Surgical Nail Tapered Insert Locking Screw Clearance
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Solution Overview
Problem
Intramedullary nails face aiming inaccuracies during locking screw insertion, resulting in clearance between the nail and bone fragments, which is undesirable in certain clinical situations and can lead to loss of function due to loose plastic inserts in existing designs.
Innovation Solution
A surgical nail with a transverse bore and a dimensioned insert having projections for secure positioning, made from a material with lower tensile and compressive strength, allowing for optional angular stability and gradual absorption for dynamization, and featuring a tapered section for prevention of axial displacement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the outside diameter of the locking screw is underdimensioned to accommodate aiming inaccuracies, then the locking screw can enter and pass through the transverse bore despite aiming errors, but clearance or play is created between the locking screw and the transverse bore, allowing unwanted movement of bone fragments
Solution Approach 1:
A tapered insert is introduced as an intermediary element between the locking screw and the transverse bore. The insert has a tapered outer surface that matches the tapered inner surface of the transverse bore, creating an interference fit that eliminates clearance while still accommodating aiming inaccuracies. This mediator allows the locking screw to be inserted easily through the tapered path while preventing unwanted movement of bone fragments.
Solution Approach 2:
The geometry of the transverse bore is changed from a simple cylindrical shape to a tapered shape, and the insert utilizes this tapered geometry to create an interference fit. By changing the dimensional parameters of the bore and insert, the system achieves both easy insertion (through the tapered path) and reliable fixation (through the interference fit that eliminates clearance).
2Measurement precision
If plastic inserts are used in window-like openings to guide locking screws, then aiming accuracy is maintained, but the inserts can be easily pushed in or out, causing loss of function
Solution Approach 1:
The insert is given a tapered geometry with a tapered outer surface that corresponds to a tapered inner surface of the transverse bore. This parameter change creates an interference fit that prevents the insert from being pushed in or out, while still maintaining the guiding function for accurate locking screw insertion. The tapered shape provides mechanical stability that prevents functional loss.
Solution Approach 2:
The insert is made from a material with lower tensile and compressive strength than the nail body, such as a biocompatible polymer or metal alloy. This material selection allows the insert to be inserted easily while providing sufficient structural integrity to maintain aiming accuracy and prevent displacement. The composite material approach balances ease of insertion with functional stability.
Data Source
AI summary
Described herein is a surgical nail in the form of an intramedullary nail with a central axis made from a material M with a tensile strength Fz, compressive strength Fd, density ρ2 and modulus of elasticity E. The nail has a transverse bore extending transversely to the central axis and having a cross-sectional profile F and a transverse axis, the cross-sectional profile F having a maximum length a that runs in the direction of the central axis and a maximum width b that runs perpendicular to the length. An insert with the longitudinal axis, made from material m and having a modulus of elasticity e that is smaller than the modulus of elasticity E of material M, can be introduced into the transverse bore. The insert also has a length L that runs in the direction of its longitudinal axis and satisfies the condition L>0.2 D.