Turbine effect on dental students' lateral pinch modulation and performances

BACKGROUND

Dental turbines can generate significant vibrations that may be transmitted to dentists' hands. The vibrations contribute to neural impairment and decrease dexterity over the long duration. It is unclear whether such vibration effects would be detected in the short time during the pre-clinical year of dentistry study.

AIMS

To investigate the effect of vibrations on lateral pinch modulation and manual performance of dentistry students.

METHODS

Four lateral pinch modulation tests by a square-shaped force sensing resistor (FSR) were administered to 30 dental students during their phantom course. Tests were performed with dominant and non-dominant hands at two time points: T0 (2 months after commencing phantom training) and T1 (end of phantom training). At each time point, students accomplished two manual performance tests on plastic teeth. Comparisons were made between lateral pinch modulation test scores at different times, considering the differences between hands and gender. The relationships between the FSR tests and manual performance were also determined.

RESULTS

The participating 19 female and 11 male students had a combined 50% response rate. At T1, the lateral pinch modulation for all tests significantly decreased for the dominant hand (P < 0.05) and non-dominant hand (P < 0.05). For the non-dominant hand at T1, the lateral pinch modulation of females significantly decreased as compared to males (P < 0.05). No significant correlations were found between lateral pinch modulation and manual tasks at T0 and T1.

CONCLUSIONS

The lateral pinch modulation of the fingers of dental students deteriorated after only 8 months of training at phantom (short-time exposure), with females at higher risk and possibly earlier symptoms manifestation.

The influence of long term water immersion on shear bond strength of amalgam repaired by resin composite and mediated by adhesives or resin modified glass ionomers

OBJECTIVE

To assess the shear bond strength between amalgam and resin composite mediated by either multipurpose adhesive systems or RMGI when subjected to long term immersion in saline.

METHODS

Part I: Cylindrical specimens (6 mm × 6 mm) composed of equal parts of sandblasted set amalgam (Oralloy) and composite (Z-100), with a thin layer of either Scotchbond Multipurpose, All Bond 2, Amalgam Bond Plus, High Q Bond Plus or Vitrebond in between were fabricated (n = 100 × 5). Each group was divided into 3 subgroups, immersed in saline at 37 °C for either 48 h, 3 or 6 months, followed by thermocycling (5000; 5/55 °C) and shear bond strength testing (SBS). Part II: Identical specimens were fabricated with intermediary of either Ketac Cem, Fuji Lining LC, Rely X Luting, Fuji Plus or Meron Plus (n = 100 × 5). Immersion periods, followed by thermocycling and SBS testing as in Part I. Two representative specimens from each subgroup were sectioned and inspected under SEM.

RESULTS

The two classes of intermediary agents yielded SBS which differed mainly in the 6 months incubation period. While multipurpose adhesives provided SBS values of ~9-10 MPa RMGI provided higher SBS of ~16 MPa. All Bond 2 and Amalgam Bond Plus exhibited deterioration of SBS during the 6 month period as well as Rely X Luting. Gap sizes between 0.5 and 3 μm exist between all intermediaries and the amalgam; on the other hand all intermediaries exhibit gap-free interfaces between the adhesives/RMGI and the composite.

CONCLUSIONS

Vitrebond in particular and RMGIs in general can serve as an excellent coupler of resin composite to amalgam, providing a durable bond.

Appropriateness of viscoelastic soft materials as in vitro simulators of the periodontal ligament

The periodontal ligament is a viscoelastic soft tissue that connects the tooth to the alveolar bone. This tissue should be simulated in numerical as well as in laboratory models. The mechanical properties of this tissue were previously determined ex vivo and in vivo. The aim of the study was to analyse the appropriateness of impression and reline materials used in dentistry to simulate viscoelastic behaviour of the periodontal ligament. Two reline [Durabase (Reliance Dental MFG, Co.) and Soft Liner (GC Corporation)] and two impression [President Plus (Coltene) and Prestige L (Vanini Dental Industry)] materials were examined in recovery and tensile relaxation tests. Recovery: This experiment simulated in vivo test. Roots of a pair of plastic maxillary premolar teeth were covered with each test material and embedded in acryl while maintaining the contact point. A 0·1-mm stainless steel strip, inserted at the contact point and maintained for 10 s, was used to tip the teeth. After removal, the tightness of dental contact point was measured over 30 min by determining the force needed to insert a 0·05-mm metal strip. Tensile relaxation: strips were elongated to 120%, 140% and 160% of their initial length and maintained at that length for 30 min. Two-phase decay function was applied. The results showed that elastic modulus and relaxation behaviour were significantly different between materials. Elastic modulus values were in the same range of those reported in the literature. However, the recovery values and behaviour showed that impression materials, especially President, are the materials of choice for this purpose because they simulated better the in vivo test.

Mechanical behavior of major connectors--part 2: influence of denture teeth height and loading direction

STATEMENT OF THE PROBLEM

When force is applied on a denture the moments may develop at the denture major connector (MC) interface and thus enabling more framework deformation.

