Dimples on the back of the knee are natural indentations, typically a depression in the skin located in the popliteal fossa. This area, behind the knee joint, is a common site for such features. For most individuals, these indentations are a normal part of their body’s structure, reflecting variations in underlying tissues rather than a medical concern.
Anatomy of the Knee’s Rear Surface
The popliteal fossa, a diamond-shaped depression behind the knee, serves as a passageway for important nerves, blood vessels, and lymphatic structures between the thigh and lower leg. Its boundaries are formed by several prominent muscles.
The upper-medial border is defined by the semimembranosus and semitendinosus muscles, which are part of the hamstring group. On the upper-lateral side, the biceps femoris muscle forms the boundary. Below, the medial and lateral heads of the gastrocnemius muscle, a major calf muscle, complete the lower borders.
The floor of the popliteal fossa consists of the posterior surface of the thigh bone (femur), the knee joint capsule, and the popliteus muscle. The space within these muscular boundaries is filled with connective tissue and fat. The interplay of these muscles, tendons, and surrounding tissues naturally creates an indentation in some individuals when the knee is extended or during movement.
Common Reasons for Dimples
Dimples on the back of the knee are influenced by genetics, the structure of surrounding muscles, and the distribution of fat. These factors contribute to the varying appearance and depth of these indentations. Such dimples are benign and a natural part of human anatomical diversity.
Genetic inheritance plays a significant role. The tendency for similar anatomical features to run in families suggests a genetic predisposition. If one parent has dimples, there is a higher chance their child may inherit them, indicating a dominant genetic trait. This genetic influence shapes the underlying muscular and connective tissue arrangements that create these indentations.
The shape and prominence of hamstring and calf muscles contribute to dimple visibility. Stronger hamstring muscles, such as the biceps femoris, semitendinosus, and semimembranosus, can create deeper depressions as they form the popliteal fossa boundaries. The size and arrangement of gastrocnemius muscle heads also influence the lower indentation. The amount and distribution of subcutaneous fat in the popliteal fossa further affect the appearance. Less fat in this area can make the underlying muscular structures and the resulting indentation more pronounced.
When Dimples Are Not Just Dimples
It is important to distinguish between a natural dimple, which is an inward depression in the skin, and a lump or swelling, which is an outward protrusion. A new lump or swelling in the popliteal fossa should be evaluated, as it may indicate an underlying condition.
A common cause of a lump behind the knee is a Baker’s cyst, also known as a popliteal cyst. This fluid-filled sac forms when excess fluid from the knee joint bulges into the popliteal bursa. Baker’s cysts often result from knee joint problems like arthritis or a meniscus tear, causing excess fluid production. While they can cause stiffness or discomfort, many Baker’s cysts do not produce symptoms and are discovered incidentally.
Other possible causes of a lump in this area include benign fatty tumors called lipomas. These are soft, movable growths of fatty tissue that are generally harmless. Less commonly, other conditions such as vascular aneurysms, enlarged lymph nodes, or rare soft tissue tumors can present as a mass in the popliteal fossa. While a natural dimple is a normal variation, any new or changing protrusion warrants attention.
When to Consult a Doctor
While most dimples on the back of the knee are normal, certain signs and symptoms warrant medical evaluation. Consult a healthcare provider if a noticeable lump or swelling develops in the popliteal fossa, especially if it is new or differs from a typical indentation.
Seek medical attention if the area behind your knee is accompanied by:
Pain, tenderness, or discomfort.
Redness or warmth, suggesting inflammation or infection.
Limited range of motion or difficulty bending or straightening your leg.
Rapid change in appearance, such as sudden increase in size or consistency of a lump.
Fever, inability to bear weight, or signs resembling a blood clot (e.g., increasing calf pain and swelling).