Review
2012, Vol. 14, no. 4, 318-325Abstract
Knee ultrasound is a very useful tool for the clinical examination of rheumatic patients. In the last years many papers have been focused on this subject, exhibiting a high degree of improvement since the first musculoskeletal ultrasound paper concerning the knee was published 30 years ago. Apart from the accurate description of anatomic landmarks and structures and also of the basic pathological findings (fluid, synovitis, enthesitis, osteophytes), rheumatologic research has focused on inflammatory findings quantification and their reaction to remissive treatments. The aim of this review is to describe the nor- mal ultrasonographic appearance of knee structures concisely and mainly to analyse the literature about pathological findings in the knee joint.
Keywords: ultrasound, knee, anatomy, pathology
Ultrasound of the knee in rheumatology
Violeta Vlad
1, Annamaria Iagnocco
21Sf. Maria Clinical Hospital, Bucharest, Romania, 2Rheumatology Unit, Dipartimento Medicina Interna e Specialità Mediche, Sapienza Università di Roma, Rome, Italy
Received 20.07.2012 Accepted 24.08.2012 Med Ultrason
2012, Vol. 14, No 4, 318-325
Corresponding author: Annamaria Iagnocco,
Dipartimento Medicina Interna e Specialità Mediche: Reumatologia,
Sapienza Università di Roma,
V.le del Policlinico 155, Rome – 00161, Italy Tel: +39 06 49974634
Fax: +39 06 49974642
Email: [email protected]
Introduction
Although the knee is widely considered to be the most accessible joint for clinical examination, knee ul- trasound (US) was proven to be superior in both accu- racy and reproducibility when compared to the clinical exam [1].
Being a relatively deep joint, the use of low freqwency probes is recommended for knee US. The first published US image of a human joint referred to the knee, specifi- cally to differentiate Baker’s cyst from thrombophlebitis, and was performed in 1972 [2]. Since then, many papers have focused on this subject and discovered elements of anatomy and main pathology of the knee.
The aim of this review is to analyse the recent lit- erature on this subject, focusing first on a brief descrip- tion of the knee anatomy, and then on the most relevant pathologic aspects of rheumatic disease.
US scanning technique
Standardized scanning of the knee is performed with a linear 5-13 MHz transducer, except for the most super- ficial structures that might need a 15 MHz probe [3]. The patient’s position depends on the area depicted: dorsal decubitus with a slight knee flexion for the anterior side and ventral decubitus with extended knee for posterior side.
US anatomy
For didactic reasons, the knee will be divided in four compartments: anterior, medial, lateral, and posterior.
The main anatomic structures from these compartments are summarized in tables I and II together with the trans- ducer position in order to maximize their visualization (table I) and the ultrasonographic correspondent image (table II). US assessments are more helpful if the ex-
aminations are dynamic, covering most of the region of interest, and if depicted structures (muscles, tendons, ligaments, nerves, cartilage, menisci, and cortical bone) are visualized in their entire length. Moreover, since pan- oramic views are available on most of the recently used US machines, it is now possible to depict in one image the whole length of a structure, regardless of its dimen- sions. US representative images of anterior and medial compartments are visualised in figures 1-3.
