Lazaridou et al.21	Skudutyte-Rysstad et al.22	Ganzetti et al.23
Country of origin	Greek	Norway	Italy
Study design	Case-control	Case-control	Case-control
Year of publication	2012	2014	2014
Cases	Cases of psoriasis were consecutively recruited from outpatient clinic of the study hospital (Hospital of skin and venereal disease in Thessaloniki, Greek) from January 2011 to May 2011. Diagnosis required histopathological confirmation. Patients who were receiving systemic therapy for psoriasis were excluded.	Cases of psoriasis were consecutively recruited from outpatient clinic of the study hospital (Oslo-University Hospital-Rikshospitalet, Norway). Diagnosis was made by study physicians. Patients with concomitant other autoimmune disorders, malignancy and pregnancy were excluded.	Cases of psoriasis were consecutively recruited from outpatient clinic of the study hospital. Diagnosis was made by study physicians. Cases never received biologic agents prior to enrollment.
Controls	Controls were sex and age-matched. They were recruited from the same outpatient clinic. Controls did not have psoriasis or other autoimmune skin diseases.	Controls were randomly selected from the Norwegian National Population Register. Controls did not have psoriasis.	Controls were sex and age-matched. They were recruited from the same outpatient clinic.
Identification and verification of periodontitis	Diagnosis of periodontitis was made by study dentists who were blinded to skin status. The evaluation was done by the measurement of the periodontal parameters according to standard method. Radiologic confirmation of bone loss was also done.	Diagnosis of periodontitis was made by study dentists who were blinded to skin status. The evaluation was done by the measurement of the periodontal parameters according to standard method. Periodontitis was defined according to the definition by CDC.	Diagnosis of periodontitis was made by study dentists.
Number of subjects (cases/controls)	100/100	50/121	60/45
Percentage of female (cases/controls)	57.0/57.0	24.0/50.0	42.0/47.0
Mean age in years (cases/controls)	57.2/57.2	44.4/48.6	44.4/48.7
Confounder assessed	None	Propensity score, smoking and dental visits	None
Quality assessment (Newcastle– Ottawa scale)	Selection: 2 stars Comparability: 1 star Outcome: 3 stars	Selection: 3 stars Comparability: 1 stars Outcome: 3 stars	Selection: 2 stars Comparability: 1 star Outcome: 3 stars