1. Pathophysiology of Pediatric Patellar Instability
Nicole A. Friel, MD MS
Orthopaedic Sports Medicine
Shriners Hospitals for Children Northern California

2. Pathophysiology of Pediatric Patellar Instability
Trochlea Dysplasia
Patellar Height
Hyperlaxity
TT-TG
Syndromic Associations
MPFL Tear Pattern
Lower Limb Alignment
Lower Limb Torsion

3. Trochlear Dysplasia
Abnormal shape and depth of the trochlear groove

4. Trochlear Dysplasia Described by Dejour et al in 1990
On a lateral knee radiograph, they identified two features:
The depth of the TG, or gorge, with respect to the height of the medial and lateral trochlear walls (the ‘‘crossing’’ sign), which represents the flattening of the groove as it is viewed from the side.
The prominence (also called the trochlear ‘‘boss,’’ ‘‘bump,’’ or ‘‘eminence’’) of the floor of the groove with respect to the anterior cortex of the distal femur.
Dejour H, Walch G, Neyret P, et al. Dysplasia of the femoral trochlea. Rev Chir Orthop Reparatrice Appar Mot. 1990; 76:45–54.

5. Trochlear Dysplasia
Femoral trochlear morphology can be improved by early (before epiphyseal closure) surgical correction in children with recurrent patellar dislocation
23 patients (aged 7 to 11)
Bilateral patellar instability
Group S (surgical; medial patellar retinacular plasty) versus Group C (control; non-operative)
Surgical patients had improvement in imaging parameters and trochlear shape

6. Patellar Height
Multiple ways to measure patellar height

7. Patellar Height Patella alta associated with patellar instability
With knee flexion, the patellar remains proximal to the trochlear groove
Patella alta is often corrected during surgery for patellar instability, as MPFL reconstruction has been shown to normalize patellar height.

8. Hyperlaxity Increased joint mobility is related to tissue elasticity
Type III collagen (increased levels in children) contributed to tissue elasticity
Beighton score is a tool used to identify hyperlaxity
Traditionally, a score of 5/9 or higher identifies joint hypermobility
However, it has been suggested that for children, a score of 7/9 or higher should identify hypermobility

9. TT-TG
Characterized the lateralization of the tibial tuberosity or the medialization of the trochlear groove (in patients with trochlear dysplasia)
Measures the distance between 2 perpendicular lines from the posterior cortex to the tibial tubercle  and the trochlear groove

10. TT-TG
Originally described as a CT finding, MRI is now used to determine TT-TG
Measurement technique (ie, MRI versus CT) produce variances in numeric values
A TT–TG of 20mm has frequently been used as a cut-off value to direct surgical treatment, although this decision is multifactorial
TTTG: bony CT: 14.4 ± 5.4
bony MRI: 13.9 ± 4.5
cart. CT: 15.3 ± 4.6
cart. MRI: 13.5 ± 4.1
Schoettle et al 2006

11. TT-TG
Correcting the TT-TG is a surgical option, but only in skeletally mature patients

12. Syndromic Associations
Common syndromes that have patellar instability:
Nail-patella syndrome
Kabuki syndrome
Down’s syndrome
Rubinstein-Taybi syndrome
Syndromes that may or may not be associated with patellar instability:
Turner syndrome
Patella aplasia
Absent patella syndrome
Treatment approach individualized for each syndrome, given that children with each of the above my have different functional levels.

13. MPFL Tear Pattern Role of the MPFL
Primary patellar stabilizer of the knee from full extension to 30° of flexion
MPFL tear location in patellar instability
Location of MPFL tears in pediatric patients is variably reported:
patellar attachment (10%-61%)
femoral attachment (12%-73%)
both (12%-35%), or
mid-substance (2.5%-15%)

14. Zone of MPFL injury was at the patella 61% of the time.
MPFL Tear Pattern
What is the location of MPFL injury?
MRI evaluation for 43 patients (under 18 years old) with a first-time patellar dislocation event
Zone of MPFL injury was at the patella 61% of the time.

15. Lower Limb Alignment Genu Valgum
Risk factor for patellar instability, as it produces an increased Q angle and an increased lateral force on the patella
At 3-4 years of age, children can have up to 20º of genu valgum, but this should normalize and be no greater than 8º by age 12.

16. Lower Limb Alignment
Genu Valgum in the setting of patellar instability
Skeletally immature
Guided growth (hemiepiphysiodesis)
Selected concomitant procedures as needed

17. Lower Limb Alignment
Genu Valgum in the setting of patellar instability
Skeletally mature
Distal femoral osteotomy (medial closed or lateral opening)
Selected concomitant procedures as needed

18. Lower Limb Torsion
Femoral anteversion + Tibial torsion = Miserable malalignment
Femoral anteversion
Increases strain in the MPFL
Increases contact pressure on the lateral aspect of the patellofemoral joint

19. Lower Limb Torsion
Femoral anteversion + Tibial torsion = Miserable malalignment
Femoral anteversion
No consensus regarding the acceptable tolerance of anteversion, but derotational osteotomy can be considered in children with symptomatic patellofemoral instability and femoral anteversion greater 25–30°

20. Thank you