This is a question that is often asked, especially in the revision or trauma setting. Titanium screws have been used successfully for many years, becoming the gold standard for implant fixation. The issue with titanium is that its durability depends on its microstructure and surface properties. Titanium is very reactive and will change its structure over time to environmental changes.
For example, titanium screws placed in the bone will rapidly become coated with calcium and phosphate. This increases the surface area of the screw, which improves its fixation. However, this coating is susceptible to corrosion and does not protect the bone from decay. The corrosion process can cause the bone to dissolve, leading to screw loosening or failure.
When placed in the bone, titanium screws will undergo a transformation process known as the surface oxide layer (SLOR). A layer of oxide forms on the surface of the screw, resulting in the formation of a titanium oxide layer. This layer protects the screw from further corrosion. Over time, the SLOR will gradually be lost, resulting in corrosion of the screw.
The length of time the screw remains in the SLOR depends on its environment. The environment that the screw is placed in can be divided into three different phases. The first phase is the integration phase, which lasts between 1- 3 months. During this time, the screw is fully surrounded by bone. This phase is crucial for screw fixation, as any micromotion of the screw in this period can lead to screw loosening. The second phase is the remodeling phase. This is when the screw is surrounded by an interfacial layer formed from the body fluids.
This layer is responsible for further transformation of the screw. The final phase is the degradation phase. When placing the screw in the bone, it is surrounded by bone. However, the bone is gradually dissolved over time, and the screw becomes exposed to the surrounding environment. Below is a chart outlining how titanium screws respond to different loads and how long they remain in the SLOR. For example, if a titanium screw is placed in a femur and subjected to 50kg, it will remain in the SLOR for approximately five years. However, if the same screw is placed in a femur and subjected to 75kg, it will only remain in the SLOR for about one year.