiltrated with macrophages, did not respond uniformly to the presence of prosthetic debris. We advance a theory that the first step toward a distractive phenomenon at the interface is micromotion between the cement and bone. Micromotion may be caused by removal of subchondral plate during total hip replacement, leading to fatigue and loss of trabeculae and resultant increase motion, thus bone loss. Bone loss may be the direct result of mechanical injury, increased osteoclasis, or direct lysis of bone by various enzymes released by the interface membrane.

Acetabulum↗
PubMed1984

A gross anatomic and histologic study of the innervation of the proximal interphalangeal joint.

The innervation of the proximal interphalangeal joint was studied by gross dissection with magnification and histologic analysis. The joint is innervated by a constant articular branch arising from the palmar proper digital nerve at the level of the midproximal phalanx. This articular nerve bifurcates adjacent to the joint, and its terminal branches enter the joint in the midlateral plane at the junction of the palmar plate and lateral capsule. No contribution to the articular innervation was found to arise from the sensory radial, dorsoulnar cutaneous, and collateral branches of the proper digital nerve that cross the joint. The innervation of the proximal interphalangeal joint is congruent with the palmar sensory innervation and has no relation to the dorsal sensory innervation.

Finger Joint↗
PubMed1983

Adjuvant chemotherapy of malignant fibrous histiocytoma of bone.

Malignant fibrous histiocytoma (MFH) is a pleomorphic sarcoma that is uncommon in children. It most frequently arises from the soft tissues; however, it has been recently established that primary bone MFH also exists. Surgical resection or amputation is the cornerstone of treatment for MFH of bone. But, with this modality of therapy alone the majority of patients develop either distant metastases or local recurrence. This study reports on three adolescent girls with MFH of bone who were successfully treated with radical resection and 18 months of adjuvant chemotherapy with vincristine, high dose methotrexate, Citrovorum Factor rescue, and Adriamycin. All three patients remain disease-free for a follow-up period of 42-48 months. The current regimen was well tolerated. Morbidity was minimal, with no patient developing any significant drug-related complications. The adjuvant chemotherapy regimen described appears to be effective in prolonging survival in patients with MFH of bone and appears to warrant further study in additional patients.

Adolescent↗
PubMed1979

The effects of a minidose of heparin on peritonitis in rats.

Sprague-Dawley rats with peritonitis treated with either a subcutaneous or intraperitoneal minidose of heparin show a significant increase in survival time when compared with controls. In the treatment groups, adhesions and abscesses were less severe and localized to the area of gangrenous bowel. Heparin also significantly reduced the incidence of recovery of viable bacteria from the blood and peritoneal cavity. These findings may be related to a decreased deposition of fibrinogen within the abdomen or to the early mobilization of fibrin.

Abdomen↗
PubMed1976

Needle biopsy of the synovium of children.

While the diagnosis of juvenile rheumatoid arthritis is based on clinical criteria and does not require confirmation by synovial biopsy, biopsy is occasionally desired to exclude other diagnoses. Needle synovial biopsy of the knee may be performed on young children as an office procedure and generally provides adequate tissue for examination. In the author's clinic this procedure has replaced open biopsy of the knee of children.

Adolescent↗