Most knee structures can be accurately targeted using ultrasound guidance. These structures are usually superficial, and the overlying soft tissues are mobile and compressible, facilitating excellent visualization with a high-frequency linear array transducer. The circumferential accessibility to the knee affords flexibility and often multiple procedural approach options. In most cases, an in-plane approach is easily achieved. Studies of ultrasonography-guided knee procedures have consistently shown high accuracy, and its use is particularly beneficial for obese patients, diagnostic injection specificity, safety, and precise targeting of pathology. More studies are needed to assess the clinical efficacy and cost-effectiveness of ultrasonography-guided knee procedures.
Key points
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The anatomy of the knee is particularly amenable to ultrasound imaging, and therefore most knee structures can be accurately targeted using ultrasound guidance.
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Studies of ultrasound-guided knee procedures have consistently shown high accuracy.
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Using ultrasound guidance for knee procedures is particularly useful for obese patients, diagnostic injection specificity, safety around neurovascular structures, and precise targeting of pathology.
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More studies are needed to assess the clinical efficacy and cost-effectiveness of various ultrasound-guided knee procedures.
Introduction
The anatomy of the knee is particularly amenable to ultrasound (US) imaging, and therefore most knee structures can be accurately targeted using US guidance. In most individuals, these structures are superficial, and the overlying soft tissues are mobile and compressible, facilitating excellent visualization with a high-frequency linear array transducer. The circumferential accessibility to the knee affords flexibility and often multiple procedural approach options. In most cases, an in-plane approach (ie, parallel to the transducer) can be easily achieved, improving needle visualization and injection safety.
Introduction
The anatomy of the knee is particularly amenable to ultrasound (US) imaging, and therefore most knee structures can be accurately targeted using US guidance. In most individuals, these structures are superficial, and the overlying soft tissues are mobile and compressible, facilitating excellent visualization with a high-frequency linear array transducer. The circumferential accessibility to the knee affords flexibility and often multiple procedural approach options. In most cases, an in-plane approach (ie, parallel to the transducer) can be easily achieved, improving needle visualization and injection safety.
Ultrasound-guided knee joint injections
Indications
General indications for US-guided (USG) knee joint injections include failure of a prior landmark-guided (LG) knee joint injection, complex postoperative or posttraumatic anatomy, obese body habitus, need for diagnostic specificity, and orthobiologic injections (eg, hyaluronic acid, platelet-rich plasma, bone marrow aspirate concentrate), in which intra-articular placement is essential for the treatment mechanism.
Cost-Effectiveness
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Although studies have generally shown superior accuracy of USG knee joint injections, there remains some debate regarding cost-effectiveness.
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Sibbitt and colleagues showed that, relative to LG knee joint corticosteroid injections, USG injections led to 13% reduction ($17) in cost per patient per year and 58% ($224) reduction in cost per responder per year.
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Because injection accuracy is likely more critical for orthobiologic versus corticosteroid injection efficacy, USG may be more cost-effective when delivering orthobiologic injections, although this has not been specifically investigated.
Knee joint injection: suprapatellar approach
Indications
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Easy access to the joint via the suprapatellar recess.
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Avoid contact with cartilage and other intra-articular structures.
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Preferred approach for visualizing and aspirating effusion.
Accuracy
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Bum and colleagues showed greater accuracy with the suprapatellar USG approach (96.0%) than LG injections (83.7%).
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Curtiss and colleagues showed 100% accuracy with the suprapatellar USG approach across experience levels, whereas LG injections showed less accuracy and more variability (55%–100%).
Safety
There are no published complications with this approach.
Clinical Efficacy
Relative to LG injections, the suprapatellar USG approach resulted in 48% reduction in procedural pain, 42% reduction in pain at outcome, and 36% increase in therapeutic duration.
Positioning
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Patient supine with knee partially flexed ( Fig. 1 A ).
