A Computational Study on the Neck-Stem Rectangular Tapered Connection: Effects of Angular Mismatch, Assembly, and Cyclic Loading.

Abstract

The bi-modular hip prosthesis is characterized by two tapered connections: a circular cross-section at the head-neck interface and a rectangular cross-section at the neck-stem interface. Even if the latter guarantees customization, it concerns a high rate of early failure. The connection resistance is relatable to machining (tolerances cause angular mismatch), implantation (hammering force or manual), and usage (Body Mass Index [BMI]). Due to the lack of literature about the neck-stem coupling, this work aims to investigate how the geometry of the rectangular taper connection and the external loads affect the fatigue strength of a bi-modular hip prosthesis through a 3D Finite Element Model (FEM). Nine combinations of neck-stem coupling are obtained considering the tolerances' limits on frontal and lateral angles as 4°+6'0'$$ {4}^{{}^{\circ}\begin{array}{c}+{6}^{\prime}\\ {}{0}^{\prime}\end{array}} $$ . The CoCr neck and the Ti6Al4V stem, studied in their halved, are constrained and loaded inspired by the standard ISO 7206: the stem is distally encastered simulating the embedding and tilted by 10° concerning the sagittal plane, while the force is applied vertically. First, the influence of the assembly is investigated using 0.3kN$$ 0.3\ \mathrm{kN} $$ , 2kN$$ 2\ \mathrm{kN} $$ , and 4kN$$ 4\ \mathrm{kN} $$ ; then, a cyclical vertical force varying from 2.67kN$$ 2.67\ \mathrm{kN} $$ to 5.34kN$$ 5.34\ \mathrm{kN} $$ is imposed. Finally, one combination is analyzed in its integrity to evaluate the effect of the out-of-plane load. The study's findings concern: (i) a positive angular mismatch, which is responsible for proximal contacts, improves fatigue life, reducing Sines stress up to 33%; (ii) the higher the assembly force the higher the neck stability and the lower the extension of the overstressed lateral area; (iii) the implant fatigue resistance is directly proportional to the patient's BMI; and (iv) the out-of-plane external load causes a 40% increment in the fatigue failure risk.