implants - international magazine of oral implantology

plants could be a fulcrum for lever action when a bending moment is applied, suggesting that im- plants could be more susceptible to crestal bone loss by mechanical force. Factorsassociatedwithincreasedbendingover- load in dental implants: _Prostheses supported by one or two implants in the posterior region (Rangert etal. 1995); _Straight alignment of implants; _Significant deviation of the implant axis from the line of action; _High crown/implant ratio; _Excessive cantilever length (>15mm in the mandible, Shackleton et al. 1994; >10–12mm in the maxilla, Rangert etal. 1989; Taylor 1991); _Discrepancy in dimensions between the occlusal table and implant head; _Para-functional habits, heavy bite force and ex- cessive premature contacts (>180µm in monkey studies, Miyata et al. 2000; >100µm in human studies, Falk etal.1990); _Steep cusp inclination; _Poor bone density/quality; and _Inadequate number of implants. The cortical bone is known to be least resistant to shear force, which is significantly increased by bending overload. The greatest bone loss was seen on the tension side.29 According to Von Recum, when two materials of different moduli of elastic- ityareplacedtogetherwithnointerveningmaterial and one is loaded, a stress contour increase is ob- servedwherethetwomaterialsfirstcomeintocon- tact.30 Photoelastic and 3-D finite element analysis studies demonstrated V- or U-shaped stress pat- terns with greater magnitude near the point of the first contact between implant and the photoelastic block, which is similar to the early crestal bone loss phenomenon.31 Misch claimed that the stresses at the crestal bone may cause microfracture or overload, result- ing in early crestal bone loss during the first year of function, and the change in bone strength from loading and mineralisation after one year al- ters the stress-strain relationship and reduces the risk of microfracture during the following years.32 Wiskott and Belser described a lack of osseointe- grationattributedtoincreasedpressureontheos- seous bed during implant placement, establish- ment of a physiological biological width, stress shielding and lack of adequate biomechanical in- tegration between the load-bearing implant sur- face and the surrounding bone.33 They focused on the significance of the relationship between stress and bone homeostasis. Based on a study by Frost,34 five types of strain levels interrelated with different load levels in the bone were described: 1) Disuse, bone resorption; 2) Physiological load, bone homeostasis; 3) Mild overload, bone mass increase; 4) Pathological overload, irreversible bone dam- age; and 5) Fracture. Theconceptof“microfracture”wasproposedby Roberts et al., who concluded that crestal regions around dental implants are high-stress-bearing areas.35 They explained that if the crestal region is overloadedduringboneremodelling,“cervicalcra- tering” is created around dental implants. The studyrecommendedaxiallydirectedocclusionand progressive loading to prevent microfracture dur- ing the bone-remodelling periods. Progressive loading on dental implants during healing stages was first described by Misch in the 1980stodecreaseearlyimplantbonelossandearly I clinical technique _ crestal bone management Table II_Studies regarding the biologic width around natural teeth or dental implants. 22 I implants2_2011 Sulcus depth (SD) Junctional epithelium (JE) Connective tissue attachment (CT) Biologic width Gargiulo etal.57 30 human skulls 0.69 mm 0.97 mm 1.07 mm 2.04 mm (JE + CT) Vacek etal. 58 10 human skulls 1.34 mm 1.14 mm 0.77 mm 1.91 mm (JE + CT) Cochran etal.68 0.16 mm 1.88 mm 1.05 mm 3.08 mm (SD + JE + CT) Berglundh etal.53 2.14 mm 1.66 mm 3.80 mm (SD + JE + CT) Abrahamsson etal.71 2.14 mm 1.28 mm 3.42 mm (SD + JE + CT) Natural teeth Non-submerged Submerged Dental Implants Table II