Bone-on-bone is a term used when joint and meniscus cartilage of the tibiofemoral joint are worn down so much that - instead of nice shiny opposing joint surfaces - exposed bone is grating on exposed bone.

bone-on-bone thinning
On the one side the normal X-ray 'gap' has gone and bone is touching bone. The normal 'gap' is filled with joint cartilage and meniscus, but on this one side that has worn away.
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Common joint surface problems in the knee

A quick overview of the problems one sees in the white joint cartilage at the end of the bones in the knee joint.

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CLINICAL PUBLICATIONS

Diffuse Tibiofemoral Cartilage Change Prior to the Development of Accelerated Knee Osteoarthritis: Data from the Osteoarthritis Initiative. Harkey MS, Davis JE, BS, Lu B, Price LL, Eaton CB, Lo GH, Barbe MF, Ward R, Zhang M, Liu S-H, Lapane KL and MacKay JW, MRCP,FRCR,13 Timothy E. McAlindon, MD,MPH,1 and Jeffrey B. Driban, PhD1 Clin Anat. 2019 Apr; 32(3): 369–378.


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