Critical Size Defect in Adult Articular Cartilage: A Preclinical Study.

Objective: Lesions of adult articular cartilage occur due to trauma or disease, such as osteoarthritis. If they do not penetrate the subchondral bone, they are called partial-thickness defects (PTDs), which are believed not to heal. However, some reports indicate that minor PTDs can be repaired. We hypothesize that a critical-size PTD exists below which spontaneous healing occurs. Design/Methods: In an adult pig model, we created PTDs of minimal width (a scalpel cut) and systematically increased their width up to 0.5 mm. Defect analyses were conducted at 1 and 3 months post-surgery using light microscopy and histomorphometry. Results: None of the defects healed by repair cartilage; therefore, all PTDs are of a critical size. Surprisingly, a critical defect-size range was identified where significant mesenchymal tissue (MT) formation occurs, specifically in defects measuring 50-100 μm in width. The presence of this MT was limited to a 1-month time window. Furthermore, physiological joint loading during the postsurgical phase was associated with substantial structural tissue deformation, often leading to an overlapping of the side walls of the smallest defects. This results in a pseudo-covering of the defect void, which may thus be invisible when observed from above. Conclusions: The main novel finding of this study is that there is no critical width below which PTDs undergo repair.