Patellar instability with increased knee flexion due to lateral femoral condyle distal dysplasia: a report of two cases.

We present two case reports with clinical and radiological assessment of the anatomical features at the distal lateral end of trochlea and femur in patients with patellar instability in higher knee flexion and how these findings could possibly be involved in the genesis of this rare type of patellar instability. Both patients underwent several (nine and seven) surgical procedures, but the patellar instability could not be successfully eliminated. Our hypothesis was that a short and flattened lateral distal condyle/trochlea may cause lateral patellar instability in higher flexion. We found considerable anatomical variations at the distal lateral femoral condyle and trochlea in both patients. Individually tailored surgical procedures were selected for each patient according to the documented variations and the previous operations. Our surgical interventions consisted of different components, such as osteotomy of the distal lateral femoral condyle/trochlea with lengthening and elevation, balancing of the medial and lateral patellar soft tissue structures, MPFL reconstruction, transposition of the medialised tibial tubercle and the patellar tendon back to lateral to the normal initial anatomical position. With these procedures, lateral patellar instability with increased flexion could be completely eliminated in both patients at the 1- and 2-year follow-up. Our experience of assessment and treatment of these patients let us conclude that variations of the distal lateral femoral condyle/trochlea morphology may be responsible for patellar instability with increased flexion. This is another type of patellar instability caused by distal condyle/trochlea dysplasia compared to the well known and often described types of patellar instability close to extension caused by proximal trochlea dysplasia.