Dislocated Patella


Repeated dislocation of patella with minimal trauma

- 15-20% of paediatric acute patella dislocations

- more common girls 

- often bilateral


Dislocation occurs unexpectedly when quadriceps contracted with knee in flexion 




Usually lateral


Medial is usually iatrogenic

- excessive lateral release

- lateral release for incorrect reasons

- overtightening of medial structures






Usually one ossification centre usually that appears at age 3 & closes soon after puberty




Retropatellar surface has 7 facets

- 3 on lateral side

- 3 on medial side

- 1 extra on medial side (odd facet)


Patella Medial and Lateral Facets


Lateral surface larger than medial

- lateral cartilage thicker than medial

- medial & lateral separated by median ridge 


Medial facet & odd facet are separated by another long ridge




Tracking is dynamic 

- lateral in full extension

- more medial & central with flexion


Relies on normal static and dynamic stabilisers


Static Constraints


1.  Bony contours of femur

- prominence of LFC anteriorly

2.  Normal rotational profile

3.  MPFL is constant / static checkrein to patella


Dynamic Constraints


Quadriceps is dynamic stabilizer

- VMO fibers attach to patella at 65° angle


Biomechanics Goodfellow 1976


0°         No PF contact


20°       Most distal part patella contacts trochlea


0-30°    Median patella ridge lies lateral to the centre of the trochlea


30-60°  Patella moves medially to be centered in groove


60-90°  Deeply engaged in trochlear groove & is held by ST tension


90°       Entire articular surface contacts except odd facet


>90°    Patella tilts so that medial facet articulates with the MFC


135°    Odd facet contacts lateral border of MFC


Aetiology Patella Instability


Complicated / Multifactorial


Valgus malalignment

Ligamentous laxity

Insufficient medial structures (MPFL rupture / medial retinaculum laxity / VMO atrophy)

Tight lateral retinaculum

Trochlea dysplasia

Patella alta

Abnormal rotational profile (femoral anteversion / external tibial torsion)



- patella alta / baja

- trochlea / patella hypoplasia / dysplasia 


Soft tissue

- VMO atrophy / medial retinaculum laxity / torn MPFL

- tight lateral structures (capsule, retinaculum, ITB)

- ligamentous laxity



- femoral anteversion

- external tibial torsion

- genu valgum






Beware unrelenting pain

- chondral damage

- patella tilt / lateral patella syndrome




Traumatic vs. atraumatic onset

Direction of instability 

Age first dislocation

Subsequent dislocations

- mechanism, frequency

- ? voluntar

Treatment to date 






Generalised ligamentous laxity  


Wynne-Davies Criteria

- positive if 3 of 5 bilateral signs

- hyperextension of the MCP joints parallel to  forearm 

- touch thumb passively to forearm

- elbows hyperextend beyond 0o

- knees hyperextend beyond 0o

- ankle DF > 45o


Patient Standing


Valgus Malalignment




1.  Squinting patella

- with femoral anteversion patellae point inwards when standing


2.  Grasshopper eyes

- patella sits high & lateral due to patella alta




In toeing

- internally rotated foot progression angle

- indicates femoral anteversion / tibial torsion


Patella Tracking


Patient sitting over side of bed

- flex and extend knee

- compare normal to abnormal side (if not bilateral)



- lateral subluxation of patella as knee approaches full extension

- patella sharply deviates laterally in terminal extension 

- indicates some degree of mal-tracking


Patella J Tracking Enlocated in FlexionPatella J Tracking Extension


Knee Examination


Previous incisions

VMO wasting



- exclude extensive mechanism tightness

- symmetrical heels to buttocks


Knee extended (3)


1.  Tenderness

- lateral retinaculum 

- retropatellar space

- Bassett's sign (tender medial epicondyle / acute MPFL avulsion)


2.  Clarke's Test / patella grind

- produces anterior knee pain with PFJ pathology

- compress patella and ask patient to contract quads

- very non specific test


3.  Patellar tilt test


Patella Tilt 1Patella TIlt Normal


Evaluates tension of lateral restraint 

- patient supine and relaxed with knees extended 

- examiner's thumb on lateral aspect of patella

- lateral edge of patella elevated from the lateral condyle and medial edge depressed 


Abnormal if unable to tilt lateral patella to horizontal


Knee flexed 30o over pillow (3)


1.  Q (quadriceps) angle 


Patella Instability Increased Q Angle



- line from ASIS to centre of patella 

- line from centre of patella to tibial tuberosity

- angle subtended is Q angle 



- normal 10o men, 15o women

- abnormal if > 15o in males and > 20o in females 


Causes increased Q angle

- femoral anteversion (squinting patellae) 

- external tibial torsion

- lateral tibial tuberosity

- genu valgum 


2.  Sage mobility


Test at 30o flexion

- move patella medially and laterally

- graded in number of quadrants patella displaces 

- > 50% displacement = insufficient restraints 


Patella Lateral HypermobilityPatella Medial Hypermobility


Lateral glide 

- >3 quadrants suggests incompetent med restraints 


Medial glide

- > 3 suggests incompetent lateral restraint / hypermobile patella

- < 1 suggests tight lateral retinaculum


3.  Apprehension test (Fairbank)


Patient supine and relaxed 

- place relaxed knee at 30 degrees & push patella laterally as flex

- can also do with knee flexed over edge of bed

- positive test is a quads contraction & apprehension


Rotational Profile




1.  Lateral border of feet

- if curved, metatarsus adductus 


2.   External tibial torsion

- intermalleolar axis > 30o

- Thigh foot angle > 15o


Thigh Foot Angle 20 DegreesThigh Foot Angle 35 Degrees


3.  Femoral anteversion

- IR > 45o

- Gage's trochanteric angle > 15 - 20o


Increased Femoral Anteversion