Abstract
In this article I will consider the basic principles of requesting, acquiring, interpreting and reporting plain radiographs of joints, including assessment of the distribution of joint abnormalities, and specific pathological changes that may occur in bone, cartilage and soft tissues. I will then move on to a more specific discussion of the major arthropathies and the role of radiographs in the diagnosis and assessment in each condition as well as reviewing the combined abnormalities that may be visible on radiographs and how these relate to underlying pathological processes.
Introduction
Conventional radiography (plain radiographs or X-rays) has been used by physicians to provide additional information in patients with musculoskeletal symptoms for over a century. Despite advances in other imaging technologies such as ultrasonography (US), magnetic resonance imaging (MRI), computed tomography (CT) and nuclear scintigraphy, including single photon emission computed tomography (SPECT) and positron emission tomography (PET), plain radiographs remain an important and widely used first-line investigation by health professionals in primary and secondary care.
This chapter will discuss the application of plain radiographs in the investigation and management of patients with arthritis.
Advantages and disadvantages of plain radiography
It is worth first considering some of the potential advantages and disadvantages of conventional radiography.
One of the main advantages is that it is probably the most readily available and least expensive imaging modality. This means that a wide range of health professionals are able to readily access these data and almost all local patient populations are within a short distance of being able to have an X-ray performed. All medical professionals have received at least some basic training in X-ray interpretation and so this immediate access to information can generally be used by the requesting physician to aid diagnosis and management in a time efficient fashion.
A wide range of pathological changes in bones, joints and cartilage can be evaluated with conventional radiography. However, it is also important to consider that many soft tissue structures, including the synovium, are not well visualised with plain radiographs and so X-rays cannot necessarily be relied upon to provide the most sensitive information for diagnostic or therapeutic decision making in patients with inflammatory arthritis. Other modalities such as US or MRI have a number of advantages in this context.
In addition, it is important to recognise that a radiograph effectively provides a two-dimensional picture obtained from one slice through a three-dimensional structure. Therefore, the plane of assessment is crucial in order to provide as much information as possible, as there may be a danger of missing abnormalities if they are not caught in the path of the X-ray beam. For example, it is for this reason that plain radiography may provide less sensitive information compared with topographical techniques in the assessment of bone erosions in rheumatoid arthritis (RA).
Performing and receiving a radiographic assessment is a well-regulated process and it is generally considered safe, although it does involve ionising radiation and the amount of radiation exposure varies dependent on the structure being visualised. For example an X-ray of the pelvis involves considerably more radiation exposure than an examination of a single limb joint, and the number of views and images obtained are proportional to the amount of radiation received.
Data are generally regarded as reproducible as there are widely recognised standards for the acquisition of images and increasingly there are also validated conventions and scoring systems for interpreting and reporting various pathological processes, all of which are aimed at improving the reliability and reproducibility of radiographic acquisition and interpretation. This is important as radiographs are often used for repeated serial evaluation and follow-up, for example, in the assessment of progression of joint damage in RA. However, local variations remain relatively common.
Advantages and disadvantages of plain radiography
It is worth first considering some of the potential advantages and disadvantages of conventional radiography.
One of the main advantages is that it is probably the most readily available and least expensive imaging modality. This means that a wide range of health professionals are able to readily access these data and almost all local patient populations are within a short distance of being able to have an X-ray performed. All medical professionals have received at least some basic training in X-ray interpretation and so this immediate access to information can generally be used by the requesting physician to aid diagnosis and management in a time efficient fashion.
A wide range of pathological changes in bones, joints and cartilage can be evaluated with conventional radiography. However, it is also important to consider that many soft tissue structures, including the synovium, are not well visualised with plain radiographs and so X-rays cannot necessarily be relied upon to provide the most sensitive information for diagnostic or therapeutic decision making in patients with inflammatory arthritis. Other modalities such as US or MRI have a number of advantages in this context.
In addition, it is important to recognise that a radiograph effectively provides a two-dimensional picture obtained from one slice through a three-dimensional structure. Therefore, the plane of assessment is crucial in order to provide as much information as possible, as there may be a danger of missing abnormalities if they are not caught in the path of the X-ray beam. For example, it is for this reason that plain radiography may provide less sensitive information compared with topographical techniques in the assessment of bone erosions in rheumatoid arthritis (RA).
Performing and receiving a radiographic assessment is a well-regulated process and it is generally considered safe, although it does involve ionising radiation and the amount of radiation exposure varies dependent on the structure being visualised. For example an X-ray of the pelvis involves considerably more radiation exposure than an examination of a single limb joint, and the number of views and images obtained are proportional to the amount of radiation received.