PURPOSE

To determine the influence of crown height and loading directions on the stiffness of different MC designs when supported by an oral cavity simulating model.

MATERIAL AND METHODS

Oral cavity models were used as support for different major connector's designs. A set of metal tooth casted crowns, 8, 10, and 12 mm in height, was prepared from molar plastic phantom teeth. The crowns were connected to premolar or molar regions of the major connectors and loaded vertically or in 45 to the occlusal plane. The stiffness of the system was measured.

RESULTS

Changing tooth-crown height resulted in small changes in stiffness values when loaded vertically to the occlusal plane. Loading in oblique to the occlusal plane, the mean stiffness value decreased 30% when crown height increased from 8 mm to 12 mm. A reduction in stiffness values of 80-90% compared to vertical loading was found.

CONCLUSION

Long denture crowns reduce the rigidity of the removable partial dentures.

CLINICAL IMPLICATIONS

The removable partial denture should be supported by the maximal oral tissues. This stresses the importance of periodical examinations for any need of denture-base relining to maintain good support.

Posterior and anterior components of force during bite loading

Late anterior crowding of teeth has been associated with the anterior component of force (ACF) developed during biting. Possible physiologic mechanisms countering ACF, including the presence of a posterior component of force (PCF), are hypothesized. In this self-controlled study, 60 subjects aged 27.05+/-3.9 years were examined for ACF and PCF that were calculated as the change in tightness of a mandibular dental contact points from non-biting to biting state. Both ACF and PCF were found to develop simultaneously. However, the PCF was 4-7 folds smaller than the ACF (p<0.001). The ACF progressively declined by 10-20 folds (p<0.001) from the posterior to anterior dentition. The lateral incisor-canine contact point had the greatest ACF decline (63-74%). ACF effect on the anterior dentition is counteracted by a protective mechanism consisted of PCF, progressive dissipation of ACF, and canine blockage.

Mechanical behavior of major connectors--Part 1: Influence of supporting tissues

STATEMENT OF THE PROBLEM

Mechanical properties of major connectors are crucial for the performance of prosthetics.

PURPOSE

To determine the stiffness of systems of major connectors when supported by a model simulating the oral cavity.

MATERIAL AND METHODS

Oral cavity models were made of plaster covered by resilient material. The stiffness of five maxillary and five mandibular major connectors were measured while loading vertically to the occlusal plane on two points, premolar or molar, directly on the frame work and when the major connectors were supported by the oral cavity model.

RESULTS

The mean stiffness values of the major connectors loaded on the oral models were 35-folds and 60-folds higher compared to non-supported results for the premolar and molar points, respectively, for both jaws. With no support, stiffness values when the major connectors were loaded on molar point were lower compared to premolar point. When major connectors were supported by the oral models, the opposite occurred: the system showed higher stiffness when the major connectors were loaded on the molar point compared to premolar point.

CONCLUSION

The influence of supporting tissues on the stiffness of major connector systems should be included in further studies.

CLINICAL IMPLICATIONS

It is recommended to end the distal portion of the dentures at the second molar and to use the whole support provided by the oral tissues of the ridge.

Mechanical resistance of biological repair cartilage: comparative in vivo tests of different surgical repair procedures

The former common knowledge that cartilage lesions do not heal has been modified over the last few years due to new technologies. For repair of deeper circumscribed lesions osteochondral press-fit grafting and tissue engineering are used in clinical application. The histological data of the hyaline-like tissue obtained by engineering are just as satisfactory as the surviving grafted hyaline cartilage on top of osteochondral cylinders. But comparative studies are still lacking. To fill the gap and with a view to repairing larger osteoarthritic defects we have performed an in vivo study on 16 goats. Three months after the creation of a full thickness wide cartilage defect on the femoral condyle with harvesting of cartilage samples for tissue cultures we performed secondary cartilage repair procedures on the installed osteoarthritis areas: 1) grafting with autogenic osteochondral press-fit cylinders from the opposite knee, 2) autologous engineered chondrocyte grafting under periosteal flaps, 3) both in combination. The harvesting defects were either left as controls or filled with a hyaluronate fleece. After eight months the repaired areas and the harvesting defects were examined for cartilage stiffness as a novel comparative parameter. Compared to normal the cartilage on top of osteochondral grafts is considerably stiffer. Engineered cartilage is weaker than normal. Spontaneously ingrown fibrous cartilage is much weaker even with a carrier fleece. A combination of osteochondral press-fit grafts with engineered autologous cells restores biomechanical qualities to repaired larger degenerative cartilage defects.

Laser soldering of rat skin, using fiberoptic temperature controlled system

BACKGROUND AND OBJECTIVE

Laser soldering of tissues is based on the application of a biological solder on the approximated edges of a cut. Our goal was to use laser soldering for sealing cuts in skin under temperature feedback control and compare the results with ones obtained using standard sutures.

