A method and set of instruments are disclosed particularly useful for carpal tunnel surgeries that allow a precision cut in the transverse carpal ligament (TCL) without direct vision or exposure of the ligament, except for its most proximal edge, but with guidance and safety of the cutting knife, eliminating or, at least very much decreasing, the probability of cutting lines in the wrong direction and inadvertent (iatrogenic) lesions to the surrounding structures. In one preferred embodiment the instrument comprises: a uniquely shaped cannulated guide rod, through which passes a flexible metal guide needle (33) which serves as a guideline for a uniquely shaped cutting knife or fasciotome (25) having a cannulated finger-like prong in the inferior edge of the blade portion of the knife.