Tears of the anterior cruciate ligament (ACL) often result from sports-related injuries and significantly compromise the stability of the knee joint. In particularly to anterior translation and rotational forces and also disrupts the proprioceptive function of the knee. Patients often require surgical reconstruction of the ligament to enable them to return to playing pivoting sports and prevent further instability and damage to the knee, the main risk being meniscal tears and chondral injuries. The significance of this injury and ligament function is highlighted by the approximate 60,000-175000 ACL reconstruction surgeries performed annually in the United states.
Structure and Function
The complicated anatomy of the knee includes the interaction of three bones, 4 major ligaments, menisci and the surrounding musculature. This hinge joint is an articulation between the femur, tibia and patella. Stabilizing and firmly connecting these bones externally are two major ligaments, the medial collateral (MCL) and lateral collateral ligament (LCL). Internally, joint stabilization can be attributed to the ACL and posterior cruciate ligament (PCL). These ligaments are the primary restraints to anterior and posterior (particularly at 90 degrees knee flexion) translation of the tibia on the femur respectively. Acting as shock absorbers are the two fibrocartilaginous menisci, each located laterally and medially on the surface of the tibia. It is important to appreciate the different morphology of the medial and lateral tibial plateaus. The medial plateau is concave and thus congruent with the medial femoral condyle whereas the lateral plateau is convex making it less congruent. The lateral meniscus, therefore, has an important role in increasing the congruence of the lateral compartment and its absence results in rapid degenerative changes. Finally, two major muscle groups surround the knee joint, the quadriceps and the hamstring muscles. The quadriceps consists of four muscles, and is responsible for knee extension. The hamstrings consist of three muscles and generates flexion at the knee.
Those presenting after the initial inflammatory phase has resolved may describe a sense of instability and giving way and feel unable to ‘trust’ the knee especially with pivoting activities. They may also have associated joint line pain and swelling due to meniscal tears (especially lateral) and chondral damage due to recurrent subluxation.
Hearing a "popping" sound, followed by immediate and severe swelling combined with instability are three symptoms highly suggestive of an ACL tear. If these symptoms are experienced, continued activity on the damaged knee leads to further consequences, such as tearing of menisci and cartilage. May also present with a locked knee (inability to fully extend the knee) – this may be due to a bucket handle tear of the medial meniscus or a bulky ACL stump and requires urgent arthroscopic surgery to remove the mechanical block and restore the range-of-motion (ROM) of the knee.
Rehabilitation is a critical factor in achieving a stable and well-functioning knee after ACL reconstruction. It generally takes around 2-3 months for graft incorporation into the bone. A brace may be worn 1-2 weeks post-surgery while initial healing takes place. A basic outline of rehab is as follows. Within the first month, passive ROM isometric exercises can begin as soon as 1 week post surgery. At the second and third month, the patient can start closed followed by open kinetic chain exercises. At the 4th-5th month mark, sport-specific training can begin and at 6-9 months the patient can return to full sports with 6 months being the earliest that this should be attempted ensuring that the patient has achieved the required goals prior to this.
ACL reconstruction has a reported success rate of over 90%1. Possible failures and complications can arise such as deep venous thrombosis, infection, graft impingement and graft rejection. Most importantly, placement of the graft insertion in particularly the femoral tunnel is the most underlying cause for ACL reconstruction failure.
Recent literature has demonstrated that there is no significant difference in outcomes based on graft choice. A systematic review by Timothy Foster, et al discussing allograft versus autograft concluded no superior outcome arises from either graft choice. Additionally, a 10- year follow-up study by Leo Pinczewski, et al comparing patella tendon versus hamstring tendon demonstrated no significant difference in functional outcomes. However, the article preferred hamstring reconstructions due to less donor site symptoms and decreased rates of osteoarthritis.
Outcomes in regards to returning to competitive sports are generally good. In a study by Shelbourne, et al of ACL reconstructions in high-school athletes, 95% recovered to participate in competitive sports. A reported 84% of females and 81% of males were able to compete at the same, if not higher level then before.
Anterior Cruciate Ligament, Anterior Tibial translation, Reconstruction, Osteoarthritis
- Understand the underlying anatomy and mechanism of ACL injury
- Being able to identify and diagnose the injury
- Understand the options for treatment including deciding on the appropriate treatment for the individual patient.