In-vitro Determination of Contamination of Low-speed Handpieces

Objectives: The purposes of this in-vitro study were to investigate whether low-speed dental handpieces with attached contra-angles become contaminated with bacteria during use and if bacteria present inside could be expelled outward during operation. Methods: This study involved two trials, each containing 25 handpiece-contra-angle sets. The first trial involved contamination of sterilized handpiece gears with 0.01 mL of a solution containing 2.0 x 10⁶ CFU/mL Geobacillus stearothermophilus endospores within 10% sheep's blood. The handpieces were operated for 5 minutes in a beaker containing 12.5 mL of phosphate buffered saline (PBS, pH 7.2). 0.05 mL aliquots of the PBS were spiral plated onto enriched trypticase soy agar plates. Aerobic and anaerobic incubation of the plates was at 37^oC for seven days. Examination of the plates for the number of bacterial colonies followed. In the second trial, operation of sterilized handpieces with burs attached occurred while submerged in beakers containing 12.0 mL of the spore suspension. Sampling the gears of the contra angle and handpiece for contamination involved sterile cotton-tipped swabs and paper points. Swabs and points went into 2.0 mL of PBS and then mixed well. Culturing of the specimens was as described previously. Results: In the first trial, spores moved from the handpiece to the PBS on 10 occasions (40%). In the second trial, spore contamination of the handpiece motor gears occurred 4 times (16%). Conclusion: The results of this in-vitro study suggest that bacteria can travel through the vents in the contra angles and spread internally towards the gears of the handpiece motors. The results also suggest that microbes placed on the gears of the motor can travel through the handpiece and then out through the contra angle. Unless properly sterilized between patient uses, slow-speed handpieces with contra-angles could become a source of cross-contamination.