Evaluation of tibial hamstring graft fixation

Following replacement of the graft, the robot returned under position control to exactly the same positions in order to determine the in situ forces. Since the motions of the joint to be tested were identical before transection of the anterior cruciate ligament (ACL) and after insertion of the graft, it was possible to apply the principle of superposition in which only one of the factors being tested is changed. Applying this principle, it was possible to measure the vector difference of the forces and thus determine the in situ force in the ACL. In force-controlled mode, on the other hand, a data set specifies the joint forces and moments and the resulting motion is recorded using the coordinate system of the software. Since, in this mode, the system can apply identical external forces and moments to a knee in both the intact and transected states, it was possible to determine and compare the differences in the kinematics for both states.

• KUKA KR 125 robot
• PC-based KUKA robot controller, including control panel with familiar Windows interface
• Universal force-moment sensor
• Two aluminum cylinders as a mounting fixture, in which the knees were embedded on both sides with bone cement. The tibial cylinder was connected to the end effector of the robot and the universal force-moment sensor. The femoral cylinder was fastened in a clamp at the base of the robot.
• Robot programming
• Start-up

• The normal kinematics of the knee can be restored using both methods
  
  

  The choice of attachment site for the graft fixation at the tibia had a significant influence on the stability of the knee and the in situ forces of the knee ligament graft. Both juxtaarticular fixation techniques gave rise to in situ forces closely approximating those of the native ACL, and ensured greater stability of the knee than the abarticular technique. There was no significant difference, however, between the two juxtaarticular, anatomic techniques using cross pins and interference screws.

  This means that both the kinematics in the primary stability and the initial fixation strength of the double cross pin technique are comparable to those of the interference screw fixation. The stiffness is similar to that of the native ACL. Anatomic knee ligament graft fixation at the tibia using two bioabsorbable pins can thus be considered as an alternative to an anatomic interference screw fixation. In other words, the normal kinematics of the knee can be restored using both methods.
  
  

• Efficient combination of jointed-arm robot and force-moment sensor
  
  Moreover, the study also showed that the combination of jointed-arm robot and force-moment sensor is ideal for use in research of this kind. There is thus every reason to expect six-axis jointed-arm robots to find even more applications in the field of biomechanical testing. Possible examples include testing of the shoulder, ankle and spine. One economically viable application for this technology would be the testing of implants, such as endoprotheses.