Bibliographic reviews

NumeriUno, 2013; 17: 9-11

PRECISION IN LASER SINTERED CO-CR ALLOYS WITH CERAMIC CASINGS PART 1

The new CAD/CAM technologies have brought a veritable revolution in the field of dentistry and dental technology in particular. In addition to new materials, such as zirconium, these technologies also offer solutions for more conventional materials like titanium and the cobalt-chrome. This latter can be worked not only by milling, but also using laser sintering systems, building up the component with the addition of microscopic spheres that are fused together by a numerically-guided laser beam.
In the search for a material that could replace the precious metal, which has by now reached intolerable price levels, at least for routine jobs, and that could maintain the long-consolidated characteristics of precision and reliability in casting techniques, the choice of this system seemed to us to be economically viable and interesting in terms of practicality. We were persuaded to explore this technique by the fact that there is no need to change the ceramic casing, that we are thus able to exploit the experience of over forty years with ceramics on base metals, and last but not least, that there is the possibility of working on pieces again with lasers and other types of welding. Our initial experiences were not particularly satisfactory in terms of precision, but all other aspects gave excellent results. But good results without precision are not good results!
The situation was frustrating. We started from the consideration that every material has its prerogatives, and these do not always coincide with conventional ways of working. Very often it can be wiser to adapt to the characteristics of new technologies than to try to adapt these technologies to our habits.
So reconsidering the problem, we found that laser-sintered edges have a “teardrop” shape that is not a defect, but is instead a specific characteristic of this technology. It would therefore be impossible to achieve precision in this area. Precision, if attainable, must be “upstream” from these teardrop edges, which must be positioned beyond the preparation during scanning to allow the teardrop shape to be eliminated.
The technique to achieve this level of precision is extremely simple:

• Locate the closure line beyond the preparation.
• Remove the “teardrop” edge.
• Finish off by trimming away excess material until the closure edge.

Clearly, the more the preparation is vertical, the greater will be the possibility of achieving high precision. Having obtained this result, we decided to verify whether this precision could be maintained on larger jobs as well. We used a step-by-step process on a small bridge of three elements to document this further experience and further develop the technique.
After these results, the construction of laser-sintered bridges became a massive part of our laboratory’s daily routine, resolving the problems encountered at the outset.
In the second part of this study, we will discuss particularly extensive cases, with the problems they present and strategies to resolve these problems.

NumeriUno, 2014;18: 16-19

PRECISION IN LASER SINTERED CO-CR ALLOYS WITH CERAMIC CASINGS PART 2

The results obtained in the protocols illustrated in the previous number led us to continue our efforts in larger jobs, with results that were initially encouraging, but that later showed signs of unexpected torsions. In some larger jobs, during the repeated firing sessions, the arch had to be readjusted, since imprecisions appeared due to the release of internal tensions. In some cases, firing for stabilization before ceramic application mitigated the problem. In jobs on milled posts without perfect closure, this could even be acceptable, but on natural teeth it would definitely not be. We need to verify exactly how, when and to what extent these problems occurred, and how we could rectify them, or at least understand their limits.
In agreement with Sweden & Martina we devised a small study to clarify this situation. We constructed a model with four milled posts, and on this we produced three types of arch, so as to determine which was most controllable. The three ferrules were adjusted cold to assess precision both on the single posts and on the entire assembly, and they were then annealed in a ceramic furnace, performing a series of firings as if they were to be ceramicized, with checks every time they were taken out of the furnace and readjustments if necessary. In greater detail, ferrule A was normally adjusted when cold, always requiring further readjustments after the various firings, and becoming stabilized around the sixth firing. The precision after the firings was very good on one side and poor on the other.
Ferrule B gave the best results, with precision when cold, becoming stabilized after four firings, and finally with good precision.
Ferrule C was the most disappointing. The stabilizer bar that we expected to provide stabilization instead caused a series of torsions that made it unusable after just two firings.
Cold adjustment was already very difficult, and even more so after the first firing.

Conclusions:

• it is evident that a structure fabricated with a series of fusions contains tensions that tend to be released during annealing;
• it was noted that a particular design of the structure mitigates this contraindication (a robust linguopalatal band);
• it was determined that these distortions emerged only on very extended structures;
• experience teaches that with up to 2/3 of a complete ferrule, distortions can be controlled if an adequate design is chosen.

The precision when cold of laser-sintered Co-Cr is of a quality that is higher and above all predictable. It is only extended ferrules that show distortions after firing, with particular characteristics: all the ferrules tended to curve inwards at the level of the more distal elements. The use of a very long firing time for stabilization (e.g. 15/20 minutes at 980°/1000°C) could probably further mitigate this contraindication.
To verify if absolute stability could be achieved in such extensive jobs, we decided to experiment with a series of annealing processes on two Co-Cr arches milled from solid metal. Two structures were prepared, exploiting the previous scans, one with a stabilizer bar and one without.
The two structures were then subjected to the same treatment as the previous laser-sintered versions, with extremely positive results. No differences of precision were found between the arch with and the bar without the stabilizer bar, and only cold adjustment required a little extra work. We can conclude with these considerations: for normal routine jobs, laser-sintered Co-Cr is able to provide excellent solutions for all needs for precision in bridge structures, until a particularly sharp curve is reached, and our advice is not to exceed 2/3 of the arch. A milled element gives greater guarantees in all jobs calling for a complete ferule. Considering that as regards dental technology, these techniques are still very recent, and that therefore notable progress will be made with the many experiences of our colleagues, I believe that very soon we will be able to achieve constant dimensional stability even with the laser-sintered Co-Cr technique.


