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Biomedical subjects

R K Clark

Publications and source records attributed to R K Clark.

At least 19 recordsLinked to original sources
PubMed2001

The contribution of imaging and digitised data to mandibular reconstruction and implant stabilised occlusal rehabilitation: a case report.

Different methods are recommended for the surgical reconstruction of the resected mandible. The advantages for implant stabilised prostheses in restoring the occlusion are recognised but few papers provide adequate data to identify the successful outcome of treatment. The literature is reviewed and the advantages of imaging together with the use of digitised data is highlighted by a case requiring rehabilitation with enhanced planning methods.

Bone Transplantation↗
PubMed2000

The physical mechanisms of complete denture retention.

The purpose of this article is to assist the practitioner to understand which factors are relevant to complete denture retention in the light of the current understanding of physics and materials science and thus to guide design. Atmospheric pressure, vacuum, adhesion, cohesion, surface tension, viscosity, base adaption, border seal, seating force and muscular control have all been cited at one time or another as major or contributory factors, but usually as an opinion without proper reference to fundamental principles. Although there has been a detailed analysis published, it seems appropriate that a restatement of the points in a collated form be made. In fact, denture retention is a dynamic issue dependent on the control of the flow of interposed fluid and thus its viscosity and film thickness, while the timescale of displacement loading affects the assessment. Surface tension forces at the periphery contribute to retention, but the most important concerns are good base adaptation and border seal. These must be achieved if full advantage is to be taken of the saliva flow-related effects.

Dental Stress Analysis↗
PubMed2000

Oral health status of patients 5-6 years after placement of cobalt-chromium removable partial dentures.

This study is a clinical survey of cobalt-chromium (Co-Cr) removable partial denture (RPD) wearers that aimed to investigate the effects of denture wearing on oral tissues. A random sample of patients who had received their dentures 5-6 years previously from a dental teaching hospital in Hong Kong was selected. Those who had been constantly wearing the RPDs were examined by one calibrated examiner under an optimal clinical setting. The patients' dental, periodontal and mucosal status were assessed. A total of 87 patients were examined. Mucosal lesions under the Co-Cr RPDs were uncommon in this study sample. However, there was a high prevalence of plaque, gingivitis and gingival recession, especially in dento-gingival surfaces in close proximity (within 3 mm) to the dentures. Thus, there is a special need for regular oral hygiene reinforcement, scaling and prophylaxis among RPD wearers. Despite a low caries incidence in the study sample, root caries were found to be associated with contact with the RPDs (P < 0.05) but coronal caries were not. It is recommended that coverage of the exposed root surfaces by RPD components should be avoided. Also, topical fluorides should be regularly applied onto exposed root surfaces as a preventive measure.

Adult↗
PubMed1999

Effect of adhesive drying time on the bond strength of irreversible hydrocolloid to stainless steel.

STATEMENT OF PROBLEM: The efficacy of adhesive material is believed to be affected by the drying time allowed. However, there is no universal consensus on the drying time required for adhesive to achieve optimal bond strength for irreversible hydrocolloid impressions. PURPOSE: This study investigated the effect of drying time on the cleavage bond strength of irreversible hydrocolloid adhesive and determined optimal adhesive drying time. MATERIAL AND METHODS: In the cleavage test, the adhesive bond was stressed at the edge. An offset tensile force was applied to 1 side of the testing plates to split the bond. Cleavage testing plates were designed and machined in stainless steel. Adhesive material (Fix) was applied thinly to the plates and allowed to dry for 1, 3, 5, 7.5, 10, and 20 minutes before loading with an irreversible hydrocolloid (Blueprint). The irreversible hydrocolloid was then allowed to set for 5 minutes before testing. Twenty specimens were tested for each selected drying time. The force at failure was measured by an Instron machine with a crosshead speed of 50 mm/min applied to the edge of the bond. A nonadhesive group was included as a control. RESULTS: The calculated cleavage bond strength at 1-minute drying (32 kPa) improved moderately up to 5-minute drying (37 kPa), but decreased on further drying. When compared with specimens in the nonadhesive group, cleavage bond strength was 3 times greater when the adhesive was applied and dried for 5 minutes. CONCLUSIONS: Adhesive is effective in improving the cleavage bond strength of irreversible hydrocolloid to stainless steel, and drying of adhesive in excess of 5 minutes is not recommended.

