Introduction

Posterolateral impression fractures of the tibial plateau seen as depression of the articular surface with breach of the cortical bone are common concomitant injuries of an ACL rupture. They are caused by the mechanism of high-energy pivot-shift trauma, in which the lateral tibial plateau subluxate anteriorly, resulting in a violent collision of the anterolateral aspect of the lateral femoral condyle and posterolateral area of the tibial plateau.

Aim

The aim of this biomechanical cadaver study was to evaluate the destabilizing effect of high-grade posterolateral tibia plateau fractures on kinematics of the ACL-deficient joint.

Material And Methods

8 human fresh-frozen cadaveric knees were tested using a six-degree of freedom robotic setup (KR 125, KUKA Robotics, Augsburg, Germany) with an attached optical tracking system (Optotrack Certus Motion Capture, Northern Digital, Ontario, Canada). After the passive path from 0-90° was established, simulated Lachman test and pivot-shift test as well external rotation (ER, 4Nm) and internal rotation (IR, 4Nm) were applied at 0°, 30°, 60°, and 90° of joint flexion under constant 200 N axial loading. All the parameters were initially tested in the intact and the ACL-deficient state, followed by 2 different types of posterolateral impression fractures in the ACL-deficient knees. The dislocation height was 10 mm and the width 15 mm constantly in both groups. The intraarticular depth of the fracture corresponded to the half of the width of the posterior horn of the lateral meniscus in the first group (Bankart 1), and 100% of the meniscus width in the second group, marked Bankart 2. A two-way repeated measures ANOVA with post-hoc Bonferroni corrections for multiple comparisons was performed to evaluate the effect of different states over different angles on tibial translation. The significance level was set to 0.05.

Results

There was a significant decrease in knee stability after both types of additional posterolateral tibial plateau fractures in the ACL-deficient specimens with increased anterior translation following the simulated Lachman test in up to 30° of knee flexion (p < 0.05). The same effect was recorded in regards to the anterolateral rotational (ALR) instability. Simulated Pivot shift test in near extension position (0° and 30°), as well as IR test (0° to 30°), showed significantly increased instability in both types of additional fractures in ACL-deprived state (p < 0.05). Following ER and posterior drawer test, ACL deficiency and concomitant fractures did not influence knee kinematics.

Conclusion

The results of this study demonstrate that high-grade impression fractures of the posterolateral aspect of the tibial plateau increase the instability of the ACL-deficient knees and result in a considerable increase in translational and ALR instability. Comparably to the shoulder with its integrity of the labrum and the glenoid as his bony ground, the posterolateral tibia plateau impression fracture seems to work like a bony Bankart lesion which results in instability of the joint by destabilizing the posterior horn of the lateral meniscus.
Considering their potential effects on the kinematics of the ACL-deprived knees, these fractures should be taken into consideration in the overall treatment decision-making process.