Data are generally regarded as reproducible as there are widely recognised standards for the acquisition of images and increasingly there are also validated conventions and scoring systems for interpreting and reporting various pathological processes, all of which are aimed at improving the reliability and reproducibility of radiographic acquisition and interpretation. This is important as radiographs are often used for repeated serial evaluation and follow-up, for example, in the assessment of progression of joint damage in RA. However, local variations remain relatively common.
Basic principles of requesting plain radiographs of joints
Which structures?
Before requesting the test, it is important to consider the capabilities of radiographs, that is, what structures and pathology may be visualised using this modality and whether an alternative imaging technique may provide more useful information. For example, radiographs can be used to evaluate changes in joint and cartilage surfaces, bone pathology, joint space narrowing, fracture, subluxation and dislocation. However, soft tissue structures such as tendons, ligaments and synovium are not well visualised and alternative imaging may provide more sensitive and specific information. Often X-rays are performed as a standard baseline imaging investigation but discussion with a musculoskeletal radiologist may help to select the most appropriate subsequent test if additional information is still required. Correlation with clinical findings and information from other investigation and imaging results is always important and radiographs should not be interpreted in isolation.
Which joints?
In most cases, the choice of site of radiograph will be strongly influenced by a prior clinical assessment, and an X-ray requested of the patient’s symptomatic joint or joints. Exceptions to this may include a patient with RA where X-rays of both hands and feet may be used to evaluate extent of disease or structural damage which may be repeated serially over time, or a skeletal survey in an oncology setting, looking for evidence of sub-clinical malignant disease, such as myeloma. Nevertheless, a fundamental principle of requesting any imaging investigation is that any findings need to be interpreted in clinical context, and it is therefore imperative that any X-ray request is accompanied by comprehensive clinical information.
Which views?
Consideration should also be applied as to which views or plains are requested of the joint. Usually it is appropriate to obtain two views at 90° apart, typically in antero-posterior (AP) and lateral or oblique plains. This is particularly important in the setting of trauma but may be less crucial in the assessment of arthropathy. There are a number of established techniques and protocols which have been developed to provide optimal imaging of most joints and for particular indications. However, it is important to remember that these only represent a two-dimensional ‘slice’ through a joint and so are likely to be less sensitive than a three-dimensional or topographical assessment. In addition, specialist views have been developed for particular indications. For example, a ‘ball-catchers’ view can be helpful to visualise more of the cartilage surfaces in the finger joints and a ‘skyline view’ may be useful for more accurate assessment of the patella-femoral joint.
Compare sides and review previous radiographs
It can often be helpful to compare findings in a symptomatic joint with those in a contralateral asymptomatic joint. For example in a patient with osteoarthritis (OA), an AP X-ray of the pelvis allows some comparison between both hip joints or in a patient with RA, an X-ray of both hands allows comparison between finger, hand and wrist joints. If there is an old X-ray of the area of interest, reviewing and comparing these images can also help to improve diagnostic certainty and evaluate any progression over time.
Patient position
The position of the patient can also be important. For example, requesting weight-bearing views can be much more informative in evaluating cartilage loss in the knees of a patient with OA or in providing additional information concerning biomechanical changes in the feet.
Correlation with clinical and other imaging findings
This is an important principle, as mentioned earlier. Any radiographic findings need to be interpreted in clinical context. Additionally further imaging may be required to provide more specific information. Discussion with a radiologist can often be helpful and a regular musculoskeletal radiology meeting can be a useful forum for specialist clinicians and radiologists to formally discuss and review more challenging cases.
Important safety considerations
It is important to remember that performing a plain radiograph exposes a patient to ionising radiation. This is particularly important in women of child-bearing age. Radiographs of deeper structures, such as the lumbar spine or pelvis, subject the patient to greater exposure than more superficial structures. It is important to be able to justify any radiation exposure on the basis of potential risk and benefit. The Department of Health Policy , (IR (ME)R 2000) covers this aspect in detail ( www.dh.gov/health/2012ionising-radiation-ragulations/ ).
Basic principles of examining and reporting plain radiographs of joints
Most clinicians will have a straightforward strategy for reviewing and interpreting skeletal X-rays, and this section aims to reprise some basic principles supplemented with the author’s experience and present some ideas that may help further develop a systematic approach.
Clearly a knowledge and understanding of anatomy and pathological effects on bone, cartilage, joints and soft tissues is crucial. Also it is important to be able to describe any obvious abnormality using simple descriptive language, words and phrases, for example, name the bone and describe the location and site of any abnormality, for example, right or left; proximal, middle or distal; head, neck or shaft and cortex or medulla.
One should start by making sure that the radiographic and patient demographic data correspond and the correct image is being viewed. Is it the correct date; is it the right or left side of the body; has the region of interest been included and is positioning optimal and is there more than one view to assess?
