Key words: prospective-retrospective study, single-unit implant-supported crown, osteotome sinus-lift technique, Renouard-Rangert biomechanical risk factors, initial biomechanical score, final biomechanical score.
The purpose of this paper is to investigate the prognostic value of the initial biomechanical score calculation for single-unit implant-supported crowns placed using the osteotome sinus-lift technique.
Materials and methods: from March 2004 to December 2018 we inserted in the load-bearing maxillary subsinusal areas 53 single-unit fixtures using the osteotome technique. All fixtures have been loaded after 8 to 10 months with permanent single-unit crowns. Prior to every implant placement we have calculated an initial biomechanical score, and biased it against a final one, e.g. after crown insertion.
Results: the initial biomechanical score matched the final one up to a maximal 1.5 – point deviation in 87% of the cases.
Discussion: the only 87% accuracy match is due to: (1) diverging from the initial treatment plan by changing implant length and/or diameter, (2) offset occlusal crown loading, (3) different subsinusal autologous basal bone heights, (4) different types of abutment-crown interfaces, e. g. cemented versus screw-retained and (5) a wide array of occlusal clearances ranging from 4 to 11mm. (!)
Conclusions: calculation of an initial biomechanic score seems to be an acceptable method for predicting the biomechanic outcome for single-tooth implant-supported crowns inserted using the osteotome sinus lift technique.
Tooth removal is always followed by tridimensional bone loss. In the maxillary subsinusal, load-bearing area this phenomena, when associated with sinus pneu-matisation and periodontal defects leads to reduced basal bone heights seriously hampering implant placement without additional procedures1,4,5,6,7,8,9. “Crestal approach” is a general term encompassing different sinus-lift techniques which use osteotomes, (the osteotome technique sinus-lift) the fixture by itself, piezo-surgery and sonic or rotary instruments9. If required, the created space can be filled with autologous bone, heterologous bone, artificial bone, a mixture of all these substances or can be left unfilled enabling various degrees of spontaneous osteoinduction / osteoconduction. It is an already classic procedure for it has been demonstrated to be effective, less invasive, and associated with a reduced morbidity. All these techniques have in common a controlled modification and elevation of the sinus floor. (of the cortical bone layer plus the schneiderian membrane) When using the osteotome technique the sinus floor is fractured in a controlled manner with a set of osteotomes and subsequently lifted up to a maximal height of 4mm, thus enabling placement of screw-type fixtures. For the new-placed fixtures an immediate stability in an autologous bone height of at least 4mm is mandatory. The present study can be defined as prospective-re-trospective because every single case data was picked up for some possible futu-re study but its subject and data evaluation have been done only after data accumulation has been completed.
The purpose of this article is to assess the prognostic value of the initial biome-chanical score for single-unit implant-supported crowns inserted on fixtures pla-ced using the osteotome sinus-lift technique.
For a better purpose understanding we will review these two concepts:
- The Renouard-Rangert biomechanical risk factors10 are an analytical approach instrument for analyzing clinical data for predictable treatment of an implant-supported superstructure. (see fig.1) They are expressed by numbers and can be deployed to calculate its initial and/or final biome-chanical score.
- The Renouard-Rangert biomechanical score10 is the final algebraic sum resulting from the values assigned to each risk factor and depicts a spe-cific initial, final or an hypothetical final clinical situation. (see fig.2) It is divided into 3 areas, mimicking a traffic light.
The first case report explains why the final biomechanical score shifted away from the initial one within an acceptable range of 1.5 points.
The second case report depicts why the final biomechanical score drifted away from the initial one way out of the acceptable range of 1.5 points.
The third case report depicts a perfect match between an initial and a final biomechanical score.
After three years of occlusal loading the biomechanical score remained unchanged, “in green”, without any alarm sign in sight. (look for fig. 22)
Materials and methods
From March 2004 to December 2018 we inserted in the load-bearing maxillary subsinusal areas 53 single-unit fixtures using the osteotome sinus-lift technique. The lowest vertical bone height was 4mm. The sinusal floor was fractured in a controlled manner and the schneiderian membrane was elevated up to 4mm. In only ten cases we introduced grafting material (Cerabone®) through the socket. All fixtures have been submerged for 6 to 9 months. No fixture was lost during the osseointegration period nor during the uncovering procedure. All fixtures have been loaded after 8 to 10 months with permanent single-unit crowns. Twenty-one crowns were cemented and 32 were screw-retained. Eighteen crowns had BelleGlass occlusal surfaces and 35 had standard PFM occlusal surfaces. One cemented PFM fixture has been lost after two years of occlusal loading and a second cemented PFM crown after four years of occlusal loading. Both cases featured single-unit implants placed in free-end positions. Both cases have been retreated by placing an implant mesial to the removed one. Both new implant-supported crowns had BelleGlass occlusal surfaces and were screw-retained. For each clinical case we created a Power Point Presentation file (PPT file) in which we stored: (1) standardized parallel technique periapical radio-graphs before implant placement, (2) after implant placement, (3) after pros-thesis delivery, (4) standardized intraoral buccal views before implant place-ment, (6) and after prosthesis delivery, (7) standardized intraoral occlusal views before implant placement, (8) and after prosthesis delivery. Prior to every im-plant placement we have calculated an initial biomechanical score and biased it against a final one, e.g. calculated immediatelly after crown insertion. A bio-mechanical score shift below 1.5 points was considered irrelevant.
The initial biomechanical score matched the final one up to a maximal 1.5-point shift in 87% of the cases.
The only 87% accuracy match is probably due to: (1) diverging from the initial treatment plan by changing implant length and/or diameter, (2) offset occlusal crown loading, (3) different subsinusal basal, respectively autologous bone heights, (4) different types of abutment-crown interfaces, e. g. cemented versus screw-retained, (5) a wide array of occlusal clearances ranging from 4 to 11mm, (!) and (5) different occlusal surface materials.
The main shortage of this study is that due to low patient compliance we were unable to assess the biomechanical score shift at standardized periods of time (at 5 and 10 years) following crown placement. The main accomplishment is that it establishes a prognostic value for the initial biomechanical score. (calculated be-fore treatment start)
- Calculation of an initial biomechanic score seems to be an acceptable method for predicting the biomechanic outcome for single-tooth crowns supported by fixtures inserted using the osteotome sinus lift technique.
- The osteotome technique sinus lift is indicated only if the initial biome-chanical score is “in green”.
- An initial biomechanical score will probably match the final one only if both clinical indication and procedure protocol are strictly taken into account, e. g.: an initial bone height of at least 4mm yielding good im-plant primary stability, deployment of a dedicated surgical guide, a corti-cal bone/schneiderian membrane lift of no more than 4 mm.
- When lifting the sinus floor up to 4mm bone grafting material seems to be unnecessary.
- Screw-retained crowns with non-ceramic occlusal surfaces are of superior performance compared to PFM crowns featuring ceramic occlusal surfa-ces or full-ceramic crowns.
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