Polyaxial Bone Plate Locking Cap Mechanism
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Solution Overview
Problem
Existing bone plate systems fail to provide reliable polyaxial insertion and fixation of bone anchors, leading to potential loosening and instability, especially in heavy loading conditions like the lumbar or sacral regions, which can compromise bone fusion and pose risks to surrounding tissues and organs.
Innovation Solution
A bone plate system with a through bore and a locking cap mechanism that allows for polyaxial insertion of bone anchors, featuring a locking fastener that shifts the locking cap into tight engagement with the bore annular surface to secure the anchor relative to the plate, preventing backout and accommodating various insertion angles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a bone plate system allows polyaxial insertion of bone anchors, then the ease of operation and adaptability are improved, but the reliability and stability of fixation deteriorate due to potential loosening under heavy loading
Solution Approach 1:
The system employs a dynamic locking mechanism where the bone anchor head can assume multiple angular positions during insertion (polyaxial flexibility) but becomes rigidly fixed once locked. The spherical bearing surface and set screw mechanism allow the anchor to dynamically adjust its angle during insertion while maintaining stable fixation after locking, resolving the contradiction between operational flexibility and fixation reliability.
Solution Approach 2:
The bone anchor is segmented into distinct functional components: a spherical head portion for angular adjustment, a bearing surface for rotation, and a threaded shaft for secure fixation. This segmentation allows the head to independently rotate and assume various angles relative to the plate while the threaded portion provides reliable anchoring, thereby achieving both polyaxial insertion capability and stable fixation.
2Reliability
If a rigid fixation mechanism is used to prevent loosening, then the reliability is improved, but the ease of operation deteriorates due to limited insertion angle flexibility
Solution Approach 1:
The system transitions from a static rigid connection to a dynamic locking mechanism. During insertion, the bone anchor head can dynamically rotate around the spherical bearing surface to achieve the desired angle. Once positioned, the set screw locks the mechanism, transforming it into a rigid fixed connection. This dynamic-to-rigid transition resolves the contradiction between operational flexibility and fixation stability.
Solution Approach 2:
The spherical bearing surface acts as an intermediary between the bone anchor head and the plate bore. It mediates the transition from flexible angular adjustment to rigid fixation by allowing rotation during insertion while maintaining a stable locked position afterward. This intermediary mechanism enables both polyaxial insertion and reliable fixation without compromising either property.
3Reliability
If the bone anchor is securely locked to the plate, then the reliability is improved, but the device complexity increases due to additional locking components
Solution Approach 1:
The locking function is extracted as a separate, simple set screw component rather than being integrated into the bone anchor or plate structure. This separate locking component interacts with the spherical bearing surface to provide secure fixation while keeping each individual component relatively simple. The extracted locking mechanism adds reliability without significantly increasing overall device complexity.
Solution Approach 2:
The spherical bearing surface and set screw mechanism are designed to be self-locking through friction and geometric constraints. Once the set screw is tightened, the spherical bearing naturally maintains the locked position through its geometry and contact surfaces, reducing the need for additional complex locking features or active maintenance mechanisms.
4Adaptability or versatility
If a polyaxial insertion mechanism is implemented, then the adaptability is improved, but the manufacturing precision requirements worsen due to tolerances in angular positioning
Solution Approach 1:
The use of a spherical bearing surface provides inherent geometric tolerance for angular positioning. The spherical geometry naturally accommodates variations in insertion angles and positions without requiring tight manufacturing tolerances. The curved surface allows the bone anchor head to rotate and settle into the optimal position, compensating for manufacturing variations and simplifying precision requirements while maintaining adaptability.
Solution Approach 2:
The system changes the geometric parameters of the bearing surface from a fixed rigid interface to a spherical curved interface. This parameter change allows for a range of angular positions (polyaxial flexibility) while the spherical geometry inherently absorbs manufacturing tolerances. The curved surface provides a tolerance buffer that maintains functional performance even with variations in manufacturing precision.
Data Source
AI summary
A bone plate system is provided that includes bone plates and bone anchor assemblies for being inserted into bone plate bores to secure the bone plates to one or more bones. In one aspect, the bone plate system includes a bone anchor assembly having a locking cap and a locking fastener connected to a head portion of a bone anchor. The bone anchor assembly is driven into a bone plate bore before the locking fastener is shifted to expand the locking cap and fix the bone anchor assembly within the bore. In another aspect, the bone plate system includes a bone anchor assembly having a locking cap that deflects radially inward due to contact with the bone plate at large bone anchor assembly insertion angles. In addition, a bone anchor assembly having a preassembled condition with a locking cap resisting back out of a locking fastener.