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About Joints¹

To understand why joints may become a problem for people with hemophilia, it is helpful to understand the parts of a joint and how they work together.

Joint anatomy

A joint is the place where two or more bones come together. Cartilage is a smooth layer of tissue that covers the ends of the bones where they come together, protecting them and acting as a shock absorber. The entire joint is wrapped in a tough elastic casing called the synovial capsule or joint capsule. The synovial membrane, which contains many small blood vessels and makes synovial fluid, lines the synovial capsule.

Joints are supported by muscles, tendons, and ligaments. Tendons attach muscles to bones. Ligaments connect bones and cartilage, preventing the bones in a joint from moving the wrong way.

How bleeds affect a joint

During a bleed, blood flows from the vessels in the synovial membrane into the joint capsule. If the bleeding is not stopped, the joint capsule fills with blood.

As the blood is reabsorbed into the surrounding tissue, the tissues swell, stretching ligaments and tendons and causing damage to the joint. Reabsorption also causes the synovial membrane to grow and thicken, which means it has more blood vessels and a greater risk of future bleeds.

When a joint experiences recurring bleeds, the synovial membrane begins to produce enzymes which cause more joint swelling. There is also an increase in iron deposits in the joint. Over time, recurring bleeds can destroy the synovial membrane, which is then replaced with scar tissue.

In addition, recurring bleeds lead to erosion of the cartilage. The enzymes and iron deposits which affect the synovial membrane also break down the smooth cartilage at the end of the bones. Eventually, the cartilage becomes rough and uneven, and arthritis develops.

Target joints

The major cause of joint damage is repeated bleeding into a single joint – often called a "target joint." Target joints are most commonly located at the knee, elbow and ankle. The Centers for Disease Control and Prevention defines a target joint as "a joint in which recurrent bleeding has occurred on four or more occasions during the previous 6 months or one in which 20 lifetime bleeding episodes have occurred."²

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References

1. Federation of Hemophilia website. Available at http://www.wfh.org. Accessed August 7, 2008.
2. UDC Report March 1999. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/ncbddd/hbd/documents/UDCReport3-99.pdf. Accessed August 7, 2008.