1 Introduction

Water-filtered Infrared A (wIRA) irradiation is a special form of heat radiation which has high tissue penetration while delivering a low thermal load to the skin surface. wIRA increases tissue temperature, blood flow, oxygen partial pressure, and HbO2 saturation in the treated tissue and may induce further non-thermal cellular effects. The range of applications is correspondingly large due to pain reduction, inhibition of inflammation and exudation, increase in local defense against infection, and regenerative properties. Therefore, wIRA is used to improve the healing of acute and chronic wounds. Even in non-critical wounds, wIRA accelerates the undisturbed “normal” wound healing and attenuates pain. The available evidence for wIRA in acute and chronic wounds is presented.

2 Historical Notes

At the Congress of the International Society on Oxygen Transport to Tissue (ISOTT) in Mainz, Germany, the successful effect of the therapeutic combination of hyperbaric oxygenation and local wIRA on chronic ulcers of the lower leg was presented for the first time by Hoffmann [1]. The concept of treating acute surgical wounds with wIRA was subsequently implemented in a randomized clinical trial, the results from which were published in 2006 [2].

3 Basic Concepts and Mode of Action of wIRA

wIRA is a special form of infrared radiation in the range of 780–1400 nm. The water filter reduces radiation components within the infrared A, as well as most parts of the infrared B and C range, which are associated with undesired thermal side effects of the skin surface. Compared to conventional halogen radiators without water filtering (e.g., “red light lamps”), which emit approximately 50–80% of their radiation in the undesired, skin-damaging infrared B and C range, less than 0.5% of this potentially damaging irradiation is delivered with wIRA radiators. As a special form of infrared irradiation (thermal radiation), wIRA exhibits a high tissue penetration and significantly increased energy input alongside low thermal loads to the skin surface. A distinction is made between thermal (associated with heat energy transfer), temperature-independent (occurring without relevant temperature change), temperature-dependent (occurring with temperature change), and non-thermal (without relevant heat energy transfer) effects [3, 4].

The tissue temperature is increased by ≈6 °C superficially and by ≈2.7 °C at a tissue depth of 2 cm, the oxygen partial pressure is increased by 30% at 2 cm tissue depth, and the tissue perfusion is increased eightfold superficially with a lasting effect up to 5 cm tissue depth [3, 4].

It is well known that pain reduction elicited by wIRA results from both thermal and non-thermal effects. The increased blood flow better eliminates accumulated metabolites (including pain mediators, lactate, bacterial toxins), and metabolism is activated by the elevated tissue temperature (improved metabolism of accumulated substances and regeneration).

Non-thermal effects include direct effects on cells and cellular structures and substances, possibly also on pain receptors (nociceptors). It is known that wIRA causes significant muscle relaxation and associated reduction in pain, an improvement in the quality of life, and reduces the risk of local infections by increasing blood HbO2 saturation and the tissue oxygen partial pressure [3, 4]. Irradiation with visible light (VIS) and wIRA presumably act in conjunction with endogenous protoporphyrin IX (or protoporphyrin IX from bacteria) and have a similar effect to mild photodynamic therapy by promoting cell regeneration and wound healing (old or pre-damaged cells are apoptotic and replaced by new cells) and having an antibacterial effect (photodynamic inactivation of bacteria) [3, 4]. The essential part of the antibacterial and antiviral effect is probably based on an improvement of the endogenous defense by increasing temperature, tissue perfusion, and oxygen partial pressure, as well as the associated provision of energy-rich substrates and oxygen (thermal wIRA effect). Presumably, this is combined with non-thermal wIRA effects on immunocompetent cells, resulting in an immunomodulatory effect and improved local immune defense [3, 4]. The limitation of wound inflammation is comparable to the pain reduction described above, i.e., via thermal as well as non-thermal effects. A reduction in wound secretion by wIRA may be explained by non-thermal, direct effects on cells [3, 4].

