Some patients experience some mechanical sensations after knee replacement surgery. This is generally a normal consequence of the "hard bearing" components of the knee replacement prosthesis. Let me explain.
Cartilage is a coating of connective tissue that cushions the ends of the bones where the come together at a joint. This material, when healthy, is very slippery and provides some shock absorption to the bones. Healthy cartilage will allow a normal joint to move silently and painlessly.
As the cartilage gradually wears out, it may fray, or pieces may break off. Now the joint surfaces are not perfectly smooth. The friction experienced with joint motion increases. As the cartilage layer thins out, its ability to cushion the bone ends also decreases. This all causes increased stress to the bones resulting in pain and stiffness. In response to the stress, the bone density often increases, bone spurs and/or areas of erosion may form, and the joint alignment may change. While these changes usually occur gradually over time, sometimes the progression may be rapid. X-rays will show joint space narrowing (loss of cartilage), sclerosis (increased bone density), osteophytes (bone spurs), cysts (areas of bone erosion). In addition to pain, a patient may experience grinding or crunching sensations from this increased friction and malalignment. These sensations are called crepitus.
When we perform total knee replacement surgery, we precisely remove a few millimeters of bone from the end of the femur, tibia, and the undersurface of the patella. This is done in such a way that the prosthetic components will precisely fit the bones, re-align the mechanical axis of the leg, and balance the ligaments that support the joint.
Because the prosthesis is metal and plastic, the bone ends no longer have any sensation where they come together. However, the joint surface is no longer soft cartilage, it is hard metal and plastic. Sometimes when these materials move relative to each other a patient may experience this as a clicking or other mechanical sensation.
Early after knee replacement surgery, there will be blood and inflammatory fluid within the joint. When the knee is fully extended and the quadriceps are relaxed, the patella may "float" up, away from the femoral component where it normally rests. When the knee is then bent, or the quadriceps are contracted the patella will be pushed firmly down against the femoral component. This will feel like a click or a clunk. The click will not be painful, and will resolve as the fluid is reabsorbed from within the knee.
Certain knee replacement models will substitute for the posterior cruciate ligament. This ligament prevents the tibia from moving posteriorly relative to the femoral component. This is accomplished using a polyethylene post on the tibial component. This post will engage against a bar on the femoral component when the knee is flexed and the tibia is pushed posteriorly. This engagement can feel like a click. Again, this is painless.
The knee joint normally has a millimeter or two of laxity when stressed sideways. Pushing the tibia inward relative to the femur, and pushing the tibia outward relative to the femur tensions the lateral collateral and medial collateral ligament, respectively. Because there might be slight separation of the femoral and tibial components when applying these stress, a patient can experience a clicking sensation when the tension is removed and the prosthetic components re-engage.
Using robotics during knee replacement surgery facilitates very accurate soft tissue balancing throughout the full knee range of motion. This accuracy of ligament balancing allows me to use a posterior cruciate retaining prosthesis (no post to engage) and minimizes side to side laxity. While some mechanical sensations can still occur (particularly with patellofemoral engagement early after surgery as described above), these sensations are minimized.
These are the most common reasons a patient may feel some clicking in their knee following knee replacement surgery. When this occurs in the absence of pain, it is likely normal. If there is any question, I recommend you see your surgeon for an evaluation and reassurance.
Orthopedic Surgeon focused on the entire patient, not just a single joint.