1. Introduction

Fibre-optic subdermal laser treatment of the lower face — delivering diode laser energy into the superficial hypodermis through a bare micro-fibre — has accumulated a genuine if immature evidence base, including one randomised controlled trial and several case series reporting improvement in jawline laxity and jowl fullness.[1–4] Protocol reporting across that literature shares a consistent weakness: total delivered energy, in joules, is almost always stated, while the power at which it was delivered is frequently absent.

This is not a pedantic omission. Energy and power describe different things. Energy (joules) is the total dose; power (watts) is the rate at which that dose is delivered. Two treatments delivering identical joules at different wattages are not the same treatment delivered twice — they differ in delivery time and, more importantly, in the competition between heat deposition and heat loss within the tissue. Whether that distinction matters clinically is precisely the kind of question a small clinic can begin to answer honestly, provided it reports its own data rather than extrapolating from marketing materials.

We report a retrospective two-cohort series from our own practice in which a protocol adjustment — raising power from 3 W to 4 W while holding total energy constant at 300 J per side — coincided with a clear shift in patient satisfaction. The series is small and its limitations are substantial, and we state them plainly. The observations are reported because the underlying physics is not speculative, and because power deserves the same protocol discipline as total energy.

2. Methods

Study design. Retrospective review of ten consecutive patients treated for jowl laxity by Dr Ahmed Haq at Cosmedocs, Harley Street, London, over a six-month period. No patient was excluded from the consecutive series. The work was conducted as a retrospective clinical service evaluation of routinely collected treatment records and satisfaction scores.

Device and technique. All treatments used a dual-wavelength endolaser platform (XL EndoLaser) at the 1470 nm wavelength. A sterile single-use optical micro-fibre was introduced into the superficial subdermal plane of the jowl without incision, under local anaesthesia, and advanced in a fan-like pattern along anti-gravitational vectors, creating micro-tunnels across the treatment zone. Both sides of the face were treated at the same sitting, with 300 J delivered per side in every case.

Cohorts. The first five consecutive patients (Cohort A) were treated at a power setting of 3 W. Following an internal protocol review, the subsequent five consecutive patients (Cohort B) were treated at 4 W. Total energy per side (300 J), fibre technique, treatment zone, anaesthetic approach and aftercare were held constant to the extent a retrospective design allows. The cohorts were sequential rather than randomised.

Outcome. Overall satisfaction with the result was recorded at review on an 11-point numeric scale from 0 (completely dissatisfied) to 10 (completely satisfied). Adverse events were recorded from treatment notes and review visits.

Analysis. Given the sample size, results are reported descriptively. No inferential statistical testing is presented; with five patients per arm, any p-value would be theatre rather than evidence.

3. Results

Ten patients completed treatment and review. Mean overall satisfaction was 7/10 in Cohort A (3 W) and 9/10 in Cohort B (4 W), despite both cohorts receiving an identical total energy of 300 J per side.

The only intended difference between cohorts was the rate of energy delivery. At 3 W, delivering 300 J requires 100 seconds of emission per side; at 4 W, the same 300 J is delivered in approximately 75 seconds. Total procedure time was correspondingly shorter in Cohort B.

No burns, skin necrosis, steatonecrosis, persistent sensory disturbance or other significant adverse events were recorded in either cohort. Transient oedema and expected early post-treatment firmness occurred in both groups and resolved without intervention.

4. Discussion: Why the Same Joules Behave Differently at Higher Power

The result that demands explanation is the apparent paradox at the centre of this series: identical energy, different outcome. The explanation is that tissue never experiences joules in the abstract — it experiences temperature, over time, at a location. Power determines how quickly temperature rises, and tissue loses heat continuously while energy is being delivered.

The physics, stated plainly. One watt is one joule per second. At 3 W, 300 J takes 100 seconds; at 4 W, 75 seconds. During every one of those seconds, the heated tissue is simultaneously losing heat to its surroundings by conduction into adjacent cooler tissue and by perfusion, as blood flow carries warmth away. Heat deposition and heat loss are in competition for the entire emission window.

