Summary

The design of current metal augmentation differs from the morphology of the proximal tibia, which has a greater tapering angle on the medial, lateral, and posterior surfaces.

Abstract

Background

Metal augmentation is a remarkable option for the treatment of tibial bone defects. However, it is not well known whether the angle of currently available blocks matches the actual shape of the proximal tibia. The aim of this study was to analyze the bony contour of the proximal tibia and measure the angle of the taper for each resection surface to examine whether the angle of currently available metal augmentation blocks matches the actual tibia. It was hypothesized that the angle of currently available metal augmentation blocks did not match the morphology of the actual tibia and that the angle of the taper could be adjusted on the basis of the thickness and the side of the metal augmentation.

Materials And Methods

Computed tomography of the lower extremity was performed on consecutive 100 knees, and three-dimensional images of the tibia were reconstructed. The primary resection level was determined on the basis of a plane 10 mm below the center of the lateral tibial plateau without a posterior slope and perpendicular to the mechanical axis of the tibia. The assumed levels were set to 5, 10, 15, and 20 mm below the primary resection level. The tapering angle was calculated on the basis of the reduction value from the primary level to each resection surface at eight areas and compared with the angle of currently available metal augmentation. The tapering angles of metal augmentation were obtained from the company for three commonly used implants. The tapering angles of the proximal tibial bony morphology were compared with the angle of currently available metal augmentation: NexGen LPS-Flex, Vanguard 360, and Persona (Zimmer-Biomet, Warsaw, IN, USA).

Results

The tapering angle on the medial and lateral sides increased with the more distal resection level, which was up to 30° at the 20-mm level. The tapering angle on the posterior side also increased with the more distal resection level, which was approximately 40° at the 20-mm level. In most of the tibial bony surface, different contour was observed between medial and lateral compartments. The tapering angles of the conventional implants (NexGen and Vanguard 360) were considerably smaller than the tibial bone surface. In Persona, the tapering angle was greater than the conventional implants. However, the tapering angle was still smaller than that of the original tibial morphology.

Conclusions

The design of current metal augmentation differs from the morphology of the proximal tibia, which has a greater tapering angle on the medial, lateral, and posterior surfaces. Surgeons should pay attention to the size mismatch between the femoral and tibial components during revision TKA because the size of the tibial component fitted to the more distal level due to bone defects can be smaller with the current metal augmentation. Asymmetric metal augmentation with a greater tapering angle is required to fit the surface during distal bone resection.