STUDY DESIGN/MATERIALS AND METHODS

Albumin solder was applied onto the approximated edges of cuts created in rat skin. A fiberoptic system was used to deliver the radiation of a CO(2) laser, to heat a spot near the cut edges, and to control the temperature. Laser soldering was carried out, spot by spot, where the temperature at each spot was kept at 65-70 degrees C for 10 sec.

RESULTS

The tensile strength of laser-soldered cuts was measured after 3-28 days postoperatively and was found comparable to that of sutured cuts. Histopathological studies showed no thermal damage and less inflammatory reaction than that caused by standard sutures (P = 0.04).

CONCLUSIONS

Temperature controlled laser soldering of cuts in rat skin gave strong bonding. The cosmetic and histological results were very good, in comparison to those of standard sutures.

Effect of electrocautery vs. scalpel on fascial mechanical properties after midline laparotomy incision in rats

BACKGROUND

The method of midline laparotomy incision and closure remains a complex surgical problem.

OBJECTIVE

To compare the mechanical properties at the interface of midline laparotomy incision made by scalpel versus electrocuting current in rats.

METHODS

A sharp midline laparotomy incision was made in 60 Wistar female rats using a scalpel or electrocautery to open the fascia. The fascial and skin wounds were closed separately with a continuous nylon. Fascial specimens were analyzed for mechanical properties at the midline incision using a loading machine. The load-extension curve was recorded during tensile loading at a steady extension rate of 15 mm/min.

RESULTS

There was no statistically significant difference between the two groups in either wound-bursting force (PPEAK) or the strain energy spent until the point of measured PPEAK. Each load-extension curve showed a characteristic pattern in all rats. Tissue stiffness was greater in the scalpel group than in the electrocautery group (P = 0.02). Correlations were found between tissue stiffness and strain energy, between tissue stiffness and bursting force, and between bursting force and strain energy.

CONCLUSIONS

While tissue stiffness was greater when a scalpel was used compared to electrocuting to incise the midline abdominal fascia in rats, there was no difference in the bursting force required to disrupt the wound.

Tightness of dental contact points in spaced and non-spaced permanent dentitions

One of the characteristics of normal occlusion is tight dental contact points (CPs). However, the magnitude and distribution of the tightness of a dental contact point (TDCP) in non-spaced versus spaced dentitions are unknown, as well as the mechanism controlling this arrangement. Two hypotheses were examined: the compression theory, i.e. the teeth touch each other in a compressive state; and the resistance theory, i.e. size and number of roots determine TDCP values. For the study, 60 subjects (27 men, 33 women), mean age 25 +/- 4.3 years, with a complete permanent dentition and no missing teeth were divided into spaced (n = 22) and non-spaced dentitions (n = 38). For each CP, four repeated measurements of peak strain were performed with a one-month interval. No significant differences were found between repeated measurements. All CPs demonstrated a continuous decreased TDCP in the postero-anterior direction. Consequently, in non-spaced dentitions TDCPs between molars were 100 per cent higher than incisors. The five anterior CPs of each jaw demonstrated similar TDCP values. Maxillary TDCPs versus mandibular antagonists were not significant. Mandibular TDCPs were significantly higher in men than in women (14 per cent). Anterior TDCPs were less in spaced than in non-spaced dentitions (55 per cent). Posterior TDCPs were also lower in spaced dentitions, however, to a lesser extent (25 per cent). With the exception of TDCPnon-spaced > TDCPspaced, which is partially explained by the compression theory, most of the findings support the resistance theory regulating TDCP characteristics of the permanent dentition.

Splints and stress transmission to teeth: an in vitro experiment

OBJECTIVE

To determine the influence of hard and soft splints with two thicknesses on the stress transmission to the tooth supporting the splint and the opposite tooth.

METHODS

Continuous vertical forces up to 500N were applied to two opposite first molar phantom teeth using a universal loading machine. Deformation was detected by strain gauges attached to the cervical area of the buccal and lingual aspects of the lower tooth. Strain, as a function of force, was collected and the slope, defined as the compliance (in microS/N) of the system, was calculated.

RESULTS

The highest compliance was found with hard splints. When splints were constructed on the upper molar, the highest compressive compliance was registered on the buccal side (2.8 microS/N) and tension compliance on the lingual side (-0.35 microS/N). When constructed on the lower tooth, the opposite was found. Soft splints resulted in compression on both the buccal and lingual sides when adjusted to the upper or lower tooth. A higher compliance was found on the buccal side (1.26 microS/N), while on the lingual side, the values varied (0.48-0.78 microS/N).

CONCLUSIONS

Soft splints are more efficient in protecting teeth against the damage of bending forces although there is an increase of compression forces. The tooth opposing a hard splint is exposed to a higher risk of bending forces.

Modeling the body/chair interaction - an integrative experimental-numerical approach

OBJECTIVE

To develop a systematic methodology towards a realistic model, of the body/chair interaction as a tool to analyze sitting posture and the cushioning system influence on the pelvis/lower back stress development.

BACKGROUND

The model concept comprises an integrative structure including both the pelvis girdle and the lower spine, as well as the surrounding soft tissues with their specific characteristic behavior during sitting.

