Intern Ultrasound of the Month: Complex Patellar Fracture Diagnosed with POCUS
The Case
50yo male with a history of diabetes, hypertension, and osteoarthritis presented to the ED for 1 day of right knee pain after fall. The day prior to presentation he had taken 40 units of insulin for a high blood sugar read, and when he stood up, he felt dizzy and fell to the floor. He landed on his right knee and had pain there ever since. The pain did not improve which prompted him to come to the ED.
His physical exam was notable for a swollen knee which was diffusely tender to palpation, particularly over the patella, without gross deformity. Active range of motion was diminished, partly due to pain. Passive ROM was intact but also elicited pain. There was no appreciable fluid shift on palpation of the joint or obvious joint instability. His sensation and motor function were otherwise intact and no other injuries were noted.
A point-of-care ultrasound of the knee was performed to evaluate for abnormalities (particularly quadriceps and patellar tendon injuries), while x-ray was pending.
POCUS findings
Quadriceps tendon appears intact
Patellar tendon with questionable slight defect at proximal aspect, otherwise appears intact. Overlying inflammatory changes are present
Suprapatellar effusion
Bony defect of the patella with echogenic fluid collection, concerning for fracture with hematoma
Step-off in proximal tibia suggestive of fracture
Case Continued: X-rays confirmed the diagnosis of a displaced patellar fracture and likely hematoma (as well as an old proximal tibia fracture). Orthopedic surgery was consulted and surgical repair was recommended. The patient opted for outpatient surgery the following week.
Patellar Fracture
Epidemiology
Common, accounts for approximately 1% of skeletal injuries [1]
Incidence of 10-16/100000/year [2]
Men have the highest incidence between the ages of 10-20, women between 60-80 years old [2]
Mechanism/Injury pattern
Direct force
Blow to the anterior knee
Ex: fall or a dashboard injury
Comminuted fracture pattern
>50% nondisplaced, however can have significant chondral damage as the extensor mechanism is typically intact [1]
Indirect force
Rapid knee flexion against contracted quadriceps
Force from the tendon exceeds strength of the bone
Ex: lands from moderate height, sudden stop after sprint
Transverse or avulsion fracture pattern [1]
Diagnosis
The mainstay of diagnosis is still plain film x-rays of the joint in both AP and lateral views. CT and MRI are occasionally indicated [1]
Ultrasound may be a useful adjunct in settings where x-ray is not immediately available or if there is a delay to obtaining x-ray [3].
Has the benefit that it can also evaluate injuries to the tendons and ligaments that plain radiographs cannot and without the added radiation or cost of MRI and CT [4,5]
Management
Nonsurgical
Indication: Intact extensor mechanism and minimal step-off (<2-3mm) or displacement (<1-4mm) [1]
Long leg splint and non-weight bearing for few days followed by partial weight bearing [1]
Early weight bearing with hinged knee brace, and PT starting at 1-2 wks [1]
Surgical
Indication: incompetent extensor mechanism, or large step-off/displacement, or intra-articular loose bodies. [1]
Point-of-Care Ultrasound Exam of the Knee
Indication
History of pain, swelling, reported trauma to the area
Physical exam revealing weakness of the extension of the knee, swelling and pain to the area
Key clinical questions include:
Quadriceps and patellar tendon injury?
Joint effusion?
Evidence of fracture?
Technique [6-7]
Probe: high frequency linear transducer
Provides good resolution and ideal for superficial structures
Position the patient with knee flexed slightly, approximately 20-30 degrees. It helps to place blankets/towels under the knee for support.
Focusing here on the anterior aspect of the knee, assess the areas proximal to, directly over, & distal to the patella in both longitudinal & transverse planes
Evaluate for the following:
— Tendon injury
Quadriceps tendon
With the transducer placed superior to the patella, identify the tendon first in its longitudinal axis (it appears as a clearly defined, echogenic fibrillary structure overlying the femur). Visualize its insertion at the superior aspect of the patella.
