As the device had a minor deviation on 2 of 4 measuring points, it generally cannot be excluded that the device has contributed to the event.However given the minor degree (5n and 7n below tolerance) of the deviation it is very unlikely that the device has contributed to the event.It can not be excluded that the combination with the third party pins contributed to the reported incident.The possibility of slippage increases when using pins with a blunt tip.The fixation of the patient is realized by the pressure applied at the pin tip.Pressure is p=f/a ;with f=force and a=area.A blunt pin results in increased area and if the force remains the same, reduced pressure is the result.This results in a reduced fixation, increasing the risk of slippage (compare zaazoue et al.2017).The customer informed us that the positioning of the patient was changed during the surgery without loosening the connections or changing the adjustment of the headrest system.It can not be excluded that this also contributed to the reported incident.We suspect, that maybe the pinning technique has been not optimal as described in the instruction manual: "adjust the skull clamp to the width of the patient's head in the manner that the two skull pins in the rocker arm are equidistant from the centerline of the head and the single skull pin at the extension assembly is in line with this centerline.".
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