Imaging in Cartilage Repair

As cartilage injuries are common and frequently affect young patients with potential for progression to osteoarthritis, treatment to alleviate symptoms and potentially delay joint degeneration is warranted. A number of surgical techniques are available to treat focal chondral defects including marrow stimulation, osteochondral auto- and allografting, and autologous chondrocyte implantation. Although arthroscopy is considered the gold standard for evaluation of cartilage pre- and post repair, it is invasive with associated morbidity and cannot adequately assess the deep cartilage layer and underlying bone.


Magnetic resonance imaging provides unparalleled non-invasive assessment of the repair site and all other joint tissues. Several techniques and methods are available to evaluate the success of cartilage repair strategies in a clinical trial setting that are offered by BICL and are performed by our leading imaging experts. 

BICL Osteoarthritis Experts
MRI Observation of Cartilage Repair Tissue (MOCART)

MOCART provides a reproducible semi-quantitative scoring system for morphological cartilage repair assessment that has been widely used as an outcome measure for longitudinal clinical trials. MOCART originally defined 9 structural variables but was modified and expanded to assess 11 variables to take advantage of higher resolution 2D images and isotropic 3D MR sequences. The -MOCART improves evaluation of repair tissue by localizing the features within the repair site and evaluating the border zones of the repair tissue-cartilage interfaces in every plane, and the relation of the repair site to the weight-bearing regions of the joint. A detailed assessment of the subchondral bone was also incorporated. Since all features of the original MOCART are assessed also by 3D-MOCART, we only describe the 3D-MOCART features.

Cartilage Repair Osteoarthritis Knee Score (CROAKS)

Although the 3D MOCART is excellent for assessment of the repair site, it largely ignores the rest of the joint, which may be of relevance especially n longitudinal follow-up. MRI osteoarthritis knee score (MOAKS) is an established semi-quantitative scoring system for whole organ assessment of the joint that is not able to integrate detailed information on the repair site and immediate surroundings. As a consequence BICL members have developed the cartilage repair OA knee score (CROAKS) instrument to combine features of these two scores to provide an a comprehensive, reproducible tool for longitudinal postoperative whole-organ assessment after surgical cartilage repair using a multi-subregion division of the joint. 


In addition to the previously described repair tissue features evaluated by 3D MOCART, the whole-organ characteristics assessed by CROAKS include; bone marrow lesions (BMLS) beyond the repair site, subchondral cysts, cartilage status beyond the repair site, osteophytes, synovitis, effusion, menisci, and anterior and posterior cruciate ligaments (ACL and PCL). Some of the other periarticular features evaluated include the pes anserine bursa, illiotibial band, prepatellar bursa, infrapatellar bursa, and presence of popliteal cysts, ganglion cysts or loose bodies. 


Cartilage Repair Osteoarthritis Knee Score (CROAKS), developed by BICL’s leading imaging experts, optimizes whole organ assessment of the joint after cartilage repair by combining features of these two scores. An overview of the different dimensions of the CROAKS score is presented below.

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Individual Features of the CROAKS system


1) Bone Marrow Lesions (BMLs) and subchondral cysts


Bone marrow edema lesions and subchondral cysts are assessed together as they share common pathophysiology. These are both graded in 15 standardized subregions of the knee.


Percentage of the volume of each BML that is BML (as distinct from cyst) is graded as; Grade 0= none

Grade 1 <25%

Grade 2= 25-50%

Grade 3 >50%                                             


If a cyst is present without an associated bone marrow lesion then cysts will be scored as a 0 for size % of lesion that is BML.

2) Cartilage 

The cartilage is graded in 14 described subregions for areal extent of cartilage loss and % of loss within this subregion, which is full thickness.

A two digit score is given to each subregion with the first digit describing size of lesion and the second digit describing percentage of subregion that is affected by full thickness loss. 

3) Osteophytes

Osteophytes are scored in 12 locations according to size from grades 0 to 3; along the trochlea, central weight bearing and posterior margins of the femoral condyles and weight bearing tibial plateaus, and along the medial, lateral, superior and inferior margins of the patella. Posterior femoral osteophytes are assessed peripherally and centrally. The larger osteophyte for either, peripheral or central location is scored.

4) Synovitis and Effusion

As contrast-enhanced sequences are not currently employed in large studies assessing cartilage repair, a surrogate of signal changes in Hoffa’s fat pad has been applied that has been shown on biopsy to represent mild chronic synovitis (159). This abnormality is best described as diffuse hyperintense signal on T2, PD, and IW fat suppressed sequences within the fat pad. In addition to synovitis these signal changes could also be attributed to other etiologies such as post-arthroscopic changes or Hoffa’s disease (160).

