Patellar Fracture: Signs and Symptoms

Signs and symptoms of a patellar fracture

• Sudden, sharp pain at the front of the knee at the moment of injury
• Swelling within minutes to hours, as blood fills the knee joint (hemarthrosis), making the knee feel tight and full
• Bruising over the front of the knee, sometimes spreading down the shin over the next days
• Tenderness when you press on the kneecap
• A palpable gap in the kneecap if the fragments have separated
• Pain on bending the knee and difficulty walking, often with a stiff-legged limp
• Unable to straighten the knee against gravity, or to lift the leg while lying flat (in displaced fractures)

Some people can still walk, especially with a hairline (undisplaced) crack where the soft tissue sheath around the kneecap is intact. Walking does not rule out a fracture.

The straight-leg raise test

The kneecap is part of the extensor mechanism: the quadriceps muscle pulls on the quadriceps tendon, kneecap and patellar tendon to straighten the leg. If the kneecap breaks and the pieces separate, this chain is interrupted.

To check, lie on your back and try to lift your injured leg straight off the bed, keeping the knee locked. If you cannot, or the knee bends as you lift, the extensor mechanism is likely disrupted. This usually means surgery is needed. Doctors perform this test at the first assessment, sometimes after draining blood from the joint and injecting local anesthetic so pain does not stop you trying.

How a fractured patella happens

• Direct blow: a fall onto a bent knee or the knee hitting a car dashboard. This often shatters the kneecap into several pieces (comminuted fracture).
• Indirect force: a sudden strong quadriceps contraction while the knee is bent, such as stumbling or landing from a jump, pulls the kneecap apart in a horizontal (transverse) break.
• Stress fracture: rare; from repeated jumping or after knee surgery.
• Weak bones: osteoporosis makes fractures from minor falls more likely in older adults.

Patellar fracture vs similar knee injuries

All three extensor mechanism injuries (fracture and the two tendon ruptures) can cause an inability to straighten the leg, so imaging is needed.

How it is diagnosed

1. Examination: swelling, tenderness, a gap, and the straight-leg raise test. The doctor also checks skin over the kneecap for wounds and the circulation and nerves in the foot.
2. X-rays: front, side and skyline (looking down along the kneecap) views show most fractures and whether the pieces are separated. Separation of more than about 2 to 3 mm, or a step in the joint surface of more than 2 mm, generally points toward surgery.
3. CT scan: maps fragments in comminuted fractures before surgery.
4. MRI: used when X-rays are normal but a tendon tear, cartilage injury or hidden fracture is suspected.

What happens next

Treatment depends on displacement and whether you can straighten the knee:

• Undisplaced fracture with an intact extensor mechanism: a brace or cylinder cast holding the knee straight for about 4 to 6 weeks, with weight bearing as comfort allows, followed by physical therapy.
• Displaced fracture or failed straight-leg raise: surgery, typically with wires, pins or screws (tension band wiring) or plates. Small shattered pieces may be removed and the tendon reattached.

Bone heals in about 6 to 8 weeks, but regaining knee bending and quadriceps strength takes months. Full return to sport often takes 4 to 6 months. For details on surgery, the recovery timeline and long-term effects such as kneecap arthritis, see our page on broken kneecap treatment and recovery.

First aid before you see a doctor

• Keep the leg straight and supported; avoid bending the knee
• Apply a cold pack wrapped in a cloth for 15 to 20 minutes at a time
• Raise the leg above heart level when resting
• Take acetaminophen (paracetamol) or ibuprofen at the label dose
• Use crutches or get help moving rather than walking on it
• Cover any wound with a clean dressing and go to the ER

Frequently asked questions

What are the signs and symptoms of a patellar fracture?

The main signs are sudden pain at the front of the knee after a fall or blow, rapid swelling, bruising, tenderness over the kneecap and pain on bending. In displaced fractures you may feel a gap in the kneecap and be unable to straighten the leg or lift it while lying down.

Can you walk with a fractured patella?

Sometimes. With a small, undisplaced crack you may be able to walk with the knee held straight, although it hurts. With a displaced fracture, walking is usually very difficult because the knee gives way. Being able to walk does not rule out a fracture, so get an X-ray.

How can I tell if my kneecap is broken or just bruised?

A bruised kneecap is tender but you can still straighten the knee fully and lift your straight leg. A fracture tends to cause rapid swelling inside the joint, sharp pain when the kneecap is pressed, and often a weak or failed straight-leg raise. Only an X-ray can tell for certain.

Does a fractured patella always need surgery?

No. Fractures where the pieces have not separated and you can still straighten your leg are usually treated in a straight-leg brace or cast for about 6 weeks. Surgery is needed when the pieces are separated, the joint surface is uneven, or you cannot straighten your knee.