Table I. Main anatomic structures and related ultrasonographic scans
Com-part-
ment Structure Transducer
position Anterior Quadriceps tendon (Qt) Longitudinal scan
Patellar tendon (Pt) Longitudinal scan Suprapatellar recess (S) Longitudinal scan Prefemoral fatpad (Pf) Longitudinal scan Suprapatellar fatpad ( Sf) Longitudinal scan
Patella (P) Longitudinal scan
Tibial tuberosity (Tt) Longitudinal scan
Cartilage ( C) Transverse scan
Medial Tibiofemoral medial joint Longitudinal scan Medial meniscus (Mm) Longitudinal scan Medial collateral ligament (MCL) Longitudinal scan Semimembranosus insertion(Sm) Transverse scan Pes anserine insertion(Pa) Longitudinal scan Lateral Lateral tibiofemoral joint space Longitudinal scan Lateral meniscus (Lm) Longitudinal scan Lateral collateral ligament(LCL) Longitudinal scan Iliotibial band(Itb) Longitudinal scan Popliteus tendon(PoT) Transverse scan Biceps femoris insertion Longitudinal scan
Peroneal nerve Transverse scan
Posterior Posterior horns of menisci Longitudinal scan Femoral condyles cartilage Longitudinal scan Gastrocnemius muscle(GM) Transverse scan Gastrocnemius tendon(Gt) Transverse scan Semimembranosus tendon(Sm) Transverse scan Popliteal artery and veins(PV) Transverse scan
Fig 1. Ultrasound of the knee in a healthy individual.
Longitudinal anterior scan at the suprapatellar recess level. Normal quadriceps tendon (↑) and normal amount of synovial fluid (*) are visualized. F: femur; P: patella
Fig 2. Ultrasound of the knee in a healthy individ- ual. Transverse suprapatellar scan. Normal hyaline cartilage of the femoral condyles (o) is visualized.
Fig 3. Ultrasound of the knee in a healthy individual.
Longitudinal scan of the medial aspect of the joint.
Normal medial collateral ligament (↑) and normal in- ternal meniscus (*) are visualized. F: femur; T: tibia
US pathology
1. Joint effusion and synovial hypertrophy An amount of fluid of more than 3 ml can be depict- ed in the knee joint with US [3] (fig 4). Three recesses of knee synovia are visualized by US on the anterior part of the knee: suprapatellar, parapatellar lateral, and parapatellar medial. As fluid is displaceable, care should be taken to avoid pressure with the probe. The
usual knee fluid assessment starts with the suprapatel- lar recess, and continues with the lateral and medial re- cesses, as they are the most dependent when the patient is in dorsal decubitus. After effusion detecting, com- pression with the transducer is recommended, in order to differentiate between fluid and synovial hypertrophy (fig 5), which is not displaceable. The development of Power Doppler US (PDUS) technique has lead to im- proving information regarding the local activity, know-
ing that PDUS detects inflamed tissues hyperemia. The knee is a relatively deep joint, so PDUS signal isl not always present in case of synovitis. However, its pres- ence is indicative of active inflammation.
The superiority of US examination over clinical exam in the swollen knee was proven first by Hauzeur et al [1], and a few years later by Kane et al regarding Rheu- matoid Arthritis (RA) fluid in the knee [4]. This applies for suprapatellar recess, parapatellar lateral and medial, Table II. Main anatomic structures depicted by ultrasound and related sonographic pattern
Com-part-
ment Structure Ultrasonographic aspect
Anterior Quadriceps tendon (Qt) Fibrillar hypoechoic structure inserting on superior pole of the patella
Patellar tendon (Pt) Fibrillar hypoechoic structure inserting on inferior pole of the patella and on tibial tuberosity Suprapatellar recess (S) Hypo/anechoic line between prefemoral and suprapatellar fatpad
Prefemoral fatpad (Pf) Hyperechoic structure on top of femoral cortex
Suprapatellar fatpad ( Sf) Hyperechoic structure under quadriceps tendon insertion
Patella (P) Hyperechoic line
Tibial tuberosity (Tt) Hyperechoic line
Cartilage ( C) Anechoic band parallel to trochlea, knee in hyperextension Medial Tibiofemoral medial joint Two bony heads ( hyperechoic) coming together
Medial meniscus (Mm) Hyperechoic triangle inside the joint
Medial collateral ligament (MCL) Fibrillar hyperechoic structure from medial epycondyle to anteromedial tibia Semimembranosus insertion(Sm) Hyperechoic ovoid in a sulcus near medial meniscus
