DISCUSSION
We hypothesized that there would be an optimal angle of entry and maximal safe distance the trocar could advance through the ilium when aspirating. Because male and female pelvic anatomy differs, we also hypothesized that there would be differences in distance and size measurements for males and females. Our results supported our hypothesis that there is an ideal approach angle. The results also showed that the maximum distance the trocar can advance and the width of the PSIS table differ significantly between males and females.
Although pelvic anatomy differs between males and females, there should be an ideal entry angle that would allow maximum advancement into the ilium without perforating the anterior wall, which we defined as the approach angle. In our comparison of 26 MRI scans, we found that the approach angle did not differ significantly between the 2 groups (13 males, 13 females). This allows clinicians to enter the PSIS at roughly 24° medial to the parasagittal line, maximizing the space before puncturing into the anterior pelvis in either males or females.
If clinicians were to enter perpendicular to the patient’s PSIS, they would, on average, be able to advance only 3.20 cm before encountering the SI joint. When entering at 24° as we recommend, the average distance increases to 7.14 cm. Although the angle did not differ significantly, there was a significant difference between males and females in the length from the PSIS to the anterior wall, with males having 7.53 cm distance and females 6.74 cm. This is an important measurement because if the anterior ilium wall is punctured, the femoral nerve and the common, internal and external iliac arteries and veins could be damaged, resulting in retroperitoneal hemorrhage.
A fatality in 2001 in the United Kingdom led to a national audit of bone marrow aspiration and biopsies.4-6 Although these procedures were done primarily for patients with cancer, hemorrhagic events were the most frequent and serious events. This audit led to the identification of many risk factors. Bain4-6 conducted reviews of bone marrow aspirations and biopsies in the United Kingdom from 2002 to 2004. Of a total of 53,088 procedures conducted during that time frame, 48 (0.09%) adverse events occurred, with 29 (0.05%) being hemorrhagic events. Although infrequent, hemorrhagic adverse events represent significant morbidity. Reviews such as those conducted by Bain4-6 highlight the importance of a study that helps determine the optimal parameters for aspiration to ensure safety and reliability.
Hernigou and colleagues2,3 conducted studies analyzing different “sectors” in an attempt to develop a safe aspiration technique. They found that obese patients were at higher risk, and some sites of aspiration (sectors 1, 4, 5) had increased risk for perforation and damage to surrounding structures. Their sector 6, which incorporated the entirety of the PSIS table, was considered the safest, most reliable site for trocar introduction.2,3 Hernigou and colleagues,2 in comparing the bone mass of the sectors, also noted that sector 6 has the greatest bone thickness close to the entry point, making it the most favorable site. The PSIS is not just a point; it is more a “table.” The PSIS can be palpated posteriorly, but this is inaccurate and unreliable, particularly in larger individuals. The PSIS table can be identified on ultrasound before introducing the trocar, which is a more reliable method of landmark identification than palpation guidance, just as in ultrasound-guided injections7-16 and procedures.9,12,17-19
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