This 57 year old furniture retailer presented with pain, swelling and giving way as a result of patellofemoral arthritis.  He failed to respond to conservative treatment and an Avon patellofemoral arthroplasty was performed.  He recovered well but was slow to mobilise.  In the follow-up period he continued to complain of pain in the lateral retinacular region.  He was unable to squat or kneel without experiencing lateral retinacular pain.

Post-operative radiographs including tangential views at 30°, 60° and 90° suggested that the prosthesis was over-sized and might be slightly overlapping on the lateral side.  The tangential views suggested inadequate external rotation.  

Pre-op AP lateral and tangential view at 30° - 60° - 90°

30°	60°	90°

Arthroscopic assessment was performed, which confirmed overlap of the prosthesis on the lateral side which was not fully inset in the lateral femoral condyle.  It was decided to revise the femoral component.

At open operation these findings were confirmed.  The femoral component was removed without difficulty and a further 1mm of bone resected from the lateral trochlea.  The femur was templated for size and the medium prosthesis appeared to fit more satisfactorily.

Operative picture showing slight overlap
of the prosthesis and being insufficently inset on the lateral side.

He made a rapid post-operative recovery regaining 120° of flexion within six days and had very little post-operative pain.  At four years he reported no pain, full function and 130° of range of flexion.

Pos-operative antero posterior, lateral and tangential X-rays showing perfect positioning of the prothesis.

AP	Lat	Tan

Lessons

The lesson of this case suggests that minor increases in tension of the lateral retinaculum, or lateral impingement against the prosthesis can cause persistent symptoms, and this is due to technical factors in the performance of the operation.  It is gratifying that these symptoms were corrected after the revision.  This emphasises the importance of choosing the correct size of prosthesis and if in doubt to undersize and ensuring that there is 3° to 6° of external rotation of the anterior bone resection and that the prosthesis is inset congruous with the articular surfaces of both medial and lateral condyles.