Ultrasound (US) is a valuable tool for imaging musculoskeletal changes in osteoarthritis. It shows early and late findings related to inflammation and structural damage. Sonography is a safe tool, which has recently registered an increasing and widespread use, it being considered as a bedside procedure in the clinical assessment of rheumatic patients. Its applications in osteoarthritis are related to easy accessibility of equipment, low cost, short duration of single examinations and the possibility of performing a multiregional joint evaluation in the same scanning session. Permitting an extensive evaluation of most joint changes present in osteoarthritis, it gives the opportunity to monitor disease progression and perform a follow-up of the response to different local and systemic treatments. US-guided procedures are commonly performed with safety, reliability and optimal patient tolerance. Development in technology and technique with improvement of new research studies will further amplify the diagnostic role of ultrasound in osteoarthritis in the near future.
Musculoskeletal ultrasound (US) represents a novel imaging modality, which has registered an increasing role in rheumatology during the recent years. This is mainly due to tremendous technical advances and progressive, relevant technological developments of US equipment occurring over the past decade . Thank to its capacity to represent different anatomic structures in the finest details with consequent possibility to detect an incredible series of changes in minute particulars, sonography seems to represent one of the most promising imaging techniques in rheumatological clinical practice as well as in research imaging .
However, until recently, the research in the field of US has mainly focussed on the evaluation of inflammatory aspects of rheumatic diseases and on the assessment of tendons and soft tissue involvement . On the contrary, it has been applied to the study of osteoarthritis (OA) less frequently . This issue has been reported in recent reviews that have analysed the published data on the validity of US in assessing inflammatory aspects of rheumatic disorders, underlining the fact that that only a few studies have been undertaken in the field of OA so far . Most of these publications refer to the evaluation of synovitis and power Doppler signal, analysing different components of the Outcome Measures in Rheumatology Clinical Trials (OMERACT) filter in rheumatoid arthritis and, in a minority of cases, in OA . However, very recent years have witnessed an increasing interest of research in this field confirming a more widespread use of this tool for imaging different aspects of rheumatic pathology, including OA .
OA is a very common rheumatic disorder affecting synovial joints. The main pathological findings are represented by dysregulation of local turnover and changes in repairing processes involving different intra-articular tissues and resulting in the typical global joint involvement. Progressive degeneration with loss of cartilage and hypertrophy of the subchondral bone, joint margin and capsule are the most representative findings of the disease . Synovitis, with a typical episodic course, usually occurs and often contributes to the appearance and worsening of symptoms. Some degrees of cartilage deterioration have also been reported in the presence of synovitis, which has characteristically non-destructive and non-aggressive features, even though it may contribute to symptom aggravation. In most cases, frequent findings are represented by synovial proliferation, joint effusion and bursitis. Usually, OA appears and progressively worsens with the advance of old age, even though it may sometimes occur earlier in life. In those cases, disability and work impairment usually appear prematurely, due to joint use-related pain, swelling, stiffness, deformity and reduced joint motion .
OA has been traditionally imaged using conventional radiographs; this has been regarded as the reference technique in OA for a long time . It has, however, clear limitations in imaging and directly visualising hyaline cartilage and other soft tissues, which are frequently involved with disease progression over the years . In addition, plain radiographs have very low sensitivity in demonstrating minimal cartilage involvement in early disease. Common radiological findings are represented by joint space narrowing, osteophytes, sclerosis and deformity . However, on the one hand, those features appear sometimes only in late disease and, on the other hand, may be present also in old and asymptomatic people, thus, generating some doubts about their real role and importance as radiographic characteristics of the disease.
Among other imaging modalities, magnetic resonance imaging (MRI) has been demonstrated to be a sensitive and non-invasive technique for evaluating distinct musculoskeletal diseases and has been used as the reference tool in the assessment of criterion validity of US in OA, demonstrating excellent soft-tissue contrasts . Different studies have demonstrated its accuracy and reliability; however, its high costs and low availability of MRI equipment limit its routine use . Arthroscopy is a powerful tool for evaluating most osteoarthritic changes, particularly for direct visualisation of cartilage surface alterations; but its invasiveness limits its use in daily clinical practice . Scintigraphy has shown its predictive value in the assessment of progressive changes in OA; however, it is scarcely available, in a way being invasive and expensive for routine use .
