Patient-Specific Mandibular Implants to Reduce Stress Shielding
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Solution Overview
Problem
Traditional bone implants and fixation hardware in musculoskeletal reconstructive surgery often cause stress shielding and stress concentrations, leading to bone resorption, damage, and implant failure due to mismatched material properties and stiffness, particularly in cases like mandibular segmental defects.
Innovation Solution
Patient-specific implants and fixation devices fabricated using additive manufacturing, with tailored geometric and mechanical properties to match the patient's anatomy, incorporating materials like magnesium or titanium alloys, cured polymers, and ceramics, and utilizing nitinol for stress-strain trajectory restoration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional metal implants with high stiffness are used, then immediate structural support and function restoration are achieved, but stress shielding and stress concentrations occur leading to bone resorption and implant failure over time
Solution Approach 1:
The implant material properties are changed by transitioning from traditional high-stiffness metals to lower-stiffness materials such as polymers, ceramics, or composite materials. This parameter change allows the implant to better match the mechanical properties of natural bone, reducing stress shielding while maintaining sufficient structural support for immediate function restoration and long-term reliability
Solution Approach 2:
Composite materials combining multiple components with different mechanical properties are used to create an implant that simultaneously provides structural support and matches bone mechanics. The composite structure allows optimization of both strength for immediate support and elasticity for long-term stress distribution, preventing bone resorption and implant failure
2Reliability
If patient's own bone is grafted to defect site, then bone supply is utilized, but the supply is very limited and removal from patient results in tissue die-back and pain at harvest site
Solution Approach 1:
An intermediary material or scaffold is introduced that can support bone growth and regeneration without requiring harvest from the patient's own body. This intermediary structure provides a framework for new bone formation while avoiding the harmful effects of donor site morbidity, tissue die-back, and pain associated with autografting
Solution Approach 2:
A temporary, resorbable implant or scaffold is used that provides structural support during the bone healing process and then degrades naturally. This disposable-like approach eliminates the need for permanent implants and avoids the complications of bone harvesting, as the temporary structure is replaced by newly formed bone tissue
3Productivity
If traditional fixation hardware with localized stiffness is used, then immediate function restoration is achieved, but stress shielding reduces bone load leading to osteopenia and osteoporosis over time
Solution Approach 1:
The implant is designed with dynamic properties that allow it to adapt its mechanical behavior over time. The material or structure can transition from providing rigid support immediately after surgery to gradually becoming more flexible as bone heals, thereby maintaining stress distribution that prevents bone density loss while ensuring rapid function restoration
Solution Approach 2:
The mechanical parameters of the implant are specifically adjusted to match or slightly exceed the stiffness of natural bone in the defect region. This parameter optimization ensures that the implant provides sufficient support for immediate function restoration while allowing adequate stress transmission to stimulate bone remodeling and prevent osteopenia and osteoporosis
Data Source
AI summary
A device used in conjunction with fixation hardware to provide a two-stage process to address the competing needs of immobilization and re-establishment of normal stress-strain trajectories in grafted bone. A method of determining a patient-specific stress/strain pattern that utilizes a model based on 3D CT data of the relevant structures and cross-sectional data of the three major chewing muscles. The forces on each of the chewing muscles are determined based on the model using predetermined bite forces such that a stiffness of cortical bone in the patient's mandible is determined. Based on the stiffness data, suitable implantation hardware can be designed for the patient by adjusting external topological and internal porous geometries that reduce the stiffness of biocompatible metals to thereby restore normal bite forces of the patient. A method of 3D printing nitinol to create a patient-specific device to facilitate the establishment of a normal stress-strain trajectory in grafted bone.