Accuracy of template-based guided dental implant placement-An in vitro comparison of different manufacturing methods.

OBJECTIVES: This study investigates effects of surgical guide manufacturing process on 3D transfer accuracy of planned dental implant position, using three production methods: additive 3D-printed (3DF), subtractive milled (MF), and analog laboratory fabricated templates (LF). MATERIAL AND METHODS: Implant position for a single-tooth gap (#26) planned digitally. 3DF and MF templates were designed digitally, while LF templates were analogously created. For each manufacturing type, 10 surgical guides were fabricated. Each guide was used for template-guided implant placement in model replicas. For evaluation of implant placement, cone beam computed tomography scans of all implanted models were superimposed, and implant positions were determined. Deviations at implant shoulder/apex were measured, and median and inter-quartile range (IQR) were determined for mesio-distal, oro-facial, coronal apico, 3D spaces, and angles. RESULTS: At implant shoulder, vertical components dominated position deviations (up to 1.04 mm, IQR 0.28 mm for 3DF). Horizontal deviations were much lower (mesio-distally up to 0.38 mm, IQR 0.36 mm (LF)). Implant apex shows similar vertical deviations, while horizontal deviations clearly increased compared to shoulder, especially in mesio-distal direction. Median angular deviations were between 2.1° (IQR 2.0 mm, max. 4.2°) for 3DF and 3.3° (IQR 1.9 mm, max. 5.3°) for MF. No statistical differences were found between manufacturing types (Kruskal-Wallis test, p = .05). CONCLUSIONS: The study showed the method of implant guide fabrication did not affect the accuracy of implant placement within the limits of an in vitro environment. All methods resulted in implant placement which did not exceed the accepted safety deviation envelope (1.5-2.0 mm).

Numerical investigation of the biomechanical effects of orthodontic intermaxillary elastics on the temporomandibular joint.

Temporomandibular joint disorder (TMD) often coincides with malocclusion, and in some cases TMDs are reported after orthodontic treatment. Intermaxillary elastics (also known as orthodontic elastics, OE) are a common way to apply force during orthodontic treatment, and they might cause mechanical effects on the temporomandibular joint (TMJ), thereby lead to joint remodeling. It is still a controversial topic whether the adapted remodeling of the TMJ or of the alveolar bone is the main cause for the alteration of occlusion after treatment with OEs. It was the aim of this study to analyze whether variations of OEs would develop harmful effects on the healthy TMJ. A TMJ model with a masticatory system based on Hill-type muscle actuators was established. Mouth opening and closure with and without OEs were simulated, and maximum principal stresses in the disc and condylar cartilage as well as the displacement of the mandible were analyzed. We found no considerably difference in the mandibular movement without and with symmetrical OEs during mouth opening and closing. At full mouth opening, stresses in the disc and condylar cartilage of some models with OEs were much smaller than without OEs, but we did not find consistency in the results from the left and right sides of the same model (e.g. the lowest compressive stress on the left side of disc from the model with Class II OEs is much smaller than without OEs, -17.3 MPa compared with -28.2, while on the right side, there was no obvious difference). Hence, we could not conclude that OEs would develop deleterious effects on the healthy TMJ.

Biomechanical analysis of different fixed dental restorations on short implants: a finite element study.

OBJECTIVES: Although the use of short implants is becoming more common for patients with atrophic alveolar ridges, their use is still quite limited. This is due to the lack of data of long-term survival compared to standard-length implants. The aim of this study was to determine the load in the bone and implant system with different superstructures. METHODS: Three kinds of prosthetic restorations were created on short implants based on CT-Data. Two short implants with different macro-geometries were used. The implants were inserted in idealised posterior lower mandibular segments and afterwards restored with a crown, a double splinted crown, and a bridge. RESULTS: The analysis was performed under load of 300 N either divided between a mesial and distal point or as a point load on the pontic/mesial crown. The different design of the implant systems had a noticeable influence on the stress in the cortical bone, in the implant system, and the displacement of the superstructure as well. CONCLUSIONS: Compared with implants of standard length, higher stresses were observed, which can lead early failure of the implant during the healing period or a late cervical bone resorption. Precise indications are essential for short implants to avoid the failure of short implants.

Trueness and precision of milled and 3D printed root-analogue implants: A comparative in vitro study.