4 Clinical Application Aspects

wIRA is a non-contact, easy-to-use, and non-painful method which has good depth effects and establishes a prolonged heat depot. Further advantages of wIRA are maintenance of the blood circulation, hygienically clean application (compared to fango/mud), use in different body positions without fixation (compared to wraps), the possibility of combining with exercise, and adequate dose adaption by varying the irradiation time and distance. The irradiation distance should be chosen so that the irradiance is perceived as comfortable (subjective comfort distance usually between 35 and 80 cm). In patients with limited sensory perception (e.g., patients with diabetic polyneuropathy), impaired ability to respond, inadequately perfused tissue, cold tissues, or subcutaneous tissue (e.g., along the tibia ridge), a lower irradiation intensity (irradiance) should be used by increasing the radiator-patient distance.

The application of wIRA to wounds is by no means limited to wound-healing disorders. The “normal” unimpeded wound-healing process can also benefit from wIRA. In this setting, an accelerated and less painful wound healing with a good cosmetic result has been described.

Irradiation of the uncovered skin/wound is carried out vertically, for at least 60 min/day (some longer application times are possible, e.g., 2–6 h/day). The recommendation is to treat more frequently and longer with lower irradiances than shorter with higher irradiances until wound healing is completed. The wIRA irradiance for wounds is recommended to be 70 mW/cm2. Wounds with reduced thermal tolerance of the irradiated tissue should be treated with 35 mW/cm2. Based on extensive findings from many years of clinical experience, the use of wIRA with adequate irradiances can be considered to be safe [3,4,5].

5 wIRA for the Treatment of Acute and Chronic Wounds

Wound healing is a highly energy-consuming process, and energy production in the tissue depends on an adequate supply of oxygen and energy-rich substrates. Tissue temperature, perfusion, and oxygen partial pressure are therefore crucial factors, all of which are increased by wIRA.

Based on the current data, it can be concluded that wIRA accelerates the healing of acute and chronic wounds, including infected wounds, via thermal and temperature-dependent, non-thermal, and temperature-independent effects, as well as normal wound-healing processes. In this context, wIRA has pain-reducing (with reduced need for analgesics), anti-inflammatory, infection-reducing, wound secretion-reducing, and regeneration-promoting effects (reviewed in [3, 4]). In total, there have been seven prospective clinical studies (overview and details in [3, 4]) which demonstrate good wound healing with wIRA at the highest level of evidence (evidence level 1a/1b), six of which were randomized controlled trials.

5.1 Acute Wounds

5.1.1 Acute Abdominal Surgical Wounds

The effects of wIRA in acute wounds have been investigated in a prospective, randomized, controlled, double-blind study in 111 patients after abdominal surgery [2]. For this, wIRA was applied twice daily for 20 min (starting on the second postoperative day). In addition to a highly significant reduction in pain in the wIRA group compared to the control group (visible light, VIS) (acute pain reduction at 230 irradiations: 18.5 vs. 0 on a visual analog scale (VAS) of 0–100, p < 0.000001), a reduction in pain medication (52–59% less pain medication versus the control groups with VIS or peridural catheter analgesia only, p = 0.00002/p = 0.00037) was demonstrated. Upon irradiation with wIRA, the partial pressure of oxygen increased by 32% and the temperature by 2.7 °C in the 2 cm depth tissue, whereas no changes were recorded in the control group (42 vs. 30 mmHg, p < 0.000001; 38.9 vs. 36.4 °C, p < 0.000001). The surgeon’s assessment of overall wound healing on a VAS scale of 0–100 was also significantly better in the group treated with wIRA (79 vs. 46.8 for the control, p < 0.000001), and the cosmetic result was comparable (84.5 vs. 76.5, p < 0.0002). Although not significant, there was a positive trend in favor of a lower rate of wound infections in the wIRA group ((3 of 46 (7%) vs. 7 of 49 (15%), p = 0.21)), including subsequent infections after discharge. Interestingly, there was a discernible trend toward a shorter postoperative inpatient stay in the wIRA group (9 vs. 11 days, p = 0.02). A key finding of this study was that postoperative irradiation with wIRA alone can also improve the normal wound-healing process.

