Preoperative MRI Underestimates Articular Cartilage Defect Size Compared With Findings at Arthroscopic Knee Surgery

Background: Magnetic resonance imaging (MRI) is widely used as a preoperative tool to estimate the size of articular cartilage defects to optimize treatment selection. However, the reliability of MRI sizing of cartilage defects is not well understood. Hypothesis/Purpose: The purpose of this investigation was to compare the size of knee articular cartilage defects on MRI to arthroscopic visualization after debridement. It was hypothesized that MRI sizing would produce measurements that were no different than those made during arthroscopic knee surgery. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: Seventy-seven patients (age [mean +/- SD], 38 +/- 10.7 years) who met inclusion criteria underwent preoperative knee MRI of at least 1.5 T within 1 year of arthroscopic knee surgery for a high-grade cartilage defect. Postdebridement defect sizes were obtained from intraoperative surgery notes and compared with retrospective MRI estimates. Results: Ninety-two total cartilage defects were analyzed with an average of 1.2 high-grade defects per knee and average postdebridement defect area of 2.99 cm(2) per lesion (95% CI, 1.63-2.26 cm(2)). Preoperative MRI analysis estimated a lesion area that was an average of 1.04 cm(2) smaller (95% CI, 0.70-1.39 cm(2); P < .0001). In 74% of the lesions analyzed, defect size was larger on arthroscopic visualization than was estimated by MRI sizing. On average, MRI underestimated the defect area by 70% compared with arthroscopic visualization. Conclusion: Magnetic resonance imaging underestimates the size of articular cartilage defects compared with final postdebridement size as measured during arthroscopic knee surgery. Thus, before arthroscopic surgery, orthopaedic surgeons should consider treatment strategies that are appropriate for a larger defect than predicted by preoperative MRI.