Should We Always Use a Metaphyseal Cone in Conjunction With Tibial Augments for Uncontained Defects? A Finite-element Biomechanical Analysis.

Abstract

BACKGROUND Choosing the appropriate implants for reconstruction in revision TKA is essential for long-term fixation. While cones and augments are routinely utilized to address tibial defects, the effect of augment location and size on the biomechanical stability of revision TKA constructs and the indications for the use of metaphyseal cones are not known. QUESTIONS/PURPOSES Is the risk of cement-implant debonding of revision TKA constructs impacted by the thickness and location (medial versus bicompartmental) of tibial augments and the presence of metaphyseal cones during (1) a demanding daily activity like stair ascent and (2) torsional loads? METHODS Under institutional review board approval, we developed patient-specific finite-element models of revision TKA from four patients (three males and one female, ages 50 to 80 years, BMI 27 to 37 kg/m2) who underwent two-stage revision and had a CT scan with no metal artifact after first-stage implant removal. For each patient, we created 5-mm, 10-mm, and 15-mm-thick medial and bicompartmental uncontained defects. We considered two situations for the metaphysis: using a metaphyseal cementless cone into which the implant was cemented or using only cement to fill the metaphyseal cavity. To answer our first question, we conducted finite-element simulations of the immediate postoperative loading scenario representative of stair ascent, while to answer our second research question, we considered an idealized torsional test consisting of 100 N of axial load and twice the axial moment experienced at the same instant of stair ascent. We calculated the risk of cement-implant debonding from an interfacial failure function (calculated as a function of the normal and shear stresses at the cement-implant interface) wherein values of interfacial failure ≥ 1 indicate debonding. Our primary outcome was the cement-implant interface area with ≥ 10% risk of debonding, which we considered to be the interface area with greater than minimal risk of debonding. RESULTS During stair ascent, we computed a decrease of the cement-implant interface area with greater than minimal risk of debonding (that is, ≥ 10% risk of debonding) with medial uncontained defects (median [IQR] 2.6% [1.4% to 3.7%] with 15-mm augment) but not with bicompartmental defects (5.2% [3.7% to 5.3%]) compared with the scenario with no uncontained defect (5.2% [3.9% to 5.9%]). Compared with using a metaphyseal cone, using cement alone in the metaphysis increased the interfacial area with greater than minimal risk of debonding, reaching a median (IQR) of 13.8% (11.4% to 14.3%) with a 15-mm bicompartmental defect. Under the torsional load scenario, the increase in the area with greater than minimal risk of debonding was small for medial defects, from a median (IQR) of 4.3% (2.5% to 5.3%) to 4.9% (3.9% to 6.2%) when using a metaphyseal cone and from 7.0% (4.0% to 9.5%) to 7.2% (6.1% to 9.8%) when only using cement in the metaphysis. However, the area at risk of failure of bicompartmental defects under torsional loads reached 23% when using a metaphyseal cone and 52% when using only cement in the metaphysis. CONCLUSION The size of bicompartmental uncontained defects treated with a bicompartmental augment (a full block) negatively affected the overall construct stability in our finite-element model. However, medial defects of the same size did not negatively influence the stability of the construct when addressed with an augment perfectly contacting the bone. In our computational finite-element model, using metaphyseal cones increased the stability of the revision TKA construct. CLINICAL RELEVANCE Our finite-element results suggest that medial augments have little impact on the stability of revision TKA constructs, but clinicians may want to combine bicompartmental augments with cones for increased stability of revision TKA constructs.