Semi-constrained Bone Screw with Repositionable Head
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Solution Overview
Problem
Bone screws used in spinal fusion procedures tend to 'back out' or loosen due to torsional and bending motions, leading to discomfort and potential separation from the vertebrae, as they fail to securely engage with the bone plate or spinal prosthetic.
Innovation Solution
A semi-constrained bone screw design featuring a tubular body with a helical thread, a repositionable head, and a shaft that couples the head to the tubular body, allowing for a gap and angular adjustment, along with a driving tool with a hexagonal configuration for secure rotation and engagement, which inhibits relative movement and provides an autogenic lock by deforming the bone plate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bone screw is used to attach a bone plate to vertebrae, then the bone plate can be secured to the bone, but the screw tends to back out or loosen due to torsional and bending motions of the spine
Solution Approach 1:
The bone screw is designed with a repositionable head that can rotate relative to the tubular body, allowing the screw to dynamically adapt to the torsional and bending motions of the spine. This dynamic capability enables the screw to maintain secure engagement with the bone plate while accommodating physiological movements without loosening
Solution Approach 2:
The screw configuration allows for angular adjustment of the head relative to the tubular body, changing the geometric parameters of the screw's engagement with the bone plate. This parameter change enables optimization of the connection stability under varying mechanical loads from spinal movements
2Adaptability or versatility
If the bone screw head is fixed relative to the tubular body, then the structure is simpler, but the screw cannot accommodate relative movement of vertebrae and may cause discomfort
Solution Approach 1:
The repositionable head design transforms the static bone screw into a dynamic device that can adapt its orientation. The head's ability to rotate and reposition relative to the tubular body provides the necessary adaptability to accommodate vertebrae movement while maintaining a relatively simple overall structure
Solution Approach 2:
The bone screw is divided into distinct functional segments: the tubular body for bone engagement, the shaft for structural support, and the repositionable head for adaptation. This segmentation allows each component to perform its specific function independently, achieving movement accommodation without excessive complexity
3Reliability
If the bone screw is designed to be fully constrained, then it provides stable fixation, but it cannot accommodate the natural torsional and bending motions of the spine
Solution Approach 1:
The bone screw implements semi-constraint through the repositionable head mechanism, which provides stable fixation in the axial direction while allowing rotational and angular adjustments to accommodate spinal motion. This dynamic semi-constrained design resolves the contradiction between secure fixation and motion accommodation
Solution Approach 2:
The screw structure separates the fixation function (tubular body with helical thread) from the adaptation function (repositionable head), allowing each to optimize its performance independently while working together to achieve both reliable fixation and motion accommodation
Data Source
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AI summary
A bone screw for attaching a spinal prosthetic or bone plate to bone includes a tubular body having open proximal and distal ends. The open proximal and distal ends define a passageway therethrough. A shaft is insertable into the passageway and includes a distal tip that extends beyond the open distal end. The shaft is insertable through a lumen of a head of the bone screw. A gap is defined between a distal surface of the head and a proximal end of the tubular body. The shaft is rotationally fixed with the tubular body and the head. The head is repositionable with respect to the shaft such that an acute angle is defined between the head and a longitudinal axis of the shaft.