Why to use sonography in OA?
General indications for using US in OA are reported in Table 1 .
| Indications Detection of joint effusion Detection of synovial thickening and hypertrophy Differentiation between active and inactive synovitis Assessment of cartilage lesions Evaluation of osteophytes Detection of erosions (erosive hand OA) Evaluation of mucous cysts (hand OA) Assessment of periarticular soft tissues abnormalities in OA (bursitis) Execution of US guided procedures Monitoring of disease progression from early to late stages Follow-up of the response to local and systemic therapies |
In OA, sonography has shown its capability in detecting and evaluating a large number of abnormalities involving hyaline cartilage, synovial fluid and synovial membrane, menisci, joint capsule and bursae as well as in the bony cortex . Recent development of high-resolution transducers and more powerful machines has rendered US an emerging and even more widely used tool to image and investigate both early and late changes in OA . Its common use as a complementary method for clinical evaluation creates an interesting and useful link between clinical and imaging assessment: it is actually considered a bedside procedure, which can be easily and quickly performed in the meantime and in the same room of the physical examination, thus reducing patient discomfort . US is a dynamic imaging modality that gives the opportunity to perform a multi-planar assessment of distinct musculoskeletal areas and most peripheral joints involved in the disease . By using sonography, the clinician is able to directly monitor the progression of the pathology and evaluate the response to therapy by repeating the US examination as many times as necessary with time and during the medical examination . It can be successfully used as a guide for fluid aspirations, injections, biopsies and other diagnostic procedures, improving the reliability and safety of those tools and resulting in an excellent patient tolerance and absence of radiation burden . The non-invasiveness and limited cost further improve its routine use in the rheumatological clinical practice during the evaluation of osteoarthritic patients .
Equipment
High-quality machines equipped with high-resolution probes represent the necessary requirements for an optimal visualisation of the different musculoskeletal structures, which are commonly involved in OA. Both joint and periarticular soft tissues may be assessed by using different transducers having peculiar technical characteristics. Then, the choice of the most appropriate probe plays a fundamental role for the most satisfactory visualisation of the structures involved in the area under assessment. Most modern US machines are equipped with broadband multi-frequency transducers but the possibility of using even less sophisticated single-frequency probes should be considered as a possible valid alternative, in accordance with the available equipment. In general, approved rules for musculoskeletal US consider high-frequency transducers (more than 12 MHz) appropriate for the evaluation of small joints and superficial areas while regarding lower-frequency probes (8–12 MHz) as suitable for the assessment of large joints and deep tissues . The choice of the more opportune probe size and shape may have a relevant role in some occasions such as while evaluating patients with functional disability and deformities or when assessing wide areas. In fact, small-size transducers, such as hockey stick ones, appear more suitable for the assessment of small joints while large footprint probes are more appropriate for evaluating large joints and wide districts, offering a more panoramic visualisation of the areas of interest .
Both gray-scale US and Doppler techniques represent fundamental requirements for the correct evaluation of the osteoarthritic joint. Those procedures represent relevant and sequential steps for the complete assessment of the target areas. After B-mode evaluation by using the most appropriate probe frequency and correct machine setting (image size and depth, gain, focus positioning), colour/power Doppler modalities are applied to assess synovial vascularity, which may be increased in case of active inflammation within the joint and other synovial periarticular structures . Appropriate gray-scale and power Doppler settings are reported in Table 2 . Even in OA as well as in inflammatory arthritis, Doppler techniques consent to demonstrate local hyperaemia due to active synovial inflammation . A fundamental aspect when using Doppler modalities consists of the application of the optimal setting, which markedly improves the capacities of US in detecting increased flow in inflammatory pathological conditions . In particular, the use of the correct Doppler frequency (high frequencies for superficial tissues and low frequencies for deep structures), the application of the lowest pulse repetition frequency, the adjustment of the appropriate colour gain, the positioning of the focus by the area of interest and the correct regulation of the colour box size represent all the fundamental aspects to be considered .
