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

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

Trochlear Dysplasia Abnormal shape and depth of the trochlear groove

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.

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

Patellar Height Multiple ways to measure patellar height

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.

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

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

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

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

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.

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%)

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.

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.

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

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

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

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°

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