METHODS

In vivo measurements were performed, in order to obtain a set of contact moduli defining the behavior of the soft tissues under tension during the sitting down process. Additionally in vivo indentation of a metal ball into the same soft tissues during sitting were performed in order to obtain the characteristic moduli. A finite element model was also developed for the specific analyses.

RESULTS

Validation of the model was achieved by comparing its results with in vivo measurements of contact stresses developed between the body and a stiff target seat area. Loading the model using two alternative cushioning materials lead to different sets of stresses within the model; as the stiffness of the seat decreased, the peak contact stresses, as well as the internal body stresses substantially decreased.

RELEVANCE

Body/chair interaction model methodology may be a practical means to analyze and design structures and materials in order to achieve more comfort during work or leisure, as well as for geriatric or impaired subjects.

Accuracy of three polyvinyl siloxane putty-wash impression techniques

STATEMENT OF PROBLEM

There is much discussion in the dental literature concerning the effect of the impression technique on the accuracy of cast restorations.

PURPOSE

This study assessed the accuracy of 3 putty-wash impression techniques using the same impression material (polyvinyl siloxane) in a laboratory model.

MATERIAL AND METHODS

The 3 putty-wash impression techniques used were (1) 1-step (putty and wash impression materials used simultaneously); (2) 2-step with 2-mm relief (putty first as a preliminary impression to create 2-mm wash space with prefabricated copings. In the second step, the wash stage was carried out); and (3) 2-step technique with a polyethylene spacer (plastic spacer used with the putty impression first and then the wash stage). For each technique, 15 impressions were made of a stainless steel master model that contained 3 complete crown abutment preparations, which were used as the positive control. Accuracy was assessed by measuring 6 dimensions (intraabutment and interabutment) on stone dies poured from impressions of the master model.

RESULTS

One-way analysis of variance showed statistically significant differences among the 3 putty-wash impression techniques, for all intraabutment and interabutment measurements (P <.001). Overall discrepancies of the 2-step technique with 2-mm relief putty-wash impression technique were significantly smaller than that in the 1-step and polyethylene putty-wash impression techniques.

CONCLUSION

The polyvinyl siloxane 2-step, 2-mm, relief putty-wash impression technique was the most accurate for fabricating stone dies.

The effects of prolonged cryopreservation on the biomechanical properties of bone allografts: a microbiological, histological and mechanical study

Bone allografting is the most common form of allotransplantation in modern medicine. Bone banking is usually the major part of most tissue banks throughout the world. Several years ago, many standards of bone banking were set empirically, and have never been evaluated. One particular parameter or standard was outdating graft materials after 5 years of storage. This study was conducted to evaluate the effect of prolonged cryopreservation on the biomechanical properties of bone allografts and establish whether graft materials become contaminated during long-term storage.Proximal humeral bone allografts were obtained from the bone bank after 1, 3 and 5 years of -80 degrees C cryopreservation. Samples of each humeral head, i.e., cartilage, subchondral bone and spongy bone were histologically examined for inter- and intra-cellular changes. A three-point mechanical bending test was used on identical pieces of cortical bone to compare fresh and cryopreserved materials. Fresh-retrieved cortical bone using identically-sized segments, served as a control. Cultures were taken from each respective sample to determine contamination or sterility.Results of both the histological and mechanical testing showed that there were no significant, qualitative histological, or quantitative mechanical differences among the samples. All the cultures were negative. Therefore, based on this study's parameters, bone allografts can safely be used after a cryopreservation period of over 5 years and should not be discarded.

Effect of changing the saddle angle on the incidence of low back pain in recreational bicyclists

OBJECTIVE

According to the literature, 30-70% of cyclists suffer from cervical, dorsal, or lumbar back pain. This study was conducted to evaluate one of the possible causes of low back pain and to suggest a solution by appropriate adjustments to the bicycle.

METHODS

Serial fluoroscopic studies were performed while cyclists sat on different types of bicycle (sports, mountain, and city). Pelvic/spine angles were measured at different seat angles, and the related force vectors analysed.

RESULTS

There was a tendency towards hyperextension of the pelvic/spine angle which resulted in an increase in tensile forces at the promontorium. These forces can easily be reduced by appropriate adjustment of the seat angle--that is, by creating an anterior inclining angle. The findings of the biomechanical analysis were then applied to a group of cyclists who were members of a cycling club and who complained of low back pain. After appropriate adjustment of the saddle angle, most of the cyclists (>70%) reported major improvement in the incidence and magnitude of their back pain.

CONCLUSIONS

The incidence and magnitude of back pain in cyclists can be reduced by appropriate adjustment of the angle of the saddle. It is important that these findings be conveyed to cyclists, bicycle salesmen, trainers, and members of the general public who engage in cycling, in order to decrease the prevalence of back pain.

Stiffness of different designs and cross-sections of maxillary and mandibular major connectors of removable partial dentures

STATEMENT OF PROBLEM

Major connectors of removable partial dentures must distribute forces bilaterally without damaging the supporting tissues.

