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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>4</mml:mn> <mml:mo>°</mml:mo> <mml:msup><mml:mspace></mml:mspace> <mml:mrow> <mml:mtable> <mml:mtr><mml:mtd><mml:mo>+</mml:mo> <mml:msup><mml:mn>6</mml:mn> <mml:mo>'</mml:mo></mml:msup> </mml:mtd> </mml:mtr> <mml:mtr> <mml:mtd><mml:msup><mml:mn>0</mml:mn> <mml:mo>'</mml:mo></mml:msup> </mml:mtd> </mml:mtr> </mml:mtable> </mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>$$ {4}^{{}^{\circ}\begin{array}{c}+{6}^{\prime}\\ {}{0}^{\prime}\end{array}} $$</mml:annotation></mml:semantics> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>0.3</mml:mn> <mml:mspace></mml:mspace> <mml:mi>kN</mml:mi></mml:mrow> <mml:annotation>$$ 0.3\ \mathrm{kN} $$</mml:annotation></mml:semantics> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>2</mml:mn> <mml:mspace></mml:mspace> <mml:mi>kN</mml:mi></mml:mrow> <mml:annotation>$$ 2\ \mathrm{kN} $$</mml:annotation></mml:semantics> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>4</mml:mn> <mml:mspace></mml:mspace> <mml:mi>kN</mml:mi></mml:mrow> <mml:annotation>$$ 4\ \mathrm{kN} $$</mml:annotation></mml:semantics> </mml:math> ; then, a cyclical vertical force varying from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>2.67</mml:mn> <mml:mspace></mml:mspace> <mml:mi>kN</mml:mi></mml:mrow> <mml:annotation>$$ 2.67\ \mathrm{kN} $$</mml:annotation></mml:semantics> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>5.34</mml:mn> <mml:mspace></mml:mspace> <mml:mi>kN</mml:mi></mml:mrow> <mml:annotation>$$ 5.34\ \mathrm{kN} $$</mml:annotation></mml:semantics> </mml:math> 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.