Bone Screw Receiving Part with Flattened Leg Edges
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Solution Overview
Problem
Polyaxial and monoaxial bone screws experience diagonal splaying of legs during final tightening due to torque, leading to instability and potential loosening of the fixation, which existing solutions like outer nuts or specific thread shapes fail to fully address without increasing the size of the bone screw.
Innovation Solution
A receiving part with flat surfaces and enhanced wall thickness at the lateral edges, forming a stiffening structure that prevents or significantly reduces diagonal splaying by distributing torque uniformly and maintaining a compact design, allowing for the use of larger diameter rods without external securing devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If outer nuts or rings are used to prevent leg splaying, then leg stability is improved, but the size of the bone screw is enlarged
Solution Approach 1:
The patent applies local quality by providing flattened surfaces at specific locations on the legs (outer sides of the legs in the transverse direction) rather than uniformly thickening the entire structure. This localized flattening prevents diagonal splaying at the critical areas where torque acts on the legs during final tightening, while maintaining a compact overall bone screw size without requiring outer nuts or rings.
2Force
If specific thread shapes (saw tooth, reverse angle) are used to eliminate radial forces, then radial force elimination is improved, but manufacturing complexity increases
Solution Approach 1:
The patent changes the geometric parameters of the thread by using a flat thread design where the upper and lower flanks of each thread make an angle of 90 degrees with the screw axis. This parameter change eliminates the outwardly directed radial forces acting on the legs during securing screw insertion, while simultaneously simplifying manufacturing compared to more complex thread shapes like saw tooth or reverse angle threads.
3Volume of moving object
If flattened surfaces are provided at the sides of the receiving part, then bone screw size in longitudinal direction is minimized, but leg splaying prevention is insufficient
Solution Approach 1:
The patent applies asymmetry by providing flattened surfaces only at specific locations (outer sides of the legs in the transverse direction) rather than making the entire receiving part symmetric and compact. This asymmetric flattening at critical locations effectively prevents diagonal splaying caused by torque during final tightening, while maintaining minimal increase in the overall longitudinal size of the bone screw.
4Volume of moving object
If the receiving part is made compact to reduce device size, then device compactness is improved, but resistance to torque-induced deformation is reduced
Solution Approach 1:
The patent resolves this contradiction by applying local quality through strategic placement of flattened surfaces at the outer sides of the legs where torque-induced diagonal splaying occurs. This localized structural modification provides enhanced torque resistance precisely where needed, while maintaining the overall compact design of the receiving part without requiring uniform thickening that would increase device size.
Data Source
AI summary
A receiving part for connecting a shank of a bone anchoring element to a rod. The receiving part has a first end, a second end and a recess having a substantially U-shaped cross section. The recess extends from the first end in a direction of the second end forming two legs open at the first end. The recess defines a channel to receive the rod. The channel has a longitudinal axis. The legs have an inner thread to receive a securing element having an outer thread cooperating with the inner thread. The legs are provided with stiffening structures. In one embodiment, the stiffening structures are formed by the wall thickness of the outer edges of the legs being larger than the wall thickness of the legs at a location closest to the central axis of the receiving part.