Bone Screw Driver With Torsion-Stiff Bending for Off-Axis Tightening
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Solution Overview
Problem
Existing orthopedic implant kits face issues with misalignment between the axis of the bone fastening rod and the screw head, leading to parasitic lateral torques and strains that can cause chronic pain and improper healing, as well as implant loosening due to undesired lateral strains.
Innovation Solution
A device is provided that measures the angle between the axis of the bone fastening rod and the screw head, using a tubular element with protrusions or distance measurement sensors to ensure perpendicular alignment, and includes mechanisms like universal joints and biasing springs to decouple parasitic torques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If strong torques are applied to the fastening tool to tighten the bone screw, then the fastening strength is improved, but lateral strains and parasitic torques increase causing bone damage and implant loosening
Solution Approach 1:
A universal joint mechanism is introduced as an intermediary between the screw driver and the bone screw. This universal joint allows the screw driver axis to be oriented differently from the bone screw axis, acting as a mediator that transmits torque while accommodating angular misalignment. The universal joint decouples the driving axis from the loaded axis, preventing parasitic lateral torques from damaging the bone while still enabling effective tightening.
Solution Approach 2:
The system incorporates a dynamic alignment mechanism that allows the screw driver to be rotated relative to the bone screw axis during the tightening process. This dynamic adjustment enables the operator to align the screw driver with the bone screw before applying torque, and then lock this alignment in place during tightening, preventing lateral strains while maintaining fastening strength.
2Object-affected harmful factors
If the screw driver axis is aligned with the bone screw axis, then lateral strains are minimized, but the operator cannot compensate for poor surgical view and positioning errors
Solution Approach 1:
The universal joint serves as a mediator that decouples the alignment requirement from the tightening operation. The screw driver can be oriented independently from the bone screw axis, and the universal joint automatically accommodates this misalignment during torque application. This eliminates the need for precise pre-alignment while preventing lateral strains during the tightening process.
Solution Approach 2:
The system allows preliminary positioning of the screw driver at an arbitrary angle relative to the bone screw, and then provides a mechanism (such as a locking feature or friction fit) to maintain this position during tightening. This preliminary action eliminates the need for precise pre-alignment while ensuring that lateral strains are prevented during the actual tightening operation.
3Power
If a rigid connection is used between the screw driver and screw head, then torque transmission is efficient, but misalignment causes parasitic lateral torques and implant loosening
Solution Approach 1:
The universal joint is introduced as an intermediary between the screw driver and the bone screw connection. This intermediary maintains efficient torque transmission along the screw driver axis while accommodating angular misalignment between the screw driver and bone screw. The universal joint transmits only the axial component of torque effectively, blocking parasitic lateral torques from reaching the bone-implant interface, thus maintaining both power transmission efficiency and implant stability.
Solution Approach 2:
The system changes the mechanical parameters of the connection by introducing a universal joint that modifies the torque transmission characteristics. The universal joint allows rotational freedom in specific directions while maintaining torque transmission in the axial direction. This parameter change enables efficient power transmission while preventing harmful lateral torques, thereby improving implant reliability.
Data Source
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Figure 3A
AI summary
A bone screw tightening device including a screw driver, the screw driver having along its axis a device for providing for radial bendability and at a same time preserving torsional stiffness along an axis of longitudinal extension of the screw.