Intracapsular Pressures After Stable Slipped Capital Femoral Epiphysis.

PURPOSE

Stable slipped capital femoral epiphysis (SCFE) has been shown to have a lower rate of avascular necrosis than unstable SCFE. A recent study found increased intracapsular hip pressures in the setting of unstable SCFE, thus increasing the risk of osteonecrosis. The purpose of this study was to measure the intracapsular pressure in stable SCFE and compare it to the intracapsular pressure in normal hips and in unstable SCFE.

METHODS

Thirteen hips with stable SCFE and 15 hips with unstable SCFE were identified. Using a side-bored needle, intracapsular hip pressures were measured at the time of surgery. Within these 2 study groups, 11 unaffected (normal) hips were also measured. Diastolic blood pressure and mean arterial pressure at the time of measurement were also recorded.

RESULTS

The average intracapsular hip pressure in the stable SCFE group was 27.0 mm Hg, whereas the average pressure in the unstable SCFE group was 48.2 mm Hg and the average pressure in the normal group was 21.8 mm Hg. There was no significant difference between the normal and stable SCFE groups. There was a statistically significant difference between the stable SCFE and unstable SCFE groups (P<0.001). We found similar trends when comparing the intracapsular hip pressure as a percentage of the mean arterial pressure as well as the difference between diastolic blood pressure and hip pressure.

CONCLUSIONS

As expected, the intracapsular pressure in the setting of stable SCFE approaches that of normal hips. This may explain why the risk of AVN in stable SCFE is significantly lower than that of unstable SCFE. It also supports the idea that capsulotomy is indicated for unstable slips to decrease the elevated hip pressure but not in stable SCFE.