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Background: There is a paucity of literature reporting clinical and magnetic resonance imaging (MRI) outcomes after allogeneic umbilical cord blood–derived mesenchymal stem cell (UCB-MSC) implantation for chondral defects of the knee. Purpose: To report clinical and MRI outcomes after UCB-MSC implantation for chondral lesions of the knee. Study Design: Case series; Level of evidence, 4. Methods: Inclusion criteria were patients aged between 40 and 70 years with focal chondral lesions of grade 3 or 4 on the medial femoral condyle, defect sizes >4 cm2, and intact ligaments. Exclusion criteria were patients who required realignment osteotomy or who had a meniscal deficiency, ligamentous instability, or a concomitant full-thickness chondral defect in the lateral or patellofemoral compartment. A mixture of human UCB-MSCs and sodium hyaluronate was implanted into the chondral defect through mini-arthrotomy. MRI at 1-year follow-up was performed to evaluate repaired cartilage hypertrophy. Repaired cartilage thickness was measured, and hypertrophy was classified as grade 1 (<125%), grade 2 (<150%), or grade 3 (<200%). Patient-reported outcomes (PROs; International Knee Documentation Committee, visual analog scale for pain, and Western Ontario and McMaster Universities Osteoarthritis Index) were evaluated preoperatively and at 1, 2, and 3 years postoperatively. Repaired cartilage hypertrophy was evaluated for a correlation with PRO scores. Results: Enrolled were 85 patients with a mean age of 56.8 ± 6.1 years and a mean chondral defect size of 6.7 ± 2.0 cm2. At follow-up, a significant improvement in all PRO scores was seen compared with preoperatively ( P < .001 for all). MRI at 1-year follow-up demonstrated that 28 patients had grade 1 repaired cartilage hypertrophy, 41 patients had grade 2, and 16 patients had grade 3. MRI performed in 11 patients at 2 years after surgery indicated no difference in repaired cartilage hypertrophy between the 1- and 2-year time points. The grade of repaired cartilage hypertrophy did not correlate with PRO scores. Conclusion: Clinical outcomes improved significantly at short-term follow-up after UCB-MSC implantation. Although all patients showed repaired cartilage hypertrophy, it did not correlate with clinical outcomes.

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