It is generally considered that SUFE occurs in susceptible growth plates exposed to appropriate shear forces, and many studies attempting to assess the resistance of growth plates to such forces have been carried out on both animal and human material (Chung et al. 1976
, Peltonen et al. 1984
A previously reported study by me (Tayton 2007b
) confirmed the presence of the epiphyseal tubercle as a significant feature of the human capital growth plate complex, and a major obstacle to any slippage occurring as a result of shear forces (which for practical purposes act in a single plane). Unfortunately, the strength of the conclusions in that study was limited by the fact that there were only 11 anatomical specimens, several of which bore the erosive changes of aging.
The study reported here was carried out to extend the investigation, and in particular to try to assess the quality of the epiphyseal tubercle in vivo. With case numbers limited by ethical constraints, its findings confirm the proposition that the tubercle is a normal feature of the capital growth plate complex, although assessment of its size from the films was found to be difficult. In the previous study, it had averaged 4 mm in length on anatomical specimens.
Although it seems probable that this epiphyseal “peg” is present in most adolescents, it should be noted that the plain radiographs illustrated here have been selected for the clarity with which they show it, and that many were examined which failed to show any convincing sign of one.
An attempt to carry out this study using MRI scanning was abandoned because currently these scans give very small images and enlargement causes loss of definition due to pixel size. Within the limitations of pixel enlargement, well-developed epiphyseal tubercles could be seen in the younger child. On the other hand, excellent images of the epiphyseal peg were produced by CT scanning, which is clearly the investigation of choice.
The findings on the plain radiographs (and MRI scans) show that in younger children the tubercle is wide-based and fairly flat, and that as maturity progresses it becomes somewhat narrower, longer, and more pointed. As a result, its anti-slipping effect seems to change from one of friction at two irregular surfaces to one of pegging. The concept of pegging is supported by the CT scan studies on the older adolescents, where the tubercle was seen to remain reasonably prominent up to the age of 16. Of the 4 cases with SUFE who were CT-scanned, the tubercle in one was clearly acting as a pivot for the epiphysis, and in one other it was just beginning to lose good contact with the metaphysis—and it seems likely that it was twisting free of its socket.
Although the value of this radiographic study is restricted by the numbers, it does confirm the presence and size of the epiphyseal tubercle as a fairly regular feature in adolescents. It also shows that using CT scanning, the tubercle can nearly always be demonstrated and that it seems to be acting as a peg.
Tempting though it is to suggest that these findings represent the norm, a larger population survey is needed to establish this. The findings do, however, support the theory that a relatively stable chronic epiphyseal slip cannot take place in a simple linear direction due to the configuration of the bone surfaces adjacent to the growth plate, and in particular to the presence of the epiphyseal tubercle.
The clinical relevance of this is two-fold. Firstly, because in early chronic SUFE the epiphysis is reasonably stable, it should not be necessary to carry out an epiphyseodesis. The insertion of a properly anchored anti-rotation pin should be sufficient to control stability and so allow the physis to continue to grow normally and the femoral head to remodel. Secondly, in cases of unilateral slippage where the risk of a contralateral slip is considered high, a CT scan of the unaffected hip will indicate the size of its epiphyseal tubercle; and if it is dysplastic, then its prophylactic stabilization should be considered.