An acute injury to the patella (kneecap) can happen from a blow to the knee or a fall. There may be damage to the soft tissues, such as a patellar tendon tear, or a fracture to the bone.
Symptoms may include pain, swelling, or a feeling of instability or that the joint is locked. Some types of injuries can be treated with bracing and rehabilitation exercises, but others may need surgery.
Types of Acute Patellar Injuries
The patella is part of the knee joint, along with the tibia (shin bone) and femur (thigh bone). It is wrapped in the patellar tendon, which connects the quadriceps muscle of the thigh to the tibia below the knee joint.
Sitting at the front of the knee joint, the patella tracks in a groove at the end of the femur (the patellofemoral joint) and enhances the extension of the knee.
The most common types of acute patellar injuries include:
Patellar tendon tear: A tear may be small, partial, or complete. A complete tear is a serious injury that requires surgery and a long recovery of at least four to six months.1
Kneecap dislocation: This occurs when the kneecap comes completely out of its groove on the femur, usually to the side. It must be put back, which is called a reduction.2 While painful, it is not as serious as a knee dislocation, in which the thigh bone and shin bone lose contact with each other.
Patellar subluxation (unstable kneecap): In this condition, the patella does not track evenly within its groove on the femur.3 This can cause discomfort with activity and pain. It might lead to partial or full dislocation.
Fracture (broken kneecap): The patella bone can be broken during a fall or an impact. It can be a complicated fracture requiring surgery.4
Acute injuries of the kneecap will produce symptoms common to soft tissue and bone injuries, such as pain, swelling, and deformity. You often will have functional symptoms as well.
Common symptoms include:5
Pain: Most acute patellar injuries are very painful. Knee pain may be more noticable during specific activities, including walking stairs (particularly going down), or kneeling. But it can be so severe that you can't place any weight on the leg at all.
Swelling: Acute injuries often produce swelling from inflammation.
Noises: You may hear a popping noise or feel a snapping sensation at the time of injury, especially with a patellar tendon tear or dislocation. With an unstable knee joint, you may hear creaking or feel grinding (crepitus).
Instability: You may not be able to support your weight on the leg after the injury. It may buckle when you try to stand up or walk.
Locked joint: You may find the joint locked up and be unable to bend or straighten your knee.
Deformity: Especially with a fracture or dislocation, your knee joint will be misshapen.
Bruising: There can be significant bruising with a fracture or tendon tear, but also with any traumatic injury.
Injuries to the patella usually result in difficulties in moving the knee, walking, or running. The patella is important functionally because it increases the leverage of the knee joint and the strength of extension of the leg.
A dislocated kneecap may spontaneously pop back into place. This can produce bruising and damage to the soft tissues. You should see your doctor as soon as possible for further assessment even if there doesn't seem to be damage.
Prepatellar bursitis (inflammation and swelling of the sac around the knee) can be a complication of traumatic injuries to the patella, either due to the injury itself or infection following an injury. In addition to swelling at the front of the kneecap, it may be tender and warm.6 If due to infection, there may be fever and chills as well.
Knee Pain: When to See a Doctor
Acute patellar injuries can occur due to trauma, sports activities, or anatomical problems within the knee joint.
Accidents and Trauma
The location of the patella at the front of the knee makes it vulnerable to fracture, dislocation, or tendon tears during falls, blows to the knee, or sharp impacts such as with the dashboard in a car accident.
Traffic accidents are the cause of 78.3% of patella fractures. Work-related accidents and accidents in the home account for 13.7% and 11.4%, respectively.7
If a person has osteoporosis, the weakness of the bone may result in a patella fracture from a minor fall or blow to the knee.4 Pathologic patella fractures may also be seen in cases of bone infection or a bone tumor.
Acute patellar injuries can also happen during sudden movements such as those that may be made during sports activities—for example, when the foot is planted and the torso rotates swiftly during a swing of a baseball bat.8
You can tear your patellar tendon when landing from a jump with the knee bent and foot planted. You may be predisposed to a patellar tendon tear if you already have patellar tendonitis (jumper's knee), which is inflammation caused by overuse of the knee joint. It is seen in people who jump on hard surfaces, such as basketball or volleyball players.
A sudden contraction of the quadriceps can pull the patella apart, resulting in a fracture.4 This can happen when you land a jump from a height, although it is uncommon.
The anatomy of the knee is another source of injury, especially instability or dislocation.
Some people are born with an uneven or shallow femoral groove. This compromises the integrity of the patellofemoral joint and can result in the kneecap being displaced due to seemingly normal activities—not just a blow or a fall.
