Aims and objectives:

It is known that posterior tibial translation (PTT) in stress radiographs will increase with a combined posterior cruciate ligament (PCL) and posterolateral (PLC) lesion. However, there is no evidence on combined PCL and posteromedial lesions (PMC) and how the PTT will change, when the foot is placed in external (ER) or internal rotation (IR). Thus, the goal of the present study was to evaluate the PTT using stress radiographs in combined PCL and posterolateral/posteromedial instability.
It was hypothesized that (1) a combined PCL and PLC/PMC injury leads to increased PTT in stress radiographs and (2) that IR and ER will alter the PTT measured in the stress radiograph.

Materials And Methods

6 paired fresh frozen human cadaveric legs (n=12) were mounted in a radiographic posterior drawer device GA-III/E (telos GmbH, Woelfersheim-Berstadt, Germany). The rotation was simulated using an attached rig which was capable of rotating the foot 30° internally and externally. The x-ray source was mounted 1m above the laterally aligned specimen and the detector was positioned under the specimen, simulating a clinical stress radiograph setup. PTT was then performed in the intact specimen with application of 15kp (147.1N) to the tibial tubercle in 90° knee flexion. This was repeated with the foot placed in 30° internal and external rotation. The PCL, the PLC (lateral collateral ligament and popliteofibular ligament) and the PMC (medial collateral ligament and posterior oblique ligament) were cut consecutively in 6 knees, whereas the PMC was cut before PLC in the other 6 knees. PTT was radiographically measured using the midpoint between the posterior lateral and medial femoral and tibial condyle parallel to the tibial plateau using Horos Viewer 3.3.6 (Horos Project, Annapolis, MD, USA). Statistical analysis was performed using a 2-way repeated measurements ANOVA

Results

The PTT significantly increased from 3.9±1.5mm to 13.3±3.1mm, when the PCL was cut. This further increased to 15.3±2.7mm and 15.1±3.1mm (p< 0.05) after additionally creating a combined PCL + PLC or PCL + PMC lesion, respectively. After completely destabilizing the knee (PCL + PLC + PMC) the PTT increased to 17.8±3.5mm (p< 0.05), respectively.
The posterior drawer in ER did not lead to a significant change of the PTT in all of the tested structures, whereas IR led to a slightly higher PTT, which was not significant (n.s.) compared to the neutral posterior drawer.

Conclusion

In PCL stress radiographs a PTT of more than 10 mm side to side difference should be suspicious to a combined PCL and PLC or PMC injury. Furthermore, a PTT of more than 14mm side to side difference indicates a combined PCL with PLC and PMC injury. Even though the posterior drawer in either IR or ER is a valid clinical tool in order to diagnose an additional PLC or PMC injury, the PTT in stress radiographs did not change significantly, when the foot was placed in IR or ER. Based on these results there is no added value in stress radiographs using rotational posterior drawer compared to the neutral position.