Overview Biomechanically complex articulation between the patella and distal femoral condyles Associated clinical conditions trauma patella dislocation fracture patella tendon rupture quadriceps tendon rupture OCD lesions chondromalacia patellae patellofemoral joint arthritis patella instability Anatomy Osseous bony constraint of the patella within the trochlear groove intracondylar groove diameter of lateral femoral condyle > medial femoral condyle bony constraint of groove is the primary constraint to lateral patellar instability when knee flexion is > 30 degrees Ligaments static stability of the patella within the trochlear groove medial patellofemoral ligament (MPFL) originates from the adductor tubercle to insert onto the superomedial border of the patella primary constraint to lateral patellar instability with knee flexion 0 to 20 degrees patellotibial ligament retinaculum Muscles dynamic stability of the patella within the trochlear groove vastus medialis = medial restraint to lateral translation vastus lateralis = lateral restraint to medial translation Tendon angular difference between the quadriceps tendon insertion and patella tendon insertion creates a valgus axis (Q angle) creates a laterally directed force across the patellofemoral joint leads to increased contact pressures in lateral patellar facet between 40-90 degrees Blood supply superior, medial and lateral, geniculate arteries inferior, medial and lateral, geniculate arteries anterior geniculate artery descending geniculate artery Biomechanics Function transmits tensile forces generated by the quadriceps to the patellar tendon increases lever arm of the extensor mechanism patellectomy decreases extension force by 30% Biomechanics patellofemoral joint reaction force up to 7x body weight with squatting 2-3x body weight when descending stairs Motion "sliding" articulation patella moves caudally during full flexion maximum contact between femur and patella is at 45 degrees of flexion Stability passive restraints to lateral subluxation medial patellofemoral ligament primary passive restraint to lateral translation in 20 degrees of flexion 60% of total restraining force medial patellomeniscal ligament 13% of total restraining force medial retinaculum 10% of total restraining force dynamic restraint quadriceps muscles Q angle definition line drawn from the anterior superior iliac spine --> middle of patella --> tibial tuberosity normal Q angle males = 13 degrees females = 18 degrees Imaging Radiographs recommended views of the knee AP, lateral, axial views findings AP = joint alignment, fracture, knee arthritis lateral = patella alta vs baja, femoral condyle dysplasia, arthritis, transverse patellar fracture axial = patella malalignment, trochlear groove depth, arthritis, vertical patellar fracture measurements patellar height (e.g. Insall-Salvati ratio) lateral patellofemoral angle (normal is an angle that opens laterally ) congruence angle (normal is -6 degrees) CT indications better visualization of the patellofemoral joint alignment fracture findings trochlear geometry TT-TG distance MRI indications best modality to assess articular cartilage views T2 best sequence to assess cartilage