PubMed Health⌕ Search

Biomedical subjects

J A Hobkirk

Publications and source records attributed to J A Hobkirk.

At least 19 recordsLinked to original sources
PubMed1998

The influence of mandibular deformation, implant numbers, and loading position on detected forces in abutments supporting fixed implant superstructures.

STATEMENT OF PROBLEM: Forces may arise in association with the use of implant stabilized mandibular fixed partial dentures as a result of functional jaw deformation, yet the phenomenon has received little attention and is rarely included as a variable when investigating implant biomechanics. PURPOSE: This in vitro investigation tested the hypothesis that functional mandibular deformation influences force distribution in the jaw/implant/superstructure complex. MATERIAL AND METHODS: Six Brånemark implants were mounted in an acrylic resin replica of an edentulous human mandible. Applied forces were measured with 4-element resistance strain gauges mounted on each of 6 standard titanium abutments. The mandible was supported either on its lower border or suspended in a frame that simulated the natural situation. A cast gold superstructure was a mounted on varying combinations of implants, loaded occlusally in different locations, and the resultant forces at each supporting transmucosal abutment measured. RESULTS: The suspended ("natural") support configuration was associated with considerable differences in patterns of force transmission as compared with bench support. Loads were more widely spread, and large extrusion forces were detected, particularly where several implants were connected. CONCLUSIONS: Functional mandibular deformation is a significant factor in the design of mandibular implant stabilized prostheses, and calls into doubt the value of modeling techniques that do not allow for this phenomenon.

Acrylic Resins↗
PubMed1998

A comparison between the wear resistance of three types of denture resin to three different dentifrices.

This study compares the abrasive effects of three dentifrices on three different denture base resins, a heat cured acrylic resin, a self cured acrylic resin and a light activated urethane dimethacrylate-based resin. Specimens of the resins were subjected to a regimen of mechanical brushing using measured amounts of paste on standard toothbrushes driven by a reciprocating device. The apparent wear of each specimen was assessed qualitatively using scanning electron microscopy and quantitatively using profilometry and reflex microscopy. The study showed that cleaning pastes play a significant role in the wear of conventional acrylic resins, the relative abrasivity of the dentifrices being substrate related. However, wear was less evident on the light activated urethane dimethacrylate resin. Brushing with water alone produced no detectable wear on any of the specimens.

Acrylic Resins↗
PubMed1997

Functional adaptability to changes in lower denture shape.

Forces exerted on the right and left lingual and buccal flanges of Kennedy Type I mandibular dentures were studied using an 8 channel recording technique, during resting, swallowing, talking, reading and simulated mastication. The effects of adaptation, head position and incremental thickening of the flanges were determined. Highest forces were recorded when swallowing, with smaller differences between the other activities. Lingual forces were higher than buccal, and posterior forces higher than anterior ones. There was a highly significant relationship between increases in flange thickness of 2 and 3mm and increases in peak forces. Evidence for adaptation and effects related to head position was weaker.

Adaptation, Physiological↗
PubMed1996

An in vivo quantitative analysis of the fit of Nobel Biocare implant superstructures.

Loss of osseointegration of implant fixtures and mechanical failure of superstructures have been attributed to failure to achieve a passive fit between the superstructure and the transmucosal abutment (TMA). The fit of five mandibular superstructures, fabricated on Nobel Biocare implants, was investigated in vivo, using a polyvinyl siloxane impression material to record the discrepancies. Following sectioning of the impressions, the vertical and horizontal discrepancies were analysed at four locations using a travelling microscope. The discrepancies obtained were measured both when the gold cylinder screws were maximally tightened by hand and when tightened with a torque driver to the recommended value of 10 Ncm. The vertical discrepancies for the hand tightened abutments ranged from 0 micron to 63 microns with a mean of 14 microns. In the mechanically tightened group, the vertical discrepancies ranged from 0 micron to 130 microns with a mean of 21 microns. The horizontal discrepancies for the mechanically tightened abutments ranged from 0 micron to 140 microns with a mean of 31 microns, compared with a mean of 46 microns and range of 0 micron to 113 microns, for the hand-tightened abutments. A close fit was seldom achieved. It is concluded that considerable discrepancies existed around superstructures that had been judged to have a clinically acceptable fit. These were reduced in the vertical direction by hand tightening of the gold screws.

