The Anterior Cruciate Ligament (ACL) is one of the four main ligaments within the knee joint, connecting the femur (thighbone) to the tibia (shinbone). It stabilizes the knee by preventing the tibia from sliding too far forward and controlling rotational movements. An ACL tear is a common sports injury, often happening during non-contact maneuvers. These maneuvers include sudden stops, rapid changes in direction, or awkward landings, which are common in sports like soccer and basketball.
Because the ACL is located deep within the knee joint, it has a poor blood supply and cannot heal itself. For active individuals who wish to return to sports, an ACL tear generally requires reconstruction. This surgical procedure replaces the torn ligament with a tissue graft to restore stability. Restoring stability prevents further damage to structures like the menisci and cartilage, which can lead to long-term issues such as osteoarthritis.
Selecting the Replacement Graft
The success of the reconstruction relies on selecting a suitable tendon, called a graft, to act as the replacement ligament. Grafts are classified as either autografts, sourced from the patient’s own body, or allografts, sourced from a deceased donor. Autografts are generally preferred for younger, highly active patients because they have a lower risk of re-rupture and failure.
The three main types of autografts are distinguished by their harvest site. The Bone-Patellar Tendon-Bone (BPTB) graft is sourced from the middle third of the patellar tendon, including bone blocks from the kneecap and the tibia. This graft is favored for its faster bone-to-bone healing and provides some of the lowest re-tear rates, making it a common choice for high-demand athletes. However, harvesting the BPTB graft carries a higher risk of harvest site morbidity, potentially leading to chronic pain when kneeling.
Hamstring and Quadriceps Autografts
The hamstring tendon autograft typically uses the semitendinosus and sometimes the gracilis tendons from the inner thigh. This option is associated with less post-operative pain at the front of the knee and a smaller incision site compared to BPTB. Potential drawbacks include a slightly higher re-tear rate and a risk of hamstring muscle weakness, though physical therapy can often mitigate this.
The quadriceps tendon autograft is harvested from the tendon just above the kneecap. It is gaining popularity due to its large size and strength, offering a robust option with less anterior knee pain than the BPTB graft. However, it can cause temporary weakness or pain in the quadriceps muscle post-surgery.
Allografts, sourced from cadaver tissue like the Achilles or tibialis tendon, eliminate the pain and weakness associated with harvesting tissue from the patient. This makes them attractive for older or less active patients, or for revision surgery. However, allografts have a significantly higher failure rate in young, active individuals and take longer to incorporate into the patient’s bone. While modern processing techniques have made the risk of disease transmission extremely low, this small possibility remains a factor.
The Steps of ACL Reconstruction
The procedure is most often performed using an arthroscopic, or minimally invasive, technique. After the patient receives general or regional anesthesia, the surgeon makes several small incisions, known as portals, around the knee. These portals allow the insertion of an arthroscope (a miniature camera) and specialized surgical instruments into the joint.
The first surgical step involves inspecting the knee structures, including the menisci and cartilage, and addressing any concurrent damage. The remnants of the damaged ACL are then removed, ensuring the attachment points are preserved to guide the anatomical placement of the new graft. If an autograft is chosen, a slightly larger incision is made at the harvest site to retrieve the tendon tissue. The graft is then prepared on a surgical back table, where it is measured and secured with sutures or specialized fixation devices to ensure the correct length and tension.
The next step is creating bone tunnels to house and anchor the replacement ligament. The surgeon uses a guide wire and a drill to create tunnels in the femur and the tibia. The precise location of these tunnels is important, as they must accurately recreate the native ACL’s attachment points to restore proper knee motion and stability. The prepared graft is then passed through the tibial tunnel and pulled into the femoral tunnel, positioning it along the natural path of the original ACL.
Once positioned, the graft must be secured under the correct tension to ensure knee stability. The fixation method depends on the type of graft used. For BPTB grafts, which have bone blocks on each end, metal or bioabsorbable interference screws are typically driven alongside the bone blocks in the tunnels, wedging the bone against the tunnel wall for strong, direct bone-to-bone healing. Soft-tissue grafts, such as hamstring or quadriceps tendons, are often secured using suspensory fixation, like small buttons that are flipped on the outside of the bone, or by a combination of buttons and interference screws. Finally, the surgeon tests the knee’s stability and range of motion before closing the small incisions with stitches or surgical tape.
Post-Surgery Recovery and Rehabilitation
Recovery begins immediately after surgery, with the patient typically returning home the same day. The immediate focus is managing pain and swelling through elevation, icing, and prescribed medication. Early movement is emphasized, and the patient is instructed to begin passive range-of-motion exercises. Achieving full knee extension (straightening) as soon as possible is prioritized to prevent stiffness.
Physical therapy (PT) is the cornerstone of a successful recovery, often starting within the first few days of the operation. The initial phase (weeks zero to two) involves protecting the graft, controlling swelling, and re-establishing quadriceps muscle control. This progresses to the intermediate phase (weeks two to twelve), which focuses on increasing range of motion and initiating strength exercises like straight leg raises and light stationary cycling.
The final stages of rehabilitation, which can last for many months, center on intensive strengthening, balance, and agility training. Milestones like walking without crutches are often reached within the first two weeks, and light jogging can begin around three to five months. However, a return to high-risk sports is typically reserved for nine to twelve months or longer. The decision to return to sport is based on objective criteria, such as strength testing and functional performance, rather than simply the time elapsed since surgery. This is because the graft requires significant time to fully mature and incorporate into the bone.
While the success rate of ACL reconstruction is high, potential complications can arise. The most common issues include stiffness of the joint (arthrofibrosis) and anterior knee pain, particularly with kneeling, which is seen more frequently with BPTB grafts. There is also a risk of graft failure or stretching, occurring in approximately 5 to 10 percent of cases, especially if the patient returns to rigorous activity too soon. Adherence to the physical therapy protocol is the most important factor in mitigating these complications and ensuring a stable, functional knee.