The central screw of the abutment is an indispensable part of an abutment to increase the tightness of the connection between the abutment and the internal cavity of the implant. It relies on the conical surface of the screw head to fit the conical surface inside the abutment. With the force of the central screw, a downward wedging force is generated, so that the cone of the screw head and the inner core of the angled abutment; the outer cone of the abutment, and the inner core of the implant closely fit.
In some special cases, the central screw may be broken (different from the central screw of the carrying body, which will be elaborated at the end of the article). The conventional feedback description is as follows: After applying a force of 30Ncm, or even less than 30Ncm, the central screw will break. The location of the fracture is mostly located at the thread (shown by the red line in the picture).
As for the reason for the fracture, according to personal opinion, it is summarized as follows: (for the bone-level abutment and non-abutment integrated crown).
1. When the abutment is in place, the abutment is separated from the central screw. If the abutment is not given the positioning pressure, the abutment will be slightly displaced when it is pushed by the soft tissue, and the screw is rushed to prevent the position of the abutment from changing, causing the central screw to be incorrectly seated.
2. When the abutment is put on, the screw is not separated from the abutment and no pressure is applied to the abutment. There is a possibility that the thrust of the soft tissue will cause a slight change in the position of the abutment. When the screw is screwed in, it is not correctly aligned with the thread inside the abutment, resulting in the wrong thread alignment. As the screw is screwed, there is a stress concentration point at the thread position. When the stress is greater than the maximum material resistance, it will break.
Generally, after a break occurs, the broken screw is stuck inside the implant due to the force, and special tools are required to take out the broken wire with probability, and the operation is more difficult. So instead of studying how to take the broken wire, it is better to avoid this situation from the source.
Based on personal opinions derived from past experience, there are the following points to note:
1. When screwing in the central screw, a certain downward pressure should be applied to the temporary abutment to ensure that the internal connection is correct and stable without movement.
2. Before screwing in the central screw clockwise, turn the central screw properly to ensure that the screw is correctly aligned with the silk track.
I hope that teachers will pay more attention to details in clinical operations. Avoid breaking the central screw.
PS: The central screw of the carrying body breaks because the implant torque exceeds the upper limit that the central screw can bear. For the protection of the internal connection, the pre-designed breakpoint will break. It is recommended that when the implantation torque of the columnar implant is close to 50Ncm, please unscrew the implant and perform some fine preparation of the cortical bone area or implant socket (for example, the rational use of shaped drills), while avoiding excessive extremes. In this way, the central screw of the carrying body generally does not break.
In addition, the initial stability of columnar implants is inherently worse than that of cone-shaped implants. Therefore, there is no need to pursue the initial stability too much, so as to avoid excessive bone compression and unnecessary bone resorption.