A screw-retained implant crown is a prosthetic tooth restoration secured directly to the dental implant or an attached abutment using a retention screw. This design includes a small access channel through the crown, allowing tools to engage the screw. Screw retention offers significant advantages over cemented restorations, primarily the ability to access the underlying components. This retrievability means the restoration can be easily removed without damage for maintenance or inspection. Furthermore, using a screw eliminates the possibility of residual cement being left below the gum line, which is associated with inflammation of the surrounding soft tissues.
Pre-Seating Protocol
Before seating, a meticulous protocol ensures a long-term, stable outcome. The restoration’s fit must be verified, starting with checking the contacts between the new crown and adjacent teeth. These proximal contacts should allow for light friction during placement.
Once the crown is seated, the marginal fit must be assessed to confirm a precise connection with the implant or abutment platform. Clinicians use a dental explorer to ensure there are no gaps where the crown meets the implant shoulder, which could harbor bacteria. For multi-unit restorations, a technique like the Sheffield test checks for passive fit, confirming the restoration sits completely without introducing strain. A complete passive fit is necessary to prevent biomechanical complications on the fixture.
A subsequent step involves the thorough cleaning of the implant connection site and the internal surface of the crown. The implant’s internal threads and abutment surface are cleaned, often irrigated with solutions like chlorhexidine or sterile saline, to remove debris introduced during the try-in process. The crown’s internal surface is also cleaned to remove oils or particles that could interfere with the final seal. Maintaining a dry and isolated field during this preparation is important to avoid salivary contamination.
The Seating and Torque Sequence
With the crown and implant site prepared, the retention screw is first hand-tightened to ensure proper alignment and initial seating. Hand-tightening provides tactile feedback, confirming the screw engages the implant threads smoothly without binding. Once the restoration is confirmed to be fully seated, the process of generating the necessary clamping force begins.
This clamping force, known as preload, is the tension created in the screw that holds the crown firmly against the implant platform, preventing micro-movement and subsequent loosening. Achieving the correct preload requires a calibrated torque wrench, which delivers a specific rotational force measured in Newton-centimeters (Ncm). Hand-tightening is insufficient, as it delivers inconsistent forces below the recommended range.
Manufacturer guidelines recommend a specific torque value, typically between 20 and 35 Ncm, to achieve adequate preload without risking fracture. When the screw is torqued, a portion of the applied force is consumed by friction and the flattening of microscopic surface irregularities, known as the settling effect. This initial settling reduces the actual preload achieved.
To stabilize the screw connection, a technique of retorquing is employed. After the initial tightening, the screw is allowed to rest for about ten minutes and then torqued again to the specified value. This two-step process results in a higher and more stable long-term preload, minimizing the possibility of subsequent loosening.
Managing the Screw Access Channel
Once the precise torque has been applied, the next step is to protect the retention screw head and seal the access channel to prevent bacterial entry. This sealing process uses a two-layer approach for both protection and ease of future retrieval.
The first layer involves placing a protective barrier directly over the screw head. Polytetrafluoroethylene (PTFE) tape, often called plumber’s tape, is the preferred material due to its high biocompatibility. A small, condensed piece of sterilized tape is placed into the channel, shielding the screw head from the subsequent restorative material. This maintains the screw’s drive feature for future access.
The barrier material must be condensed to a depth that leaves adequate space for the final restorative layer, typically about 3 millimeters of clearance from the crown surface. This remaining space is then filled with a material designed to restore the crown’s contour and seal the opening. A flowable or conventional composite resin, matched to the crown’s color, is commonly used.
Before placing the composite, a bonding agent may be applied to the internal walls of the channel to promote retention. The composite is then placed in increments, shaped to match the anatomy, and light-cured. This achieves a smooth, integrated seal that prevents food impaction and is aesthetically acceptable.
Post-Insertion Assessment
After the access channel is sealed, the final assessment ensures the crown functions harmoniously within the patient’s bite. The primary objective is to adjust the occlusion so the implant crown does not receive excessive force, which could strain the implant-bone interface. Unlike natural teeth, implants are rigidly integrated with the bone, making them vulnerable to biomechanical overload.
Occlusal contact is checked in both centric occlusion (the normal bite) and in all eccentric movements. Specialized thin articulating film is used to gauge force distribution. The implant crown should have slightly lighter contact than the adjacent natural teeth when the patient bites down lightly. During chewing, the restoration should generally be free of interference to minimize lateral forces.
The soft tissues surrounding the implant must also be assessed for signs of health, such as the absence of redness, swelling, or bleeding upon gentle probing. This confirms the seating procedure did not cause trauma and that the patient’s oral hygiene is adequate. The final steps involve providing the patient with specific hygiene instructions and scheduling follow-up appointments to monitor the restoration and tissue health.