On (b)(6) 2023, nakanishi received an email from a distributor (nsk europe) about a nsk handpiece overheating.The details are as follows: - the event occurred on (b)(6) 2023.- the dentist was performing a surgical extraction procedure on a lower right wisdom tooth of a patient using the x-sg65 serial no.(b)(4).- during the procedure, the handpiece heated up, and the patient received a burn to their lower lip.- the injury required no further medical attention other than instructions to keep area clean.
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On may 22, 2023, nakanishi received detailed information on the event from the distributor (nsk united kingdom).- at the time of the event, the patient was under local anesthesia.- the patient suddenly jumped and the dentist found a minor burn inside of the patient's left cheek (buccal mucosa).The epithelium of the patient was white and extended about 2cm in diameter.- the patient was advised to take local measures for the injury such as salty mouthwash and paracetamol for 2 or 3 days.
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Upon receiving the device involved in the mdr event from the distributor, 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-sg65 device [02160066].There were no problems observed during manufacturing or testing noted in the dhr.There were also no repair history records since the device was shipped.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) and (3)).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 an abnormal temperature rise at the test point (1) 4 minutes into the test.Temperature measurements about 5 minutes after the start of the test were as follows: - test point (1): 55.1 degrees c - test point (2): 46.4 degrees c - test point (3): 40.7 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 bearing was soiled, discolored, and abraded.- the bearing balls were deformed and metal stripped.B) nakanishi took photographs of all the disassembled parts and kept them in the investigation report no.(b)(4).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 broken bearing.Nakanishi considers the possibility from many years of experience that the cause of the broken 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 distributor and directed the distributor to remind the user of the importance of maintenance as instructed in the operation manual.
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