Effect of low intensity laser irradiation on surgically created bony defects in rats.

Low intensity lasers have been used by clinicians to improve healing and reduce pain in humans. Lasing also results in new bone formation around hydroxyapatite implants and a significant increase in the total bone area. However, the exact mechanism of cell biostimulation by laser is still unclear. This study biochemically assessed the effects of low intensity laser (Gallium-Arsenide) using 4 and 22.4 mW cm(-2) power density on the bone healing process after surgically creating bony cavities in rat mandibles. Rats (n = 24) were divided into two groups each treated with specific energy, 4 or 22.4 mW cm(-2), for 3 min each day post-surgery. Surgical cavities were created on both sides of the mandible: the left served as an untreated control, the right was treated with laser. All rats were sacrificed after 1, 2 and 4 weeks of treatment. In the newly formed callus, accumulation of radiocalcium and alkaline phosphatase activity was measured to indicate osteogenic activity. One-way anova with repeated measures showed that the low intensity laser using 4 mW cm(-2) power density significantly increased radiocalcium accumulation from 2 weeks post-surgery, whereas 22.4 mW cm(-2) had no effect. No changes were noted in the activity of alkaline phosphatase with the laser treatment. These results suggest that laser therapy of low power density is effective on the bone healing process in artificially created osseous cavities by affecting calcium transport during new bone formation.

[Full arch impression techniques utilizing addition type poly vinyl siloxane for fabrication of tooth born fixed partial dentures].

A precise impression is necessary for fabricating an accurately fitting cast restoration. For this purpose, Polyvinyl Siloxane (PVS) impression materials are extremely popular because of their combination of excellent physical properties, handling characteristics and dimensional stability. Its excellent clinical features remain unaffected if simple measures are guarded. This review presents several impression techniques using PVS and recommends the one that provides the most accurate impression, utilizing the superior qualities of the PVS. The one step impression technique where no control of wash bulk and thickness exists, is considered to be the least accurate impression method with measured discrepancies as large as 7 times the original inter preparation distance and 40 times the original cross arch dimensions. Furthermore, the direct contact between the less refined putty material and the tooth preparation, as well as the high prevalence of air bubble entrapment, seriously compromises restoration longevity. The two stage impression technique has proved to produce the most accurate and reliable impressions due to complete control of the wash bulk and thickness entailed. The ideal wash bulk thickness should range between 1 to 2.5 mm all around the abutment tooth in order to minimize distortion of its subsequent die. Using a "Putty Tray" at the first stage with a predetermined space encircling the abutments will allow the wash to flow to its ideal uniform bulk size at the second stage. A uniform bulk size will prevent differential setting contraction and uneven changes at the dimensions of the die. The easiest and most clinically applicable method to achieve the desired space around the preparations is by loading the Putty material with the temporary crowns in place, followed by their removal at the second stage and occupation of the created space by the wash. In general, less control of wash bulk will result in either insufficient or excessive wash material which will determine uneven dimensional changes in the impression. This, in turn, will produce ill fitting cast restoration.

Chewing side preference as a type of hemispheric laterality.

Chewing side preference is a factor that could effect prosthodontic treatment. The purpose of this study was to determine whether chewing side was another type of hemispheric lateralization comparable with footedness, handedness, eyedness and earedness. Chewing side preference was tested in 189 subjects of whom 84 were partially edentulous, 98 had a full compliment of dental units (81 included implant-supported restoration restoring the missing teeth and 17 with fully intact dentitions), and seven were fully edentulous, restored with complete dentures. Laterality tests were carried out for the first cycle of mastication, handedness, footedness, earedness and eyedness and patient questionnaire. Most patients preferred chewing on the right side (78b3%) and were right sided. Chewing side preference correlated with other tested hemispherical lateralities. Missing teeth, occlusion type, lateral guidance, gender, implant-supported restorations and complete dentures do not affect the side preference for chewing. This presents a strong argument that chewing side preference is centrally controlled and provides food for thought regarding its significance in prosthodontics.

Effect of wash bulk on the accuracy of polyvinyl siloxane putty-wash impressions.

