Stardrive a new AO screw drive connection
Some 40 years ago the AO selected the hexagonally recessed screw drive connection. In comparison to single and cruciate slots, the HEX offers a good lateral guidance that allows blind insertion and removal. In comparison to Phillips, the torque transmission is largely independent of axial thrust, which may compromise initially unstable reduction of the fracture fragments. Furthermore, to avoid collision of sequentially inserted plate or lag screws the surgeon knew the inclination of a HEX screw of which only the head protrudes from the bone because the screw driver by necessity aligns with the screw axis.
Still, the flats of the HEX screw-driver and -recess are oriented rather tangentially to the force applied. Such torque transmission may strip and results in a tendency to expand the screw head with application of torque. This is the case especially when the screw driver is worn. The new AO Stardrive maintains the advantages of the HEX but offers a better resistance to stripping, as the flats are orientated more perpendicularly to the force applied. A further advantage of the new AO Stardrive is that the size of the drive connection now conforms to general technical standards.
The Stardrive has been successfully introduced in the area of maxillo- facial and spine surgery. Further applications such as in the long bones are under close investigation.
Comparison of force transmission upon application of torque at insertion/removal between Stardrive and HEX
Stardrive: the force is exerted nearly perpendicularly to the flats of the screw, therefore the transmission of torque is superior
HEX: the arrows show that force during application and removal is exerted rather tangentially to the flats of the screw drive.
Appearance of the two different drive connections of screws according to the function of the screw used in the LCP. The locked screws are fitted with Stardrive, the conventional screws are fitted with HEX drive.
Stardrive recess compared to hexagonal recess - A finite element analysis
The Stardrive is the new AO screw drive connection. This drive has several advantages when compared to the conventional HEX drive. The Stardrive is also used in locked screws. The drive diameter of the Stardrive is larger than the HEX drive, while the HEX drive is deeper. The influence of these parameters on the stresses in the transition zone between the head and the body of the screw, which is the weakest region of any screw, was not predictable. The AO Research Institute performed a mathematical analysis of the situation.
To compare the von-Mises-stresses in the transition zone between the screw head and the screw body for the two screw drive designs.
A Finite Element Analysis using the ABAQUS Software was carried out. Two models with up to 250,000 elements and linear elastic material properties were analyzed under a bending and a torsion load.
The maximum von-Mises-stresses in the transition zone of the Stardrive screw are 4% lower under bending and 8% lower under a torsion load than those in the HEX drive screw. The main reason for this result is the depth of the drive. In the case of a locked screw, the screw head is supported by the surrounding plate. For the Stardrive design the weakest diameter lies in the supported area inside the head, but for the HEX drive design, the drive reaches into the unsupported area of the transition zone and thus the weakest diameter lies in the unsupported area.
Despite the geometrical differences of the drive, a locking head screw with Stardrive recess offers at least the same resistance to bending and torsion loads than a locking head screw with hexagonal recess.
Hazards and labeling
Due to varying countries’ legal and regulatory approval requirements, consult the appropriate local product labeling for approved intended use of the products described on this website. All devices on this website are approved by the AO Technical Commission. For logistical reasons, these devices may not be available in all countries worldwide at the date of publication.
This work was produced by AO Foundation, Switzerland. All rights reserved by AO Foundation. This publication, including all parts thereof, is legally protected by copyright.
Any use, exploitation or commercialization outside the narrow limits set forth by copyright legislation and the restrictions on use laid out below, without the publisher‘s consent, is illegal and liable to prosecution. This applies in particular to photostat reproduction, copying, scanning or duplication of any kind, translation, preparation of microfilms, electronic data processing, and storage such as making this publication available on Intranet or Internet.
Some of the products, names, instruments, treatments, logos, designs, etc referred to in this publication are also protected by patents, trademarks or by other intellectual property protection laws (eg, “AO” and the AO logo are subject to trademark applications/registrations) even though specific reference to this fact is not always made in the text. Therefore, the appearance of a name, instrument, etc without designation as proprietary is not to be construed as a representation by the publisher that it is in the public domain.
Restrictions on use: The rightful owner of an authorized copy of this work may use it for educational and research purposes only. Single images or illustrations may be copied for research or educational purposes only. The images or illustrations may not be altered in any way and need to carry the following statement of origin “Copyright by AO Foundation, Switzerland”.
Check www.aofoundation.org/disclaimer for more information.
If you have any comments or questions on the articles or the new devices, please do not hesitate to contact us.
“approved by AO Technical Commission” and “approved by AO Foundation”
The brands and labels “approved by AO Technical Commission” and “approved by AO Foundation”, particularly "AO" and the AO logo, are AO Foundation's intellectual property and subject to trademark applications and registrations, respectively. The use of these brands and labels is regulated by licensing agreements between AO Foundation and the producers of innovation products obliged to use such labels to declare the products as AO Technical Commission or AO Foundation approved solutions. Any unauthorized or inadequate use of these trademarks may be subject to legal action.
AO TC Innovations Magazine
Find all issues of the AO TC Innovations Magazine for download here.
Recognizing outstanding achievements in development and fostering excellence in surgical innovation.