MR Imaging-based Semi-quantitative Methods for Knee Osteoarthritis

Abstract

Magnetic resonance imaging (MRI)-based semi-quantitative (SQ) methods applied to knee osteoarthritis (OA) have been introduced during the last decade and have fundamentally changed our understanding of knee OA pathology since then. Several epidemiological studies and clinical trials have used MRI-based SQ methods to evaluate different outcome measures. Interest in MRI-based SQ scoring system has led to continuous update and refinement. This article reviews the different SQ approaches for MRI-based whole organ assessment of knee OA and also discuss practical aspects of whole joint assessment.

Fig. 1.

Typical image examples for different types of cartilage damage. (A) A focal superficial defect (arrow) not reaching the subchondral plate is shown in this coronal intermediate-weighted MRI (arrow). Lesion will be coded as a grade 1.0 lesion in MOAKS or as grade 2 in WORMS. (B) Coronal intermediate-weighted MRI shows a focal defect (arrow) that reaches the subchondral plate and is consequently defined as a grade 1.1 lesion using MOAKS. In WORMS this lesion would be scored as a 2.5 lesion. A 2.5 lesion is not a reflection of a within-grade coding but a distinct grade by itself. (C) Sagittal intermediate-weighted fat-suppressed MRI depicts diffuse full thickness cartilage damage in the central subregion of the medial femur and the central medial tibia (large arrows) representing grade 2.2. lesions in MOAKS, and grade 5 lesions in WORMS. There are associated subchondral bone marrow lesions (small arrows). MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; WORMS, Whole Organ Magnetic Resonance Score.

Fig. 2.

Example of longitudinal assessment of bone marrow lesions (BMLs) in the lateral tibio-femoral compartment. (A) Baseline sagittal intermediate-weighted fat-suppressed MRI shows a grade 2 MOAKS/grade 3 WORMS BML in the anterior lateral femur displaying high signal intensity, comprised of an ill-defined (edema-like) component (large arrows) and a well-defined cystic component (small arrows). In addition, there are small cystic BMLs in the subchondral anterior and posterior lateral tibia (small arrows). (B) Follow-up MRI 1 year later shows slight decrease of overall lesion size (within-grade change for MOAKS, and change from grade 3 to grade 2 for WORMS) in the femur (large arrows, black-filled) but increase of size of femoral cystic component (small arrows). Note regression of cystic lesion in the posterior lateral tibia and increase of ill-defined (edema-like) portion of BML in the anterior lateral tibia (large arrow, gray-filled). MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; WORMS, Whole Organ Magnetic Resonance Score.

Fig. 3.

Longitudinal evaluation of BMLs. Relevance of lesional vs. subregional scoring. (A) Baseline sagittal intermediate-weighted fat-suppressed MRI shows two distinct ill-defined BMLs at the central subregion of the medial femur (arrows). Overall lesion size in subregion qualifies as a grade 1 MOAKS/grade 2 WORMS lesion. (B) Follow-up MRI 1 year later shows within-grade increase in overall subregional lesion size in the same subregion. In contrast to image A, now three distinct lesions are observed (arrows). The single anterior lesion has split into two lesions with a decrease in lesion size, while the previous posterior lesion shows now an increase in lesion size. (C) Two-year follow-up MRI shows a decrease in overall femoral BML size with now a total subregional score of 1 using WORMS and MOAKS. There are two distinct lesions now with the most anterior lesion from image B showing complete regression. No cystic component of any of the lesions is observed. There is a large (grade 3 WORMS and MOAKS) incident lesion in the posterior medial tibia. BML, bone marrow lesion; MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; WORMS, Whole Organ Magnetic Resonance Score.

Fig. 4.

Meniscal maceration is commonly observed in OA knees. (A) Baseline coronal dual echo at stead state (DESS) image shows a normal body of the medial meniscus without evidence of a tear of substance loss but little extrusion (arrow; grade 1 MOAKS). (B) Two years later, there is evidence of substance loss (arrow) in the central part of the body region (also referred to as the “white zone”). This finding is also termed partial meniscal maceration. MOAKS, Magnetic Resonance Imaging Osteoarthritis Knee Score, OA: osteoarthritis.

Fig. 5.