Some children (especially girls) may be more prone to kneecap dislocations because they have looser ligaments.2
Anatomy of the Knee
Depending on the circumstances, you may see your primary care physician or seek emergency treatment. Urgent assessment is best if the knee injury was sustained during a fall or accident, or if a kneecap dislocation has not popped back into place.
The healthcare provider will take your history and report of what led to the knee pain, symptoms at the time (such as hearing a pop), and your current symptoms.
During the physical examination, the healthcare provider will observe whether you are able to walk and are able to bend or unbend your knee. They will palpate the knee to see if there are areas that show defects, displacement, or localized pain. A visual exam of the knee assesses whether there is swelling, deformity, or obvious displacement.
Manual tests and maneuvers are used to assess damage to the kneecap or tendons, including trying to extend your knee against gravity. The straight leg raise test can reveal disruption of the extensor mechanism, which includes the quadriceps tendon, patella, and patellar tendon.
Imaging and Labs
An X-ray is often the first imaging test used, as it can show whether there is a fracture, which is important to find or exclude early.
It may be the only imaging done, or the provider may order magnetic resonance imaging (MRI) to further assess damage to the ligaments, tendons, or cartilage.
Blood tests are often done to assess your general health, but may specifically look for signs of inflammation if an infection is suspected. If there is significant swelling, arthrocentesis can be used to extract fluid from the knee for evaluation in the lab, looking for infection or blood from an injury.
The differential diagnosis may include arthritis and chondromalacia patellae (runner's knee).9 These can be the source of knee pain without a traumatic incident, or they may be present in addition to other patellar injuries.
Treatment of these various acute kneecap conditions depends on the diagnosis. However, there are some general guidelines that can be followed. Your doctor will advise you accordingly based on your diagnosis.
Acute kneecap injuries require self-care, which you can do until you get medical treatment and afterward as recommended by your doctor:
Rest the injured knee to prevent further injury and allow time for inflammation to subside.
Apply ice to the knee to reduce inflammation. Ensure there is a cloth barrier between the ice pack and the skin, and do not ice the injury for more than 15 minutes at a time.
Nonsteroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen) and Aleve (naproxen) to help with inflammation and alleviate some of the pain.
When you do return to activity (with your doctor's OK), do so gradually. Cross-training can help you maintain your fitness.
A kneecap dislocation needs to be reduced (the kneecap is returned to the femoral goove). If this didn't happen spontaneously, the physician will do the reduction as soon as possible.
A reduction might require pain medication so the provider can extend the leg and manipulate the kneecap. Often, it will snap back into place with gentle pressure when the leg is extended.2
Any of the acute patellar injuries may need to be immobilized with a cast, splint, or brace to allow healing of the bone, tendon, or other soft tissues. Your physician will recommend which of these is appropriate, if any.
Your doctor may recommend you not bear weight on the affected leg during the immobilization period. You may need to use crutches or other mobility aids until this period is complete.
A fractured kneecap may require surgery if any pieces of bone are out of place.4 This often means installing wires, screws, plates, or pins to unite the bone pieces and keep them in place while you are healing.
Patellar tendon tears, unless very small, usually require surgical repair as soon as possible.1 Sutures are placed in the tendon and anchored to holes or surgical anchors on the patella. This can be done as an outpatient surgery.
There are several surgical kneecap stabilization procedures that can be done for recurring patella dislocations or subluxations. These may involve adjusting or reconstructing the ligaments that hold the knee in place.10 Recurring partial and full dislocations can damage the medial patellofemoral ligament (MPFL) and it might need repair as well.
Weight-Bearing Restrictions and Activity After Surgery
For any acute patellar injury, physical therapy is likely to be recommended after the initial inflammation has subsided and immobilization is no longer needed. Immobilization itself will result in stiffness and loss of muscle strength. Your physical therapist will work to restore the knee's range of motion and muscle strength.4
If weight bearing was not allowed, you will gradually progress to being able to support weight on the injured leg. This starts with toe-touch weight bearing for a couple of weeks, then progresses to 50% weight bearing, and finally full weight bearing after four to six weeks.1
The function of the knee joint requires balancing the strength of the muscles around the knee joint. Most importantly, the quadriceps and hamstring muscle groups should be flexible and balanced. Your physical therapist will recommend specific exercises depending on your condition.
A Word From Verywell
An acute kneecap injury is not only painful but frustrating, as it will mean restrictions in your activities while you heal. Talk to your doctor and physical therapist so you understand what you can do to ensure a full recovery. Most people are able to return to their previous activities following a fracture, tendon tear, or dislocation.