Is There "A Best" Centric Relation Record? Centric Relation Records, Condyle Positions, and Their Practical Significance.

PURPOSE: To investigate if and how the positions of condyles in centric relation (CR) varied from each other using different CR records. MATERIALS AND METHODS: Condylar positions in CR were repeatedly recorded by four licensed clinicians in 81 stomatognathically healthy, fully dentate subjects with six different types of CR records. The maxillary casts were mounted in articulators after individual facebow transfer, and the mandibular casts were mounted with a central bearing point (CBP) registration on the tip of the Gothic arch. Mean values from three registrations of the centric condylar positions with each of the six methods were calculated, and comparative calculations were made. RESULTS: The median spatial distances of the condylar positions between the CBP and all other CR records was 0.73 ± 0.24 mm (0 to 3.19 mm). The median spatial values between the condylar positions of the different CR records ranged from 0.48 to 0.79 mm. Statistically, the positions of almost all types of registration were slightly different from each other. CONCLUSION: Different CR records reproduce slightly different positions of the mandible, which is especially true for CBP registration, whereas reproducibilities and SDs between registrations are similar. Unrefined wax-wafer registrations using bimanual manipulation have proven particularly successful with regard to precision and time requirements, as has CBP registration with regard to condylar position.

Investigation of the retention forces of secondary telescopic crowns made from Pekkton® ivory in combination with primary crowns made from four different dental alloys: an in vitro study.

The aim of this study was to investigate the retention forces of secondary telescopic crowns made of polyetherketoneketone (PEKK) in combination with primary crowns made of four different dental alloys and to determine whether the retention forces change in the course of up to 5000 simulated wear cycles. A total of four groups of telescopic crowns were investigated: group 1: 10 primary crowns made of a gold alloy (NEOCAST®3), group 2: 10 primary crowns made of a non-precious metal alloy (Girobond NB), group 3: 10 primary crowns made of zirconium (Cercon®base) and group 4: 10 primary crowns made from PEKK (Pekkton®ivory). The corresponding secondary crowns were made from PEKK in all the four groups. Each pair was fixed axially in a wear simulator specifically designed for the study. Overall, 10,000 joining and separating cycles were performed for each group. To simulate intraoral conditions, a saliva substitute served as a lubricant. Force transducers were used to record the retention forces and after completing 10,000 cycles, the surface of each primary crown was examined using a scanning electron microscope (SEM). All groups showed an increase in the retention force for the first 2000 cycles which stayed constant for the remaining 8000 cycles. The Pekkton®ivory/Pekkton®ivory and NEOCAST®3/Pekkton®ivory combinations displayed mean retention force values of 16 N after a slight increase in the retention force. The Cercon®base/Pekkton®ivory and Girobond NB/Pekkton®ivory combinations displayed an initially high increase in the retention force and then showed a mean retention force of up to 29 N. All primary crowns displayed surface wear. Zirconium primary crowns showed the least wear compared to PEKK and the gold and non-precious metal alloys.

The influence of cyclic shear fatigue on the bracket-adhesive-enamel complex: an in vitro study.

OBJECTIVES: To describe the effect of fatigue on the strength of the bracket-adhesive-enamel complex and characterize the fatigue behavior of the materials tested. METHODS: Upper central incisor brackets (Discovery(®), Dentaurum) were bonded with a light-curing (Transbond XT™, 3M Unitek) and a chemically-curing adhesive (Concise™, 3M Unitek) on bovine teeth embedded in cylindrical resign bases and stored in water at 37(±2)°C for 24 (±2)h. The first 15 specimens were tested with a universal testing machine ZMART.PRO(®) (Zwick GmbH & Co. KG, Ulm, Germany) for ultimate shear bond strength according to the DIN-13990-2-standard. The remaining three groups of 20 specimens underwent fatigue staircase testing of 100, 1000 and 3000 cycles at 1Hz with a self-made testing machine. The survived specimens were subjected to shear strength testing. RESULTS: The fatigued specimens showed decreased shear strength with both adhesives at all cycle levels. The shear strength after fatigue for 100, 1000 and 3000 cycles was in the Concise™-groups 34.8%, 59.0%, 47.3% and in the Transbond™ XT-groups 33.6%, 23.1%, 27.3% relative to the ultimate shear strength. The fatigue life of the Concise™-groups decreased with increasing stress and Transbond™ XT showed lower fatigue ratio with no obvious trend. The specimens bonded with Transbond™ XT showed typically favorable fracture modes in contrary to Concise™. SIGNIFICANCE: Fatigue of the bracket-adhesive-enamel complex decreased its shear strength. The staircase method can provide a standardized experimental protocol for fatigue studies, however testing at various cycle numbers is recommended.

