This surgical instrumentation assembly is for positioning a shoulder prosthesis, the shoulder prosthesis comprising a patient-specific shoulder implant adapted to fit onto a glenoid cavity of the scapula of a patient. The assembly comprises a patient-specific impacting device having an underside surface congruent with the glenoid cavity of the scapula of the patient, said underside surface being provided with protrusions adapted to perforate the cortical bone of the scapula upon impact of the impacting device against the scapula by a one-sided translation movement.
