Caton Deschamps Index

http://radiopaedia.org/cases/caton-deschamps-index

Methodology slightly different to "Patella Height Ratio" here:

MR Imaging of Patellar Instability: Injury Patterns and Assessment of Risk Factors, Radiographics July 2010

Transient Patella Disclocation

With torn medial patello-femoral ligament and medial patella retinaculum. Note that impaction injury can occur almost anywhere at the lateral femoral condyle (although commonly anterolaterally).

Lateral Patella Subluxation, Patella Tilt, Shallow Trochlea Groove and Borderline Patella Alta

MRI criteria for patella alta and baja

Patellar length (PL) and patellar tendon length (TL) were measured .....on sagittal images by a line connecting the superior and inferior patellar poles and the shortest length of the inner margin of the tendon respectively. TL/PL ratio was subsequently calculated

Ratios defined for patella alta and baja were 1.52 and 0.79 respectively in females and 1.32 and 0.74 respectively in males

Skeletal Radiol. 2004 Aug;33(8):445-50. Epub 2004 Jun 24.

http://www.ncbi.nlm.nih.gov/pubmed/15221214


Factors predisposing to Patella Instability:
1. Trochlea dysplasia
2. Patella  alta (TL/PL > 1.3 abnormal)
3. Lateralisation of tibial tuberosity

MR Imaging of Patellar Instability: Injury Patterns and Assessment of Risk Factors
http://radiographics.rsna.org/content/30/4/961.full

Patella Dislocation and PT-LFCFS