2-Propanol↗
PubMed1999

Effect of processing method on the dimensional accuracy and water sorption of acrylic resin dentures.

STATEMENT OF PROBLEM: Acrylic resin complete dentures exhibit certain unavoidable dimensional changes. Processing shrinkage and expansion due to water uptake are 2 important aspects influencing dimensional accuracy. PURPOSE: This study investigated linear dimensional changes and water sorption of dentures processed by dry and wet heat with different rates of cooling. METHODS: Fine crosses marked on tinfoil inserts were placed at the incisive papilla and tuberosity regions of edentulous maxillary casts and incorporated into the dentures during polymerization by 3 processing techniques. A traveling microscope was used to measure the distances between the reference points to determine dimensional changes. Water uptake and content were determined by the mass changes of the dentures with an electronic balance. Data of linear dimensional change and water sorption were analyzed by multivariate analysis of variance and analysis of variance, respectively. Bonferroni simultaneous confidence intervals (95%) were applied for multiple comparison. RESULTS: Dry heat-processed and water bath-processed acrylic resin dentures did not exhibit significant differences in shrinkage (0.42% to 0.58%) at water saturation. Amounts of water sorption of dentures processed by dry and wet heat (0.50 and 0.48 mass%, respectively) were not significantly different, and their associated expansion did not entirely compensate for the processing shrinkage. The initial water content of dry heat-processed dentures (1.77 mass%) was unexpectedly slightly higher than that of wet heat-processed dentures (1.68 mass%). The rate at which the dentures cooled did not affect their initial water content and subsequent water uptake. CONCLUSIONS: Water uptake of dry and wet heat-processed acrylic resin dentures after deflasking was in both cases low, and the dentures did not reveal significant differences in shrinkage at water saturation. Air oven-processed and water bath-processed acrylic resin dentures show similar dimensional shrinkage at water saturation.

Acrylic Resins↗
PubMed1999

Somatosensory-evoked potentials during aortic coarctation repair.

OBJECTIVE: To determine the incidence of somatosensory-evoked potential (SSEP) changes and the interventions based on these changes during aortic coarctation repair. DESIGN: Retrospective review. SETTING: Single-institution, university hospital. PARTICIPANTS: Eighty-four children who had undergone surgical repair of aortic coarctation from January 1984 to May 1996. INTERVENTIONS: SSEPs were monitored in all patients throughout the procedure. A persistent decrease in amplitude greater than 50% from baseline was considered significant. Duration of SSEP changes in relation to the time course of surgical repair and whether a surgical or anesthetic intervention resulted from a change in SSEPs were documented. MEASUREMENTS AND MAIN RESULTS: Eighty-four patients underwent 87 surgical procedures. SSEP changes occurred in 40% of the procedures: 38.5% with repair and 15% with test clamp, with 9% occurring during both test clamp and repair. Interventions, which included repositioning the aortic cross-clamp, elevating blood pressure, and aborting surgery, occurred in 26.4% of all procedures based on SSEP changes. No patient sustained a neurologic deficit. CONCLUSION: This is the largest series to date describing the use of SSEPs in aortic coarctation repair. These SSEP changes were often immediately amenable to changes in surgical and anesthetic management. SSEP changes and interventions based on these changes occurred with a considerable frequency.

Adolescent↗
PubMed1999

Trypanosoma cruzi does not induce apoptosis in murine fibroblasts.

