The following subclassification to indicate the site of involvement is provided for optional use with appropriate categories in Chapter XIII. As local extensions or specialty adaptations may vary in the number of characters used, it is suggested that the supplementary site subclassification be placed in an identifiably separate position (e.g. in an additional box). Different sub-classifications for use with derangement of knee, dorsopathies, and biomechanical lesions not elsewhere classified are given at M23, before M40 and at M99 respectively.