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Planing Chip Phenomenon – Clinical Sign of Psoriasis

The planing chip phenomenon is a clinical sign of psoriasis: when scales are scraped off, thin lamellae detach like wood shavings. It aids in diagnosis.

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Things worth knowing about "Planing Chip Phenomenon"

The planing chip phenomenon is a clinical sign of psoriasis: when scales are scraped off, thin lamellae detach like wood shavings. It aids in diagnosis.

What Is the Planing Chip Phenomenon?

The planing chip phenomenon (German: Hobelspanphänomen) is an important clinical examination sign observed in the skin condition psoriasis. It describes the characteristic detachment of silvery-white scales in thin, cohesive layers when a spatula or fingernail is gently scraped across an affected skin area. The detaching scales resemble thin wood shavings produced during planing – hence the name.

The planing chip phenomenon is one of the classic dermatological step signs of psoriasis and forms part of the so-called Auspitz triad, a sequence of three typical skin reactions observed upon mechanical manipulation of a psoriatic plaque.

Clinical Context: The Psoriasis Triad

The planing chip phenomenon is the first of three signs that appear in sequence when a psoriatic plaque is examined systematically:

1. Planing chip phenomenon: Gentle scraping causes scales to detach in thin, cohesive lamellae resembling wood shavings.
2. Last membrane phenomenon: After full removal of the scales, a thin, shiny membrane remains that can also be peeled off as a coherent sheet.
3. Auspitz sign (bloody dew): After removal of the last membrane, pinpoint bleeding appears on the exposed skin, caused by the characteristically dilated capillaries in the dermal papillae.

Cause and Pathogenesis

The planing chip phenomenon results from the pathologically accelerated cell division of the epidermis (outer skin layer) in psoriasis. While healthy skin renews itself approximately every 28 days, this cycle is reduced to just 3 to 7 days in psoriasis. The skin cells do not mature fully and undergo incomplete keratinization, leading to the formation of the typical thick, silvery-white scales. These scales have a lamellar structure, which is why they detach in characteristic thin layers.

Diagnostic Significance

The planing chip phenomenon has high diagnostic relevance during clinical skin examination. It helps to differentiate psoriasis from other scaling skin conditions, such as:

Seborrhoeic dermatitis: Scales are greasy and yellowish; the planing chip phenomenon is absent.
Tinea corporis (ringworm): Ring-shaped skin lesions without the characteristic scaling pattern.
Pityriasis rosea: Fine, bran-like scaling without lamellar detachment.

The presence of the planing chip phenomenon, together with the other signs of the psoriasis triad, is considered strong clinical evidence for psoriasis and can substantially support the diagnosis without the need for complex laboratory investigations.

How the Examination Is Performed

The examination for the planing chip phenomenon is carried out by a physician, usually a dermatologist. A blunt instrument, typically a wooden spatula or fingernail, is gently drawn across a scaly plaque. The procedure is generally painless and takes only a few seconds. A positive result is indicated by the characteristic lamellar detachment of the scales.

Treatment of the Underlying Condition: Psoriasis

The planing chip phenomenon itself requires no treatment, as it is purely a clinical examination sign. The underlying condition of psoriasis, however, does require treatment. Common therapeutic options include:

Topical therapy: Corticosteroids, vitamin D analogues (e.g., calcipotriol), dithranol, retinoids, and salicylic acid for scale removal.
Phototherapy: UVB irradiation or PUVA therapy (psoralen + UVA).
Systemic therapy: Methotrexate, ciclosporin, and fumaric acid esters for moderate to severe psoriasis.
Biologics: TNF-alpha inhibitors, IL-17 or IL-23 blockers for severe, treatment-resistant psoriasis.

References

Braun-Falco's Dermatology. Springer, 3rd edition (2000). ISBN 978-3-642-97933-0.
Nast A. et al. - S3 Guidelines for the Treatment of Psoriasis Vulgaris. Journal of the German Society of Dermatology (JDDG), 2021. Available at: https://www.awmf.org
Griffiths C.E.M. et al. - Psoriasis. The Lancet, 397(10281): 1301-1315 (2021). DOI: 10.1016/S0140-6736(20)32549-6

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