After going through the procedure of knee replacement surgery it can be devastating to find that the pain in the knee has persisted. This paper discusses the possible reasons why this might occur.

There are a number of problems which which are the commoner causes of persistent pain after a knee replacement -

• pain originating from within the knee joint itself, such as infection, internal scarring (arthrofibrosis), loosening of the prosthesis or wear of the polyurethane component - all worrying problems, and often associated with one another.
• pain originating from outside the knee joint, but experienced in the region of the knee joint - commonly from the hip (eg hip arthritis) or lower spine (eg spinal stenosis - a narrowing of the spinal canal - or a 'popped' disc). This kind of pain is called referred pain.

Early Persistent Pain

Pain early after the original surgery is often due to one of the four 'I's -

Infection

Infection is always at the top of the list in the surgeon's mind (or should be) when investigating continuing pain after knee replacement. Infection within the joint after a knee replacement is a dreaded complication, but if diagnosed early it can frequently be brought under control. However, florid early - so-called acute infection, is less commonly found than late cryptic - so called chronic infection - which is harder to diagnose and manage. With acute infection the patient is clearly ill, the joint hot, red, acutely painful and swollen, the blood tests support the picture of acute infection and laboratory culture of the joint fluid demonstrates the infective organism(s). Indicators of chronic infection include persistent pain or discomfort, weepy wounds which just won't heal, skin redness and persistent swelling.

Instability

Instability can be associated with continued pain after knee replacement. The cause of sideways 'axial'instability may be due to failure by the surgeon to establish correct 'balance' in the collateral ligaments - the 'guy ropes' at both sides of the knee, or it may be due to a fall during rehab with damage to one of the collateral ligaments. The other type of instability - 'flexion instability' - is suspected when patients complain of difficulty getting up from sitting or going up stairs. This may be caused by rupture of the posterior cruciate ligament if it had not been removed during surgery, but is more often a mistake in surgical technique. With the posterior stablised knee prosthesis (where the PCL was deliberately removed) if there is a laxity problem it is possible for there to be dislocation of the prosthesis with the one component of the stabilising mechanism to jump over the other, resulting in a sudden inability to fully straighten the knee.

Impingement

Impingement causes a specific type of pain - a sudden severe stabbing pain on movement - because of soft tissues getting nipped in the prosthesis. The popliteal tendon may impinge at the back of the knee, causing a snapping on the outer aspect of the back of the knee. A clunk in the kneecap area when straightening the knee may be due to catching of a lump of fibrous material which gets in the way in this position.

Incorrect indications

Incorrect indications implies an error of diagnosis - a failure to exclude other problems which may be the actual cause of the pain originally experienced in the knee - usually due to initial failure to consider the hip or lower spine in the original assessment of the knee pain.

Regarding the last 'I' we have already mentioned the hip and spine. Other causes of referred pain may be more obscure, including -

• complex regional pain syndrome (previously called RSD or reflex sympathetic dystrophy)
• inflammation in the pes anserine bursa, a lubricating pouch under the tendons on the medial aspect of the knee
• pain arising from the leg muscles due to inadequate blood supply (blocked arteries)
• bone breaks related to the prosthesis, or in the bone immediately above or below the prosthesis due to the change in the stresses applied to the bone
• neuroma - a bunch of nerve fibres which have 'gone astray' and locked up into a tender ball
• tendon irritation of the patellar tendon below the knee or the quadriceps tendon above the knee

Complex Regional Pain Syndrome

Of all of these, one needs special mention, and that is complex regional pain syndrome (CRPS). It is extremely uncommon, but it is imporant for patients to know about, as the symptoms are bizarre and often dismissed by the family doctor as ‘neurotic’. The over-riding symptom is pain out of proportion to the apparent problem, and distinguishing features include skin blotching and alteration in colour in the limb (blue mottling), changes in temperature - hot or cold, skin sensitivity, skin thinning and dryness, swelling of the outside of the knee, and knee stiffness. Symptoms may be inconsistent. The reason why it is essential that diagnosis be made early is that treatment is most effective when started early - and failure to make the diagnosis in time may lead to permanent wasting of the leg and disability.

Late Persistent Pain

Pain late after surgery is invariably due to loosening of the prosthesis.