Background

 

 

Definition

 

Recurrent lateral dislocation /subluxation of patella with minimal trauma

 

Epidemiology

 

Females > Males

2nd - 3rd decade

 

Anatomy

 

Facets	Static constraints	Dynamic constraints
Medial and lateral facet	Bony - prominence of lateral femoral condyle	Quadriceps / VMO
Lateral larger	MPFL

 

 

Etiology Patella Instability

 

Multifactorial / Combination of

- valgus malalignment

- ligamentous laxity

- MPFL tears

- Trochlea dysplasia

- Patella alta

- Abnormal rotational profile (femoral anteversion / external tibial torsion)

 

Examination

 

Ligamentous laxity / Beighton score > 4

- hyperextension of the MCP joints beyond 90^o

- touch thumb passively to forearm

- elbows hyperextend beyond 0^o

- knees hyperextend beyond 0^o

- palms flat on floor while knees straight (1)

 

 

Patella Tracking / J Sign

- patient sitting over side of bed

- flex and extend knee

- lateral subluxation of patella as knee approaches full extension

 

 

Q (quadriceps) angle 

- knee flexed 30^o over pillow or examiner knee

- line from ASIS to centre of patella 

- line from centre of patella to tibial tuberosity

- angle between lines is Q angle 

- abnormal: > 15^o in males and > 20^o in females 

 

 

 Patella mobility

- knee flexed 30^o over pillow or examiner knee

- move patella medially and laterally

- graded in number of quadrants patella displaces 

- lateral glide: >3 quadrants suggests incompetent medial restraints 

- medial glide: < 1 suggests tight lateral retinaculum

 

 

Apprehension test 

- push patella laterally

- patient feels apprehension due to patella instability

 

External tibial torsion

- intermalleolar axis > 30^o

- thigh foot angle > 15^o

 

Thigh-foot angle

 

Femoral anteversion

- patient prone

- hip internal rotation > 15^o

 

Increased femoral anteversion on left

 

Investigations

 

Patella alta

 

 30^o flexion lateral xray

 

Blumensaat's line	Blackburn-Peele ratio	Insall ratio
Cross inferior pole of patella	Distance between - tibial and patella articular surface - divided by length of patella articular surface	Length of patella tendon / length patella
Pole above line - alta Pole below line - baja	Patella alta > 1	Patella alta 1.2 Patella baja < 1

 

TT-TG

 

 Tibial Tuberosity Trochlea Groove (TTTG)

- superimpose 2 axial slices

- axial slice of trochlea

- line perpendicular through trochlea groove

- perpendicular line through tibial tuberosity

- distance between two lines

 

There are no absolute cut offs for instability, or to guide surgery

 

 

Park et al AJSM 2023

- MRI of 87 patella instability patients and 500 non patella instability in pediatric patients

- control group:  mean TTTG 8 mm

- instability group: mean TTTG 16 mm

 

Tan et al J Knee Surg 2020

- systematic review of TTTG CT versus MRI

- measured TTTG on CT greater than on MRI

- CT control group:  mean TTTG 13 mm

- CT instability group: mean TTTG 18 mm

- MRI control group:  mean TTTG 10 mm

- MRI instability group: mean TTTG 15 mm

 

 Trochlea Dysplasia

 

Dejour Crossover Sign

- lateral x-ray at 30^o with condyles superimposed

- identify base of trochlea

- normal: clearly defined trochlea groove

- abnormal / Crossover: line of floor of trochlea crosses lateral lip of condyle

 

Normal trochlea / no crossover

 

Crossover with supratrochlear spur

 

Crossover with large supratrochlear spur

 

Trochlea dysplasia / Sulcus angle

- normal < 140^o

- > 140^o flattened

 

Normal trochlea / sulcus

 

Trochlea dysplasia with reduced sulcus angle

 

Dejour trochlea dysplasia classification

 

Type A:  Shallow trochlea

Type B:  Flat trochlea groove with trochlea spur

Type C:  Lateral trochlea convex, hypoplastic medial trochlea

Type D:  Trochlea spur + cliff like fall off between lateral and medial trochlea

 

 

Type A

 

Type B

 

Type C

 

Type D

 

Other findings on imaging

 

Xray

 

Bony MPFL avulsion

 

 

 Patella subluxation

 

Patella should be well engaged in groove

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

 

Patella well engaged in groove versus laterally subluxed patella

 

CT

 

Lateral subluxation with Type C trochlea dysplasia

 

MRI

 

Lateral subluxation with trochlea dysplasia type B

 

Articular cartilage damage on patella