Why do so many knee replacement patients end up with a stiff knee?

By 'knee stiffness' in this context, we really mean loss of range-of-motion, not just an achy stiff feeling on getting up out of a chair.

Loss of range-of-motion may imply a mechanical cause such as scar tissue inside the joint, or around the soft tissues or between the muscle bundles. Or there may be a mis-fit of an internal implant.

It's common for the knee replacement patient to have to wait many years before undergoing their surgery because the classical wisdom is that the knee replacement will eventually wear out sufficiently to require revision, so the longer it can be put off the smaller the likelihood of such a revision becoming necessary.

By the time the person makes the top of the list for a knee replacement, it is likely that there have been several years of pain and reduced activity in addition to arthritic bony changes of the knee. Inactivity may have made the knee muscles weak, and the range of motion is likely to already be reduced. So the probability of perfect function and range is not good to begin with, but despite this patients generally have high expectations.

Avoiding triggers for knee stiffness

The knee replacement surgery itself has potential triggers for stiffness - a tourniquet may be used to prevent bleeding during surgery, drains may be inserted into the joint after surgery, it may be a challenge to optimally size and align the knee replacement, pain management may not be optimal, hospital stay may be minimised and rehabilitation classes may not commence for some weeks. I won't go into all the reasons for this in detail, but suffice it to say that these factors make this a high risk group for post-operative stiffness.

Let's take a look at each of these potential triggers:

Tourniquets

The tourniquet used for knee replacement is an inflatable band that is placed high up on the thigh and pumped above the blood pressure once the leg has been lifted high and the blood squeezed out with a rubber bandage. 

In the majority of knee replacements it is pumped up before the surgery begins and only released once the prosthesis has been fixed in place. Because the end of the bone are cut off, most surgeons would say that the tourniquet is essential to prevent severe bleeding during surgery. But high tourniquet pressures tourniquets can damage the tissues, long periods without oxygenation can, as well. Also when a tourniquet is released, surgical diligence is important in catching any bleeders before the wound is closed. 

Excessive internal bleeding after surgery

A number of excellent knee surgeons advocate a different approach, and that is one of chemically dropping the blood pressure during the anaesthesia, most likely by using an epidural catheter close to the spine and titrating carefully to maintain the blood pressure at a level optimal for bloodless surgery. Other pharmacologic agents may be used as an adjunct to the epidural to keep the blood pressure low. As the patient is brought back to normal but before the wound is closed, such surgeons will be meticulous about sealing any bleeding vessels and may also infiltrate the wound edges with adrenaline, which causes constriction of the capillaries.

By paying attention to these details the use of drains is avoided, so the potential for infection is reduced, and infection is a potent trigger of arthrofibrosis in the knee, as is the presence of blood.With regard to the sizing of the knee replacement, this can be a challenge, because the patient may have arthritic deformity and the original shape of the ends of the bones may be uncertain. However the experienced surgeon is able to remove the bony spurs and adjust the tension in the soft tissues to allow the new knee replacement optimal movement.

Poor pain management

Good pain management is extremely important in allowing the patient the emotional freedom to get the knee moving after surgery, and these days the preference is for a balanced 'cocktail' of medications that each have a different effect on the pain pathway.​

Immobilisation

Early mobility is important, too, and surgeons may choose to use a CPM (continuous passive motion) machine for the first 24 hours to keep the knee movement and minimise muscle inhibition.​

I think it is a shame that these patients are sent home so early after surgery rather than being able to benefit from early rehabilitation instruction in the ward, but I do understand that today the risk of hospital-acquired infection as well as cost have to be taken into consideration.