A 27-year-old female presents with left knee pain after a low-speed motor vehicle collision in which her knee hit the dashboard. She is tender over the patella without significant effusion and has an intact extensor mechanism. The above x-ray was obtained (Image 1. X-ray left knee. Case courtesy of Dr. M. Mourits, Radiopaedia.org, rID: 14476).
Patella alta, which in this case is a congenital/developmental abnormality not caused by the recent trauma.
Pearl: Patella alta is diagnosed using the Insall-Salvati ratio, which compares the length of the patellar tendon to the pole-to-pole length of the patella (Image 2). A normal ratio (Tendon/Patella) is 0.8-1.2, with anything greater indicating patella alta and anything less patella baja.
Image 2. Insall-Salvatti ratio. Case courtesy of Dr. Wael Nemattalla, Radiopaedia.org, rID: 10329
Given an intact extensor mechanism and no other findings on exam, this patient likely suffered a contusion and can be made weight-bearing as tolerated and discharged with follow-up as needed. Surgical treatment is generally reserved for patients with severe symptoms.
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Stefanik JJ, Zhu Y, Zumwalt AC, Gross KD, Clancy M, Lynch JA, Frey Law LA, Lewis CE, Roemer FW, Powers CM, Guermazi A, Felson DT. Association between patella alta and the prevalence and worsening of structural features of patellofemoral joint osteoarthritis: the multicenter osteoarthritis study. Arthritis Care Res (Hoboken). 2010 Sep;62(9):1258-65. PMID: 20506169.