Pes anserine insertion(Pa) Hyperechoic structure inserting together with MCL, on top of it Lateral Lateral tibiofemoral joint space Two bony heads coming together
Lateral meniscus (Lm) Hyperechoic triangle inside the joint
Lateral collateral ligament(LCL) Hypoechoic structure over the joint space with sinuous traject Iliotibial band(Itb) Hyperechoic structure inserting on Gerdy’s tubercle on tibia Popliteus tendon(PoT) Hyperechoic ovoid in a sulcus next to lateral meniscus Biceps femoris insertion Hyperechoic structure inserting on fibular head, on top of LCL Peroneal nerve Hyperechoic ovoid with dots inside near fibular head
Posterior Posterior horns of menisci Hyperechoic triangles on lateral and medial joint spaces Femoral condyles cartilage Anechoic band parallel to bony cortex
Gastrocnemius muscle(GM) Hypoechoic pennate structure
Gastrocnemius tendon(Gt) Hyperechoic triangle on top of gastrocnemius muscle Semimembranosus tendon(Sm) Hyperechoic ovoid next to gastrocnemius
Popliteal artery and veins(PV) Anechoic ovoids with Doppler signal inside, veins compressible
and Baker cysts. On the other hand, clinical exam proved itself to underestimate knee inflammation.
Effusion is defined by OMERACT as an abnormal hypoechoic or anechoic intraarticular material that is compressible and displaceable and does not exhibit Dop- pler signal. Synovial hyperthropy is defined as an ab- normal hypoechoic intra-articular tissue that is nondis- placeable and poorly compressible and may exhibit the Doppler signal [5]. The dimensions can be evaluated by the direct measurement of knee effusion at its greatest point of thickness inside the suprapatellar recess [6], and in that case a >3mm diameter of suprapatellar recess is considered pathologic [7].
The presence of the PDUS signal inside the knee joint can be used to differentiate between hypervascular and fibrous pannus, but it might not always be accurately vis- ualized, mainly because of the deep location of the joint.
Fiocco et al proved that by using contrast enhanced Dop- pler (CED), the method has enhanced reliability, consist- ently reducing artefacts, when comparing to arthroscopy as the gold standard [8]. However, the use of contrast agents can be considered time consuming and therefore cannot be used in daily practice.
Knee synovitis in RA is sometimes refractory (per- sistent for more than 6 months of aggressive local and systemic treatment) or relapsing during the treatment, and that may cause great damage to the joint, leading fre- quently to arthroplasty. This is another important reason for the indication of repeated examinations of the knee joint by US especially in RA. As quantification of dis- ease activity in RA is an important issue nowadays for starting or monitoring biologic treatment, US became very important together with clinical scores for global evaluation. More than 10 different US scores were de- fined in the last years, including different joints, mostly hand joints. The knee was also included in some of these scores [9-11]. The possibility of developing US scores emerged from the description of the semiquantitative scale as a quantifying method with high reproducibility [12]. US scores are based on adding the values of semi- quantitative grades in specific joints (from a scale 0-3).
This modality of quantification being relatively easy, can be applied to large joints as knee as well.
US is frequently used to evaluate the therapeutic in- tervention in the RA knee. Fiocco et al tested the spe- cific action of Etanercept on RA and Psoriatic arthritis patient’s knees, in patients with persistent knee synovitis after methotrexate and other DMARDs. The conclusion was that Etanercept can suppress persistent knee synovi- tis in most patients [13]. Regarding PDUS, the study re- ported the decrease in vascularization first in the superfi- cial layer of knee pannus, and later in the deeper one. The
separation between the two layers of inflammation inside knee synovitis was reanalysed by Kasukawa et al [14].
Superficial flow signals were defined as located in the su- perficial half of the pannus and fluid space whereas deep flow signals were located in the deep half of the pannus.
Joints with superficial pattern had a higher flow signal and a higher synovial effusion grade, whereas joints with deep pattern had a higher grade of synovial proliferation.
