Bending strength and holding power of tibial locking screws

The bending strength and holding power of two types of specially designed t ibial locking devices, a both-ends-threaded screw and an unthreaded bolt, w ere studied and compared with four types of commercially available tibial i nterlocking screws: Synthes, Howmedica, Richards, and Osteo AG, To test ben ding strength, the devices were inserted into a high molecular weight polye thylene tube and loaded at their midpoint by a materials testing machine to simulate a three point bending test, Single loading yielding strength and cyclic loading fatigue life were measured, To test holding power, the devic es were inserted into tubes made of polyurethane foam, and their tips were loaded axially to measure pushout strength, The devices were tested with tw o different densities of foam materials and two different sizes of pilot ho les. Insertion torque and stripping torque of the screws were measured firs t, Pushout tests were performed with each screw inserted with a tightness e qual to 60% of its stripping torque, Test results showed that the yielding strength and the fatigue life were related closely to the inner diameter of the screws. The stripping torque predicted the pushout strength more relia bly than did the insertion torque, All tested devices showed greater holdin g power in the foam with the higher density and with the smaller pilot hole s. The both-ends-threaded screw had the highest pushout strength and a sati sfactory fatigue strength, The unthreaded bolt had the highest fatigue stre ngth but only fair holding power. Clinical studies of the use of these two types of locking devices are worthwhile.