Q: What is a sleeve fracture of the knee?

A: You are probably not familiar with the term sleeve fracture of the patella (kneecap) because this is a very rare injury. Of all the bone breaks children have, the kneecap is only involved in about one per cent of the cases. And sleeve fractures make up about half of those patellar injuries.

What's a sleeve fracture? A little anatomy will help explain what happens. The patella or kneecap sits in front of the knee joint. It isn't attached by a piece of bone or bone bridge. Instead, it moves freely up and down, gliding along a set pathway or patellar track. The kneecap is held in the track by the quadriceps tendon.

The quadriceps tendon is wrapped around the kneecap to hold it in place. At the upper end, the tendon is attached to the large four-part quadriceps muscle along the front of the thigh. Its job is to straighten the knee. The quadriceps tendon continues down below the knee cap where it inserts or attaches to the tibia (lower leg bone).

With a sleeve fracture, the quadriceps tendon is torn so severely, it separates from the muscle and takes a piece of the cartilaginous patella with it. It also takes the top layer of bone called the periosteum. When the periosteum is peeled away with a fragment of the underlying bone still attached, it is called a sleeve avulsion.

Sleeve fractures of the patella can actually occur at the top of the kneecap (called the superior pole) or at the bottom (inferior pole). Most sleeve fractures involve the inferior pole.

In children who are not fully grown yet (we say they are skeletally immature), the patella is still more cartilage than bone. The softer cartilaginous patella in the skeletally immature child tears more easily than solid, hardened bone in a skeletally mature individual.

Sleeve fractures were first described in the literature in 1979. Boys are affected five times more often than girls. Most are between the ages of eight and 16 years old. Increased high-intensity sports activity may be one reason this type of injury has started to show up.

You will be relieved to know most children are able to recover fully. They resume full participation in all activities and sports. There may be some occasional knee pain with certain activities like running and jumping. But for the most part, the fracture heals, the kneecap tracks normally, and the quadriceps muscle bulks up again.

Reference: F. Keith Gettys, et al. Superior Pole Sleeve Fracture of the Patella. In The American Journal of Sports Medicine. November 2010. Vol. 38. No. 11. Pp. 2331-2336.