Special Considerations for Pediatric Patellar Instability

PATELLAR HEIGHT

The role of patellar height in patellar instability has been well established.³⁸ In patients with patella alta, the patella remains proximal to the TG during the greater arc of knee motion, which predisposes it to patellar instability. Calculation of patellar height in children could be challenging due to incomplete ossification, as well as asymmetric ossification of the patella and the tibial tubercle (TT). Since the patella ossifies from proximal to distal, most radiographic methods that measure the patellar height from the distal aspect of the patella provide a spurious elevation of the measurement.

The Caton-Deschamps (CD) method measures the length of the patellar articular surface and the distance from the inferior edge of the articular surface to the anterosuperior corner of the tibial plateau.³⁹ A ratio >1.3 signifies patella alta. The CD ratio has been verified as a simple and reliable index for measuring patellar height in children.⁴⁰ Two other methods have been described for determining patellar height in children.^41,42 Based on anteroposterior (AP) radiographs of the knee in full extension, Micheli and colleagues⁴¹ calculated the difference between the distance from the superior pole of the patella to the tibial plateau and the length of the patella. A positive difference signified patella alta. The Koshino method involves the ratio between a reference line from the midpoint of the patella to the midpoint of the proximal tibial physis and a second distance from the midpoint of the distal femoral physis to the midpoint of the proximal tibial physis on lateral knee radiographs.⁴² Normal values range from 0.99 to 1.20 with the knee in >30° flexion, in children 3 to 18 years of age.

HYPERLAXITY

In contrast to adults, children have increased levels of collagen III compared with collagen I, which is responsible for tissue elasticity.⁴³ Tissue elasticity leads to increased joint mobility, which is more common in children. Joint hypermobility or hyperlaxity has to be differentiated from symptomatic instability. The traditional Beighton score identifies individuals as having joint hypermobility with a score of 5/9 or higher in school-aged children.^44-46 Smits-Engelsman and colleagues⁴⁴ suggested using stricter criteria with scores of 7/9 or higher being indicative of hyperlaxity in school-aged children. A study of 1845 Swedish school children noted that females have a higher degree of joint laxity.⁴⁵ Maximal laxity was noted in females at 15 years of age.⁴⁵ Hyperlaxity has been demonstrated to be greater on the left side of the body⁴⁴ and can be part of generalized syndromes including Down’s syndrome, Marfan’s syndrome, or Ehlers-Danlos syndrome.

LIMB TORSION

Staheli and colleagues⁴⁷ described the normative values of a lower extremity rotational profile, including femoral anteversion and tibial torsion. Children normally have increased femoral anteversion, which decreases with growth. Miserable malalignment is a term used to denote increased femoral anteversion and increased external tibial torsion.^48,49 These rotational abnormalities can increase the Q angle and the lateral forces on the patella. Femoral anteversion or internal rotation of the femur of 30° significantly increases strain in all areas of the MPFL.⁴⁸ This increased strain may lead to MPFL failure and patellar instability.⁴⁸ Increased internal rotation of the femur also increases contact pressure on the lateral aspect of the patellofemoral joint.⁴⁸ Miserable malalignment frequently manifests following a pubertal growth spurt and may require femoral and tibial osteotomy.⁵⁰

SYNDROMIC ASSOCIATIONS

Several syndromes have patellar instability as a part of their manifestation. The more common syndromes include nail-patella syndrome, Kabuki syndrome, Down’s syndrome, and Rubinstein-Taybi syndrome.^51-54 Other syndromes less commonly associated with patellar instability include Turner syndrome, patella aplasia, or absent patella syndrome. Since many patients with syndromic patellar instability are functionally limited, they may not require an aggressive approach to treatment. When treating these patients, it is important to recognize the unique features of a specific syndrome, which may affect the anesthesia risk profile, management decisions, rehabilitation, and prognosis.

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