The apparent critical isotherm for cryoinsult-induced osteonecrotic lesions in emu femoral heads

Abstract

Cryoinsult-induced osteonecrosis (ON) in the emu femoral head provides a unique opportunity to systematically explore the pathogenesis of ON in an animal model that progresses to human-like femoral head collapse. Among the various characteristics of cryoinsult, the maximally cold temperature attained is one plausible determinant of tissue necrosis. To identify the critical isotherm required to induce development of ON in the cancellous bone of the emu femoral head, a thermal finite element (FE) model of intraoperative cryoinsults was developed. Thermal material property values of emu cancellous bone were estimated from FE simulations of cryoinsult to emu cadaver femora, by varying model properties until the FE-generated temperatures matched corresponding thermocouple measurements. The resulting FE model, with emu bone-specific thermal properties augmented to include blood flow effects, was then used to study intraoperatively performed in vivo cryoinsults. Comparisons of minimum temperatures attained at FE nodes corresponding to the three-dimensional histologically apparent boundary of the region of ON were made for six experimental cryoinsults. Series-wide, a critical isotherm of 3.5 degrees C best corresponded to the boundary of the osteonecrotic lesions.

Figure 1

A.) Two-dimensional (axisymmetric) mesh used for finite element analysis. The geometry of the probe (see inset for detail) reflects the geometry of the outer portion of the cryoprobe used intraoperatively. Ultem is a low-thermal-conductivity polymer used to insulate the stainless steel (SS316) tip from the actively heated shaft. The epoxy and the Teflon tubing seal the cryoprobe. During intraoperative simulations, the area around the probe marked as air was modeled as blood. B.) Schematic of thermocouple arrangement during bench top experimental cryoinsult. Thermocouples glued to Ultem rods were positioned with their junctions near the tip of the cryoprobe. Following testing, the locations of the external thermocouples were measured with respect to the tip of the cryoprobe. The Ultem mounting rods were not represented in the FE model.

Figure 2

Illustrative example (for a single thermocouple channel) of the effects of changing emu bone thermal properties on the agreement between FE generated and experimentally generated temperature information. Because specific heat is treated as a temperature-dependent property in the model, the above values for specific heat of emu cancellous bone (c_b) are averages over the range of temperatures defined in the model. The center image shows the best agreement between measured and FE generated temperatures. The right and left images show the effects of a specific heat value that is too low (left) or too high (right).

Figure 3

Fluoroscope image of intraoperative cryoprobe positioning (left). The depth of the femoral head within the emu body required imaging through large amounts of soft tissue, thereby degrading image clarity. Because it is difficult to discern the outline of the emu femur, it has been traced on the right.

Figure 4

Illustration of the 3D viability mapping assembly. Two-dimensional maps of osteocyte viability were stacked in 3D to get a volumetric data set of osteocyte viability. The lesion (shown in dark) edge was determined relative to the drill tract (white space in the center of the necrotic lesion).

Figure 5

Identification of FE nodes corresponding to the edge of a necrotic lesion. Prior to identification of the nodes, the boundary of the necrotic lesion was rotated and translated so as to be aligned with the distal end of the probe tip.

Figure 6

Representative comparisons between four different thermocouple-measured temperatures and the corresponding FE temperatures. While there was very good agreement for the close-to-probe site (A), FE estimated temperatures at other more remote thermocouple locations had some larger disparities (C). Series-wide, the maximum instantaneous error in any one of the channels in any of the FE runs was 4.3°C. Maximum errors in all of the other FE-thermocouple pairs (13 pairs) were always less than 4.3°C, and the instantaneous errors over the freeze and thaw averaged 0.51°C (about 4.5% of the total temperature change).

Figure 7

Prediction of lesion boundary in the −30°C trial by identification of the critical isotherm at the nodes in the FE model. The nodes most closely corresponding to the histological lesion boundary are circled in black, and the nodes identified by the critical temperature are circled in gray. The lesion boundary is a 4^th order polynomial fit to nodes with the critical isotherm.