To understand how ankle arthritis causes pain it is helpful to understand ankle anatomy:
- The ankle is where the long tibia and fibula bones of the shin meet with the foot’s talus bone.
- The talus bone is an irregular shaped bone that rests on the heel bone, or calcaneus.
- Strong, slippery and flexible articular cartilage covers the bones’ surfaces where they meet, providing cushion and facilitating joint movement.
Typically, ankle arthritis refers to the joint degeneration between the talus, tibia and fibula, but occasionally people use the term to refer to degeneration between the talus and the calcaneus or other bones. This article focuses on where the talus, tibia and fibula bones meet, but many concepts discussed here apply to other joints in the foot.
The ankle's cartilage is different from other weight-bearing joints in two ways:
- The ankle's articular cartilage is uniformly thin. Cartilage in a healthy ankle ranges from about 1 to 1.7 mm thick, whereas cartilage in a healthy knee is thin in some areas and plush in others, ranging from 1 to 6 mm thick.1-3
- The ankle cartilage is especially dense and tough, and holds up comparatively well under weight-bearing stress.
Differences in cartilage and joint structure may help explain why ankle arthritis is relatively uncommon in the absence of a previous injury or an underlying condition. In fact, osteoarthritis is 9 times less likely to develop in the ankles than in the knees or hips, even though the ankles bear more weight per square inch.4
How Ankle Arthritis Causes Pain
An arthritic ankle has articular cartilage that is thinned, damaged or entirely worn away. When the ankle cartilage has deteriorated in such a manner, the following process ensues:
- New cartilage may be produced, but the new cartilage cells will grow in irregular, bumpy patterns rather than the original smooth form of cartilage. The result is that the ends of the tibia and talus bones may rub and grind against each other.
- To compensate for the deteriorated or missing cartilage, the bones in the joint may produce small scalloped growths called osteophytes, or bone spurs. In turn, the bone spurs can create even more friction in the ankle joint.
- As the body attempts to compensate for the functional abnormalities of a degenerating ankle joint, tendons and ligaments can be stretched and compromised, leading to further loss of ankle stability and function as well as ankle pain and stiffness.
It is important to note that cartilage does not contain nerves, so damaged cartilage is not the primary source of pain in ankle osteoarthritis. Likewise, bone spurs are a normal sign of aging and the presence of bone spurs alone is not a cause for concern. However, the resulting friction between bones can cause discomfort, pain, and functional abnormalities.