The intracellular cycle of Trypanosoma cruzi in mammalian host cells involves the differentiation of dividing amastigote forms into flagellated trypomastigote forms. The mechanism(s) regulating the growth and differentiation of the intracellular parasites is (are) not known. The number of parasites in infected cells can be several hundred and may be enough to induce apoptosis, a suicide-like death programme, generating products (e.g. nuclear proteins) that could function as signals to initiate the differentiation of amastigotes into trypomastigotes. Murine fibroblasts infected with T. cruzi were examined during a 5-day course of infection for evidence of apoptosis. However, characteristics of apoptosis, including degeneration of nuclear structure, condensation of chromatin, loss of plasma membrane integrity, or the cleavage of DNA into nucleosomal fragments, were not observed. Therefore, it is unlikely that products resulting from host cell apoptosis function to induce parasite differentiation. The possibility that T. cruzi might inhibit host cell apoptosis by increasing intracellular levels of Bcl-2, an endogenous inhibitor of apoptosis, was then investigated. Analysis of infected cells by flow cytometry did not demonstrate a significant amount of intracellular Bcl-2. This suggests that if the parasite is inhibiting host cell apoptosis, it is by a method that does not involve increasing levels of Bcl-2.

Animals↗
PubMed1999

Custom-made titanium mandibular reconstruction tray.

Reconstruction of the mandible after ablative surgery can be achieved by using preformed trays or trays formed from models produced by computer-assisted modelling systems. The former presents difficulty in matching the required facial contour, jaw relationship and condylar position; while the latter is expensive. This paper presents a simple and inexpensive method of fabricating a custom-made titanium bone grafting tray. The dimensions of the patient's mandible are obtained by clinical measurement. Such measurements are used to construct a mandibular replica. The region to be reconstructed is carved to produce the ideal shape and dimensions of an edentulous segment. The tray is made either by casting or by swaging. Twenty-one custom-made titanium bone grafting trays have been fitted in patients with encouraging results. This method of bone grafting tray construction is a simple, inexpensive technique for achieving excellent facial contour and functional reconstruction after mandibulectomy.

Bone Transplantation↗
PubMed1998

A new murine model for mammalian wound repair and regeneration.

Regeneration is generally considered to be a phenomenon restricted to amphibians in which amputated limbs reform and regrow. We have recently noted a strain of mouse, the MRL, which displays a remarkable capacity for cartilagenous wound closure and provides an example of a phenomenon previously considered to be a form of regeneration. Specifically, through-and-through ear punches rapidly attain full closure with normal tissue architecture reminiscent of regeneration seen in amphibians as opposed to scarring, as usually seen in mammals. Histologically, we have demonstrated normal cell growth and microanatomy, including angiogenesis and chondrogenesis, as opposed to control C57BL/6 mice which have ear holes that contract minimally but do not close. Finally, this phenomenon is a genetically definable quantitative trait.

Amphibians↗
PubMed1998

Linear dimensional change of heat-cured acrylic resin complete dentures after reline and rebase.

STATEMENT OF PROBLEM: Relining and rebasing of complete dentures are common procedures to improve the fit of the prostheses. After relining or rebasing, adjustments may be required because of dimensional change that occurs during reprocessing of the denture when new denture base material is polymerized. PURPOSE: This study analyzed the linear dimensional change of heat cured acrylic resin that occurs during reline and rebase procedures for complete dentures. MATERIAL AND METHODS: Twenty-two maxillary and mandibular complete denture bases with artificial teeth were saturated by immersion in distilled water at 37 degrees C +/- 1 degree C. Fine crosses were marked on the incisal edges of the central incisors and the supporting cusps of the second molar teeth. Distances between the marks were measured with a high resolution traveling microscope. Heat-cured acrylic resin was processed by a long curing cycle of 72 degrees C for 6.5 hours, then heated to 100 degrees C for over 30 minutes and kept at 100 degrees C for 1 hour. The relined or rebased denture was allowed to cool slowly in the water bath for 48 hours to reach ambient temperature. The reference-crosses were measured immediately after careful deflasking. RESULTS: In the relining procedure, all incisor-molar and intermolar distances exhibited shrinkage of 0.3%, despite different shapes of maxillary and mandibular dentures. Results of the rebasing procedure were similar to that of the reline expect that only 0.1% intermolar shrinkage was found on the maxillary denture. CONCLUSION: Shrinkage was approximately 0.15 mm for an interarch distance of 50 mm. This degree of change is both clinically undetectable and insignificant. Using this processing cycle for acrylic resin in reline or rebase procedures did not cause clinically significant dimensional changes to complete dentures.

