Chills or shivering are a common physical reaction for many patients recovering from knee replacement surgery (TKA). This involuntary muscle tremor is how the body attempts to regulate its internal temperature, and it can occur at different points during recovery. The timing and nature of the chills determine if they are a temporary effect of the procedure or a potential signal of a complication. Understanding the reasons behind this phenomenon, from immediate surgical effects to the body’s healing response, helps patients manage expectations.
Immediate Causes: Post-Anesthesia Shivering
The most frequent cause of chills that patients experience immediately after their knee replacement is a condition known as post-anesthesia shivering. This reaction is a direct result of the body’s core temperature dropping during the surgical procedure, a state called hypothermia. The operating room environment is often kept cool, which contributes to heat loss from the patient’s exposed skin and internal body cavities.
Anesthetic medications, whether general or regional, significantly interfere with the body’s ability to maintain a stable temperature. These drugs suppress the central nervous system, which normally controls the body’s thermal set point. Anesthetics also cause the blood vessels near the skin’s surface to widen (vasodilation), which increases the amount of heat lost to the environment.
When the anesthesia begins to wear off, the body’s temperature control mechanisms attempt to correct the core temperature deficit. This rapid attempt to rewarm the body triggers involuntary muscle contractions (shivering) to generate heat. Shivering can be intense, but it is typically a transient response that subsides within 20 to 30 minutes as the patient is warmed with blankets in the recovery room. Non-thermoregulatory factors, such as post-operative pain or the effects of certain anesthetic agents, can also contribute to the shivering sensation.
Systemic Responses to Trauma and Healing
Chills that occur hours or days after the procedure, distinct from the immediate post-anesthesia effects, are often a natural reaction to the trauma of surgery itself. Knee replacement involves significant tissue and bone manipulation, which initiates an inflammatory cascade throughout the body. When tissues are damaged, they release chemical messengers called cytokines.
These substances travel through the bloodstream and can temporarily reset the body’s internal thermostat in the brain, leading to a mild fever. Chills are experienced as the body attempts to reach this new, elevated temperature set point. This mild, temporary fever is common, often peaking around the first post-operative day and usually returning toward normal by the fifth day. Certain pain medications, particularly opioids, can also influence the body’s thermoregulation during early recovery.
Identifying Chills as a Sign of Infection
While many instances of chills are benign, they can also be an important sign of a serious complication, such as a surgical site infection. Chills that occur alongside a high or persistent fever are a significant warning sign that the body may be fighting an infection, which requires immediate attention.
The presence of chills combined with a fever above 100°F (37.8°C) or 101°F is a major concern, particularly if the fever lasts for more than a few days. Other concerning symptoms include persistent or worsening pain in the knee, especially pain that is not relieved by medication.
Patients should also watch for increased redness, warmth, or swelling around the incision site, or any gray, foul-smelling drainage that lasts longer than a few days. If these physical signs are accompanied by general malaise, excessive fatigue, or persistent, shaking chills, contact the surgical team right away. Early identification and treatment of an infection are important for a successful recovery and to preserve the new knee joint.