OBJECTIVES: The present study aimed to evaluate the accuracy (trueness and precision) of titanium and zirconia multi-rooted root analogue implants (RAIs) manufactured by milling and 3D-printing. METHODS: A multi-rooted RAI was designed based on a mandibular second molar segmented from cone-beam computed tomography (CBCT). The manufactured RAIs were divided into four groups: 3D-printed titanium (PT) and 3D-printed zirconia (PZ) (n=10 each), as well as milled titanium (MT) and milled zirconia (MZ) (n=5 each). The specimens were scanned with a high-precision scanner, and the scanned data were imported into 3D-measurement software to evaluate the precision and trueness of each group. Root mean square (RMS) deviations were measured and statistically analysed (One-way ANOVA, Tukey's, p≤0.05). RESULTS: PZ showed the highest precision with RMS value of 21±6 µm. Nevertheless, there was no statistically significant difference in precision among the other groups. Regarding trueness, MZ showed the highest trueness with RMS value of 66±3 µm, whereas MT showed the lowest trueness result. Inspection sections showed that MT had significantly high RMS deviation in the furcation area (612±64 µm), whereas PZ showed significantly high RMS deviation at the apical area (197±17 µm). CONCLUSIONS: The manufacturing process significantly influenced the RAI accuracy. PZ exhibited the highest precision, whereas MZ exhibited the highest trueness, followed by PT. Finally, our results suggest that 3D-printing can reproduce concave surfaces and less accessible areas better than milling. CLINICAL SIGNIFICANCE: Milled and 3D-printed RAIs showed promising results in terms of precision and trueness. However, further clinical research is needed to advocate their use as immediate implants. Additionally, the inherent volumetric changes of the various materials during manufacturing should be considered.

Three-dimensional finite element analysis of anterior fixed partial denture supported by implants with different materials.

BACKGROUND: Implant restoration in the anterior region was a great challenge for dentists. The aim of the study was to use three-dimensional finite element analysis to analyze the feasibility of the fixed partial denture supported by two diverse material implants in the maxillary anterior region, to provide a theoretical basis for clinical therapy. METHODS: Based on CBCT data, a three-unit fixed partial denture supported by two endosseous implants with a diameter of 3.5 mm and a length of 10 mm inserted in the upper left central incisor and canine was created. The materials of the implant were verified in seven models. Titanium (Ti), zirconia (Zr) and titanium-zirconia (TiZr) were used for the implant materials. A total load of 150 N was applied on the prosthesis units at 45˚ to the long axis of the abutment. RESULTS: The maximal displacement of the implant was 6.1 µm in the titanium implant at the canine region in the TiZr-Ti model. The stresses in the cortical bone and strains in the spongious bone were below the physiological upper limits. The highest value of the maximal stress in cortical bone was 78 MPa, observed in the left central incisor region of Zr-TiZr model. And the highest value of the maximal strain in the spongious bone was 1738 µstrain around the titanium implant of the Ti-TiZr model. CONCLUSIONS: From the biomechanical point of view, fixed partial denture supported by two diverse material implants in the maxillary anterior region is an acceptable treatment option.

Comparison of two resilient attachment systems for implant-/mucosa-supported overdentures with a PEKK framework: a clinical pilot study.

OBJECTIVES: The aim of the study was to determine differences between Locator and CM LOC attachment systems regarding patient satisfaction and wear of the abutments and their inserts. Plaque accumulation onto the polyetherketoneketone (PEKK) framework and polymethylmethacrylate (PMMA) was investigated for the implant-supported overdentures. METHODS: Seventeen edentulous patients were randomised to receive either Locator or CM LOC system for the first year. The total number of implants was 53. After the randomisation, 25 implants received Locator system, and 28 implants received CM LOC system in the first year. After a period of 12 months, the attachment system was exchanged from either Locator to CM LOC or vice versa. Oral Health Impact Profile (OHIP-14) questionnaires were used to evaluate patient satisfaction, chewing comfort, and pressure lesions. Prosthesis hygiene on the PMMA and PEKK surfaces was evaluated by using Stark plaque index. After the exchange of the abutments, they were stored until the end of the 24 months, and the surface wear of the abutments was analysed using a scanning electron microscope. RESULTS: Three patients (10 implants) died shortly before the end of the first year. Two patients (7 implants) received only Locator system since CM LOC was not indictable for their implant system. Patient's satisfaction was increased when the attachment system was changed from Locator to CM LOC after 12 months of wearing time. Chewing ability and comfort were increased when the attachment system was changed from CM LOC to Locator after 12-month wearing time. There was no influence of the change of the attachment system on pressure lesions. The observed plaque accumulation was higher on the PMMA than on the PEKK surface. For the 8 investigated Locator abutments, the wear was within low and middle level. For the 28 investigated CM LOC abutments, the wear was within middle and high level for the terminal implants and between low and middle for the central implants (for patients who received 4 implants). CONCLUSIONS: Patient's satisfaction and wearing comfort can be improved with implant-supported overdentures with CM LOC abutments in comparison to Locator. There was no clear difference between both attachment systems concerning the chewing ability of the patients. Plaque accumulation was observed on both attachment systems in different areas. Plaque accumulation on PEKK surface was less than on PMMA surface. CLINICAL RELEVANCE: The CM LOC attachment system offers stable and comfortable wearing conditions for implant-supported overdentures. The use of PEKK as a framework material could reduce the incidence of pressure lesions.