Stabilization firings
Before consigning the job, a cycle of “distension” firings was performed in an argon atmosphere, consisting in:

• First firing from 0 to 450°C in 60 minutes, maintaining this temperature for 45 minutes. Heating up to 750°C in 45 minutes, maintaining this temperature for 60 minutes.
• Furnace off and cooling down to 600°C, then opening the furnace.
• Cooling down to 300°C, closure of the argon flow, extraction of the pieces.

This firing certainly optimizes the properties of the metal, but in dimensional terms it is absolutely insufficient.
Various experiences and a number of tests led us to consider a more incisive firing to be more advantageous: 1000°C for 10 minutes in a vacuum. This firing almost completely releases all the tensions, giving the piece 95% stability. Small corrections will be necessary as work proceeds, but these will be only minor, very similar to those to be encountered with fused precious metals.

Journal of Clinical and Experimental Dentistry 2015;7(1):e175-9

Numeri Uno, 8, 04-05, 2009

IMPLANT-PROSTHETIC REHABILITATION OF THE TWO ARCHES WITH IMMEDIATE LOADING: WORK PROTOCOL WITH THE USE OF THE SINGLE-MODEL AND P.A.D. SYSTEMATICS

In this clinical case, the single-model technique allowed rehabilitating the patient with one surgery and a minimal number of sessions for fi nalising the screwed prosthesis, obtained with the CAD-CAM Echo protocol on Kohno implants.

Numeri Uno, 6, 04-05, 2009

ZIRCONIUM PROSTHESIS ON IMPLANTS AND NATURAL ELEMENTS

The implementation of the Echo prosthetic protocol allowed obtaining an entire meso-structure in zirconium with excellent functional and aesthetic characteristics; the structure is also suitable for maintaining the morphostructural integrity of the remaining teeth.

TeamWork, 11(6), 63:75, 2009

SOFTWARE-ASSISTED SURGERY WITH THE GUIDE MODEL TECHNIQUE: 3D PROJECT ? PLACING OF THE GUIDED IMPLANT

The use of software for assisted surgery allows the three-dimensional diagnosis of the bone volumes and correct planning of the implant insertion, taking full advantage of the anatomy of the existing bone. This case includes the insertion of highly angled implants on which was constructed a Toronto screwed prosthesis using PAD abutments.

Numeri Uno, 4 (4-6):04-05, 2009

USE OF CAD/CAM TECHNOLOGY FOR TREATING CEMENT-RETAINED SINGLE-TOOTH PROSTHESIS IN THE AESTHETIC ZONE

This clinical case demonstrates how the use of the CAD-CAM Echo technique allows obtaining aesthetic results by creating abutments entirely in zirconium oxide on which a zirconium-ceramic crown was cemented.

Numeri Uno, 2014;18: 6-9

A MINIMALLY INVASIVE TECHNIQUE: IMPLANTS WITH A REDUCED DIAMETER AND CAD/CAM TECHNIQUES FOR LONG-TERM TEMPORARY SOLUTIONS

Numeri Uno, 2013; 16, 14-15

IMPLANT-SUPPORTED REHABILITATION OF BOTH EDENTULOUS JAWS WITH IMMEDIATE LOADING

In May 2011, a patient aged 69 years presented for generalized mouth pain, and to be able to assess the stability of the existing prosthetic work carried out by another dentist. The general medical anamnesis was negative. The patient declared that he had had several teeth extracted due essentially to caries, which were substituted with a number of implants some years before.
On objective examination, the situation of oral hygiene was very poor. No lesions were noted to the oral mucosae, and neither were there alterations to the main lymph node stations. In greater detail, intraorally it was possible to observe a skeletal class III malocclusion with an inward bite. The presence of periodontal disease affecting all the remaining teeth was also evident, with level 2–3 mobility on all teeth and some implants. After impressions, waxes and the facial arch were taken, the models were fitted to an articulator. The required wax-up was constructed using facial photographs.
A preliminary treatment plan was then formulated, with the aim of alleviating the pain and rehabilitating the mouth:

• Extraction of all teeth by now impaired and removal of the implants without adequate bone support.
• Cleaning of treatable implants (16, 17, 33)
• Positioning of upper and lower mobile prostheses to permit healing of mucous and bone tissues in keeping with the diagnostic wax-up.

Numeri Uno, 2013, 17:14-17

IMPLANT POSITIONING AND RECONSTRUCTION OF HARD AND SOFT TISSUES: A SIMPLIFIED PROTOCOL

Infective endodontic or periodontal pathologies and root fractures can often cause serious bone defects, involving in particular the buccal wall.
Residual post-extraction bone volumes are therefore often affected, in both their apicocoronal and their vestibulopalatal dimensions, with evident deficits and asymmetries also involving the mucogingival tissues. The aim of this case report is to present a simplified protocol for the morphological restoration of hard and soft tissues simultaneously with the insertion of an implant in the aesthetic area.