Glossary

For many decades, dental implants have been one of the most desirable ways to replace one or more missing teeth. Their outcomes are generally predictable, and the failure rate of dental implants is low overall. However, failure can still occur both in the short and long term. Screw fracture, or the fracture of the screw implanted into the bone to hold the dental prosthetic, is a common reason dental implants fail within the first 10 years. Although nearly half of dental implant patients suffer a screw fracture, removing it is a complex procedure, especially if the screw is fractured or broken in more than one place. Considered a mechanical complication, the fracture of a dental implant screw is often caused by high occlusal loads. In cases of bruxism, where there is constant pressure on the teeth from clenching or grinding them (usually at night), screw fractures are more common.

A screw implant is a threaded root-form dental implant which can either be parallel-sided or tapered. A screw implant is placed into the bone of the jaw to replace the root of a missing tooth. After the screw implant is positioned, gum tissue is placed over it and allowed to heal, sometimes for up to 6 months. A second procedure is usually then required to affix the abutment and the crown. The screw implant is generally made of a material such as titanium that is biocompatible and capable of fusing with the surrounding bone during osseointegration. At times, a patient may require a bone graft prior to the placement of the screw if the existing bone is not substantial enough to support the implant by itself. Complications such as peri-implantitis may occur following a screw implant. This condition causes inflammation and can lead to bone loss around the implant and to implant failure.

A screw joint is the junction of two parts held together by a screw (e.g., implant-abutment screw joint). In implant dentistry, the screw joint is an essential part of the implant or prosthesis, however, it is also susceptible to loosening over time. This can cause implant failure at the screw joint. A screw at the screw joint may become loose for several reasons which include incorrect fit of the prosthesis, excessive loading, poor screw design, improper placement, inadequate torque on placement, poorly made prosthesis components, and improper design of the restoration. For those factors which can be controlled by the dental professional, exact adherence to protocols should be maintained to ensure screw joint failure isn’t caused by an error on placement. A basic understanding of the physical forces that act on the screw joint in a prosthesis is extremely helpful for the dental professional as it provides information on how screw joint loosening can occur.

Screw loosening is a prosthetic complication whereby a screw loses its preload causing the loosening of a restoration or abutment. Due to the many forces exerted on a screw in an implant, a variety of issues may occur that contribute to screw loosening. These forces include the bite angle of the patient, loading of the implant, and the functional tension, rotation, and force on the screw. There are other factors that can contribute to the loosening of an implant or abutment screw as well. Some of these factors include the design of the implant, the design of the abutment, the amount of torque used to secure the screw, the efficiency of the implant’s manufactured parts, and the fit of the implant. While manufacturer errors may not be something the dental professional can control, they should follow all protocols when placing the implant and the screw to avoid improper torque, positioning, or incorrect loading.

Screw preload is the clamping or stretching force that occurs across the interface of the implant components that are being attached via screw tightening. Screw preloading has been shown to increase the screw’s fatigue strength as well as improve the locking effect. To set the screw preload within its optimal range, the correct amount of torque must be applied. However, this can be a complicated process as the torque applied in preloading a screw can be affected by a variety of factors. These factors include the material which the screw is made of, the screw’s stiffness, construction, and shape, the integrity of the screw and joint, and the condition of the surrounding surfaces. A screw’s level of preloading should be more than the biting force but less than the elastic limit and may fall within a 75-90 percent range of the material’s elastic limit to prevent the screw from loosening under moderate loads.

The term screw-retained refers to the use of a screw for retention of an abutment or a prosthesis. Screw-retailed abutments generally include a screw joint. The screw joint is the junction of two parts held together by a screw and is an essential part of the abutment and prosthesis. However, the screw joint can become loose and cause the implant to fail. Loosening of a screw-retained abutment or prosthesis is a complication that may occur over time and may be caused by an incorrect fit of the prosthesis, excessive loading, flaws in the screw design, improper placement, poorly made prosthesis or abutment components, and improper design of the restoration. Though some loosening may be expected in the years following the placement of a prosthesis, early loosening can be prevented through compliance with proper placement protocols. The patient should also maintain good oral hygiene and practice consistent oral care to avoid placing additional strain on the screw-retained prosthesis.