Bone Screw with Segmented Thread Diameters for Vertebral Fixation
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Solution Overview
Problem
Conventional bone screws face challenges in securing fixation to bones with varying dimensions and densities, particularly when inserted through the narrow pedicle, as the cortical bone provides a secure connection but the cancellous bone requires a larger thread diameter for stability, compromising the fixation in vertebrae.
Innovation Solution
A bone screw design featuring a lead portion and a tail portion with approximately constant diameters, where the tail portion has a greater thread diameter and a multi-start thread on the lead portion optimized for cortical bone, allowing secure fixation in both regions by varying the thread pitch and aspect ratio, and being constructed as a single body to prevent rotational separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bone screw is inserted through the narrow pedicle into the vertebral body, then the screw can be fixed to the vertebra, but the thread diameter is restricted by the pedicle width which compromises the fixation security in cancellous bone tissue
Solution Approach 1:
The bone screw is divided into two distinct portions: a lead portion with a first thread diameter optimized for cortical bone fixation in the pedicle, and a tail portion with a second thread diameter optimized for cancellous bone fixation in the vertebral body. This segmentation allows each portion to have independently optimized thread dimensions suitable for the specific bone tissue it engages.
Solution Approach 2:
Different regions of the bone screw are given different thread properties to match the local characteristics of the bone tissue. The lead portion has a smaller thread diameter suitable for the narrow pedicle corridor, while the tail portion has a larger thread diameter to provide secure fixation in the cancellous bone of the vertebral body.
2Reliability
If the thread diameter is increased to improve fixation in cancellous bone, then the fixation security improves, but the screw cannot be inserted through the narrow pedicle
Solution Approach 1:
The screw is segmented into lead and tail portions with different thread diameters, allowing the lead portion to fit through the narrow pedicle while the tail portion provides large-diameter fixation in the vertebral body.
Solution Approach 2:
The solution transitions from a single-dimension (uniform diameter) approach to a two-dimension (varying diameter along length) approach, enabling the screw to navigate the narrow pedicle corridor while providing adequate fixation diameter in the vertebral body.
3Device complexity
If a conventional uniform thread screw is used, then the screw structure is simple, but the fixation is compromised in bones with varying dimensions and densities
Solution Approach 1:
The screw features local quality variations with different thread diameters and pitch characteristics in different regions, allowing optimization for both cortical and cancellous bone fixation while maintaining a relatively simple overall screw structure.
Solution Approach 2:
The thread parameters (diameter, pitch, lead) are changed along the length of the screw to match the varying bone characteristics. The lead portion has parameters optimized for cortical bone, while the tail portion has parameters optimized for cancellous bone.
Data Source
AI summary
A bone screw (10) having a lead portion (12) and a tail portion (14), each comprising a root and a thread (22, 24) (having a thread lead) formed on the root. The thread on each of the lead and tail portions has an approximately constant diameter along a significant portion of its length. The diameter of the thread on the tail portion is greater than that of the thread on the lead portion, and in which the thread lead of the thread on the lead portion is equal to the thread lead of the thread on the tail portion. The screw can be used to achieve fixation to a vertebra with an anterior approach.