The patella is a large Sesamoid bone – that is a bone which is found in a tendon. It acts as a pulley, allowing the quadriceps muscle to pull the knee straight, and as such it has enormous forces go through it. Because of this, the cartilage under the patella is the thickest in the body.

Causes

A broken patella typically occurs in one of 2 ways :

A direct blow to the front of the knee can break or shatter the patella. This mechanism also happens in a fall directly onto the front of the knee. In this mechanism, the different parts of the kneecap tend to stay close together as the soft tissues around it are not torn apart. This is sometimes called a stellate fracture.

Another common injury mechanism is a sudden rapid contraction of the thigh against resistance, with or without a blow to the knee at the same time.  In this mechanism, which is more common, the top and bottom parts of the patella tend to be split from each other. The strong pull of the quadriceps muscle then displaces the top of the bone, constantly pulling it away from where it should be.

This is a transverse patella fracture – with the 2 parts widely separated.

Symptoms

A broken patella causes pain, swelling and bruising over the knee. It hurts to move the knee, and in most cases it is too painful to stand or walk. If the fracture fragments are displaced, then it is impossible to straighten the knee against gravity or any resistance, as the quadriceps mechanism is no longer attached below the knee.

Diagnosis

To diagnose a broken kneecap, your doctor will ask you questions about how you injured the area, and will examine your knee. Simple x-rays will almost always show this fracture, but at times a CT scan may help as well.

Treatment

The majority of these injuries need surgery, to re-attach the pull of the quadriceps muscle and allow the knee to function normally. This fracture also damages the joint cartilage under the kneecap, and so if fragments are displaced they need to be replaced back together to allow good function of the joint in the future.

There are some patella fractures however where the fracture fragments do not separate, and if the joint cartilage underneath is preserved, as well as the ability to straighten the knee, then non-operative management can be successful. This usually means use of  a hinged knee brace for several weeks.

Surgery

If surgery is required, then the aim of this is to replace the bone and cartilage fragments back where they came from, and hold them there (typically with metal wires or screws) until the injury heals. The surgery is straightforward, takes less than an hour, and is done through a vertical incision directly over the front of the kneecap.

After surgery, patients are always allowed to weight-bear fully on the injured knee, but using a strong splint to support the surgical repair. How quickly knee bending is allowed depends on the type of fracture, bone strength and type of repair, and so will vary from case to case.

The fracture has been reduced (fragments repositioned) and fixed with internal wires.

Rehabilitation / return to function

Whether treated surgically or not, the fracture itself tends to heal well over the first few weeks. The difficulty with this injury, and specifically affecting recovery, is the muscle wasting and knee joint stiffness that develops whilst the bone is healing. After a short period of time, patients are usually allowed to bend the knee by a few degrees (typically 30-45) at first, protected in a hinged knee brace, and then over several weeks the allowed amount of knee flexion is increased. Physiotherapy is essential to minimise muscle loss and knee joint stiffness, and as the knee flexion increases, to re-build muscle bulk and strength. A full recovery from this injury takes at least 6 months! Most patients however make a good recovery to near normal function.