5.1.2 Burn Wounds

A prospective, randomized, controlled, double-blind study investigated the effect of wIRA in 45 severely burn injured children (overview and details in [3, 4, 6]). Daily wIRA irradiation was applied for 30 min from day 1 of the burn injury. Irradiation with wIRA significantly increased the rate of wound area reduction and epithelialization (90% wound area reduction after 9 vs. 13 days, p = 0.00001) compared to control (VIS only). After 5 days, the physician decided whether surgical debridement of necrotic tissue was indicated or whether conservative therapy could be continued. Surgery was performed on 11 of 21 patients in the wIRA group and 14 of 24 in the control group. Furthermore, the wIRA group showed better results in the overall surgical assessment of the wound and with regard to the assessment of the irradiation effect versus the control (not significant, but tending to show better effects up to 3 months after the burn injury). However, a difference in stratum corneum formation between the 4 treatment arms was verified by laser scanning microscopy, especially for days 5–7. The fastest formation of the stratum corneum was seen in wounds treated with wIRA and dexpanthenol ointment, followed by wIRA alone. Analysis of wound bacterial counts performed every 2 days showed that wIRA and the combination of wIRA with dexpanthenol ointment prevented colonization with physiological skin bacteria by day 5 versus the other two treatment arms. Among other effects, bacterial colonization was more distinctively suppressed upon wIRA than following treatment with wIRA and dexpanthenol ointment.

5.1.3 Experimental Wounds

In this prospective randomized controlled trial, 4 experimental superficial wounds (each 5 mm in diameter) were induced in each of 12 subjects (overview and details in [3, 4]). Subsequently, 4 different therapies were applied for 10 days: (1) no treatment, (2) wIRA only with 30 min of irradiation daily, (3) dexpanthenol ointment only once daily, and (4) wIRA and dexpanthenol ointment once daily.

It was found that wound healing was very good from a clinical point of view with all four types of treatment. There were only small differences between these treatment options, with small advantages for the combination wIRA and dexpanthenol ointment and only for dexpanthenol ointment in terms of relative wound area changes.

5.1.4 Other Aspects and Perspectives in Acute Wounds

Reviews report on further positive effects of wIRA after endoprosthetic treatment of knee and hip joints [4, 6]. During the rehabilitation process, wIRA irradiation rapidly enhances the resorption of wound seroma and wound hematoma, which was objectively demonstrated clinically and sonographically. Pain relief was demonstrated in parallel.

Successful treatment with wIRA has been documented after urological surgery, with rapid wound healing and resorption of recurrent wound seroma. wIRA has also been shown to have a positive effect on wound healing after cesarean surgery. An amelioration of postoperative pain has also been successfully observed after thoracotomy.

5.2 Chronic Wounds

It is well known that the wound center is often hypoxic and relatively hypothermic and that this combination is unfavorable for wound healing. As optimal wound healing requires an adequate supply of energy-rich substrates and oxygen to the tissue, temperature, oxygen partial pressure, and blood flow are indispensable central factors in wound healing. Since wIRA irradiation increases temperature, tissue perfusion, and the tissue oxygen partial pressure, the wound healing is improved by wIRA application. The clinically beneficial effect is based, among other things, on an improved energy supply (increase in metabolic status) and oxygen supply [3, 4].

5.2.1 Chronic Venous Stasis Leg Ulcers

wIRA irradiation has positive effects on wound healing in three prospective, randomized, controlled clinical studies in patients with chronic venous stasis leg ulcers (overview and details in [3, 4]).

A Swiss study has shown significantly faster wound healing and a significantly lower need for analgesics in 40 patients with 3 × 30 min of wIRA irradiation over 6 weeks versus the control. Wound closure was objectifiable in the wIRA group after 14 days and in the control after 42 days (p < 0.000005). After 42 days, 19 of 20 (95%) of the wIRA group and 9 of 20 (45%) of the control showed completed wound healing (p < 0.001).

