BACKGROUND TO THE INVENTION
The present invention relates to a method of reducing wrinkles from a superficial area of mammalian skin tissue, and apparatus therefor.
The application of laser technology in healthcare is well known, and the use of lasers in medical applications has been studied extensively since the early 1960's. In recent years an increasing interest has been shown in cosmetic applications. Two such cosmetic applications are skin resurfacing and wrinkle removal; in this field lasers can be used as an alternative to surgical facelifts.
There is a distinct difference between wrinkle removal and skin resurfacing. Skin resurfacing is where laser energy vaporizes thin layers of the epidermis without breaking through the basal layer into the dermis. This is essentially a superficial process primarily used to give the skin a “fresher” appearance. However, wrinkle removal as a more aggressive technique where tissue is removed layer by layer, invading the dermis and effectively inducing a second degree burn. Heat is deposited in the dermis shrinking the collagen and tightening the skin.
In young skin, the collagen just beneath the surface of the skin forms an organized lattice with good elasticity and flexibility. During aging, the collagen changes its structure impacting negatively on the cosmetic appearance of the skin. Several techniques have been developed to induce a “controlled injury” to the dermis in an attempt to generate rejuvenation of the collagen structure returning the skin to an earlier cosmetic appearance. During the 1990's a laser approach to wrinkle removal has been introduced.
For known wrinkle removal techniques, the wavelength is chosen so that the laser energy is highly absorbed in water, the current lasers of choice being the CO2 laser at 10.6 μm wavelength and the Erbium YAG laser at 2.94 μm wavelength. In this non-selective process, pulses of laser energy are applied to the skin surface, each pulse vaporizing a layer of tissue between 30 μm to 60 μm in thickness. Normally, the first pass of the laser removes a thin layer of the epidermis without damaging the basal layer. Successive passes over the same area penetrate into the dermis and heat the collagen. The laser operator sees this thermal build-up “shrink” the skin in “real time”, tightening up the skin's appearance. When the desired clinical outcome is achieved, the operator ceases applying laser pulses. It is therefore apparent that the quality of the cosmetic result is highly dependent upon the experience and skill of the operator.
In the case of CO2 laser wrinkle removal, post-treatment supervision of the patient is a necessity. Immediately after treatment, the skin is essentially an open wound requiring dressings in place for 2-10 days. Additionally, topically applied lotions are required for patient comfort and prevention of infection. Post-operative infection is common, primarily due to removal of the natural protective barrier of the skin, with a reported incidence of between 4.5 to 7%.
On average, with CO2 laser wrinkle removal, post-treatment erythema is present for 4-5 months. This compares to 2-3 months following a Chemical Peel. Also, the incidence of side effects is significant, the most common being hyperpigmentation occurring in 30-40% of cases. Higher incidences are reported in darker skin types. A delayed hypopigmentation, which can occur up to a year after the procedure was performed, has recently emerged as a complication of aggressive laser resurfacing. Many of the eminent laser resurfacing surgeons have resorted to less aggressive techniques.
The effect of known procedures is two fold:
(a) the laser induces denaturing of the collagen in the dermis, and the formation of cross links, which results in a tightening effect stretching the skin, reducing or removing the wrinkles (it is thought that the thermal threshold for this effect is a temperature of 70° C.); and
(b) the changes to the dermis induce the generation of new collagen which develops using the matrix created by the denatured collagen as a foundation.
The skin-resurfacing and wrinkle removal procedure outlined above is considered by many experts in the field as a significant improvement over previously used surgical methods. The procedure uses the laser's ability to deliver high energy density at the surface of tissue and hence ablate the surface tissue in a well controlled manner. Continuing to remove the tissue, layer by layer is designed to damage the collagen and hence induce wrinkle removal. This second stage of the procedure is primitive; the skin weeps, scabs form and redness of the skin appears for many weeks.
OBJECT OF THE INVENTION
It is therefore the primary object of the present invention to provide a technique for removing wrinkles from a superficial area of mammalian skin tissue without causing secondary burns and other problems associated with traditional wrinkle removal.
SUMMARY OF THE INVENTION
The present invention provides a method of removing wrinkles from a superficial area of mammalian skin tissue. The dermal layer of the tissue is irradiated through the basal layer by radiation selected to be absorbed by a chromophore in the dermal layer such that collagen present in the dermal layer is heated, while the basal layer remains intact so as to substantially inhibit contact of the dermal layer with ambient air.
