Advances in light-curing units: four generations of led lights and clinical implications for optimizing their use: part 2. from present to future

Ratio of consumption to power supplied

The ideal light would convert all the electrical power into light of the desired wavelength. As the power conversion improves less heat is generated. Because LED LAUs need less electrical energy for the same optical power output than QTH and PAC lamps, it has meant that small, lightweight battery-operated units have become practical and popular. Broad spectrum QTH and PAC LAUs require wavelength restricting photon and heat filters to suppress dangerous and inactive radiation, such as UV, red and IR. Only about 1–2% of the energy used to power a QTH LAU is converted into useful curing energy. Filters used by traditional lamps return the filtered radiation as heat. Noisy and bulky fans are required to remove this heat and they have the added drawback of consuming energy themselves. LED units are inherently more efficient in this respect. Putting together all these energy losses (heat/fan) or these levels of unused energy (halogen/plasma), a third-generation LED unit with equal power may be assumed to consume between 5 to 10 times less energy than a halogen lamp and 20 times less than a xenon plasma lamp. In these circumstances, a simple battery can replace the mains electricity supply.

Cordless battery or mains powered corded LED units

Third-generation LED lights are either battery powered or corded (Figure 1) and may be integrated into a complete dental unit. Battery powered units offer extreme portability and convenience, freeing the handpiece from a mains power supply. Battery life may range from 30 minutes to over three hours. As batteries have developed in parallel with LEDs, from Nickel cadmium to Nickel-metal-hydride and now Lithium-ion (thanks partly to mobile phones and MP3 players), several cordless units now offer over two hours of battery activation life, allowing greater convenience and improved ergonomics for the dental team. The consequences of this overflow of energy, in terms of consumption and hence battery life, remains to be seen. A second-generation LED unit could operate for 3 to 4 days without needing to be recharged. Third-generation lights are generally only required to operate half as long overall as second-generation lights, giving us ~10–15% longer running times in everyday practice. Ni-Cad batteries and their memory effect are a thing of the past for well-made lights. These lights now only use Ni-Mh or Li-ion, whose durability of performance is acknowledged over more than 5 years (loss of capacity is noticeable as a very fast decline in their ability to retain charge over time). The new cordless version of Valo, VALOCL® (Ultradent) may be used either with non-rechargeable camera batteries or Lithium iron phosphate batteries, which are stated to last for approximately 400 cure cycles or a week and can be recharged >1,000 times before replacement. Although the energy of most of these LED units is supplied by batteries, increasingly versions are appearing which are integrated into dental units (Adec, Kavo, Sirona, Planmeca, etc) or are adaptable to accessory equipment such as ultrasonic generators (Satelec, EMS). This is not surprising because contemporary LED lights have designs and electronics that facilitate this integration. A pen-or wand-type light is hardly any more bulky than a turbine or a surgical aspirator. However, batteries may add signficantly to unit bulk and weight. In addition, because rechargeable batteries have a finite lifespan and they are relatively expensive items to replace; many practitioners still prefer mains powered corded units.

Design

The second-generation lights had changed considerably in comparison with the first-generation, but the same cannot be said of the arrival of the third-generation lights. They have retained the familiar outlines endorsed by practitioners: pen shape with optic fibre (such as MiniLed^™), pen shape without optic fibre-LED at the end, micro light type such as Coltolux^® (Coltene), SmartLite PS^® (Dentsply) or gun design with fan – Bluephase^® (Ivoclar Vivadent) and SmartLite IQ^® (Dentsply).

Problems for the clinician

A number of questions have arisen since the advent of light curing in dentistry. Thus there have been, and continue to be, debates about the power employed and the radiation spectrum, shrinkage or stress of composites or the source and precise role of the heat generated. Even though several excellent studies have been published on these subjects,^1-9 no precise answer to these issues has ever been provided. This is entirely understandable because these analyses were, and still are, highly dependent on factors that are constantly changing: the energy source (light-curing lamp) and the material to be activated (essentially restorative composites, sealants and bonding agents).

It therefore makes sense to look to this third generation to help solve these problems. Even if these suggestions do not solve all the current problems, they do at least raise the issues and demonstrate that today there are certain routes which are as yet unexplored.

