Shape Memory Compression Screw for Orthopedic Fixation

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Solution Overview

Problem

Current orthopedic fixation devices fail to consistently bring bone fragments into close proximity and maintain compressive loads over time, leading to delayed healing due to rapid dissipation of compressive force as the bone relaxes and remodels around the threads.

Innovation Solution

A compression screw made from shape memory materials like Nitinol, featuring a proximal and distal threaded region with a differential pitch and a hollow central bridge that can be strained and elongated, providing additional therapeutic compression by foreshortening after implantation to maintain compressive force.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Force

If conventional screws with differential pitch are used to generate compression, then initial compressive load is created, but the compressive load dissipates rapidly as bone relaxes and remodels around the threads

Engineering Contradiction:

Improvecompressive loadVSAvoidduration of compressive load

Core Design Contradiction:

ForceVSDuration of action of stationary object

Solution Approach 1:

The hollow central bridge is pre-strained in a stretched condition before implantation. This preliminary action stores elastic energy in the bridge, which is then released after implantation to generate and maintain compressive force across the fracture site, counteracting the natural relaxation of bone around the threads

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The invention changes the physical state of the hollow central bridge from a relaxed to a strained (stretched) condition. By controlling the strain parameter of the shape memory material, the system can generate sustained compressive forces that adapt as the bone heals and remodels around the threads

Inventive Principle:

Principle #35Parameter changes

2Length of moving object

If headless bone screws with two separated threads are used, then fracture reduction is achieved, but gap reduction is limited by relatively small pitch differential and short thread length

Engineering Contradiction:

Improvethread lengthVSAvoidcompressive load generation

Core Design Contradiction:

Length of moving objectVSForce

Solution Approach 1:

The screw is segmented into three functional regions: a proximal threaded region, a distal threaded region, and a hollow central bridge connecting them. This segmentation allows each region to be optimized independently - the threaded regions for bone engagement and the elongated bridge for generating sustained compressive force through elastic recovery

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention adds a spatial dimension to the compression mechanism by introducing a hollow central bridge that extends the effective length of the compression-generating structure. This bridge provides an additional dimension for storing and releasing elastic energy, thereby increasing the overall compressive load generation capability beyond what short threaded regions alone could achieve

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution effectively maintains a prolonged compressive load across the fracture site, enhancing bone healing by reducing gaps and motion between fragments, with a reported 20% compression loss over 12 hours compared to 43-55% for conventional screws, achieving a steady state of 100 N or greater compressive force.

Implementation Method 1

The hollow central bridge can be strained and reversibly elongated, e.g., up to about 8% where the compression screw is formed from Nitinol. The contracting hollow central bridge can aid in fracture reduction and provide additional therapeutic compression to the bone fracture

Methodology Applied

Scientific EffectElastic energy storage and release: Elasticity

Implementation Method 2

The compression screw is designed to engage bone fragments and to generate compression between the bone fragments. The compression screw has a proximal threaded region and a distal threaded region. The thread pitch on the proximal threaded region is finer than the thread pitch on the distal threaded region

Methodology Applied

Scientific EffectShape memory effect: Shape Memory Alloy

Implementation Method 3

The thread pitch on the proximal threaded region is finer than the thread pitch on the distal threaded region (i.e., the thread pitch on the proximal threaded region has more threads per inch than the thread pitch on the distal threaded region). This pitch differential aids in reducing fractures and in generating compression between the bone fragments

Methodology Applied

Scientific EffectMechanical advantage through differential pitch: Mechanical Advantage

Data Source

PatentUS9861413B2Screws for generating and applying compression within a body

Publication Date: 2018.01.09 ARTHREX INC

AI summary

A compression screw system, said compression screw system comprising:a compression screw comprising a shaft, a screw thread formed on said shaft at a distal location, and a bone-engaging feature formed on said shaft at a proximal location, wherein at least a portion of said shaft disposed between said screw thread and said bone-engaging feature is capable of being stretched; anda holding element connectable to said compression screw for releasably holding said at least a portion of said shaft in a stretched condition.