Knee cancer refers to the presence of a malignant tumor in or around the knee joint, typically involving the distal femur or proximal tibia. While relatively uncommon, this condition is usually characterized by primary bone cancers like osteosarcoma or soft tissue sarcomas that occur near the joint capsule. Understanding what this condition looks like involves recognizing both observable physical signs and subjective internal symptoms. The appearance and symptoms vary based on the tissue of origin. This information is intended to provide general insight and should not replace a consultation with a qualified healthcare professional for persistent symptoms.
Types of Cancer Affecting the Knee
The presentation of a malignancy near the knee is often determined by whether it originates in the bone or the surrounding soft tissues. Primary bone cancers are rare, but the area around the knee is the most frequent site for several types. Osteosarcoma, the most common type of primary bone cancer, often develops in the long bones around the knee, primarily affecting adolescents and young adults.
Chondrosarcoma, another primary bone cancer, arises from cartilage cells and typically affects older adults, sometimes occurring near the knee. Ewing sarcoma is less common but can also occur around the knee, generally affecting individuals between the ages of five and twenty. Soft tissue sarcomas, such as Synovial Sarcoma, originate in connective tissues like fat, muscle, or blood vessels. These distinctions influence whether the resulting mass is hard and fixed to the bone or softer and more pliable in the surrounding soft tissues.
Observable Physical Changes
One of the most noticeable external signs of knee cancer is the development of localized swelling or a palpable mass. This swelling is usually subtle at first and often appears weeks after the onset of pain. A mass arising from a bone tumor, like osteosarcoma, may feel firm or hard to the touch because it is attached to the rigid underlying bone structure.
In contrast, a soft tissue sarcoma, such as synovial sarcoma, often presents as a lump or bump under the skin that may be slightly softer or rubbery. Cancerous masses generally feel fixed in place and do not easily move under the skin, distinguishing them from benign, mobile lumps like lipomas.
The skin overlying the mass may show signs of inflammation, such as warmth or redness, especially with fast-growing tumors like Ewing sarcoma. As the tumor enlarges, the skin may appear stretched or shiny due to the underlying pressure.
Subjective Symptoms and Functional Impairment
The primary and most consistent symptom of malignancy near the knee is pain, which has distinct characteristics compared to mechanical joint issues. This pain often begins mildly and intermittently but progresses to become constant and severe over weeks or months. A particularly distinguishing feature is pain that is worse at night or occurs while resting, meaning it is non-mechanical and does not improve with typical rest or pain relief medications. The tumor growth causes discomfort by invading the bone structure and increasing pressure.
Functional impairment often accompanies the pain as the disease progresses, leading to difficulty bearing weight on the affected leg. Many patients develop a noticeable limp. The mass or the pain itself can limit the knee’s normal range of motion, making it challenging to fully straighten or bend the leg. In some cases, the cancer weakens the bone structure, potentially leading to a pathological fracture, where sudden, severe pain occurs after minimal trauma or spontaneously.
Distinguishing Characteristics from Common Knee Conditions
Differentiating knee cancer from common, benign knee problems like arthritis or meniscal tears relies heavily on the nature and persistence of the symptoms. Common knee conditions are typically preceded by an acute injury or are directly related to activity, with pain often improving with rest and anti-inflammatory drugs. The pain associated with a tumor, however, is characteristically non-mechanical, persisting even when the joint is not being used.
The presence of pain that is predominantly severe at night is a significant warning sign that suggests a neoplastic process rather than a typical orthopedic issue. Furthermore, the mass associated with cancer often develops without a preceding event like a fall or twist. If any knee pain is constant, progressively worsens over time, or is accompanied by a fixed lump, it warrants immediate medical investigation. A physician will use advanced imaging studies and potentially a biopsy to determine the exact cause of the symptoms.