In a systematic review focusing on the responsive- ness of knee arthritis to therapy, Keen et al [15] reviewed the studies in which US synovitis of the knee was as- sessed (Gray Scale and PD) before and after a therapeutic intervention. The authors defined internal responsiveness as the ability of an outcome tool to demonstrate temporal changes in response to therapy and external responsive- ness as the extent to which changes in an outcome tool correlate with other referenced measures [15]. US of the knee demonstrated internal responsiveness with regard to synovial thickness, effusion size and popliteal cyst size Fig 4. Ultrasound of the knee. Longitudinal anterior
scan of the suprapatellar recess showing the pres- ence of enthesophytes at the quadriceps tendon’s enthesis (↑) and mild effusion (*). F: femur; P: pa- tella
Fig 5. Ultrasound of the knee. Longitudinal anterior scan of the suprapatellar recess showing the pres- ence of mild effusion (*) and mild synovial hyper- trophy (o). F: femur; P: patella
in 3 studies, while other 4 studies found correlations be- tween US and subjective measurements of health status [15]. Unfortunately, the authors found a great heteroge- neity of working methods in knee US examintaion, lead- ing to the conclusion that US still needs a great work for standardization.
In a pilot study on 104 knees, Vlad et al revealed that effusion dimensions measured in RA knees are strongly correlated to the pain expressed by the patient quantified using the VAS score in the same day as the US examina- tion in a blind manner [16].
Trying to study the prevalence of US pathologic ab- normalities encountered in RA knees, Riente et al discov- ered effusion in 70% of the 200 knees; 82% of these had synovial hypertrophy accompanying the effusion, with only 19% a PD positive signal [17]. In a psoriatic arthri- tis group comprising 186 knees, Delle Sedie et al discov- ered in 84.3% of joints at least one sign of inflammation (effusion and/or synovial hypertrophy) [18].
Regarding knee osteoarthritis, a report of the EULAR on US use in this pathology discovered in 600 patients examined, 53.7% with no signs of inflammation (effu- sion or synovitis), 29.5% with joint effusion alone, 14.2%
with both synovitis and effusion, and 2.7% with synovitis alone [19,20]. Inflammation found by US correlated well to advanced radiographic disease on the Kellgren-Law- rence scale, but not to pain intensity during recent physi- cal activity. The explanation could be linked to the pain source in osteoarthritis, which may be especially linked to bone oedema, seen only on MRI. However, the pres- ence of inflammatory signs on US suggests an inflamma- tory pathway for osteoarthritis, too.
Proofs regarding inflammation in osteoarthritis are increasing; in a recent study, Clockaerts et al showed cy- tokine production by infrapatellar fatpad [21]. It is still unclear how inflammation can affect the natural evolu- tion of osteoarthritis. Chao et al [22] examined clinically and by US 79 patients with symptomatic knee osteoar- thritis, and then infiltrated their knees with corticoster- oids/placebo after randomization. The conclusion was that corticosteroids have a short time effect on knee pain compared to placebo. They also defined the concept of US inflammatory signs in osteoarthritis by the presence of intraarticular synovial hypertrophy with or without effusion. „Inflammatory patients” were called patients with knee effusion and they were proven to experience a shorter benefit from steroids injection than the patients with a dry knee joint. In conclusion, lack of synovitis on US may be a good prognostic sign, showing that symp- toms might be easily controlled by intraarticular steroids.
This was the first study to prove the ability of US exam to predict response to a treatment.
2. Baker’s cyst and other knee bursitis
Popliteal cysts were first described by Adams in 1840, but Baker in 1877 established the causality relation with joint effusions and offered the definition [23]. Gastrocne- mius and semimebranosus bursitis is called Baker cyst.
