Articular Cartilage Defect
For an isolated "articular cartilage defect" the most specific code would be M94.8X _: Other Specified Disorders of Cartilage (of joint). Although this code set includes the knee (lower leg: 6) and does not appear to exclude the Patella, I think that for the Patella, M22.8 _ (Other Disorders of the Patella) would be more correct. M24.8 _: Other Specified Derangement of Joint NEC seems to me to be far less specific. This is the simplest answer to the question, but this can be only a part of the joint problem. Other concerns are the presence or absence of a Cartilaginous Loose Body(s) originating from the "defect," and/or is there other articular cartilage disease of the joint, such as Chondromalacia?
Chondromalacia is "softening" of the articular cartilage, with varying degrees of depth and severity of involvement. It can progress to the point of producing an articular cartilage defect all the way to the bone underneath. For all joints and areas other than the Patella, M94.2 _ _ would apply to the associated Chondromalacia, if present. For Patellar Chondromalacia, if associated with an articular cartilage defect, then M22.4 _ would apply. However, the presence of Chondromalacia in any joint or area does not necessarily mean there is an articular cartilage defect, but the two can occur simultaneously/concurrently.
An articular cartilage defect can also be associated with &/or the source of a Cartilaginous Loose Body in the affected joint. For the knee joint, the code for an associated Loose Body would be M23.4 _; but for other joints, it would be M24.1 _ _.
I would be careful about "Cartilage Derangement" as regards this problem/issue. As it applies to the knee joint, "Cartilage Derangement" applies to meniscal tears, not articular cartilage disorders.
I hope this is more helpful than confusing.
Respectfully submitted, Alan Pechacek, M.D.