It may be that there is an obvious abnormality visible on the radiograph, in which case I would probably move straight on to describing and interpreting this. If not (and it is probably good practice to still do this, in case there is an additional, perhaps less obvious abnormality present), a coordinated systematic approach of formally looking at specific structures can be useful. This mental checklist should ensure that the likelihood of missing any abnormalities is reduced.
- 1.
Consider bony alignment – are there any changes that may suggest fracture or dislocation?
- 2.
Consider bone cortices – follow the outline of each bone as any breach in the cortex may indicate a fracture or arthropathy.
- 3.
Consider bone texture – normally one expects to see a trabecular pattern within the substance of bones but any distortion of this may indicate pathology.
- 4.
Joint space – a careful look at the joint space may demonstrate changes such as joint space narrowing due to cartilage loss or calcification of the cartilage (chondrocalcinosis), or new bone formation, for example, osteophytes.
- 5.
Soft tissues – changes in the soft tissue densities adjacent to a joint may indicate a joint effusion.
It is important to also review adjacent non-musculoskeletal structures looking for any pathological changes.
Remember to look at all views, compare both sides and adjacent joints, review any previous images, consider clinical findings and correlate with other imaging and test results.
Ask for a second opinion if any uncertainty, particularly a musculoskeletal radiologist, or consider presenting the case at a multidisciplinary radiology conference.
A summary of the important factors to consider when interpreting a musculoskeletal radiograph is provided in Table 1 .
|
| Ask for a second opinion – discuss case and clinical context with a specialist musculoskeletal radiologist |
Radiographic abnormalities in bone, cartilage and soft tissues
In each of the major arthropathies, different pathological processes can cause changes to bone, cartilage and soft tissue structures which may be demonstrable on radiographs. Whilst some of these pathological changes may be specific, one should recognise an overlap across different types of arthritis.
In bone, changes include osteopenia, erosion, osteophyte and new bone formation, subchondral sclerosis and cyst development. There may be cartilage loss and calcification (chondrocalcinosis). Soft tissue swelling, calcification and deformity may be visible. Many of these changes may be in evidence on radiographs of affected joints in the different forms of arthritis ( Table 2 ).
| RA | OA | PsA | AS | Gout | CPPD | |
|---|---|---|---|---|---|---|
| Bone | Periarticular osteopenia Erosions adjacent to the joint | Subchondral cysts Subchondral sclerosis Osteophytes | New bone proliferation Erosion Periostitis Osteolysis | New bone formation Erosion | Erosion away from the joint | Subchondral cysts Subchondral sclerosis Osteophytes |
| Cartilage | Joint space loss (usually uniform) | Joint space loss (usually asymmetric) | Preserved unless severe | Chondrocalcinosis Joint space loss | ||
| Soft tissue | Soft tissue swelling | Enthesitis Dactylitis | Enthesitis | Soft tissue swelling Tophi | Calcification | |
| Joint | Ankylosis Deformity | Bony remodelling Deformity | Ankylosis Deformity | Ankylosis Deformity | Deformity | Soft tissue swelling |
Assessment of distribution of joint abnormalities
When assessing a patient with arthropathy, it is often useful to consider not only which joints are affected but also the pattern of distribution of joint involvement ( Table 3 ). This can provide helpful information not only in the initial stages of diagnosis but also when thinking about prioritising which joints to assess during subsequent follow-up. These typical distributions of joint abnormalities are also important to take into account when choosing which joints to evaluate with plain radiographs and also when interpreting these images.
| Type of arthropathy | Distribution of pathology |
|---|---|
| Rheumatoid arthritis | Typically an inflammatory polyarthritis affecting the small joints of the hands, feet and wrists in a symmetrical distribution. |
| Osteoarthritis | A mono, oligo or polyarthritis typically affecting DIP and PIP joints, first CMC joint, axial skeleton and large weight bearing joints e.g. hips and knees. |
| Psoriatic arthritis | Usually one of five sub-types:
|
| Ankylosing spondylitis | Usually involves the axial skeleton (sacroiliac joints and spine) (axial spondylitis). May have asymmetrical involvement of medium and large joints particularly shoulders and hips (peripheral spondylitis). Inflammation at sites of bone insertion of tendons and ligaments is common (enthesitis) e.g. iliac crests, gluteal and tibial tuborosities and heels. |
| Reactive arthritis | Typically asymmetrical mono or oligoarthritis of weight bearing lower limb joints, but other joints may be involved and may present as a polyarthritis. |
| Gout | Usually a monoarthritis, most commonly first MTP joint, then mid-foot, ankle and knee; lower limb > upper limb. |
| Acute calcium pyrophosphate arthritis “pseudogout” | Usually a monoarthritis, most commonly knee then wrist, shoulder, ankle and elbow joints. |
| OA with CPPD (previously often called chronic pyrophosphate arthropathy) | Usually an oligoarthritis, most commonly knee then wrist, shoulder, elbow, hip, midtarsal and MCP joints. |
| Septic arthritis | Usually a monoarthritis most commonly affecting lower limb large/medium joints e.g. knee or less commonly hip although upper limb joints may be affected. May occasionally present as a polyarthritis if immunocompromised or underlying arthropathy such as RA. |
Combined radiographic abnormalities that may occur with each major arthropathy
Rheumatoid arthritis
Plain radiography remains the gold standard for the assessment of structural joint damage in RA even though this may not necessarily be the most sensitive imaging investigation in this setting. Nevertheless, radiographs remain very important in the evaluation of these patients and have been historically used as a primary outcome measure in RA. Indeed, much of the evidence we commonly apply in the diagnosis and management of our RA patients continues to be based on longitudinal radiographic information .
