Advances and Perspectives on Tissue Repair and Healing *

General Aspects of Tissue Repair

W ound healing is a complex process that involves both local and systemic responses, and these responses will widely reflect the etiology of the lesion, type of tissue, systemic condition of the subject, etc.

Despite being essentially the same for different wounds, the pattern of healing may change due to both intrinsic and/or extrinsic factors. These factors include age, gender, hormonal factors, nutritional deficiencies, the use of drugs, and some diseases. It is well known that some diseases or conditions such as uncontrolled diabetes, malnutrition, hypothyroidism, anemia, burns, etc., may cause delayed wound healing. The type of tissue has also to be considered as, despite showing similar phases, skin and mucosal wounds heals quickly than bone.

Several therapeutic approaches have been used to improve healing, and these include the use of stem cells, ¹ growth factors, ² drugs, ³ hyperbaric oxygen, ⁴ and phototherapies. ^{5 –8} Several light therapies have been used to improve both healing and repair. Laser, light emitting diodes (LEDs), and lamps have been used worldwide, both experimentally and clinically, to improve healing. It has been shown elsewhere that different phototherapies are able to stimulate the repair of skin, ligament, tendon, bone, and cartilage in laboratory animal wounds and human ulcers.

Phototherapy and Wound Healing

Closing Remarks

Tissue repair is a chemical, physical, and biological phenomenon that occurs after any kind of tissue lesion. These phenomena are of the utmost importance in the repair of tissues damaged by a wide range of pathologies. It is absolutely necessary to know what factors could prevent complete tissue repair. The repair process is influenced by a series of local and general conditions. Local conditions are good vascularization, absence of pathogenic microorganisms, absence of foreign bodies, and lack of trauma in area. Furthermore, systemic conditions also play a role on healing.

Several factors that are affected by LPT have been pointed out in the literature; these include: transforming growth factor-β, platelet-derived growth factor, Ca²⁺, enhancement of ATP production, prostaglandin reduction, improved cell wall permeability, and reactive oxygen species (H₂0₂, O₂ ²⁺). These factors are not only increased, but are modulated according to the oxidative state of the cell. For instance, cells in 2% fetal calf serum react best, those in 5% react well, and those in 10% hardly react at all. This is a key to understand the effect of light on living tissues. Some kind of deficiency has to be present. LPT is optimal when the function of the cell, that is the redox situation, is suboptimal. Healthy cells react little or do not react at all to light energy.

There are several specific effects of LPT that are widely reported on the literature; these include altered nerve conduction; altered blood flow and neoangiogenesis; increased metabolism of endorphins, acetylcholine, serotonin, and cortisol; enhanced ATP production; enhanced cell function (release of growth factors, cytokine reactions, replication); increased synthesis of DNA and proteins; and acceleration of healing. Secondary effects may be due to the fact that the laser-excited form of ATP behaves biochemically different than the nonexcited form. The former has an increased kinetics.

LPT is one of the areas of controversy in the use of lasers, and most of its applications are related to soft tissues. The true mechanism, which leads to a positive effect of laser light on different tissues, is not fully understood. Several hypotheses have been suggested. It has been suggested that the most probable hypothesis is that the laser energy stimulates porphyrins and cytochromes to increase cellular activity, increasing the concentration of ATP and the release of Ca. The magnitude of the biomodulative effect depends on the physiologic status of the cell at the irradiation time. This may explain why the biomodulative effect is not always detectable. The stimulant effect of the laser light occurs during the initial phase of proliferation and initial differentiation of undifferentiated cells. However, this does not occur during more advanced stages.

The dose is the most important parameter in LPT. At first glance, the dose seems very simple and obvious: simply give some joules per square centimeter of skin. A two-dimensional power distribution of the light on the tissue surface leads to a three-dimensional energy distribution in the tissue. This distribution depends on many factors.