Osteophytes are one of the hallmark features of OA on imaging and part of the disease definition on X-rays. While WORMS uses a complex approach of osteophytes scoring on a 0–7 scale at 16 articular anatomical locations, MOAKS applies a somewhat simplified scheme on a 0–3 scale at only 12 different locations omitting the scores of the anterior and posterior medial and lateral tibia. (A) Sagittal fat-suppressed intermediate-weighted image of the lateral tibio-femoral compartment shows a moderate sized MOAKS grade 2/WORMS grade 4 osteophyte at the anterior femur, a MOAKS grade 3/WORMS grade 5 osteophyte at the posterior femur (short white-filled arrows), and a WORMS grade 5 osteophyte (long black-filled arrow) at the anterior lateral tibia (location not considered in MOAKS). (B) Marginal osteophytes in the coronal plane are similarly considered in MOAKS and WORMS. Example shows femoral osteophytes (small arrows; MOAKS grade 2/WORMS grade 4 medial; MOAKS grade 3/WORMS grade 6 lateral) and a moderate osteophyte at the medial tibia (large arrow; MOAKS grade 2/WORMS grade 4). There is diffuse cartilage loss at the central lateral tibial and femur with moderate lateral tibial plateau remodeling (attrition). (C) Sagittal dual-echo at steady-state (DESS) MRI of the medial tibio-femoral compartment shows moderate-sized (MOAKS grade 2/WORMS grade 3) osteophytes at the anterior and posterior medial femur (small white-filled arrows). At the tibia (large white-filled arrows) there is a tiny anterior osteophyte (WORMS grade 1) and a moderate-to-large sized posterior osteophyte (WORMS grade 5). Tibial locations are not scored in the sagittal plane using MOAKS. MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; OA: osteoarthritis; WORMS, Whole Organ Magnetic Resonance Score.

Fig. 6.

MRI of markers of inflammation in OA. Fluid sensitive sequences are capable of delineating intraarticular joint fluid. However, a distinction between true joint effusion and synovial thickening is not possible as both are visualized as hyperintense signal within the joint cavity. For this reason the term effusion-synovitis has been introduced, which is scored based on the distension of the joint capsule for both systems, WORMS and MOAKS, and is graded collectively from 0 to 3 in terms of the estimated maximal distention of the synovial cavity with 0 = normal, grade 1 = <33% of maximum potential distention, grade 2 = 33–66% of maximum potential distention, and grade 3 = >66% of maximum potential distention. Axial dual-echo at steady-state (DESS) MR images show (A) grade 1 effusion-synovitis, (B) grade 2 effusion-synovitis (asterisk), and (C) grade 3 effusion-synovitis (asterisk). MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; OA: osteoarthritis; WORMS, Whole Organ Magnetic Resonance Score.

Fig. 7.

Signal changes in Hoffa’s fat pad are commonly used as a surrogate for synovitis on non-contrast-enhanced MRI. While these structural changes have been used for a long time they have not been described in the WORMS system but have been incorporated in the MOAKS system. Although synovitis can only be visualized directly on contrast-enhanced sequences, it has been shown that Hoffa’s signal changes are a sensitive but non-specific surrogate of synovitis. (A) Sagittal proton density-weighted MRI shows a discrete ill-defined hyperintense signal alteration in Hoffa’s fat pad consistent with grade 1 Hoffa-synovitis (arrow). (B) A grade 2 signal change within the fat pad is shown in this example (arrows). (C) Severe, grade 3 signal alterations almost occupying the entire fat pad are seen in this image (arrows). MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; WORMS, Whole Organ Magnetic Resonance Score.

Fig. 8.

Comparison of inflammatory manifestations of disease using non-enhanced and contrast-enhanced sequences. (A) Axial intermediate-weighted fat-suppressed MRI shows homogeneous hyperintensity within the joint cavity consistent with grade 2 effusion-synovitis by MOAKS and WORMS (asterisk). Note BML in the posterior lateral femoral condyle consistent with traction edema at the insertion of the anterior cruciate ligament (arrows). (B) Corresponding T₁-weighted fat-suppressed image after intravenous contrast administration clearly differentiates between intraarticular joint fluid depicted as hypointensity (asterisk) and true synovial thickening visualized as hyperintense contrast enhancement of the synovial membrane (large arrows). Note that BMLs are similarly depicted on T₂-weighted fat-suppressed and T₁-weighted contrast-enhanced fat-suppressed MRI (small arrows). BML, bone marrow lesion; MOAKS, MRI Osteoarthritis Knee Score; MRI: magnetic resonance imaging; WORMS, Whole Organ Magnetic Resonance Score.