It communicates with the joint through the bursal’s neck – the joint fluid accumulates in the bursa in knee flexion and cannot go back due to the one-way valve mechanism [3]. Sometimes, Baker cysts may be giant and ruptured into the calf, causing inflammation resembling throm- bophlebitis. The differential diagnosis between these two entities can be easily done by US examination. Any dis- ease causing fluid accumulation inside the knee can lead to a Baker cyst formation.
The general prevalence of Baker cysts in population is evaluated with many variations. In a study compar- ing MRI with US in the detection of Baker’s cysts, US detected 100% of the MR detected cysts [24]. The inci- dence of Baker’s cysts in a group of 99 consecutive pa- tients with RA was 33.8% [ 25]. In a group of 100 patients programmed to knee arthroscopy for various reasons the incidence of Baker’s cyst was 20% [26]; in a study by Ward et al, out of 36 evaluated pathological knees, 58%
had Baker’s cysts [24]. Although the incidence is not clearly established yet, the popliteal fossa must be exam- ined every time when knee US is performed, especially when fluid is found in the anterior recesses.
The content of a cyst may be variable depending on the base pathology- cysts may contain fluid (anechoic im- age), synovial hypertrophy (hypoechoic images inside, sometimes with cauliflower aspect resembling knee syn- ovitis), calcifications, osteochondral fragments. If Baker cysts develop slowly, the patient may be asymptomatic.
Baker cysts were proven to regress following remis- sive treatment in RA together with knee synovitis [24- 26].Other knee bursitis are prepatellar and infrapatellar bursitis. Prepatellar bursitis is superficially located, and it is mainly posttraumatic. Infrapatellar bursitis is super- ficial (in continuation with prepatellar bursitis, over the last third of the patellar tendon), or deep (between the patellar tendon and Hoffa’s fatpad). Deep infrapatellar bursitis can accompany tendon and entheseal pathology of that area. An important aspect to mention is that while no fluid is normally found in prepatellar bursitis, a small amount in deep infrapatellar bursa is common in healthy subjects [3].
3. Tendon and entheseal pathology
The most encountered knee tendon pathology in rheumatology is the inflammation of their bony insertion known as enthesitis, widely considered as the hallmark
for spondylarthropathies (SpA). It is also known that en- thesitis may affect primarily lower limbs, knee together with the heel being the most frequently affected joints.
US definition of enthesopathy includes an abnormally hypoechoic and/or thickened tendon at its bony attach- ment, occasionally with calcifications, seen in two per- pendicular planes that may exhibit a Doppler signal, and also bony changes – entesophytes, erosions or irregulari- ties [5] (fig 4). The inflammatory enthesopathy is known as enthesitis and it has been widely studied with US, mostly since biologic treatment emerged in SpA and had to be objectively monitored.
The quadriceps tendon insertion and the patellar ten- don/ligament both insertions, as well as suprapatellar and infrapatellar bursitis were included in the GUESS score, one of the first available US scores to quantify enthesitis [27]. The study found numerous subclinical enthesitis in SpA and proved US to be more sensitive and more spe- cific than clinical examination. GUESS score, consisting only in Gray Scale examinations, was proven reproduc- ible for treatment evaluation of SpA patients, but values obtained did not correlate to systemic parameters of dis- ease activity, like in RA.
Adding PDUS to knee enthesitis evaluation, D’Agostino et al found abnormal vascularization in 81%
out of 164 consecutive SpA patients; in the knee, quadri- ceps tendon was less affected than patellar tendon [29].
An important conclusion of the study was that US shows enthesitis signs ( PDUS) mostly in SpA patients with pe- ripheral form of disease – psoriatic arthritis and reactive arthritis [28,29].
Regarding enthesitis response to treatment, Naredo et al investigated a group of 327 patients with SpA before and after anti-TNF alfa treatment. They concluded that enthesis morphologic abnormalities, PD signal and bur- sitis are active inflammatory lesions responsive to bio- logic therapy, and calcifications and bone lesions are not responsive, being them as structural damage lesions [30].
