Data for this retrospective cohort study were obtained from the PHARMO Record Linkage System (PHARMO RLS), a population-based patient centric data tracking system including high quality and complete information linked on a patient level of, among other things, patient demographics, drug dispensing records from community pharmacies and hospital discharge records of approximately 2.5 million individuals from 1998 and still ongoing in defined areas throughout the Netherlands. The hospital records are obtained from the Dutch National Medical Register (LMR) , which comprises all hospital admissions in the Netherlands, i.e. admissions for more than 24 hours and admissions for less than 24 hours for which a bed is required. These records include detailed information concerning the primary diagnosis, procedures, and dates of hospital admission and discharge. All diagnoses are coded according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). Procedures are coded according to the Dutch classification of procedures ("CvV - Classificatie van Verrichtingen"). No approval was required to access the databases, because the study was performed by employees of the PHARMO Institute (owner of the PHARMO RLS).
The source population included all patients with a primary hospital admission for Dupuytren's contracture (ICD-9-CM code 728.6) between January 1, 2004 and December 31, 2007. The date of the first primary hospital admission for Dupuytren's contracture in this study period was defined as the cohort entry date; consequently the source population included patients with a first hospitalization for Dupuytren's contracture, but also patients with previous hospitalizations for Dupuytren's contracture. Patients were included in the study cohort if they had a history of at least 12 months in PHARMO RLS before cohort entry date, in order to determine their co-morbidities and co-medication at the time of the first known hospitalization in the study period. Patients were followed from cohort entry date to end of data collection in the PHARMO RLS (i.e. the patient moves out of the PHARMO RLS catchment area), death, or end of the study period (December 31, 2008), whichever occurred first.
The yearly number of patients with a hospitalization for Dupuytren's contracture in the Netherlands between January 1, 2004 and December 31, 2007 was estimated by extrapolating the numbers derived from the source population, standardized for age and gender . Numbers are given per 100, 000 inhabitants, rounded off to 5, stratified by gender and age, and include 95% confidence intervals (CI).
For all study patients the following characteristics were determined at cohort entry date: gender, age, and co-morbidities/co-medication based on hospitalizations and/or drug use in the year prior to cohort entry date including diabetes mellitus (ICD-9-CM code 250 and/or use of antidiabetics), epilepsy (ICD-9-CM code 345 and/or use of antiepileptics), and use of lipid modifying agents. In addition, the following characteristics of the hospitalization at cohort entry date were determined: type of admission (day-case or inpatient), length of stay, recorded type of procedures (fasciotomy ("CvV"-procedure codes 5-820.2 and/or 5-821.2), fasciectomy ("CvV"-procedure codes 5-823.5 and not 5-884.2, 5-892, 5-893, 5-895, or 5-896 (skin related procedures) during same hospitalization), and dermatofasciectomy ("CvV"-procedure code 5-823.5 and 5-884.2, 5-892, 5-893, 5-895, or 5-896 (skin related procedures) during same hospitalization)), and treating specialty.
Rehospitalizations for Dupuytren's contracture (primary discharge diagnosis ICD-9-CM code 728.6) were assessed in the period between cohort entry date and end of follow-up. The number of rehospitalizations was determined, as was the time to first rehospitalization and the treating specialty during the first rehospitalization.
The 95% CI around the prevalence of patients with a hospitalization for Dupuytren's contracture (N per 100, 000) was calculated using the formula N ± 2*√N . Characteristics of patients with a hospitalization for Dupuytren's contracture were presented descriptively. As the latest data on national trends in hospitalization show that lengths of hospital stays are declining, a Cochran-Armitage test for trend  was used to assess whether the proportion of day-case admissions changed in a specific direction over time.
Survival functions describing the proportion and 95% CI of patients without a rehospitalization for Dupuytren's contracture over time were computed using Kaplan-Meier survival analyses censoring patients who were considered lost to follow-up .
Data management and analyses were conducted using SAS version 9.1 within SAS Enterprise Guide version 4.0 (SAS Institute Inc., Cary, NC, USA).