Acrylic Resins↗
PubMed1998

Tensile, shear and cleavage bond strengths of alginate adhesive.

OBJECTIVES: This study determined the effect of alginate adhesive on various bond strengths of alginate to stainless steel. METHODS: Three test assemblies were designed and machined in stainless steel for tension, shear and cleavage tests. Alginate adhesive (Fix) was applied thinly and dried for 5 min. Alginate (Blueprint) was then loaded and allowed to set for 5 min before testing. The force at failure was measured by an Instron machine with a cross-head speed of 50 mm min(-1). RESULTS: Breaking stresses of alginate without adhesive were found to be 65 kPa (tension), 31 kPa (shear) and 10 kPa (cleavage). The bond strengths of Blueprint with Fix were 100 kPa (tension), 42 kPa (shear) and 37 kPa (cleavage) giving improvements of 53%, 37% and 270% respectively (p < 0.05). CONCLUSIONS: Alginate adhesive increases the bond strength of alginate, particularly cleavage, to stainless steel.

Adhesives↗
PubMed1997

A bone graft condensing syringe system for maxillofacial reconstructive surgery.

A new system of metallic syringes and pluggers specifically designed for delivery and condensation of particulate bone graft is reported, and the clinical application in maxillofacial reconstructive surgery is described. This bone graft condensing system has advantages over the conventional plastic injection syringes in enabling much better bone graft condensation as well as easier bone delivery and insertion of the syringe to the depth of the reconstruction crib.

Bone Transplantation↗
PubMed1995

Discovery of adrenomedullin in rat ischemic cortex and evidence for its role in exacerbating focal brain ischemic damage.

Focal brain ischemia is the most common event leading to stroke in humans. To understand the molecular mechanisms associated with brain ischemia, we applied the technique of mRNA differential display and isolated a gene that encodes a recently discovered peptide, adrenomedullin (AM), which is a member of the calcitonin gene-related peptide (CGRP) family. Using the rat focal stroke model of middle cerebral artery occlusion (MCAO), we determined that AM mRNA expression was significantly increased in the ischemic cortex up to 17.4-fold at 3 h post-MCAO (P < 0.05) and 21.7-fold at 6 h post-MCAO (P < 0.05) and remained elevated for up to 15 days (9.6-fold increase; P < 0.05). Immunohistochemical studies localized AM to ischemic neuronal processes, and radioligand (125I-labeled CGRP) displacement revealed high-affinity (IC50 = 80.3 nmol) binding of AM to CGRP receptors in brain cortex. The cerebrovascular function of AM was studied using synthetic AM microinjected onto rat pial vessels using a cranial window or applied to canine basilar arteries in vitro. AM, applied abluminally, produced dose-dependent relaxation of preconstricted pial vessels (P < 0.05). Intracerebroventricular (but not systemic) AM administration at a high dose (8 nmol), prior to and after MCAO, increased the degree of focal ischemic injury (P < 0.05). The ischemia-induced expression of both AM mRNA and peptide in ischemic cortical neurons, the demonstration of the direct vasodilating effects of the peptide on cerebral vessels, and the ability of AM to exacerbate ischemic brain damage suggests that AM plays a significant role in focal ischemic brain injury.

Adrenomedullin↗
PubMed1995

Time-related changes in myeloperoxidase activity and leukotriene B4 receptor binding reflect leukocyte influx in cerebral focal stroke.