Experimental and numerical investigations of fracture and fatigue behaviour of implant-supported bars with distal extension made of three different materials.

The aim of this study was to investigate experimentally the fatigue and stability of three bar materials with distal extension at the molar region and to numerically analyse the biomechanical properties of the bar materials connected to overdentures in a patient individual model. A milled bar was designed for the mandible on four implants in the canine and second premolar region. Three bar materials were investigated: titanium (Ti), cobalt chromium (CoCr), and polyetherketonketon (PEKK). Firstly, static and fatigue tests were performed based on EN ISO 14801 in a commercial permanent loading set-up. Unilateral axial force was applied on the distal extension of the bars. Secondly, numerical models were created. Different bar materials and loading scenarios were analysed. The static fracture limit of the three materials was 1,750 N, 780 N, 310 N for Ti, CoCr, and PEKK, respectively. The Wöhler curves showed comparable fatigue limits of 200 N, 160 N, and 150 N for titanium, CoCr, and PEKK, respectively. The stress at the distal extension was 2,600 MPa (Ti), 1,000 MPa (CoCr), and 270 MPa (PEKK). All loading simulations with the PEKK bar showed higher stresses in the implants and in the bone bed as well as higher displacements of the over denture in comparison to metal bars. PEKK showed different mechanical behaviour compared to Ti and CoCr. The distribution of stresses within the PEKK bar was wider than the area of loading which probably leads to fatigue of the whole bar and not only the part under load.

The all-on-4 concept in the maxilla-A biomechanical analysis involving high performance polymers.

A finite element analysis was used to determine the loading behavior of a full "All-on-4" bridge in the maxilla made from different framework materials. The model was generated based on data from an edentulous maxilla. Four implants were positioned according to the All-on-4 concept and a full arch restoration was created on top of the implants. Separate thrust dies were modeled directly above each of the units of the bridge. Forces of 500 N were applied to each thrust die separately. Simulations were repeated with the following variations and results were compared to determine the influence of each variation: point of force application (from tooth 16 to 26), framework material (high performance polymer HPP, titanium Ti, cobalt chromium CoCr), and healing state of the bone/implant interface. Depending on the point of force application, the bone below the HPP framework showed increased stresses (median: 180 MPa at 13) compared to the CoCr and Ti frameworks (110 MPa at 16) under immediate loading. After osseointegration, the framework material had no obvious influence on the stresses in the bone. In the osseointegrated state, maximum stresses are decreased by the tenfold. Stresses in the framework were clearly decreased with HPP compared to CoCr and Ti (75, 125, 115 MPa). Loading the anterior teeth induced higher stresses than loading the molar region due to torsional deformation. This effect increased with decreasing framework material stiffness. The framework stiffness had less distinct influence on the stresses in the restoration than the point of force application.

Fatigue behaviour of dental crowns made from a novel high-performance polymer PEKK.