The same three enthesis points were included from the knee. Together with enthesitis, the presence of synovial effusion was also considered as inflammatory, being re- versible to treatment.
Enthesitis of the quadriceps tendon was found also in RA patients [31], less common than in psoriatic patients, and always accompanied by effusion. Also, inflamma- tory signs previously described appear more often in RA whereas in psoriatic arthritis patients exhibit more struc- tural damage.
4. Bone and cartilage abnormalities
Knee osteoarthritis is one of the most encountered diseases among general population, with a women pre-
dilection. The most specific US sign for osteoarthritis are the osteophytes, defined as step-up bony prominence at the end of the normal bony contour or at the margin of the joint seen in two perpendicular planes, with or with- out acoustic shadow [27] (fig 6). In the knee joint, the place to look for osteophytes is the tibiofemoral joint. In advanced disease, osteophytes can permanently compro- mise the joint structures (capsule, menisci or ligaments), leading to secondary pathology (ligament rupture, menis- cal clefts or meniscal cysts). The indication for US is in the early stage of the disease, mainly for diagnosis.
Cartilage at the knee joint has a thickness of about 3mm [27] (fig 2). Signs of deterioration appear on US as blurring, loss of sharp contour, and margin irregularities.
Usually in osteoarthritis the cartilage is asymmetrically thinned, being more symmetrically affected in RA. Knee cartilage is best visualized at the patellar level, with hy- perflexed knee, and also from the posterior view, along femoral condyles.
US was recently proven to be very accurate in car- tilage depiction of knee condylar cartilage in cadaver specimen [32].
5. Knee joint injection
Intraarticular joint injections of the knee are per- formed frequently in rheumatologic daily practice. The placement of the needle must be strictly inside the joint, no matter if the maneuver is for aspirating fluid or for therapeutic agents injection (corticosteroids or viscosup- plementation substances). Only 56-85% of intraarticular (IA) injections are correctly placed [33,34], without using an imaging modality to guide the needle. Other studies demonstrate a higher rate for non guided IA knee injec- tion – up to 93% [35-37]. Sonographic needle guidance has been proven to enhance clinical outcomes (pain con- trol) and cost-effectiveness of the procedure [36]. Balint et al reported a 4/10 rate of success in a blinded aspira- tion of the knee, compared to 18/19 when using US [37].
Fig 6. Ultrasound of the knee. Longitudinal scan of the medial aspect of the joint showing the presence of an osteophyte (↑). Normal internal meniscus (*) is also visualized. F: femur; T: tibia
2. McDonald DG, Leopold GR. Ultrasound B-scanning in the differentiation of Baker’s cyst and thrombophlebitis. Br J Radiol 1972;45:729–732.
3. Bianchi S, Martinolli C. Ultrasound of the musculoskeletal system. Springer Berlin Heidelberg New York 2007:637- 4. Kane D, Balint PV, Sturrock RD. Ultrasonography is supe-638.
rior to clinical examination in the detection and localization of knee joint effusion in rhaumatoid arthritis. J Rheumatol 2003;30:966-971.
5. Wakefield RJ, Balint PV, Szkudlarek M, et al; OMERACT 7 Special Interest Group. Musculoskeletal ultrasound in- cluding definitions for ultrasonographic pathology. J Rheu- matol 2005;32:2485-2487.
6. Court-Payen M. Sonography of the knee: intra-articular pa- thology. J Clin Ultrasound 2004;32:481-490.
7. Naredo E, Bonilla G, Gamero F, Uson J, Carmona L, Laffon A. Assessment of inflammatory activity in rheumatoid ar- thritis: a comparative study of clinical evaluation with grey scale and power Doppler ultrasonography. Ann Rheum Dis 2005;64:375-381.
8. Fiocco U, Ferro F, Cozzi L, et al. Contrast medium in power Doppler ultrasound for assessment of synovial vascularity:
comparison with arthroscopy. J Rheumatol 2003;30:2170- 2176.