Controversies regarding dose is still a hot topic among scientists, and previous reports have pointed out that the use of low doses result in positive effects on living organisms. On the other hand, higher doses have not been shown to produce positive effects in stimulating living tissues. Cellular proliferation is more intense 24 hours after the irradiation and decreases, in an energy-dependent way, up to 72 hours. If the same power output is kept and time is increased, the amount of energy within the tissue will increase in the same ratio, so a larger volume of tissue will receive a dose within the therapeutic window.

The efficacy of LPT depends on: amount of energy delivered to the tissue, exposure time, and delivery method. It is important to start the treatment using lower doses and intensity, particularly in elderly patients whose cellular response is relatively weak. Dose may be adjusted during treatment. Care has to be taken in order to avoid inhibitory threshold and overexposure during treatment. Controversies regarding dose continue, although previous reports have pointed out that low doses result in positive effects on living organisms. ⁴⁶ On the other hand, higher doses have not been shown to have positive effects on stimulating living tissues.

Lower doses are recommended when irradiating mucous membrane or wounds due to the lack of skin (optical barrier). Dose and time interval will depend on the status of the process (chronic or acute), patient systemic status, and skin pigmentation. The recommended doses must be considered when using the contact method. When the noncontact technique is used, the loss of energy is difficult to calculate because of reflection. Usually the tissue reacts well to higher doses using HeNe and GaAlAs lasers; higher doses may cause inhibition or over-reaction.

Another question that is frequently asked regards the frequency of irradiation in clinical settings. So far there is no study that supports the theory that more frequent irradiation will further improve the final results. Dose has been found to be cumulative. Also, we will always get systemic effects. Our clinical experience shows that patients respond very well to treatment carried out every other day. There is no difference if the patient is treated at daily basis. Our clinical protocols always follow three sessions per week.

It is known that the nutritional status influences the healing process as well as the quality of the healed tissue and that the use of both visible and IR wavelengths result in a positive biomodulatory effect on undernourished subjects, and the effect is more evident with the use of 635 nm and SAEF of 20 J/cm² or 780 nm with spatial average energy fluence (SAEF) of 40 J/cm². ^21,22

Most studies on soft tissues suggested that visible light produces more pronounced effects on cutaneous wounds than IR light. In our experience, in burns, lesions which deeply affects the tissues, IR wavelengths seems to be the better option since surface tissues are thermally damaged and less susceptible to be affected by light. Thus, the use of a wavelength with deeper penetration associated with pulsing, which has been shown to increase the penetration, may be accountable for the findings of this study. We have found that the use of polarized light emitting on both visible and invisible wavelengths positively affected the healing of burns. ^19,20

On surgical flaps, we found the best responses in 790-nm irradiated subjects, with intense angiogenesis and fibroblastic proliferation. Angiogenesis is essential for wound healing and for the reestablishment of local blood flow on ischemic areas, such as on flaps where tissue necrosis is a common feature. In this case, we have reduction in tissue necrosis and less and precocious inflammatory reaction. ²³ In cases in which some therapeutical approach may led to impairment of the healing, such as the use of some drugs, the use of LPT resulted in less inflammation and increased fibroblast proliferation and collagen deposition, favoring healing. ²⁴

Our group has shown, using different models, that association of bone grafts, bone morphogenetic proteins, and guided tissue regeneration does improve the healing of bone tissue. ^{36 –45} Many studies indicated that irradiated bone, mostly with IR wavelengths, shows increased osteoblastic proliferation, collagen deposition, and bone neoformation when compared to nonirradiated bone. ^{36 –45}

Several groups worldwide have been studying the effects of the use of different phototherapies on the healing and repair of both soft and mineralized tissue. From these reports and our own experience we have no doubt whatsoever that the use of phototherapies, carried out with appropriate parameters, promotes quicker tissue repair. Phototherapy is a highly effective form of treatment with increased supporting evidence from both experimental animal research and human studies. Controlled multicenter trials are necessary and mandatory for a worldwide acceptance of this therapeutic by the health authorities.