Ortho Hors D'oeuvres (Answer)
To review, this is a 26 year-old male presenting with right knee pain after landing forcibly on it during basketball.
Questions and Answers:
1) What are the findings on radiographs?
This above radiograph demonstrates bony opacities just distal to the distal pole of the patella as well as a possibly high-riding patella-both of these findings are consistent with patellar tendon rupture. Radiographic diagnosis of patellar tendon dislocation (and associated patella alta) is made using the Insall-Salvati ratio.
To calculate the Insall-Salvati ratio, the patellar tendon length (from the distal end of the patella to the tibial tuberosity) is divided by the patellar length along the articular surface of the patella. A normal ratio is considered between 0.8 and 1.2. An Insall-Salvati ratio higher than 1.2 is classified as patella alta and highly indicative of patellar tendon rupture. The above patient has a patellar tendon length of 64mm (dashed line) and a patellar length of 45mm (solid line), yielding an Insall-Salvati ratio of 1.4, with likely patella alta and associated patellr tendon rupture.
The Insall-Salvati ratio should generally be calculated with lateral films and the knee in 30 degrees of flexion, and small studies have shown that interobserver reliability is poor (kappa = 0.6) for diagnosis of patella alta in patients with a normal knee. Because of this, several alternative diagnostic rules have been introduced, including an Insall-Salvati ratio used for MRI diagnosis of rupture and a modified Insall-Salvati ratio using only the articular surface of the patellar tendon. Unfortunately, these diagnostic rules have yet to be validated and the Insall-Salvati ratio is still the clinical diagnostic gold standard.
Other signs of patellar tendon rupture on radiograph include bony fragments along the patellar tendon and a heterogeneous appearance of the infrapatellar fat pad, but these are neither sensitive nor specific.
2) What are the next steps in diagnosis?
A thorough physical exam can often confirm a diagnosis of suspected tendon rupture, even in the absence of radiographs. Patients with patellar tendon rupture will often have a palpable defect inferior to the patella, and will lack the ability to actively extend their leg at the knee. In some situations, the medial and lateral patellar retinaculum, the ligamentous and tendinous sheath extending from the quadriceps and around the patella, can be intact and allow the patient to weakly extend the knee despite a tendon rupture, but this is very rare.
3) What is the appropriate emergency department management?
Patellar tendon ruptures require operative fixation, generally in a short time frame to prevent tendon retraction, and these patients are often admitted for next-day fixation and repair. The patient should be placed in a knee immobilizer in full extension in order to best approximate the ends of the patellar tendon and be made non-weightbearing to prevent activation of the quadriceps muscle and associated tendinous separation.
References:
Radiographs and case courtesy of Gretchen Fuller, MD.
Kadakia NR, Ilahi OA. Interobserver variability of the Insall-Salvati ratio. Orthopedics, 2003, 26:321-324.
Fazal MA, Moonot P, Haddad F. Radiographic Features of Acute Patellar Tendon Rupture. Orthop Surg. 2015 Nov;7(4):338-42.
Kadakia NR, Ilahi OA. Interobserver variability of the Insall-Salvati ratio. Orthopedics, 2003, 26:321-324.
Shabshin N, Schweitzer ME, Morrison WB et-al. MRI criteria for patella alta and baja. Skeletal Radiol. 2004;33 (8): 445-50.
Yousef AMA, Rosenfeld S. Acute traumatic rupture of the patellar tendon in pediatric population: Case series and review of the literature. Injury. 2017 Nov;48(11):2515-2521.
