Do I need an MRI or ultrasound to diagnose a knee tendon rupture?

The examination

The surgeon looks for swelling and bruising, feels for a gap above or below the kneecap, and checks the kneecap height. The key test is whether you can straighten the knee against gravity or perform a straight-leg raise. Inability to do so, with a palpable gap, strongly suggests a complete rupture. Swelling and pain can make the examination harder in the first hours, which is one reason imaging helps.

What each scan adds

• X-ray: shows whether the kneecap sits too high, as in patellar tendon rupture, or too low, as in quadriceps rupture, and rules out a kneecap fracture or a small bone fragment pulled off with the tendon
• Ultrasound: quick and widely available; useful to confirm a tear, assess its location and see whether it is partial or complete
• MRI: gives the most detail on the extent of the tear, tendon quality and any associated injuries inside the knee, such as ligament or cartilage damage

Why partial versus complete matters

A partial tear in which you can still straighten the leg may sometimes be treated without surgery, with a brace and gradual rehabilitation. A complete rupture usually needs surgical repair. Getting this distinction right, quickly, is the main purpose of imaging.

Other tests that may be useful

Because tendon ruptures from small injuries are often linked with medical conditions, your doctor may also suggest blood tests, such as blood sugar, kidney function or uric acid. This can help both with planning surgery and with protecting the other knee.