The knee is a complex, modified hinge joint designed primarily to allow the leg to bend and straighten. Achieving full flexion, the motion of bringing the heel toward the buttock, requires the joint to bend significantly, often up to 135 to 150 degrees in a healthy individual. When this full range of motion (ROM) is restricted, it becomes difficult to perform basic activities like squatting, climbing stairs, or even sitting comfortably. This inability to fully bend the knee is a common symptom that points to several distinct underlying issues, ranging from physical blocks within the joint to soft tissue tightness surrounding it.
Physical Blockages Within the Knee Joint
One clear reason for limited knee flexion is a mechanical obstruction that physically prevents the joint from closing fully. The sensation often described is a sudden, hard stop, as if something is caught in the hinge mechanism. This blockage frequently involves internal derangements located within the joint capsule itself.
A common culprit is a torn meniscus, the C-shaped cartilage that acts as a shock absorber between the thigh bone and shin bone. A bucket-handle tear, where cartilage flips into the joint space, can become trapped and impede the smooth motion required for deep flexion. This displaced tissue acts like a wedge, locking the knee at a certain angle.
Loose bodies, which are fragments of bone or cartilage floating freely in the joint fluid, can also cause mechanical locking. These fragments migrate and become lodged between the femur and tibia, suddenly halting the bending motion. In advanced osteoarthritis, the formation of osteophytes (bone spurs) can physically narrow the joint space. These bony growths collide during deep flexion, creating a physical barrier to further movement.
Joint Swelling and Acute Inflammation
A different category of limitation arises from the accumulation of fluid and the body’s protective response to inflammation. Joint effusion, often called “water on the knee,” occurs when excess synovial fluid builds up inside the joint capsule. This fluid accumulation creates a volumetric restriction, meaning the joint runs out of physical space to bend fully.
Conditions like acute bursitis, injury, or synovitis can trigger this swelling. The distended joint capsule creates a feeling of stiffness and puffiness, and the knee may appear visibly larger. This limitation feels softer and tighter than a mechanical block, stopping movement due to pressure and discomfort.
Accompanying this swelling is pain inhibition, where the nervous system reflexively limits muscle activation to protect the joint. Pain signals prevent surrounding muscles, particularly the quadriceps, from relaxing enough to allow full bending. This protective guarding contributes significantly to the reduced range of motion.
Restrictions from Muscles and Scar Tissue
Beyond issues within the joint, the soft tissues surrounding the knee can impose external restrictions on its ability to flex. The quadriceps muscle group must lengthen significantly to allow the heel to approach the buttock. If these muscles are chronically tight, weak, or in spasm, they resist the bending motion, limiting the final degrees of flexion.
Tightness in the hamstrings and calf muscles, which cross the knee joint posteriorly, can also contribute to restriction. These muscle imbalances and lack of flexibility often develop from inactivity or an altered gait following an injury. The shortened muscles do not have the necessary length to accommodate the full range of angular change required for deep bending.
A complex cause of stiffness is arthrofibrosis, the excessive formation of dense scar tissue inside and around the joint, often occurring after surgery or significant trauma. This fibrous adhesion binds structures together, preventing the gliding and rolling needed for smooth movement. This rigid, fibrotic tissue significantly limits both straightening and bending, making it a challenging form of stiffness to overcome.
Seeking Diagnosis and Treatment
Identifying the specific cause of limited knee flexion is necessary to determine the correct course of action. A medical professional performs a physical examination to assess the quality of the limitation—whether it is a hard mechanical stop or a softer, painful restriction. They measure the exact degree of flexion, compare it to the unaffected knee, and check for signs of swelling or tenderness.
Diagnostic imaging confirms the suspected cause. X-rays reveal bony issues like advanced arthritis and bone spurs, while a Magnetic Resonance Imaging (MRI) scan visualizes soft tissues. An MRI is useful for identifying meniscal tears, loose bodies, or extensive scar tissue not visible on a standard X-ray.
Treatment pathways are tailored to the underlying problem. Physical therapy focusing on stretching and mobilization is the primary approach for soft tissue tightness or mild arthrofibrosis. Swelling and acute inflammation are managed with anti-inflammatory medications or aspiration of excess fluid. If the limitation is a true mechanical block, such as a displaced meniscal tear, surgical intervention may be required to remove the obstruction.