Acknowledgement
The authors thank Dr John W. Read for his contribution to this chapter in giving permission and rights to many of the images, which are taken from the book Atlas of Imaging in Sports Medicine, second edition, edited by J. Anderson and J. W. Read (McGraw-Hill, 2008).
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. This chapter describes each radiographic view, alongside its uses, before progressing into trauma, pathological conditions and finally surgical considerations.
Introduction
Plain radiographs result from the passage of x-rays from a focused source, across the body, to a receiver. The image results from a variation in the radio-opacity of the tissue through which it passes. The origin of radiography and fluoroscopy can be traced to 8 November 1895, when Wilhelm Conrad Röntgen, a German physics professor at Würzburg University, discovered the x-ray. Using a cathode-ray tube, he noted that although x-rays could pass through human soft tissue, they could not pass through bone or metal. Clinical applications rapidly followed, and despite many advances in imaging technology, plain radiography continues to be a major diagnostic and surgical tool today.
Technological improvements with the advent of digital radiography using a panel detector rather than x-ray film have allowed immediate image preview, elimination of film costs, a wider dynamic range and the ability to apply special image processing techniques to enhance image quality while reducing the radiation dose. The ability to manipulate image contrast and size and perform quantitative measurement has been facilitated by the use of Digital Imaging and Communications in Medicine (DICOM) viewers using a picture archiving and communication system (PACS).
Plain radiography remains the primary investigation for acute knee trauma and most chronic knee pathological conditions. To complement routine views of the knee, numerous specialised views and techniques have been developed to give the maximum amount of information for a suspected pathological condition.
Radiographic Views of the Knee
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 mobilise a patient or even move the limb, the views are limited by the patient remaining supine. 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.
General Approaches: Tips for Plain Film Review and Reporting
A systematic approach to evaluation of each x-ray will optimise the information obtained, minimise the chance of missing subtle features and ensure the maximum utility of each view. Starting with the soft tissues acts as a useful guide to the site of injury, ensuring this feature of the x-ray is not forgotten. Swelling will occur adjacent to sites of trauma, whereas joint effusions can be appreciated in the suprapatellar bursa on a lateral projection. Joint spaces can then be measured and their relative appearance, congruity and alignment can be assessed. Osseous assessment should include evaluation of the cortical margins of the visualised bones, assessing the density and margins for fracture and periosteal reaction, osteophyte formation and overlying soft tissue swelling. The trabecular pattern should be assessed for disturbance and the subchondral bone evaluated for sclerosis, thickening or fracture. A fracture can be subtle and seen as a small cortical break or radiolucent area within the trabecular pattern depending on its position relative to the x-ray beam. Evaluation should be on the basis of at least two orthogonal views. Pathological osseous lesions can be lytic, sclerotic, or mixed, and characteristic patterns of bone destruction, expansion or deformity can result from specific pathological processes.
Although each individual may take his or her own specific approach to evaluating a plain knee x-ray, the key is that this approach is systematic, consistent and logical.
AP View
An AP view can be taken with the patient either supine or weightbearing. 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. For a weightbearing AP view, the patient stands with both knees in full extension with the beam horizontal and perpendicular to the centre of the image receptor. The beam should be centred 1.3 cm below the lower pole of the patella. A technical evaluation of the image to ensure correct rotation and alignment should reveal symmetrical femoral condyles, with the head of the fibula positioned 1 cm below the tibial plateau with a 25% overlap with the tibia ( Fig. 2.1 ). Radiographer judgement is required when landmarks are lost as a result of pathological changes such as bone loss or deformity.
This view allows good visualisation of the tibia, femur and tibiofemoral joint. The medial and lateral tibial plateaus, the intercondylar eminence, femoral condyles and intercondylar notch can all be readily identified. The bony contours of the fibula and fibula head can be seen. The superimposed image of the patella is more difficult to visualise but can be seen, and when carefully outlined, the synchondrosis of a bipartite patella and displaced fractures can often be identified.
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. Occasionally an ossicle can be seen here within the popliteus called a cyamella (see Fig. 2.1 ).
The tibiofemoral joint space should be slightly greater than 5 mm, but quantification can be inaccurate and inconsistent. Systems that use knee osteoarthritis (OA) computer-aided diagnosis have been shown to be superior and can be more useful as an objective, accurate and simple evaluation of the severity of OA.
Weightbearing views are mandatory for accurate evaluation of the tibiofemoral joint. This can be taken further with single-leg standing radiographs, which will increase load through the knee and has been reported to give an even better representation of the actual functional joint space. In a study of 100 knees with medial OA there was a mean decrease in joint space from 2.4 mm to 1.8 mm when patients transferred full weight to the pathological knee, with an increase in Kellgren-Lawrence OA grade severity in 32% of cases.
