This is a population based study of 212 children with CP followed until 9–16 years of age. Most of the children have not completed growth. However, it is known that most dislocations in children with CP occur before seven years of age [15
]. This is also supported by the decreasing number of children with debut of displacement after 3 years of age in the present study (Figure ).
Lateral displacement of the femoral head is common without acetabular dysplasia and acetabular dysplasia develops at a later stage than femoral head lateralisation [16
]. Measurement of the lateral displacement is sufficient for hip screening in CP [7
]. The MP is a valid and reliable measure of lateral displacement [13
], and probably the most commonly used. We choose the two levels of MP, 33 and 40% in the follow-up programme, CPUP, and in this study. When introduced, a hip with MP > 33% was the definition of a subluxated hip [18
]. Some hips with MP 33–40% do not need preventive surgery, and these hips should be regarded as hips at risk, needing intensified observation, and operative intervention if increasing MP is seen. Hips with MP ≥ 40% have a high risk of further displacement, indicating need for surgical intervention [16
]. The surgery performed in this materal has been described earlier [6
The most common age at first registration of MP > 33% or > 40% was 3–4 years (Figure ). Some of the children showed displacement at the first radiographic examination in the follow-up, so the real age at development of lateral displacement could be even lower. Some children showed lateral displacement already at two years of age. This means that it is of the utmost importance that children with CP are identified early, and that children at risk are examined radiographically as early as possible. Scrutton and Baird [19
] recommended that children with spastic diplegia or quadriplegia should have a first radiograph at age 30 months. Based on our findings we recommend that children with highest risk should have their first radiographic examination even earlier, if possible.
Hip displacement in relation to CP-subtype could be divided into three levels of risk. Children with spastic hemiplegia or ataxia had very low risk, children with spastic diplegia or the dyskinetic types (dystonia and atetosis) had an intermediate risk, and children with spastic tetraplegia had the highest risk (Figure ). There are, however, several problems using CP-subtype as indicator of hips at risk in screening programme. It is sometimes difficult to distinguish spastic hemiplegia from spastic diplegia with asymmetric severity, or pure ataxia from ataxic diplegia. The descriptors for the motor types of CP have not been universally agreed on; new definitions are under development, and the definitions may be difficult to apply in a reliable manner [20
]. It is sometimes impossible to determine the CP-subtype before 4 years of age.
Hip displacement was directly related to the level of GMFCS (Figure ). Similar results have been shown from the Victorian Cerebral palsy register [21
]. The GMFCS has proved to be a valid and reliable tool [12
] and has been reported to remain relatively stable over time [22
]. According to the designers of the GMFCS, most children will remain at the same level from age 2 [24
]. By using GMFCS, instead of CP-subtype, as an indicator of hips at risk in screening programme, all the problems discussed with subtypes seem to be avoided.
When the CPUP screening programme was initiated in 1994, the GMFCS was not available. Based on the findings in this and earlier studies we now have changed our programme for radiographic hip screening, and use GMFCS instead of subdiagnosis (Table ). Most children with spastic hemiplegia and ataxia are at level GMFCS I, and all children with spastic tetraplegia are at level V. Children with spastic diplegia represent about one third of the population of children with CP and are distributed at all levels of function according to GMFCS. By changing to the new screening programme, those with spastic diplegia and GMFCS level I or II no longer need yearly radiographic examinations. The new screening programme results in about 35% fewer radiographic examinations up to 8 years of age in a total population of children with CP, compared with our previous programme.
The radiographic prevention programme 2007
The increased risk of hip displacement related to range of motion found in the crude estimates (Table ) disappeared in the adjusted estimates. The increased risk described is therefore likely to be influenced by either the distribution of age, GMFCS-level or CP-subtype in the study population. The narrow confidence intervals corresponding to the adjusted estimates indicate that no change in risk seems to follow an increase in either measurement of ROM. This implies that the measurement of the range of motion is a poor indicator of risk and cannot replace radiographic examinations for hip screening. However, a decreasing ROM over time in an individual child could warrant radiographic hip examination.