international magazine of oral implantology

I research _ implant geometries Fig. 7_Equivalent of total strain ob- tained for the short implants and the corresponding standard implants. (a) Maximum values obtained, (b) strain distribution. compared to longer implants. The use of short im- plants in the posterior jaw region reduces the need forboneaugmentationprocedurespriortoorincon- junction with implant placement in the maxilla and the mandible. Shorter implants reduce the surgical risk of sinus perforation or mandibular paresthesia, with an overall reduction in surgical complications. Owing to the decreased length of the drills and im- plants,theosteotomypreparationimplieslessriskof overheatingthebone.Insertionofdrillsandimplants are also easier in small intra arch spaces. In the case of apical root proximity short implants can be the onlypossiblechoice.Fromthepatient’spointofview, shorterimplantsreducetreatmenttime,discomfort, and overall costs related to graft procedures. All thesefactorsmakeshortimplantsahighlyattractive restorative option. In a recent study, Hagi and coworkers14 concluded that dental implant surface geometryisamajordeterminantinhowwelltheim- plants perform in short lengths, which were defined in that study as being shorter than 7mm. Threaded implants showed higher failure rates in short vs. longer length, sintered porous surfaced implants performed well in short lengths. Moreover, various researchers using FE analysis have demonstrated thathorizontalandverticalocclusalforcesplacedon implants were distributed primarily in the crestal bone, rather than along the entire implant-bone in- terface. These findings led the Lum group to con- clude that short implants serve as favourable as longer implants.17, 21, 22 Implant diameter should also beconsideredasanimportantclinicalvariable.Ithas been suggested that increasing implant diameter couldcompensatefordecreaseoflength.Himmlova and colleagues showed that an increase in the im- plant diameter decreases the stress around the im- plant neck more than an increase in the implant length, as a result of a more favourable distribution of the simulated masticatory forces.16 Concerning the strain, the values obtained with the short implants were relatively high (above 10,000 µstrain) in comparison with long implants (up to 5,000 µstrain). The same behaviour was ob- served for implant displacements. Short implants haveadisplacementofapproximately290mm.This behaviour could be explained by discussing the ma- terial properties and cortical bone thickness that were used in this analysis which were based on the typicalregionfortheclinicalapplicationoftheseim- plants (posterior region), where the bone quality is poor.Theretentionoftheimplantswasmainlyatthe cortical layer which had a thin thickness of 0.5mm, whereas the rest of the implant length was in the cancellous bone which had a low stiffness of 300MPa, causing a high deformation range. The present study confirms these results. Short implantsshowedhigherstressvaluesthanlongim- plants and less homogeneous distribution than long implants. However, the magnitude of stresses was clearly above the physiological ranges as sug- gested by Frost.8 Maximum physiological stresses andstrainsdefinedbyFrostareintheregionsof100 MPa and 3,000 µstrain for cortical and cancellous bone,respectively.Consequently,thepresentedre- sultsindicateahigh-riskofoverloadingthebonein certain selected cases. Nonetheless, although sev- eral studies in the literature have shown that short implantshaveriskfactorsandthereforehigherfail- ureratecomparedtolongerimplants,19, 27, 36 several recent studies seem to prove the good long-term prognosisofshortimplants.Ithasalsobeenshown that the crown/implant ratio does not seem to be a majorriskfactorinthecaseoffavourableforceori- entationandloaddistribution.Tawilandcoworkers in 2006 evaluated the bone loss around short im- plants (>10mm) and concluded that these im- plants are a long-term viable solution in sites with reduced bone height even when the prosthetic pa- rameters exceed the normal values but under force parafunction control.32 Gentile et al.10 estimated the survival rate of short (5.7mm in length) Bicon dental implants and compared it to Bicon implants of greater length (8mm and longer). The authors reportednodifferenceintheshortimplantsurvival rate when compared to implants of greater length. Essential condition for all implant uses, conse- quently mini implants as well, is successful os- seointegrationthatcanbeconfirmedonlywiththe long-term studies of success and survival of MDIs under load in masticatory function. Shatkin et al.29 12 I implants1_2011 Fig. 7a Fig. 7b