Mechanisms of Cryonecrosis

Freezing mammalian tissue results in direct and indirect cell destruction. Direct cell injury occurs by formation of intracellular and extracellular ice crystals, which destroy cell walls and cause intracellular dehydration, respectively. Intracellular dehydration causes severe electrolyte concentration and pH shifts, which damage lipoprotein membranes and organelles. Loss of cellular homeostasis results in cell death. Indirect cell injury occurs by damage to the endothelium of arterioles and venules, causing increased vascular permeability, edema, and hemoconcentration. Local tissue damage occurs from thrombosis and infarction of small vessels. Two rapid-freeze, slow-thaw cycles are used. Rapid freezing maximizes intracellular crystal formation and crystal size. Slow thawing causes additional cell damage by a process of recrystallization, during which time crystals increase in size before melting. Precooled tissue freezes faster than normal tissue; therefore a second freeze-thaw cycle optimizes the processes of tissue destruction.

Because cryotherapy causes tissue destruction in situ, fibrous structures such as epineurium remain intact. This allows for regeneration of large myelinated nerves. Experimental percutaneous cryotherapy of equine palmar and plantar digital nerves resulted in neuropraxia (temporary ablation of nerve function, with the axons remaining intact) or axonotmesis (wallerian degeneration of axonal tissue, with the fibrous supportive tissue remaining intact). All nerves regenerated.⁵ The degree of nerve damage is temperature dependent, with lower temperatures resulting in longer duration of analgesia.⁶

Presumably, percutaneous cryotherapy causes destruction of local type C nerve fibers. Type C fibers are small, unmyelinated, nociceptive nerve fibers that contribute to chronic pain. The inflammatory response to cryotherapy appears to cause thickening and fibrosis of certain soft tissues, such as the suspensory ligament and subcutaneous tissues, and although this effect has not been studied, the clinical result is a stronger ligament and one less likely to be reinjured in the same location. Cryogens must be used judiciously on the distal aspect of the limb to avoid cryonecrosis of tendon, ligament, joint tissues, or cortical bone.

Basic Technique

Cryotherapy instruments and cryogens are described elsewhere, and only basic principles are discussed here.^7-9 Instruments used to apply cryogens vary from simple cryoprobes to cryounits with continuous closed-system flow of cryogen liquid. We use liquid nitrogen that is stored in a commercial 10- to 20-L tank. For all techniques we use individual brass probes, 1.5 cm in diameter (Figure 89-1), precooled in liquid nitrogen and positioned on the lesions for a double freeze-thaw cycle consisting of freezing (60 seconds), thawing (60 seconds), and freezing (60 seconds). Local edema formation is minimal and has no effect on the second freezing cycle. Earlier work suggested that a thaw duration of 15 seconds was optimal, but our modified cycle appears effective.⁴ We prefer to use solid metal probes, because consistent freezing to a specific depth is easy to control (Figure 89-2). The cooling ability of solid metal probes has been questioned, but in one study solid metal probes were most effective in freezing to specific depths.⁷

Fig. 89-1 Solid brass cryoprobes commonly used for cryotherapy in the horse.

Fig. 89-2 Cryotherapy is being used to manage pain associated with an exostosis of the fourth metatarsal bone in a Standardbred racehorse. A double freeze-thaw cycle has been used in one site, and the probe is being applied at a distal site.