Influence of the lubricant and the alloy on the wear behaviour of attachments.

OBJECTIVES: Wear of attachments leads to a loss of retention and reduces the function of overdentures. This study evaluated the retention force changes of an attachment system for overdentures. The influence of the lubricant and the alloy on wear constancy was examined. METHODS: Cylindrical anchors of the Dalbo(®) -Z system were tested (Cendres+Métaux SA). Three groups of alloy-lubricant combinations were generated 1.Elitor(®) /NaCl-solution (EN) 2.Elitor(®) /Glandosane(®) aquadest. (EG) and 3.Valor(®) /Glandosane(®) /aquadest. (VG). Ten samples of each group were subjected to 10 000 insertion-separation cycles. RESULTS: For the EN-group, this led to a large increase in retention force. The EG- and VG-group showed a constant decrease after an initial increase in retention force at the beginning of the wear simulation. The change of the alloy caused no statistically significant differences. The use of a more viscous lubricant reduced the retention force increase significantly. CONCLUSIONS: The use of a lubricant which simulates clinical conditions is an absolute need for wear simulation because the retention force changes are influenced enormously. The change of the alloy at the Dalbo(®) -Z system did not influence the wear behaviour. As a slight decrease in retention force was recorded, it is useful for an attachment system to allow compensation with an adjustable matrix.

Numerical investigation of the mechanical loading of supporting soft tissue for partial dentures.

The aim of this research was to study the impact of loading on partial dentures within the supporting soft tissue with respect to different attachment techniques. A finite element model was developed to calculate the stress and strain distribution in this tissue. The model consisted of the left half of a mandible with three remaining teeth that had suffered an atrophy in the anterior region, and a partial denture over the toothless area that was connected at the left mandibular canine using an attachment system. Resulting stress/strain distributions are presented for different load cases using a commercially available prefabricated attachment system.

Comparative study of four retentive anchor systems for implant supported overdentures--retention force changes.

OBJECTIVES: Wear of attachments leads to a loss of retention and potentially reduces the function of complete dentures. This study evaluated the retention force changes of different prefabricated attachment systems for implant-supported overdentures to estimate the wear constancy and applicability in clinical practice. METHODS: Four prefabricated attachment systems were tested [Group SG: retentive ball attachment (Straumann, Switzerland) with gold matrix, Group ST: retentive ball attachment (Straumann, Switzerland) with titanium spring matrix, Group IB: UNOR i-Ball with Ecco matrix (UNOR, Switzerland) and Group IMZ: IMZ-TwinPlus ball attachment with gold matrix (DENTSPLY Friadent, Germany)]. Ten samples of each system were subjected to 10,000 insertion-separation cycles. RESULTS: Results showed that all types of attachments showed wear, which led to a loss of retention force after an initial increase at the beginning of the wear simulation. Attachments with a plastic retention insert or gold matrices underwent the smallest changes in retention force. The titanium spring system showed the largest changes in retention force and a greater variation between the different cycles and specimen. This behaviour is probably caused by a large fitting tolerance of the titanium spring. CONCLUSIONS: Attachment systems which possess a male and female component of different material composition are preferable. They show smaller changes in the retention force. For retention force increase and wear compensation, an attachment system should be adjustable.

