Preoperative measurement of cartilage defects by MRI underestimates lesion size

Objective: Anecdotal evidence suggests that MRI frequently underestimates the size of cartilage defects when compared with final lesion size after debridement of all degenerated tissue. This has potential implications for the choice of cartilage repair technique since most treatment algorithms are primarily driven by defect size. We conducted an investigation comparing size estimates based on preoperative MRI with final defect size after debridement. Our aim was to provide surgeons with more objective data to assist in predicting true defect size based on MRI scanning. Design: Patients were included in this retrospective study if they had undergone preoperative MRI and open cartilage repair within 12 months to minimize potential confounding by defect progression on MRI. Defect sizes measured after debridement were obtained from surgical notes and compared with MRI size estimates by 2 musculoskeletal radiologists. Results: Thirty-eight patients were enrolled with a median age of 37 years, median number of 1.7 defects, and a total median defect area of 6 cm ² per knee. Preoperative MRI scanning had predicted a median defect area of 3.6 cm ². This reflected a difference of 65% (P < 0.001) between MRI and final defect area after debridement when 85% of all individual defects were larger than predicted by preoperative MRI. Conclusions: Our study compared the size of cartilage defects measured by preoperative MRI with surgical measurements after debridement. On average, the final total defect area per knee was 65% larger than estimated preoperatively by MRI. Individual defects were larger than predicted by 47% to 377%, depending on defect location.