genius protective layer 

Healthy cartilage is functionally a highly specialised tissue, which can not be directly engineered. This special role and structure has been known for a long time. "Cartilage is a troublesome thing", said Hunter brothers in 1790s. Since then it has been known that torn cartilage will not heal spontaneously, but can cause severe discomfort, mechanical problems and swelling of the joint. An intact cartilage layer is needed for normal function. In short, cartilage layer can be thought as a bullet proof vest, or as a shin pad protecting the underlying bone from high focal pressure peaks. In addition it serves as a gliding surface with very low friction.
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It is still quite commonly thought that cartilage rupture could not be related to pain, since cartilage itself does not have nerve endings. Literally, that is true at anatomic level, but in everyday life the pain sensation comes from the underlying bone after the protective cartilage layer is damaged. This is comparable to glove or shin pads. If you have a broken glove and you touch a hot thing, or you get a kick on the shin area without protective layer of shin pads. And it is true that neither gloves nor shin pads have nerve endings themselves.

injury OR degeneration?

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Injury types

Degenerative lesions

Articular cartilage can be damaged in joint distortion. Especially shear forces combined with compressive force can tear cartilage in a mangle like manner.  That is called delamination cartilage injury. In children and young people all cartilage layers are usually torn apart from the bone. In adults, delamination injury is more often limited to  the deepest calcified layers at the bottom. These layers do not have gliding properties. In practice this means that 2/3 to 3/4 of the cartilage thickness is lost, but functionally that is a 100% damage. Most accurate imaging methods like CBCT can even reveal this layer, but it's role as a functional cartilage should not be overestimated. 

​One of the difficult injuries for many imaging techniques are delaminated flaps which have a base. They stay on place as a hinged bonnet, and can only be seen if there is enough joint fluid between loose cartilage and bone surface. If the rupture is total, then sharp shouldered cartilage defect can be seen more easily, and in many cases detached cartilage flap  can be seen as a loose body inside the joint. In either case, it can cause also mechanical symptoms.

Another type of cartilage injury is a gradual degeneration after initial hit. This can most often be seen in connection to  ACL injuries. Cartilage appears normal in initial imaging and during ACL reconstrution surgery, but it has got a compressive load too high to stand. Tissue degeneration starts, and consequences are  seen with a couple of months delay. In these cases patient recovers well from the index procedure, knee is stable, rehabilitation proceeds well, but joint starts to react to increased exercises with swelling, stiffness and pain during the first months after initial injury. It is not exactly known what happens there inside joint and cartilage layers,  but cartilage is destroyed very fast. At the moment we do not have knowledge enough to say if something protective could be done after this kind of injuries.

Diagnostics of osteoarthrosis (OA) degree is very much based on old Kelgren-Lawrence classification. It includes notable changes in the joint space width (JSW) in early osteoarthrosis, followed by subchondral bone plate thickening, osteophyte formation and bone cysts formation. This is a widely available way to describe OA degree, but it is also very simplified model of OA. It is now known, that osteoarthrosis is a multifactorial disease of the whole joint. Cartilage thinning can be partly reason, but also a consequence of other processes in joint. These processes remain to be understood in coming years, but currently late phase OA findings are  mostly seen as an indication to even total joint replacement if the patient has OA symptoms.
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One of the biggest questions in cartilage related problems is if we can distinguish between traumatic cartilage lesions and degenerative cartilage thinning. It is not easy, but there is now increasing data that modern imaging methods can give answers to this in the near future. Increasing spatial resolution with cone beam CT (CBCT) scanner and contrast media reveals cartilage layers with ultra high 0.2 resolution in any given direction. This offers a novel insight to cartilage layers in clinical work. Another way is to combine physiological parameters to different imaging methods, giving some information on functional condition of cartilage like in dGEMRIC MRI, or penetration of contrast media inside cartilage in delayed CT scans.

Subchondral bone plate is a crucial player in OA.  It's mineral density would be of interest, and it has been measured in a clinical setting. It seems to correlate to ICRS OA grading,  but much further work is needed on that area.