INFLUENCE OF IMPLANT NUMBER, IMPLANT LENGTH AND CROWN HEIGHT ON BONE STRESS DISTRIBUTION FOR THREE-UNIT BRIDGES IN THE POSTERIOR MANDIBLE: A 3D FINITE ELEMENT ANALYSIS.
Background and aim: Reduced alveolar bone height due to ridge resorption represents a major limitation in the use of dental implants, in particular in the posterior sectors of jaws.
It increases the probability of an invasion with related possible damage to some anatomical structures, such as the inferior alveolar nerve. Short dental implants' placement has been proposed as an alternative to surgical bone augmentation procedures and recent studies indicated that short implants could present survival and success rates similar to conventional implants in short and medium term. However doubts about biomechanical performances were risen because higher crowns are sometimes necessary to compensate the bone resorption, leading to a less favourable crown to implant ratio.\u2028Recently image-based approaches combined with Finite Element Analyses (FEA) have allowed effective stress\u2013strain investigations in biological systems and in particular stress distribution in bone.
The objective of this study was to evaluate the stress transmitted to surrounding bone by different implant number, implant length and crown configurations in a three-unit bridge by means of finite element analysis.
Material and methods: The 3D geometry of the edentulous mandible was reconstructed from computerized tomography (CT) scans. The symmetry of the structure permitted the reconstruction of a half maxilla.\u2028Bone material mechanical properties have been assigned to each tetrahedral element based on the Grey Value. The meshes of the implants (Astra Tech AB OsseoSpeedTM TX, Dentsply Implants) were placed in second premolar, first molar and second molar position for the three implants configurations and in second premolar and first molar position for the two implants configurations. A superstructure representing a porcelain three unit bridge was built using beam elements for each configuration. Six different implant configurations were compared: LS2) two 4 mm diameter x 11 mm long implants with 8 mm long crowns; LS3) three 4 mm diameter x 11 mm long implants with 8 mm long crowns SS2) two 4 mm diameter x 11 mm long implants with 8 mm long crowns; SS3) three 4 mm diameter x 11 mm long implants with 8 mm long crowns; SL2) two 4 mm diameter x 6 mm long implants with 13 mm long crowns; SL3) three 4 mm diameter x 6 mm long implants with 13 mm long crowns. A 200 N axial and 45\ub0 oblique loads were applied to each crown. For each configuration the effect of both loading scenarios was evaluated in terms of state of stress in the bone-implant interface (Von Mises stress, maximum and minimum principal stresses).
Results: Under oblique load the stress distribution is more concentrated around the coronal part of the implant and it is several times higher than under axial load. In particular the tension represented by the maximum principal stress is from 15 to 35 times higher.
In all configurations the stress was more concentrated in the cervical area of the peri-implant bone.
Considering axial load the higher values of peri-implant stress were found in the SS2 and SL2 configurations while the lower values around in LC2 and LC3 configurations.
Under oblique load the maximum peri-implant stress was found in the SL2 configuration while the minimum peri-implant stress was found in the LC3 configuration.
The increase of stress parameters values in SS configurations respect to LS configuration were on average of the 40%. Even the average increase of stress values in SL configurations respect to SS configuration was about the 42% under tilted load.\u2028Configurations with 2 implants were recorded to undergo about the 50% more of stress on average than the respective 3 implants configurations.
Conclusions: Crown heigh, implant number and implant length seem to be all influencing factors on implant bone stress, however the augmentation of crown heigh seems to have a greater effect than a reduction of implant length.\u2028Even if the stress observed in all configurations was within a physiological range, a three-unit bridge with 13 mm long crowns supported by two implants may be biomechanicaly hazardous in the presence of horizontal forces, and the addition of another short implant or increase of bone volume may be suggested to dissipate the stress at bone-implant interface. In conclusion the use of short dental implants to support a three unit bridge in the posterior mandible can be considered a potential alternative to standard length implants, but crown heigh and lateral forces have to be carefully analyzed in every patient