PURPOSE

This study investigated which design and cross-sectional shape of major connectors most favorably influence rigidity and flexibility.

METHODS AND MATERIAL

Five designs for maxillary removable partial denture major connectors and 5 lingual bar major connectors of different cross-sectional forms were cast in chrome cobalt alloy on a master cast. Points M and P, which represented the position of the first premolar and second molar teeth, were positioned 20 mm apart on the casting. Vertical and horizontal forces were applied to each point while the opposite side was gripped in an Instron testing machine. A force-deflection curve was obtained for each loading point. Mean stiffness values were obtained for loading in compression and torsion.

RESULTS

Values for torsional loading simulating vertical forces were lower when compared with values obtained for compression loading that simulated horizontal occlusal forces. Differences in stiffness were greater in mandibular major connectors loaded at M and P. The half pear-shaped cross section was the stiffest.

CONCLUSIONS

In the maxillary arch, the most rigid major connector was the anteroposterior palatal bar combination placed on different horizontal and vertical planes. The most flexible was the U-shaped design. In the mandibular arch, the most important factor in achieving rigidity was the cross-sectional shape of the major connector. The half pear-shaped cross section proved to be the most rigid.

Analysis of strength properties of light-cured resin composites

OBJECTIVES

To determine and correlate the compressive and tensile strengths of resin composites, to scale their failure probability and to analyze their failure mode under combined state of stresses.

METHODS

Ten brands of composites were tested for compressive and diametral tensile strengths. A recently introduced device for testing of pure shear stresses was modified to adapt to smaller specimens. Uniformity of pure shear stress distribution in the significant section was verified by a photoelastic model. Loading specimens in pure shear up to failure determined their mode of fracture under combined state of stresses.

RESULTS

Diametral tensile strength yielded values that were 20% of their respective compressive strength. Multiple comparison test indicated that strength properties of the tensile strength test were much more sensitive in predicting differences between resin composites when compared to a compressive strength test. Pertac (Espe) had the highest compressive strength, Graft LC (GC) and Z-100 (3M) had the highest diametral tensile strength. No correlation was found between tensile and compressive strengths. The Weibull modulus disclosed differences in the liability of the materials to fracture. When combined state of stresses were applied through the pure shear test, failure of each specimen occurred at the principal tensile planes.

SIGNIFICANCE

Compressive strength cannot predict the ability of the resin composite to withstand tensile stresses. The importance of compressive strength is limited as failure of a brittle material occurs in tension.

Rapid palatal expansion. Part 3: strains developed during active and retention phases

Patterns of strain accumulation/dissipation during the active and retention phases of rapid palatal expansion treatment were studied in a preliminary animal model (5 cats) followed by clinical study (14 patients). Two uni-axial strain gauges were bonded to the arms of a hyrax screw. The strain gauges were wired intraorally to a common male connector and protected against salivary assault. For strain registration, the connector was hooked to a switch and balance instrument and a digital strain indicator. The screw was activated (4 x 1/4 turn) every 3 to 4 days and strain and interarch distances were measured during rapid palatal expansion active and retention phases. Interarch distances were also measured during the relapse phase. Both studies share the same results. Strains and expansion increased progressively during active and retention phases of rapid palatal expansion. No difference between anterior and posterior strain was found. An immediate dental strain response that occurred during screw activations (intrasession strain) was related to tooth compression in the PDL. During the 3 to 4 day pause, a delayed skeletal strain response (intersession strain) developed that was inverted or continuous to the preceding intrasession strain. Complete dissipation of residual strains was limited to the first 1 to 3 active phase sessions. Strain accretion resulted essentially from intersession strain build-up. Strain level was preserved during the retention phase, apparently due to relapse strains. Relapse strains could not be measured but are inferred from the predominant interarch rebound measured during the relapse phase. Clinically, an extension of the intersession intervals and the retention phase are recommended.

Cusp reinforcement by bonding of amalgam restorations

The purpose of the present study was to assess the effectivenes of several adhesives in bonding amalgam in order to recover tooth stiffness. A non-destructive experimental methodology was adopted, using strain gauges bonded to the midbuccal surfaces of 40 teeth, with sequential evaluation of loaded intact, prepared and restored stages of the same tooth. Continuous strain measurement as a function of the applied load was acquired by A/D equipment and a data acquisition programme. The strain-force behaviour of the sound teeth under non-axial force up to 97.5 N served as the baseline. The five experimental groups (8 x 5) consisted of control (no adhesive) and four different adhesives. One-way analysis of variance with repeated measures was calculated for the deformation ratio, relative stiffness and recovery values. Reductions in tooth structure by cutting a mesio-occlusal-distal preparation, width one-third intercuspal distance, resulted in 39-52% loss of buccal cusp stiffness. Non-bonded amalgam produced negligible increase (5%) in the stiffness recovery values of the buccal cusps. The adhesives splinted the cusps together, thereby decreasing cuspal flexure and increasing relative stiffness values. Recovery values obtained ranged from 39% to 61%. Assuming that cusp fracture occurs as a result of brittle tooth structure fatigue, amalgam adhesives may contribute to the strengthening of weakened cusps.

