To explore the effect of the shape of foramen ovale and the size of the external orifice on percutaneous microballoon compression (PMC) of the semilunar ganglion.

Three hundred and eighty-two patients with primary trigeminal neuralgia (TN) who admitted to the Neurosurgery Department of Nanjing Drum Tower Hospital, the Affiliated Hospital of Nanjing University Medical School, from January 2017 to June 2018 were underwent thin-layer CT scanning of skull base before operations, then the images were reconstructed in three dimensions, the shape of foramen ovale was observed and the area of foramen ovale was measured from the direction of surgical puncture subsequently. Finally evaluate the effect of the shape of foramen ovale and the size of external orifice on the operations according to the results of intraoperative puncture.

According to the balloon was placed into Meckel’s cavity and showed a typical "pear shaped" imaging, which was regarded as successful and included in the successful group. The failure of foramen ovale puncture or the success of foramen ovale puncture, but the balloon failed to be placed into Meckel’s cavity and failed to show a typical "pear shaped" , which was regarded as failure and included in the failure group. There were 356 patients in the successful group, the success rate was 93.19%, the oval foramens look like oval in 324 cases (91.01%), and the areas of external orifice were 23.42-34.23 mm², the median area was 30.27 mm², oval foramens with round shape in 32 cases (8.99%), the areas of the external orifice were 24.15-33.00 mm², median area was 30.15 mm². There were 26 patients in the successful group, the failure rate was 6.81%. Among them, 1 failed case due to unsuccessful puncture of foramen ovale, the remaining 25 cases failed to Meckel’s capsule. Twenty cases (76.92%) with oval outer opening, and the areas were 11.67-48.84 mm², the median area was 24.15 mm² in this group, otherwise 6 cases(23.08%) with round outer opening, their areas were 14.38-43.51 mm², the median area was 25.40 mm². Although there was one patient in the failure group with an orifice area of 48.84 mm², which was larger than all cases in the successful group, but the number of median areas in the failure group was significantly smaller than the successful puncture group (P<0.05). The proportion of circular external orifice of foramen ovale in the failure group was larger than successful group (P<0.05). One special case with oval-likely foramen ovale was failed to puncture the foramen ovale, the area of external orifice was 20.00 mm², but the inner and outer part of the foramen ovale were angled, and the puncture needle couldn’t pass through.

The success rate of puncture in the treatment of TN by PMC is very high, the reasons of failure related to foramen ovale were mostly due to the small external area and round shape of foramen ovale, and a few related to the large external diameter of foramen ovale. In special cases, the folding and angulation of the internal and external part of foramen ovale would also lead to the failure of operation.