Painting acid into the base of a boxcar scar
Trichloroacetic Acid Paint for Boxcar and Polymorphic Acne Scars
A strong acid, painted into the base of a scar with an artist's brush and nowhere else. Every one of the 37 patients who completed three sessions improved, 57% of them by more than 70% — and darker skin, usually the cautious case in scar treatment, did better than fair skin.
Four numbers worth knowing
What we found
The technique is focal by design.
Ninety per cent trichloroacetic acid was applied only to the base of the scar, using a fine cosmetic brush, with light strokes until frosting appeared — which took 8 to 16 seconds in every scar. The surrounding healthy skin is deliberately left untouched, because that is where the risk of a deeper peel lies.
Every patient improved.
Thirty-seven of 41 recruited patients completed all three sessions. All of them had a good or excellent response on the Jacob four-grade scale: 22 excellent (more than 70% of lesions cleared) and 15 good (50–70%). All had severe scarring at baseline, Goodman grade 3 or 4.
Darker skin did better, not worse.
Of the 23 patients with skin types 3 to 6, 13 achieved an excellent outcome, against 9 of the 13 with types 1 and 2. The authors attribute this to the more vigorous fibroblastic response in those skin types — a reversal of the usual expectation in scar treatment.
Complications were rare and temporary.
Two of 37 patients (5%) had a complication: one post-inflammatory erythema and one post-inflammatory hyperpigmentation, in a skin type 6 patient. Both resolved spontaneously within 8 weeks of finishing treatment. No scarring, no scar extension.
Isotretinoin did not delay healing.
Five patients (14%) continued low-dose isotretinoin at 0.2–0.4 mg/kg/day throughout. Mean re-epithelialisation was 6.4 days against 6.2 days for the other 32, with no complications in that group. The authors believe this is the first CROSS study to include patients on isotretinoin.
Partly-treated scars are treated partly.
Broad boxcar scars over 2 mm had acid applied only to the margin, to avoid painting a large surface that would heal slowly and risk extending the scar. Polymorphic scars — those that do not fit any classification — were treated only on the side with the sharp border, leaving the sloping edge alone for the same reason.
The brush may be doing more than convenience.
The technique is conventionally performed with a wooden applicator. The two studies in the literature reporting high scar-extension rates — 17% and 13.3% — both used wooden applicators, which the authors suggest may spill more acid onto healthy skin. No scar extension occurred here.
This fills a specific gap in the evidence.
TCA CROSS is well established for ice pick scars, with 73–80% of patients achieving over 70% improvement in a systematic review. Its use in other scar types was barely studied: only two prior studies had assessed it outside ice pick scars, and both pooled results across scar types rather than reporting them separately.
What this means for you
The skill is in where the acid does not go
Frosting appears within 8 to 16 seconds and the acid is placed only at the scar base. Almost every complication associated with this technique comes from spillover onto healthy skin.
Aftercare is part of the protocol
Paraffin moisturiser three times daily and gentle washes for six days, doxycycline for seven, SPF 50+ started three weeks before and resumed on day five. Antiviral cover for anyone with a cold sore history, and hydroquinone for skin type 4 and above.
Being on isotretinoin may not rule you out
Procedures have historically been deferred until well after isotretinoin. Five patients here stayed on it with no delay in healing and no complications — a small number, but consistent with a 2017 systematic review on superficial peels.
Why this matters
Focal high-concentration TCA has a good evidence base for ice pick scars and almost none for anything else, even though most people do not have one tidy scar type — they have boxcar scars, partial boxcar scars, and scars that fit no classification at all. In practice those get treated with energy devices, which are expensive and not obviously better for this pattern.
This study was designed to test the technique on exactly those scars, and to test a brush in place of the traditional wooden applicator. The brush is not a gimmick: it covers a scar base faster and more accurately, and the two studies reporting high scar-extension rates both used wooden sticks.
Why it matters to you
If you have boxcar scars and have been quoted for laser, this is a cheaper technique with, in this group, a complication rate of 5% and every patient improving. It is not right for rolling scars, and the person holding the brush matters more than the acid.
What this study cannot tell you
Every study has a boundary, and it is fairer to state it than to leave you to guess.
- Small, single-centre and single-operator. One dermatologist applied every treatment, so the results reflect that operator's technique as much as the method.
- There was no control group and no comparison against energy-based treatment, so no claim about which is better can be drawn from it.
- The scars treated were predominantly classical and partial boxcar scars on the cheeks and temples. Rolling scars, saucer scars, hypertrophic and bridging scars were excluded.
- The midline — forehead, nose and chin — was not assessed and may respond differently.
- The sample was too small to test statistically whether skin type predicts response, so that finding is descriptive only.
- Only five patients were on isotretinoin, which is too few to settle the question of procedural safety on it.
Questions people actually ask
What is TCA CROSS?
Chemical reconstruction of skin scars. A high concentration of trichloroacetic acid is placed into the base of an individual scar and nowhere else, so the surrounding skin is untouched.
The acid causes controlled injury at the scar base, and the skin then re-epithelialises from the preserved hair and oil gland units, laying down new collagen as it heals.
Does it work on boxcar scars, or only ice pick scars?
Ice pick scars are where the established evidence sits. This study was done specifically because the technique's use in other scar types was barely studied.
In 37 patients with classical and partial boxcar scars and polymorphic scars, every one improved after three sessions — 57% by more than 70% of lesions.
Is it safe in darker skin?
In this study darker skin did better, not worse: 13 of 23 patients with skin types 3 to 6 had an excellent result, against 9 of 13 with lighter skin.
One skin type 6 patient developed post-inflammatory pigmentation, which resolved within 8 weeks. Hydroquinone was prescribed beforehand for skin type 4 and above as part of the protocol.
Can I have it while taking roaccutane?
Five patients in this study stayed on low-dose isotretinoin throughout, healed in 6.4 days against 6.2 for everyone else, and had no complications. A 2017 systematic review reached a similar conclusion about superficial peels.
Five patients is a small number and the authors say so. It is a question for the clinician treating you, not a general clearance.
Cite this paper
Sun C, Lim D. Trichloroacetic Acid Paint for Boxcar and Polymorphic Acne Scars. Dermatologic Surgery. 2022. doi:10.1097/DSS.0000000000003339
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Published 2022 · Summary written and reviewed by Dr Davin Lim, FACD · General information, not personal medical advice.
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