Dental Abutments↗
PubMed1996

Load distribution on implants with a cantilevered substructure: an in vitro pilot study.

The forces distributed on six Nobelpharma implants were analyzed by strain gauges fixed to various implant combinations that were placed in a replica of a human edentulous mandible and connected to a cast gold alloy superstructure. The mandible was supported on its lower border and the superstructure loaded in different locations. Although force distribution was found to be asymmetrical, statistical analysis revealed no significant differences. Loads applied to one side of the superstructure were largely carried by the implants on that side, demonstrating torquing of the casting in the midline. Loads applied to the cantilever resulted in large extension forces in implants on the opposite side of the fulcrum. The magnitudes of the loads distributed on the screw varied as a result of the combinations, inter-implant spaces, and implant locations.

Dental Implantation, Endosseous↗
PubMed1996

The influence of occlusal scheme on masticatory forces using implant stabilized bridges.

The objectives of this study were to determine the effects on masticatory forces of using implant stabilized mandibular bridges with occlusal schemes providing either balanced or unbalanced articulation. The investigation was carried out in a group of six subjects who had dentate, or restored dentate maxillary arches and who used implantstabilized mandibular prostheses. Peak masticatory forces, chewing frequency and load rate were determined when chewing bread, carrots and nuts. The mean peak masticatory force varied markedly from subject to subject, and, together with the masticatory load rate were lowest when eating bread and highest when chewing nuts, with values for chewing carrots between the two. There was no detectable difference in mean peak masticatory force or load rates associated with chewing bread with the two occlusal schemes; however, these were lower with balanced occlusion when chewing nuts and carrots. The ranking of subjects in terms of mean peak masticatory force and load rate was consistent and peak load rates of up to 2000 N/sec were recorded. Chewing frequency was relatively consistent for a particular subject irrespective of the foodstuff.

Aged↗
PubMed1996

The effects of superstructure fit and loading on individual implant units: Part 2. The effects of loading a superstructure with varying degrees of fit.

This study investigated force transmission in an implant supported bridge with varying degrees of fit. The project utilised an in vitro model of five implant fixtures mounted linearly and rigidly. These supported a well-fitting cast gold superstructure via implant abutments fitted with strain gauges so as to function as force transducers. Gaps of 10-110 microns were created at the middle and one terminal abutment and loads of 50N applied over the cantilever and 230N between the terminal abutments. Force distribution was uneven, compression, tension and torque observed, and loading of the cantilever noted to generate large forces.

Bite Force↗
PubMed1995

An in vivo investigation of seating and removal forces associated with recording impressions in dentate patients.

This in vivo study examined forces exerted during the recording of impressions of dentate maxillary arches. This study consisted of 10 subjects; four impression materials in metal stock trays and disposable plastic trays were used. The load rate and the maximum insertion and removal forces were measured with strain gauges mounted on the tray handles. The results indicated considerable variations in forces and load rates related to material, subject, and tray type. Lower forces were measured with the disposable trays. The greatest mean peak removal forces were 36.3 N with a metal tray/elastomer combination and 19.9 N with the same material used in a plastic tray.

Alginates↗
PubMed1995

Load transmission in implant superstructures supported by natural teeth and osseointegrated dental implants. A preliminary report.

There is considerable debate as to the effects of using bridges which are supported simultaneously by teeth and implants, due to the marked differences in their displaceability. This in-vivo experiment was carried out on three subjects, and measured the vertical and horizontal movement of implants and teeth, linked with a fixed superstructure, when this was loaded. The results suggest that natural abutment teeth may contribute significantly to bridge support in these circumstances.

Dental Abutments↗
PubMed1995

The effects of superstructure fit and loading on individual implant units: Part I. The effects of tightening the gold screws and placement of a superstructure with varying degrees of fit.

This in vitro study investigated forces associated with the assembly of rigidly mounted Nobelpharma implants and a 'passively' fitting cast gold superstructure. The effects of modifying the fit by 10-110 micrometers were also investigated. Measurements were made using linear resistance strain gauges mounted longitudinally on the abutments. Tightening the gold screws produced high compressive strains. These were unevenly distributed, and inconsistent. Tensile strains were produced by gaps as small as 10 micrometers between the superstructure and an abutment while the ability of a torque of 10 N-cm in the gold screw to close the gaps was dependent on their dimensions and locations.