Hoffa-synovitis score is scored on a mid-line sagittal image as one single score for assessment of degree of hyperintensity in Hoffa’s fat pad based on the region outlined in Scoring is based on size:

Grade 0= normal

Grade 1= mild

Grade 2= moderate

Grade 3= severe
Knees that have undergone a cartilage repair procedure often show scarring in Hoffa’s fat pad with concurrent signal alterations on MRI. These signal changes cannot be distinguished from true synovitis, as only contrast-enhanced MRI is able to differentiate between active inflammation and inactive scar tissue.

5) Menisci

Changes in meniscal position and meniscal morphologic changes manifesting as tears or loss of substance have both been shown to predispose to cartilage loss (161, 162).


Meniscal extrusion is graded in four locations as follows:

Medial Meniscus: Medial extrusion relative to medial tibial margin (coronal image)

Medial Meniscus: Anterior extrusion (sagittal image) where extrusion is maximum

Lateral Meniscus: Lateral extrusion relative to lateral tibial margin (coronal image)

Lateral Meniscus: Anterior extrusion (sagittal image) where extrusion is maximum


The reference for measurement is the edge of the tibial plateau (excluding osteophyte). Extrusion is scored as follows:

Grade 0 - no extrusion

Grade 1 - < 2mm

Grade 2 - 2-5 mm

Grade 3 - > 5mm


Morphology is assessed for the medial and lateral meniscus at the anterior, body and posterior horn. The anterior and posterior horn regions are scored using the sagittal sequences and the body is scored using the coronal sequences. Morphologic features are scored as follows:


Grade 0: Normal

Grade 1 - High signal not extending through meniscal surface i.e. not a tear

Grade 2 - Horizontal tear: grade 0 - absent, grade 1 – present

Grade 3 - Radial Tear: grade 0 - absent, grade 1 – present

Grade 4 - Longitudinal tear: grade 0 - absent, grade 1 – present

Grade 5 - Complex tear: grade 0 - absent, grade 1 - present

Defined by high signal that extends to 2 surfaces and > 3 points

Grade 6 - Root tear (posterior horn): grade 0 - absent, grade 1 - present

Grade 7 - Partial maceration: grade 0 - absent, grade 1 - present

Defined by loss of morphological substance of the meniscus

Grade 8 - Progressive partial maceration: grade 0 - absent, grade 1 – present

Progressive partial maceration as compared to the previous visit.

Grade 9 - Complete maceration: No more meniscal substance visible.

Meniscal cyst: grade 0 - absent, grade 1 - present

Meniscal hypertrophy: grade 0 - absent, grade 1 – present

Defined as definite increase in meniscal volume in given subregion when compared to normal

6) Ligaments

The ligaments are graded as follows:


a)     Anterior Cruciate Ligament (ACL)

        Grade 0: normal

        Grade 1: partial tear

        Grade 2: complete tear


b)     Associated with BML/cyst at site of ACL insertion or origin

        Grade 0: absent

        Grade 1: present


c)     ACL Repair

        Grade 0: absent

        Grade 1: present


d)     Posterior Cruciate Ligament (PCL)

        Grade 0: normal

        Grade 1: complete tear


e)     Associated with BML/cyst at site of PCL insertion or origin

        Grade 0: absent

        Grade 1: present


f)     Patellar tendon

       Grade 0: no signal abnormality

       Grade 2: signal abnormality present

7) Periarticular features

Some of the other periarticular features assessed are as follows:


a)    Pes anserine bursitis

       Grade 0- absent

       Grade 1-present


b)    Illiotibial band signal (bursitis)

       Grade 0- absent

       Grade 1- present


        Popliteal cyst

        Grade 0- absent

        Grade 1- present


c)    Infrapatellar bursa signal (bursitis)

       Grade 0-absent

       Grade 1-present


d)    Prepatellar bursa signal (bursitis)

       Grade 0 - absent

       Grade 1 – present


e)    Ganglion cyst

       Associated with the tibio-fibular joint: grade 0 -  absent, grade 1 - present

       Associated with PCL and ACL: grade 0 - absent, grade 1 - present

       Other: grade 0 - absent, grade 1 – present


f)     Loose bodies

       Grade 0 - absent

       Grade 1 - present

Compositional MRI Assessment

Furthermore, advanced compositional MRI sequences including T2, T2* and T1rho quantification, and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), diffusion weighted imaging and diffusion tensor imaging, sodium imaging, magnetization transfer contrast (MTF) and glycosaminoglycan chemical exchange saturation transfer imaging (gagCEST) are available for biochemical assessment of cartilage and cartilage repair tissue. These quantitative MRI techniques assess collagen content and orientation, water content and glycosaminoglycan (GAG)/proteoglycan content in the repair tissue as it matures, and also within the non-operated cartilage. An overview of the different techniques to be applied in a clinical trial setting is given below.