When a patient with inflammatory arthritis is initially assessed in the rheumatology outpatient clinic, as well as a clinical, biochemical and immunological investigations, it is usual to perform plain radiographs of the hands and feet. This is supported by most national rheumatology guidelines . Even if symptoms are predominantly in the hands, it is important not to neglect the feet and perform a radiographic assessment of the hands, wrists and feet, and any other affected joints at the physician’s discretion, as erosive changes may be present in the feet and not the hands in early disease . This may improve the diagnostic utility of radiographs in the baseline evaluation of patients with early disease. Large joint radiographs of RA patients have been less frequently studied; however, studies including large joints of patients with established disease indicate good correlation between radiographic erosions of large and small joints . This information can be useful to not only help establish a diagnosis but also determine the extent of disease and help ascertain risk of progression and prognosis. For example, the presence of radiographic erosions often confers more severe disease and worse prognosis and a number of studies have demonstrated a correlation between joint damage seen on radiographs and disability in long-standing RA, although this link is perhaps less strong in patients with early disease .
It is important to note that X-rays are often normal at onset and during the early stages of the condition and the more characteristic radiographic features may only develop in more established disease. Studies have estimated that less than half of new RA patients attending their first rheumatology clinic visit may have visible radiographic erosions and in the remainder of patients X-rays are often normal . Moreover, a third of new RA patients may not develop radiographic erosions within the first 2 years of disease onset . It is therefore important to also evaluate other non-radiographic markers of disease severity when planning individual patient management. This has led to some authors questioning the utility of radiographs in the assessment of early RA, as bony changes visible on X-ray often lag behind clinically detectable joint and soft tissue inflammation. In addition, studies comparing radiographs with other imaging techniques such as US, MRI and CT have all confirmed the reduced sensitivity of X-rays at detecting early erosive changes .
Other studies suggest that the majority of patients will have developed some radiographic changes within 2 years of diagnosis and most national recommendations support the value of a baseline radiographic assessment although caution should be applied when interpreting the significance of normal X-rays in this situation.
Radiographs may therefore perhaps be more usefully applied in the serial assessment of joints affected by RA, looking for evidence of progression of joint damage over time. Nevertheless, most specialists would suggest that radiographs of hands and feet should be performed at baseline and every 6–12 months in early RA and perhaps every 1–2 years in more established disease.
This is reflected in recent modifications to the RA classification criteria. Previous criteria, which included radiographic changes , were mainly applied to clinical and epidemiological research studies but have only been shown to be sensitive and specific for the diagnosis of active, established RA. Their sensitivity may be low in early RA, particularly <12 weeks duration , as the classical features of RA such as radiographic erosions and rheumatoid nodules, which are included in these criteria, are often absent at disease onset. In response to this, a collaborative initiative undertaken by American College of Rheumatology (ACR) and European League Against Rheumatism (EULAR) was launched which has resulted in the publication of the 2010 Rheumatoid Arthritis Classification , which focusses on features at an earlier stage of disease, rather than defining the disease by late stage features, and does not include radiographic changes.
A range of pathological abnormalities may be visualised radiographically in RA. These include soft tissue swelling, periarticular (juxta-articular) osteopenia, erosions, joint space narrowing (usually uniform) and deformity such as subluxation and dislocation ( Figs. 1 and 2 ). It is important to remember that radiographs only provide limited information on soft tissue structures and US or MRI are the imaging modalities of choice to directly visualise these structures and provide important objective information on pathological changes such as synovitis and tenosynovitis. For this purpose, radiographic assessment relies on the interpretation of soft tissue shadows corresponding to, for example, elevation of the fat pad in the elbow joint or suprapatellar pouch of the knee, which may suggest a joint effusion or synovitis, and is therefore not necessarily sensitive or specific for assessing inflammation.
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