Anteroposterior radiograph of the patient
Lateral radiograph of a 31-year-old woman’s knee suggests a complete tear of the patella tendon or patella ligament
A 31-year-old woman presents with a left knee injury that occurred 2 days earlier. She tripped while coming down stairs and her knee struck a railing. She had immediate pain and has been unable to bear weight ever since. Following the injury, she also noticed a depression in the front of her knee beneath her knee cap. Anteroposterior and lateral radiographs are taken in the emergency department the day of injury.
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Lateral radiograph reveals patella alta and increased Insall-Salvati ratio. Patella alta describes a patella positioned too high or more proximal than normal. Conversely, patella baja describes a low-lying patella or patella positioned more distal than normal. The Insall-Salvati ratio is a measurement of patella position taken on a lateral radiograph with the patient’s knee ideally at 30 degrees of flexion. The ratio is equal to the patella tendon length (TL) divided by the length of the patella (PL). Patella TL is measured from the inferior pole of the patella to its insertion site at the tibial tubercle. PL is measured as the greatest length between the superior and inferior pole of the patella. A normal ratio (TL/PL) is 0.8 to 1.2. A ratio <0.8 is considered patella baja. A ratio >1.2 is considered patella alta.1
The patient’s radiographic findings suggest a complete tear of the patella tendon or patella ligament. No further imaging is necessary because patient history, physical examination, and radiographic findings all suggest patella tendon rupture. The patient underwent surgery 5 days after injury for potential primary repair of the patella tendon or patella tendon reconstruction with tendon allograft. Fortunately, the tear was mid-substance and a primary repair was performed. Rehabilitation generally includes wearing a knee immobilizer for approximately 8 weeks after surgery with weight bearing as tolerated. Patients can begin heel slides immediately postoperatively and can begin gentle passive range of motion with physical therapy.2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Gaillard F. Insall-Salvati ratio. Radiopaedia.org. https://radiopaedia.org/articles/insall-salvati-ratio. Accessed February 21, 2017.
- Watts E. Patella Tendon Rupture. OrthoBullets.com. http://www.orthobullets.com/sports/3024/patella-tendon-rupture. Accessed February 21, 2017.