Microfracture (MFX) is a cartilage repair technique that depends on cell migration from marrow-rich trabecular bone cavities into the cartilage lesion. This study tested the hypothesis that MFX awls with distinct geometry generate different hole shapes and variable bone marrow access in condyles with Grade III to IV lesions. Lateral and medial condyles from total knee arthroplasty (N = 24 male and female patients, 66 ± 9 years) were systematically microfractured ex vivo to 2 and 4 mm deep and the bone holes analyzed by micro-computed tomography. Subchondral bone in lesional condyles showed different degrees of sclerosis up to 2 mm deep ("porous," sclerotic, extremely dense). MFX holes ranged from 1.1 to 2.0 mm in diameter, and retained the awl shape with evidence of slight bone elastic rebound and bone compaction lining the holes that were increased by wider awl diameter and deeper MFX. Marrow access was significantly diminished by sclerosis for all three awls, with an average marrow access varying from 70% (nonlesional bone) to 40% (extremely dense bone). This study revealed that subchondral bone sclerosis can reach a critical limit beyond which MFX creates bone compaction and fissures instead of marrow access.
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