Craniomandibular pain, bite force, and oral health-related quality of life in patients with jaw resection.

The present study evaluated the connection between temporomandibular disorders (TMD), maximum voluntary bite force and oral health-related quality of life (OHRQoL) in patients with oral cancer. Twenty-six prosthetically rehabilitated patients with partial resection of the upper and/or lower jaw with segmental mandibulectomy, with and without reconstruction, were examined. The examination comprised the Research Diagnostic Criteria for TMD (RDC/TMD), determination of the individual pain threshold, evaluation of the maximum voluntary bite force, and OHRQoL according to the Oral Health Impact Profile (OHIP) questionnaire. Male pain thresholds were significantly higher than female pain thresholds (P=0.003). Patients with maxillary resection showed higher pain threshold values than patients with (segmental) mandibulectomy. Bite force was significantly (P=0.000014) lower in resected jaw areas than in healthy ones. Patients with resections of the maxilla showed higher voluntary bite forces than patients with resections of the mandible; males showed higher bite forces than females. The prevalence of negative responses ("often"-2 and "very often"-1) on the OHIP items was 10.39%, whereas the prevalence of positive responses ("from time to time"-3, "seldom"-4, and "never"-5) was 89.60%. The prevalence for negative responses was 2.46% in male patients and 8.07% in female patients. Patients with resections in the maxilla suffered from negative responses of 3.92%, patients with resection of the mandible of 6.64%. According to the OHIP, the psychological disposition of these patients clearly referred to coping mechanisms.

Investigation of the wear of prefabricated attachments--an in vitro study of retention forces and fitting tolerances.

OBJECTIVE: To quantify wear processes by measuring the retention force changes and the fitting tolerance at different prefabricated attachment systems to estimate the wear constancy and applicability in clinical practice. METHOD AND MATERIALS: Seven prefabricated attachment systems (Dalbo-Classic, Dalbo-PLUS, Dalbo-Z, Mini-Gerber-PLUS, Stufenexzenter, SpharoLock, and Degussa-Kugelankersystem) with different construction and alloy composition were tested. Twenty samples of each system were subjected to 10,000 insertion-separation cycles in a wear simulator with a periodontium-simulating specimen holder. The simulator was designed to record the force needed to insert and to separate the attachment and the distance moved during the insertion and separation cycles. RESULTS: All types of anchors showed wear that led to a loss or to an increase in retention force at the beginning of the wear simulation. Anchors with a plastic retention insert showed the slightest changes in retention force. The wear does not have an effect on the fitting tolerance. CONCLUSION: Anchor systems that possess an adjustable matrix should be preferred. They allow decreasing or increasing the retention force of an anchor if this force is changed by wear. There is no risk of a loss of support if forces in occlusal direction are exerted because there is no clinically relevant change in the fitting tolerance.

Anterior crowding--just an esthetic problem?

BACKGROUND: One frequent late sequel of frontal crowding is the development of gingival recessions manifesting themselves especially at rotated anterior teeth. MATERIAL: This correlation is discussed with regard to the literature and to the authors' own clinical findings and experimental studies. RESULTS: According to present-day knowledge the following statements can be made: In cases with frontal crowding the tangential stiffening of the anterior teeth is inadequate. Thus translatory and rotational force components resulting from the elastic deformation of the mandible under functional loading effect the movement of rotated front teeth during maximum jaw opening and mandibular protrusion. Also tongue thrusting and frontolateral bruxism lead to force vectors in vestibular direction affecting the delicate periodontal tissues. These statements are confirmed by the authors' own in-vivo experimental studies measuring the amount of tooth displacement during test persons' functional movements. Furthermore, in the long-term anterior crowding will mostly trigger incorrect anterior guidance, i.e. guidance predominantly by the rotated or ectopically positioned front tooth with subsequent overloading of the antagonist. CONCLUSION: Orthodontic therapy of frontal crowding is not just an esthetic problem but can be of special importance in the prevention of stress-related reactions in the masticatory system.