Nowadays what we choose as the maximum energy reference value and the way the energy is delivered by a LAU should be dictated by the needs of different clinical situations. The light-curing unit and irradiation protocol (radiation time, spectrum and energy delivery sequence) should be tailored to meet diverse clinical demands. For example:

Optimal radiant exposure, wavelength and irradiance delivered over time will vary greatly according to clinical circumstances. Here are a few examples:

One manufacturer has recommended an eight-fold difference in radiant exposure for curing their composites (6-48 J/cm²) based on product choice and shade selection. It is generally recommended that a 2 mm increment of composite should receive 12–36 J/cm² radiant exposure to be adequately polymerized, but energy quality and material properties are also crucial.

A light does not have the same irradiance if it is placed 1 mm from the luting cement in orthodontics and up to 8 mm from the bottom of a deep cavity that needs to be reconstructed in multiple layers.¹⁰ Collimation varies with unit design and output irradiance declines by 50% or more at 4 mm distance for some, but not until 8 mm or more for others. Deep restorations may not be adequately cured if the irradiation time is based on data when the curing light is positioned adjacent to the material. Undercut cavities may further complicate adequate light energy delivery. Manufacturers recommend radiation times which are often based on the latter ideal situation and the light guide is fixed normal and stationary to the composite surface. Clinically this is almost impossible to achieve. Turbo light guide tips have a greater disparity between the entrance and exit diameters of the light guide (higher R-value) than standard light guides and increase irradiance when the tip is close to the material surface, but suffer at distances over 5 mm as they have poorer collimation (Figure 2).¹¹ In general, units with standard light guides are better collimated than units without optic-fibre guides or micro light types. Some manufacturers now address this problem by using convex or Fresnel (as used in Lighthouse lamps) light collimating lenses to improve collimation with distance. Price and co-workers have recently reported the effect of distance on irradiance for four popular LED LAUs.¹² The useful beam diameter of two of the four units tested (defined as the beam diameter having a minimum irradiance of 400 mW/cm²) varied between 7.1 mm and 6.8 mm at a 2 mm distance, whereas at 8 mm distance this had declined to 5.3 mm and 2.6 mm, respectively. Over a 3.9 mm target diameter representative of a Class I restoration, this corresponded to a 60% and 80% irradiance decline for the units in question. The superior collimation of the former unit (Fusion) was attributed to the wider collection angle of its lens enabling ‘soliton-like’ propagation of the optical beam.

Controlling the power

The first solution provided by third-generation LED lights regards power: these lights are capable of offering irradiance ranges that can be described as unlimited in both low and high energy. Some units may allow less than 300 mW/cm² for 40 seconds without any noticeable heating effect. These lights, via a simple menu choice, may also allow 1,000–3,000 mW/cm² or more which can provide ‘flash’ curing for orthodontics.

Controlling time/power curves

This unlimited provision of the energy required for all clinical situations leads to a second form of control, namely that of the profiles and modulations linking time and power. Until the present day, low powers as well as high powers were totally incompatible. Some lamps (xenon plasma in particular) could never emit below 80% of their power rating. The same problem occurred with the first LEDs to some extent and many halogen units. The light was, so to speak, an all-or-nothing lamp. The arrival of new, custom-made LEDs offers all the desired options with the same emission area, and the responses to ‘menu instructions’ are almost instant.

It is thus possible nowadays to have a program linking time and power with the desired profile and giving free rein to the clinician's imagination. Soft and step menus are likely to become more complex but without increasing the practitioner's task in the process.

