Notch sensitivity of titanium causing contradictory effects on locked nails and screws

The purpose of this biomechanical study was to compare the mechanical properties of specially designed locked nails and screws with the same structures and made from either stainless steel or titanium alloy. The structural factors investigated included inner diameter and root radius for locking screws and outer diameter and nail hole size for locked nails. The mechanical properties investigated included yield load, cyclic bending stiffness, and fatigue life. Finite element models were used to simulate the mechanical tests and compute the stress concentration factors. Increasing the root radius and the inner diameter could effectively increase the fatigue life of the locking screws. The fatigue life of titanium screws was higher (by 1.4- to >6-fold) than that of stainless steel screws, especially when the inner diameter was increased. In contrast, the fatigue life of titanium locked nails was lower (by about 1/4 to 1/3) than that of their stainless steel counterparts. Finite element models could closely predict the results of the biomechanical tests with a Pearson correlation coefficient that ranged from -0.58 to -0.84 for screws and was -0.98 for nails. The stress concentration factors ranged from 1 to 1.97 for screws and from 3.89 to 4.99 for nails. The present study suggested that with larger root radius and inner diameter, titanium locking screws could provide much higher fatigue life than stainless steel counterparts. However, titanium locked nails might lose their advantages of superior mechanical strength because of high notch sensitivity, and this limitation should be taken seriously during the design process.