Effects of screw types in cementless fixation of tibial tray implants: stability and strength assessment

Objective. To test the stability and strength of tibial trays fixed with cortical screws and tibial trays fixed with cancellous screws under axial and anterior–posterior shear loading conditions. Design. Five matched human fresh frozen cadaver knees without prior pathology were used as paired specimens for comparison of cortical to cancellous screw fixation of tibial trays. Background. Current cementless fixation of tibial trays in total knee arthroplasty is obtained by using a combination of stems, keels, posts, pegs, and screws. To date there has been no study comparing isolated cortical and cancellous screw fixation in cementless arthroplasty. Methods. An Instron machine and a video digitizing system were used to examine tibial tray lift-off, subsidence, and anterior–posterior load to failure at the bone–implant interface in five matched pairs of knees and compare the difference between cancellous and cortical screw fixation. Results. Non-destructive cyclic testing showed a significant difference between the two types of screws for tibial tray subsidence with cancellous fixation greater than cortical (P < 0.05), but not for lift-off (P > 0.05). Load to failure testing in anterior–posterior direction did not result in a significant difference between the two types of screws for initial stiffness and ultimate load (P > 0.05), but did result in a significant difference of energy absorbed to failure (P < 0.01). Conclusions. The difference in amount of subsidence between cortical and cancellous fixation could possibly be due to microfractures of the underlying trabecular bone with cancellous fixation. Neither screw type was able to completely fix the tibial tray to bone and prevent micromotion. Although the load at failure for anterior–posterior shear was similar for the two screw types, the energy absorbed was greater for the cortical screws which may be a result of additional strength provided by the cortical bone surrounding the screw.