Spinal fusion is a surgical procedure designed to permanently join two or more vertebrae, transforming a mobile spinal segment into a single, solid bone structure. The primary goal is to stop motion between painful vertebral segments, which helps to alleviate symptoms caused by conditions like spinal instability, deformity, or severe degenerative disease. To achieve this, a bone graft material is placed between the vertebrae. Specialized metal hardware, composed of screws, rods, and sometimes plates, is temporarily implanted to stabilize the spine while the graft heals and grows across the gap, creating a stable environment for successful biological fusion.
Standard Dimensions of Spinal Screws
The screws used in spinal fusion are standardized and measured in millimeters (mm), reflecting the precise scale of the anatomy they fit. These devices come in a wide array of dimensions to accommodate the differing sizes of individual vertebrae throughout the spine. Typical pedicle screw lengths, which anchor into the bony structures of the vertebra, generally fall within a range of 25 mm to 60 mm.
Screw diameter is equally variable, commonly ranging from 3.5 mm for smaller applications up to 8.5 mm for the largest parts of the spine. Selecting the correct length is a fine balance for the surgeon. The screw must be long enough to achieve maximum purchase for stability, but not so long that it breaches the far side of the vertebral body, risking injury to surrounding tissues or blood vessels.
The outer diameter of the screw shaft is a significant factor in determining the pull-out strength. Manufacturers produce screws in specific increments, such as 5 mm for length and 0.5 mm or 1.0 mm for diameter, allowing surgeons to closely match the implant to the patient’s anatomy. This ensures the hardware provides the necessary mechanical stability while bone growth completes the fusion.
Anatomical Factors Influencing Screw Selection
The selection of a specific screw size is highly individualized, determined by the patient’s unique anatomy and the specific location within the spine. The vertebral level dictates the available bone size; screws for the cervical (neck) spine are the smallest, while those for the lumbar (lower back) and sacral regions are the largest. For example, the smaller pedicles in the thoracic spine often require smaller diameter screws compared to the robust pedicles found in the lumbar spine.
Pre-operative imaging, typically using computed tomography (CT) scans, is performed to create precise measurements of the pedicle width and length for each vertebra. Surgeons use these images for templating, meticulously planning the screw trajectory and size to maximize bone contact and fixation strength. This detailed planning ensures the screw achieves optimal purchase without compromising the spinal canal or nearby neurological structures.
Another significant anatomical factor is the patient’s bone density, particularly in cases of osteoporosis. Reduced bone density can compromise the screw’s hold, potentially leading to loosening or failure. To counteract this, surgeons may choose screws with a larger diameter or specialized thread designs to increase surface area contact and improve fixation strength in softer bone.
Types of Spinal Fusion Hardware
Screws anchor the construct to the bone but are only one component of the spinal instrumentation system. The most common type is the pedicle screw, inserted through the pedicle (the bony bridge connecting the back of the vertebra to the front) and used primarily in the thoracic and lumbar spine. For the smaller cervical vertebrae, surgeons typically use lateral mass screws, which anchor into the bony mass on the side of the vertebra instead of the pedicle.
The screws function primarily as anchors for the rods or plates. These rods are contoured to the natural shape of the spine and span the distance between the implanted screws, providing structural stability that holds the vertebrae motionless. Screws are often categorized by their head design: monoaxial or polyaxial.
Monoaxial vs. Polyaxial Screws
Monoaxial screws have a fixed head that does not swivel, creating a stiffer construct that provides stronger leverage for correcting spinal alignment. Polyaxial screws are used more frequently and feature a head that can pivot or toggle. This flexibility makes it easier for the surgeon to connect the rod to multiple screws that may not be perfectly aligned, simplifying the surgical process.
Composition and Longevity of Implants
The materials used for spinal implants must possess a specific combination of strength, durability, and biological compatibility. The vast majority of modern spinal hardware, including screws and rods, is manufactured from Titanium or Titanium alloys, such as Ti-6Al-4V. Titanium is favored because it is highly biocompatible, meaning the body does not typically reject it, and it exhibits excellent corrosion resistance.
Titanium’s non-ferromagnetic properties are a major advantage, making the implants safe for patients who may require magnetic resonance imaging (MRI) scans after surgery. Stainless Steel is now less frequently used for primary spinal implants because it can create significant interference and artifacts on post-operative MRI images.
The implanted hardware is generally intended to be permanent, acting as an internal cast until the biological fusion of the bone is achieved. Once the bone graft has solidified and the fusion is mature, the metal hardware is no longer mechanically necessary. It is typically left in place unless it causes pain or complications. The screws and rods provide the temporary, stable framework required for this biological process to take place.