Variations in the bulk of wash in a putty-wash impression technique can result in dimensional changes proportional to the thickness of the wash material during setting. The purpose of the study was to determine the amount of wash necessary to achieve accurate stone models while using a two-step putty-wash impression technique with polyvinyl siloxane (PVS) impression material. A total of 45 impressions were made of a stainless steel master model, 15 impressions for each wash thickness (1, 2 and 3 mm). The model contained three full-crown abutment preparations, which were used as the positive control. Accuracy was assessed by measuring six dimensions (occlusogingival and interabutments) on stone dies poured from impressions of the master model. One-way analysis of variance (ANOVA) showed statistically significant differences amongst the three wash bulk groups, for all occlusogingival and interabutment measurements (P < 0.001). The overall discrepancies of the groups using wash thickness of 1 and 2 mm were smaller than the group with 3 mm wash thickness. Therefore, wash bulks of 1 and 2 mm were most accurate for fabricating stone dies, using PVS impression materials. This can be achieved by using the temporary crown to create the desired wash space in the preliminary putty impression. Wash thickness > 2 mm was inadequate to obtain accurate stone dies.

The use of reinforced composite resin cement as compensation for reduced post length.

STATEMENT OF PROBLEM: Cements that yield high retentive values are believed to allow use of shorter posts. PURPOSE: This study investigated the use of reinforced composite resin cement as compensation for reduced dowel length. MATERIAL AND METHODS: The retention values of stainless steel posts (parallel-sided ParaPost and tapered Dentatus in 5-, 8-, and 10-mm lengths) luted with Flexi-Flow titanium-reinforced composite resin and zinc phosphate cements were evaluated. Single-rooted extracted human teeth with crowns (n = 120), removed at the cementoenamel junction, were randomly divided into 4 groups of 30 samples each. Different post lengths were luted with either Flexi-Flow or zinc phosphate. Each sample was placed into a specialized jig and on a tensile testing machine with a crosshead speed of 2 mm/min, applied until failure. The effect of different posts and cements on the force required to dislodge the dowels was evaluated with multiple analyses of variance (ANOVA). One-way ANOVA with Scheffé contrast was applied to determine the effect of different post lengths on the retentive failure of posts luted with the 2 agents. RESULTS: Flexi-Flow reinforced composite resin cement significantly increased retention of ParaPost and Dentatus dowels (P<.001) compared with zinc phosphate. One-way ANOVA revealed no statistically significant difference (P>.05) between mean retention of both dowels luted with Flexi-Flow for all posts length used (5 mm = 8 mm = 10 mm). Mean retention values of the groups luted with zinc phosphate showed a statistically significant difference (P<.001) for the different post lengths (10 > 8 > 5 mm). Parallel-sided ParaPost dowels demonstrated a higher mean retention than tapered Dentatus dowels (P<.001). CONCLUSION: In this study, Flexi-Flow reinforced composite resin cement compensated for the reduced length of shorter parallel-sided ParaPost and tapered Dentatus dowels.

Stress levels for well-fitting implant superstructures as a function of tightening force levels, tightening sequence, and different operators.

STATEMENT OF PROBLEM: Unfavorable stress distribution and occlusal overload have been reported to result in failures ranging from screw loosening to loss of osseointegration. PURPOSE: The purpose of this study was to assess the effect of different tightening forces and sequences, with different operators, on stresses generated on an accurately fitting implant superstructure on multiple working casts made with a splinted impression technique. MATERIAL AND METHODS: The effects of different tightening forces (10 and 20 Ncm) were assessed with the use of 30 stone casts made from a metal master model with a splinted impression technique. Stresses generated were recorded by 4 strain gauges attached to the superior surface of the master framework. A multiple analysis of variance with repeated measures was performed to test for significant differences among the groups. RESULTS: Tightening force values at 10 Ncm ranged from 150.43 to 256 Ncm. At 20 Ncm, microstrain values ranged from 149.43 to 284.37 Ncm. Microstrain values related to the sequence of tightening ranged from 150.8 to 308.43 Ncm (left to right) and 154.63 to 274.80 Ncm (right to left). For the different operators, microstrain values ranged from 100.13 to 206.07 Ncm. No statistically significant differences among the variables of tightening force, tightening sequence, and operators were found ( P >.05). The interaction between groups and strain gauges was also found to be nonsignificant (P >.05). CONCLUSION: The potential of variable tightening force and tightening sequence to generate unfavorable preload stresses can be minimized through use of the splinted impression technique, which ensures an accurately fitting superstructure.

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.

