A 73-year-old woman attended the emergency department after an accidental fall due to tripping on the stairs at home.
The left limb presented with the knee in fixed flexion of 40º and external rotation of the hip. The femoral condyles were clearly palpable, with a 'pseudo-hip impingement' proximal to the patella. The attitude in flexion was not changed by pain when attempting to change the position. The patella was fixed and tight. Any attempt at flexion-extension resulted in severe pain and tightness of the quadricipital tendon and goose foot.
The radiological study showed a low patella with its proximal pole in contact with the intercondylar area, compatible with lower horizontal dislocation of the patella.

Reduction by manipulation was unsuccessful, so epidural anaesthesia was performed, producing a sonorous audible 'clunk' or patellar snap when performing patellar traction and knee extension, after which the patella was anatomically arranged.
The patellar and quadricipital tendons were found to be undamaged and functional, without haematoma, with correct gliding and tension. The anterior tibial tuberosity was also found to be in normal condition. The joint was stable in both varus and valgus in extension and 30° semi-flexion, with no haemorrhagic focus in LLE, LLI or goose foot. External arthrotomy was performed with extraction of 30 cc of sero-haematic synovial fluid, where the anterior and posterior cruciate ligaments were found to be normal. There was evident femoro-patellar arthrosis with chondyle and patellar osteophytosis (Ahlbach grade III). We performed excision of the patellar osteophytes and the intercondylar groove.
The patient was immobilised with a knee orthosis in extension for 2 weeks with support, after which physiotherapy was started for 2 weeks.
One month after the trauma he presented flexion of -30º and full extension, with absence of pain. Two months after the accident she presented full flexion-extension. The radiological study at that time showed knee with predominantly femoro-patellar arthrosis (Ahlbach grade III).

A characteristic of the patient that helps to explain the injury is that the patient had generalised hyperelasticity.
Four years later, she has suffered no recurrence and only reports discomfort in the anterior area of both knees when descending stairs, compatible with her femoro-patellar osteoarthritis. This symptomatology was already present before the dislocation.