review of knee radiographs
More than one million patients with the complaint of acute knee injury are seen annually in U.S. emergency departments . Even though very few of these patients prove to have fractures, the majority undergo plain radiography. Many individuals will also visit outpatient clinics with knee pain and plain knee radiographs are performed frequently.
This post will evaluate which knee radiographs are available and which may be appropriate for certain conditions. There is debate on whether knee radiographs are needed for certain conditions. Proposals have been made to utilize guidelines similar to the Ottawa ankle rules, but no guidelines have become as popular as the ankle radiograph rules or guidelines. There has been recent evaluation for the Ottawa knee rules .
There are two main radiographic protocols for investigation of the knee. A nonweightbearing series should only be performed if the patient is unable to bear weight or there is potential harm in doing so. Usually after major trauma, where it would be unsafe to mobilize a patient or even move the limb, the views are limited by the patient’s supine position. In these cases a shoot-through anteroposterior (AP) view and cross-table lateral view constitute a standard nonweightbearing trauma series .
For patients who are mobile, a more comprehensive examination can be performed, including a number of different views, with different amounts of knee flexion, with and without weightbearing. Typically this would consist of an AP view, weightbearing semiflexed posteroanterior (PA) (Rosenberg) view, lateral view and a tangential (axial/sunrise/skyline) patellofemoral view .
The most common and standard for knee radiographs is the AP view or anteroposterior view. The knee is normally in extension and internally rotated 3-5 degrees. The beam is aimed 1.5 cm distal to the apex of the patella .
Image 1. An anteroposterior view with the beam perpendicular to the receiver and centred just below distal pole of the patella. Correct rotation is confirmed by the tibia having a 25% overlap with the head of the fibula. Normal anatomical landmarks include the adductor tubercle (adductor magnus attachment) on the superomedial aspect of the medial condyle (small arrowhead) and the groove for the popliteus is seen on the lateral profile of the lateral condyle (large arrowhead), with an adjacent popliteal tendon cyamella. Adopted from .
If tolerated by the patient, a weightbearing view is superior because it allows for not only an evaluation of the static osseous structures of the knee but also an assessment of the tibiofemoral joint space and coronal plane alignment. When a PA projection is combined with varying degrees of knee flexion during weightbearing, it may provide a higher sensitivity and specificity to detect tibiofemoral joint space narrowing than a fully extended AP view. For this reason, it is preferred by many orthopedic providers.The PA/Rosenberg view is with the knees erect and knees flexed to 45 degrees. This is usually done on both knees and the aim is the same as the AP view at 1.5 cm distal to the apex of the patella .
For providers and interpreters, identification of normal bony anatomical landmarks is important. On the AP view the adductor tubercle, the site of the attachment of the adductor magnus tendon, can be seen as a bony protrusion just above the medial border of the medial femoral condyle and a groove in the lateral profile of the lateral femoral condyle is formed by the popliteus sulcus .
The second view usually performed are lateral knee radiographs. They allow identification of the medial and lateral femoral and tibial surfaces, trochlea groove and the anterior borders of the trochlea, the lateral facet and ridge of the patella and the fibula head and neck. Specific soft tissue structures that can be identified include the extensor mechanism and patella tendon. One thing that can be specific of this view is the possible detection of a knee joint effusion, which can be very useful in sports medicine and orthopedics .
For positioning, the patient is in a lateral decubitus position on the ipsilateral side with the knee flexed 20 to 30 degrees. The aim of the beam is 2.5 cm distal to the medial epicondyle with a tilt of 5 to 7 degrees cephalad. Indications include evaluation for fracture, osteoarthritis, patella alta or baja, trochlear dysplasia and to evaluate for a joint effusion.
The profile of each femoral condyle should be superimposed. The medial condyle is rounded with a groove that separates the anterior and middle thirds of the articular surface, whereas the lateral condyle has a flattened groove known as the terminal sulcus . This represents the junction zone on the lateral femoral condyle where the tibiofemoral and patellofemoral radii of curvature meet, and this is seen more readily if the knee is externally rotated. External rotation of the knee also provides a view of the tibiofibular joint. An internally rotated film allows demonstration of the adductor tubercle proximal to the medial condyle. The bony roof of the intercondylar notch, the Blumensaat line, can be seen on a proper lateral view.
