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
A patellar tendon rupture can present with the same finding, as the ruptured tendon allows the patella to move proximally (Image 3).
Image 3. Ruptured patellar tendon – note the avulsed fragments at the inferior pole of the patella. Case courtesy of Assoc Prof Frank Gaillard, Radiopaedia.org, rID: 7514
- Pearl: To verify tendon integrity, an intact extensor mechanism, exhibited by a straight leg raise, and/or ultrasound can be used (Image 4).
Image 4. Intact patellar tendon, courtesy of Joy English, MD
Resources & References:
Don’t forget to brush up on these other can’t miss adult knee injuries!
- Insall J, Goldberg V, Salvati E. Recurrent dislocation and the high-riding patella. Clin Orthop Relat Res. 1972;88:67-9. PMID: 5086583.
- 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.
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