Final answer:
If a torn anterior cruciate ligament (ACL) is not properly repaired, it can lead to an inability to prevent knee hyperextension and instability, as the ACL is vital for knee joint stability.
Step-by-step explanation:
If a torn anterior cruciate ligament (ACL) is not properly repaired, the consequences could include the inability to prevent hyperextension of the knee and knee instability. ACL injuries can occur from a forceful blow to the anterior knee, causing the knee to extend beyond its normal range of motion, or when making a rapid change in direction with the foot planted, resulting in both twisting and hyperextension. The ACL is an intracapsular ligament of the knee that extends from the anterior, superior surface of the tibia to the inner aspect of the lateral condyle of the femur and is crucial for stabilizing the knee when moving.
Furthermore, an unrepaired ACL can lead to a series of knee injuries known as the "terrible triad," which includes damage to the tibial collateral ligament, the medial meniscus, and the ACL itself. This set of injuries commonly results from a powerful blow to the lateral side of the knee. An intact ACL is essential to prevent excessive forward movement and rotation of the tibia, thereby maintaining overall stability of the knee joint.