OBJECTIVES: The aim of this study was, firstly, to analyse the long-time fatigue behaviour of crowns constructed from a novel polyetherketoneketone (PEKK) polymer, using artificial prepared teeth. Secondly, to determine the effect of the material's stiffness that used as an artificial prepared tooth on the fatigue life of the PEKK crowns in comparison to human prepared teeth. METHODS: Veneered crowns with a PEKK framework were constructed on three different prepared teeth: artificial polymethyl methacrylate (PMMA) teeth, artificial CoCr teeth and extracted human teeth. As far as applicable, the loading protocol was based on EN ISO 14801:2007 for fatigue testing of dental implants. After initial static fracture tests on three specimens from each group, the remaining crowns were loaded with different force levels until fracture or until 2 × 106 loading cycles were reached. The number of loading cycles until failure was recorded. Wöhler curves were created to display the fatigue limits. RESULTS: Static fracture limits as well as fatigue limits differed for all three core materials. The static fracture tests resulted in fracture limits of 1200 (± 293) N for the PMMA group, 1330 (± 219) N for the CoCr group and 899 (± 96) N for the human tooth group. Fatigue limits of 770 N, 840 N and 720 N were determined for the PMMA group, CoCr group and human tooth group, respectively. CONCLUSIONS: The determined fatigue limit of above 720 N (depending on the core material) is sufficiently high and a good performance of this crown material is expected in the clinical loading life. The results showed that using artificial teeth instead of natural teeth for fatigue testing of crowns might result in an overestimation of the fatigue limits of the crown material. CLINICAL RELEVANCE: PEKK-made crowns offer a stable and priceworthy treatment for patients, in particular those that suffer from metal allergy.

Quantitative appraisal of bilateral sagittal split osteotomy impact on the loading of temporomandibular joint.

Bilateral sagittal split osteotomy is one of the most frequently performed operations in orthognathic surgery. The health of the temporomandibular joint (TMJ) is an important prerequisite for its functionality. The aim of this finite element study was to assess the developed stresses during mouth opening after bilateral sagittal split osteotomy. Different osteotomy gap widths and disc positions were evaluated. Computed tomography and magnetic resonance data of a dentulous cadaver head were used in order to create two finite element models simulating split distances of 5 and 10 mm, respectively. The fixation of the distal and proximal segments was made by a four- or a six-hole titanium mini plate and four monocortical screws respectively. For both models, three different situations of the articular disc were created: a physiological disc position, anterior disc displacement and posterior disc displacement. The mandible was vertically displaced in the midline in order to simulate a mouth opening of 20 mm. The simulation showed high stresses in the area of the titanium plates (up to 850 MPa), implying an increased risk of material failure. High stresses were found within the discs in the models with normal disc position and anterior disc displacement as well (up to 8 MPa), indicating a higher risk of developing craniomandibular disorders. Regarding the stresses within the fixation screws, the highest values were recorded in the area of the upper thread. The degree of mandibular advancement after a bilateral sagittal split osteotomy affects the stress balance in the mandible and the articular discs during mouth opening.

Clinical investigations of the comparability of different methods used to display occlusal contact points.

AIM: The aim of the present study was to evaluate the number, strength, and position of occlusal contacts shown using an intraoral scanner (IOS) and a digital occlusal analysis system (T-Scan) compared with the current gold standard using occlusal foil (OF). MATERIALS AND METHODS: Occlusal contacts were analyzed for 70 volunteers using OF in maximum intercuspation (MI). The contact points obtained using the IOS were evaluated using a screenshot from Zirkonzahn.Modellier CAD software. Finally, the volunteers were asked to bite on the sensor sheet of the T-Scan system. For the evaluation of these data, the contact points of the OF and the IOS were graded as light, medium, and strong. Furthermore, the positions of the contact points were analyzed for the anterior region (premolars and molars). Parametric statistical tests were applied to analyze the differences among the three methods. RESULTS: The mean number of all contact points was similar: 29 ± 8 with the OF, 30 ± 12 with the IOS, and 24 ± 10 with the T-Scan. However, results were different in terms of the grading of the strength of contact points: mean number of light contacts: 8 ± 4 OF vs 17 ± 8 IOS and 17 ± 6 T-Scan; medium contacts: 12 ± 5 OF vs 8 ± 4 IOS and 5 ± 4 T-Scan; and strong contacts: 9 ± 5 OF vs 6 ± 6 IOS and 4 ± 2 T-Scan. The positions of the occlusal contact points were also different. CONCLUSION: The data sets showed that there were differences in the distribution of occlusal contact points evaluated using the OF, the IOS, and the T-Scan system. Although the number of detected occlusal contacts was similar, different occlusal contact protocols were determined by the three different methods.