The holding power of orthopedic screws in vivo

Great difficulty is encountered in choosing screws for internal fixation, for screws differ in material, geometry, dimension, and in insertion technique. Recently claims have been made that self-tapping screws cause necrosis of bone and result in fibrous tissue formation, with loss of holding power. This investigation was undertaken to study the healing of bone about screws inserted for a period of 3 months. Self tapping and non-self tapping screws were employed. The Vitallium screws not only differed in dimension, but also in configuration of the thread, with the smaller of the self-tapping screws exhibiting a "V" thread in distinction to the buttress thread of the larger screw. The histologic data were correlated with the holding power of the screws as obtained by means of a push out test, performed with the aid of an Instrom testing machine. The largest screw tested, the 4.5 mm non-self tapping stainless steel AO screw (Type 1), provided the greatest safety factor to push out loading over the period tested in the unloaded system. The self tapping and non-self tapping screws of similar material and size were found to maintain comparable holding power at all intervals tested in vivo in the unloaded system. No histological differentiation could be made with regard to both death or tissue reaction around the implant, between the stainless steel or cobalt chromium alloy materials, nor between the self tapping and non-self tapping insertion methods.