Accuracy of implant impression splinted techniques: effect of splinting material.

Three implant impression techniques, using 3 different splinting materials, were assessed for accuracy in a laboratory model that simulated clinical practice. For group A, an autopolymerizing acrylic resin was used to splint transfer copings. In group B, a dual-cure acrylic resin was used, and for group C, plaster, which was also the impression material, was used. A metal implant master cast with an implant master framework was made to accurately fit to the cast. This cast was the standard for all impressions. For each group, 15 impressions were made. Polyether impression material was used for groups A and B. The accuracy of the stone casts with the implant analogues was measured against the master framework, using strain gauges. A multiple analysis of variance with repeated measures was performed to test for significant differences among the 3 groups. Additional analyses of variance were carried out to locate the source of difference. The statistical analyses revealed that a significant difference existed between groups A and B and between groups B and C but not between groups A and C. Impression techniques using autopolymerizing acrylic resin or impression plaster as a splinting material were significantly more accurate than dual-cure acrylic resin. Plaster is the material of choice in completely edentulous patients, since it is much easier to manipulate, less time consuming, and less expensive.

Analysis of load transfer and stress distribution by an implant-supported fixed partial denture.

This study simultaneously examined the load transfer and stress distribution by an implant-supported fixed partial denture. A mandibular implant framework with implants connected to the abutments was embedded in a three-dimensional photoelastic model of a mandible. Strain gauges were attached on the superior surface of the framework, and a vertical load of 7.5 kg was applied to seven points on the framework. The measurements derived from this simulation revealed that (1) there was a direct proportion between the stress distribution in the metal framework and stresses created in the supporting structure around the implants; (2) the mode of load transfer and stress distribution was directly proportional to the distance of the components from the loading point; and (3) when the cantilever was loaded, the major part of the stress was distributed within the cantilever in the connection to the distal abutment. In this simulation, stress was distributed over the two, or maximum three, closest implants with the distal implant the most stressed.

Accuracy of implant impression techniques.

Three impression techniques were assessed for accuracy in a laboratory cast that simulated clinical practice. The first technique used autopolymerizing acrylic resin to splint the transfer copings. The second involved splinting of the transfer copings directly to an acrylic resin custom tray. In the third, only impression material was used to orient the transfer copings. The accuracy of stone casts with implant analogs was measured against a master framework. The fit of the framework on the casts was tested using strain gauges. The technique using acrylic resin to splint transfer copings in the impression material was significantly more accurate than the two other techniques. Stresses observed in the framework are described and discussed with suggestions to improve clinical and laboratory techniques.

Biomechanical considerations in restoring endodontically treated teeth.

Various concepts for dental treatment have been established without appropriate documentation, such as restoration of endodontically treated teeth. Some researchers and dentists strongly recommend including a post with the restoration to strengthen the root. Other studies have indicated that posts may substantially weaken the roots and should be avoided. An additional approach suggested that the post did not improve the resistance to fracture during occlusion and did not support the restoration. Biomechanical problems are analyzed, and a recommended clinical approach is presented.

A modified impression technique for implant-supported restoration.

An impression technique is proposed for construction of implant-borne restorations that uses a modified autopolymerizing resin custom tray to allow splinting of the impression copings directly to the tray. This method eliminates the use of the dental floss-autopolymerizing resin complex, thus decreasing resin distortion and simplifying the clinical procedures.

Effect of post design on resistance to fracture of endodontically treated teeth with complete crowns.

The effect of post design on the fracture resistance of endodontically treated premolars restored with cast crowns was examined in vitro. The experimental model used cast posts and cores to test the effect of post design in a post-core system with identical rigidity. Samples loaded on an Instron testing machine until failure revealed that post design did not influence the fracture resistance of endodontically treated premolars restored with complete cast crowns. There was also no statistically significant difference between restored teeth with or without cast posts and cores.

The degree of zinc phosphate cement coverage of complete crown preparations and its effect on crown retention.

This investigation examined the amount of retentive area covered by cement under complete coverage crowns and its effect on retention. Sixty crowns of self-cured acrylic resin were prepared on 60 identical brass dies and were divided into six groups of 10 crowns each according to different cement applications. The results showed that retention was dependent on the amount of retentive area covered by the cement. The amount of cement in the occlusal part of the cementation space did not affect retention.

The cantilever fixed partial denture--a literature review.

