ON AUGUST 5, 2015, A NSK HANDPIECE, SGA-ES (SERIAL NO. (B)(4)) WAS RETURNED FROM A DENTIST TO NAKANISHI FOR REPAIR. THERE WAS A NOTE COMING WITH THE HANDPIECE REFERRING TO THE HANDPIECE OVERHEATING. NAKANISHI CONTACTED THE DENTIST FOR MORE INFORMATION. THE INFORMATION NAKANISHI OBTAINED IS DURING TREATMENT, THE PATIENT COMPLAINED ABOUT THE HANDPIECE OVERHEATING. ACCORDING TO THE DENTIST, THE HANDPIECE WAS OVERHEATED BUT THE OVERHEATING DID NOT CAUSE A BURN INJURY TO THE PATIENT. ON AUGUST 24, 2015, NAKANISHI MADE A PHONE CALL REQUESTING ADDITIONAL INFORMATION. NAKANISHI TRIED TO ACQUIRE THE PATIENT INFORMATION SUCH AS AGE, SEX, ETC. FROM A DENTAL HYGIENIST IN THE DEPARTMENT OF ORAL SURGERY IN THE HOSPITAL. DUE TO THE PATIENT NOT BEING BURNED, THERE IS NO RECORD KEPT TO IDENTIFY THE PATIENT TO WHICH THE HANDPIECE WAS USED.
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Patient 1
UPON RECEIPT OF THE DEVICE INVOLVED IN THE MDR EVENT, NAKANISHI CONDUCTED A FAILURE ANALYSIS OF THE RETURNED DEVICE THAT INCLUDED MEASUREMENT OF THE TEMPERATURE OF THE OPERATING DEVICE (B)(4). THESE ACTIVITIES ARE DESCRIBED IN MORE DETAIL BELOW. METHODOLOGY USED : NAKANISHI EXAMINED THE DEVICE HISTORY RECORD FOR THE SUBJECT SGA-ES DEVICE [SERIAL NUMBER (B)(4)]. THERE WERE NO PROBLEMS OBSERVED DURING THE MANUFACTURING OR TESTING NOTED IN THE DHR. NAKANISHI CONDUCTED A VISUAL INSPECTION OF THE RETURNED DEVICE AND PERFORMED A SIMPLE MOVEMENT TEST. NAKANISHI OBSERVED DISCOLORATION AT THE END OF THE HANDPIECE NEAR A PATIENT. NAKANISHI THEN SET A TEST BUR IN THE HANDPIECE AND ROTATE IT BY HAND TO SEE BEARING CONDITION. NAKANISHI CONFIRMED THAT THE BUR ROTATED SMOOTHLY. NAKANISHI CONDUCTED A TEMPERATURE TESTING OF THE RETURNED DEVICE IN THE FOLLOWING MANNER. TEMPERATURE SENSORS WERE FIRST ATTACHED TO THE EXTERIOR OF THE DEVICE AT VARIOUS TEST POINTS (E.G., MOST PROXIMAL TO THE PATIENT AND ALONG POINTS FURTHER TOWARD THE DISTAL END OF THE DEVICE). THE TEST SETUP WAS PREPARED TO TAKE TEMPERATURE MEASUREMENTS AT ALL POINTS SIMULTANEOUSLY, INCLUDING A REFERENCE MEASUREMENT AT AMBIENT ROOM TEMPERATURE. NAKANISHI ATTACHED A THERMOCOUPLE (SENSOR TO MEASURE A TEMPERATURE) TO EACH OF THE TESTING POINTS (1), (2), (3) AND (4). NAKANISHI ROTATED THE HANDPIECE AT 40,000RPM, WHICH IS MAXIMUM RPM FOR THE MOTOR THAT DRIVES THE HANDPIECE (40,000 PRM FOR THE HANDPIECE), WITH WATER SPRAY AND MEASURED THE EXOTHERMIC SITUATION. NAKANISHI MEASURED THE TEMPERATURE RISE OF THE RETURNED HANDPIECE SET AT 40,000 RPM (MOTOR REVOLUTION 40,000RPM). NAKANISHI CONFIRMED THE FOLLOWING TEMPERATURE RISE AT THE TEST POINTS AFTER THE BEGINNING OF THE MEASUREMENT ; 38.2 DEGREES C, 34.9 DEGREES C, 33.3 DEGREES C AND 33.5 DEGREES C. ALL THE TEMPERATURES OBSERVED IN THE MEASUREMENT WERE WITHIN THE NAKANISHI SPECIFICATION RANGE. IDENTIFICATION OF THE SPECIFIC FAILURE MODE(S) AND/OR MECHANISM(S) AND THE ASSOCIATED DEVICE COMPONENT(S) INVOLVED: NAKANISHI WASHED THE INSIDE OF THE HANDPIECE USING NAKANISHI PANA-SPRAY. NAKANISHI OBSERVED DIRT/DEBRIS BEING EXPELLED FROM THE HEAD OF THE HANDPIECE BY USING A WHITE FILTER TO CATCH ANYTHING THAT WAS EXPELLED. NAKANISHI DISASSEMBLED THE HANDPIECE AND PERFORMED A VISUAL INSPECTION OF THE INSIDE PARTS. DIRT/DEBRIS WAS OBSERVED ON THE BEARING INCORPORATED IN THE MOST PROXIMAL AREA TO THE PATIENT. NAKANISHI TOOK PHOTOGRAPHS OF THE BEARING MENTIONED ABOVE AND KEPT THEM IN A FILE. CONCLUSIONS REACHED BASED ON THE INVESTIGATION AND ANALYSIS RESULTS: NAKANISHI WERE NOT ABLE TO REPLICATE THE TEMPERATURE RISE AT THE TIME OF THE EVENT, HOWEVER NAKANISHI IDENTIFIED THAT THE CAUSE OF OVERHEATING OF THE RETURNED DEVICE WAS DUE TO DIRT/DEBRIS INGRESS. A LACK OF MAINTENANCE CAUSES ACCUMULATION OF DIRT/DEBRIS (ABRASIVE POWDERS) IN THE HEAD, WHICH CAUSES DIRT/DEBRIS INGRESS INTO THE BEARING WHILE ROTATING. THIS WILL CONTRIBUTE TO THE HANDPIECE OVERHEATING. IN ORDER TO PREVENT A RECURRENCE OF THE HANDPIECE OVERHEATING, NAKANISHI TOOK THE FOLLOWING ACTIONS. NAKANISHI FIRST REVIEWED THE OPERATION MANUAL AND ENSURED CLARITY, UNDERSTANDABILITY AND FEASIBILITY OF THE INSTRUCTIONS. NAKANISHI REPORTED THE ABOVE EVALUATION RESULTS TO THE DENTIST AND REMINDED THE DENTIST OF THE IMPORTANCE OF MAINTENANCE AND PRIOR-TO-USE CHECKUPS AS INSTRUCTED IN THE OPERATION MANUAL.