Addition of a short central extension to surface cemented tibial trays in primary TKA: An in vitro study of the effect on initial fixation stability and its relationship to supporting bone density

Background Short central extensions which do not enter the tibial medullary canal are incorporated to cemented tibial components to increase initial stability in primary total knee arthroplasty. Their role in tibiae of differing preoperative mechanical quality has been little studied. Methods Twelve embalmed cadaveric tibiae were paired and divided into two groups, receiving a similar cemented tibial component with or without a non-cemented short central extension (10 mm diameter, 35 mm length). The specimens were subjected to 6000 cycles of a medially applied 1350 N load. Relative bone–tray displacements were measured and the evolution of inducible and permanent micromotions were computed. The apparent density of the cancellous bone under the tibial tray and at the area to support the extension was computed from computed tomography images of each specimen. Findings No significant differences between groups were detected for any parameters. For the group with extension, a significant negative linear correlation (P = 0.009, r2 = 0.849) was found between the inducible tilt of the tray and the bone density at the zone of the extension. Also a trend towards a negative linear relation (P = 0.07, r2 = 0.59) was observed for the same group between maximum subsidence and apparent density at the zone of the extension. Interpretation The study did not find that the addition of a non-cemented short central extension provides any overall improvement of the initial fixation stability. However, it was found that short extensions may enhance tilting stiffness of the bone–implant construct if bone of sufficient mechanical quality is located around its supporting area.