This study utilized a subsample of prospective, population-based Northern Finland Birth Cohort 1966 (NFBC1966) (University of Oulu, 1966; Nordström et al. 2021). Initially, NFBC1966 comprised pregnant women living in Northern Finland (i.e. the provinces of Oulu and Lapland) whose expected delivery dates fell between Jan 1 and Dec 31, 1966. The cohort included 12,068 mothers and 12,231 children, with a coverage of 96% of all births during 1966 in Northern Finland. Prospective data collection began in the 16th gestational week, and the NFBC1966 participants have been followed ever since. Broad questionnaires and clinical examinations have been used to gather information on the participants' health status and lifestyle habits.
At the age of 46, a total of 5,861 NFBC1966 participants responded to postal questionnaires and attended clinical examinations. In addition, 1946 individuals underwent radiography of the knee joint. A total of 1131 individuals were excluded due to 1) male sex, 2) missing reproductive or confounder data, 3) previous knee surgery, 4) bone pathologies in the radiographs (mostly osteoarthritic changes), or 5) technically inadequate radiographs. Thus, the final sample of this study comprised 815 women.
Knee breadth measurements were taken from digital radiographs of the right knee joint by an author of the study (A.K.). A detailed description of the procedure has been given in a previous publication (Keisu et al 2019). Radiographs were accessed and measured using neaView Radiology software version 2.31 (Neagen Oy, Oulu, Finland). Posteroanterior radiographs were utilized, with individuals positioned in fixed flexion view (Kan et al., 2017; Niinimäki et al. 2010).
The following measurements were taken from each radiograph: 1) mediolateral breadth of the articular surface of the femoral condyles (FCML), and 2) mediolateral breadth of the articular surface of the tibial plateau (TPML). FCML was measured by drawing a line tangential to the inferiormost points of the femoral condyles; this line was transposed to the widest part between the femoral condyles. TPML was measured as close to the border of the tibial plateau as possible. Measurements were recorded to the nearest 0.1 mm. The initial measurements were converted into true sizes with the help of a metal calibration disc of 30 mm in diameter attached on the participant’s right leg. The technical error of measurement (TEM) and relative technical error of measurement (rTEM) were reported by Keisu and colleagues (2019), and the repeatability was high for all the measurements (TEM 0.1–0.5 mm, rTEM 0.1–0.6%).
In the 46-year follow-up questionnaire, women were asked about the number of deliveries, ectopic pregnancies, miscarriages and abortions they had undergone during their lifetime. As described in a previous publication (Oura et al 2018), gravidity was calculated as the overall number of pregnancies, and parity as the number of deliveries of each woman. Women with no deliveries were classed as ’nulliparous’, those with one delivery as ’primiparous’, and those with a history of several deliveries were classed as ‘multiparous’. Those with ≥ 5 deliveries were classed as ‘grand multiparous’.
In the clinical examination at the age of 46, a study nurse systematically measured the height and weight of each individual. Body mass index (BMI) was calculated as weight divided by height squared.
Education, smoking history and leisure-time physical activity were elicited in the 46-year follow-up questionnaire. Education years, a proxy for socioeconomic status, was determined by asking: ‘What is your basic education? 1) Less than nine years of elementary school, 2) elementary school, 3) matriculation examination’; and ’What is your vocational education? 1) None, 2) occupational course, 3) vocational school, 4) vocational college, 5) polytechnic, 6) university, 7) other, 8) unfinished course’. The responses were classed according to the Finnish education system as follows: < 9 years, 9–12 years, or > 12 years.
Smoking history was elicited using two questions: 1) ‘Have you ever smoked cigarettes (yes/no)?’ and 2) ‘Do you currently smoke (yes/no)?’. Individuals were classed as non-smokers, former smokers, or current smokers.
Leisure-time physical activity was elicited by asking: ‘How often do you participate in brisk physical activity/exercise [defined as causing at least some sweating and breathlessness] during your leisure time? 1) Daily, 2) 4–6 times a week, 3) 2–3 times a week, 4) Once a week, 5) 2–3 times a month, 6) Once a month or less often.’ The responses were regrouped as follows: < 1 times/week, 1—3 times/week, or ≥ 4 times/week.
SPSS software (IBM, Armonk, NY, USA) version 27, 64-bit edition was used for the statistical analyses. P values < 0.05 were considered statistically significant. Means with standard deviations (SDs), medians with interquartile ranges (IQRs) and percentages with frequencies were used as descriptive statistics. Characteristics of the sample were presented before and after stratification by parity.
The associations of gravidity and parity with knee breadth (i.e. FCML and TPML in mm) were analyzed using general linear models. Beta coefficients, 95% confidence intervals (CIs) and P values were documented from the output. Models were first constructed without adjustments (unadjusted models), and then confounder variables were added (adjusted models). Gravidity and parity were modelled as continuous variables (where beta coeffcients are interpreted relative to one pregnancy/delivery), and by comparing groups with each other (e.g. multiparous vs. others, where beta coefficients represent mean difference between groups).
The study adhered to the principles of the Declaration of Helsinki, with voluntary participation and signed informed consent. Sensitive details were replaced by anonymous identification codes. The Ethics Committee of the Northern Ostrobothnia Hospital District approved the study.