9. Naredo E, Rodriguez M, Campos C, et al. Validity, re- producibility and responsiveness of a twelve-joint simpli- fied power doppler ultrasonographic assessment of joint inflammation in rheumatoid arthritis. Arthritis Rheum 2008;59:515-522.
10. Iagnocco A, Fillippucci E, Perella C, et al. Clinical and ultrasonographic monitoring response to adali- mumab tratment in rheumatoid arthritis. J Rheumatol 2008;35:35-40.
11. Hammer HB, Sveinsson M, Kongtorp AK, Kvien TK. A 78 joints ultrasonographic assessment is associated with clinical assessments and is highly responsive to improve- ment in a longitudinal study of patients with rheumatoid arthritis starting adalimumab treatment. Ann Rheum Dis 2010;69:1349-1351.
12. Szkudlarek M, Court-Payen M, Jacobsen S, Klarlund M, Thomsen HS, Østergaard M. Interobserver agreement in ultrasonography of the finger and toe joints in rheumatoid arthritis. Arthritis Rheum 2003;48:955-962.
13. Fiocco U, Ferro F, Vezzu M, et al. Rheumatoid and psori- atic knee synovitis: clinical, grey scale and power doppler ultrasound assessment of the response to etanercept. Ann Rheum Dis 2005;64:899-905.
14. Kasukawa R, Shio K, Kanno Y, et al. Doppler ultrasono- graphic characteristics of superficial and deep- flow signals in the knee joint pannus of patients with rheumatoid arthri- tis. Ann Rheum Dis 2007;66:707-708.
15. Keen HI, Mease PJ, Bingham CO 3rd, Giles JT, Kaeley G, Conaghan PG. Systematic review of MRI, ultrasound and scintigraphy as outcome measures for structural pathology in interventional therapeutic studies of knee arthritis: focus on responsiveness. J Rheumatol 2011;38:142-154.
In a systematic review, Hermans et al [38] included nine studies on the matter of needle placement inside knee joint.
The most accurate (and the most used) approach of the knee is the superolateral one, with the knee in extension, resulting in a 91% pooled accuracy. Other approaches are lateral mid- patellar, anterolateral and anteromedial approach, which showed a lower accuracy. A medial patellar approach was used by Sang Hee et al to guide intraarticular hyaluronan in a dry knee- the guided injections had an accuracy of 95,6%
compared to 77.3% in blinded injections [39].
The main agents used for knee IA injections are cor- ticosteroids, both in refractory synovitis in inflammatory conditions (RA, SpA, PsA) and for symptomatic relief in OA. IA corticosteroids are recommended by ACR guide- lines of knee RA/ OA treatment since 2002/2000 [40,41].
US guided knee injection is able to improve the ac- curacy from 82% in a blinded manner to 91%, even if the rheumatologist was a young doctor with basic US training in the Cunnington et al study [42]. The minimal difference between the two types of procedure was only obtained for the knee, suggesting that for the other joints the need for US guidance is higher.
The use of viscosupplementation for knee OA is also comprised in ACR guidelines for treatment [40]. Hy- aluronan treatment is mostly indicated in painful knee OA with no or mild effusion [43]. US highly improves IA needle placement in such conditions, and its importance is augmented by the fact that the periarticular hyaluronic acid injection showed no improvement in knee pain.
Conclusion
US of the knee is a very frequently encountered proce- dure, both for clinical practice and for research purposes.
Knee US, should be performed in all patients with RA as well as in OA, US evaluation of the knee enhances the possibility of detecting and extracting fluid. Recognizing the presence of knee effusion is not always easy at clinical examination. With US, even a beginner can obtain very good results in detecting knee effusion. In a recent study, 21 medical students were able to detect knee injected fluid on cadavers, after a few hours of teaching [44]. This is a strong reason for introducing US in rheumatology teaching process at all levels.
Conflict of interest: none
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