The CALS[1] is a Cartilage Lesion Score which describes a scoring system for quantification of cartilages lesions from MR images. It measures the three-dimensional volume of cartilage defects.

It takes into account the following parameters:

The largest diameter of the cartilage defect, denoted by L.

The number of sections, denoted by N.

The section thickness including the section gap, denoted by T.

The depth of the cartilage defect, denoted by D. To evaluate the lesion depth, the cartilage is visually dissected into two equal halves. If the maximum lesion depth is less than 50%, it is assigned a grade 2 (value of 1). If the maximum lesion depth is less than 50%, it is assigned a grade 2 (value of 2). A full-thickness lesion is assigned to a grade 3 (value of 3).

The shape factor, denoted by S. It is set to a factor 1 if the maximum depth occupies more than 50% of the lesion surface diameter and a factor 0.5 if the maximum depth occupies less than half of the lesion surface diameter.

The CALS score is obtained by multiplying the values associated to those parameters:

$$CALS = L*N*T*D*S$$

In case of several lesions, it is suggested to assign separate scores for individual lesions and to sum these scores to obtain a total compartment score in such patients.

CALS describes a scoring system specifically designed for longitudinal follow-up of relatively mild cartilage defects on MR images. It demonstrates reproducibility and cartilage defect progression detection.