ICD-10-CM > M00-M99 > M20-M25 > M25 > M25.6 > M25.66 M25.66 Stiffness of knee, not elsewhere classified Non-Billable/Non-Specific Codes M25.661Stiffness of right knee, not elsewhere classifiedM25.662Stiffness of left knee, not elsewhere classifiedM25.669Stiffness of unspecified knee, not elsewhere classified