Abstract
INTRODUCTION
Total knee arthroplasty (TKA) is an effective technique to treat end-stage osteoarthritis of the knee. One important goal of the procedure is to restore physiological knee kinematics. However, fluoroscopy studies have consistently shown abnormal knee kinematics after TKA, which may lead to suboptimal clinical outcomes. Posterior slope of the tibial component may significantly impact the knee kinematics after TKA. There is currently no consensus about the most appropriate slope. The goal of the present study was to analyze the impact of different prosthetic slopes on the kinematics of a PCL-preserving TKA. The tested hypothesis was that the knee kinematics will be different for all tested tibial slopes.
MATERIAL
PCL-retaining TKAs (Optetrak Logic CR, Exactech, Gainesville, FL) were performed by fellowship trained orthopedic surgeons on six fresh frozen cadaver with healthy knees and intact PCL. The TKA was implanted using a computer-assisted surgical navigation system (ExactechGPS®, Blue-Ortho, Grenoble, FR). The implanted tibial baseplate was specially designed (figure 1) to allow modifying the posterior slope without repeatedly removing/assembling the tibial insert with varying posterior slopes, avoiding potential damages to the soft-tissue envelope.
METHODS
Knee kinematics was evaluated by performing a passive range of motion (ROM) from full extension to at least 100 degrees of flexion. Passive ROM was repeated three times at each of the 4 posterior slopes selected: 10°, 7°, 4°, and 1° using the adjustable tibial component (figure 1). Respective 3D positioning of femur and tibia implants was recorded by the navigation system. Hip-knee-ankle (HKA) angle, femoro-tibial antero-posterior (AP) translation and internal-external (I/E) rotation were plotted according to the knee flexion angle.
RESULTS
HKA angle (figure 2B): all 4 different tibial slopes induced a physiologic motion curve, and the kinematic differences between 10°, 7°, 4°, and 1° of posterior slope with the native knee were small. All slopes induced a varus angle beyond 60° of flexion, most likely was due to the external rotation of the femoral component. Femoro-tibial AP translation (figure 2C): all 4 different tibial slopes induced a physiologic motion curve and all slopes induced a large posterior translation before 80° of flexion, which was proportional to the slope. I/E rotation (figure 2A): all slopes induced an excessive internal rotation before 60° of flexion.
DISCUSSION
A change in the tibial slope may impact significantly the TKA kinematics. Slopes of 1° and 4° seemed to be the better compromise with the specific implant used. Navigation systems are able to assess the knee kinematics after TKA. The test protocol has been assessed for reproducibility in a separate study with satisfactory results. Changing the tibial slope significantly impacted the TKA kinematics. With the specific implant used, rotational and coronal kinematics was only marginally impacted by the change in tibial slope. AP kinematics was significantly impacted by the change in tibial slope. These changes may be related to a change in the PCL strain. Slopes of 1° and 4° induced the more physiologic compromise.
