Combined PCL instabilities cannot be identified using posterior stress radiographs in external or internal rotation: A cadaveric study.

PURPOSE Posterior stress radiography is recommended to identify isolated or combined posterior cruciate ligament (PCL) deficiencies. The posterior drawer in internal (IR) or external rotation (ER) helps to differentiate between these combined instabilities. The purpose of this study was to evaluate posterior stress radiography (PSR) in isolated and combined PCL deficiency with IR and ER compared to PSR in neutral rotation (NR) for diagnosing combined PCL instabilities. METHODS Six paired fresh-frozen human cadaveric legs (n = 12) were mounted in a Telos device for PSR. The tibia was rotated using an attached foot apparatus capable of rotating the foot 30° internally and externally. A posterior tibial load of 15 kp (147.1 N) was applied to the tibial tubercle at 90° knee flexion, and a lateral radiograph was obtained. This was repeated with the foot in 30° IR and ER. The PCL, posterolateral complex (PLC), and posteromedial complex (PMC) were sectioned in six knees, while the PMC was sectioned before the PLC in the other six knees. Posterior tibial displacement (PTD) was measured radiographically. Statistical analysis was performed using a two-way ANOVA and a mixed model with Bonferroni correction, and the significance was set at p < 0.05. Furthermore, intra- and interobserver reliability was determined. RESULTS Cutting the PCL significantly increased the radiographic PTD by 9.8 ± 1.8 mm (side-to-side difference compared to the intact state of the knee, n = 12; p < 0.001). This further increased to 12.2 ± 2.3 mm (n = 6; p < 0.01) with an additional PLC deficiency and to 15.4 ± 3.4 mm (n = 6; p < 0.05) with an additional PMC deficiency. A combined PLC and PMC deficiency resulted in an increase of the PTD to 15.9 ± 4.5 mm (n = 12; p < 0.01). In the PCL/PLC deficient state, ER did not demonstrate a higher PTD, compared to the NR and IR posterior drawer. In the PCL/PMC deficient state in IR, PTD was 1.6 ± 0.7 mm (p < 0.01) higher compared to NR and 3.2 ± 1.9 mm (p < 0.05) higher compared to ER. We showed excellent intra- and interobserver reliability (0.987-0.997). CONCLUSION Combined PCL instabilities resulted in a significant increase in posterior tibial displacement in posterior stress radiographs. However, PSR in IR or ER was unable to differentiate between these combined instabilities. Based on our data, additional stress radiographs in rotation are unlikely to provide any diagnostic benefit in the clinical setting. LEVEL OF EVIDENCE There is no level of evidence as this study was an experimental laboratory study.