Clinical Approach to Knee Radiographs

Early in the morning, you begin your day in your local emergency department. After getting yourself situated, a slow trickle of patients begin to appear on the board. It appears to be a normal morning, all except for the fact that five patients appear, one after the other, who have the same chief complaint: “Knee pain”. It is a good thing you brushed up on reading knee x-rays recently!

Patient 1

A 30 year old male walks into your emergency department. He reports he has had left knee pain since twisting it while in his aerobics class this morning. He has been ambulatory since the incident. He can fully range the knee, and has isolated tenderness to palpation only over the medial knee. There is no palpable effusion, swelling, or ecchymosis. 

+ Should We Get an X-ray in this Patient?

No. But why?

There are two primary clinical decision tools by which we can use to aid in the decision on whether to get knee x-rays on patients.

The Ottawa Knee Rule [1] states radiographs are needed if patients:

  • Are 55 years of age or older,
  • Have palpable tenderness over the head of the fibula,
  • Have isolated patellar tenderness,
  • Cannot flex the knee to 90°,
  • Cannot bear weight immediately following the injury, or
  • Cannot walk in the emergency room (after taking 4 steps).

The Pittsburgh Decision Rule states radiographs are needed if patients:
Have blunt trauma or fall as a mechanism of injury, and:

  • Are < 12 or >50 years of age
  • Are unable to take four weight bearing steps in the emergency department

The Ottawa Knee Rule is more widely used than the Pittsburgh Decision Rule. The Ottawa Knee Rule has been found to have a higher sensitivity than the Pittsburgh Decision rule, with sensitivities of 100% [2] and 92% respectively[3]. However, from a resource utilization perspective, while the Ottawa Rule has a relative reduction in radiographs of 28%[1], the Pittsburgh Decision Rule decreased Emergency Department knee radiographs by 52%(3).

Both of these decision rules would suggest knee x-rays are unneeded in this patient. Additionally, he has no immediate effusion, warmth, ecchymosis, or swelling of the knee post-injury. Lastly, blunt trauma rather than our patient’s twisting injury causes up to 86% of all knee fractures, for which a radiograph would be more useful. [4]

Patient 2

Your 2nd patient is brought in by EMS. He is a 45 year old male who was involved in a motor vehicle collision. He was a restrained driver traveling 35 miles per hour who rear-ended the car in front of him. He has significant tenderness to palpation diffusely across the knee. His knee has palpable effusion. Additionally, he was non-ambulatory at the scene. You obtain the left knee x-ray which shows the following.

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+ What's the Diagnosis

This patient has a comminuted medial tibial plateau fracture. Tibial plateau fractures, while the second most common fracture of the knee in adults, is missed in 16% of emergency department patients[5].

Tibial plateau fractures are frequently caused by the femoral condyle impacting the tibial plateau. The lateral tibial plateau has more articular surface, so is more prone to fracture. Therefore, patient’s with mechanisms such as MVC or falls, where there is significant axial loading, should be suspected of having tibial plateau fractures.

This AP knee shows cortical disruption along the medial tibial plateau. On the AP view, it is important to follow the surface of both medial and lateral tibial plateaus to look for any cortical disruption, as is shown in this image. Additionally, one can draw a line straight down the lateral margin of the femoral condyle. If the most lateral surface of the fibula is > 5mm outside this line, suspect tibial plateau fracture.

One should also inspect for tibial plateau fractures with two or more views to ensure the fracture line not missed because it is parallel to the radiograph. On the lateral view, pictured above, lipohemarthrosis can be another clue to look for fracture. This is when an intra articular fracture can cause fat and blood to escape the marrow cavity. Fat, which is less dense, will cause a fluid level and float above the blood. Just superior to the patella one can see a darker hypo-dense fluid stripe superficially and a brighter more hyper-dense stripe deeper.

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Patient 3

Your 3rd patient is wheeled in to the emergency department. He is a 39 year old male. He reports severe left knee pain after being struck in the knee by a baseball bat. He too was non-ambulatory after the incident. X-rays were obtained. 