Rapid palatal expansion. Part 2: Dentoskeletal changes in cats with patent versus synostosed midpalatal suture

Intercanine expansion (C-C) following rapid palatal expansion is made up of sutural displacement (Sd-Sd), tooth tip (Tt-Tt), tooth displacement (Td-Td), and alveolar process tipping and bending (At+b-At+b). The involvement of these four components was studied on 10 rapid palatal expansion treated and two control cats during an active phase (25 days), a retention phase (60 days), and a relapse phase (60 days). The midpalatal suture was analyzed for linear measurements, radiopaque versus radiolucent zones and optical density from occlusal radiographs. Nine treated cats exhibited sutural split and one treated cat showed no split as a result of synostosis of the suture. The contribution of the four constituents [(Sd-Sd):(Tt-Tt):(Td-Td):(At+b-At+b)] to the C-C expansion changed from active to relapse phase from [45%:15%:25%:15%] to [50%:25%:25%:0%] in the animals with sutural split and from [0%:40%:60%:0%] to [0%:0%:100%:0%] in the cat without sutural split, implying the major role of sutural displacement in patent suture, and tooth displacement in synostosed suture. The latter indicates the potential buccal corticalis fenestration, dehiscence or perforation in synostosed suture undergoing RPE. In patent suture (animals with sutural split), optical density increased during rapid palatal expansion (soft tissue build-up) and decreased during retention (remineralization) and relapse phases (medial convergence of the palatal processes). In the animal without sutural split, a continuous decrease in the optical density (predetermined ossification) was found. The progressive six-fold surge in coefficient of variation of C-C expansion during the relapse phase indicates limitation in predicting rapid palatal expansion stability. Clinically, the use of serial occlusal radiographs during rapid palatal expansion is recommended to evaluate patency and extent of retention period.

Rapid palatal expansion: Part 1. Mineralization pattern of the midpalatal suture in cats

The mineralization pattern of the midpalatal suture after rapid palatal expansion was investigated in 10 treated and 2 control cats, in light of the tendency of RPE to relapse. The rapid palatal expansion treatment consisted of active (25 days), retention (60 days), and relapse (60 days) phases. Standardized occlusal radiographs were taken periodically and analyzed for suture width, suture optical density in anterior vs. posterior regions, and suture area measurements of radiopaque vs. radiolucent zones. Nine cats exhibited suture splitting. During the active phase, the radiolucent zone (nonmineralized tissue) increased 12-fold and the increase in optical density was 50% greater in the anterior over the posterior suture region, demonstrating increased formation of loose connective tissue at the anterior region. During the retention period, the suture's radiopaque zone (mineralized tissue) increased by 62%, the radiolucent zone declined (64%) and the suture width decreased (65%) indicating reorganization of mineralized tissue. The decrease in optical density (increased mineralization) was 2.5 times greater in the posterior over the anterior suture region, indicating that the remineralization (closure) pattern of the expanded suture is analogous to a zipper closed in a posteroanterior direction. During the relapse phase, the reduction in total suture area (41%) and in the radiopaque zone (32%) indicates medial convergence of the maxillary horizontal processes. From our findings we extrapolated that the retention of the suture anterior region should be longer than the posterior region to catch up the lag in rebuilding and maturation of the newly deposited hard tissue.

The influence of abutment angulation on strains and stresses along the implant/bone interface: comparison between two experimental techniques

STATEMENT OF PROBLEM

Preangled abutments produce different stress distribution compared to straight abutments.

PURPOSE

The objectives of this study were to (1) test the hypothesis that preangled abutments produce different stress distribution than straight abutments by using strain gauges attached to implants embedded in a medium simulating bone to determine strain distribution along the implant/bone interface; (2) test this hypothesis by photoelastic method; and (3) compare the two experimental techniques.

MATERIALS AND METHODS

Five Integral Omniloc cylindrical implants 13 x 4 mm were polished to remove the hydroxyapatite coating, then six linear miniature strain gauges were attached, three on each side of the implant's surface. Two similar implants were embedded in a photoelastic material. Three abutments, straight, 15 degrees, and 25 degrees, were connected to each implant; strain versus applied compressive forces were recorded. Strain response to force parameter was defined as the slope of the strain-force curve. Isochromatic fringe patterns were also recorded.

RESULTS

The strain gauge measurements showed higher, threefold and 4.4-fold, compressive strain concentration in the coronal zone of the implant when 15-degree and 25-degree angulated abutments were used, respectively, compared with the straight abutment; whereas the photoelastic method showed an increase of only 11% in fringe order. Tensile strains were also measured from the coronal contralateral position on the implant, where photoelastic models did not show a change in stress type.

CONCLUSIONS

Data obtained from strain gauges bonded to implants embedded in a medium can represent a precise simulation of the clinical condition when analyzing stress distribution along the implant/ bone interface. Photoelasticity provides different information and therefore should be regarded as a complementary method.

