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

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 $4°{}^{\begin{array}{c}+6^{\prime }\\ 0^{\prime }\end{array}}$. 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 $0.3\mathrm{kN}$, $2\mathrm{kN}$, and $4\mathrm{kN}$; then, a cyclical vertical force varying from $2.67\mathrm{kN}$ to $5.34\mathrm{kN}$ 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.