Background

Definition

 

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 

 

Direction

 

Usually lateral

 

Medial is usually iatrogenic

- excessive lateral release

- lateral release for incorrect reasons

- overtightening of medial structures

 

Anatomy

 

Ossification

 

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

 

Facets

 

Retropatellar surface has 7 facets

- 3 on lateral side

- 3 on medial side

- 1 extra on medial side (odd facet)

 

 

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

 

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)

 

Bony

- patella alta / baja

- trochlea / patella hypoplasia / dysplasia 

 

Soft tissue

- VMO atrophy / medial retinaculum laxity / torn MPFL

- tight lateral structures (capsule, retinaculum, ITB)

- ligamentous laxity

 

Alignment

- femoral anteversion

- external tibial torsion

- genu valgum

 

History

 

Pain 

 

Beware unrelenting pain

- chondral damage

- patella tilt / lateral patella syndrome

 

Instability

 

Traumatic vs. atraumatic onset

Direction of instability 

Age first dislocation

Subsequent dislocations

- mechanism, frequency

- ? voluntar

Treatment to date 

 

Effusions

 

Examination

 

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 0^o

- knees hyperextend beyond 0^o

- ankle DF > 45^o

 

Patient Standing

 

Valgus Malalignment

 

Patella

 

1.  Squinting patella

- with femoral anteversion patellae point inwards when standing

 

2.  Grasshopper eyes

- patella sits high & lateral due to patella alta

 

Gait

 

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)

 

J-sign

- lateral subluxation of patella as knee approaches full extension

- patella sharply deviates laterally in terminal extension 

- indicates some degree of mal-tracking

 

 

Knee Examination

 

Previous incisions

VMO wasting

Effusion

ROM

- 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

 

 

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 30^o over pillow (3)

 

1.  Q (quadriceps) angle 

 

 

Measurement

- line from ASIS to centre of patella 

- line from centre of patella to tibial tuberosity

- angle subtended is Q angle 

 

Values

- normal 10^o men, 15^o women

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

 

Causes increased Q angle

- femoral anteversion (squinting patellae) 

- external tibial torsion

- lateral tibial tuberosity

- genu valgum 

 

2.  Sage mobility

 

Test at 30^o flexion

- move patella medially and laterally

- graded in number of quadrants patella displaces 

- > 50% displacement = insufficient restraints 

 

 

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

 

Prone

 

1.  Lateral border of feet

- if curved, metatarsus adductus 

 

2.   External tibial torsion

- intermalleolar axis > 30^o

- Thigh foot angle > 15^o

 

 

3.  Femoral anteversion

- IR > 45^o

- Gage's trochanteric angle > 15 - 20^o