A Tightening Technique to Minimize Loosening of Prosthetic Screws

While the use of torque wrenches has greatly decreased its occurrence, screw loosening remains a prosthetic complication. When a screw is tightened by the application of torque, the screw elongates and a clamping force, called preload, is produced under the screw head. Preload prevents the implant prosthetic screw joint from separating under occlusal loading. As long as the screw preload is greater than the external forces, the screw joint will not loosen.

Insufficient preload of the screw joint can result in screw loosening and fa- tigue failure of the pros- thetic assembly. Excessive preload can exceed the elastic limit of the screw, causing screw breakage or damage to the screw threads and prosthetic hardware. Preload is pro- portional to the torque applied, and the use of a torque wrench provides a simple means of control (Fig. 1).

Because loss of preload or insufficient preload to resist the external forces acting on the screw joint is a major cause of screw loosening, careful attention to the implant manufacturer's screw tightening parameters is a must. An additional factor to consider when tightening prosthetic screws is embedment relaxation of the prosthetic screw joint once tightened.

The surfaces underneath the prosthetic screw head and of the screw seat are not perfectly smooth. When the screw is tightened to the recommended torque setting, the opposing surface imperfections experience localized stress concentrations above the yield strength of the material. At these points the material will deform until the stress is sufficiently relieved, diminishing the screw preload. Although embedment relaxation occurs to some degree during prosthetic screw tightening, loss of preload due to embedment relaxation can be mitigated by technique.1,3

One technique is to tighten the prosthetic screw to the manufacturer's recommended torque, then wait approximately 10 minutes and re-torque the screw.3 This allows time for embedment relaxation to take place. The second time the screw is tightened, there should be less decay in the screw preload.2

If loosening still occurs, the clinician should go through a troubleshooting list to determine possible causes, including3:

• Passive fit of the prosthesis (this should have been evaluated at the try-in/delivery appointment)
• Occlusion
• Parafunctional habits
• Excessive transverse loading.

Once the probable cause is identified and ad- dressed, the screws can be retightened.

By understanding basic engineering principles and then incorporating techniques around them, you can minimize potential complications on your implant cases. The recommendation is to use a torque wrench and to tighten the screws at least twice, preferably waiting at least 10 minutes between each tightening.

References

1. Siamos G, Winkler S, Boberick KG. The relationship between implant preload and screw loosening on implant-supported restora- tions. J Oral Implantol. 2002;28:67-73.
2. Byrne D, Jacobs S, O'Connell B, Houston F, Claffey N. Preloads generated with repeated tightening in three types of screws used in dental implant assemblies. J Prosthodont 2006;15(3):164-171.
3. Winker S, Ring K, Ring JD, Boberick KG. Implant screw mechanics and the settling effect: an overview. J Oral Implantol. 2003;29:242-245.