The treatment of patellofemoral conditions is notoriously difficult.  It is essential to make a specific diagnosis of the mechanical and pathological conditions and to plan the treatment appropriately.  

There are certain conditions which are much more difficult to treat and they may constitute relative or absolute contra-indications for a patellofemoral arthroplasty.

1. Patella infera (baja)
   
   This condition is seldom successfully treated by arthroplasty because of the abnormal loading of the patellofemoral joint.  This can be a developmental condition but may also be acquired following previous surgical treatment.
   
   
2. Patella alta
   
   This condition is often responsible for significant patellofemoral disease with either instability or arthritic damage.  It may be appropriate to lower the patella with a tibial tubercle transfer prior to or at the same time as the patellofemoral arthroplasty. These cases may have significant proximal mal-alignment and great care must be taken with tracking and with the design of the proximal trochlea to ensure that the prosthetic patella engages satisfactorily with the trochlea in extension or as is sometimes the case if there is hyperextention.
   
   
3. Algodystrophy – Regional pain syndrome
   
   These cases are notoriously difficult and great caution must be taken when considering an arthroplasty.
   
   
4. Arthrofibrosis
   
   This is often a secondary effect of an algodystrophy, or an abnormal response to previous surgery. If their has been malposition of the prosthesis leading to overstuffing this will restrict flexion and may lead on to arthrofibrosis.  It is generally unwise to attempt to treat such cases unless the surgeon has considerable experience and extremely good rehabilitation is available.
   
   
5. Axis mal-alignment
   
   If there is significant varus or valgus mal-alignment an isolated patellofemoral arthroplasty is unwise.  There will be an increased risk of developing disease progression in the tibio femoral joint.
   
   
6. Fixed flexion deformity
   
   It is not possible to correct a fixed flexion deformity by a patellofemoral arthroplasty.  This constitutes a specific contra-indication to an isolated patellofemoral arthroplasty if the deformity is more than 10°.
   
   
7. Restricted flexion
   
   Flexion may be restricted due to pain inhibition.  In such a case free movement of the knee under anaesthesia will confirm an appropriate indication for an arthroplasty.  If the range of flexion is less than 90 degrees due to capsular contracture or arthrofibrosis, satisfactory result may not be obtained.
   
   
8. Early chondral disease (anterior knee pain syndrome)
   
   This should never constitute an indication for a patellofemoral arthroplasty. When there is significant chondral damage in younger patients, (at least those under fifty), they should usually be considered for minimally invasive surgery,with chondrectomy, patella realignment or cartilage autograft, before being considered for an arthroplasty.  In cases of failed previous surgery for significant chondral damage careful counselling of the patient should be undertaken before performing an arthroplasty. Arthroplasty may be a salvage procedure in the 30-50 age group but should seldom be considered under age 30 unless their is no other option.
   
   

Cautions

This Patellofemoral Arthroplasty is generally fairly forgiving and will tolerate minor mal-alignments and technical errors without adverse effect.  Our experience to date, however, has shown that there are some aspects that are less well tolerated:

1. Elevation of Trochlea
   Elevation of the trochlea leads to overstuffing of the patellofemoral joint and is not well tolerated.  The anterior cut should be parallel to the anterior cortex of the lower quarter of the femur.  The trochlear component should be inset so that it is level with both medial and lateral femoral condyles unless there is significant loss of bone on the lateral side as in severe cases.  The tip of the prosthesis is inset above the intercondylar notch to allow a bridge of 2-5mm of articular cartilage depending on the size of the knee joint.  If this gap is large it will indicate relative extension of the prosthesis suggesting overstuffing.  In the lateral post-operative radiograph the tip of the prosthesis should be at the level of, or just above Blumenstadt`s line.
   
   Over stuffing of the femoral component  can occur due to excessive cement. It is best to apply only a very thin layer of cement with pressurisation into the cancellous bone surfaces. No cement is applied to the under surface of the femoral component. (see technique )
   
   a)   b)
   
   a) Femoral component positioned in 15° extension leading to a tight Patellofemoral joint    
   
   b) Post-operative view showing correction of femoral component after revision (see case 1)
   
   
2. Femoral external rotation
   
   Failure to externally rotate the femoral component on the femur by 3-6° will cause lateral overstuffing and reduce the ability of the patella to track satisfactorily.  This is an important feature of the Avon patella (see highlighted case).
   
   
3. Chondral and arthritic damage of tibio-femoral joint
   
   Patellofemoral arthritis is a very slowly progressive disease and in many cases the condition will have been present for five to ten years or more.  The long-term effects of a chronic synovitis with continuing release of attrition products from both articular cartilage and bone are likely to have a deleterious effect on the articular cartilage structure of the tibio femoral joint.  It is, therefore, a matter of fine judgement at the time of arthroplasty as to whether there is sufficient damage to the tibio-femoral joint to justify a total knee replacement.  Such is the success of a modern total knee replacement that one can be assured of an excellent long-term result, although there will inevitably be a higher morbidity from this significantly more invasive procedure. This should be carefully considered in patients over 70 years of age.
   
   Experience to date suggests that if there is more generalised damage to the articular surfaces of the tibio-femoral joint, damage to the menisci or grade two or more damage in the critical weight bearing areas of the tibial plateau, it may be wiser to perform a total knee replacement.  On the other hand if there are only localised areas of chondral damage less than 1cm a simple chondrectomy will be adequate and allow a successful patellofemoral arthroplasty with the expectation of a longevity of ten or more years.  In the younger patients under the age of 60 it is clearly desirable to carry out a compartmental arthroplasty rather than a total joint replacement.