We thank Dr. Edward for his comments. In this study,1we used the Philips S12 5–12 MHz real-time linear-array ultrasound transducer (Philips Medical Systems, Andover, MA). The contact surface area of the transducer head is 4.73 cm2.
We tried both the linear-array and the curvilinear-array transducers. We found that the linear-array transducer was sufficient to depict the sacral cornua and the base of sacrum even in obese patients. The above-mentioned transducer is capable of penetrating to a depth of 8–10 cm. In patients weighing more than 100 kg, the distance from the skin to the base of the sacrum is far less than 8 cm. During the entire injection process, we adjusted the patient’s posture (such as flexing the hip more) to obtain the most suitable position for placement of the ultrasound probe and injection. The best sonogram can be obtained when full contact is made between the transducer head and the examined area. We found the curvilinear-array transducer inconvenient because full contact with the examined area cannot be achieved most of the time.
I agree with Dr. Huang that ultrasound has the limitation of not being able to observe the inadvertent placement of the needle intravascularly. During the initial draft of the manuscript, the Discussion section contained more content. I even mentioned the possible sonogram findings for patients without sacral hiatus. One of our authors wanted to include in the Discussion section the fact that ultrasound cannot be used to observe the inadvertent needle placement into the vessels. The reviewers suggested that we shorten the manuscript and focus mainly on the application of ultrasound in locating the sacral hiatus. Therefore, after revision, the entire article focused mainly on the sonograms of the sacral hiatus, and how ultrasound can be used as an adjuvant tool in caudal needle placement. Content about the drugs used and complications of caudal injections was not included in the article.
We were fortunate that the Christmas tree–like appearance (fig. 4 in the article) was observed under fluoroscope in all of our patients after locating the sacral hiatus accurately first by ultrasound. This symbolizes the fact that intravascular injection did not occur. Fluoroscopic guidance and contrast dye administration is still the standard in the assessment of the spread of the injected drugs into the desired target levels during caudal epidural injection.
Chang Gung Memorial Hospital, Tao-Yuan County, Taiwan. firstname.lastname@example.org