The anterior cruciate ligament (ACL) is a band of tissue that helps stabilize the knee by connecting the shinbone (tibia) to the thighbone (femur). ACL injuries are common, especially in sports that involve sudden stops, pivots, or jumps, such as basketball, volleyball, soccer, and football. When this ligament tears, individuals may experience pain, swelling, and reduced range of motion. ACL reconstruction surgery is a common procedure to restore knee stability and function. This surgery often involves the use of fixation devices, such as screws, to secure the new ligament graft.
Understanding ACL Reconstruction and Screw Purpose
ACL reconstruction surgery involves replacing the torn ACL with a new piece of tissue, known as a graft. This graft can come from another part of the patient’s own body (autograft) or from a deceased donor (allograft). The surgeon creates tunnels in the thighbone and shinbone to position the new graft where the original ACL was located.
An ACL screw, often called an interference screw, is designed to hold this new graft within these bone tunnels. Its primary purpose is to provide immediate mechanical stability by compressing it against the tunnel walls. This stability is important for the initial healing phase, allowing the graft to integrate with the surrounding bone over time and become a functional ligament.
Types of ACL Screws
ACL screws are primarily categorized by their material composition, each offering distinct properties for graft fixation. Metallic screws, commonly made from titanium or stainless steel, provide robust strength and durability. Titanium screws are often chosen for their high biocompatibility, which minimizes adverse reactions with the body’s tissues, and their radiopacity, allowing them to be visible on X-rays for postoperative monitoring. Some metallic screws may also feature a biodegradable coating.
Bioabsorbable, or bioresorbable, screws gradually break down and are absorbed by the body, eliminating the need for future removal. These screws are typically made from polymers such as PLLA, PDLLA, PGA, or composites like β-TCP and PLGA. However, some bioabsorbable screws may be associated with a higher risk of complications, such as inflammatory reactions or tunnel widening, compared to metallic screws.
A third category includes screws made from polyether ether ketone (PEEK). PEEK screws combine properties of both metallic and bioabsorbable materials, offering biocompatibility and mechanical stability. These screws are inert, meaning they are not absorbed by the body, but they are radiolucent, making them less visible on X-rays than metallic screws, which can be an advantage for postoperative MRI imaging.
The Role of Screws in ACL Fixation
ACL screws function by creating an “interference fit” within the bone tunnels. Once the graft is positioned in the bone tunnel, the screw is inserted alongside it. This insertion compresses the graft against the inner walls of the bone tunnel.
This compression provides immediate mechanical stability, which is important for holding the graft firmly in place. The design of interference screws, often with sharp lead threads and a tapered tip, helps facilitate their insertion and engagement with the bone. This initial fixation allows the graft to begin the biological process of healing and integrating with the surrounding bone tissue, eventually forming a strong, stable connection.
Life of an ACL Screw Post-Surgery
The long-term fate of an ACL screw after surgery depends largely on its material composition. This process can take approximately one to two years for many bioabsorbable screws, though some types may take longer. As they absorb, these materials are replaced by new bone tissue, ideally leaving no foreign material behind.
Metallic screws are permanent implants and typically remain in the bone indefinitely unless a specific reason for removal arises. These screws generally do not cause problems, but they can sometimes interfere with future magnetic resonance imaging (MRI) studies by creating artifacts on the images.
While screw removal is not a routine procedure, it may be necessary in rare instances if complications occur. Reasons for removal can include persistent pain or irritation at the screw site, screw prominence that causes discomfort, infection, or other screw-related issues like loosening or migration. Bioabsorbable screws, despite their intended absorption, have also been linked to issues such as inflammatory reactions, cyst formation, or late extrusion, which might necessitate removal. Such removals are usually performed if the symptoms are significant and affect the patient’s quality of life, and typically result in the resolution of symptoms.