Asymmetric metaphyseal cones versus medial modular block augment with central cone for medial tibial defects : a biomechanical comparison in varying bone quality.

Abstract

Aims: Managing large medial tibial bone defects during total knee arthroplasty (TKA) remains technically demanding, particularly in elderly patients. This study quantitatively compares two reconstructive strategies: 'bridging' the defect using an asymmetric metaphyseal cone versus 'anatomical reconstruction' employing a symmetrical cone with a metallic block augment, and assesses their sensitivity to osteoporotic bone quality.

Methods: A finite element model of a tibia with a standardized medial defect (Anderson Orthopaedic Research Institute (AORI) Type 2a) was adapted to simulate both configurations under physiological loading cycles in healthy and severe osteoporotic bone. Cumulative interface micromotion was quantified using a coordinate-based post-processing algorithm that decouples normal separation from tangential shear. A parametric sweep of the tangential contact clearance evaluated near-surface biological interaction zones without additional simulations.

Results: In healthy bone, both strategies demonstrated adequate stability with micromotions predominantly below 50 μm. In osteoporotic bone, the asymmetric cone exhibited a loss of stability, characterized by intermittent lift-off during walking and excessive tangential shear during stair descending. This instability induced a toggle effect at the distal stem. The augmented symmetrical cone maintained stable fixation regardless of bone quality. Expanding the tangential contact clearance revealed considerable unrecognized shear risk surrounding the asymmetric cone, while the augmented construct remained insensitive to this parameter in the high-risk range (> 100 μm).

Conclusion: Asymmetric cones are viable in good bone stock but exhibit instability in osteoporosis due to the cantilever effect. Anatomical reconstruction with a modular augment and central cone provides a predictable, load-sharing structural column, making it the preferred strategy for large medial defects in osteoporotic patients.