Evaluate in its transverse plane
Patellar tendon
Sliding the probe caudally, identify the patellar tendon as it attaches at the inferior aspect of the patella. Evaluate the entirety of the tendon until it attaches to the proximal tibia. Also assess in transverse view
Tendon tears appear as a disruption of its normal fibrillary structure; in addition to a defect in the tendon, there’s often associated hypoechoic areas representing hematoma
Complete tears appear as full thickness disruptions & there may be retraction of the intact portion of the tendon
— Joint effusion
Commonly seen in the suprapatellar recess deep to the quadriceps tendon and separating the suprapatellar and prefemoral fat pads
Flexing the knee can help augment visualization of this
A heterogenous fluid collection with an echogenic fat-fluid layer is indicative of a lipohemoarthrosis which occurs in the setting of a fracture [5]. See image below.
— Fracture/bone abnormalities
Normal bone appears as a bright hyperechoic line with hypoechoic shadowing posteriorly
A fracture will show a discontinuation of the hyperechoic bone surface
Small fractures may just appear as a step off
Larger fractures of the patella may show a displacement and an isoechoic/hypoechoic hematoma between the segments of the bone, as seen in this case.
Pearls and pitfalls
Be sure to scan in both longitudinal and transverse planes. Remember one view is no view!
Evaluate not only for the presence of an effusion but the consistency of the effusion.
2-3% of the population may have a biphasic patella which can easily be confused for a fracture
Of these people, 50% have it bilaterally
If there is suspicion, compare to the contralateral side
Evidence
US can be used to diagnose fractures particularly in bones that are difficult to identify on x-ray, such as sternum, ribs, metacarpals, and scaphoid [4]
One study compared US to XR in the setting of concern for intra-articular knee fracture (not specific to the patella) [5]
48 participants were evaluated, and fracture was confirmed on CT scan
They evaluated for the presence of lipohemoarthrosis as that is only present in the setting of fracture
US was found to be more sensitive, with similar specificity (94%, 100%) when compared to traditional XR (55%, 100%).
Although MRI is the traditional choice for ligamentous and tendon injuries in the knee, US is still highly sensitive [4]
Take Home Points
Although XR and CT/MRI are the mainstays of diagnosis for a patellar fracture and other acute knee injuries, POCUS can be just as, if not more, effective. POCUS is more rapid, convenient, and less costly than CT/MRI and has the advantage over XR of evaluating for tendon (& other soft tissue) injuries.
A fracture can be visualized as a discontinuation of the hyperechoic cortex
Associated hematoma can be seen between the fractured segments of the patella which is concerning for displacement of the fracture
Heterogenous joint effusion is highly sensitive and specific for a fracture, even if a fracture is not clearly visualized.
POST BY: DR. SOFIA CHINCHILLA (PGY1)
FACUTY EDITING BY: DR. LAUREN MCCAFFERTY (ULTRASOUND FACULTY)
References
1. Melvin JS, Mehta S. Patellar fractures in adults. J Am Acad Orthop Surg. 2011; 19:198.
2. Larsen P, Court-Brown CM, Vedel JO, Vistrup S, Elsoe R. Incidence and Epidemiology of Patellar Fractures. Orthopedics. 2016;39(6):e1154-e1158.
3. Wilkerson RG, Stone MB. Ultrasound identification of patella fracture. Wilderness Environ Med. 2009; 20:92.
4. Carter K, Nesper A, Gharahbaghian L, Perera P. Ultrasound Detection of Patellar Fracture and Evaluation of the Knee Extensor Mechanism in the Emergency Department. West J Emerg Med. 2016;17(6):814-816.
5. Bonnefoy O, Diris B, Moinard M, et al. Acute knee trauma: role of ultrasound. Eur Radiol 2006; 16:2542.
6. Dewitz A (2014). Musculoskeletal, Soft Tissue, and Miscellaneous Applications. In OJ Ma, JR Mateer, RF Reardon, SA Joing (eds), Ma and Mateer’s Emergency Ultrasound (3rd ed). McGraw-Hill Education. pp 503-568.
7. Alves TI, Girish G, Brigido MK, Jacobson JA. US of the Knee: Scanning Techniques, Pitfalls, and Pathologic Conditions. Radiographics. 2016; 36(6): 1759-1775.