Polyaxial Bone Anchoring Device with Frictional Lugs
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Solution Overview
Problem
Current polyaxial bone anchoring devices face challenges in providing improved handling during surgery, ensuring safe fixation, and being simple to manufacture, while allowing for adjustable angular positioning of the anchoring element relative to the receiving part without free pivoting, and accommodating tolerances between components.
Innovation Solution
A polyaxial bone anchoring device with a pressure element that exerts a predefined preload on the head of the anchoring element, allowing temporary clamping and adjustment of the receiving part's angular position relative to the bone anchoring element, using a pressure element with lugs that create frictional forces to maintain the head in a desired position before final locking, facilitating easier alignment and insertion of a stabilization rod.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the anchoring element is allowed to pivot freely in the receiving part, then ease of positioning is improved, but stability of the connection is worsened
Solution Approach 1:
The device transitions from a static locked state to a dynamic adjustable state. The pressure element can be moved axially to release the lugs from the head, allowing the anchoring element to pivot freely within the receiving part. This dynamic mechanism enables the connection to switch between stable (locked) and easily repositionable (unlocked) states, resolving the contradiction between stability and ease of positioning.
Solution Approach 2:
The friction force between the lugs and the head is changed by varying the axial position of the pressure element. When the pressure element is in the engaged position, high friction maintains the anchoring element at a fixed angular position. When the pressure element is moved axially to the disengaged position, friction is reduced, allowing free pivoting. This parameter change enables transition between stable and easily repositionable states.
2Stability of the object's composition
If the head is locked in a fixed angular position, then stability is improved, but adjustability is worsened
Solution Approach 1:
The locking mechanism is made dynamic through the movable pressure element. The system can switch between a locked state (providing stability) and an unlocked state (providing adjustability). The pressure element's axial movement allows the lugs to engage or disengage from the head, enabling the connection to transition between fixed and adjustable angular positions as needed during surgery.
Solution Approach 2:
The pressure element is pre-configured with lugs that can engage the head to provide friction-based holding. This preliminary engagement mechanism allows the anchoring element to be temporarily held at any angular position during adjustment, providing both stability during positioning and ease of adjustment when needed, before final locking occurs.
3Stability of the object's composition
If friction force is increased to prevent free pivoting, then stability is improved, but ease of adjustment is worsened
Solution Approach 1:
The friction force parameter is dynamically changed by moving the pressure element axially. When adjustment is needed, the pressure element is moved to reduce friction, allowing easy pivoting. When stability is needed, the pressure element is moved to increase friction, preventing free pivoting. This parameter control resolves the contradiction between stability and ease of adjustment.
Solution Approach 2:
The pressure element acts as an intermediary between the locking mechanism and the anchoring element. It controls the friction force applied to the head through its axial position, mediating between the need for stability (high friction) and the need for ease of adjustment (low friction). This intermediary mechanism allows smooth transition between states without requiring complete loosening or tightening.
4Measurement precision
If precise angular positioning is enabled, then alignment accuracy is improved, but device complexity is worsened
Solution Approach 1:
The positioning function is segmented between the receiving part (providing the seat and defining angular positions), the anchoring element (with head and shank), and the pressure element (with lugs for frictional engagement). This segmentation allows each component to be relatively simple while achieving precise angular positioning through their interaction. The receiving part can be a simple cylindrical structure with a conical seat, avoiding complex positioning mechanisms.
Solution Approach 2:
The anchoring element's spherical head self-aligns within the conical seat of the receiving part, providing inherent angular positioning without requiring complex external positioning mechanisms. The pressure element with lugs provides friction-based holding that maintains the self-aligned position. This self-service mechanism achieves precise positioning through the geometry of the components rather than complex active control systems.
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 precise and adjustable angular positioning of the receiving part relative to the bone anchoring element, allowing for easier alignment and insertion of a stabilization rod, while maintaining the device in a stable yet adjustable state until final locking, and simplifies manufacturing by accommodating tolerances between components.
Implementation Method 1
the head is clamped by friction between the head and the lugs, causing the anchoring element to be maintained at a desired temporary angular position
Data Source
AI summary
A polyaxial bone anchoring device includes an anchoring element having a shank and a head having an outer surface portion shaped as a segment of a sphere, a receiving part having a seat portion for receiving the head, and a pressure element having a head contacting surface portion having at least two circumferentially distinct projections and defining a recess for holding the head, the recess having a first region configured to receive the head and a second region defined by at least part of one of the projections and having an undersize compared to the shape of the head, such that when the head is held in the recess, the projections extend over a portion of the head with the largest outer diameter, and the head contacting surface portion corresponding to the second region is expanded from a neutral position to clamp the head by friction.