The influence of surface loading and irradiation time during curing on mechanical properties of a composite

PURPOSE

The aim of this study was to determine the influence of different surface loadings during curing with various irradiation times on hardness and diametral tensile strength of a light-cured composite.

MATERIAL AND METHODS

A mold was fabricated to allow loading during curing of cylindrical specimens of a composite. Four surface loadings of 0, 0.35, 0.87, and 1.73 MPa and four irradiation times of 20, 40, 60, and 180 seconds were used (n = 15). Each specimen was subjected to a microhardness test and to a diametral tensile strength test.

RESULTS

Surface loading during curing affected both hardness and strength properties, whereas irradiation time influenced only the hardness of the material. Both parameters gained between 15% and 20% improvement when the material was loaded with 0.87 MPa surface pressure and cured by 60-second irradiation time. Higher loading or longer irradiation times did not improve these properties.

CONCLUSION

Loading composite during curing improves its mechanical properties, probably through decreasing flaws and air voids of the material.

Effect of irradiation time on tensile properties of stiffness and strength of composites

PURPOSE

We compared the effect of irradiation time on the diametral tensile strength and stiffness of three visible light-cured composites. The sensitivity of the two mechanical properties to differentiate between the materials was also investigated.

MATERIAL AND METHODS

Cylindrical specimens of three composites were cured for 20, 40, 80, and 120 seconds (n = 10) and loaded up to failure in a diametral tensile strength test. Stiffness and diametral tensile strength values were recorded.

RESULTS

Irradiation time significantly influenced diametral tensile strength (p = 0.0017) and stiffness (p = 0.0002). With the same irradiation times the three tested materials demonstrated no significant difference when diametral tensile strength was studied (p = 0.31). However, stiffness values did show a significant difference (p = 0.0002).

CONCLUSIONS

This study found that stiffness is more sensitive in disclosing differences in tensile properties between the materials than diametral tensile strength.

Effect of combinations of surface treatments and bonding agents on the bond strength of repaired composites

STATEMENT OF PROBLEM

Enhancement of bond strength between new and old composite usually requires increasing the surface roughness to promote mechanical interlocking and coating of old composite with unfilled resin bonding agents to advance surface wetting and chemical bonding.

PURPOSE

The purpose of this study was to evaluate the effect of combinations of surface treatments and bonding agents on the shear bond strength between new and old composite.

MATERIAL AND METHODS

Six surface treatments, two bonding agents, and an untreated control comprised 18 different subgroups.

RESULTS

The use of unfilled resin, alone or combined with silane, was the most effective procedure to enhance the shear bond strength of the repaired composite specimens, irrespective of the surface pretreatment processes. Silanation and unfilled resin slightly but not significantly improved the repair strength compared with unfilled resin alone.

CONCLUSIONS

Different combinations of surface treatments and bonding agents affect shear bond strength differently. The highest shear bond strength values were achieved by grinding the surface with green Carborundum stone or sandblasting, whereas the lowest values were obtained with hydrofluoric acid as the surface treatment agent.

Long-term durability of adhesive systems bonded to fresh amalgam

A strong durable resin bond to fresh amalgam is desired in composite-veneered amalgam restorations and in adhesive amalgam restorations. The purpose of this study was to evaluate the influence of long-term water storage on the durability of the shear bond strength of new adhesive systems bonded to fresh amalgam. Sixty cylindrical specimens composed of equal parts of amalgam and composite, with a layer of bonding material in between, were prepared for each adhesive system: All-Bond 2, Amalgambond Plus, High-Q. Bond, and Comspan. Specimens were divided into three subgroups and immersed in saline at 37 degrees C for either 48 hours, 3 months, or 6 months. After the immersion period, specimens were thermocycled and subjected to shear bond strength testing. Shear bond strength of Comspan and High-Q-Bond adhesives did not deteriorate significantly during the 6-month experiment and maintained a mostly mixed mode of failure. All-Bond 2 and Amalgambond Plus adhesives exhibited deterioration of the shear bond strength as a function of immersion time and shifted from dominantly mixed mode of failure to totally adhesive (All-Bond 2) or mostly adhesive failure (Amalgambond Plus). Incubation in saline for long periods should be a standard test in evaluating the bond of new adhesive systems to fresh amalgam, whereas short exposure time to water might be misleading.

Shear modulus--measurement methodology with application to light-cured resin composites

OBJECTIVES

This study was conducted to test a novel methodology for shear modulus determination of light-cured resin composites.

METHODS

Using a butterfly-shaped specimen, a special loading device was able to apply pure uniform shear stress on a significant region of the specimen. Validation of the method was proven by photoelastic experiments. Shear modulus was directly obtained through shear stress-strain curve where the strains were measured by a shear strain gauge rosette. The composite materials analyzed were one microfill (Silux Plus, 3M Dental Products) and four hybrids (Brilliant, Coltene; Herculite XRV, Kerr; Z-100, 3M Dental Products; Graft LC, GC). An analysis of variance was used to evaluate the G modulus (p<0.01).

