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

## TL;DR

This study uses a 3D model to explore how design and loading factors affect the durability of hip implants, finding that certain design changes and patient factors can significantly improve implant performance.

## Contribution

The paper introduces a 3D FEM analysis of a bi-modular hip prosthesis's neck-stem connection, revealing novel insights into fatigue strength under various conditions.

## Key findings

- A positive angular mismatch improves fatigue life by reducing stress up to 33%.
- Higher assembly force increases neck stability and reduces overstressed areas.
- Implant fatigue resistance is directly proportional to the patient's BMI.

## 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 . 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, 2kN, and 4kN; then, a cyclical vertical force varying from 2.67kN to 5.34kN 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.

This work aims to investigate how the geometry of the neck‐stem rectangular taper connection, the assembly force, and the external load affect the fatigue strength of a bi‐modular hip prosthesis through 3D FEM. The study's findings concern: (i) a positive angular mismatch improves fatigue life; (ii) the higher the assembly force the higher the neck stability; (iii) the implant fatigue resistance is directly proportional to the patient's BMI; and (iv) the out‐of‐plane external load increases by 40% the fatigue failure risk.