Cresco™ is the easy-to-use solution for screw-retained implant bridges, providing freedom and a perfect fit every time. Cresco is available in different framework materials for all major implant systems. The technique is carefully described and clinical result are presented in the following summaries.
Structure and mechanical properties of Cresco-Ti laser-welded joints and stress analyses using finite element models of fixed distal extension and fixed partial prosthetic designs
Purpose: The Cresco Precision'" (CP) method takes advantage of laser welding technology and the knowledge that due to its material properties and its higher laser beam absorption titanium will establish a deeply penetrating weld that is as strong as the unwelded material itself. However no data exists as to the fatigue resistance of the welded joints or indeed the stresses induced within the CP frameworks, which was the purpose of this study using bench test methods and finite element calculations.
Material and Methods: 20 cylinders of grade 3 c.p. titanium were milled for bench analysis and testing under tension. Ten cylinders were left as unwelded controls while the other 10 were sectioned and laser welded using established conditions and welding protocols. One test specimen was sectioned and analyzed under stereo microscope to determine penetration depth and cross sectional area of welding. All remaining specimens were subjected to a uniaxial tensile test, with data on load versus percentage elongation (๐/o strain) recorded by strain gauge. Once 0.2% proof stress was reached the samples were then loaded through to failure and both yield strength and elastic modulus calculated.
All fracture surfaces were tested for hardness and then subject to analysis under scanning electron microscope in order to classify type of fracture into one of 3 categories: 1) Fracture within spot weld 2) Fracture between weld spots 3) Fracture within parent metal with weld joint preserved. Results were subject to statistical analysis.
In addition to the above, a finite element model was designed to replicate the CP titanium cylinder supports which are welded to the framework to create a passive fit in vivo. Two prosthetic scenarios were modeled for the edentulous mandible. In model 1 (Ml) a full arch beam 3.3 mm in height and 5.0 mm in bucco-lingual width was supported by 5 evenly distributed implants with 12 mm cantilevers bilaterally. In model 2 (M2) a short span beam with the same beam dimensions were used without cantilevers supported by 2 implants, one of which was angled by 30° offset to the other. Cortical and cancellous bone, as well as material properties, were assumed to be isotropic, homogenous and lineary elastic. All com¬ponents and frameworks were modeled as deformable bodies in contact. The implants were modeled with an ankylotic attachment to bone to represent osseointegration and no pretensile stresses were assumed within the frameworks to represent a passive fit. All calculations for stress were under load conditions of 400N with 15 " linguoaxial angulations applied in Ml on the distal implants, as well as 4 mm and 10 mm along the cantilever. In M2 point of load application was first on the angled implant and then on the upright implant. Stress values were analyzed and compared to the known maximal normal stress obtained from the bench test data.
Results: Control specimens revealed a completely ductile type of fracture while test specimens show a combination of ductile fracture and cleavage with fracture always occurring between the weld joint and the parent metal. No porosities were observed. Comparison of data revealed that mean ultimate tensile strength (776MPa v's 574MPa) as well as yield strength, % elongation, and hardness were all significantly superior for the laser welded joints, p <0.001.
The finite element calculation revealed that for both models Ml and M2 the maximum normal stresses under load were significantly lower than the ultimate tensile strength of both the welded and unwelded material. Indeed the stresses induced rarely exceeded 10% of the ultimate tensile strength. The highest stresses were recorded in the distal implants in Ml and the angulated implants in M2.
Discussion and Conclusions: Laser welding under previously established conditions and protocols revealed a penetration depth of 0.64mm with an absence of porosities. Furthermore the mechanical data indicated a highly significant increase in ultimate tensile strength of the welded joint which failed at the boundary with the parent metal and not within or between spot welds. This can be directly related to the optimal welding conditions of the Cresco Precision method.
In the Cresco Precision method, prefabricated titanium cylinders secured to abutment replicas are laser welded to paralleled cuts made through the chimneys above each implant at the base of the superstructure, thus incorporating perfectly fitting passive cylinders into the superstructure. In the current study it was demonstrated that when this approach was modeled, a finite element calculation predicted that maximum stresses under a load of 400N at 15° would induce a peak stress at the welded joints which did not far exceed 10% of the ultimate tensile strength of the welded titanium.
On this basis it can be deduced that given the constraints and assumptions set within the model, not least the isotropic, homogenous nature of the bone, the ankylotic nature of an osseointegrated implant and the passive fit of a Cresco prosthesis, it is unlikely that the welded joint of a Cresco-Ti framework would fail.
Further research is necessary to evaluate the fatigue life from dynamic load of these laser welded joints.
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