AANA Lab Course 913 - Advanced Knee Course-Assessment of Patellar Instability_ Consider These Factors Faculty Lecture

Assessment of Patellar Instability_ Consider These Factors Faculty Lecture_ Patrick Kane, MD

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This presentation by Dr. Patrick Kane at the AANA Complex Knee Course (2019) addresses key factors involved in patellar instability, a condition affecting 2-3% of knee injuries seen in orthopaedics, mostly in young adults. Redislocation rates vary widely, and up to 60% of cases involve osteochondral lesions.<br /><br />Patellar instability is influenced by both bony and soft tissue anatomy. Critical bony factors include limb alignment, patella height (commonly measured on lateral radiographs using Insall-Salvati or Caton-Deschamps methods), trochlear morphology (assessed by Dejour classification and sulcus angle on axial imaging), and rotational alignment (involving femoral anteversion, tibial torsion, and the tibial tubercle–trochlear groove (TT-TG) distance, with values above 20mm considered pathological).<br /><br />Patellar engagement depends on patella height and trochlear groove shape/depth; abnormalities like patella alta and trochlear dysplasia reduce stability. Medial soft tissue restraints, especially the medial patellofemoral ligament (MPFL), provide up to 60% of resistance to lateral patellar translation; MPFL reconstruction is a cornerstone of surgical treatment. Other soft tissue contributors include the medial patellotibial and patellomeniscal ligaments. Dynamic stabilizers, such as hip abductors and the vastus medialis obliquus (VMO), also play a role.<br /><br />Ligamentous laxity from generalized hypermobility (Beighton score) or connective tissue disorders like Ehlers-Danlos further predispose to instability.<br /><br />Successful management hinges on addressing anatomical and functional factors: ensuring proper patellar engagement (“bringing the train to the tracks”) by correcting patella alta and rotational deformities, providing adequate trochlear morphology (“appropriate tracks”) and restoring medial soft tissue stabilizers (“the conductor”). Limb alignment assessment is critical, with valgus deformities warranting correction. Overall, comprehensive evaluation of osseous parameters, soft tissue structures, and limb mechanics is essential for optimizing outcomes in patellar instability.

Keywords

Patellar instability

Patella alta

Trochlear dysplasia

Medial patellofemoral ligament (MPFL)

Osteochondral lesions

TT-TG distance

Limb alignment

Rotational deformities

Ligamentous laxity

MPFL reconstruction