In previous studies, we have used histological methods to characterize cellular changes, and validated the use of the myeloperoxidase (MPO) activity assay to quantitate increased neutrophil infiltration in ischemic stroke. We also identified increased leukotriene B4 (LTB4) binding sites as a potential marker for neutrophil infiltration into focal ischemic tissue. However, these studies were conducted at only one time-point, 24 h after ischemia. In the present study, we examined the full time-course of MPO activity and LTB4 receptor binding following middle cerebral artery occlusion (MCAO) made permanently (PMCAO) or transiently (160 min followed by reperfusion; TMCAO) in spontaneously hypertensive rats, and compared the results to previously characterized histologic changes in these models. Ischemic and contralateral (control) cortical tissue samples were assayed for MPO (U/g wet wt) and [3H]LTB4 receptor binding (fmol/mg protein). Following PMCAO, MPO activity significantly increased as early as 12 h and continued to increase over the next 5 d (p < 0.05). Following TMCAO, MPO activity was significantly elevated already after only 6 h of reperfusion and also continued to increase over the next 5 d of reperfusion (p < 0.05). LTB4 receptor binding and MPO activity were highly correlated during periods when both measures increased together (i.e., 0.5-5 d; p <0.01). However, by 15 d post-MCAO, LTB4 receptor binding remained elevated after MPO activity levels had returned to normal. This persistent LTB4 binding was associated with the significant gliosis that was characterized previously to persist in these models. The time-course of increased MPO activity and initially increased LTB4 binding post-MCAO correspond very well to our previous histological data that characterized the time-course for leukocyte infiltration under these conditions. Therefore, the increased MPO activity over time was associated with initial neutrophil and later mononuclear cell infiltration into ischemic tissue in these models. In addition, the present studies utilized histochemical analysis to demonstrate peroxidase activity in macrophages within the cerebral infarct following MCAO, thus validating that MPO activity originates from the later infiltrating mononuclear cells in addition to the early infiltrating neutrophils that had been previously characterized in the same manner. TMCAO produces a significantly larger and earlier increase in ischemic cortex MPO activity and a similar later increase in MPO activity compared to PMCAO treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of Variance↗
PubMed1995

Contamination of titanium castings by aluminium oxide blasting.

OBJECTIVES: It is desirable that the surfaces of surgical implants be uncontaminated by foreign materials to avoid untoward tissue reactions, and grit blasting is widely assumed to leave clean metal surfaces. SEM examination and X-ray microanalysis of a recovered 'pure' titanium implant casting that was associated with tissue breakdown revealed embedded particles of alumina. The casting had been cleaned of investment by blasting with alumina grit. METHODS: A variety of treatments of cast titanium plates was used: (a) to establish that the observed aluminium was due to the blasting grit, and (b) to determine whether removal of investment could be achieved effectively by other means. SEM examination and X-ray microanalysis were used. RESULTS: The detected aluminium was associated with embedded fragments identified as coming from the blasting grit. Acid-pickling and mechanical (rotary instrument) trimming produced minimally contaminated surfaces. CONCLUSIONS: Whilst unproven, the presence of the alumina is viewed with great concern as a possible causative agent in the observed tissue breakdown and procedures avoiding alumina blasting are recommended as a precautionary measure.

Aluminum Oxide↗
PubMed1995

Temperature and dimensional changes in the two-stage processing technique for complete dentures.

OBJECTIVES: This study concerned the temperature and linear dimensional change of heat-cured acrylic resin in the two-stage processing technique for complete dentures. METHODS: Thermocouples were incorporated in the acrylic resin for recording temperatures. Measurements between reference marks were made by a high-resolution digital measuring microscope. RESULTS: No increase in temperature associated with the exothermic nature of the polymerization reaction was recorded. The temperature of the resin followed the waterbath temperature closely. The temperatures recorded at various regions were in phase with each other. The total linear shrinkage of the base after two processing cycles was less than 1% and compares favourably with studies on the single-stage processing technique. CONCLUSIONS: Temperature differential is excluded as a reason for the warpage of dentures. The dimensional changes of the denture base resulting from the two-stage processing technique cannot be considered to be of any clinical significance.

Acrylic Resins↗
PubMed1995

Prosthodontic aspects of a new method for functional reconstruction following maxillectomy.

The prosthodontic aspects of a new method of reconstruction after maxillectomy are described. A custom-made titanium mesh tray filled with crushed autogenous hip bone is interposed between two layers of a pedicled temporal muscle flap. The titanium mesh tray is fabricated on a cast carved to represent the anatomy of the reconstructed maxilla. The tray is designed so that, after healing, osseointegrated implants can be placed in preplanned positions in the reformed maxilla. The buccal sulcus is recreated with a free mucosal graft from the contralateral cheek at the second implant surgical stage. The prosthodontic restoration is completed after healing of the free mucosal graft.

Bone Transplantation↗