Composite Prosthesis with Hard Inner and Soft Outer Sections

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Solution Overview

Problem

Existing motion restoring intervertebral devices and artificial joint replacements face issues with wear debris due to point contact and nonuniform stress distribution, leading to bone absorption and implant loosening, as they rely on hard materials for articulation and rigid plates for bone attachment, which causes stress shielding and uneven load distribution.

Innovation Solution

A motion restoring prosthesis with two components, where the inner section is made of a hard, stiff material for articulation and the outer section is a softer, less stiff biocompatible polymer like PEEK or CFRPEEK for bone attachment, with a thin coating of titanium or calcium phosphate to enhance bone integration and even stress distribution, preventing motion between sections and reducing wear.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If hard material is used for articulating surfaces, then wear is reduced and life expectancy increases, but stress shielding occurs and bone absorption leads to implant loosening

Engineering Contradiction:

Improvewear resistanceVSAvoidstress shielding

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The prosthesis is divided into two distinct segments: an inner hard articulating component and an outer soft bone-contact component. This segmentation allows each material to perform its optimal function - the hard inner layer provides wear resistance for articulation while the soft outer layer distributes stress evenly across the bone interface, preventing stress shielding and bone absorption.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention employs a composite structure combining hard material (ceramic or cobalt chromium) for the inner articulating surface with soft material (biocompatible polymer like PEEK) for the outer bone-contact surface. This composite design resolves the contradiction by integrating materials with complementary properties - the hard core provides durability while the soft shell provides stress distribution and bone compatibility.

Inventive Principle:

Principle #40Composite materials

2Object-affected harmful factors

If softer material is used for bone attachment, then stress shielding is reduced, but bone attachment is compromised and wear increases

Engineering Contradiction:

Improvestress shieldingVSAvoidbone attachment

Core Design Contradiction:

Object-affected harmful factorsVSReliability

Solution Approach 1:

Different regions of the prosthesis have different material properties optimized for their specific functions. The inner articulating region uses hard material for wear resistance, while the outer bone-contact region uses soft material for stress distribution and bone attachment. This local differentiation of material quality allows the prosthesis to simultaneously achieve bone attachment and reduce stress shielding without compromising either function.

Inventive Principle:

Principle #3Local quality

3Strength

If rigid plates are used for bone attachment, then structural strength is improved, but nonuniform load distribution causes hard spots and soft spots leading to bone absorption

Engineering Contradiction:

Improvestructural strengthVSAvoidload distribution uniformity

Core Design Contradiction:

StrengthVSStress or pressure

Solution Approach 1:

The invention changes the material parameter of stiffness from rigid to compliant for the outer bone-contact layer. By using soft material with appropriate elasticity, the prosthesis can conform to the bone surface and distribute loads uniformly across the interface, eliminating the hard spots and soft spots that occur with rigid plates, while maintaining sufficient structural strength through the inner hard articulating layer.

Inventive Principle:

Principle #35Parameter changes

4Device complexity

If point contact articulation is used, then device complexity is reduced, but wear debris increases and implant life decreases

Engineering Contradiction:

Improvearticulation structureVSAvoidimplant life

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The use of hard composite material (ceramic or cobalt chromium) for the inner articulating layer provides a durable, low-wear surface that maintains implant life even with simplified point contact or ball-and-socket articulation geometries. The hard material's inherent wear resistance compensates for the simplicity of the articulation structure, reducing wear debris without requiring complex multi-component designs.

Inventive Principle:

Principle #40Composite materials

Data Source

PatentUS7799083B2Prosthesis for restoring motion in an appendage or spinal joint and an intervertebral spacer

Publication Date: 2010.09.21 SEASPINE ORTHOPEDICS CORP

AI summary

A motion restoring prosthesis to be interposed between the ends of two bones adjoining a mammalian appendage or spinal joint is formed of two components with the components having inner cooperating articulating surfaces and outer bone engaging surfaces. At least one of the components has an inner section made of relatively hard, stiff material defining one or the articulating surfaces and an outer section made of a softer material defining the bone engaging surface. The softer material having a hardness/stiffness characteristic compatible with the bone to reduce stress shielding and more evenly distribute the forces from the articulating surfaces to the associate bone interface.