Overview and definition
The orthokeratinized odontogenic cyst (OOC) is a benign developmental odontogenic cyst of the jaws lined by orthokeratinized stratified squamous epithelium.[1][2] It arises from remnants of the dental lamina and was formerly considered a variant of the odontogenic keratocyst (OKC); however, it is now recognized as a distinct entity owing to its different histopathologic features, non-aggressive biologic behavior, and low recurrence rate.[3][4] The OOC characteristically presents as a well-defined, corticated, unilocular radiolucency in the posterior mandible, commonly associated with an impacted tooth.[1][5] Unlike the OKC, the OOC is not associated with nevoid basal cell carcinoma syndrome (NBCCS).[6] Management is conservative, typically by enucleation, with an excellent prognosis.[1][5]
Terminology and classification
The 5th edition WHO Classification of Head and Neck Tumours (2022) lists the orthokeratinized odontogenic cyst as a separate developmental cyst of the jaws, distinct from the odontogenic keratocyst.[7][8] The 4th edition WHO classification (2017) first formally separated the OOC from the OKC (then reclassified from "keratocystic odontogenic tumour" back to "odontogenic keratocyst").[4]
- Current accepted name: Orthokeratinized odontogenic cyst (OOC)
- Obsolete terminology: The OOC was previously grouped under "odontogenic keratocyst, orthokeratinized variant" or "orthokeratinized variant of keratocystic odontogenic tumour." These terms should no longer be used, as they incorrectly imply a subtype relationship with the OKC.[1][3]
Epidemiology
The OOC is an uncommon developmental cyst, comprising approximately 10% of cases historically coded as odontogenic keratocysts.[1]
- Age: Broad age range (13–75 years), with peak incidence in the third and fourth decades of life.[1][5]
- Sex: Male predilection, reported at approximately 1.4:1 to 2.6:1 male-to-female ratio across large series.[1][5]
- Anatomic distribution: Marked mandibular predilection; the mandible accounts for approximately 83–90% of cases, with the posterior body and ramus being the most common sites. Maxillary involvement is uncommon.[1][5]
- Multiplicity: Synchronous multiple OOCs are rare and show a predilection for young adult males; unlike multiple OKCs, multiple OOCs have not been associated with NBCCS.[2][9]
Clinical presentation
Presentation and symptoms
Most OOCs are discovered incidentally on routine dental imaging or present with painless jaw swelling.[10] Swelling is the most frequent presenting symptom.[10] Pain and discharge occur occasionally. The interval between symptom awareness and clinical presentation tends to be short.[10]
Examination findings
- Firm, non-tender bony expansion of the affected jaw region
- Overlying mucosa is typically intact and normal in color
- An associated impacted tooth is present in approximately half of cases[1]
Tempo and complications
Growth is slow and expansile. Paresthesia of the inferior alveolar nerve is rare. Secondary infection may occur but is uncommon. Rare cases of malignant transformation to intraosseous squamous cell carcinoma have been documented in recurrent OOCs, though this is exceptional.[2][11]
Radiographic findings
Location and distribution
The OOC occurs predominantly in the posterior mandible, particularly the molar and ramus region.[1][5] Maxillary lesions are uncommon and, when present, tend to involve the posterior maxilla.[1] The cyst is characteristically associated with the crown of an impacted tooth in approximately half of cases, mimicking a dentigerous relationship.[1][10]
Periphery and shape
- Periphery: Well-defined, corticated border is characteristic. The corticated rim is typically continuous and sharply demarcated from surrounding bone.[1][5][10]
- Shape: Most lesions are round to ovoid. Scalloped margins, which are common in OKC, are not a typical feature of OOC.[12]
Internal architecture
- The internal structure is characteristically a homogeneous radiolucency.[1][5]
- Occasional radiopaque foci within the lumen have been described in a large series, a feature not previously recognized in OOC.[2]
- Multilocular internal architecture is uncommon; the large majority of OOCs are unilocular. In one series of 61 cases, approximately 87% were unilocular and 13% multilocular. All 48 cases in another series were unilocular.[1][5][12]
Effects on teeth and adjacent structures
- Impacted teeth: Association with an impacted tooth (dentigerous relationship) is common, occurring in roughly half of cases.[1]
- Tooth displacement: Rare; OOC infrequently displaces adjacent erupted teeth, which is a distinguishing feature from OKC.[12]
- Root resorption: Uncommon but documented in small numbers of cases across multiple series. This feature was previously considered absent in OOC but has been confirmed in recent large studies.[2][5]
- Inferior alveolar canal: Displacement and loss of visibility of the canal have been observed in mandibular cases.[10]
Growth pattern, cortical involvement, and extent
- OOC tends to grow in an expansile pattern with buccolingual expansion of the cortical plates, in contrast to the anteroposterior medullary growth pattern typical of OKC.[5][12]
- The cortical bone expansion rate of OOC is greater than that of unilocular OKC.[12]
- Loss of continuity (perforation) of the buccal or lingual cortex is common on cross-sectional imaging, observed in the majority of cases evaluated with CT or CBCT.[5]
- Large lesions may displace or erode the inferior border of the mandible. Maxillary lesions may extend into the maxillary sinus.[10]
Variants and uncommon features
- Multilocular presentation, though uncommon, should not exclude OOC from the differential.[1]
- Radiopaque foci within the cyst lumen are rare but have been documented.[2]
- Periapical presentation mimicking a radicular cyst has been reported occasionally.[13]
Differential diagnosis
The OOC presents as a well-defined, corticated, unilocular radiolucency in the posterior mandible, frequently associated with an impacted tooth. This appearance overlaps substantially with several odontogenic cysts and cystic neoplasms, and definitive distinction requires histopathologic examination.[14]