Summary of MRI Compositional Techniques

Compositional MRI Technique

Cartilage Component Assessed


T2 Mapping




T2* Mapping






T1 Rho Imaging







Sodium Imaging















Diffusion-Weighted Imaging






Ultrashort TE Imaging

Collagen network,

Water content



Collagen network,

Water content





Collagen network,






















Collagen Network,






Collagen Network, Water content,


Well validated

Easy to implement

Does not require contrast administration


Faster acquisition than T2 mapping

Can be used with UTE to assess calcified cartilage at the osteochondral junction

Does not require contrast administration



Sensitive to early degeneration

May complement T2/T2* mapping

Does not require contrast administration





Correlates directly with GAG content

Does not require contrast administration




Indirect assessment of GAG content

Well validated

Clinically useful


Does not require contrast administration






Provides additional information regarding cartilage microarchitecture

Does not require contrast administration




Can be used to assess tissue with intrinsic short T2 such as cartilage near osteochondral junction.

Can be used in conjunction with T2, T2* and T1rho imaging


Long acquisition times using MESE sequence, cannot assess calcified cartilage at osteochondral junction


Not well validated

Susceptible to magnetic field inhomogeneities and magic angle effect




Nonspecific in terms of cartilage components assessed

Special pulse sequences only available at a select number of academic institutions

Acquisition can be time consuming



Requires specialized hardware

Long examination times

Low spatial resolution



Requires administration of IV contrast




Difficult to implement due to technical complexity

Requires high field MRI

Not well validated



Semi-quantitative image processing is demanding

Susceptible to movement artifacts

Illustrative Cases

Multi-tissue assessment using CROAKS grading scheme. Coronal dual echo at steady state (DESS) image shows 24 months follow up examination after MACI of  the medial weight bearing femur (large arrows). Partial underfilling of the repair zone is noted. In addition large marginal osteophytes characteristic of definite osteoarthritis are noted at the medial and lateral tibio-femorla joint (small arrows). These osseous bone outgrowths are not assessed in grading schemes focusing on the repair zone only.

Cartilage assessment using CROAKS grading scheme.

Sagittal proton density weighted image shows the lateral tibio-femoral compartment 18 months after MACI of the lateral weight bearing femur. The repair zone is demarcated by long thin arrows and hypertrophy of the repair tissue is noted. At the posterior lateral tibia, there is a focal areaq of superficial cartilage thjinning and marked intrachondral hyperintensity of the native cartilage (large arrow). This finding of pathology distant to the repair zone is not covered by commonly applied cartilage repair grading schemes but might be relevant for long term outcome of the joint.

  1. Brittberg M, Lindahl A, Nilsson A, Ohlsson C, Isaksson O, Peterson L. Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation. The New England journal of medicine. 1994;331(14):889-95.

  2. Marlovits S, Singer P, Zeller P, Mandl I, Haller J, Trattnig S. Magnetic resonance observation of cartilage repair tissue (MOCART) for the evaluation of autologous chondrocyte transplantation: determination of interobserver variability and correlation to clinical outcome after 2 years. European Journal of Radiology. 2006;57(1):16-23.

  3. Welsch GH, Zak L, Mamisch TC, Resinger C, Marlovits S, Trattnig S. Three-dimensional magnetic resonance observation of cartilage repair tissue (MOCART) score assessed with an isotropic three-dimensional true fast imaging with steady-state precession sequence at 3.0 Tesla. Investigative Radiology. 2009;44(9):603-12.

  4. Hunter DJ, Guermazi A, Lo GH, et al. Evolution of semi-quantitative whole joint assessment of knee OA: MOAKS (MRI Osteoarthritis Knee Score). Osteoarthritis Cartilage 2011;19(8):990-1002.

  5. Roemer FW GA, Trattnig S, Apprich S, Marlovits S, Niu J, Hunter DJ, Welsch GH. Whole joint MRI assessment of surgical cartilage repair of the knee: Cartilage Repair OsteoArthritis Knee Score (CROAKS). Osteoarthritis Cartilage 2014;22(6):779-99

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