Clinical precision of paralleling instruments

A method for measuring pin- or pinhole-deviation was developed and tested on nine consecutive bridge cases with parallel pin retention, in order to assess paralleling precision of guiding instruments used clinically. Measurements were performed on rubber base impressions including pinholes, involving the use of a stereomicroscope. The maximum pinhole error angle for the bridges investigated ranged between 5.9 and 16.1 degrees. The corresponding interval for abutments was 1.3-10.8 degrees. The experimental observations concerning the rather large values for the greatest pinhole deviation of fitting bridges could be satisfactorily explained theoretically. The results indicated that the magnitude of the largest permissible angular deviation for a restoration with parallel pin retention was significantly larger than that previously presented in the literature. Compared with results from a previously performed model study carried out during ideal preparatory conditions, the current investigation suggested that the paralleling precision of guiding instruments used clinically in fact decreases considerably, remaining, however, at an acceptable level.