| Entity | Why it is considered | Best discriminator |
|---|---|---|
| Odontogenic keratocyst | Well-defined posterior mandibular radiolucency, often associated with impacted tooth, may be unilocular or multilocular | OOC shows greater buccolingual expansion and is characteristically unilocular; OKC more often grows in an anteroposterior direction with scalloped margins and multilocularity; definitive separation requires histopathology[1][3][5][12] |
| Dentigerous cyst | Unilocular pericoronal radiolucency enveloping the crown of an impacted tooth | OOC may show greater expansion and cortical perforation; dentigerous cyst lacks keratinized lining on histopathology[5][14][15][16] |
| Unicystic ameloblastoma | Well-defined unilocular radiolucency in the posterior mandible, often pericoronal | Ameloblastoma more frequently causes root resorption and may show intraluminal or mural nodularity; biopsy is essential to exclude this more aggressive entity[14][17][18] |
| Radicular cyst | Periapical radiolucency associated with a nonvital tooth | OOC may rarely present in a periapical location; vitality testing and histopathologic identification of orthokeratinized lining distinguish the two[13][15] |
The highest-risk diagnostic pitfall is misclassifying an OOC as an OKC, which carries substantially different recurrence risk and may prompt unnecessarily aggressive treatment, or failing to identify a unicystic ameloblastoma, which requires different surgical management. Incisional biopsy or complete histopathologic evaluation of the excised specimen is essential for definitive diagnosis.[1][14]
Histopathology
Diagnostic microscopic features
The OOC is lined by a thin, uniform stratified squamous epithelium with a prominent granular layer and a surface of orthokeratinized (anucleate) keratin.[1][19] The epithelial lining is typically four to eight cell layers thick with a flat epithelial-connective tissue interface (flat rete ridges).[19] The luminal surface contains laminated orthokeratin, often with an "onion-skin" pattern.[19] The basal cell layer is flat or cuboidal, lacking the palisaded, hyperchromatic columnar basal cells that characterize the OKC.[1][20]
The fibrous cyst wall is typically thin and non-inflamed. The cyst lumen contains keratinous material.
Inflammatory changes
When inflamed, the epithelial lining may become thickened with rete peg formation and focal loss of keratinization, potentially mimicking a non-specific odontogenic cyst.[19] Extensive sampling is important when inflammation obscures the diagnostic orthokeratinized areas.
Histologic variants and unusual findings
- A unique histiocytic lining has been described in some OOCs.[2]
- Rare cases show features suggestive of multipotentiality of the odontogenic epithelium, including association with ameloblastoma.[2]
Radiologic-pathologic correlation
The well-defined corticated border corresponds to a fibrous capsule with reactive bone at the periphery. The homogeneous radiolucent interior reflects the keratinous cyst contents. Radiopaque foci, when present, may correspond to dystrophic calcification or other intraluminal material.[2]
Immunohistochemistry and molecular features
- Ki-67 and p63 expression are significantly lower in OOC than in OKC, reflecting lower proliferative activity.[1][5]
- Cyclin D1 expression is also significantly lower in OOC compared with OKC.[5]
- Keratin profiling shows OOC expresses keratins 1, 2, and 10 and loricrin, consistent with differentiation toward normal epidermis, in contrast to OKC which expresses keratins 4, 13, 17, and 19.[21]
- GLI2 (hedgehog pathway effector), mTOR, and BCL2 are not significantly expressed in OOC, further distinguishing it from OKC and supporting a different histopathogenesis.[21]
- OOC is not associated with PTCH1 mutations or hedgehog pathway activation.[6][21]
Management and prognosis
Treatment
Conservative surgical enucleation is the standard treatment for OOC.[1][5] For large cysts, decompression or marsupialization followed by enucleation is an effective alternative that reduces surgical morbidity.[5] Aggressive adjunctive measures used for OKC (Carnoy's solution, peripheral ostectomy, resection) are generally not warranted given the low recurrence rate of OOC.[5]
Recurrence
The recurrence rate is low. In a series of 42 patients followed for a mean of 76.8 months, no recurrences were observed.[1] Another series of 45 patients reported a recurrence rate of approximately 4% over a mean follow-up of 32.5 months.[5] This contrasts sharply with the OKC recurrence rate of approximately 25%.[7]
Surveillance
Periodic clinical and radiographic follow-up after surgery is recommended. Although recurrence is uncommon, rare cases of malignant transformation (intraosseous squamous cell carcinoma) arising in recurrent OOC have been documented.[2][11] A shared p53 mutation between a dysplastic OOC and a subsequent intraosseous squamous cell carcinoma has been identified in one case, suggesting that p53 mutation may be involved in this rare event.[11]
Prognosis
The overall prognosis is excellent. OOC behaves as a benign, non-aggressive cyst with low proliferative activity and minimal recurrence potential after adequate surgical removal.[1][5]
Special considerations
Distinction from nevoid basal cell carcinoma syndrome
Unlike the OKC, the OOC is not associated with NBCCS (Gorlin syndrome).[6][9] Multiple OOCs have been reported but occur without syndromic features.[2][9] Therefore, the identification of an OOC does not warrant screening for NBCCS, which is an important distinction from the OKC where such evaluation is indicated, particularly in children and young adults with multiple or recurrent lesions.[6]
Rare neoplastic associations
A single case of OOC combined with a BRAF-mutated ameloblastoma has been reported, suggesting that oncogenic mutations may occasionally arise in odontogenic epithelium associated with OOC.[2] This remains an isolated observation and does not alter standard management.