Special Considerations for Pediatric Patellar Instability

EPIDEMIOLOGY

In a prospective 2-year study of Finnish children, the annual incidence rate of patellar instability was 43/100,000 pediatric population.¹ In patients 9 to 15 years of age, the incidence was approximately 1/1000.¹ In another study, patients at highest risk for a first-time patellar dislocation were females aged 10 to 17 years.² In a study in patients with traumatic hemarthrosis, 36% in the younger age group (10-14 years) and 28% in the older age group (15-18 years) had sustained patellar dislocation. In contrast, 22% in the younger age group and 40% in the older age group had sustained an anterior cruciate ligament tear.³

Approximately one-half of patients who dislocate their patella suffer from long-term complications.^4,5 These complications include recurrent instability, patellofemoral pain, osteochondral lesions, and eventual arthritis.^1,4,5 Young, active individuals are more prone to these issues.⁶ Also, 39% or more of patellar dislocation patients have an associated osteochondral fracture that might influence the management.¹ Thus, patellar instability in young patients is an area of concern.

DEVELOPMENTAL ANATOMY

At 4-week gestation, the patellofemoral joint is an ectodermal sac filled with mesenchyme of the somatic mesoderm.⁷ Mesenchymal condensations then appear at 4 to 5 weeks gestation, followed by chondrification of both the femur and patella.⁷ The joint space is present by 6 weeks, and the patellar and distal femoral condyles are present at 7 weeks gestation.⁷ By 8 weeks gestation, the basic knee anatomy resembles that of an adult with the chondroepiphysis forming the articular surfaces of the femur, tibia, and patella.⁷ By this time, the extensor mechanism is formed, and active joint motion has begun, facilitating the development of the trochlear sulcus.⁷ The secondary ossification center in the distal femoral epiphysis forms around 36 weeks gestation.⁸ Postnatally, both the patella and distal femur grow through endochondral ossification.^9,10

The patella is the largest sesamoid bone in the human body.¹¹ The patella begins as a dense consolidation of cells that differentiate as the quadriceps mechanisms develop.^12,13 The patellar anlage becomes distinguishable within the quadriceps tendon around 7.5-week gestation.¹² The morphology of the patella conforms to the distal femur.¹² The patella molds or re-models as the knee begins to move in response to mechanical stresses.⁷ The patella increases in relative size during the first 6 months of gestation, then enlarges proportionately to the rest of the bones.⁷ Ossification begins around 3 years of age for females and 4 to 5 years of age for males.^8,14 The ossification center may appear irregular as it rapidly expands.¹⁴ Ossification proceeds in a proximal to distal direction, thus giving a spurious estimation of patellar height on radiographs in children. The overall morphology of the cartilaginous patella during development is comparable to the final mature shape.¹⁴ Abnormal contact stresses on the articular surface of the patella during skeletal immaturity can lead to deformation.⁷

Ultrasonographic measurements in normal patients show that trochlear groove (TG) morphology is present early and becomes more radiographically apparent as distal femoral ossification is completed.¹⁵ Anatomic dissections of aborted fetuses have verified the morphology of the TG as it remains constant during growth and the groove morphology is the same for both fetuses and adults.¹⁶ An ultrasound study performed on patients aged 12 to 18 years showed the cartilaginous sulcus angle (CSA) remained constant throughout all age groups (146°).¹⁷ The CSA however, differed in patients who suffered a patellar dislocation (average, 164°; range, 154°-195°) compared with normal knees (average CSA, 145°; range, 131°-158°).^15,17,18 The osseous sulcus angle, on the other hand, appears flat at birth and the TG deepens with age. This increase in depth is more of a reflection of progressive ossification of a well-formed cartilaginous trochlea, rather than a true deepening of the sulcus.¹⁷ Thus, the axial radiographic view of the patella provides misleading information about the sulcus angle in children and should not be used to define trochlear morphology.

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