Ultrasound CPPD: OMERACT 0–3 Semi-Quantitative Scale

Why the OMERACT Scale is Needed for CPPD

Calcium pyrophosphate dihydrate (CPPD) arthropathy is often described on ultrasound as chondrocalcinosis: hyperechoic deposits in hyaline cartilage, fibrocartilage, and other assessable joint structures. Before a unified scale, conclusions were often qualitative: “single inclusions,” “pronounced chondrocalcinosis,” “multiple deposits.” Such formulations are poorly comparable between physicians and between visits.

OMERACT 2023 Consensus proposed a semi-quantitative 0–3 system to describe the extent of crystalline deposits at two levels: individual joint and overall patient. The practical purpose of the scale is not to replace the clinical diagnosis but to provide a reproducible ultrasound language for protocol, dynamics, and research.

What Counts as a CPPD Deposit on Ultrasound

For the scale, it is crucial not just to see a bright echo signal but to localize it in the correct anatomical structure. CPPD deposits are described as hyperechoic formations of various shapes located within fibrocartilage, hyaline cartilage, tendinous, or other evaluated structures. They should be interpreted considering the movement of the structure during dynamic scanning and anatomical context.

An acoustic shadow, the size of an individual inclusion, and subjective “brightness” are not independent steps of the 0–3 scale. The key parameter is the extent: how many structures in the joint contain deposits and how many joints are involved in the patient.

OMERACT 0–3 Semi-Quantitative Scale

The scale uses the same counting principle: at the joint level, the number of involved structures is considered, and at the patient level, the number of involved joints. Therefore, it is advisable to explicitly indicate in the protocol which joints and structures were actually examined.

It is important not to mix the two levels. For example, grade 3 for one joint means widespread intra-articular distribution of deposits but not necessarily widespread disease at the patient level. Conversely, a patient-level grade 3 may consist of several joints with minimal local deposit volume.

Application Algorithm in the Ultrasound Room

1. Determine the clinical task: confirmation of CPPD, mapping of deposits, dynamic observation, or research protocol.
2. Record the list of examined joints before starting the description. For the patient level, the scale applies only to the actually examined set of joints.
3. In each joint, sequentially assess the available structures relevant to CPPD: hyaline cartilage, fibrocartilage, tendinous, or other areas included in the protocol.
4. For each structure, note the presence or absence of typical deposits.
5. Assign a joint grade 0–3 based on the number of involved structures.
6. After assessing all joints, assign a patient grade 0–3 based on the number of joints with deposits.

This order reduces the risk of overestimating the degree due to multiple inclusions in one structure. If there are many hyperechoic signals in one meniscus or one area of hyaline cartilage, it still counts as involvement of one structure until other structures of the joint contain deposits.

How to Formulate a Conclusion

The optimal formulation should contain three elements: localization of deposits, joint grade, and final patient-level grade. For example: “Ultrasound signs of CPPD deposits in the fibrocartilage and hyaline cartilage of the right knee joint; joint OMERACT CPPD grade 2.” If several joints are examined, the conclusion adds: “At the patient level, OMERACT CPPD grade 2: two examined joints involved.”

If the study is limited to one joint, the patient-level grade should be used cautiously and with a caveat: “patient-level assessment not performed, one symptomatic joint examined” or “patient-level grade applies only to examined joints.” This protects the conclusion from being misinterpreted as a complete assessment of CPPD extent.

What the Scale Does Not Assess

• Does not assess inflammation activity. Synovitis, effusion, hypervascularization, and pain are described separately.
• Is not an independent diagnosis. The ultrasound picture should be correlated with clinical, laboratory data, and, if available, crystal microscopy.
• Does not replace differential diagnosis. Hyperechoic structures may correspond to artifacts, calcifications of another nature, enthesopathy, or postoperative changes.
• Does not rank deposit size. The step depends on the number of involved structures or joints, not on the thickness, length, or brightness of inclusions.

Typical Errors in Assessing Chondrocalcinosis

The first error is counting the number of individual point inclusions within one structure and translating it into a grade. In the OMERACT scale, the count is based on involved structures at the joint level, not the number of echogenic points. The second error is assigning a patient-level grade without indicating which joints were examined. This is especially critical in a focused study of one painful joint.

The third error is using the term “pronounced CPPD” without grading. If the scale is applied, it is better to write “OMERACT CPPD grade 3 at the joint level” or “patient-level grade 3,” as these formulations have a specific meaning. The fourth error is mixing CPPD deposits with signs of osteoarthritis: osteophytes, irregular cortical contour, and cartilage thinning are described separately and do not increase the CPPD grade.

Minimal Protocol Template

For practical work, a short structured block is convenient:

• “Joints examined: …”
• “Structures with CPPD deposits: …”
• “Joint OMERACT CPPD grade: right …, left …”
• “Patient-level OMERACT CPPD grade: …, considering examined joints”
• “Associated ultrasound findings: effusion, synovitis, osteoarthritic changes — separately”

This template makes the conclusion suitable for repeat comparison. In dynamics, it is important to repeat the same set of joints and structures; otherwise, a change in patient-level grade may reflect not the biological dynamics of CPPD but the expansion or contraction of the scanning area.

Clinical Interpretation

Grade 0 means that no ultrasound signs of CPPD deposits were detected in the examined structures. This does not exclude CPPD outside the scanning area or with limited visualization. Grades 1–3 reflect increasing deposit extent: from one structure or one joint to involvement of more than two structures or joints.

For a rheumatologist, the value of the scale is that it separates “there is a deposit” from “how widely it is spread.” For an ultrasound diagnostician, the value lies in a reproducible protocol: consistent criteria, explicit indication of the assessment level, and fewer ambiguous epithets. This is why the OMERACT 0–3 scale has become a basic guideline for standardizing ultrasound descriptions of CPPD and chondrocalcinosis.