The inability to fully straighten the knee (lack of full terminal extension) can significantly affect daily life. Even a slight deficit, such as a few degrees of lost motion, compromises the knee’s stability and alters the way a person walks. This loss forces the leg to remain slightly bent, causing an abnormal gait that stresses other joints and potentially leads to issues elsewhere in the body. Restoring this motion is often considered one of the most important goals after a knee injury or surgery, as its absence can increase the risk of developing early arthritis.
Acute Issues Causing Temporary Loss of Extension
Immediate inability to straighten the knee often follows a recent injury or a sudden flare-up. One common cause is joint effusion, the accumulation of fluid within the joint capsule, often called “water on the knee.” A large volume of this fluid increases pressure inside the joint, physically preventing the bones from achieving full extension. If the fluid is blood (hemarthrosis), the effect is typically more pronounced and linked to significant trauma, such as an Anterior Cruciate Ligament (ACL) tear.
The pain and swelling from an acute injury also trigger a protective neurological response called arthrogenic muscular inhibition (AMI). AMI causes the brain to reflexively limit the firing of the quadriceps muscle, which is responsible for pulling the leg into full extension. This protective reflex is meant to safeguard the injured joint but results in the inability to straighten the knee. Patients instinctively keep the knee in a slightly bent position, typically around 15 degrees of flexion, because this is the position of least pressure and discomfort inside the joint.
Physical Obstructions Inside the Joint
Sometimes, the inability to straighten the knee is due to a mechanical block. This is often described as a true “locking” sensation where the knee suddenly gets stuck and cannot move past a certain point. A frequent culprit is a torn meniscus, particularly a specific type called a “bucket-handle tear.”
In a bucket-handle tear, a large, U-shaped fragment of the cartilage flips into the central joint space, acting like a wedge that jams the knee and prevents full extension. Another mechanical cause is the presence of loose bodies, sometimes referred to as “joint mice.” These are fragments of bone or cartilage that have broken off due to trauma or degenerative conditions like osteoarthritis. When lodged between the femur and tibia, they cause a sudden, sharp block to motion.
Long-Term Stiffness and Tissue Restriction
If the inability to straighten the knee persists for weeks or months, it may be due to chronic tissue adaptations. A major cause of persistent stiffness is arthrofibrosis, characterized by the excessive formation of dense scar tissue within and around the joint. This phenomenon, sometimes called “stiff knee syndrome,” frequently occurs following major knee surgery, such as an ACL reconstruction or total knee replacement, but can also follow severe trauma.
The uncontrolled production of collagen in arthrofibrosis creates thick, restrictive scar bands, resisting any attempt to achieve full extension. This chronic restriction is often termed a flexion contracture, where the knee is permanently held in a bent position. Contractures of the surrounding muscles, like persistent tightness in the hamstring muscles, can also passively resist the final degrees of extension. These long-term issues require a different approach than acute swelling, often involving intensive, prolonged stretching to remodel the resistant collagen fibers.
When to Seek Professional Evaluation
It is important to seek prompt medical attention if you experience certain warning signs alongside the inability to straighten your knee. Immediate evaluation is necessary if:
- You cannot bear weight on the leg.
- The knee is acutely locked and will not move.
- You notice signs of infection such as fever, extreme redness, or warmth.
These symptoms can indicate a serious injury like a large ligament tear, a trapped meniscus, or septic arthritis. If the lack of full extension is persistent, even without severe pain, consultation with a physical therapist or orthopedic specialist is advised.