Variable Angle Bone Fixation via Segmented Insert
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Solution Overview
Problem
Existing orthopedic bone fixation systems face limitations in achieving rigid locking between screws and plates, particularly when screws need to be angled, due to reduced surface contact and mechanical advantage in current mechanical designs, which often involve weaker materials and reduced surface contact.
Innovation Solution
A variable angle locking system that uses a machined threaded interface between the bone screw and plate, with an insert that allows the screw to be positioned at various angles relative to the plate hole axis and circumference, providing a strong and rigid fixation by utilizing locking tabs and expansion relief grooves for enhanced frictional contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If variable angle locking is achieved using self-tapping or frictional crush-lock features, then the screw can be positioned at various angles, but the strength and rigidity of the interface between screw head and plate is reduced due to reduced surface contact area and mechanical advantage
Solution Approach 1:
The system divides the plate hole into two functional components: a cylindrical portion containing machined threads for strong screw engagement, and a conical portion providing variable angle positioning. This segmentation allows each portion to optimize its specific function while working together to achieve both strength and versatility.
Solution Approach 2:
The conical portion acts as an intermediary element between the screw head and the cylindrical threaded portion. It enables angular adjustment and positioning while transferring loads to the strong machined thread interface, thereby mediating between the need for variable angles and the requirement for strong fixation.
2Adaptability or versatility
If variable angle locking is achieved using cross-threading or self-tapping designs, then the screw can be inserted at offset angles, but the mechanical advantage and surface contact area are reduced
Solution Approach 1:
The plate hole is segmented into a cylindrical threaded portion for secure engagement and a conical positioning portion for angular adjustment. This division simplifies the overall mechanism by separating the functions of positioning and fixation, avoiding complex cross-threading or self-tapping designs.
3Strength
If machined threads are used in both screw and plate for rigid fixation, then the interface strength is maximized, but the ability to angle the screw offset from the central axis is lost
Solution Approach 1:
The fixation system is segmented into two zones: a conical zone that provides angular positioning flexibility and a cylindrical threaded zone that provides strong rigid fixation. This allows the screw to be angled during insertion while maintaining strong threaded engagement in the final position.
Solution Approach 2:
The conical portion introduces an angular dimension to the otherwise axial threaded connection. By tapering the upper portion at a specific angle (e.g., 15 degrees), the system enables screw insertion at offset angles while the lower cylindrical portion maintains strong axial threaded fixation.
4Ease of manufacture
If a single collar is used for self-tapping in the plate hole, then the design is simple, but the material must be weaker than the screw material, reducing overall strength
Solution Approach 1:
Instead of using a single collar design, the system segments the plate hole into a conical upper portion for positioning and a cylindrical lower portion with machined threads for strong engagement. This allows both components to be made from equally strong materials, eliminating the need for a weaker collar material.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables secure and rigid fixation of screws at various angles without compromising strength, allowing for precise orientation and alignment, thereby improving the stability and effectiveness of bone fixation in orthopedic applications.
Implementation Method 1
Mating of machined screw threads on the head of the screw and in the plate is preferred for rigid screw/plate fixation
Implementation Method 2
threaded engagement between the screw and the insert hole offsets a longitudinal axis of the screw from the insert central axis
Implementation Method 3
locking tabs and expansion relief grooves for enhanced frictional contact
Data Source
AI summary
An orthopedic fixation assembly for connecting to a bone includes a fixation plate having a hole, an insert securable to the hole and a screw securable to the insert. The fixation plate includes a shoulder recessed from its upper surface and an aperture. The insert includes a base portion and a flange extending laterally from an upper end of the base portion. The insert defines an insert central axis extending in a direction generally perpendicular to the flange. The screw has a screw head and the base portion has an insert hole with machined internal threads for engaging the screw. The insert hole defines an insert hole central axis extending in a direction at an offset angle relative to the insert central axis. The aperture of the fixation plate is dimensioned to receive the base portion of the insert and the recessed shoulder is dimensioned to receive the flange.