| Gray-scale setting | Power Doppler setting |
|---|---|
| Higher probe frequency (≥ 14 MHz) for superficial tissues and small joints | Higher Doppler frequency (7.5–12 MHz) for superficial tissues |
| Lower frequency (≤ 12 MHz) for deep structures and large joints | Lower Doppler frequency (5–7 MHz) for deep structures |
| Maximal image size (depth) to visualise even minimal details | Lowest possible pulse repetition frequency (0.5–1 KHz) |
| Medium image size (depth) to obtain a more panoramic view of the area of interest | Optimal colour gain (just below the level causing noise artefacts) |
| Gain adjustment to low-medium levels to obtain optimal visualization of different tissues, avoiding artefacts | Correct positioning of the foci, at the level of the area of interest |
| Variable number of foci (usually from 1 to 3) according to the extent of the target area | Appropriate modification of the colour box size according to the extent of the studied area, including the upper part of the image to avoid reverberation artefacts |
| Correct positioning of the foci, at the level of the area of interest |
In general, it is actually possible to assert that past technological advances in US equipment, which have led to recent considerable improvement in software and hardware and the production of high-resolution transducers have sensibly amplified the diagnostic abilities of US in rheumatology .
Equipment
High-quality machines equipped with high-resolution probes represent the necessary requirements for an optimal visualisation of the different musculoskeletal structures, which are commonly involved in OA. Both joint and periarticular soft tissues may be assessed by using different transducers having peculiar technical characteristics. Then, the choice of the most appropriate probe plays a fundamental role for the most satisfactory visualisation of the structures involved in the area under assessment. Most modern US machines are equipped with broadband multi-frequency transducers but the possibility of using even less sophisticated single-frequency probes should be considered as a possible valid alternative, in accordance with the available equipment. In general, approved rules for musculoskeletal US consider high-frequency transducers (more than 12 MHz) appropriate for the evaluation of small joints and superficial areas while regarding lower-frequency probes (8–12 MHz) as suitable for the assessment of large joints and deep tissues . The choice of the more opportune probe size and shape may have a relevant role in some occasions such as while evaluating patients with functional disability and deformities or when assessing wide areas. In fact, small-size transducers, such as hockey stick ones, appear more suitable for the assessment of small joints while large footprint probes are more appropriate for evaluating large joints and wide districts, offering a more panoramic visualisation of the areas of interest .
Both gray-scale US and Doppler techniques represent fundamental requirements for the correct evaluation of the osteoarthritic joint. Those procedures represent relevant and sequential steps for the complete assessment of the target areas. After B-mode evaluation by using the most appropriate probe frequency and correct machine setting (image size and depth, gain, focus positioning), colour/power Doppler modalities are applied to assess synovial vascularity, which may be increased in case of active inflammation within the joint and other synovial periarticular structures . Appropriate gray-scale and power Doppler settings are reported in Table 2 . Even in OA as well as in inflammatory arthritis, Doppler techniques consent to demonstrate local hyperaemia due to active synovial inflammation . A fundamental aspect when using Doppler modalities consists of the application of the optimal setting, which markedly improves the capacities of US in detecting increased flow in inflammatory pathological conditions . In particular, the use of the correct Doppler frequency (high frequencies for superficial tissues and low frequencies for deep structures), the application of the lowest pulse repetition frequency, the adjustment of the appropriate colour gain, the positioning of the focus by the area of interest and the correct regulation of the colour box size represent all the fundamental aspects to be considered .
| Gray-scale setting | Power Doppler setting |
|---|---|
| Higher probe frequency (≥ 14 MHz) for superficial tissues and small joints | Higher Doppler frequency (7.5–12 MHz) for superficial tissues |
| Lower frequency (≤ 12 MHz) for deep structures and large joints | Lower Doppler frequency (5–7 MHz) for deep structures |
| Maximal image size (depth) to visualise even minimal details | Lowest possible pulse repetition frequency (0.5–1 KHz) |
| Medium image size (depth) to obtain a more panoramic view of the area of interest | Optimal colour gain (just below the level causing noise artefacts) |
| Gain adjustment to low-medium levels to obtain optimal visualization of different tissues, avoiding artefacts | Correct positioning of the foci, at the level of the area of interest |
| Variable number of foci (usually from 1 to 3) according to the extent of the target area | Appropriate modification of the colour box size according to the extent of the studied area, including the upper part of the image to avoid reverberation artefacts |
| Correct positioning of the foci, at the level of the area of interest |
In general, it is actually possible to assert that past technological advances in US equipment, which have led to recent considerable improvement in software and hardware and the production of high-resolution transducers have sensibly amplified the diagnostic abilities of US in rheumatology .
