Patient-specific finite element analysis of chronic contact stress exposure after intraarticular fracture of the tibial plafond

Abstract

The role of altered contact mechanics in the pathogenesis of posttraumatic osteoarthritis (PTOA) following intraarticular fracture remains poorly understood. One proposed etiology is that residual incongruities lead to altered joint contact stresses that, over time, predispose to PTOA. Prevailing joint contact stresses following surgical fracture reduction were quantified in this study using patient-specific contact finite element (FE) analysis. FE models were created for 11 ankle pairs from tibial plafond fracture patients. Both (reduced) fractured ankles and their intact contralaterals were modeled. A sequence of 13 loading instances was used to simulate the stance phase of gait. Contact stresses were summed across loadings in the simulation, weighted by resident time in the gait cycle. This chronic exposure measure, a metric of degeneration propensity, was then compared between intact and fractured ankle pairs. Intact ankles had lower peak contact stress exposures that were more uniform and centrally located. The series-average peak contact stress elevation for fractured ankles was 38% (p = 0.0015; peak elevation was 82%). Fractured ankles had less area with low contact stress exposure than intact ankles and a greater area with high exposure. Chronic contact stress overexposures (stresses exceeding a damage threshold) ranged from near zero to a high of 18 times the matched intact value. The patient-specific FE models represent substantial progress toward elucidating the relationship between altered contact stresses and the outcome of patients treated for intraarticular fractures.

Figure 1

A depiction of the process for generating patient-specific models. Inferior views of intact and fractured (reduced) source CT images, raw tibial bone surfaces from CT, smoothed tibial bone surfaces, and FE meshes of the cartilage volumes. (NOTE: these are for patient #4.)

Figure 2

Antero-superior (subchondral) view of the contact stress distributions of the 13 instants of the stance phase of gait for the intact and fractured ankles of patient #4.

Figure 3

Antero-inferior view of the patient-specific tibial articulating surfaces for intact and (reduced) fractured ankles of all 11 patients. Patient and fracture characteristics were as shown.

Figure 4

Inferior view of the contact stress exposure distribution on the tibial articulating surfaces for the intact and (reduced) fractured ankles of all 11 patients.

Figure 5

Series-wide average, area engagement histograms for the intact and fractured ankles.

Figure 6

Plot of peak contact stress over-exposure (damage threshold P_d = 6 MPa – see Eqn. [1]) for the intact and (reduced) fractured ankles of 11 patients.