Peripheral facial nerve palsy (PFNP) has a substantial physical, psychological and social impact on patients. Neurophysiological study quantifies the degree of nerve injury and assesses prognosis. We present the case of a woman with a 3-month history of left PFNP after a dental implant, with facial functionality of 85.5% and with a normal neurophysiological study performed according to the standard protocol. By modifying the technique centred on the orbicularis oris in its upper portion, the procedure showed an asymmetry of amplitude and signs of denervation. This allowed us to detect a deficit and differentiate a possible asymmetry or simulation by the patient.