The anterior cruciate ligament (ACL) is a band of tough connective tissue in the center of the knee joint, acting as a primary stabilizer against excessive forward movement of the shin bone relative to the thigh bone. A complete tear, common in high-risk sports, generally requires surgical reconstruction because the native tissue has limited healing potential. This procedure involves replacing the torn ligament with a tendon graft, which is secured within bone tunnels drilled into the femur and tibia. The ACL screw is introduced to provide the necessary immediate mechanical support for the new ligament.
The Primary Function of Fixation Hardware
The ACL screw provides high initial fixation strength for the new tendon graft while biological healing occurs. This hardware is most often an interference screw, designed to achieve a fixation mechanism known as aperture fixation. The screw is driven into the bone tunnel alongside the graft, compressing the tendon against the tunnel wall. This technique creates an interference fit, which offers robust, rigid attachment at the entrance of the bone tunnel.
The screw’s immediate stability resists the forces of early rehabilitation, such as range-of-motion exercises. Without this rigid mechanical hold, the graft could slip or elongate before it integrates with the surrounding bone. This early fixation prevents the graft from healing in a lengthened position, which leads to persistent knee instability and surgical failure. The hardware is a temporary anchor, maintaining proper graft tension until the body’s natural healing mechanisms take over.
Classification of ACL Fixation Screws
ACL fixation screws are categorized by their material composition, which dictates their long-term behavior.
Metallic Screws
Metallic screws, commonly made of surgical-grade titanium, are known for their strength and durability. Titanium provides reliable, robust initial fixation that is generally well-tolerated by the body. However, metallic hardware is permanent and visible on post-operative imaging, which can create significant artifacts that distort magnetic resonance imaging (MRI) studies of the knee.
Bioabsorbable Screws
Bioabsorbable screws are designed to degrade slowly over time and be replaced by native bone tissue. These screws are typically composed of polymers like Poly-L-Lactic Acid (PLLA) or Polyglycolic acid (PGA), sometimes with additives to promote bone growth. The advantage of these materials is that they eliminate permanent foreign material and avoid interference with future imaging studies. The slow breakdown process gradually transfers the mechanical load back to the healing graft and surrounding bone.
PEEK Screws
Another distinct type of hardware is the Polyetheretherketone (PEEK) screw, which is often grouped with bioabsorbable options but behaves differently. PEEK is a high-performance, non-metallic polymer that is biocompatible and mechanically stable, offering strength comparable to some metals. Unlike PLLA or PGA screws, PEEK is not designed to resorb or degrade; it is a permanent implant. Because it is non-metallic, PEEK screws cause significantly less distortion on MRI scans, offering a middle ground between the strength of titanium and the imaging benefits of bioabsorbable materials.
The Fate of the Screw
The ultimate destiny of the ACL screw depends entirely on its material composition.
Permanent Hardware
Permanent hardware, such as titanium or PEEK screws, is intended to remain in place indefinitely, acting as an inert component of the knee joint. These screws are generally well-tolerated and are not removed unless a specific medical complication arises. Reasons for surgical removal are rare but include persistent localized pain over the screw site, screw migration, or interference with a necessary future surgery, such as a revision ACL reconstruction.
Bioabsorbable Hardware
The fate of bioabsorbable screws is more complex, as their design promises complete material disappearance. The goal is for the polymer to be absorbed and replaced by bone through osteointegration, but this timeline is often much longer than expected. While some degradation begins within months, clinical studies show that remnants of the screw material are frequently visible on imaging three to five years after surgery. Complete absorption can take up to ten years, depending on the specific polymer used and individual biological factors.
During this slow absorption process, the breakdown of some polymer materials can occasionally trigger a localized inflammatory response. This reaction may lead to the formation of small, sterile fluid-filled sacs or cysts within the bone tunnel, or contribute to bone tunnel widening. Although rare, these responses can sometimes necessitate a secondary procedure to address the material breakdown products.