A Norwegian-Danish study [7] prospectively analyzed the effect of wound healing in 10 patients with thermographic follow-up. In 7 patients, the therapy resulted in complete or almost complete healing of the therapy-refractory chronic lower leg ulcers and in 2 other patients in a significant reduction of the ulcer size. In addition, pain relief with a substantially reduced need for analgesics was demonstrated. The thermographic picture typically showed a hyperthermic ulcer border, hypothermic wound center, and temperature differences up to 4.5 °C before the start of therapy, which normalized in the course of treatment. Of special interest are the results in a study participant, who had an ulcer on one leg treated with wIRA, and another ulcer on the other leg treated with VIS as control. There was a clear difference in favor of wIRA. Furthermore, a significant improvement in the effect of irradiation (reported by patient and examiner), assessment of wound healing (by the examiner), the cosmetic result (by patient and examiner), and an improved quality of life was demonstrated for the wIRA group (using a visual analogue scale).

In another prospective, randomized, controlled, blinded study involving 51 patients with non-healing chronic venous leg ulcers [8], there was a trend toward faster wound healing, better healing tendency, granulation, reduced exudation, and diminished wound coatings by combining compression treatment, wound cleaning and non-adhesive wound pad, and 30 min of wIRA irradiation (5 × 30 min per week for 9 weeks) versus control (VIS only).

A lower wound infection rate was observed after a single preoperative irradiation with wIRA (evidence level 1b). Wound infections occurred in 5.1% (9 of 178 patients) in the intervention group and in 12.1% (22 of 182 patients) in the control group (p < 0.02). A wound infection during postoperative days 9–30 was noted in 1.7% (3 of 178) of the irradiation group versus 7.7% (14 of 182) of the control group [9].

5.2.2 Other Indications

Indications for improved wound healing upon wIRA irradiation have been described in case reports for arterial-venous ulcers or arterial ulcers (at low irradiance), ulcers due to external pressure, i.e., decubitus ulcers (preventive and therapeutic with reduction of the wound area and better granulation of the wound area), diabetic foot (preventive and therapeutic, with thermographic proof of effect), healing of fistulas (in dentistry), and for improved absorption of topically applied substances on wounds (overview in [3, 4]).

5.3 Variable Irradiations Used in Different Studies

All studies listed above differed significantly in the wIRA application time. The latter ranged from 9 to 40 min/day, with one exception not exceeding 30 min/day. Interestingly, one study [5] recommended a significantly longer application time of up to 60 min/day, although this was also increased to up to 2–6 h/day. It was found that longer daily irradiations were associated with better effects on wound healing. Thus, more frequent and prolonged irradiations at low irradiances are preferable over shorter application times at higher irradiances.

It is possible that the therapeutic effects in the studies conducted can be further improved by longer irradiation times and this possibility should be considered in future studies.

5.4 Conclusions and Perspectives

Based on the data presented, the positive effects of wIRA-irradiation in wound treatment can be summarized as follows:

  • wIRA is a useful therapeutic option recommended for the treatment of acute and chronic wounds. wIRA can provide significant pain relief with substantially reduced need for analgesics;

  • wound exudation and inflammation are reduced;

  • wound healing, clinical outcome, and cosmetic result are improved;

  • wIRA application is reasonable before and after surgical procedures;

  • wIRA is a positive adjunct to pre- or postoperative routine administration of antibiotics (under certain conditions, wIRA may even replace antibiotics, although this has not yet been evaluated);

  • for chronic ulcers of the lower legs, wIRA is a positive adjunct to common therapies for wounds of various etiologies;

  • wIRA can be used to improve the absorption of topically applied substances.

Furthermore, some perspectives for the implementation of wIRA-irradiation in wound-related indications have only been described in smaller studies or casuistics:

  • preoperative wIRA (e.g., over 1–2 weeks) for preconditioning sites for removal and transplantation of skin grafts, transplants, and split-skin grafts;

  • postoperative wIRA to promote wound healing and reduce pain, exudation, inflammation, and infection at the mentioned locations;

  • wIRA as non-invasive alternative to the punction of wound seromas and wound hematomas, and to wound revisions;

  • wIRA for prophylaxis and therapy of decubitus ulcers.

In conclusion, wIRA-irradiation offers a valuable therapeutic option in the overall therapy concept for the treatment of acute, chronic, infected, and burn wounds.