Abstract
Dear Colleagues,
A Letter to the Editor has expressed some concerns that need to be clarified regarding our paper ‘Prognostic value of severity of dislocation in late-detected developmental dysplasia of the hip’. 1 Firstly, we would like to thank the readers for their interest in this study.
The patients in Group 1 were treated with preliminary traction and closed reduction, which was the most common method for late-detected developmental dysplasia of the hip (DDH) around 1960. The aims were to obtain satisfactory hip reduction, maintain good femoral head coverage during childhood and adolescence, and to avoid complications. It is of course not possible to obtain these ambitious goals in all the patients, which is the reason that there are cases of less than optimal reduction and residual dysplasia with or without subluxation in all studies in this field.
We agree that it might seem strange that all the primary radiographs were still available in Group 1 whereas more than half the radiographs in Group 2 were missing. The reason was that the old radiographs in Group 1 had been kept in special files and were still available, whereas most of the radiographs in Group 2 had been discarded during reorganization of the hospital's radiograph archives. However, the lateral metaphysis height (LMH) method was available in all 54 hips in Group 2 because the first author (TT) had measured the radiographs prospectively from diagnosis to skeletal maturity.
‘Residual dysplasia’ was defined in our paper as: “Additional surgery to correct hip dysplasia/subluxation during childhood and adolescence or centre-edge angle < 20° at skeletal maturity”. We think this definition is sufficient.
We were asked about the position of the lower limbs at the point of radiography. Standard procedure was used: one anteroposterior radiograph of the pelvis with the child in the supine position, and care was taken to position the child with the legs parallel and to avoid rotation of the pelvis and hips. Even if a moderate internal or external rotation of the leg should occur, this would hardly affect LMH significantly, since the method is based on the most lateral point of the metaphysis, which is usually the same as the most proximal point, in relation to Hilgenreiner's line.
We emphasized that the LMH classification is especially useful in severe grades of DDH because it is less dependent on the superolateral margin of the acetabulum (SMA) than the International Hip Dysplasia Institute (IHDI) and Tönnis classifications. In a severely dislocated hip, there is usually bony resorption around the SMA, which makes it difficult and often unreliable to use the Tönnis classification, because the distinction between Grades II, III, and IV is entirely dependent on a correct location of the SMA. This problem could also cause unreliable distinction between Grades II and III of the IHDI classification. The LMH method does not use the SMA as a landmark for the distinction between the most severe (Grade III) and less severe dislocation (Grade II). Since Hilgenreiner's line is easy to identify even in severe dislocations, the distinction between Grades II and III in the LMH classification and Grades III and IV in the IHDI classification is quite reliable. Perkin's line is dependent on the position of SMA and is used to distinguish between Grade I and Grade II in all three classifications. However, as we wrote in the paper, this is not usually a significant problem since SMA is easier to identify in normal hips and hips with slight dislocation.
The LMH classification predicted that 50% of the Grade III hips in Group 1, treated with closed reduction, underwent total hip arthroplasty during long-term follow-up (Table 5). Table 3 showed that 72% of Grade III hips in Group II were treated with open reduction. We can anticipate that more modern treatment of severe late-detected DDH, that is more often open reduction, will lead to lower incidence of osteoarthritis and less need for total hip arthroplasty in the future. However, this important question is open to discussion until we see more long-term studies.
It would be positive if a multicentric reliability check of the LMH classification was carried out. In the meantime, we recommend our colleagues use the method in clinical practice.