Technique
Guidelines for US in rheumatology have been published in 2001 and represent the reference point in sonographic assessment of the joints in OA . An adequate knowledge of the scanning technique for the various anatomic areas to be examined is mandatory to correctly evaluate the different joints.
A standard scanning protocol, including multi-planar, dynamic and bilateral assessment, should be always followed to perform a complete study of the various anatomic structures included in the examined joint .
The use of generous amounts of ultrasonographic gel is necessary for improving the visualisation of the structures included in the target area and reducing occurring artefacts.
Correct patient positioning is fundamental for the best visualisation of different joint tissues. In particular, for the imaging of the hyaline cartilage, joints should be kept in well-defined and standardised positions to enable the ultrasonographic beam to penetrate through the most suitable acoustic windows . A tailored protocol for a complete study of the joints in OA includes the assessment of hyaline cartilage, bony cortex, synovial fluid and synovial membrane for detection of cartilaginous lesions, osteophytes, erosions and synovitis . In target areas where bursitis may appear in OA with the presence of Baker’s cysts and bunion bursitis, those particular pathological conditions should be investigated . The documentation of all lesions by two perpendicular planes is mandatory .
US of the normal joint
The normal joint space is imaged by US as an area containing minimal amounts of hypo-anechoic synovial fluid, being limited, on the one hand, by hyperechoic and regular margins that correspond to the bony cortex, and, on the other hand, by a homogeneously echoic band representing the joint capsule . The assessment of synovial fluid is possible by performing multi-planar and dynamic scans and comparing findings with the contralateral side . This useful scanning protocol, which should always be applied, facilitates a correct evaluation of the joint and avoids errors or misinterpretations due to the fact that minimal amounts of fluid are commonly present in the joints of healthy individuals. Standard reference values for measurements of normal joints have been recently reported and are useful for differentiations in clinical practice .
By using specific acoustic windows through which the ultrasonographic beam penetrates and keeping the joints in the correct position, articular cartilage may be imaged in most articular sites. Usually, the proper patient position for the assessment of the cartilage consists of keeping the joint either in maximal flexion (hand and knee) or in extension (elbow, wrist, ankle and foot) or in intra-/extra-rotation (hip and shoulder). Due to its high water content, it typically appears as a homogeneously anechoic band with curvilinear shape . Its anterior margin is typically sharp, regular and continuous and represents the interface between cartilage and soft tissues. It is thinner than the posterior edge, which is more echoic and thicker and corresponds to the interface between cartilage and bony profile . Thanks to its typical ultrasonographic characteristics, which depict the cartilage as a well-defined anechoic structure lacking internal echoes, it is possible to measure its thickness that sensibly differs in the various sites, according to the size of the joint where it is measured; it usually varies between 0.1 and 0.5 mm in the small hand joints to 3 mm in the knee . In some deep joints lacking appropriate acoustic windows, hyaline cartilage cannot be imaged by US, except in limited portions that may not be representative of the entire structure. To optimally visualise the cartilage, a correct, perpendicular insonation of the US beam is mandatory .
In some joints, menisci can be visualised as homogeneously echoic triangular structures located in the inner part of the joint space, between the bones .
US of the osteoarthritic joint
Sonography demonstrates a large set of changes involving the hyaline cartilage from early to late disease ( Fig. 1 a). Initial findings are represented by blurring of the edges, which become irregular and lose their normal sharpness . Firstly, they involve the superficial cartilaginous margin and correspond to the micro-cleft formation due to tissue deterioration . Later, changes in the echotexture appear, with evidence of loss of homogeneity and transparency . With disease progression, focal and asymmetric narrowing is usually present; subsequently, diffuse thinning is charted, up to the complete absence of the cartilaginous layer that corresponds to cartilage breakdown and bony denudation . In the presence of joint effusion, fluid placing over the superficial margin of the cartilage may create pseudo-thickening; this particular finding should be correctly identified to avoid errors and misinterpretations . All cartilaginous changes need to be assessed by using a correct US scanning technique, based on appropriate patient positioning to concentrate the sonographic beam to penetrate the joint, adequate probe location to obtain perpendicular insonation of the US beam and assessment of the contralateral site to perform complete and deep comparisons .
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