According to a further aspect, the invention provides apparatus for cosmetic reduction of wrinkles on a superficial area of mammalian skin, the apparatus comprising a radiation delivery system for delivering a radiation beam of predetermined monochromatic wavelength or narrow wavelength bandwidth to the skin, the radiation delivery system including a pulsation system for pulsing the radiation delivered according to a predetermined regime, and an optical arrangement for focussing the beam such that the total radiation energy density delivered to the skin is substantially in the range 0.5 J/cm2 to 5 J/cm2 per pulse.
The irradiation of the dermal layer in the method according to the invention is tailored to shrink the skin tissue without damage to the dermis (in other words, without causing second degree burns) because the barrier provided by the basal layer remains intact. This is achieved by selecting the required radiation wavelength to match the characteristic absorbtion wavelength of the chromophore whilst being absorbed to an insignificant degree in the epidermis and basal layer. The energy delivered per pulse is also tailored to ensure that ablation does not occur of the target structure, but rather that energy absorbed in the target provides sufficient heating that heat energy diffusing outwards from the target heats the surrounding tissues to a degree sufficient to thermally induce shrinkage of the surrounding tissue and also stimulate the production of new tissue components such as elastin and collagen.
If the target for the laser has an appropriate chromophore (a substance that absorbs a specific wavelength and transmits or scatters at other wavelengths) then the laser can be used to modify that target selectively within an inhomogeneous volume of tissue. Occasionally, the desired target does not have a suitable chromophore of its own but exists in close proximity to another material which has such a chromophore which can be selectively targeted. Such interaction is called secondary selective interaction.
An artificial chromophore may be introduced into the desired area for wrinkle reduction, or a naturally occurring chromophore may be selected. In a preferred embodiment of the technique, the naturally occurring chromophore selected is oxyhemoglobin of the dermal plexus which has wavelength absorbtion peaks at 585 nm and 815 nm, at which wavelengths absorbtion in surrounding tissue components is relatively low.
According to a further aspect, the invention therefore provides apparatus for cosmetic reduction of wrinkles on a superficial area of mammalian skin, the apparatus comprising a radiation delivery system for delivering substantially monochromatic radiation in a bandwidth of substantially 15 nm or less in at least one of the ranges 570 nm to 600 nm and 750 nm to 850 nm, the delivery system including a pulsation system for pulsing the radiation delivered according to a predetermined regime in which the energy density of the substantially monochromatic radiation in the bandwidth of substantially 15 nm or less delivered to the skin is substantially in the range 0.5 J/cm2 to 5 J/cm2 per pulse.
The method according to the invention is non-invasive and non-ablative and can readily be performed by non-medical personnel. The total energy delivered per pulse is sufficient to effect the required physical change in the tissue surrounding the target chromophore without causing ablation of the target or other skin components through which the radiation passes.
DESCRIPTION OF PREFERRED EMBODIMENTS
The radiation is preferably substantially monochromatic or at least of a relatively narrow wavelength bandwidth to ensure that it is preferentially selectively absorbed by the target chromophore. A laser source may be used to produce the required wavelength, or a light source, such as an LED may be used with appropriate filtering to permit the selected wavelength (or narrow wavelength band) to pass.
The irradiation may be by means of a source of visible or infra-red radiation (suitably filtered to remove deleterious ultra-violet radiation if necessary). The radiation may be coherent (that is from a laser source). Such a laser source may be, for example, a dye laser, a ruby laser, or a semiconductor laser. If a dye laser is used, its wavelength is preferably such that it is absorbed by oxyhemoglobin (as naturally occurring chromophore present in blood vessels in the dermis). Alternatively, the superficial area may be treated with an artificial chromophore which is absorbed into the dermal layer. Such an artificial chromophore may be applied to the epidermal layer in the form of a liposome-containing topical formulation. The chromophore may then permeate through the basal layer for delivery to the dermal layer.
When a laser is used, it may be arranged to scan the superficial area and/or to irradiate the dermal layer in pulses. When the laser is in pulsed mode, the pulses typically have duration of 10 μsec to 10 msec (more preferably 200 μsec to 1 msec).
It is sometimes desirable to remove part of the epidermis prior to irradiating the dermal layer according to the invention. Such epidermis removal (known as skin resurfacing) may be effected mechanically (for example by abrasion), or by means of laser radiation. When laser radiation is used for this purpose, it is typically a scanner controlled CO2 laser source.
The energy density per pulse is preferably accurately controlled to ensure that a maximum threshold level (substantially of 5 J/cm2) is not exceeded.