THE CASE

A 44 year old male with a history of right ACL repair 1 year ago presents with right knee pain and swelling after a fall. He was helping his neighbor fix their snowblower when he slipped and fell backwards. He denies hitting his knee on the ground but felt a pop in his right knee during the fall. He reports feeling as though his patella was “out of place” but then it popped back in and his pain improved somewhat. He now has worsening swelling of his leg, an inability to extend at his knee and an inability to weight-bear. He has no numbness or tingling radiating down his leg. No ankle or hip weakness or pain. There was no head strike or any other injuries incurred in the fall.

PHYSICAL EXAM

Vitals signs: BP 144/117, HR 93, T 97F, RR 16, O2 sat 100%

Musculoskeletal exam: His right knee is slightly flexed with significant swelling noted. Tenderness was present over the anterior knee and patella. There was a deformity/depression over the quadriceps tendon. No tenderness to palpation over the medial or lateral joint line. The patient was able to isometrically contract the quadriceps muscles but not able to extend the lower leg. He has 5/5 strength to dorsi and plantar flextion bilaterally. No overlying erythema or increased warmth. 2+ PT and DP pulses. Sensation to light touch is intact distally.

Bedside musculoskeletal ultrasound of his knees was performed.

His left (normal) knee is shown below:

Labeled combined still frames of the above ultrasound pointing out landmarks That should BE noted on every knee ultrasound. (*Please excuse the choppiness of the image conglomeration)*

Right (affected) knee:

Labeled combined still frames of the above ultrasound noting a large hematoma between two distinct pieces of the patella

A follow up xray was performed confirming the patellar fracture seen on ultrasound

CASE RESOLUTION

He was placed in a knee immobilizer in the emergency department with instructions to weight bear as tolerated and follow up with orthopedic surgery for surgical planning. He had an operative repair of his patellar fracture 10 days after his initial visit.

ULTRASOUND PEARLS

This case highlights an excellent use of musculoskeletal ultrasound for the diagnosis of patellar fracture. Based on the clinical presentation there were multiple diagnoses possible including quadriceps tendon rupture, patellar fracture, patellar tendon rupture, joint effusion, long bone fracture. While an xray will obviously be useful in this case to evaluate for bony injury, a point of care ultrasound is extremely useful in the evaluation of an acute post-traumatic knee. In acute knee trauma, musculoskeletal ultrasound has been shown to have increased sensitivity (94% vs 84%), compared to xray in diagnosing intra-articular fractures[1]. Literature has shown that point of care ultrasound performed by ED physicians can be used to accurately diagnose quadriceps and patellar tendon ruptures [2,3].

Make sure you always scan the opposite extremity to get a sense of your patient’s baseline/normal anatomy. Do this first so when you scan the affected side the abnormal findings stand out more.
For musculoskeletal knee ultrasound, use a linear probe oriented in the sagittal plane starting anterior and midline over the quadriceps and then slide inferiorly over the quadriceps tendon and patella as show above in the clips.
When evaulating for joint effusion in the knee, look for an anechoic collection directly below the quadriceps tendon. If you are seeing anechoic fluid directly above the intact tendon, that is indicative of bursitis or other fluid outside of the joint.

Want to learn more about diagnosing tendon injuries with bedside ultrasound? Check out Dr. Fried’s lecture below

References

Bonnefoy O, Diris B, Moinard M, et al. Acute knee trauma: role of ultrasound. Eur Radiol. 2006;16:2542–8 [Pubmed]
Bianchi S, Zwass A, Abdelwahab IF, Banderali A. Diagnosis of tears of the quadriceps tendon of the knee: value of sonography. AJR. Am. J. Roentgenol. 1994; 162:1137–40 [Pubmed]
Wu, Teresa S., et al. “Bedside Ultrasound Evaluation of Tendon Injuries.” The American Journal of Emergency Medicine, vol. 30, no. 8, Elsevier Inc, 2012, pp. 1617–21, https://doi.org/10.1016/j.ajem.2011.11.004. [Pubmed]