This cadaveric study evaluated the mechanical properties of screw placement in simulated scaphoid waist fractures. Eleven matched pairs of cadaveric scaphoids underwent a linear osteotomy with a Herbert-Whipple cannulated screw to secure the osteotomy. The specimens were transfixed in the testing apparatus to deliver a dorsal to volar cantilever bending load. Stiffness, load at 2mm displacement, load at failure and the mechanism failure were measured. Central placement of the screw in the proximal fragment of the scaphoid exhibited superior mechanical properties compared to those after eccentric positioning of the screw. Central placement of the screw demonstrated 43% greater stiffness, 113% greater load at 2mm of displacement, and 39% greater load at failure. The mode of failure for all specimens was screw migration and fracture at the screw-bone interface with dorsal wedge opening.
This study confirms the biomechanical advantage of central screw placement in the proximal fragment of a scaphoid waist fracture in a linear osteotomy model. These results confirm long held tenants of scaphoid treatment which were previously unconfirmed biomechanically. The advent of cannulated screws allow for optimal positioning and placement with maximum stability imparted to the scaphoid fracture. The use of more minimally invasive techniques of scaphoid fracture fixation should not occur at the expense of adequate fracture reduction and correct screw positioning.
J Bone and Joint Surg