This investigation was undertaken to identify the structures torn within the medial retinaculum and localize the injury site anatomically following acute lateral dislocation of the patella in a cadaver model. The patellae of 10 fresh-frozen cadavers were translated laterally 135% of the patella width on a universal testing instrument. Magnetic resonance imaging (MRI) was performed on all specimens prior to testing and immediately following testing. Anatomical dissection also was performed on the medial retinaculum following testing. Dissection revealed avulsion fractures from the inferomedial border of the patella in 8 of the 10 knees. The medial patellofemoral ligament was injured in 8 of the 10 knees; the location of the injury varied. Tears of the medial patellofemoral ligament from the femur in 6, a midsubstance tear in 1, and stretch in 1 knee were noted. In a knee with a femoral-sided tear, an avulsion fracture of the medial patellofemoral ligament was identified. None of the cadaver knees demonstrated tears of the lateral retinaculum or medial patellotibial ligaments on dissection. Review of the MRIs revealed a medial retinaculum tear in 6 of the 10 knees. Two tears from the femur, 3 from the patella, and 1 tear from both the patella and femur were noted. An avulsion fracture was noted from the inferomedial patellar border in 3 of the 10 knees. No pathology was noted on 4 of the MRIs. When anatomically correlated, the 3 patellar retinacular tears and 3 avulsion fractures noted on MRI represented a tear of the medial patellomeniscal ligament from the patella. The femoral-sided tear represented a tear of the medial patellofemoral ligament from the femur. An appreciation of the spectrum of injury to the medial retinaculum may aid in the diagnosis of an acute dislocation of the patella and help establish the anatomical structures damaged. The pathology demonstrated in this study may explain the diversity of injury seen clinically. Whereas an avulsion fracture from the patella may represent the medial patellomeniscal ligament, a femoral-sided retinacular tear may represent the medial patellofemoral ligament. This may lead to future refinements of surgical options and anatomic restoration of the damaged structure.