AP / Long Leg Views


Quantify Valgus Malalignment


Patella Instability Long Leg Views Valgus MalalignmentValgus Knee


Lateral Xray


1.  Assess Patella Alta


30o flexion


A.  Blumensaat's line / Inaccurate


Knee flexed to 30o

- line should just touch inferior pole of patella

- pole above line - alta

- pole below line - baja


Patella Height Normal Blumensaat's LinePatella Alta Blumensaat's LinePatella Alta Blumensaat's Line


B.  Blackburn-Peele ratio / Best and Most accurate


Distance between tibial and patella articular surface

- divided by patella articular surface

- patella alta > 1


Patella Baja Blackburn PeelePatella Alta Blackburn Peele


C.  Insall ratio

- less accurate, probably because more difficult to measure

- ratios also difficult to remember and calculate

- length of patella tendon v length patella

- patella alta LT : LP 1.2

- patella baja LT : LP <1


Patella Height Normal Insall RatioPatella Alta Insall Ratio


2.  Assess Trochlea Dysplasia


Dejour Crossover Sign

- lateral x-ray at 30o with condyles superimposed

- identify base of trochlea



- clearly defined trochlea groove


Trochlea Anatomy NormalTrochlea Normal Anatomy Diagram


Abnormal / Crossover

- line of floor of trochlea crosses lateral lip of condyle

- indicates trochlea is deficient proximally


Trochlea Crossover


Trochlea depth

- < 8 mm shallow


Dejour grading system 1 - IV


Patellofemoral view


1.  Skyline view



- 45o

- shoot throught film


Look for


- bony avulsion MPFL


MPFL Bony Avulsion


2.  Laurin view / patella tilt



- knee 20o, camera at bottom


Assessment patella tilt

- first line anterior aspect both condyles

- line lateral facet

- should diverge laterally


Patella tilt

- lines parallel or open medially


Patella Laurin View NormalPatella Tilt Laurin ViewPatella Tilt


3.  Merchant view / patella subluxation



- 40o flexion, beam from top

- patella should be well engaged

- central ridge should lie at or medial to bisector of the trochlea groove


Congruence angle

- draw sulcus angle

- bisector of sulcus angle

- line to central ridge of patella

- should be - 10o (i.e. medial)

- lateral direction is positive




Patella non SubluxedPatella Medial Congruence Angle




Patella Lateral SubluxationPatella Lateral Congruence AnglePatella Subluxation


4.  Trochlea dysplasia




Patella Normal TrochleaPFJ Normal Sulcus Angle


Sulcus angle

- > 140o flattened


Trochlea Dysplasia


5. Excessive Lateral Pressure Syndrome


Ficat and Hungerford


A.  Indirect signs of excessive lateral pressure

- thickened subchondral plate

- increased density lateral facet

- lateralisation of trochlea

- medial facet osteoporosis

- hypoplasia lateral condyle


Patella Excessive Lateral Pressure 1Patella Excessive Lateral Pressure 2


B.  Indirect signs of excessive lateral ligament tension

- fibrosis lateral retinaculum

- calcification lateral retinaculum

- lateral osteophyte

- bipartite patella

- lateral facet hypoplasia

- medial compartment hypoplasia


Patella Excessive Lateral TensionPatella Excessive Lateral Pressure




1.  Skyline View


Assess for

- lateral tilt

- subluxation

- trochlea dysplasia


PFJ Axial CT


2.  Lateralisation of tibial tuberosity




Jones et al Skeletal Radiology


Superimpose 2 axial slices


A.  Axial slice of trochlea

- line of posterior condyles

- line perpendicular through trochlea


Axial CT PFJ


B.  Slice through tibial tuberosity

- perpendicular line through TT


CT Axial Tibial Tuberosity


Calculate Distance between two points / TTTG


10 - 15 mm normal, > 15 abnormal


Pandit et al Int Orthop 2011

- normal 10 +/-1 on MRI




Articular Cartilage Damage

MPFL integrity


Loose Bodies


MPFL Femoral Tear




Assess chondral surfaces

Removal of Loose Bodies


- not particularly valid

- patient is relaxed / knee filled with fluid