Spinal Tethering Cord Clamping Mechanism for Stress Distribution
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Solution Overview
Problem
Current spinal tethering systems face issues such as screw migration, cord/tether failure, and difficulties in implantation, which affect the stability and effectiveness of spinal deformity correction procedures.
Innovation Solution
The development of improved vertebral implants, including bone screws and anchors with enhanced designs, such as dynamic cord slip mechanisms and poly-axial heads, to reduce stress on the cord, prevent overcorrection, and increase construct strength, along with cord clamping mechanisms that distribute tension and reduce implantation time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional bone screws and anchors are used in spinal tethering systems, then the implantation procedure is simpler, but screw migration occurs and construct strength is reduced
Solution Approach 1:
The implant system is divided into separate components: an anchor that is first implanted into the vertebral body, and a bone screw that is subsequently inserted through the anchor. This segmentation allows each component to be optimized for its specific function - the anchor provides stable anchorage while the screw provides cord attachment, thereby preventing screw migration without requiring a single complex monolithic implant.
Solution Approach 2:
The anchor is implanted into the vertebral body before the bone screw is inserted. This preliminary action creates a pre-prepared receptacle that guides and secures the bone screw in the correct position, preventing migration. The anchor serves as a preliminary structure that facilitates the subsequent screw implantation and ensures proper positioning.
2Reliability
If cord tension is concentrated at single points, then the cord clamping mechanism is simpler, but cord failure risk increases
Solution Approach 1:
The cord clamping function is segmented into multiple cord contact surfaces distributed around the bone screw. Instead of a single clamping point, the cord is contacted at multiple locations (anterior, posterior, and lateral surfaces), which distributes the tensile load across multiple contact points. This segmentation of the clamping function reduces stress concentration on the cord while maintaining effective tension control.
Solution Approach 2:
The cord clamping mechanism transitions from a single-point (zero-dimensional) or line-contact (one-dimensional) approach to a multi-surface (two-dimensional) contact system. The bone screw and anchor create multiple contact surfaces that engage the cord in different spatial dimensions, distributing forces across a larger area and reducing stress concentration.
3Strength
If rigid cord clamping is used, then construct strength is increased, but cord stress and potential failure increase
Solution Approach 1:
The cord itself acts as a flexible element that is clamped by the rigid bone screw and anchor assembly. The flexible nature of the cord allows it to distribute stresses along its length rather than concentrating them at single clamping points. The combination of rigid clamping structures and flexible cord creates a system that maintains construct strength while reducing harmful stress concentrations on the cord.
Solution Approach 2:
The system allows for dynamic adjustment of cord tension through the polyaxial head capability, which enables the bone screw to be inserted at various angles relative to the anchor. This dynamic positioning capability allows optimization of cord tension distribution, maintaining construct strength while minimizing cord stress through proper geometric alignment.
4Adaptability or versatility
If polyaxial heads are used in bone screws, then adaptability and positioning flexibility are improved, but device complexity increases
Solution Approach 1:
The polyaxial positioning capability is segmented into a separate polyaxial head component that attaches to the bone screw. This head portion provides the angular adjustment capability while the screw shaft remains relatively simple. The segmentation of the polyaxial function into a distinct component adds adaptability without significantly complicating the overall bone screw structure.
5Strength
If traditional cord clamping methods are used, then implantation time is reduced, but construct strength and tension distribution are insufficient
Solution Approach 1:
The cord clamping function is merged with the bone screw structure itself. The bone screw includes integrated cord contact surfaces and clamping mechanisms as part of its basic structure, eliminating the need for separate clamping devices. This merging of functions maintains implantation efficiency while achieving superior construct strength and tension distribution through the multi-surface cord engagement.
Data Source
AI summary
Various implants and surgical techniques for dynamic spinal tethering systems are discussed. In an example, a spinal tethering system can comprise a flexible elongate spinal tethering cord and a plurality of vertebral implants connecting the cord across at least four spinal levels. In this example, each vertebral implant of the plurality of vertebral implants can include a dynamic head coupling each vertebral implant to the cord. The dynamic head of each vertebral implant can be configured to share cord tension across multiple spinal levels by releasing a first tension generated at a first level between two vertebral implants of the plurality of vertebral implants to generate a second tension across two spinal levels between three vertebral implants of the plurality of vertebral implants, wherein the second tension is lower than the first tension.