Experiencing a warm or hot sensation in the knee following surgery is a common concern for many patients navigating the recovery process. This localized warmth may signal a normal part of the body’s repair mechanism or, in some instances, the onset of a serious complication. Understanding the biological reason behind the temperature change is important for distinguishing between typical post-operative healing and issues that require immediate medical attention.
The Expected Warmth: Inflammation as Part of Healing
The body initiates a predictable biological response immediately following any surgical procedure, which is the process of inflammation. This is not a malfunction but rather a necessary, highly organized reaction designed to clean the surgical site and begin tissue repair. The heat felt on the skin surface is a direct result of increased blood flow, a process medically termed vasodilation, where blood vessels expand near the surgical site.
The expanded blood vessels deliver a concentrated supply of immune cells, oxygen, and nutrients to the area of injury. This surge of metabolic activity and circulating blood volume generates a detectable rise in local temperature. Furthermore, the release of inflammatory signaling molecules, such as the cytokine Interleukin-6, contributes to this localized thermal response as part of the systemic reaction to trauma.
This expected warmth gradually reduces over time but can persist for several months. Some studies note a noticeable temperature difference between the surgical and non-surgical knee for up to 6 to 12 months after the operation. To manage this warmth, surgeons often recommend elevating the leg above the heart and consistently applying ice packs, which helps mitigate swelling and reduce the local temperature.
When Heat Signals Danger: Recognizing Post-Operative Infection
While some warmth is normal, a significant increase in temperature often signals a post-operative infection. Infectious heat is typically more intense, localized directly around the incision site, and worsens over time rather than gradually improving. This heat is driven by the body’s immune system actively fighting invading bacteria.
Infection is rarely an isolated symptom, and the distinguishing feature is the presence of accompanying systemic and localized signs. Patients may develop a persistent or high fever, generally defined as a temperature above 100°F or 101.6°F, often accompanied by chills. The skin surrounding the incision may show rapidly spreading redness that extends far beyond the immediate wound edges, along with severe pain that is not relieved by prescribed medication or rest.
A sign of likely infection is purulent drainage from the wound, which is a thick, cloudy discharge, or persistent wound leakage lasting more than a few days. Recognizing these concurrent symptoms is important, as an acute joint infection can be devastating to the surgical outcome and requires immediate attention.
Other Causes of Localized Heat: Deep Vein Thrombosis (DVT)
Localized warmth can also be a symptom of Deep Vein Thrombosis (DVT), a blood clot typically forming in the deep veins of the calf or thigh. Knee surgery significantly increases the risk of DVT due to reduced mobility and trauma to blood vessels during the operation. The warmth associated with DVT is usually not centered at the knee incision but is often felt lower down, in the calf or back of the leg.
The heat in this context results from the partial or complete blockage of blood return to the heart, which causes localized congestion and inflammation. This warmth is usually accompanied by sudden, significant swelling in the affected leg that is often more pronounced than the expected post-surgical swelling. Patients often describe the pain as a sharp, cramping sensation or a persistent tenderness in the calf or thigh that does not improve with elevation or rest.
The skin over the affected area may also exhibit a change in color, sometimes appearing noticeably red, pale, or bluish due to circulatory compromise. Since a DVT can lead to life-threatening complications if the clot travels to the lungs, it is necessary to differentiate this warmth and swelling from the general inflammatory response.
Triage and Next Steps: When to Contact Your Surgeon
Determining whether the warmth in your knee is normal or concerning involves systematically assessing the severity and presence of other related symptoms. You should contact your surgeon’s office immediately if you observe any combination of signs suggesting an emerging complication. Urgent medical evaluation is necessary for symptoms such as:
- A fever persistently higher than 101.5°F.
- Sudden, severe, and worsening pain not managed by medication.
- Spreading redness, or an increase in swelling that extends rapidly.
- Thick, discolored drainage from the wound.
- Signs of DVT, including sudden, intense calf pain, tenderness, or swelling in the leg with skin discoloration.
If the warmth is mild, stable, or gradually improving, and is not accompanied by concerning signs, it likely represents the body’s expected inflammatory response. Mild, improving warmth and swelling without systemic signs can typically be discussed during a scheduled follow-up appointment. Always adhere strictly to post-operative instructions and monitor the surgical site daily for subtle changes.