Types

What types of knee fractures can occcur?

Fractures of the Femur at the Knee

Avulsion Fractures

The medial epicondyle is a small bump of bone on the inner side of the end of the femur (thigh bone). The medial collateral ligament attaches there and passes to the shin bone below the knee. This ligament prevents abnormal inwards movement of the knee. If the knee is forced into an inward bend the ligament either stretches, ruptures or pulls off the medial epicondyle.

This type of avulsion fracture is not very common and may accompany other serious injuries of the knee such as a dislocation. If a fracture of the medial epicondyle is seen the knee will be unstable side-to-side. Instability in other planes such as front-to-back needs to be checked. This fracture normally heals without trouble but the knee may be unstable if the ligament has been stretched.

An avulsion fracture of the lateral epicondyle on the outer side of the end of the femur is slightly more common. This is the point of attachment of the lateral collateral ligament. The bony bump is quite easily felt on the outside of the knee and the ligament may be felt if you sit cross-legged. The tight ligament can then be felt as a tight band passing down to the upper end of the fibula.

This ligament prevents the knee from bending outwards. If an outward force is too strong the ligament may tear or may tear off the lateral epicondyle. This type of avulsion fracture heals well and the ligament component of the injury does not usually cause major long term problems.

Osteochondral Fractures

In some twisting knee injuries the kneecap can dislocate and chip off a portion of the outer joint surface of the femur. These osteochondral fractures involve the joint surface and a small piece of the underlying bone. Osteo means bone and chondral means cartilage, so osteochondral usually refers to something containing both bone and cartilage.

Osteochrondral fractures are significant for several reasons. First, they indicate that the kneecap came out of position and the restraints that prevent dislocation were torn. This means that another dislocation may easily happen. Second, the fragment may be loose in the joint and may get between the weight bearing bones and damage the joint surface. Third, the extent of the damage to the joint surface may be significant even though the bone fragment appears very small on x-ray. The long term result of the injury will be worse if there is a larger area of joint surface damage. Lastly, these injuries are sometimes difficult to diagnose. The bone fragment is so small that it may be difficult to see on x-ray. Special views or CT scans may be needed to make the diagnosis.

In most osteochondral fractures the loose fragment is too small to heal back into position. The usual management is to remove the loose piece to prevent it from damaging the rest of the joint and to treat the dislocation of the kneecap.

Supracondylar Fractures of the Femur

The rounded ends of the femur are called condyles. There is a type of fracture of the lower end of the femur that starts above the condyles and may pass down into the joint. Supra means above, so supracondylar means "above the condyles".

There is a great deal of variation in the pattern of these fractures. At one extreme is a simple transverse fracture which does not enter the joint. At the other there may be splintering of the bone and multiple fragments including joint surface. The more severe injuries are caused by high energy trauma and may be open fractures. Fractures of the kneecap and tibial plateau may be involved as well. These injuries are amongst the most challenging fractures to treat.

Fractures of the Tibia at the Knee

Avulsion Fractures

Either the anterior or the posterior tibial spine may be pulled off in knee ligament injuries. The anterior spine is the attachment of the anterior cruciate ligament to the tibia. The posterior cruciate ligament attaches to the posterior spine. These avulsion fractures are rare in adults but indicate that there is a serious injury to the ligaments. Even if the fracture fragment is brought back down to its anatomical position and heals in place, the stretched ligament may not be functional.

The Segond fracture is an avulsion fracture of the anterolateral rim of the tibia. It is an indication that the anterior cruciate ligament (ACL) has been ruptured.

A severe outward bending force on the knee may cause an avulsion fracture of the tip of the fibula. It is pulled off by the lateral collateral ligament.

The point of attachment of the patellar ligament is a small raised area of bone called the tibial tubercle. It can be felt in the front of the knee at the top of the tibia. When the quadriceps muscles are contracting and the knee is straightening all the force is applied to the tubercle. If a sudden extra load is added the bone may fail and a piece of the tibial tubercle is pulled off. This disrupts the mechanism to straighten the knee and is a disabling injury. It often requires surgery as the pull of the muscle tends to distract the bone fragment from its correct position and inhibits healing.

Tibial Plateau Fractures

The flat parts of the tibia which form a joint with the femur are called the tibial plateau. On the outside, or lateral side, of the knee is the lateral tibial plateau and on the inside, or medial side, is medial tibial plateau. Compression forces may be too great for this region and the lower end of the femur may be driven into the the tibial plateau on the inner or outer side - or occasionally both.

These are severe and troublesome injuries which damage the weight bearing surfaces of this major joint. The focus of treatment of a tibial plateau fracture is to restore the smoothness of the joint surface as best as possible. Like supracondylar fractures of the femur there is a wide spectrum of tibial plateau fractures ranging from simple injuries which damage one plateau to extensive fractures with many fragments and involvement of the shaft of the tibia.

Fractures of the Kneecap

Avulsion Fractures

If the extensor mechanism is overloaded the muscle can tear off the patella taking small fragments of the bone with it. This can happen with quadriceps rupture which occurs at the upper end of the patella where the thigh muscle attaches to the kneecap. It can also occur at the lower pole of the patella where the patellar ligament begins. In both these situations it is the damage to the muscle mechanism that straightens the knee that is important. The fracture is not usually a major problem. The injury usually requires surgery to repair the extensor mechanism.

Osteochondral Fractures

When the kneecap dislocates to the outer, or lateral, side the inner, or medial, ligaments tear. Sometimes a piece of bone is pulled off from the inner (medial) aspect of the kneecap. More commonly, a piece of the inner side of the kneecap may be chipped off against the outer side of the femur. This piece is referred to as an osteochodral fragment. It contains both bone and articular cartilage and is often floating free in the joint. The key to management is to make the diagnosis, recognize that the kneecap has dislocated and remove the loose fragment from the joint before it damages the joint surfaces.

Transverse Patellar Fractures

A common pattern of fracture of the kneecap is a transverse fracture. This means that the fracture line travels straight across the kneecap from side to side. The pull of the quadriceps is too great for the strength of the bone so it is pulled apart. This can happen when there is a sudden increase in load when you are straightening the knee. A more complex injury can occur as a result of an impact against the kneecap. The kneecap may shatter with this type of force and there may also be extensive crushing damage to the underlying joint surface. High energy trauma may cause an open fracture of the kneecap.