A screw-retained implant crown is a permanent prosthetic restoration attached directly to an implanted fixture in the jawbone via a fastener. This design offers a significant advantage for long-term maintenance because the entire crown can be easily removed by accessing the internal screw. Understanding the components and steps involved in seating this restoration is necessary for its long-term success.
Understanding the Hardware
The screw-retained system consists of three primary physical parts that must fit together with high precision. The foundation is the implant fixture, a titanium or zirconia post surgically placed into the jawbone that acts as the artificial tooth root. This fixture remains permanently integrated with the bone through osseointegration, providing a stable anchor.
The second component is the crown unit, the visible, tooth-shaped restoration that often includes the abutment, or connector post, fabricated as a single piece. This integral design ensures a precise fit and seal at the connection point to the implant platform. The crown unit contains a narrow access channel extending through its center or side, which provides a direct path to the underlying fixture.
The third element is the retention screw, the fastener that mechanically joins the crown unit to the internal threads of the implant fixture. Its proper engagement and tightening are paramount to prevent the crown from loosening over time. The design of all three parts, particularly the connection interface, is engineered to withstand the forces of chewing.
The Seating Procedure
The process begins with cleaning the implant platform once the temporary healing abutment is removed. The connection surface of the implant fixture must be thoroughly debrided and irrigated to ensure no debris or bacteria are trapped before the final crown is seated. The new retention screw should also be cleaned to remove any debris from the threads before use.
The crown unit is then aligned over the implant platform and gently seated into place, ensuring the anti-rotational features of the connection interface are fully engaged. The fit is checked visually and tactilely to confirm the crown is sitting flush with the gum line and that the margins are accurately adapted to the tissue. Radiographic verification may also be used at this stage to confirm the complete and passive seating of the crown on the implant.
Once seated, the retention screw is inserted through the access channel in the crown and hand-tightened using the appropriate driver. Hand-tightening provides an initial gentle engagement, but it is insufficient to secure the crown against chewing forces. The screw must then be mechanically tightened to a specific tension using a specialized torque wrench.
The manufacturer of the implant system specifies a specific torque value, typically ranging from 20 to 35 Newton-centimeters (Ncm). Applying this torque creates a preload force that stretches the screw. This preload prevents the screw from backing out and prevents micromovement at the interface, which is a common cause of screw loosening and potential bone loss.
Some protocols recommend a second tightening approximately 10 to 15 minutes after the initial application of force. This practice accounts for the “settling effect,” where microscopic irregularities on the screw and abutment surfaces flatten under the initial load. The second torque application re-engages the connection, maximizing the contact and maintaining the desired preload.
Securing the Screw Channel
The final step involves sealing the screw access channel. Before filling the void, the head of the retention screw must be protected to ensure the crown can be retrieved later. Without this protective layer, the final filling material would bond directly to the screw head, making future removal difficult and risking damage to the crown.
A small piece of barrier material is condensed directly over the screw head, leaving a space of about two to three millimeters between the barrier and the top surface of the crown. Polytetrafluoroethylene (PTFE) tape is frequently used because it is hydrophobic and easily condensable. Due to its non-filamentous structure, PTFE limits the potential for bacterial adhesion, a concern associated with materials like traditional cotton pellets.
Once the barrier material is placed, the remaining void is filled with a restorative material, most commonly a tooth-colored composite resin. If the access hole is deep or if a metal substructure is present, an opaque composite may be used first to mask any discoloration before applying a final layer that matches the crown’s shade. The crown material itself may be treated with a bonding agent or silane before the composite is placed to facilitate a strong seal.
The composite resin is then cured, contoured, and polished to ensure proper bite alignment. This sealing process completes the restoration, protecting the underlying screw while preserving the crown’s aesthetics and function.