Surgical screws are specialized medical implants used primarily in orthopedic and trauma surgery to stabilize broken bones. These devices act as rigid internal fixation, holding bone fragments securely while the body’s natural healing processes occur. They provide the necessary mechanical stability for the bone to mend properly, ensuring alignment and reducing movement at the fracture site. The screw’s design and material are matched to the specific bone being fixed and the biomechanical forces it will endure during recovery.
The Materials Used in Surgical Screws
The composition of a surgical screw is determined by the required strength, location, and intended permanence. Metallic options offer the highest strength and have historically been the standard for internal fixation. Titanium and its alloys are highly favored due to their excellent biocompatibility and superior strength-to-weight ratio. Titanium is also non-ferromagnetic, which is a practical benefit because it does not interfere with advanced imaging techniques like Magnetic Resonance Imaging (MRI).
Stainless steel, typically a medical-grade alloy, provides high mechanical strength and remains a cost-effective choice for many applications. However, stainless steel is denser than titanium and can sometimes cause artifacts on post-operative imaging. Beyond metals, high-performance polymers like Polyether Ether Ketone (PEEK) are used because their stiffness is closer to that of natural cortical bone. This mechanical similarity helps reduce stress shielding, where a much stiffer implant takes too much load, causing the underlying bone to weaken.
The third major category is bioabsorbable polymers, such as poly-L-lactic acid (PLLA) and polyglycolic acid (PGA), which are designed to dissolve over time. These materials are gradually broken down by the body, eliminating the need for a second surgery to remove the implant. While not as strong as metallic counterparts, these polymers are suitable for lower-load applications and are frequently used in small joint fixation or pediatric cases.
Classifying Surgical Screws by Design and Function
Surgical screws are classified based on the type of bone they are designed to engage. Cortical screws are engineered for the dense, outer shell of bone found in the shafts of long bones. They feature fine, closely spaced threads along their entire length to maximize purchase in this compact bone structure. Conversely, cancellous screws are designed for the spongy, porous bone found at the ends of long bones and in the pelvis. These screws have coarse, widely spaced, and deeper threads, providing a better grip in the less dense, trabecular bone.
The cannulated screw features a hollow central shaft. This design allows the surgeon to insert the screw precisely over a pre-placed guide wire or pin, facilitating accurate placement, especially in minimally invasive procedures. The choice between a locking and a non-locking screw relates to how the screw interacts with a fixation plate. A non-locking screw relies on the threads gripping the bone for stability, while a locking screw threads directly into the plate head, creating a fixed-angle construct.
This fixed-angle system makes locking screws valuable for stabilizing complex fractures or fractures in bone of poor quality, such as in patients with osteoporosis. Screws are also classified by their function, operating either as position or lag screws. A position screw simply holds two bone fragments in alignment, preventing them from moving relative to each other. In contrast, a lag screw creates compression by threading only into the far bone fragment, pulling the fragments tightly together to enhance stability and healing.
The Fate of Surgical Screws in the Body
The long-term outcome for a surgical screw depends on whether it was intended for permanent or temporary fixation. Metallic screws are generally designed to remain in the body indefinitely after the bone has healed. However, these permanent implants may require removal if the patient experiences chronic pain, soft tissue irritation, or if an infection develops around the hardware. In some cases, the screw may be removed because it is no longer structurally necessary after the bone has fully recovered.
For specific patient populations, particularly children, metallic hardware is often removed to prevent potential interference with the growth plates. Bioabsorbable screws are intended to be temporary, offering the advantage of degradation over time. These polymer devices gradually transfer the mechanical load back to the healing bone as they dissolve. After the material is completely absorbed by the body, the need for a follow-up surgery is eliminated.
However, even bioabsorbable devices may occasionally require early removal. The breakdown products of the polymers can sometimes cause a localized inflammatory reaction or a late-onset infection. The decision to leave a screw in place or to remove it is always a clinical judgment based on the patient’s symptoms and the implant’s continued necessity.