Finger joint laxity, number of previous pregnancies and pregnancy induced back pain in a cohort study

All pregnant women living in two districts of the city of Sundsvall, Sweden, were identified through check-ups at the antenatal care units in the metropolitan area, at the offices of practicing gynecologists and at the outpatient clinic at the local hospital. All Caucasian and Swedish speaking women attending during early pregnancy in 1991 were sampled for this study. Two hundred and twenty-seven pregnant women fulfilled the sampling criteria, of whom 222 attended the antenatal care units serving the two districts. These 222 women were invited to participate in the study. Twenty-two declined participation, which left 200 (88.1%) women as the final study population. All women were apparently healthy, and none had ongoing medication. During the follow-up period, 10 women left the study because of spontaneous abortion, two women declined further participation, and one woman moved from the area.

The methods for this back pain study have been described in detail previously [14]. Briefly, data were collected at three appointments during pregnancy, on average at 11 (range 6 to 19), 24 (range 21 to 27) and 36 (range 34 to 38) completed gestational weeks, and at a fourth appointment 13 (range 4 to 29) weeks postpartum. Early pregnancy was defined as gestational week 11. At each appointment, the women completed a questionnaire and underwent a general clinical examination including measurement of the passive abduction angle of the left fourth finger. For all women, the duration of pregnancy was confirmed by ultrasonography in estimated gestational week 19 and was registered as completed weeks of gestation.

The questionnaire included an instrument for measuring ongoing pain, its location, intensity, and consequences in terms of disability. In addition, there were questions about previous obstetric history, previous back pain problems and current smoking habits. The questionnaire was completed in privacy with no time limit and was checked for completeness.

Women who reported pain were instructed to indicate the location of the pain on a pain drawing. More than one location could be indicated. The back locations were coded as cervical spine, thoracic spine, lumbar spine, and sacral spine. In the present study, women who reported any of these four back pain locations were pooled into a group labeled “back pain” and women who reported neither of these back pain locations were pooled into a group labeled “no back pain”, at each study visit. The women were also asked to estimate the date of onset of pain for each pain location. Only one onset time was registered at each appointment. If several back pain locations were indicated, the time of onset of the lowest back pain location was registered.

The intensity of pain at the moment and the worst pain during the past week, was described on two visual analogue scales (VAS): 0 mm indicating no pain and 100 mm indicating intolerable pain [25].

Twelve disability ratings measuring the ability to perform activities of daily living were recorded on VAS scales, where 0 mm indicated no restriction on the activity and 100 mm indicated inability to do the activity. The mean score of the 12 ratings was used as the disability rating index (DRI) [26].

At each of the four appointments, the passive abduction angle of the left fourth finger was measured with a well defined and constant abduction force of 1.7 Newton applied to the medial side of the distal phalanx, with the forearm and hand in horizontal position and the second finger immobilized (Figure 1). The angle between the fourth and second finger was measured using a protractor to the nearest degree by the same physician throughout the study. The same method has been used before assessing abduction angle of the left fourth finger in early pregnancy [27] and a significant correlation between abduction angle of left fourth finger and Beighton score, was reported (r = 0.55, p < 0,001) [28]. The reliability of the abduction angle measurements was calculated by the intra-individual coefficient of variance. The coefficients of variance between the first and second measurement was 0.077, between the second and third 0.070 and between the third and fourth 0.071. The coefficient of variance of all four angle measurements across the nine months study period was 0.085.

At study inclusion weight and height were measured. Height was measured barefoot with a wall-mounted tape measure to the nearest centimeter. Weight was measured with indoor clothing on a balance lever scale with digital presentation of kilograms to one decimal point. Body mass index was calculated as weight (kg)/height² (m²).

No systematic pain treatment was given, but women with severe pain were offered sick listing or physiotherapy, which included sacroiliac belt and pool training.

Permission for this study was obtained from the Research and Ethics Committee of the University of Umeå and all women gave their informed consent. The procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation and with the Helsinki Declaration of 1975, as revised in 1983.

Descriptive data were given as frequencies, percentages, means and standard deviations (SD). Spearman correlation was used to test association of inter-individual abduction angles at different time intervals. Differences between the dependent intra-individual abduction angles at different time intervals were tested with paired t-test. Comparison between groups regarding continuous data was performed using t-test and comparisons of categorical data with χ². Only two-tailed tests were used and a p-value <0.05 was considered statistically significant. No account was taken of multiple comparisons.

To control for possible confounding, multiple logistic regression analysis was used and P-values and confidence intervals (CI) were given. Time since last delivery was excluded from the regression analysis since it excessively reduced the total number of women included in the analysis. The set of possible determinants in early pregnancy were: passive abduction angle of the left fourth finger, age, weight, height, number of previous pregnancies, current smoking habits and reported previous back pain problems. The categorization of the nominal and ordinal factors used in the model were 0, 1, 2, 3 or ≥4 previous pregnancies, no current smoking (0), smoking <10 cigarettes/day (1) and smoking ≥10 cigarettes/day (2) and yes/no for previous back pain problems. For the analyses of the regression surface in Figure 2, the logistic regression model was used to compute expected mean incidence estimates of back pain postpartum, based on the passive abduction angle of the left fourth finger in early pregnancy and the number of previous pregnancies. To estimate the ability of the passive abduction angle of the left fourth finger measurement to predict the risk of developing pregnancy-induced back pain, the receiver operating characteristic curve and logistic regression were used. In a post-hoc power calculation with a β-risk of 0.80, a significance level of 0.05 and a standard deviation of 6.5° indicated that a study population of 200 women would detect a difference in the abduction angle of the left fourth finger of 2.6°. The results were obtained using the Statistical Analysis System, version 9.3, SAS Institute Inc, Cary, NC, USA.