Anterior Cruciate Ligament Tibial Footprint Size as Measured on Magnetic Resonance Imaging: Does It Reliably Predict Actual Size?

BACKGROUND: Measuring the size of the anterior cruciate ligament (ACL) tibial footprint on magnetic resonance image (MRI) is common for preoperative planning of ACL reconstruction. However, the accuracy of such measurement has not been well documented. PURPOSE: To investigate whether the actual size of the ACL tibial footprint could be predicted by its measurement on MRI and to develop equations to improve the accuracy of predicting the actual size based on MRI measurement. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 2. METHODS: A total of 164 patients with normal visualized ACL in gross evaluation and MRI were included (mean ± SD age, 67.3 ± 8.3 years). Cases with ACL tear, severe mucoid degeneration, osteophyte around the ACL tibial insertion, or intervals >12 months between MRI and actual measurement were excluded. The ACL tibial footprint was carefully dissected and measured during total knee arthroplasty. The length of the ACL tibial footprint on MRI was measured on a sagittal image, while the width was measured on an oblique coronal image. For the ACL tibial footprint, the association between measurement on MRI and actual measurement of length and width was analyzed via univariable and multivariable regression analyses. Reliability of measurements on MRI was also evaluated. RESULTS: The length and width of the ACL tibial footprint as measured on MRI showed strong correlation with the actual length and width (coefficients: ρ = 0.904 and ρ = 0.808, respectively). There were differences between ACL size on MRI and its actual size: length, 12.4 mm (range, 9.7-15.3 mm) vs 13.8 (10.6-17.8) ( P < .001); width, 8.8 mm (range, 7.0-12.1 mm) vs 7.2 (5.8-10.4) ( P < .001). Based on sex, there were also differences between the size per MRI and the actual size ( P < .001 for all): length in men, 12.6 mm (range, 10.9-15.3 mm) vs 14.2 (12.3-17.8); length in women, 12.4 mm (range, 9.7-14.5 mm) vs 13.7 (10.6-15.8); width in men, 9.3 mm (range, 8.0-12.1 mm) vs 7.6 (5.8-10.4); width in women, 8.7 mm (range, 7.0-10.4 mm) vs 7.2 (5.8-9.7). The actual length of the ACL tibial footprint could be predicted by its length on MRI and sex ( R2 = 0.83, P < .001). Similarly, actual width could be predicted by the width on MRI and sex ( R2 = 0.75, P < .001). All intraclass correlation coefficients were >0.8, indicating good reliability. CONCLUSION: Measurements of the size of the ACL tibial footprint on MRI showed strong correlation with its actual size. Prediction equations showed good concordance correlation coefficients.