Methodological concerns using intra-cortical pins to measure tibiofemoral kinematics.
The complexity of human tibiofemoral joint motion is now better understood with the advancement of new methodologies to measure tibiofemoral kinematics in vivo. Marker clusters anchored to stainless steel bone pins inserted directly into the femur and tibia provide the most sensitive and accurate means for directly measuring skeletal tibiofemoral joint motion. Despite its invasiveness, this technique has been successful, although complications have been reported with the femoral pin and its insertion site. The purpose of this technical report is twofold: to review the difficulties with the femoral pin and its insertion site from a historical perspective, and to identify the load force required from biological tissue to permanently deform the pin. In addition, proposals in the advancement of this method are discussed in the context of reducing impingement with the femoral pin and the Iliotibial band. Because stainless steel exhibits plastic behaviour with no sharp yield point, Apex self-drilling/self-tapping bone pins underwent incremental loading on an Instron materials testing machine. Loads were transmitted perpendicular to the pin with the threads partially exposed and fully secured in vice. Since the accuracy of our combined stereophotogrammetry and Optoelectric motion analysis was less than 0.4 mm, it was decided that plastic deformation occurred after deflections of 0.4 mm. With exposed threads, deflections larger than 0.4 mm were observed at 150 N and 100 N when loads were applied at 15 mm and 20 mm from the vice (representative of where the tissue came in contact with the pin). Loads greater than 200 N produced deflections less than 0.2 mm when threads were fully inserted. The 90 Hz resonant frequency for the marker cluster-bone pin complex is beyond the spectrum of human movement and can be lowpass filtered. To reduce impingement and pin bending, one solution may be to implant pins with a shorter threaded section. By completely penetrating the bone, only the smooth surface of the pin is exposed which is more resistant to bending. Otherwise pins with larger diameters and longer longitudinal incisions about the femoral insertion site are an alternative. Lengthening the longitudinal incisions about the insertion site, and correctly aligning and inserting the femoral pin between the Iliotibial band and quadriceps tendon may diminish impingement. Performing dynamic open chain flexion and extension movements while on the operating table may aid in aligning the pin at the incision site. This may stretch the IT band and quadriceps tendon and may guide the femoral pin into a more optimal position prior to it being inserted into the cortex of the bone.