ANIMAL MODELS

TEMPERATURE FIELDS SURROUNDING A NEW INSTRUMENT FOR A CRYOGENICALLY- INDUCED OSTEONECROSIS ANIMAL MODEL

Reed, KL¹ Brown, TD¹, Conzemius, MG^2
1University of Iowa Orthopaedic Biomechanics Lab, 2181 Westlawn, Iowa City, IA 52242
²Veterinary Teaching Hospital, Iowa State University, Ames, IA 50014

INTRODUCTION: Various animals have been used as models for osteonecrosis, including dogs, rabbits, and sheep. While these species have exhibited various histological attributes of the condition, none progresses to femoral head collapse. The emu (Dromaius novaehollandiae) appears to overcome that difficulty³. The liquid nitrogen spray used in piloting that model achieved lesion collapse, but involved a larger than desired necrotic region.

MATERIALS AND METHODS: A 3 mm-diameter cryogenic probe has been designed for better freeze control. It contains a small tube within its outer tube to allow liquid nitrogen circulation and exhaust. The probe and 5 copper-constantan thermocouples at sites 0,1,2,5, and 8 mm from the probe tip were embedded in a 1% agarose block. Liquid nitrogen was circulated through the probe and temperatures recorded (Figure 1).

Figure 1: Temperature as a function of time

The probe's heat removal rate⁴ q_r was calculated with: k/q_r,=[l/r₁-l/r₂]/4T where k is thermal conductivity, and r₁ and r₂ are radii of two points having a temperature differential T.

RESULTS AND DISCUSSION: For cancellous bone₂, k is approximately 0.35 W*deg/m, similar to that of water₁ (k==0.34). Assuming k=0.35 for

(water based) agarose gel, the probe's heat removal rate, q_r, averaged 458.9 Watts (s.d. 376.3), The theoretical radius of the corresponding visual ice ball at steady state, 6 mm (Figure 2), agreed closely with experimental observation.

Figure 2: Steady-state temperature as a function of radial distance

REFERENCE:

1. Avallone, E.A. et al, Mark's Standard Handbook for Mechanical Engineers McGraw Hill 1996
2. Biyikii, S. et al J Biomed. Mater. Res.20(9) p. 1335-45 1986
3. Conzemius et al ORS 2000
4. Incropera et al. Fundamentals of Heat and Mass Transfer John Wiley & Sons, Inc. 1996