Thermal and non-thermal effects off capacitive-resistive electric transfer application on the Achilles tendon and musculotendinous junction of the gastrocnemius muscle: a cadaveric study

Our objective was to analyze the thermal behavior and transmission of electric current in the Achilles tendon and the musculotendinous junction of the gastrocnemius muscle with different CRet protocols, using non-living specimens and invasive temperature measurements.

The study material included 5 fresh frozen cadavers: 4 male and 1 female (10 legs). The age range at the time of death was 60–80 years (mean 69.80 ± 6.04). The bodies were stored at 3 °C and brought to room temperature a day before the test to make it stable. The basal superficial, Achilles tendon and musculotendinous junction temperatures were measured prior to any intervention to ensure the same starting values. None of the cadaveric specimens used for this study had evidence of traumatic injuries or surgical scars on the lower limbs.

To better understand the temperature behavior and passage of current in conditions similar to rehabilitative treatments, we applied a power limit similar to that typically applied with a T-Plus device during real-life treatments. This was based on the power level, which is easily identifiable and controllable by the therapist during therapy, and the watts (absorbed power) shown by the device during the therapy.

The power range of a very large T-Plus device ranges from 1 to 300 watts in resistive and from 1 to 450 Volt-Ampere (VA) in capacitive mode.

Two thresholds were identified: high power and low power, based on the real powers that the therapist typically applies when she/he wants to generate a thermal or non-thermal reaction. On this basis, high-power thresholds were set at 90VA in capacitive mode (HPC) and 60 watts in resistive mode (HPR), while low-power thresholds were set at 20VA in capacitive mode (LPC) and 10 watts in resistive mode (LPR). In real-life use, on average, thresholds of 10 watts and 20 VA respect the limit of 0.3 A, while applications at 60 watts and 90 VA are widely-used for a thermal effect.

The 4 interventions (capacitive and resistive mode; low- and high-power) were performed for 5 min each, by a physiotherapist with experience in the use of T-Plus. Dynamic movements similar to those used with real patients were performed with constant pressure to the posterior region of the heel (Fig. 1).

Cadavers were placed in the prone position. Hips were placed in neutral rotation, with the knee in 30° of flexion, and a thermoplastic splint maintained the ankle joint position. The skin was cleaned with chlorhexidine-isopropyl alcohol [23].

The order of the 4 treatment protocols and the specimen (leg side) were both randomized generating a pre-listing through Random.org. by one of the researchers not involved in the recruitment. The temperature generated in the specimen was allowed to return to normal before the next application.

All instrumentation received a calibration certificate prior to the study. Thermocouples “Hart Scientific PT25 5628-15” were used to measure the musculotendinous junction and Achilles tendon temperature, and a digital thermometer “Thermocomed” was used to measure the superficial temperature of the calf (Fig. 2a). Thermocouples were placed under ultrasound guidance “US Aloka Prosound C3 15.4”, with a high-frequency linear transducer (USTTL01, 12 L5), by a researcher expert in the use of the instrument (Fig. 2b). One thermocouple was placed in the middle of the Achilles tendon and the other in the musculotendinous junction (Fig. 2c).

This protocol increases the superficial temperature with a small increase in Achilles tendon and musculotendinous junction temperature. However, despite the small thermal effect, we observed a current flow (0.039 A ± 0.02): this has been associated with cell proliferation in deep structures [14, 16]. This protocol could hypothetically be interesting in acute inflammatory Achilles tendinopathy or acute muscle strain in which it is important to increase cell proliferation [14, 16] and tissue reconstruction without increasing temperature too much [28].

This protocol is similar to the LPC; however, we can see that it has a lower superficial thermal effect, a greater thermal effect at the Achilles tendon and a similar effect at the musculotendinous junction. LPR has a greater current flow (0.063 A ± 0.02) than the LPC, which suggests it may be better at generating cell proliferation [14, 16]. This protocol could be useful in acute inflammatory Achilles tendinopathy or acute muscle strain in which it is important to improve cell proliferation [14, 16] and tissue reconstruction without increasing temperature too much [28]. Previous studies have reported good clinical results with a combination of capacitive and resistive modes [9, 22].

With this protocol, we found an increase in the thermal effect at all depths, especially superficial. In addition, we observed a high current flow (0.095 A ± 0.03), which is associated with a cell proliferation effect [14, 16]. This protocol may be useful in more chronic phases in which the main objective is to improve the viscoelasticity of tissues, especially in chronic tendinopathies or in fibrous scars after sprains such as “tennis leg” [28‐30].

This protocol achieved the greatest temperature increase at the Achilles tendon and musculotendinous junction. It also registered the highest current flow (0.120 A ± 0.03), which is associated with a cell proliferation effect [14, 16]. This protocol has a greater effect on deeper structures than HPC and could be combined with it to generate further increase in superficial temperature. It could be interesting to combine them if you want to work on chronic superficial and deep pathological structures of the same region [9, 22]. These thermal and current effects may generate mechanical effects on the viscoelastic properties of the structures, which are mainly related with chronic tendinopathies or fibrous scars after sprains [28‐30].

As this was a cadaveric study, in which the bodies did not have thermoregulatory blood circulation, it is possible that the effects in living subjects may be minor. It is likely that the living population would not have such a large temperature increase, as circulating blood dissipates heat toward adjacent areas, maintaining the temperature of the treated structures within the desired limits. This process avoids unwanted hyperthermia in nearby tissues, as well as excessive heat during treatment, which can be enough to cause a skin burn [13]. In this type of treatment, patient feedback is important; clearly in this study that was impossible. The temperature increase recorded in this study is probably higher that which would occur in living subjects. In addition, despite being fresh corpses, it is very likely that the muscular properties were not the same as those of living subjects and the average age of the corpses is considerably high compared to the average age of the patients who suffer injuries in this region. However, using body donors allowed us to measure the deep temperature, at the Achilles tendon and musculotendinous junction and make hypotheses about what happens when we apply these treatments in living real patients.