A PA view is comparable to an AP view for most clinical indications; however, 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; this is described in a separate section later.
Lateral View
Lateral knee radiographs allow identification of important bony landmarks: 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 ( Fig. 2.2 ). Specific soft tissue structures that can be identified include the extensor mechanism and patella tendon.
The profile of each femoral condyle should be reviewed, with a ‘true’ lateral image obtained when the two are 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 condylopatella , or 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 true lateral.
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. The universal key point is that the measurement is made with the extensor mechanism, and the patella tendon specifically, under tension. This is achieved by having the knee flexed to a minimum of 30 degrees (see Conditions Affecting the Patellofemoral Joint).
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 ( Fig. 2.3 ).
If there is an effusion present, this can be seen as fluid accumulation below the quadriceps tendon within the suprapatellar pouch. When a fracture is clinically suspected, a cross-table lateral view is obtained to avoid weightbearing or the need to move the patient into a lateral position. An additional benefit of this approach is that a fat-fluid level may be seen in the suprapatellar joint recess. This important sign is formed by the escape of marrow fat through the cortical breach, which floats on the hemarthrosis and indicates that a fracture is present. Two fluid levels can be occasionally seen when a lipohaemarthrosis layers the marrow fat, serum and separated red cell sediment ( Fig. 2.4 ).
If the soft tissues are not adequately shown on routine lateral view, a separate soft tissue–sensitive view can be taken.
Tangential (Axial/Sunrise/Skyline) Patellofemoral View
Tangential views 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.
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 ( Fig. 2.5 ).
The addition of this view significantly improves overall detection of OA, with a population-based study of 777 participants demonstrating an increased sensitivity for the detection of OA from 87.0% to 98.7% when using three views. It is recommend that this is routinely added in standard evaluation for OA.
45-Degree Oblique Views
Additional views can be useful in the trauma setting, improving fracture detection at sites not tangential to the beam in the aforementioned AP and lateral views ( Fig. 2.6 ). 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.
Weightbearing Semiflexed PA (Rosenberg) View
A weightbearing semiflexed PA view is a useful addition for the full evaluation of the tibiofemoral joint, with a technique description with the knee flexed to 45 degrees published by Rosenberg et al. in 1988. This knee joint position can place the most clinically relevant part of the joint under load and in profile. When the weightbearing knee is flexed, the tibiofemoral joint contact point changes, allowing demonstration of joint space loss at the articulation between the posterior weightbearing aspect of the femoral condyles and posterior aspects of the tibial plateaus ( Fig. 2.7 ).
When detecting changes in tibiofemoral joint space narrowing, this view is significantly more reproducible and more likely to demonstrate significant changes in joint space measurements compared with the extended knee positions, increasing the sensitivity of the investigation. Radiographs of the knee in 30 degrees of flexion have consistently showed more advanced bone loss in both the medial ( P = .001) and the lateral ( P = .0001) tibiofemoral compartments compared with those in full extension, with Ahlbäck classifications changing in half of the 50 radiographic assessments. It is worth noting, however, that some apparent changes in joint space narrowing with an AP view may be attributed to inherent differential thickness of articular cartilage and change in areas of contact between femoral and tibial condyles rather than indicating actual cartilage loss. Deep et al. evaluated the joint space height in normal tibiofemoral joints in semiflexion and full extension. They reported that there may be a difference of up to 2 mm in the two views.
Obtaining this view as part of the routine assessment of the tibiofemoral joint is now a widespread practice. In a survey of 990 members of the British Orthopaedic Association, there was a consensus for AP and lateral radiographs, with a further 86% also using a weightbearing PA. This same survey demonstrated a debate in the optimal knee flexion angle. Davies et al. concluded that a standing PA x-ray with the knee flexed to 30 degrees is satisfactory and, importantly, is easy to obtain and well tolerated by patients. Vignon et al. concluded that PA imaging of the knee in the Schuss position (20 to 30 degrees flexion) increases the reproducibility of radiographic joint space width measurements in arthritic knees. A comparison between a Rosenberg view at a full 45 degrees and the 20/10 view where the knee flexion is limited to 20 degrees and the tube is angled to 10 degrees demonstrated no significant difference in the amount of joint space narrowing in 80 participants. Radiography departments need to be aware of these variations to ensure that they use a uniform, reproducible technique.
