We studied the prevalence of hip pain related to age, gender and gross motor function level, and the associations between hip pain and; age, gender, gross motor function level, range of hip and knee motion, spasticity in the muscles around the hip joint and degree of hip displacement. The main findings were that hip displacement and lower range of motion, but not spasticity, were associated with hip pain.
The prevalence of hip pain and pain at any site was similar to other studies based on CPUP data [10, 28], and less than half compared to the population-based SPARCLE study, conducted in seven European countries [9]. This was also the case for the prevalence of pain at any site, which was almost twice as common in the SPARCLE study. In the SPARCLE study, 54% of the children between 8 and 12 years experienced pain in the previous week, [4] a percentage that increased to 74% in a follow up at 13–17 years [9]. Differences in age structure and GMFCS distribution between the two studies might explain some of the differences. Less sensitive pain screening in CPUP, without specified recall time when asking about pain, might also help explain the differences. However, the lower pain prevalence might, to some extent, be contributed to successful prevention of secondary musculoskeletal complications in CPUP [17]. As expected, proxy reports were more common in younger children and for children with lower motor ability. Hip pain was more frequently reported in the proxy reported group than the self-reported group. Part of the reason for this could be that the prevalence of hip pain is higher in children at higher GMFCS levels, and that children reported by proxy generally were at higher GMFCS levels. However, to report on somebody else’s pain is difficult.
There was a trend of increasing prevalence of hip pain with increasing age. However, in the multivariable analysis, no significant association was found. This differs from what is usually observed for pain in this population where pain and age are generally positively associated [13]. Regarding gender, there was no significant difference in hip pain prevalence and no association with hip pain in the multivariable analyses. The lack of age and gender effects on hip pain specifically have been observed previously [14].
The prevalence of hip pain at GMFCS IV and V was higher than at GMFCS I-III. In the multivariable analysis, with children at GMFCS II as reference, only being at GMFCS V was borderline significantly associated with hip pain. In the additional analysis, with children at GMFCS I as reference, being at GMFCS V was significantly associated with hip pain. Possible causes are inactivity, extended periods of time spent in the same position or poor positioning. As the majority in this group did not answer the pain items themselves, it is also possible that the proxy reporters misinterpreted the pain or the pain site, e.g. abdominal pain might have been interpreted as hip pain or vice versa.
There was a trend of increasing prevalence of hip pain with higher MPs. Compared to MP 0–30, the OR for hip pain were significantly higher at MP 31–40% and 41–100%. An association between hip pain and MP > 50% has previously been found in children with CP [14]. In adults with CP, associations between hip pain and MP ≥ 33 has been found [29]. Our study confirms the association between hip pain and higher MPs and implies that the risk for hip pain is increased already from the 31–40 range. In Sweden, surgeries to prevent hip dislocations are sometimes recommended in children with MP > 33%. Our findings do indicate that pain seems to occur at low MPs and that interventions should be considered early to prevent pain. However, the majority of painful hips had MP < 30%, indicating that other factors than hip displacement also drive hip pain.
Decreased ROMs in abduction, flexion and inwards hip rotation of the hip were significantly associated with hip pain. The association between hip pain and decreased ROM has previously been reported in adults with CP [12]. One possible explanation is that decreased ROM causes painful strenuous movement patterns and body positions. It is also possible that children with decreased ROMs are closer to their maximum ROM for longer periods and that this causes pain. However, it is also likely that the maximum ROM is not measured in a painful hip.
Higher spasticity level was not significantly associated with hip pain for any muscle group, except for MAS level 2 in knee extensors. However, wide confidence intervals and absence of significant associations for the other spasticity levels in this muscle group implies that the association could be due to chance and not clinically important. Association between pain and spasticity is not well substantiated in the literature. Children with spastic CP have not been found to report pain more frequently than children with other subtypes [28]. However, spasticity has been related to deterioration in ROM during growth and as such might indirectly affect pain [30].
As this was a cross sectional study, causality cannot be inferred. The pain items in CPUP were created to mainly screen for pain. More information of the pain experience is needed and to that end, more detailed pain items have now been included in CPUP. The lack of data on side of body, type, intensity and duration of pain limited the analysis. Due to some of the children’s comorbidities and/or young age/s, self-report was not always possible. The effect of proxy reports is unclear, in other studies both under- and over reporting of pain have been reported [13, 31]. It should be noted that it is very difficult to know for sure that the pain came from the hip joint even if that is where the pain seemed to originate. It is possible that the pain radiated from other areas, such as the stomach or the spine. Moreover, with respect to generalizability, the rather low MP reported in children with CP in Sweden might limit this to populations with similarly low MPs.
The major strengths of this study come from the large cohort analysed, including almost all children with CP in Sweden at all GMFCS levels. Since almost the entire population of children with CP were included, there is little risk for selection bias and our results are likely generalizable.