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The dentist refused to provide information about the patient's gender and weight.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 z95l device [(b)(6)].There were no problems observed during manufacturing or testing noted in the dhr.The repair history showed 1 service record since the device was shipped.The repair details are as follows: march 2022: the cartridge, drive shaft, and dog clutch were replaced.With respect to the repair in the above list, the service record indicates 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 (200,000min-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 200,000min-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 5 minutes into the test were as follows: test point (1): 38.4 degrees c, test point (2): 45.1 degrees c, test point (3): 33.4 degrees c, test point (4): 34.2 degrees c.C) nakanishi cleaned the inside of the handpiece using nakanishi pana spray plus.Nakanishi then conducted temperature testing of the device in the same manner as above yet again.There was no abnormal rise in temperature during the 5-minute test period.Temperature measurements 5 minutes into the test were as follows: test point (1): 32.4 degrees c, test point (2): 35.6 degrees c, test point (3): 29.9 degrees c, test point (4): 29.9 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 internal parts were soiled but was not broken.Abrasive debris was adhering to the ball bearing on the rear side of the cartridge.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 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 debris from the soiled and abraded internal parts or residual liquid (excessive water or lubricating oil), which interfered with rotation.B) a lack of maintenance caused the accumulation of debris and the residual liquid on the internal parts, which caused debris or liquid 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 as instructed in the operation manual and checking of the handpiece prior to use to prevent overheating, as instructed in the operation manual.
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On february 19, 2024, a dentist visited the central japan dental show and there complained about an nsk handpiece overheating.On march 25, 2024, two nsk z95l handpieces were returned from a distributor to nakanishi for repair.Nakanishi made a phone call to the dentist and obtained detailed information on the adverse event.According to the dentist, there are two devices suspected to be involved in the event, but the dentist could not identify which one of the devices actually caused the following event.Therefore, nakanishi is submitting two separate mdrs for this event.This mdr is regarding the handpiece with the serial number (b)(6).The event occurred on september 7, 2023.The dentist was performing #6 and #7 crown removal procedure on a patient using z95l handpiece (serial no.(b)(6)).The patient was under local anesthesia, and then the patient and the dentist did not find that the handpiece overheated.The patient has had a follow-up visit with the dentist on the next day and the dentist found that the patient received a circular whitish burn injury about 1cm in diameter to their left buccal mucosa.The patient is reported to have received laser treatment for the injury and to have been healed.According to the dentist, patients have occasionally complained of feeling hot during the procedures.
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