Quantitative Evaluation of the Thermal Heterogeneity on the Surface of Cryotherapy Cooling Pads

Cryotherapy is frequently prescribed following orthopedic surgeries and by physical therapists and athletic trainers to reduce inflammation, swelling, pain, secondary hypoxic injury, and bleeding and to allow early mobilization in both acute and chronic phases of soft tissue injury [1,2]. However, in contrast to its therapeutic benefits, cryotherapy also presents a risk factor for causing ischemic-induced injuries such as nerve palsy [3–5] and tissue necrosis [6,7]. Furthermore, no single thermal application cryotherapy protocol has been identified that provides an optimum risk/benefit ratio, resulting in a broad spectrum of cryotherapy practices reported in the literature [8]. Cryotherapy protocols are generally defined in terms of temperature and duration of application and may be run either continuously or with temporal modulation. Recommendations for the duration of application vary from minutes [9,10] to days [11–13]. It is claimed that skin temperature must drop below 13.6 °C (56.5 °F) to realize the analgesic effect of cryotherapy and that a 50% reduction in tissue metabolism requires tissue temperatures in the range of 10–11 °C (50–51.8 °F) [14]. Moreover, the ability of hemoglobin to release bound oxygen strongly relates to temperature, and below about 12 °C (53.6 °F), the residence time of blood flowing through capillaries is shorter than the time constant for oxygen dissociation [15,16]. This condition could result in the creation of tissue hypoxia that may also be exacerbated by ischemia. In summary, the level of temperature produced by a CTU system may affect multiple aspects of the procedure outcome. The components of a CTU system typically consist of an insulated chest that is filled with ice and water, a submersible pump, a cooling pad that is applied to the treatment area, and insulated tubing that connects the pump to the pad in a closed ice water circulation loop. The cooling pad generally embodies a flexible polymeric bladder with molded conduits through which water is pumped via inlet and outlet ports. Cooling pads employ distinct patterns for directing water flow through the pad. These flow patterns are reflected on the pad surface as areas with higher and lower temperatures due to the low thermal conductivity of the pads providing only minimal lateral heat transfer. Manufacturers provide user instructions that commonly relate to a target operating temperature but include a paucity of specific implementation guidelines. Alternatively, the responsibility for defining a thermal treatment protocol may be assigned to health care providers. The following are examples of temperature-related instructions provided with CTUs. DeRoyal recommends a target temperature for water flowing to the pad within the range of 7.2–15.6 °C (45–60 °F) [17], with specific operating temperatures of 7.2 ± 2.8 °C (45 ± 5 °F) and 6.1–18.3 °C (43–65 °F) for models T505 and T600 [18], respectively. Game Ready [19] claims that the greatest benefit from cryotherapy takes place at 4.5–15.5 °C (40–60 °F). Bledsoe [20,21] states that their devices are designed for the temperature of water circulating within the pad to remain at or above 4.4 °C (40 °F) with other specific recommendations for particular treatment sites, the highest being 10–15 °C (50–59 °F) for the ankle and the lowest 7–12 °C (44.6–53.6 °F) for the back, knee, and shoulder [21]. DonJoy warns against using their CTU with a water temperature lower than 5 °C (40 °F) [22]. Breg specifies different temperature ranges depending on the protocol. The temperature range of cooling water for continuous long term operation is 7.2 °C–12.8 °C (45 °F–55 °F), and the temperature may be below 7.2 °C (45 °F) for intermittent exposures of 20 min or less [23]. EBIce only stipulates that a licensed health care practitioner must prescribe the appropriate treatment details [24] while warning that cooling pads may have wide temperature variation and that cold spots create the potential for tissue damage, nerve palsy, or other vasospastic conditions while hot spots may create areas of lessened therapeutic effectiveness. The therapeutic effect of cryotherapy is temperature driven, and our experiments show that temperature may vary greatly over a single pad and also among different pad/CTU systems. The goal of this study is to measure and characterize quantitatively the two dimensional temperature profiles produced on the surface of various cooling pads when exposed to ambient air during normal CTU operating protocols. Our hypothesis is that temperature variation exists across the surface of cryotherapy cooling pad and that this variation occurs as a function of the water flow pattern through the pad.