Fig. 1
( A ) Setup for a right knee sonographically guided, lateral-to-medial, suprapatellar joint recess injection. Proximal is left. ( B ) Sonographic longitudinal view of an effusion in the suprapatellar joint recess between the suprapatellar fat pad and quadriceps tendon. Note that this is a different orientation from that depicted in Fig. 1 A. ( C ) Sonographic transverse view of a lateral-to-medial injection, in plane with the transducer into the suprapatellar joint recess between the suprapatellar fat pad and prefemoral fat pad. Medial is left. ( D ) Injectate distending the suprapatellar joint recess. Medial is left. ANT, anterior, FEM, femur, LG, longitudinal; MED, medial; PAT, patella; PF, prefemoral fat pad, QT, quadriceps tendon, SP, suprapatellar fat pad, TR, transverse.
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Transducer in anatomic transverse plane over suprapatellar recess.
Preprocedural Scan
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Visualize suprapatellar recess deep to the quadriceps fat pad/tendon and superficial to the prefemoral fat pad.
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If effusion present, this makes for an effective target ( Fig. 1 B). Small effusions can be enhanced with knee flexion. Check dependent portions of joint recess.
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Note depth of target for planning skin entry point.
Needle Approach
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In plane relative to transducer ( Fig. 1 C, D).
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Advance lateral to medial or medial to lateral.
Pearls
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To confirm plane of suprapatellar recess, use external pressure to mobilize prefemoral fat pad and visualize differential motion relative to quadriceps tendon.
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Do not confuse hypoechoic fat pad or synovitis with an effusion; the latter is typically compressible and displaceable.
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During injection, confirm injectate flow distally into patellofemoral joint by visualizing suprapatellar recess in anatomic sagittal plane.
Knee joint injection: patellofemoral approach
Indications
No effusion present and/or suprapatellar recess is difficult to visualize.
Accuracy
Ninety-five percent using lateral patellofemoral approach (out of plane, or perpendicular, relative to transducer), but there have been no direct comparisons with other USG approaches or LG injections.
Safety
No published complications with this approach.
Clinical Efficacy
No published studies have evaluated the clinical efficacy of this approach.
Positioning
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Patient supine with knee extended ( Fig. 2 A ).
Fig. 2
( A ) Setup for a right knee sonographically guided lateral patellofemoral joint injection, out of plane with the transducer. Proximal is left. ( B ) Sonographic oblique transverse view of the lateral patellofemoral joint space deep to the lateral patellofemoral retinaculum between the patella and femur showing an injection out of plane with the transducer. Arrows identify the needle tip in short axis adjacent to the femoral articular cartilage (asterisks). ( C ) Sonographic coronal view of lateral patellofemoral joint space injection in plane with the transducer. Right is lateral/distal. Note this is a different orientation than that depicted in Fig. 2 A. COR, coronal; LAT, lateral; LAT RET, lateral patellofemoral retinaculum.
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Transducer in anatomic axial plane over the anterolateral knee with visualization of the lateral patella and lateral femoral epicondyle.
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Positioning is similar on the medial side for a medial patellofemoral approach.
Preprocedural Scan
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Lateral (or medial) patellofemoral recess adjacent to patellofemoral joint.
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Check dependent regions of recess for small effusion.
Needle Approach
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Out of plane: advance proximal to distal or distal to proximal ( Fig. 2 B).
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Use walk-down technique until needle descends into patellofemoral joint.
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In plane: advance lateral to medial or medial to lateral ( Fig. 2 C).
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Can inject into patellofemoral recess or directly into patellofemoral joint.
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Pearls
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If no effusion to target in joint recess, it is essential to visualize needle tip pass deep to patellofemoral retinaculum and periretinacular tissue before injecting.
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During injection, confirm intra-articular flow by visualizing injectate flow into suprapatellar recess.
Knee joint injection: posteromedial approach
Indications
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No effusion present and/or suprapatellar recess is difficult to visualize.
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Patellofemoral osteoarthritis limits patellofemoral approach.
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Performing Baker cyst aspiration in conjunction with knee joint injection (allows for single sterile preparation without changing patient position).
Accuracy
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One study showed 100% accuracy of this approach, but made no direct comparisons with other USG approaches or LG injections.
Safety
No complications with this approach in the series mentioned earlier (n = 67).
