It is a common instruction given after knee or lower leg orthopedic surgery: do not place a pillow directly under the knee while resting. This advice is frequently given following procedures like total knee replacement, but also after certain hip or lower leg surgeries. While placing a pillow under the joint feels temporarily comforting and helps alleviate tension, following this guideline is an important step that directly influences the success of your recovery. Ignoring this instruction, particularly in the critical first weeks, can create a significant obstacle to regaining the full range of motion necessary for normal daily life.
The Mechanism of Flexion Contracture
The primary reason for avoiding a pillow directly beneath the knee is the risk of developing a complication known as a flexion contracture. This condition occurs when the soft tissues surrounding the knee joint—including the muscles, ligaments, and the joint capsule—shorten and tighten. These tissues adapt to the position the joint is held in for prolonged periods, which is a slightly bent or flexed position.
Even a minimal degree of knee flexion, maintained over several hours a day, can encourage this shortening process. When the knee is bent, it places the structures at the back of the leg, like the hamstring muscles, in a relaxed, shortened state. If this becomes the default resting position, the tissues start to heal in that shortened configuration.
The result is a physical limitation that prevents the knee from achieving full extension, or complete straightness. After surgery, the main goal for range of motion is to achieve “terminal extension,” which is a perfectly straight knee, ideally 0 degrees of flexion. A flexion contracture makes achieving this zero-degree extension extremely difficult and painful. This is a mechanical problem where the tightened soft tissues physically block the joint from straightening out completely.
Consequences for Physical Therapy and Long-Term Mobility
The inability to fully straighten the knee has serious consequences for both rehabilitation and long-term function. Full knee extension is a fundamental requirement for the biomechanics of normal walking. If the knee cannot fully extend, the leg is functionally shortened, forcing the patient to walk with a bent-knee gait, often called a “crouch gait.”
This altered walking pattern requires significantly more energy because the muscles, especially the quadriceps, must constantly work harder to support the body’s weight. Studies have shown that a flexion contracture exceeding 20 degrees can notably increase the energy cost of walking. This increased energy expenditure leads to accelerated fatigue and often results in a noticeable limp or uneven gait.
Physical therapy (PT) is specifically designed to restore full joint function, with a major focus on achieving and maintaining terminal extension. A pre-existing contracture will slow down PT progress, as the therapist must spend extra time stretching the shortened tissues before working on strength goals. Patients with a persistent contracture may experience increased anterior knee pain and lower functional scores, sometimes requiring more aggressive interventions like manipulation under anesthesia.
Safe Alternatives for Elevation and Comfort
While avoiding a pillow directly under the knee is important, managing post-operative swelling and finding a comfortable resting position are necessary for recovery. Elevation is still recommended to reduce fluid buildup and inflammation. The proper technique involves elevating the entire lower leg while ensuring the knee joint remains straight.
The support should be placed under the ankle, heel, and calf, allowing the knee joint itself to hang free or rest on a firm, flat surface. This placement keeps the knee in a fully extended position, actively counteracting the development of a flexion contracture while still allowing gravity to assist in fluid drainage. You can use a specialized foam wedge or a stack of firm pillows placed only under the lower calf and foot.
The goal is to raise the leg so the ankle is positioned above the level of the heart, which is generally considered the most effective angle for reducing swelling. During rest periods, maintaining this supported, straight position is a simple but effective way to support the surgical outcome. Using ice packs is also an excellent complementary measure for controlling pain and swelling without compromising your knee extension.