In recent years, we have seen increased interest in bone lesions of the glenoid rim as acute fractures (Bony-Bankart) and as chronic bone defect in instability. This derives from three main clinical and statistical findings: a significant incidence of bony Bankart lesion after a first dislocation, a high percentage of glenoid bone defects in chronic instability, and, finally, a close relationship between bone defect and incidence of recurrence after arthroscopic stabilization. The authors agree on determining glenoid bone defect that exceeds 15-20% as the main contraindication to arthroscopic stabilization. It is thus necessary to accurately calculate bone defect in order to be able to plan the most suited type of surgery. The authors report their simple, accurate and reproducible CT method known as Pico to quantify and measure bone defect in terms of percentage bone area and in terms of square mm of defect.