Dental implants offer a durable and effective solution for missing teeth, but they are not without potential challenges. For those with screw-retained restorations, understanding and managing these complications is key to long-term success. The most common technical problems encountered with screw-retained implants are the loosening and, in more severe cases, the fracture of the abutment screw.
The Anatomy of a Screw-Retained Implant
Before delving into the problems, it's helpful to understand the components of a screw-retained implant. This type of restoration consists of three primary parts:
- Implant Fixture: A titanium post surgically embedded into the jawbone, acting as the root of the artificial tooth.
- Abutment: A connector piece that screws into the implant fixture.
- Crown or Prosthesis: The artificial tooth or bridge that attaches to the abutment via a retention screw, visible through a small access hole.
This design's key advantage is its retrievability. The dentist can easily access the underlying components for repair or cleaning by removing the crown. However, the design also introduces potential points of failure at the screw joint.
Causes of Screw Loosening and Fracture
Multiple factors can contribute to the eventual loosening or fracture of an implant abutment screw:
- Excessive Force: Patients who grind or clench their teeth (bruxism) place a significant amount of stress on the implant and its components. This repeated overloading can weaken the screw over time, leading to loosening or breakage.
- Inadequate Preload: The preload is the clamping force created when the screw is tightened. If the screw is not torqued to the manufacturer's specified level during placement, or if the initial "settling effect" isn't accounted for with a follow-up tightening, the preload can be insufficient.
- Poor Occlusal Design: If the implant crown is not designed to distribute occlusal forces properly, excessive lateral or off-axis loading can be transmitted to the screw, causing it to loosen. Narrowing the occlusal table and centering contact points can mitigate this risk.
- Framework and Component Misfit: In multi-unit restorations, an inaccurate fit of the framework onto the abutments or implant fixtures can create stress points, which are relieved by the screw loosening or breaking. Modern CAD/CAM technology has improved passive fit, but it remains a consideration.
- Material Fatigue: Even with high-quality titanium alloys, constant stress from mastication can cause the metal to weaken and eventually fracture. This is more likely with older designs or in cases of persistent, heavy loading.
Symptoms and Diagnosis
Patients with screw-retained implants should be aware of potential warning signs that may indicate a loose or fractured screw:
- A sensation of looseness or wobbling in the crown.
- Clicking or popping sounds while chewing.
- Pain or discomfort in the implant area.
- Inflammation or swelling of the gums around the implant.
- Development of a visible gap between the crown and the gumline.
Diagnosis typically involves a combination of clinical examination and radiographic imaging. A dentist may manually check for mobility, and X-rays or CBCT scans can reveal fractures, component separation, or bone loss around the implant.
Prevention and Treatment Strategies
Prevention
Preventing screw complications is preferable to treating them. Effective prevention involves:
- Night Guards: Patients with bruxism should wear a custom night guard to protect the implant from excessive nocturnal forces.
- Proper Torque: Clinicians must use calibrated torque-limiting devices to ensure screws are tightened to the precise, manufacturer-recommended preload.
- Ideal Occlusal Design: The restorative surface should be designed to direct biting forces axially, avoiding excessive lateral loads.
- Regular Check-ups: Routine dental visits allow for early detection of issues before they escalate.
- Patient Education: Informing patients about proper oral hygiene and the risks of chewing on hard objects or neglecting symptoms is vital.
Treatment
Treatment depends on the severity of the problem. For a simple loose screw, the dentist can often remove the crown, re-tighten the screw to the correct torque, and restore the access hole. If the screw is fractured, the process is more complex. Specialized retrieval kits may be used to extract the broken piece without damaging the implant's internal threads. In the worst-case scenario, particularly if the implant threads are damaged or the fracture is at the fixture level, the entire implant may need to be removed and replaced.
Screw-Retained vs. Cement-Retained Restorations
The choice between screw- and cement-retained restorations involves a trade-off of advantages and disadvantages. While cement-retained crowns are generally more aesthetic due to the lack of an access hole, they carry a significant risk of peri-implantitis from residual cement.
| Feature | Screw-Retained | Cement-Retained |
|---|---|---|
| Retrievability | Easily removed for maintenance and repair. | Difficult to remove without destroying the crown. |
| Peri-implantitis Risk | Lower risk, as no intraoral cement is used. | Higher risk due to potential for residual cement. |
| Aesthetics | Can be compromised by the visible screw access hole, especially in the front teeth. | Superior aesthetics with no access hole. |
| Mechanical Complications | Prone to screw loosening and fracture. | Prone to cement failure and crown dislodgement. |
| Occlusal Space | Requires less restorative space. | Requires more restorative space for abutment retention. |
Conclusion
Screw-retained implants are a highly successful restorative option, but it's important to recognize that a common problem for a screw-retained implant is screw loosening and fracture. By understanding the causes, recognizing the symptoms, and prioritizing proactive care, patients can significantly minimize the risk of these complications. Regular dental check-ups, good at-home hygiene, and the use of protective devices like night guards for bruxism are essential for preserving the integrity and function of the implant. When issues do arise, the retrievability of screw-retained restorations allows for a less invasive and more straightforward repair than often possible with cement-retained alternatives, reinforcing their long-term value.