Clinical Efficacy
No published studies have evaluated the clinical efficacy of this approach.
Positioning
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Patient prone with knee extended ( Fig. 3 A ).
Fig. 3
( A ) Setup for a right knee sonographically guided posteromedial knee joint injection, in plane with the transducer. Proximal is left, posterior is top. ( B ) Sonographic transverse view of a posteromedial-to-lateral knee joint injection, in plane with the transducer, and deep to semimembranosus (SM) and gently contacting the posterior medial femoral condyle articular cartilage ( asterisk ). POSTMED, posteromedial.
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Transducer in anatomic axial plane over the posteromedial femoral condyle.
Preprocedural Scan
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Identify chondral surface of posteromedial femoral condyle.
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Identify and avoid popliteal neurovascular bundle and saphenous nerve.
Needle Approach
In plane: advance medial to lateral between semimembranosus (SM) and gracilis just superficial to articular surface of medial femoral condyle ( Fig. 3 B).
Pearls
During injection, confirm intra-articular flow along superficial surface of hypoechoic medial femoral condyle cartilage.
Baker cyst aspiration/fenestration
Indications
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Complete aspiration of cyst, including multilocular cysts.
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Avoid injury to popliteal neurovascular bundle.
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Targeting walls and/or stalk of cyst with fenestration.
Accuracy
No published studies have evaluated USG relative to LG popliteal cyst aspirations.
Safety
Smith and colleagues reported no complications in a series of 47 USG Baker cyst aspiration, fenestration, and cortisone injection procedures.
Clinical Efficacy
Smith and colleagues reported significant clinical improvement at a mean 90.2 weeks of follow-up after USG Baker cyst aspiration, fenestration, and cortisone injection procedures.
Positioning
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Patient prone with knee extended.
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Transducer in anatomic transverse ( Fig. 4 A ) or sagittal ( Fig. 4 B) plane over dependent part of cyst.
Fig. 4
( A ) Setup for a right knee sonographically guided Baker cyst aspiration, in plane with the transducer and transverse to the leg. Proximal is bottom left, posterior is top. ( B ) Setup for a left knee sonographically guided Baker cyst injection, in plane with the transducer and longitudinal to the leg. Distal is left, posterior is top. ( C ) Sonographic transverse view of a Baker cyst (outlined by + and ×) between the SM tendon and medial head of gastrocnemius (MHG). ( D ) Sonographic transverse view of a lateral-to-medial Baker cyst aspiration, in plane with the transducer, between the SM tendon (SM) and MHG. ( E ) Sonographic longitudinal view of a distal-to-proximal Baker cyst aspiration, in plane with the transducer and superficial to the MHG. DIST, distal; MED, medial; POST, posterior.
Preprocedural Scan
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Identify and avoid popliteal neurovascular bundle.
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Confirm Baker cyst (vs tumor, aneurysm, or ganglion cyst) ( Fig. 4 C).
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Anechoic, compressible cyst with stalk emanating from posteromedial knee joint between medial gastrocnemius and SM.
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Do not mistake anisotropic tendon for cyst/fluid. Toggle the transducer to confirm, particularly for the SM tendon, which may appear round and hypoechoic in this region because of anisotropy.
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Cyst may be simple, multiloculated, or ruptured. May have hyperechoic synovial debris.
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Assess with Doppler. May see cyst wall hyperemia, but be wary of extensive hyperemia or flow within cyst, which may suggest a soft tissue mass.
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If cyst is in atypical location or has atypical soft tissue features, further investigation (eg, MRI with and without intravenous contrast) may be warranted to evaluate for the presence of a soft tissue tumor before consideration of a procedure.
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Needle Approach
In plane: advance medial to lateral (or lateral to medial) ( Fig. 4 D) or distal to proximal ( Fig. 4 E), depending on shape and orientation of cyst.
Pearls
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Fenestration more accurately performed if done before aspiration.
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Aspirate with needle tip in most dependent portion of cyst (in prone position).
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Consider addressing source of cyst fluid with intra-articular injection (eg, corticosteroid).
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