Intercondylar (Tunnel) View
Similar to the 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 ( Fig. 2.8 ). Disadvantages over a Rosenberg view are the lack of weightbearing information and the greater distance between the knee and the detector; however, it can still 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. In a comparative study of 240 subjects with knee pain (aged 19 to 93) assessed with weightbearing AP and tunnel views, the tunnel view had a greater ability to detect abnormal intercondylar notch and tibial spine osteophytes. In a retrospective review of 100 weightbearing AP and tunnel views, the combination significantly increased detection of joint space narrowing in the lateral ( P < .001) and medial ( P = .006) compartments over the AP view alone. Additionally, subchondral cysts and sclerosis at the lateral plateau and moderately sized osteophytes were all well demonstrated.
Tibial Plateau (Tillman-Moore) View
A variation of the standard AP, the Tillman-Moore view is useful for the assessment of a tibial plateau fracture. A 15-degree caudal tube angulation allows for the native slope of the tibia, making the beam closer to parallel to the sagittal slope and placing it in clean profile. A normal AP view may overestimate fracture depression of the subchondral plate by as much as 50% compared with the plateau view.
Long-Leg Alignment Views
Currently the most accurate way to evaluate physiological coronal plane alignment is with a weightbearing long-leg alignment view that extends from hip to ankle. This technique will accurately characterise varus and valgus malalignment. A number of different techniques can be used for long-leg radiography. Film-screen systems use separate exposures centred over the hip and proximal thighs, knees and ankles, and the resulting images are manually stitched together. Digital imaging can automate this image stitching process ( Fig. 2.9 ).
With the advent of computed tomography (CT) and the ability to gain low-exposure, full-length scanogram images, it was speculated that long-leg radiography would be superseded. However, in a comparison study on patients with and without unilateral knee arthroplasty, statistically significant and clinically relevant differences were found in the alignment measurements in both groups, with a clinically significant mean difference of 1.2 to 3.4 degrees more valgus when using a supine CT scanogram compared with long-leg weightbearing views. This demonstrates the importance of functional weightbearing (which is not available in routine CT) and how it influences leg alignment measurements. Specialised radiography units such as EOS (EOS Imaging, Paris, France) have made it possible to obtain weightbearing biplanar AP and lateral digital images from pelvis to ankles, allowing assessment of both coronal and sagittal plane alignment and creation of three-dimensional (3D) models from which rotational profiles can be generated.
Stress Views
Stress views are radiographs where forces are applied across the joint in order to stress the different ligament complexes and thus determine their integrity.
Instability can be in the coronal, sagittal, or axial plane. There are a number of anatomical structures providing stability in each of these planes and in the majority of cases the clinician will order these views to assess the degree of functional instability under load. A systematic review identified 16 different stress techniques around the knee with excellent reliability reported for the diagnosis of anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) and varus and valgus knee injuries. Twelve studies were identified as comparing stress radiography with alternate diagnostic techniques such as arthrometry, magnetic resonance imaging (MRI) and physical examination. Stress radiography was either as good as or superior to arthrometry results for the diagnosis of AP instability and also correlated well with the pivot shift test. For the assessment of a multiligament injury, stress radiography was more accurate than both examination under anaesthesia and clinical examination, making this an important step in preoperative surgical planning.
Anterior laxity of the knee after ACL injury can be identified using a gravity-assisted lateral radiograph in prone position with the knee flexed at 15 degrees, providing more information than the simple lateral view.
Five different techniques for obtaining evidence of PCL insufficiency have been compared: Telos device, hamstring contraction, kneeling view, gravity view and axial view. The Telos 30-degree and 80-degree views accurately distinguished between the different types of lesions and permitted grading of posterior knee laxity in a cadaveric model. The kneeling technique for PCL stress radiography has been shown to provide a reproducible method of quantifying posterior knee instability and is the most commonly used stress view in routine clinical practice ( Fig. 2.10 ).
With the additional information that can be gained from stress views being somewhat offset by the challenges of obtaining them, novel techniques have been developed to ease evaluation. A stress radiographic device offers the ability to apply a standardised varus or valgus force compared with standard weightbearing views of the knee. The reliability and reproducibility is high, and it has been shown to be suitable for clinical practice and a valuable tool in research.
Varus stress radiographs can give information regarding both ACL and posterolateral corner (PLC) knee injuries. The extent of injury can be correlated with the degree of joint space widening, with a mean of 2.2 mm indicative of a grade 3 LCL tear found on subsequent surgical exploration. A mean of 2.7 mm for isolated complete LCL injuries was found to increase to 4.0 mm when the PLC was involved, with the degree of joint space widening correlating with the extent of injury seen on MRI. In a similar way, the valgus stress view evaluates the medial side of the knee and specifically the MCL ( Fig. 2.11 ).
