Two-Step Locking Screw Assembly for Spinal Stabilization
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Solution Overview
Problem
Surgeons performing spinal stabilization procedures face challenges with polyaxial screws, as their maneuverability interferes with applying compression, distraction, or reduction, while monoaxial screws lack the positional adjustment benefits of polyaxial screws.
Innovation Solution
A two-step locking screw assembly that includes a receiving member, a polyaxial screw with an enlarged head and shank, an insert, an outer fastener with ramped flanges, and an inner fastener, allowing for controlled immobilization of the screw head and fixation rod, thereby restricting polyaxial motion and enabling precise manipulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If polyaxial screws are used to provide maneuverability and positional adjustment, then the ability to position the stabilization construct in multiple directions is improved, but the freedom of motion interferes with applying compression, distraction, or reduction
Solution Approach 1:
The locking mechanism is divided into two distinct steps: first locking the polyaxial screw head to the receiver body, then separately locking the fixation rod. This segmentation allows the screw to maintain polyaxial adjustability during positioning, then be securely locked in place, and finally allows independent rod manipulation and locking without interference from screw motion
Solution Approach 2:
The system transitions from a dynamic state (polyaxial screw with freedom of motion for positioning) to a static locked state (screw head secured to receiver body). The two-step locking mechanism enables this transition, allowing the screw to be mobile during the positioning phase, then securely fixed when the locking mechanism is engaged, resolving the conflict between maneuverability and operational control
2Ease of operation
If monoaxial screws are used to avoid maneuverability problems, then the ability to apply compression and distraction is improved, but the positional adjustment benefits of polyaxial screws are lost
Solution Approach 1:
The polyaxial screw is first inserted and positioned in the desired orientation before the locking mechanism is engaged. This preliminary positioning action allows the surgeon to take advantage of the polyaxial design for optimal screw placement, then the two-step locking mechanism secures the screw in that position, enabling subsequent manipulation and locking of the fixation rod without compromising the initially achieved positional adjustment
3Ease of operation
If a locking mechanism is added to control polyaxial screw motion, then the interference with compression and distraction is reduced, but the device complexity increases
Solution Approach 1:
The locking mechanism components are nested within the receiver body structure. The first locking mechanism integrates the screw head retention features into the receiver body, and the second locking mechanism integrates the rod locking features into the same structure. This nesting approach provides the necessary locking functionality while minimizing the addition of external components, thus reducing the overall device complexity increase
Data Source
AI summary
A two step locking screw assembly includes a receiving member for receiving a fixation member and an anchor having an enlarged portion and a shank. The enlarged portion of the anchor may be seated in the receiving member. An insert may also extend inside the receiving member. An outer fastener may be positioned in the receiving member. The outer fastener may include a central opening and a pair of ramped flanges. An inner fastener may be disposed inside the central opening of the outer fastener.