Spinal fusion is a widely performed surgical procedure designed to stabilize segments of the spine and alleviate chronic pain caused by instability or deformity. This operation permanently connects two or more vertebrae into a single, solid bone unit. Achieving this stability relies on specialized internal hardware. Patients are often curious about the metallic implants that hold the spine in place while the biological fusion process occurs, providing the necessary internal scaffolding until healing is complete.
The Role of Pedicle Screws in Fusion
The primary stabilizing components used in modern spinal fusion procedures are pedicle screws. These implants are precisely inserted into the pedicles, which are the short, bony projections connecting the back part of a vertebra to the main vertebral body. The screw itself does not directly cause the vertebrae to fuse; instead, it acts as a secure anchor point within the bone.
Once the screws are seated, they are connected by rigid longitudinal rods to form a fixed internal frame. This rod-screw system provides immediate, rigid stability to the spinal segment. This stability prevents unwanted movement, creating an optimal environment for the bone graft material to successfully grow and bridge the gap between the spinal segments.
Typical Dimensions of Spinal Fusion Screws
The length and diameter of the implants are highly variable, as they must be tailored to the patient’s unique anatomy and the specific spinal level being treated. Pedicle screw length typically ranges from 25 millimeters (mm) up to 60 mm, measured from the tip to the top of the screw head. The most common lengths used often fall between 30 mm and 50 mm.
The diameter of the screws ranges from about 4.5 mm to 7.5 mm, which is critical for achieving adequate purchase, or grip, within the bone. Surgeons select the largest diameter screw that can safely fit inside the pedicle canal without breaching the surrounding bony wall. Polyaxial screws are common because their heads can swivel, allowing for easier connection to the rigid rods at various angles during the surgery.
Factors Determining Screw Size Selection
The selection of the precise screw size is a detailed process that begins long before the surgery, relying heavily on pre-operative imaging. Surgeons use computed tomography (CT) scans and X-rays to measure the dimensions of the vertebral pedicles and bodies. This imaging allows for accurate measurement of the patient’s individual anatomy, which dictates the maximum safe size of the hardware.
The spinal level being fused is a major factor in determining the required dimensions. For instance, the vertebrae in the lumbar spine (lower back) are significantly larger than those in the thoracic spine (mid-back). Consequently, screws used in the lumbar region are typically longer and wider than those inserted into the thoracic spine.
A patient’s bone quality and underlying pathology also influence the final size decision. Conditions like osteoporosis, which reduces bone density, require a larger diameter and often a longer screw to ensure maximum fixation strength and prevent loosening. Selecting a screw diameter that is approximately 80% of the pedicle width is a common guideline. This maximizes the surface area for bone purchase while minimizing the risk of pedicle wall fracture.
Components Used Alongside Spinal Screws
Pedicle screws are just one part of a larger fixation system designed to achieve a successful fusion. Rigid rods are the second major component, acting as the connecting bridge between all the implanted screws. These rods, often 5.5 mm or 6.0 mm in diameter, are contoured by the surgeon to match the spine’s natural curvature, maintaining proper alignment while the vertebrae fuse.
Interbody fusion devices, commonly called cages or spacers, are also frequently used in conjunction with screws and rods. These devices are inserted into the space where the intervertebral disc was removed. The cages maintain the necessary height between the vertebrae and create a contained space for the bone graft material, which is the actual substance that promotes the biological fusion.
The entire system is typically manufactured from biocompatible materials, most commonly titanium or titanium alloys, and sometimes medical-grade stainless steel. Titanium is favored for its excellent strength, low weight, and ability to be safely integrated into the human body. Some cages are made from polymers like PEEK, which is radiolucent, meaning it does not interfere with post-operative imaging used to assess the progress of the bone fusion.