The cantilever fixed partial denture (FPD) is a restoration with one or more abutments at one end and unsupported at the other end. Forces transmitted through the cantilevered pontics can cause tilting and rotational movements of the abutments. In a cross-arch unilateral cantilever FPD, the distal cantilevered unit is subjected to comparatively less force than the contralateral posterior abutment. The unilateral lack of terminal abutments causes lateral bending forces activate peripheral inhibitory feedback reactions from the periodontal and/or temporomandibular mechanoreceptors. The greatest strain in distal cantilevered FPDs is recorded mesial to the most distal retainer because most fractures occur in this location. To improve the prognosis of the FPD cantilever, the number of abutments should be increased and the number of pontics decreased. The abutment teeth need long roots and acceptable alveolar support. Prepared abutments require adequate length and parallel axial walls. An equilibrated and harmonious occlusion is necessary, as well as exemplary oral hygiene. A cantilevered FPD with adequate periodontal support can replace any tooth in the dental arch, but is especially useful as an alternative to a removable partial denture. The cantilevered FPD requires at least two abutment teeth. The only documented exception permitting a single abutment is the replacement of a maxillary lateral incisor with the canine as an abutment. An alternative to the cantilevered FPD is the osseointegrated implant. As osseointegrated implants become more popular, the need for the tooth-supported cantilevered FPD may decline, but it will remain an alternative treatment modality.

Comparative accuracy of implant impression procedures.

In this study, four impression procedures were assessed for accuracy in a laboratory model that simulated clinical practice. The accuracy of stone casts with brass implant analogs was measured against a standard framework. The fit of the framework on the casts was tested by manual and visual judgment and by microscopic measurement. The measurements supported the clinical judgments. When acrylic resin was used to splint transfer copings in an impression, all casts were acceptable and were more accurate than the best of the two other procedures, which did not use an acrylic resin splint. When an impression material was used to orient the transfer copings, the accuracy was better if undercut copings remained in the impression. When impressions were made by removing an impression from smooth transfer copings and replacing the copings in the impression, the majority of casts were unacceptable.

Restoring teeth following crown lengthening procedures.

Crown lengthening procedures are often necessary to successfully restore teeth that have been mutilated at or below the level of the bone crest. Forced eruption is preferred to surgical removal of supporting alveolar bone, since forced eruption preserves the biologic width, maintains esthetics, and at the same time exposes sound tooth structure for the placement of restorative margins. To properly construct a crown, the minimal distance from the alveolar crest to the coronal extent of sound tooth structure should be 4 mm. Before initiation of forced eruption, the restorability of the root after completion of the orthodontic phase must be considered. A technique is suggested to calculate the root-to-crown ratio that will be created after root extrusion with respect to the coronal level of sound tooth structure before treatment.

A controlled putty-wash impression technique.

A precise impression is imperative for the construction of an accurately fitting indirect cast restoration. The putty-wash technique is commonly used in making impressions with silicone elastomers. Errors in manipulation may lead to inaccurate impressions. A technique is presented ensuring exact reseating of the putty impression tray and creation of a uniform wash space, which are essential for accurate results.

Cuspal flexure associated with amalgam restorations.

Cavity preparation causes cuspal flexure under simulated occlusal loads. During amalgam condensation, the dentist exerts forces on the tooth. After condensation, dental amalgam undergoes dimensional changes. We measured possible changes in the cuspal position of premolars during and after their restoration with dental amalgam. Strain gauges were attached to the buccal surfaces of the teeth, and a direct reading of the strain and a simultaneous time-strain curve were obtained. Measurements were taken at the onset of amalgam condensation and continued for 24 hours. The amalgam was then removed from the teeth, and a subsequent reading was made. On the basis of this model, we found that the use of amalgam as a restorative material caused a static load on the cusps of the teeth brought about their consequent permanent deformation. After amalgam removal, we observed complete elastic recovery for all the treated teeth.

Photoelastic analysis of stress transfer by endodontically treated teeth to the supporting structure using different restorative techniques.

A photoelastic model was used to examine the influence of different types of restorations placed following endodontic therapy, emphasizing the way in which forces applied to the occlusal surface were dispersed to the supporting structures of the teeth. Stresses were photographed in the polarized light field. Findings indicate that distribution and patterns of stresses vary depending on the direction of the loads and the nature of the involved restorative procedures.