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+ What's the Diagnosis

The patient has a right non-displaced vertical patella fracture, best seen on the AP view of the knee. Patella fracture is one of the most common types of knee fractures. However, it is missed up up to 6% of patients on X-ray [5]. The AP view is needed to rule out vertical patellar fractures, where-as the lateral view will often better visualize horizontal patellar fractures.

Of note, this particular fracture does look similar to a normal anatomic variant of the patella called a bipartite patella. This is present in 2% of the population, where the patella is composed of two bones [6]. However, a bipartite patella typically has a more rounded appearance with the disruption in the superolateral portion of the patella, rather than a vertical split down the center of the patella as pictured here.

On the lateral view, the fracture is not well visualized. However, it is important to note the presence of a fabella, the small area of ossification just posterior to the femoral condyle. This is a normal sesamoid bone within the tendon of the gastrocnemius, frequently confused for an avulsion fracture. It too has a more rounded, regular appearance, which can distinguish it from an avulsion fracture.

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Patient 4

Your 4th patient is a 60 year old male who was struck by a motor vehicle when crossing the street. He was struck at a low speed, and complains of isolated knee pain. He has not been ambulatory since injury. Additionally, he complains of numbness distal to the knee. X-rays were obtained.

See answer for image credit

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+ What's the Diagnosis

The patient has a transverse proximal fibula fracture, best visualized on the AP view of the knee. These injuries are frequently nonoperative, but they are are significant because of their association with neuromuscular injury. The common peroneal nerve transverses across the proximal fibula. If directly injured, symptoms such as numbness in the lateral leg and on top of the foot, as well as foot drop can be expected. Additionally, the fibula is in close proximity to the popliteal artery, so a thorough vascular exam of the distal extremity is indicated.

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Patient 5

The patient is a 22 year old male who presents after feeling a “pop” after landing on a flexed knee when playing basketball. Exam of the affected knee reveals diffuse swelling. Additionally, he has an inability to lift his knee of the bed. X-rays were obtained. 

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+ What's the Diagnosis

This patient has patella alta, or high riding patella, visualized on the lateral view of this knee x-ray. This is evidence of patellar tendon rupture.

While this is a quite dramatic example, patellar tendon rupture can be much more subtle. In order to evaluate for it on the lateral knee x-ray in suspected patients, one should measure the distance between the tibial tuberosity and the inferior pole of the patella. If this is greater than the length of the patella +/- 20%, measured superior to inferior, one should suspected patellar tendon rupture.

For some more reading on patella alta and patellar tendon disruption, check out our recent Annals of B Pod Quick Hit Case.

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  1. Stiell IG, Greenberg GH, Wells GA, et al. Derivation of a decision rule for the use of radiography in acute knee injuries. Ann Emerg Med. 1995;26(4):405-413.
  2. Jenny JY, Boeri C, El Amrani H, et al. Should plain X-rays be routinely performed after blunt knee trauma. A prospective analysis. J Trauma. 2005;58(6):1179-1182.
  3. Cheung TC, Tank Y, Breederveld RS, Tuinebreijer WE, de Lange-de Klerk ES, Derksen RJ. Diagnostic accuracy and reproducibility of the Ottawa Knee Rule vs the Pittsburgh Decision Rule. Am J Emerg Med. 2013;31(4):641-645.
  4. Stiell IG, Wells GA, McDowell I, et al. Use of radiography in acute knee injuries: need for clinical decision rules. Acad Emerg Med. 1995;2(11):966-973.
  5. Freed HA, Shields NN. Most frequently overlooked radiographically apparent fractures in a teaching hospital emergency department. Ann Emerg Med. 1984; 13(10) 900-904.
  6. Atesok K, Doral MN, Lowe J et-al. Symptomatic bipartite patella: treatment alternatives. J Am Acad Orthop Surg. 2008;16 (8): 455-61

Written by: Issac Shaw, MD

Posted and Edited by: Jeffery Hill, MD MEd