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
a
Stardrive: the force is exerted nearly perpendicularly to the flats of the screw, therefore the transmission of torque is superior
b
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
Situation
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.
Goal
To compare the von-Mises-stresses in the transition zone between the screw head and the screw body for the two screw drive designs.
Method
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.
Results
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.
Conclusion
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.
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