Total Knee Arthroplasty (TKA), or knee replacement surgery, is a standard procedure used to relieve pain and disability caused by advanced arthritis. While patients often anticipate greater activity and subsequent weight loss, a common and frequently unexpected outcome is weight gain in the months and years following the operation. Studies indicate that a significant number of patients, sometimes over 60%, gain an average of 9.5 to 14 pounds within the first two years. This phenomenon stems from a complex interplay of physical, physiological, and behavioral factors.
The Immediate Caloric Imbalance
The most immediate cause of weight gain following Total Knee Arthroplasty is a sudden decline in energy expenditure. Major surgery requires a period of limited activity for tissues to heal and swelling to subside. This necessary reduction in movement means the body burns significantly fewer calories than before the operation. Even non-exercise activity thermogenesis—the calories burned through daily tasks like walking and chores—drops drastically. If a person maintains the same dietary intake as before surgery, this reduced caloric output inevitably results in a caloric surplus. This imbalance persists even with physical therapy, as recovery-focused exercises do not fully compensate for the reduced general activity level. The body’s metabolism slows in response to the extended period of rest, requiring the patient to drastically reduce caloric intake to maintain weight.
Physiological Responses and Medication Side Effects
The body’s internal response to surgical trauma also contributes to weight gain. A major operation triggers a systemic stress response, temporarily elevating hormones like cortisol. This stress hormone can alter metabolism, sometimes promoting fat storage, particularly in the abdominal area. The inflammatory process necessary for healing also causes postoperative edema, or fluid retention. This initial, rapid weight gain is not fat mass but an accumulation of extra fluid, which is generally temporary. Medications used for pain and inflammation can prolong these issues. Many non-steroidal anti-inflammatory drugs (NSAIDs), such as Celecoxib, commonly prescribed post-TKA, cause fluid retention and swelling. Certain pain medications can also slow metabolism and increase appetite.
Behavioral Changes in Diet and Routine
The recovery process often disrupts established eating habits and routines. Pain and limited mobility make it difficult for patients to prepare healthy meals, leading to increased reliance on convenience foods or caregiver-prepared meals. These options are often higher in calories and less nutrient-dense. The shift in daily structure also contributes to unmindful eating. Boredom, frustration, or emotional discomfort stemming from pain and slow rehabilitation can trigger increased snacking or emotional eating. Patients may inadvertently consume more calories because their focus shifts away from diet management toward comfort and pain relief.
Increased Strain on the New Joint
Weight gain following Total Knee Arthroplasty is a significant concern because it directly impacts the function and longevity of the artificial joint. The body acts as a system of levers, meaning every pound of added weight translates into a much greater mechanical load across the knee joint during movement. This load multiplier effect is substantial, increasing force by two to three times the body weight during walking. Increased body mass places disproportionate stress on the new implant and surrounding tissues. This greater mechanical burden can slow rehabilitation by causing increased post-operative pain and swelling. Over the long term, excessive loading can reduce the implant’s lifespan by accelerating the wear of components or increasing the risk of mechanical loosening, compromising surgical success.