BONE DEFORMATION BY THREAD-CUTTING AND THREAD-FORMING CORTEX SCREWS

In the operative treatment of complex midfacial fractures by rigid int ernal fixation, the maxillofacial surgeon often feels that a screw whi ch does not require pretapping would be an advantage. Nowadays almost every craniofacial implant system consists of self-tapping screws and the corresponding plates. Operation time could be decreased by using l ess instruments. Self-tapping screws are subdivided into thread-cuttin g and thread-forming screws, essentially depending on the design of th e screw tip and thread (1). In this in vitro experiment, the morpholog ical effects of five different thread-cutting/thread-forming non-flute d screws and one non-self-tapping screw, all with an outer diameter of 2.0 mm, were investigated. The implants, varying mainly in the design of the tip, were inserted into predrilled standardized slices of bovi ne cortical bone (1 mm and 2 mm thick) and into sheep skulls post mort em. During implantation the applied axial force and insertion torque w ere measured. After screw removal and histological preparation, the sp ecimens were investigated according to specific morphological criteria . The cortex screw with separate tap caused the least amount of morpho logical alteration both quantitatively and qualitatively whereby the l owest insertion torque was also measured. Of all the thread-cutting an d thread-forming screw designs tested, the trocar tip with sharp cutti ng edges caused the least deformation of the bone, requiring the lowes t insertion torque and applied axial force.