Dental Abutments↗
PubMed1994

Presenting complaints and findings in a group of patients attending a hypodontia clinic.

A survey is presented of the complaints and findings in a group of 451 patients referred to a multidisciplinary clinic for the management of hypodontia. The most common complaints were missing teeth, spacing in the dental arches, and poor appearance. The majority of patients were referred by their dental practitioners, and more than half were over 12 years old when first seen. Delayed referral can have educational and social consequences. Changing attitudes to dental care and the maintenance of an intact dentition are resulting in hypodontia, particularly in its more severe forms, becoming of increasing concern to children and their parents. The management of this condition is best organised via multidisciplinary specialist clinics.

Adolescent↗
PubMed1994

Behaviour of fibroblasts during initial attachment to a glass-ceramic implant material in vitro: a time-lapse video-micrographic study.

The in vitro behaviour of 3T3 fibroblasts plated on a glass-ceramic implant material, Apoceram, and on tissue culture polystyrene (TCP) was monitored using phase-contrast, time-lapse video microscopy and scanning electron microscopy. Significant differences were observed in the short-term cellular response to TCP and Apoceram in the timing of the fibroblasts' morphological changes, with earlier spreading and stabilization of the cells occurring on TCP. Cell surface ruffling activity was greater on the glass-ceramic, reflecting weaker adhesions to the substratum. The experimental method used facilitates direct analysis of the dynamic activity of cells attaching to artificial substrata.

Animals↗
PubMed1992

The influence of occlusal surface material on peak masticatory forces using osseointegrated implant-supported prostheses.

Masticatory forces associated with porcelain and acrylic resin occlusal surfaces on osseointegrated implant-supported prostheses opposing natural teeth were studied in five healthy subjects. A three-channel electronic force transducer carrying a fixed prosthesis with either acrylic resin or porcelain occlusal surfaces and balanced articulation was used to record masticatory forces while the subjects chewed various foods. It was found that mean peak masticatory forces varied considerably from subject to subject but were consistent within each subject. No differences related to tooth material could be detected in the load rates. Forces directed away from the implants were noted in all subjects.

Acrylic Resins↗
PubMed1991

Mandibular deformation in subjects with osseointegrated implants.

Relative movement and force transmission between osseointegrated (OI) implants in the premolar regions of the edentulous mandible have been measured using intraoral transducers linked to OI implants. These have shown deformations of up to 420 microns and force transmission of up to 16 N as a result of jaw movement from the rest position. Greater displacements and forces were observed in active opening and protrusion than in lateral excursions. There were wide variations from subject to subject, and while the effects of these phenomena are not known, they may be potentially harmful to the interfaces between the implants and bone and the various components of the implant superstructure.

Aged↗
PubMed1989

Bone response to a matched modulus endosseous implant material.

The aim of this study was to assess bone response to a novel matched modulus material using more rigid titanium as a control. Implants subjected to different loading conditions were retrieved from rabbit tibiae after various time intervals, and new bone growth was assessed by histomorphometric methods. Block face microscopy was employed for histologic purposes. From a statistical analysis of the histomorphometric results, it could be concluded that titanium implants encouraged the greatest overall amount of bone formation around them when they were unloaded, when they were loaded immediately, and after a healing period.

Animals↗
PubMed1987

Immunoglobulin A1 and A2 subclass of salivary antibodies to Candida albicans in patients with oral candidosis.

Immunoglobulin A antibody titres to a cytoplasmic protein extract of Candida albicans were determined by ELISA in saliva from 20 patients with oral candidosis and 21 controls. Patients had significantly increased levels of salivary IgA anti-Candida antibodies when compared with controls (P less than 0.001). Antibody levels were associated with IgA1 subclass in 90% of the patients; in contrast, IgA2 subclass was predominant in 67% of the controls. Antifungal therapy resulted in significantly decreased IgA1 titres (P less than 0.05) whilst the mean IgA2 antibody titre remained unchanged. The results indicate that Candida infection may change the subclass pattern of salivary IgA antibodies.

Adult↗