RESULTS

Modulus values varied between 2.54-8.01 GPa. The microfilled material presented the lowest value, and Graft LC presented the highest value. A statistically significant difference was found between all materials (p<0.01).

SIGNIFICANCE

The described methodology can also be applied to other restorative materials, as well as to adhesion strength measurements.

Mechanical properties of bone-implant interface: an in vitro comparison of the parameters at placement and at 3 months

Four biomechanical parameters--peak force, vertical displacement, interface stiffness, and strain energy--were defined to evaluate bone-implant interface properties. These parameters were measured at placement and after 3 months of healing during push-in tests on commercially pure titanium implants placed in the mandibles of dogs in a one-phase nonsubmerged procedure. Comparison of the results showed that peak force, interface stiffness, and strain energy increased after 3 months of healing, but vertical displacement decreased. These findings suggest that the interface stiffness, which is considered a major factor for implant success, increases during 3 months of healing in dogs, which corresponds to a 4- to 6-month healing period in human mandibles.

Shear bond strength of composite resin to fresh amalgam

The shear bond strength between fresh amalgam and composite resin using three adhesive systems was assessed. Amalgambond (5.19 MPa), All-Bond (3.45 MPa), and Clearfil New-Bond (4.37 MPa) had comparable shear bond strengths higher than Enamel Bond (1.27 MPa) after 48 hours of water immersion. This bond was hydrolytically degraded during 100 days of immersion in water. The greatest deterioration was observed for Clearfil New-Bond (0.81 MPa). Amalgambond provided the best results, whereas All-Bond and Clearfil New-Bond had comparable bond strength to Enamel Bond at the end of the experiment.

The effect of irradiation time on the shear strength of composites

OBJECTIVES

The purpose of this study was to determine the shear strength of composites at a specific depth by a double shear test and to relate it to irradiation time.

METHODS

Aluminum molds filled with three different composites were irradiated for the manufacturer's recommended time, as well as for three longer and two shorter times. Fifteen cylindrical specimens were prepared for each combination of material and exposure time and subjected to a double shear test based on the principle of rivets connecting various structural members. The shear planes were symmetrically located 1.5 mm from both outer surfaces.

RESULTS

Shear strength vs. irradiation time was directly but nonlinearly related. All test materials attained a maximum shear strength value which was not exceeded by a further increase of the exposure time. Maximum values obtained were 42.5 MPa for Heliomolar (Vivadent) and Durafill (Kulzer) and 66.9 MPa for P-50 (3M). Only the shear strengths using the shortest light activation time were significantly different for each product.

SIGNIFICANCE

Shear strength of composites at a specific depth is a function of the combination of light energy and the attenuating power of the specific material. A double shear test might serve as an adjunct to microhardness in determining the extent of cure of composites at a specific depth. The advantage of this testing is the extra information to be gathered regarding the ability of composites to withstand internal stresses at a predetermined distance.

A new intratracheal stent made from nitinol, an alloy with "shape memory effect"

Temporary or permanent tracheal splinting in pediatric patients may be indicated in tracheomalacia or bronchomalacia, repair of congenital tracheal stenosis, and after tracheal resection. This study presents the results of the development of a new intraluminal airway stent made from titanium alloy, a metal with "shape memory effect". At low temperatures (martensitic state) the titanium alloy stent can be fashioned into a specific shape; then when heated to a higher temperature (austenitic state) the stent alters its shape, only to regain its original shape when recooled to the lower temperature. The stent, connected to a small electric power supply, was introduced into 20 young rabbits with the use of a 2.5 cm rigid bronchoscope. After implantation in the martensitic state the stent was warmed to 40 degrees C, the austenitic state, by an electric current of 1.5 to 3 ampere for 1 to 2 seconds. After a period of 8 to 10 weeks the stent was removed (in its martensitic state) through the same-sized bronchoscope after being cooled with 3 to 4 ml of 80% alcohol solution at 6 degrees C. No signs of airway obstruction developed in any of the animals after implantation or extraction of the stent. The biomechanical properties of the trachea, as shown by strain measurements with the use of incremental forces, showed significant differences between the stented and unstented segments (p < 0.005). The titanium alloy intratracheal stent adequately fulfilled the requirements of a temporary intraluminal airway splint, and because of its unique feature of shape memory effect the stent could be inserted, fixed, and removed easily, even in very small airways.

Toward early detection of the tendency to stress fractures

Certain characteristics of the foot structure represented by the contact pressure display method provide a clear indication of the tendency to experience stress fractures. The main results of the research project are (i) the static foot-ground pressure diagram may supply basic information about the foot structure, needed to understand its behaviour, (ii) a new concept of stress intensity parameters was introduced and appeared to be basic for the foot structure evaluation, (iii) a set of criteria was established to classify the foot structure and its tendency to stress fractures, (iv) a simple computerized screening technique can calculate and classify the values of above parameters and relate them to the established criteria. The results may enable a scientific and practical approach in characterizing foot orthopaedic problems and might help to rate the stress fracture risk.