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On february 13, 2024, nakanishi received a phone call from a dealer about an nsk handpiece overheating.The details nakanishi obtained are as follows: - the event occurred on (b)(6) 2024.- the dentist was extracting a horizontal impacted wisdom tooth of a patient using the sgs-es handpiece (serial no.(b)(6)).- during the procedure, the patient complained of feeling pain to their lip but the dentist continued the procedure.- after removing #8 tooth, the dentist removed a muscle hook out of the patient's mouth and found a 3rd-degree burn injury about 1cm to the patient's right lower lip.- the patient was under intravenous sedation (midazolam and propofol) and conduction anesthesia at the time of the injury.- the dentist applied terra-cortril ointment to the burn injury of the patient.- a dermatologist in the same hospital prescribed a steroid ointment for the patient the next day.- the patient visited a dermatologist the next day and the dermatologist applied dexaltin and azunol ointment to the patient's injury.
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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.C240213-01].These activities are described in more detail below.Methodology used: a) nakanishi could not examine the device history record for the subject sgs-es device [08300118] because nakanishi had discarded the dhr for the device that had exceeded 15-year document retention period in accordance with the nakanishi's regulation.Nakanishi examined the repair history for the subject sgs-es device [08300118].The repair history showed 7 service records since the device was shipped.The repairs details are as follows: - october 2012: the bur pusher retainer and bearings were replaced.- july 2015: the bearings were replaced.- july 2018: the bur pusher retainer, bearings, spindle sub assy and spacer bearing were replaced.- february 2019: the bur pusher retainer and bearings were replaced.- december 2019: the bur pusher retainer, bur lock ring, bearings, and spindle sub assy were replaced.- january 2021: the bur pusher retainer, bearings, nose body, slider, and bur pusher pipe were replaced.- august 2022: the bur pusher retainer, bearings, nose body, slider, spindle sub assy, es nose, and middle body 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), without 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): 34.6 degrees c - test point (2): 32.3 degrees c - test point (3): 32.2 degrees c - test point (4): 33.0 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 bearings were soiled and dried.- the inner and outer races, the bearing retainer were soiled.- the bearing balls were discolored and metal-stripped.- the slider and spindle were soiled and 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 was not able to replicate the temperature rise at the time of the event, but based on the findings in the visual inspection, as well as many years of experience, nakanishi considers the possibility that the cause of the handpiece overheating was abnormal resistance during rotation due to the soiled bearings, which interfered with rotation.B) nakanishi also considers another possibility that the cause of the reported handpiece overheating was temporal friction heat generated by the dried internal parts.C) 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.D) in order to prevent a recurrence of the handpiece overheating, nakanishi took the following actions: d.1) nakanishi reviewed the operation manual and reconfirmed the clarity and understandability of the instructions.D.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.
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