Oral Frictional Hyperkeratosis
- Author: Jose Luis Tapia, DDS; Chief Editor: William D James, MD more...
The oral mucosa is lined by stratified squamous epithelium and has topographic differences that correlate with physical demands or a higher degree of specialization. For example, the epithelium lining the floor of the mouth, the ventral side of the tongue, the buccal mucosa, and the soft palate is nonkeratinized; however, the epithelium associated with the gingiva and hard palate is usually keratinized. The dorsal surface of the tongue is also keratinized, but it is referred to as specialized mucosa because of the presence of papillae. The dorsum of the tongue, the hard palate, and the gingival tissues are keratinized to better respond to masticatory demands.
Hyperkeratinization (excessive formation of tenaciously attached keratin) may be present in a variety of clinical conditions, including genetic, physiologic, inflammatory, immunologic, premalignant, and malignant conditions. The change may result from a local insult, including chemical, thermal, or physical irritants. This article focuses on the oral hyperkeratinization that results from friction. Friction (the constant rubbing of 2 surfaces against one another) in the oral cavity may result in the development of clinically observable white patches.
Various names have been used to describe particular examples of frictional keratosis (FK). These include frictional keratosis arising from excessive force while brushing the teeth (toothbrush keratosis); the constant rubbing of the tongue against the teeth (tongue thrust keratosis); the constant sucking, pressure, and irritation of the teeth against the buccal mucosa along the plane of occlusion (linea alba); and the habit of chronic cheek, tongue, or lip biting (cheek- or lip-bite keratosis). Injuries to the oral mucosa, using items such as a pen, toothpicks, or fingernails, may result in frictional keratosis.
The white patches of frictional keratosis that develop in the oral cavity represent a chronic, low-grade, mechanical process that is analogous to the formation of a callus on the skin. The most common local factors involved in this process are tissue chewing (mainly on the buccal mucosa or lips), ill-fitting or irregularly surfaced removable dental prostheses (dentures), fractured or malposed teeth, poorly adapted dental restorations, orthodontic appliances, improper toothbrushing, and constant mastication on edentulous alveolar ridges. The constant irritation stimulates the production of excessive keratin, with a subsequent change in the thickness and the color of the involved mucosa.
Few large epidemiologic studies documenting the prevalence of various oral lesions, including oral frictional keratosis, have been published.
The most comprehensive survey on the prevalence of oral mucosal lesions is the Third National Health and Nutrition Examination Survey (NHANES III). Oral examinations were performed on 17,235 noninstitutionalized civilian adults. Cheek and lip biting had a point prevalence of 3.05% and ranked third in oral lesion prevalence, while frictional keratosis had a point prevalence of 2.67% and ranked fourth.  In the same national survey, when 10,030 children aged 2-17 years were evaluated, the point prevalence for cheek and lip biting was 1.89% and 0.26% for frictional keratosis. 
In another extensive survey of 23,616 white American adults from Minnesota that evaluated a wide range of oral lesions, the number of cases of cheek-biting keratosis was 1.2 cases per 1000 individuals.  In this same study, frictional keratosis was not differentiated from leukoplakic lesions, so the prevalence of frictional keratosis alone cannot be determined.
Linea alba is a common mucosal variation that is rarely singled out as a specific entity in prevalence studies. In a limited study of young men, 13% had this mucosal alteration. 
In a Danish study of 20,333 people aged 15 years and older, the prevalences of cheek and lip biting and frictional keratosis were slightly higher than those reported in the US studies. The prevalence for cheek and lip biting was 5.1%, and the prevalence for frictional keratosis was 5.5%. Similarly, the prevalence for frictional keratosis from a small study sample of Kenyan adults was 5.5%. In Slovenia, the prevalence was 2.7% for cheek and lip biting and 2.2% for frictional keratosis. In a study of Turkish adolescents, linea alba was the second most common lesion, with a prevalence of 5.3%.
When studies were limited to individuals seeking care in oral medicine clinics, a wider frequency of occurrence was noted. In a limited study of patients treated at a dental school in Spain, the rate was 11.5% for frictional keratosis, 10.7% for linea alba, and 6.8% for cheek biting. In an India dental school study, frictional keratosis was the most common oral lesion detected, occurring in 5.8% of the patients. When referred hospitalized and clinic patients were evaluated in an Australian oral medicine clinic, hyperkeratotic lesions, including tobacco-induced lesions, were documented in 11.6% of the hospitalized patients and 10.3% of the clinic patients.
The largest study of 23,785 patients, attending a Mexican dental school clinic, found frictional keratosis to be the third most common oral mucosal finding, with a prevalence rate of 32 cases per 1000 patients, while cheek-biting lesions were ranked fifth, or 21.7 cases per 1000 patients.
Frictional keratosis and its variants do not cause symptoms and are benign mucosal lesions that remain localized with no associated mortality or morbidity.
No racial predilection seems apparent for oral frictional keratosis.
In general, frictional keratosis has no known sex predilection, except for cheek biting and lip biting, which are twice as prevalent in women compared with men.
Oral frictional keratosis affects persons from a wide range of ages, and contributing factors determine which age group is more commonly affected. In general, oral frictional keratosis lesions are more common in adults.
Neville BW, Damm DD, Allen CM, Bouquot JE. Physical and Chemical Injuries. Oral and Maxillofacial Pathology. 3rd ed. St. Louis, Mo: WB Saunders; 2009. 285-329.
Cam K, Santoro A, Lee JB. Oral frictional hyperkeratosis (morsicatio buccarum): an entity to be considered in the differential diagnosis of white oral mucosal lesions. Skinmed. 2012 Mar-Apr. 10(2):114-5. [Medline].
