Bone does grow over screws, plates, and other hardware used in fracture fixation as a natural part of the healing response. This internal fixation is surgically placed to hold broken bone fragments in correct anatomical alignment, acting as a temporary scaffold while the bone heals. The body’s reaction to the metal dictates the success of the bone repair and affects the need for future procedures. This article explains the biological interaction between bone and implant materials and details the clinical implications of this growth.
The Biological Interaction Between Bone and Implants
Orthopedic hardware, such as screws and plates, is typically made of high-grade stainless steel or titanium alloys due to their biocompatibility. These materials are non-toxic and do not provoke a significant immune reaction, allowing bone cells to interact directly with the metal surface. This interaction involves two processes: osteoconduction and osseointegration.
Osteoconduction describes the implant acting as a passive scaffold, guiding the formation of new bone across its surface or into its pores. Bone-forming cells, or osteoblasts, migrate along the implant’s contours and lay down new bone matrix, using the metal as a foundation.
Osseointegration is a direct structural and functional connection between living bone and the surface of a load-bearing implant without an intervening layer of soft tissue. This direct bone-to-metal contact results in a strong, stable biological fixation that anchors the hardware firmly within the skeleton. Modern implants often have rougher surface textures designed to increase the area available for this direct bone attachment.
The Process of Bone Growth Encapsulation
Successful healing often results in bone tissue growing over the heads of screws and the edges of plates, a process known as encapsulation. This occurs because osteoblasts treat the stable, fixed hardware as a rigid structure upon which to build new bone. The new bone matrix forms a dense biological seal around the metal, effectively burying the screw heads and making them flush with the healed bone surface.
This encapsulation confirms successful healing and robust osseointegration, showing the implant provided the necessary mechanical stability. If the internal fixation is unstable and allows for excessive relative movement (micromotion), the body’s response changes. Instead of forming a direct bone connection, the body often creates a layer of fibrous tissue around the metal, which can lead to implant loosening and failure of the fracture to heal.
When and Why Hardware is Removed
Most internal fixation hardware is designed to remain in the body permanently, but removal is often necessary. Common reasons for removal include patient-reported symptoms, such as pain or discomfort from hardware prominent under the skin. Other indications are infection, mechanical issues like a loose or broken implant, or migration.
The bone encapsulation that signals successful healing also makes hardware removal more complex. When bone has grown over the screw head, the surgeon must first uncover the head to access the recess for the screwdriver. This often requires using a high-speed surgical burr to carefully remove the thin layer of bone. This step ensures the driver can engage the screw head properly, preventing stripping and making extraction possible.
In certain cases, the hardware is left in place when the risks of removal outweigh the potential benefits. This includes hardware placed in deep, high-risk locations near major nerves or blood vessels, or in elderly patients where the stress of another surgery carries increased risk. However, hardware is often routinely removed in younger patients, especially children, to prevent future issues with growth and development.
Variables That Affect Bone Integration
The speed and quality of bone integration around surgical hardware depend on several patient-specific and material-related factors. Younger patients typically exhibit a faster rate of bone remodeling and integrate implants more quickly and robustly.
Factors Influencing Integration
- Underlying health conditions: Patients with diabetes or osteoporosis may have compromised bone quality and a slower healing response.
- Smoking: Nicotine restricts blood flow, starving the bone of the oxygen and nutrients needed for cellular activity and repair.
- Implant material: Titanium alloys generally promote better osseointegration than stainless steel.
- Fracture location: Bone in highly vascularized areas integrates faster than bone in areas with poor blood supply.