The patella, commonly known as the kneecap, is a small, flat, triangular bone located at the front of the knee joint. It is the largest sesamoid bone in the body, embedded within the quadriceps tendon. The patella acts as a shield for the knee joint and functions as a powerful lever. It increases the mechanical efficiency of the quadriceps muscle, allowing for greater force when extending the lower leg. A patellar fracture, or broken kneecap, is a serious injury that can compromise the entire extensor mechanism of the leg.
Mechanisms of Patellar Fracture
Patellar fractures typically result from two distinct mechanisms: direct trauma or indirect trauma. Direct trauma occurs when a forceful blow is delivered straight to the kneecap, such as falling directly onto the knee or an impact during an accident. This high-energy compressive force often causes the bone to break into multiple pieces.
Direct impacts frequently result in comminuted, or shattered, fracture patterns as the bone is compressed against the underlying femur. While fragments may not be widely separated, the articular cartilage on the back of the kneecap is often significantly damaged. Damage to this surface increases the long-term risk of developing post-traumatic arthritis.
Indirect trauma is caused by a sudden, violent contraction of the quadriceps muscle while the knee is bent. This action generates extreme tension that pulls the patella apart. Injuries resulting from indirect force are frequently transverse fractures, a clean break across the middle of the bone. The powerful quadriceps muscle can pull the upper fragment away from the lower fragment, causing significant separation.
Immediate Signs and Symptoms
Following a patellar injury, severe, localized pain in the front of the knee is an immediate symptom. This pain is quickly accompanied by rapid and pronounced swelling around the joint, often due to the accumulation of blood within the joint space.
A person with a patellar fracture will often notice significant bruising, which may spread down the leg over time. A specific and concerning symptom is the inability to actively straighten the leg against gravity or perform a straight leg raise. This inability indicates that the extensor mechanism, which allows leg extension, has been compromised by the fracture.
In some cases, especially with displaced fractures, a gap or defect may be felt in the kneecap when gently palpated. This palpable defect signifies that the fractured bone segments have separated. Any combination of these signs requires prompt medical evaluation, as the integrity of the knee’s primary movement system is at risk.
Classification of Patellar Fractures
Medical professionals classify patellar fractures based on the pattern of the break and the degree of separation between the fragments. Understanding the fracture’s characteristics directly influences the treatment plan.
A non-displaced or stable fracture is one where the bone is cracked, but the fragments remain closely aligned, often separated by only a millimeter or two. These stable breaks usually maintain a smooth joint surface.
A displaced fracture means the bone pieces have shifted out of their correct anatomical position, disrupting the joint surface. If the displacement exceeds a few millimeters or the joint surface has a noticeable step-off, the extensor mechanism is compromised. Displaced fractures are typically unstable and often require surgery to restore function.
The pattern of the break provides further detail. Transverse fractures are a common type involving a horizontal line across the bone. These are frequently caused by indirect trauma and are often displaced due to the pull of the quadriceps tendon. Comminuted fractures occur when the patella shatters into three or more pieces, often resulting from high-energy direct trauma.
Treatment Pathways and Rehabilitation
The treatment for a patellar fracture depends on the classification, specifically whether the fracture is displaced and whether the extensor mechanism remains intact.
Non-surgical treatment is reserved for stable, non-displaced fractures where fragments are in good alignment and the patient can still perform a straight leg raise. This approach involves immobilizing the knee in a cast or a hinged knee brace locked in full extension.
Immobilization typically lasts about six to eight weeks to allow for bone healing. Weight-bearing may be permitted depending on the specific fracture pattern and physician instructions. The goal of non-surgical management is to ensure the fragments heal without separation while protecting the knee joint.
Surgical treatment is necessary for most displaced or severely comminuted fractures to restore function. The procedure often involves Open Reduction and Internal Fixation (ORIF), where the surgeon realigns the bone fragments and secures them with hardware. Fixation methods include using wires, pins, screws, or plates, often in a “figure-of-eight” tension band configuration for transverse breaks. The primary goal of surgery is to anatomically restore the joint surface and re-establish the extensor mechanism.
In cases of severe comminution where fragments are too small to fix, a partial removal of the patella may be necessary. Following the initial treatment phase, rehabilitation is a long process, regardless of whether surgery was performed. Physical therapy (PT) begins early to combat joint stiffness and muscle atrophy that occur after immobilization. Initial PT focuses on restoring a functional range of motion, followed by exercises aimed at strengthening the quadriceps and surrounding leg muscles. The full recovery timeline can range from four to six months, with some patients requiring longer to regain pre-injury strength and mobility.