Thermal confinement. For a photothermal effect to occur, tissue temperature must cross a threshold and stay there long enough for the intended change — immediate collagen contraction and the trigger for subsequent neocollagenesis. When energy is deposited faster than the tissue can shed it, temperature rises efficiently and the threshold is crossed with most of each joule still contributing. When deposition is slower, a larger fraction of the same 300 J is carried away before it can raise local temperature to threshold: the joules are delivered, but they are spent warming a slowly leaking volume rather than driving the target tissue across its thermal threshold. This is the principle of thermal confinement, and it is why power is a therapeutic variable in its own right rather than a footnote to total energy.

An everyday analogy holds reasonably well. Putting the same quantity of heat into something slowly and putting it in rapidly do not produce the same temperature history, and biological tissue responds to the temperature history rather than to the final reading on the energy counter. Where 1470 nm energy is absorbed principally by water, that difference in temperature history is exactly where collagen contraction, coagulation and subsequent remodelling are decided.

Applied to this series, the 4 W protocol plausibly achieved a higher peak temperature within the fibre track and fibroseptal network per joule delivered — more effective collagen contraction per unit of identical energy — and did so over a shorter procedure, which may itself contribute to how patients score their experience. Both effects point in the same direction and neither can be separated in a series of this size.

The safety boundary of the same physics. The argument cuts both ways, and should be stated as plainly as the efficacy argument. The same thermal confinement that makes higher power more effective also narrows the margin for error: burns, steatonecrosis and nerve injury reported in the wider endolaser literature are explicitly linked to poor dosimetry control and inadequate temperature discipline.[5] Raising power without compensating technique — movement speed, pass planning, awareness of fibre tip position — is not a free upgrade. Our protocol change was accompanied by the same governance-led parameter discipline taught on our own training programme, and the absence of adverse events in this series should not be read as evidence that none are possible.

5. Limitations

This series has limitations substantial enough that they should be read before the results, not after. The sample is very small (n=10; five per cohort). Cohorts were sequential rather than randomised, which introduces a specific and serious confounder: the operator treated every 3 W patient before every 4 W patient, so any improvement attributable to growing technical experience is indistinguishable from improvement attributable to the power change itself. Satisfaction was a single subjective outcome without blinding, standardised photography or biometric measurement; the wider literature supports biometric assessment for this treatment, and its absence here is a genuine weakness.[2] Follow-up interval and patient characteristics were not controlled between cohorts. Finally, satisfaction scores cannot separate the aesthetic result from the experience of a shorter procedure.

One further parameter deserves specific mention because it may be more informative than total joules. If power was increased while fibre movement speed also increased — consciously or otherwise — then the energy delivered per centimetre of fibre travel, and per micro-tunnel, may have changed considerably even though the total delivered energy did not. Linear energy density (J/cm of fibre travel), number of passes, treated area and any adjunctive 980 nm use were not prospectively standardised in this series, and any of them could contribute to the observed difference.

None of this invalidates the observation, but all of it constrains the claim. We present the series as a hypothesis-generating clinical audit with a physically coherent explanation, not as evidence that 4 W is universally superior to 3 W.

6. Conclusion

In a retrospective series of ten patients treated for jowl laxity with subdermal 1470 nm laser at a fixed dose of 300 J per side, raising power from 3 W to 4 W was associated with a rise in mean satisfaction from 7/10 to 9/10. The finding is consistent with the physics of thermal confinement: at equal total energy, higher power deposits heat faster than the tissue can dissipate it, increasing the proportion of energy that contributes to threshold-level heating of the target structures.

Stated conservatively, the finding of this series is as follows: at a constant total delivered energy of 300 J per jowl, increasing 1470 nm laser power from 3 W to 4 W was associated with an increase in mean patient-reported satisfaction from 7/10 to 9/10. This suggests that power — and therefore the temporal rate of energy deposition — may influence clinical effect independently of cumulative delivered energy. It does not establish causation.

The next study follows directly from that sentence. A prospective comparison of 3 W versus 4 W with total energy fixed at 300 J per side, standardising fibre movement speed, number of passes and treated area, with photography at fixed intervals, blinded assessors and a validated improvement scale alongside satisfaction scoring. That design would begin to answer whether there is an optimum power–energy–time relationship rather than simply an optimum number of joules.

Two practice points follow. First, endolaser protocols — in this journal and elsewhere — should report power, energy, and by implication delivery time, as a complete set; energy alone describes only half the treatment. Second, the same physics that makes higher power attractive makes parameter discipline non-negotiable. A prospective, standardised comparison with biometric endpoints is the obvious next step, and we intend to conduct it.