A total knee arthroplasty (TKA), commonly referred to as a total knee replacement, is a surgical procedure where the damaged surfaces of the knee joint are removed and replaced with prosthetic components. Patients frequently ask about the weight of the implant, as a foreign object is being placed inside the body. Understanding the weight, composition, and physiological effects of the knee implant provides clarity regarding this common and successful surgery.
The Typical Weight Range of the Implant
The total weight of a knee replacement implant, including the bone cement used to secure it, is relatively light, often less than one pound. For men, the average total implant weight is around 1.12 pounds, while for women, the average is closer to 0.76 pounds, with both figures including the fixation cement. The metal components alone typically weigh between 15 and 20 ounces, which is less than 1.25 pounds.
The weight is not a fixed number and varies based on the size of the implant, which is selected to match the patient’s anatomy. Larger individuals require larger components, which contain more material and therefore weigh slightly more. This variation also depends on the specific design and manufacturer of the prosthesis used.
Composition: Materials and Components
The total mass of the implant is a combination of three main components, chosen for their strength, wear resistance, and biocompatibility. The weight is primarily concentrated in the metal pieces due to the high density of the alloys used.
Femoral Component
The femoral component resurfaces the end of the thigh bone and is typically made from cobalt-chromium alloys. This material is favored because it can be polished to a smooth, scratch-resistant surface, which helps reduce friction. The density of cobalt-chromium contributes significantly to the overall weight.
Tibial Component and Spacer
The tibial component covers the top of the shin bone and is usually a metal tray made from either titanium or cobalt-chromium alloys. Titanium alloys are often used for the tibial tray due to their good integration properties with bone. Nested within this metal tray is the third component, the polyethylene spacer, which acts as the new cartilage. This spacer is made from ultra-high-molecular-weight polyethylene (UHMWPE), a medical-grade plastic that is substantially lighter than the metal alloys.
The Physiological Impact of the Added Mass
While the implant adds mass to the knee, the body does not perceive this new weight as a heavy load during daily movement. During the total knee arthroplasty, the surgeon removes damaged bone and cartilage before inserting the prosthesis. The weight of the removed tissue is often only slightly less than the weight of the implanted components and cement.
Studies show that the net weight gain resulting from the surgery is generally insignificant, often increasing the leg’s mass by only about half a pound. The new mass is integrated directly into the skeletal structure, replacing the organic material that was removed. Because the mass is deeply integrated, it does not feel like carrying an external object or placing strain on the joint.
The density of the metal and plastic materials is higher than that of the native bone they replace. However, this small difference is not noticeable to the patient and does not negatively affect mobility or overall balance. The primary effect of the surgery is the reduction of pain and the restoration of function.