Purpose: The accurate and passive fit of dental prostheses supported by endosseous implants is of primary importance in securing long-term restorative success. In the clinical setting, adequate visual and radiographic assessment of joined implant components can be limited. Mechanical engineering principles show a linear relationship between tightening and the degree of rotation of a precision bolted assembly. At a constant torque, with certain variables controlled, a threaded fastener should return to the same rotational end position on repeated tightenings. This study evaluated the terminal screw positions of joined implant components as a potential aid to the clinician in confirming the fit of a fixed and removable prosthesis. There were three areas of experimental inquiry: (1) How reproducible are the various clinical means by which torque is applied to the fastening screws, both in absolute and relative value? (2) How reproducible are the rotational end positions of the gold (attachment) and titanium (center) screws when a controlled torque is applied? (3) Do changes in screw position occur as a function of the magnitude of artificially introduced discrepancies?
Materials and methods: Three different torque delivery devices were evaluated: a hand-held screwdriver (DIB 048; NobelpharmaUSA, Chicago, IL), a manual torque wrench (DIA 250; NobelpharmaUSA), and an electronic Torque Controller (DEA 020; NobelpharmaUSA), using a calibrated torque measuring dynamometer (Magtrol, Inc, Buffalo, NY). The reproducibility of turning limits were determined for both the titanium and gold screws contained in five Brånemark implant assemblies. Each assembly was subjected to six trials, tightening to recommended torque. The position of each screw head was recorded with a special scribe on acetate sheets and transferred to graph paper. Five implant assemblies were invested in dental stone within a die form mold. A casting was made supported by three implant analogues. Stainless steel shims of 12.7-microns, 25.4-microns, 38.1-microns, and 50.8-microns thickness were used to create impingement and space discrepancies. Controlled trials were conducted, and changes in rotational limits for each screw were recorded.
Results: The following values were measured, intending to achieve a torque of 10 Ncm, based on 10 trials for each implement: hand driver, 6.48 (+/- 0.85) Ncm; torque wrench, 7.77 (+/- 0.56) Ncm; and the Torque Controller, 8.54 (+/- 0.19) Ncm. The electronic Torque Controller proved to be the most reproducible instrument and was selected as the delivery vehicle for the remainder of the study. The titanium center screws had a rotational limit that was reproducible to within 0.6 degrees (+/- 0.2 degrees). For the gold screws, it was found that at least two trials had to be conducted for each assembly before the rotational limits conformed to a reproducible position within 1.85 degrees (+/- 1.87 degrees). A linear relationship of approximately 0.9 degrees/micron was observed between the changes in rotational limit and each subsequent shim thickness.
Conclusions: This study suggests that screw position can be an indicator of fit in dental implant prostheses if the end point of screw rotation is adequately indexed, specific to each assembly and screw.