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Upon receiving the device involved in the mdr event, nakanishi conducted a failure analysis of the returned device, which included measuring the operating temperature of the device [report no.(b)(4)].These activities are described in more detail below.Methodology used: a) nakanishi examined the device history record and the repair history for the subject x-sg65l device [(b)(6)].There were no problems observed during manufacturing or testing noted in the dhr.The repair history showed 3 service records since the device was shipped.The repair details are as follows: - march 2019: the chuck, bearings, slider, pin, and chuck retainer were replaced.- november 2019: the chuck, body, bearings, slider, pin, and chuck retainer were replaced.- december 2020: the chuck, bearings, slider, pin, and chuck retainer were replaced.With respect to the repairs in the above list, the service records indicate that nakanishi performed all of the necessary operation checks and confirmed that all of the criteria were met.B) nakanishi conducted temperature testing of the returned device in the following manner: b.1) temperature sensors were attached to the exterior of the device at various test points.This included the point most proximal to the patient (testing point (1)) and points further toward the distal end of the device (testing points (2) through (4)).The test setup was prepared to take temperature measurements at all points simultaneously, including a reference measurement at ambient room temperature.B.2) nakanishi attached a thermocouple (sensor to measure temperature) to each of the testing points.Nakanishi rotated the device's motor at 40,000min-1, which is the maximum rpm for the motor that drives the handpiece (40,000min-1 for the handpiece), with water spray, and measured the exothermic response.B.3) nakanishi measured the temperature rise of the returned handpiece set at 40,000min-1 (motor revolution 40,000min-1).Nakanishi observed rises in temperature at the test points as shown below; however, the temperatures were not high enough to cause a burn injury.The maximum temperature measured 10 minutes into the test were as follows: - test point (1): 37.2 degrees c, - test point (2): 37.1 degrees c, - test point (3): 37.7 degrees c, - test point (4): 35.1 degrees c.B.4) nakanishi also rotated the device's motor at 40,000min-1, which is the maximum rpm for the motor that drives the handpiece (40,000min-1 for the handpiece), without water spray, and measured the exothermic response.B.5) nakanishi also observed rises in temperature at the test points as shown below; however, the temperatures were not high enough to cause a burn injury.The maximum temperature measured 10 minutes into the test were as follows: - test point (1): 38.1 degrees c, - test point (2): 38.7 degrees c, - test point (3): 39.3 degrees c, - test point (4): 35.2 degrees c.Identification of the specific failure mode(s) and/or mechanism(s) of the associated device components was conducted as follows: a) nakanishi disassembled the handpiece and performed a visual inspection of the internal parts.Nakanishi observed the following: - the ball bearing was not damaged.- the bearing balls, chuck, and the bearing balls were discolored.B) nakanishi took photographs of all the disassembled parts and kept them in the investigation report no.(b)(6).Conclusions reached based on the investigation and analysis results: a) nakanishi determined that the cause of the handpiece overheating was abnormal resistance during rotation due to the damaged bearing.Nakanishi considers the possibility from many years of experience that the cause of the damaged bearing was the ingress of undesirable materials into the bearing, leading to abrasion.B) a lack of maintenance caused the accumulation of debris on the internal parts, which caused debris ingress into the bearing during rotation.This contributed to the handpiece overheating.C) in order to prevent a recurrence of the handpiece overheating, nakanishi took the following actions: c.1) nakanishi reviewed the operation manual and reconfirmed the clarity and understandability of the instructions.C.2) nakanishi reported the above evaluation results to the dentist and reminded the dentist of the importance of maintenance and checking of the handpiece prior to use to prevent overheating, as instructed in the operation manual.A) nakanishi could not identify the exact cause of overheating of the returned device because nakanishi was not able to replicate the temperature rise at the time of the event and did not observe any abnormalities in the visual inspection.B) in spite of the fact that nakanishi could not identify the cause, nakanishi took the following actions to be safe: c.1) nakanishi reviewed the operation manual and reconfirmed the clarity and understandability of the instructions.C.2) nakanishi reported the above evaluation results to the dentist and reminded the dentist of the importance of peruse check and of maintenance, as instructed in the operation manual.
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