A 13 yo M presents to the ED c/o knee pain. He was playing basketball and states he jumped up and felt his knee "give out." He is unable to bear weight. On exam, there is tenderness and swelling of the anterior knee and his ROM is limited. An xray is obtained. What's the diagnosis?  (scroll down for answer) 

Answer: Avulsion fracture of the tibial tuberosity

• In pediatric patients, tendons/ligaments and their insertion sites are stronger than bone
• Avulsion fractures occur when a strong mechanical force is applied to a tendon or ligament-- and instead of a tendon or ligamentous injury, the patient experiences a bony injury
• Occur after seconday ossification centers develop in adolescence
• Therefore avulsion fractures rarely seen before age 8
• In this particular patient jumping likely caused quadriceps contraction --> force transmitted along quadriceps and patellar tendons --> patellar tendon "pulled" on tibial tuberosity, causing avulsion
• Similar mechanism in adult would likey result in patellar or quadriceps tendon rupture
• Management determined in consultation w/ orthopedics

References:

Black KL, Duffy C, Hopkins-Mann C, Ogunnaiki-Joseph D, Moro-Sutherland D. Musculoskeletal Disorders in Children. In: Tintinalli JE, Stapczynski J, Ma O, Yealy DM, Meckler GD, Cline DM. eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8eNew York, NY: McGraw-Hill; 2016.

Silva Júnior AT, Silva LJ, Silva Filho UC, Teixeira EM, Araújo HR, Moraes FB. Anterior avulsion fracture of the tibial tuberosity in adolescents - Two case reports. Rev Bras Ortop. 2016;51(5):610–613. Published 2016 Aug 10. doi:10.1016/j.rboe.2016.08.001