Shulman JD, Beach MM, Rivera-Hidalgo F. The prevalence of oral mucosal lesions in U.S. adults: data from the Third National Health and Nutrition Examination Survey, 1988-1994. J Am Dent Assoc. 2004 Sep. 135(9):1279-86. [Medline].
Shulman JD. Prevalence of oral mucosal lesions in children and youths in the USA. Int J Paediatr Dent. 2005 Mar. 15(2):89-97. [Medline].
Bouquot JE, Gorlin RJ. Leukoplakia, lichen planus, and other oral keratoses in 23,616 white Americans over the age of 35 years. Oral Surg Oral Med Oral Pathol. 1986 Apr. 61(4):373-81. [Medline].
Kashani HG, Mackenzie IC, Kerber PE. Cytology of linea alba using a filter imprint technique. Clin Prev Dent. 1980. 2:21-4.
Axéll T. Occurrence of leukoplakia and some other oral white lesions among 20,333 adult Swedish people. Community Dent Oral Epidemiol. 1987 Feb. 15(1):46-51. [Medline].
Macigo FG, Mwaniki DL, Guthua SW. Prevalence of oral mucosal lesions in a Kenyan population with special reference to oral leukoplakia. East Afr Med J. 1995 Dec. 72(12):778-82. [Medline].
Kovac-Kovacic M, Skaleric U. The prevalence of oral mucosal lesions in a population in Ljubljana, Slovenia. J Oral Pathol Med. 2000 Aug. 29(7):331-5. [Medline].
Parlak AH, Koybasi S, Yavuz T, et al. Prevalence of oral lesions in 13- to 16-year-old students in Duzce, Turkey. Oral Dis. 2006 Nov. 12(6):553-8. [Medline].
Martinez Diaz-Canel AI, Garcia-Pola Vallejo MJ. Epidemiological study of oral mucosa pathology in patients of the Oviedo School of Stomatology. Med Oral. 2002 Jan-Feb. 7(1):4-9, 10-6.
Mathew AL, Pai KM, Sholapurkar AA, Vengal M. The prevalence of oral mucosal lesions in patients visiting a dental school in Southern India. Indian J Dent Res. 2008 Apr-Jun. 19(2):99-103. [Medline].
Farah CS, Simanovic B, Savage NW. Scope of practice, referral patterns and lesion occurrence of an oral medicine service in Australia. Oral Dis. 2008 May. 14(4):367-75. [Medline].
Castellanos JL, Díaz-Guzmán L. Lesions of the oral mucosa: an epidemiological study of 23785 Mexican patients. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2008 Jan. 105(1):79-85. [Medline].
Dabrowa T, Dobrowolska A, Wieleba W. The role of friction in the mechanism of retaining the partial removable dentures with double crown system. Acta Bioeng Biomech. 2013. 15(4):43-8. [Medline].
Natarajan E, Woo SB. Benign alveolar ridge keratosis (oral lichen simplex chronicus): A distinct clinicopathologic entity. J Am Acad Dermatol. 2008 Jan. 58(1):151-7. [Medline].
Sarifakioglu E, Gunduz C, Gorpelioglu C. Oral mucosa manifestations in 100 pregnant versus non-pregnant patients: an epidemiological observational study. Eur J Dermatol. 2006 Nov-Dec. 16(6):674-6. [Medline].
Scully C. Cannabis; adverse effects from an oromucosal spray. Br Dent J. 2007 Sep 22. 203(6):E12; discussion 336-7. [Medline].
Flaitz CM. Differential diagnosis of oral mucosal lesions in children and adolescents. Adv Dermatol. 2000. 16:39-78; discussion 79. [Medline].
Coleman GC, Flaitz CM, Vincent SD. Differential diagnosis of oral soft tissue lesions. Tex Dent J. 2002 Jun. 119(6):484-8, 490-2, 494-503. [Medline].
Scully C, Felix DH. Oral Medicine--update for the dental practitioner: oral white patches. Br Dent J. 2005 Nov 12. 199(9):565-72. [Medline].
Miller RL, Gould AR, Bernstein ML. Cinnamon-induced stomatitis venenata, Clinical and characteristic histopathologic features. Oral Surg Oral Med Oral Pathol. 1992 Jun. 73(6):708-16. [Medline].
Messadi DV, Younai FS, Liu HH, Guo G, Wang CY. The clinical effectiveness of reflectance optical spectroscopy for the in vivo diagnosis of oral lesions. Int J Oral Sci. 2014 Sep. 6 (3):162-7. [Medline].
[Guideline] Rethman MP, Carpenter W, Cohen EE, et al. Evidence-based clinical recommendations regarding screening for oral squamous cell carcinomas. J Am Dent Assoc. 2010 May. 141(5):509-20. [Medline].
Flaitz CM, Felefli S. Complications of an unrecognized cheek biting habit following a dental visit. Pediatr Dent. 2000 Nov-Dec. 22(6):511-2. [Medline].
Jones KB, Jordan R. White lesions in the oral cavity: clinical presentation, diagnosis, and treatment. Semin Cutan Med Surg. 2015 Dec. 34 (4):161-70. [Medline].
Gabrić D, Vrdoljak DV, Boras VV. Extensive oral mucosal hyperkeratosis caused by over-the-counter long lasting snoring relief agent. Br J Oral Maxillofac Surg. 2015 Aug 1. [Medline].
Makino T, Mizawa M, Inoue S, Noguchi M, Shimizu T. The expression profile of filaggrin-2 in the normal and pathologic human oral mucosa. Arch Dermatol Res. 2016 Feb 8. [Medline].