The fabella is a normal sesamoid in the tendon of the lateral head of gastrocnemius seen immediately posterior to the lateral femoral condyle with a flattened anterior surface where it articulates with the condyle
Assessment of the morphology and height of the patellofemoral joint can be made. There are several different techniques for measuring patella height, each with their own specific advantages and disadvantages. Much of this was covered in a previous post in regards to patellar instability. The universal key point is that the measurement is made with the extensor mechanism/patellar tendon under tension. This is achieved by having the knee flexed to a minimum of 30 degrees.
The third most common radiograph ordered are tangential views and have many names and variations. They are called sunrise views, skyline views, Merchant views, and axial views. They are designed to provide the best assessment of the patellofemoral articulation and are used most commonly to look for joint space narrowing, bone erosion and osteophyte formation in a pathological arthritic process . It is also used to evaluate the trochlea groove depth, lateral patellar tilt and to evaluate for a patella fracture.
There are a number of techniques for obtaining this view, with the classic descriptions by Laurin and Merchant. A single radiograph taken at 30 degrees of knee flexion, giving the patella a ‘skyline’ or ‘sunrise’ appearance over the ‘horizon’ of the trochlea, provides the optimal flexion for detecting pathological condition at the patellofemoral joint.
Image 4. The tangential (axial/skyline/sunset) view of the patellofemoral joint. The size and shape of the patella, its orientation within the trochlea groove and the shape of the trochlea groove can all be evaluated with relevance to patellofemoral tracking. Other conditions, such as osteochondritis dissecans of the patella and trochlea and subtle patellofemoral fractures, can be seen only on this view. Adopted from .
Though there are many other specialized views, we will discuss the oblique and intercondylar views briefly. The intercondylar view is similar to the PA/Rosenberg view but taken with the patient supine and resting the knee flexed at 45 degree with an AP image taken at 90 degrees to the tibia. The tunnel view projects along the ‘tunnel’ created by the resulting outline of the intercondylar fossa. It can be a valuable addition in the routine assessment of the knee joint in OA, especially when a patient is unable to comfortably hold a weightbearing flexed position. Additionally, subchondral cysts and sclerosis at the lateral plateau and moderately sized osteophytes were all well demonstrated.
Image 5. An intercondylar view. The primary beam is parallel to the tibial articular surface and the posterior area of the femoral articulation is profiled. This view demonstrates changes such as OA, osteochondritis dissecans, osteochondral fractures or subchondral avascular changes that occur in the weightbearing region. The walls and roof of the intercondylar notch are also well displayed. Adopted from .
In the trauma setting, some will also used the 45 degree oblique view. The addition of two AP 45-degree oblique views to a routine AP and lateral was demonstrated to significantly increase fracture detection sensitivity from 79% to 85% on retrospective independent review of 94 trauma cases by three musculoskeletal radiologists.
Image 6. Oblique views can highlight subtle fractures or bone fragments not seen on the standard views. Here a shearing force has produced an osteochondral fracture of the articular surface of the lateral femoral condyle (arrowhead). There has been considerable displacement of the fragment, which now lies in the anterior recess of the knee joint. Adopted from .
It is noted that there are other views for pediatric patients that may be utilized. Other views are also used to evaluate for total knee arthroplasty and to also evaluate for alignment before and after surgery. Stress views were also covered in a previous post. Each provider should evaluate each patient to see if there is a diagnostic benefit from ordering and views knee radiographs.
In summary, plain radiography remains an important aspect in the diagnosis and treatment of knee conditions. A thorough understanding of which views to order and how to interpret them will remain an invaluable and accurate diagnostic tool for sports medicine and orthopedic providers.
– More Knee Pain: https://www.sportsmedreview.com/by-joint/knee/
– Knee Radiographs @ Wiki Sports Medicine: https://wikism.org/Standard_Radiographs_Knee
- Coris, Eric Emmanuel, et al. “Imaging in sports medicine: an overview.” Sports Medicine and Arthroscopy Review 17.1 (2009): 2-12.
- Sims, Jordan I., Minh T. Chau, and Josephine R. Davies. “Diagnostic accuracy of the Ottawa Knee Rule in adult acute knee injuries: a systematic review and meta-analysis.” European Radiology 30 (2020): 4438-4446.
- Orchard, John W., John W. Read, and Ian (Jock) F. Anderson. “2. The use of diagnostic imaging in sports medicine.” Medical journal of Australia 183.9 (2005): 482-486.
- Anderson, Jock F., John W. Read, and Jeff Steinweg. “Atlas of imaging in sports medicine.” (No Title) (2008).