Engagement and Stress Concentration Evaluation of a Novel Two-Part Compression Screw: A Preliminary Finite Element Analysis

Abstract

Background/Objectives: This novel compression screw design offers potential benefits due to its two-part structure and can be used for various bone types, much like a conventional single-piece compression screw. However, full engagement may not always occur after final compression in clinical practice. This study aimed to verify the hypothesized optimal mechanical strength when the two parts are nearly fully combined and to determine a recommended engagement range based on stress distribution and concentration using finite element analysis. Methods: Ten models representing different combinations of the two screw parts (ranging from 10% to 100% of the engagement length, at 10% intervals) were simulated to determine the acceptable engagement percentage. Pull-out and bending load simulations were performed using finite element models. Extreme clinical loading conditions were simulated, including 1000 N pull-out forces and a 1 Nm bending moment at the screw head. Results: Finite element analysis revealed two stress concentration points. The pull-out load simulation showed that combinations with 100% engagement merged the two stress concentrations into one without force superposition, while combinations with less than 30% engagement should be avoided. In the bending load simulation, higher stress was observed for combinations with less than 90% engagement. A lower level of engagement increases the bending moment, which might be the major factor affecting the von Mises stress. Conclusions: Surgeons should be instructed on how to use the screw correctly and select the most appropriate screw size or length for the two parts to achieve an effective combination. Excessive bending or pull-out forces, or improper use with poor combinations, may cause the middle interval to strip or the screw to break or pull out. An engagement of more than 90% is recommended, while less than 30% is considered dangerous. This study provides biomechanical insights into this novel two-part screw design and its important clinical implications.

Keywords: engagement assessment; engagement percentage; finite element analysis; sleeve-nut screw; stress concentration; two-part compression screw.

Figure 1

(A) Prototype of the novel two-part compression screw. (B) Schematic diagram of the two-part compression screw and implantation procedures.

Figure 2

(A) Three-dimensional models of the screw were converted into finite element models. (B) Schematic illustration of the applied loading conditions, including a 1000-N axial pull-out force and a 1-Nm bending moment applied at the screw head to simulate extreme mechanical loading scenarios.

Figure 3

Finite element models showing two stress concentration points (Points A and B).

Figure 4

Pull-out Load Simulation in Finite Element Analysis. Ten models representing different combinations of the screw parts (ranging from 10% to 100% of the engagement length, at 10% intervals) were simulated. Red arrows indicate the point of highest stress. The major stress concentration point shifted from Point A to Point B when the combination was less than 30%.

Figure 5

Von Mises Stress at Points A and B for Different Combinations in the Pull-out Load Simulation. The safer range of combination is suggested as at least 30%.

Figure 6

Bending load simulation in finite element analysis. Ten models representing different combinations of the two screw parts (ranging from 10% to 100% of the engagement length, at 10% intervals) were simulated. Red arrows indicate a clear transition between high and low stress, which corresponds to Point B.

Figure 7

Stress at points A and B for different combinations in the bending load simulation. The length of engagement did not influence the stress distribution for combinations between 10% and 90%. The optimal combination is suggested to be greater than 90% to minimize the potential bending force. Higher von Mises stress, exceeding 1000 MPa, occurred at both Point A and Point B when the engagement length was less than 90%.