Background: Anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) reconstruction with bone patellar tendon bone graft (BPTB) are commonly performed. Lack of precision in patellar tendon length measurement may result in a graft-tunnel mismatch. Presently, preoperative planning needs to evaluate the limb alignment from knee extended weight bearing radiograph and the length of the patellar tendon from knee 30 degrees flexion radiograph.
Objective: To compare knee extended weight bearing radiograph and knee 30 degrees flexion radiograph to find which one can predict the patellar tendon length with more precision.
Study Design: Cross-sectional diagnostic study.
Material and Method: Thirty-six patients who received a cruciate ligament reconstruction with BPTB graft at Phramongkutklao Hospital were enrolled. Patellar tendon lengths were measured from knee extended weight bearing radiograph and knee 30 deegrees flexion radiograph and then compared to a real intraoperative patellar tendon length. Analysis using intra-class correlation coefficient and paired t-test was done.
Results: The mean patellar tendon length from radiographic taken at 30 degrees of knee flexion and extended weight bearing were 43.90 mm and 42.95 mm respectively. The difference was less than 1 mm which might not be clinically significant. The prediction of patellar tendon length from 30° flexed film is statistically closer (ICC 0.760: 0.717, p-value < 0.001). By the way, they were both in intermediate reliability level (0.6 < ICC < 0.8). It also seems not clinically different.
Conclusion: Preoperative film knee extended weight bearing alone is enough for evaluation both limb alignment and length of the patella tendon leading to decreasing cost, time and radiation exposure of patients.

Keywords: Anterior cruciate ligament, Posterior cruciate ligament, Bone patellar tendon bone graft, Preoperative planning, Patellar tendon length prediction