The Anterior Cruciate Ligament (ACL) is a thick band of fibrous tissue located deep within the center of the knee joint. It extends diagonally, connecting the femur (thigh bone) to the tibia (shin bone), forming a cross with the Posterior Cruciate Ligament. This arrangement stabilizes the knee during movement.
The primary role of the ACL is to prevent the tibia from sliding too far forward relative to the femur, a motion known as anterior translation. It also provides rotational stability, controlling the pivoting and turning motions common in sports and daily activities. The ligament’s structure ensures that some portion remains taut throughout the knee’s range of motion, providing continuous stability.
The Immediate Answer: Straightening the Leg
The ability to fully straighten the knee, known as full extension, immediately after an ACL tear varies significantly. Some individuals achieve full extension almost immediately following the injury, experiencing only instability and pain. For these people, the ACL tear itself does not necessarily create a mechanical block to motion.
However, many others find themselves unable to fully straighten the leg after the injury. This inability is often a sign of an associated injury or the body’s protective response, rather than a direct result of the torn ACL. Since the ACL rupture causes instability, not a physical obstruction, an inability to straighten the knee points to a more complex mechanical or physiological issue within the joint capsule.
Mechanical Obstructions to Full Extension
The most significant cause of a true mechanical block to straightening the leg is the displacement of another structure within the joint. A common culprit is a meniscal tear, particularly a large “bucket-handle” tear, where a piece of the C-shaped cartilage flips into the central joint space. This displaced tissue acts as a physical wedge, preventing the femur and tibia from achieving the full contact needed for terminal extension.
Another major obstruction is severe joint effusion or swelling. The rapid accumulation of fluid and blood, called hemarthrosis, creates internal pressure within the joint capsule, physically limiting movement. Furthermore, the nervous system often triggers a protective muscle guarding response, causing the hamstring muscles to contract involuntarily and keep the knee flexed.
Diagnostic Significance and Initial Care
The inability to achieve full knee extension is a symptom that raises immediate concern for clinicians evaluating an acute knee injury. When a patient presents with a “locked knee,” it suggests a true intra-articular pathology, such as a displaced meniscus, requiring timely intervention. This mechanical blockage can place abnormal pressure on the articular cartilage surfaces, potentially leading to long-term joint damage if not resolved quickly.
Initial Management
Initial care focuses on controlling the acute inflammatory response to reduce swelling and pain. The RICE protocol (Rest, Ice, Compression, and Elevation) is commonly recommended to reduce the hemarthrosis contributing to the extension deficit. Clinicians confirm the ACL tear using specific physical exam techniques, such as the Lachman test, which is a separate diagnosis from the extension issue. If the lack of extension persists despite swelling reduction, surgery may need to be delayed until motion is restored, or an earlier procedure may be necessary to clear the mechanical block.
Recovery of Full Extension
Restoring the ability to fully straighten the leg is one of the most important goals in the initial treatment phase, regardless of whether surgery is planned. Regaining full extension before ACL reconstruction is strongly linked to better outcomes and helps prevent arthrofibrosis, or excessive scar tissue formation. Failure to recover this motion preoperatively is a major predictor of stiffness after the procedure.
Physical therapy employs specific stretching techniques to restore passive extension. Common methods include the heel prop, where gravity gently stretches the knee, and patellar mobilization to ensure the kneecap glides properly. Once passive extension is achieved, rehabilitation shifts to activating the quadriceps muscle so the patient can actively hold the leg straight. Achieving symmetrical full extension compared to the uninjured leg is a fundamental benchmark that must be met before a patient can safely progress to higher-level strengthening and return to sport.