Surgical screws are specialized medical devices that provide internal fixation, a technique used to stabilize bone fragments following a fracture or to correct skeletal deformities. These implants are designed to hold bone pieces together securely while the body’s natural healing process, known as osteosynthesis, takes place. Surgical screws are selected based on the specific location of the injury, the type of bone involved, and the required duration of support. Their primary role is to ensure the bone heals in the correct anatomical position by providing mechanical stability, often working in conjunction with metal plates or rods.
The Materials Used in Surgical Screws
Material selection prioritizes two main factors: biocompatibility and mechanical strength. Biocompatibility ensures the material will not provoke a toxic or inflammatory response, while strength guarantees the screw can withstand forces during healing. The most common materials fall into two major categories: metallic alloys and bioabsorbable polymers.
Metallic screws are primarily manufactured from medical-grade stainless steel, specifically 316L, or titanium alloys, such as Ti-6Al-4V ELI. Stainless steel 316L is a strong, corrosion-resistant alloy containing iron, chromium, nickel, and molybdenum, and it remains a popular and cost-effective choice for many applications. Titanium alloys are often preferred for their superior biocompatibility, lighter weight, and ability to better integrate with bone tissue, a process called osseointegration.
A growing number of screws are made from bioabsorbable (or bioresorbable) polymers, which are designed to dissolve safely within the body over time. These materials, such as Poly-L-lactic acid (PLLA) and Polyglycolic acid (PGA), eliminate the need for a second surgery to remove the hardware. Bioabsorbable screws maintain their mechanical strength long enough for the bone to heal, after which they are broken down through hydrolysis into natural compounds, like water and carbon dioxide.
Classifying Surgical Screw Designs
The physical shape and thread pattern of a surgical screw are engineered to match the specific type of bone it is designed to fix. Bone tissue is composed of two primary layers: the dense, hard outer layer called the cortex, and the spongy, porous inner layer known as cancellous bone. Screws are categorized based on which of these bone types they are intended to engage.
Cortical screws are characterized by their fine, shallow, and closely spaced threads that run along the entire length of the shaft. This design is optimized to gain maximum purchase in the dense, compact cortical bone that forms the shaft of long bones.
In contrast, cancellous screws feature coarse, deep, and widely spaced threads, similar to a traditional wood screw. This larger thread pattern is necessary to anchor effectively in the softer, less dense cancellous bone found near the ends of long bones and in the pelvis. Cancellous screws are frequently used to achieve compression across a fracture site.
Specialized Screw Designs
Several specialized designs address unique surgical requirements. Cannulated screws feature a hollow central shaft that allows the surgeon to insert the screw precisely over a pre-placed guide wire, often called a K-wire. This technique is particularly valuable in minimally invasive procedures where pinpoint accuracy is required. Locking screws are designed for use with specific plating systems, where the screw head threads directly into the plate, rather than relying on compression against the bone surface. This creates a highly stable, fixed-angle construct, which is advantageous when fixing fractures in poor quality bone, such as in patients with osteoporosis.
The Fate of Surgical Implants
The ultimate fate of a surgical screw is determined by its material composition and the original location and purpose of the fixation. Many metallic implants are intended for permanent fixation, particularly those used in high-load areas like spinal fusion or joint replacements, where the hardware provides long-term structural support.
However, in many cases, hardware serves as temporary fixation and may be removed in a secondary procedure after the fracture has fully consolidated. The decision to remove an implant is typically driven by patient symptoms, such as persistent pain, discomfort, the physical prominence of the hardware beneath the skin, or a delayed infection.
The time between initial surgery and hardware removal is variable but often averages around 18 months. Removal is not without risk; potential complications include wound infection, nerve damage, and the risk of a new fracture through the empty screw holes. The alternative to metal removal is the use of bioabsorbable screws, which eliminate the need for a second surgery.
Bioabsorbable screws degrade through hydrolysis. They generally maintain sufficient mechanical strength for the first few months, allowing the bone to heal. The material then begins to lose mass and is gradually resorbed by the body, with the entire process often taking between one to three years, depending on the polymer ratio, implant size, and anatomical location. By the end of this process, the screw is replaced by native bone or connective tissue, leaving no foreign body behind.