Pedicle Screw Bone Alignment Device
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Solution Overview
Problem
Pedicle screw systems with polyaxial screws lack the ability to introduce force for correcting vertebral alignment due to their design, which restricts the application of corrective forces and prevents techniques like segmental derotation, whereas monoaxial screws allow force application but are difficult to insert due to their immobile screw head.
Innovation Solution
A pedicle screw system with a ball joint-mounted screw head and a bone alignment device that limits the screw head's rotational freedom to one or two degrees, allowing direct force transmission and enabling the screw to function similarly to a monoaxial screw while maintaining the insertion ease of polyaxial screws.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a polyaxial screw is used with a ball joint-mounted screw head, then the insertion of connecting elements is facilitated and the screw head can be oriented in any way relative to the screw shank, but force cannot be introduced for corrective maneuvers and alignment correction is not possible
Solution Approach 1:
The screw head is designed to be dynamically adjustable during insertion (polyaxial freedom) but becomes kinematically constrained during alignment correction through the bone alignment device, which limits rotation to one degree of freedom. This dynamic transition resolves the contradiction between ease of insertion and capability for corrective force application.
Solution Approach 2:
The bone alignment device acts as an intermediary between the surgeon's corrective forces and the polyaxial screw. It couples to the screw head and transmits forces for alignment correction while the screw head remains polyaxial, enabling force application without requiring the screw itself to be monoaxial.
2Ease of manufacture
If a monoaxial screw is used with an immobile screw head, then force can be introduced for alignment correction, but the screw head cannot be oriented in any way relative to the screw shank making insertion difficult
Solution Approach 1:
The system transitions from a static monoaxial constraint to a dynamic polyaxial joint that can be temporarily constrained. The bone alignment device provides temporary monoaxial constraint during alignment correction, then can be removed to restore full polyaxial freedom for connecting element insertion, resolving the contradiction between force application and insertion ease.
3Adaptability or versatility
If the screw head has three degrees of freedom of rotation in a ball joint, then the screw head can be oriented in any way relative to the screw shank, but the ability to directly transmit force for alignment correction is lost
Solution Approach 1:
The bone alignment device dynamically reduces the screw head's three degrees of freedom to one degree of freedom during alignment correction, enabling direct force transmission. After correction, the device can be removed to restore full orientational freedom, resolving the contradiction between adaptability and force transmission capability.
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 direct force application for vertebral alignment and correction, combining the benefits of polyaxial and monoaxial screws by restricting rotational freedom, thus facilitating the insertion of connecting elements and allowing for effective alignment and stabilization of the spinal column.
Implementation Method 1
a screw head (30) mounted in a ball joint on the screw shank (26)
Implementation Method 2
mobility of the screw head (30) and the screw shank (26) is limited to three degrees of freedom of movement, of which the number of degrees of freedom of movement can be reduced to one or two degrees of freedom of movement
Implementation Method 3
introduce forces for moving, in particular rotating, deformed vertebrae of a spinal column directly onto the screw shank
Data Source
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AI summary
To improve the handling of a pedicle screw system (22) comprising a pedicle screw (24) with a screw shaft (26) having an external thread (28) and a screw head (30) mounted on the screw shaft via a ball joint, the screw head including a connecting element receptacle (34) for a connecting element (14) of a spinal stabilization system, it is proposed that it include a bone alignment device (62) and a coupling device (64) for force-locking and/or form-locking coupling of the bone alignment device and the pedicle screw in an alignment position in which the movement of the screw head and the screw shaft is reduced by at least one rotational degree of freedom from three degrees of freedom of movement of the screw head mounted on the screw shaft via a ball joint. Furthermore, an improved spinal stabilization system is proposed.