Indications for Radiographs
Not every patient presenting with knee symptoms requires radiographs, and in those who do it is not practical or sensible to perform all possible views. Despite the potential additional clinical information that multiple knee radiographic views can provide, it should be noted that they carry a significant time and cost burden and expose the patient to radiation. The use of plain radiography should therefore not be mandatory. To determine a safe and effective method to determine who needs which radiographs, several authors have designed, evaluated and implemented an algorithmic approach to the indications for taking a plain radiographic series.
Suggested pathways have been developed for the two main indications for knee radiographs. If arthritis is suspected, functional views that best demonstrate loss of joint space are indicated; weightbearing AP and/or weightbearing semiflexed PA, a lateral view, and a tangential (axial) view are recommended ( Fig. 2.12 ).
A similar algorithmic approach can be adopted for patients after knee trauma ( Fig. 2.13 ). A number of decision rules have been developed for the evaluation of posttraumatic knee pain, with the aim to increase efficiency and reduce healthcare costs without leading to an increase in missed fractures. The best known guidelines are the Pittsburgh Decision Rules and the Ottawa Knee Rules, and in a comparison of the two, the Pittsburgh Decision Rules were found to be more specific than the Ottawa Knee Rules, with equal sensitivity and better interobserver agreement. A study of 106 patients found 25% of radiographs could have been avoided with Ottawa rules applied and 30% with Pittsburgh, demonstrating clinical efficiency without adverse clinical outcome.
When faced with patients presenting nonacutely, adaptation of pathways may need to be made, with emphasis on the likely diagnosis. In a review of 499 patients attending an outpatient sports medicine centre with the chief complaint of knee pain, it was found that the most common working diagnoses were meniscal tears, OA, patellofemoral syndrome, ACL sprain/tear and collateral ligament sprain/tear. Plain knee radiographs had no impact on clinical management in 97.3% of patients younger than 40. They concluded that factors supporting obtaining screening radiographs include age greater than 40, knee pain for greater than 6 months, the presence of medial or diffuse knee pain and the presence of mechanical symptoms.
Trauma
In the setting of knee trauma, plain radiographs are of appropriate sensitivity in identifying fractures. In a high-energy incident, the knee joint can undergo any combination of compressive, bending or torsional forces to cause a variety of different types of fracture pattern. Compressive forces tend to affect the osteochondral tibiofemoral joint surfaces, whereas bending forces can cause fractures in the metaphysis or diaphysis and torsional forces, leading to spiral fractures along the diaphysis long bones.
When describing a fracture around the knee, it is important to be able to describe the overall alignment of the joint and the anatomical location, orientation, fracture pattern and number of fracture fragments and their relative displacement. If a fracture has a displaced intraarticular component, there are major implications for future joint dysfunction, with the amount of chondral separation or degree of depression important considerations.
Specific fracture types often have individual technical classification systems to help descriptions and guide clinical management. An overall standard for the classification of all long bone fractures was developed by Müller et al. and formed the original AO classification.
Osteochondral Fractures
Intraarticular fractures can occur throughout the surfaces of the tibiofemoral and patellofemoral joints after trauma. A common mechanism of injury is excessive axial loading to the tibiofemoral joint which leads to depression injuries of the tibial plateau, though torsional or angular forces initiating extraarticular fractures can also undergo intraarticular extension.
Traditionally, tibial plateau fractures have been radiologically categorised using the Schatzker classification, which uses a single AP radiograph. However, because of the 3D nature of the plateau, it has its limitations. The complexity of different fracture patterns, difficulty in properly appreciating the degree of displacement and depression on plain films, and the significance of these features to long-term joint function have led to further CT imaging being recommended in most cases ( Fig. 2.14 ).
Fractures of the patella may result from either direct or indirect mechanisms. A direct blow to the patella tends to result in comminuted fractures with multiple fragments. Indirect mechanisms are either patella dislocations with shear injuries to the osteochondral surface of the patella or trochlea or overload of the tensile properties of the extensor mechanism. The most common mechanism leading to the latter is after a quadriceps-driven overload with excessive eccentric contraction required to suddenly decelerate the body, resulting in a transverse fracture of the patella. This is usually readily identifiable on the lateral view ( Fig. 2.15 ) as is the lipohaemarthrosis that would raise suspicion of a more subtle osteochondral injury. Failure can also occur at the quadriceps insertion to the proximal pole of the patella, or the patella tendon at its patella or tibial insertions, all of which may occur with a small avulsion fracture as described later.
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