Controlling the irradiance (power density) as target distance changes

When people talk about power they do not necessarily mean power density or irradiance. It is well known that the further away one is and the more the light diverges, the lower the effective irradiance is on the illuminated surface. Until now this correction for distance has been done in a purely ‘empirical’ way by the practitioner during the clinical irradiation phase, at the risk of underestimating the energy loss. A long-term retrospective study of the survival rate of endodontically-treated posterior teeth restored with MOD composite resin restorations reported that teeth restored with a light-activated resin had a much lower survival rate than teeth restored with a chemically-activated material. The cause was attributed to inadequate polymerization of the light-activated restorations.¹³ Accordingly, several publications have recommended that researchers position their lamp between 4 and 8 mm from the composite surface during their experiments to avoid ‘underestimating’ their lamp's power. This power reduction has never been integrated into the power or exposure time, despite a number of recommendations in the past. It is the third-generation LEDs and, more particularly, their small dimensions and speed of response to electrical impulses received via menus that have made it possible to offer and launch onto the market new functions such as ‘auto-focus’. The principle of the auto-focus developed for a third-generation LED unit the MiniLed™ AF^® from Acteon¹⁴ aims to provide a solution to the loss of energy caused by this natural or deliberate distancing of the light source exit window from the surface of the material being cured. The auto-focus on the unit being used correlates the light irradiation time as a function of the distance measured between the end of the light guide and the composite surface, like the auto-focus on a camera which correlates the sharpness of the picture as a function of the distance between the lens and the object being photographed. Nowadays, the correlation is no longer done by measuring the distance but by measuring the light reflected from the composite surface. The light sends a narrow beam of light or emits its curing light at low intensity. After being reflected from the target the beam returns to the guide and then strikes the surface of a selective photodiode cell (specific for detecting the indicator beam). This photodiode will allow more or less current to pass, depending on the irradiance received, and it is this information that enables a linked, calibrated micro-calculator to deduce the distance as a function of the measured light reflected by the composite. If the light level is weak, the time will be lengthened, and vice versa.

Ideal light orientation and stabilization

The shorter the irradiation time, the more important it is that the beam is positioned in the right direction on the composite surface. Moving away from the optical projection axis is seen to reduce the irradiance received at the surface of the material. Achieving stable and accurate light source alignment throughout polymerization during intra-oral curing is not always as easy and, unfortunately, may be neglected. In 2002, Shortall and co-workers¹⁵ demonstrated that operator variability is a critical issue for successful intra-oral curing. In 2010, this finding was confirmed by Price and colleagues,¹⁶ who showed that cavity location and light source type are also important variables. When access is restricted, relatively small changes in light guide alignment may result in considerable reduction in irradiance on the target material surface. Many light units have designs which make it difficult to see and position the light beam optimally. Accurate and stable light source alignment and position are required during the entire irradiation period to optimize intra-oral cure – something that should be obvious! This is central to the use of light curing but may be taken for granted or relegated to an assistant who might not be completely aware of the scientific nuances.

There are hence two types of reduction in power:

The latter factor is the one that the dentist varies in clinical use. A second factor may have a significant influence on the power of the light beam and hence on curing of the composite. This is the divergence of the rays exiting the light guide. Moreover, the further away, the greater is the divergence. It should also be noted that the stated real power of a light being used is always measured in a central, well circumscribed area. The dental surgeon cannot really see where this area is when starting to operate his/her unit because of the dazzling power of the light on a particularly shiny tooth. It therefore seemed crucial to combine this auto-focus feature with an indication of the optimum power area (and hence measuring area) before starting actual polymerization. To do this, a red aiming circle (the laser aiming function) was created, which precedes emission of the visible blue light. The low power laser aiming system emits at 650 nm. All the practitioner has to do is position this circle (of non-curing light) at the centre of the restoration before starting polymerization to ensure that he/she is going to illuminate in the right place (Figure 3). The two factors ‘auto-focus and laser function’ improve the time and spatial positioning of the light as a function of the distance between the optical guide exit and the composite.

Provide broad spectrum output

Halogen lights offer broad spectrum output for universal curing of photo-activated dental resins irrespective of initiator formulation, unlike second-generation monowave blue LED LAUs. Manufacturers have added violet as well as blue LEDs to some third-generation units to address this limitation. To date these polywave LED units do not possess the spectral spatial homogeneity of their QTH predecessors.

Acknowledgements

The authors acknowledge the advances made to the development of light-curing technology in dentistry by dental manufacturers and research workers. Their contributions will significantly benefit patient care.

Disclaimer

The authors confirm that they have no financial interest in any of the products mentioned in these articles and have only been reimbursed expenses for test evaluation purposes concerning a prototype of the Scanwave^TM (Acteon) light-activation unit. In this two part series the authors have classified LED LAUs by generation in order to highlight major developmental steps. No commercial product ranking for quality or efficacy is necessarily implied by this classification.