Knee

Patellar instability

Patellar instability assessment centres on patellar height, measured as a ratio on a lateral radiograph, with patella alta above roughly 1.2 a recognised and correctable risk factor.

What you are actually deciding

Choose the index by what surgery the patient has had. Insall-Salvati references the tibial tubercle and is invalid after tubercle osteotomy; Caton-Deschamps references the tibial plateau and remains valid. Patellar height is one factor among several — trochlear dysplasia and TT-TG distance matter too.

Patient-reported outcome measure1

IKDC Calculator18 items
International Knee Documentation Committee Subjective Knee Evaluation FormThe IKDC Subjective Knee Evaluation Form is an 18-item patient-reported questionnaire covering knee symptoms, sports and daily activity and current knee function, whose raw total is transformed to a 0 to 100 score where 100 means no symptoms and no activity limitation — it requires permission from the AOSSM, so the items are not reproduced here.

Clinical calculator2

Classification system1

Outerbridge Classification1 items
Outerbridge Classification of Chondral LesionsThe Outerbridge classification grades articular cartilage damage from 0 to IV at arthroscopy, progressing from normal cartilage through softening, fissuring under and over 1.3 centimetres in diameter, to full-thickness erosion exposing subchondral bone.

Patient-reported outcome measure1

Clinical calculator2

Classification system1

Related conditions