Mechanical strength and optimal site of placement of a threaded bone screw assessed on the basis of the screw breakage for non-union of the scaphoid: a biomechanical study.

We have performed the surgical procedure on the patients with the fracture of the scaphoid using mini-acutrak screws (ACUMED, Inc.). A 25-year-old man was injured in a fall. Surgical treatment for non-union of the scaphoid using a mini-acutrak screw was performed. However, this screw was broken, despite the absence of any obvious trauma mechanism. A structural examination was performed with a metal microscope and a hardness test was performed. A hardness evaluation was conducted on this broken screw and similar screws. Cannulated screws having diameters of 2.7 and 3.5 mm were also evaluated in the strength test as controls for comparison. The breakage of screw occurred due to fatigue. The screw provides the strength equivalent to that of a 3.5 mm cannulated screw but the bending strength of the head end portion in which the breakage occurred was assumed to be the strength of about one-third that of the intermediate portion.