Editorial comment: Craniocaudal bull’s eye technique for superior calyceal access during percutaneous nephrolithotomy: Determination of angle, efficacy, and outcomes

A well-made percutaneous renal access is probably the most important step in percutaneous nephrolithotomy, and the authors deserve credit for tackling a problem that every endourologist who has accessed the upper pole encounters, without quite naming it.[1] The calyces of the middle and lower renal poles are often paired in an anteroposterior plane, while the superior calyx, draining through a single craniocaudally directed infundibulum (in about 95% of kidneys),[2] usually has two calyces directed medially and laterally. Puncturing the upper calyx often does not comply with the geometry of the classical “bull’s eye” technique. Applying the bull’s eye technique without any modification to the upper pole creates an oblique, torque-generating percutaneous tract. The present study attempt to formalise a craniocaudal bull’s eye (CCBE) by first defining the anatomically ideal angle on cross-sectional imaging, then testing a rib-constrained version of it clinically, is a genuinely useful contribution, and the reported outcomes (23 of 24 successful punctures, 21 on the first attempt, no transfusions, no postoperative complications)[1] are reassuring for a technique still early in its clinical utilization.

A closer reading of the operative technique, together with the accompanying intraoperative fluoroscopic images, raises a geometric concern that the discussion does not address. Ko et al. had described the bull’s-eye technique.[3] In that account, the C-arm manoeuvre is explicitly two-staged and asymmetric for either pole: the image intensifier is first angled toward the surgeon by 20°–30° in the axial, mediolateral plane, and only then is a much smaller 5°–10° tilt added in the cranial or caudal direction, depending on whether the upper or lower pole is being accessed.[3] It is the larger, axial rotation that does the principal work of bringing the desired posterior calyx “end-on” and circular on the screen; the craniocaudal component is a secondary refinement to gauge the depth and not the primary maneuver. The technique described in the present manuscript inverts this relationship: the C-arm is held at 0° in the axial plane throughout, and the entire “bull’s eye” is generated by craniocaudal rotation alone, of a magnitude (up to 68°–69° in the anatomically “ideal” Phase 1 data, 15° as actually achieved in Phase 2) far larger than the 5°–10° secondary tilt that the classic technique reserves for this axis. With the axial, toward-the-surgeon rotation omitted altogether, the maneuver that Ko et al. identify as responsible for selecting the correct calyx is simply absent, and the resulting “end-on” alignment, by simple geometry, will result in a more medial puncture[3]. This is exactly what Figures 4 and 5C of the manuscript appear to show[1]: a needle trajectory directed at the medial-most calyx of the upper pole group, close to the line of the infundibulum, rather than at a laterally placed calyceal cup.

This distinction is not academic. Sampaio’s endocast studies, on which the authors’ own introduction draws, established that puncture through or near the infundibulum rather than the papilla carries a disproportionately higher risk of hemorrhage from interlobar and segmental vessels,[4] and this risk is highest, of all calyceal groups, in the upper pole: the posterior segmental artery lies in direct relation to the posterior surface of the upper-pole infundibulum in over half of kidneys, a fact the authors themselves cite in their introduction.[5] Thus, a medial calyx puncture of the upper pole predisposes to injury of the posterior segmental artery.[6]

The same single-axis geometry compounds the second, and arguably more immediate, risk of upper-pole access in this cohort: pleural violation. Eighty-three percent of the Phase 2 punctures were supracostal, above the 12^th rib, which is already a recognized risk factor for hydrothorax, and which the discussion acknowledges by citing the CROES data on supra-11^th access.[7] What the discussion does not address is that the mediolateral position of a puncture, independent of its craniocaudal level, is itself an established predictor of thoracic complications. As Ko et al. note, it is specifically the medial portion of the lower ribs that carries pleural attachment – the medial half of the 12^th rib and the medial three-quarters of the 11^th rib,[3] so a puncture displaced medially is mechanically more likely to traverse pleura even at an identical rib level. This is borne out clinically in a series of 347 supracostal punctures; a puncture site medial to the mid-scapular line was an independent predictor of hydrothorax on multivariable analysis, and pleural injury can largely be avoided by staying lateral to the mid-scapular line and above the lateral half of the rib.[8,9] A technique that, by virtue of its single-axis rotation, systematically biases the surgeon toward a more medial supracostal puncture is therefore combining two independent risk factors for hydrothorax: supracostal level and medial tract position in the same access. That the present series recorded no intrathoracic complications is reassuring, but a small cohort of 24 patients cannot be taken as proof of safety against an event whose reported incidence in comparable supracostal series runs into several per cent.

None of this argues against the central premise of the paper that the superior calyx deserves its own, anatomically congruent bull’s eye geometry. What we feel is that a dominant 20°–30° rotation toward the surgeon, refined by a secondary 5°–10° craniocaudal tilt for the upper pole,[3] for this superior-calyx-specific context would, in principle, preserve the torque-reducing benefit the authors are seeking while restoring the lateral displacement of the puncture trajectory that protects both the posterior segmental artery and the pleura.

A related methodological point is worth raising. The conclusion that CCBE “may produce less torque than a mediolateral puncture” is plausible, but was not directly tested in this study; there was no comparator arm, and torque or intrarenal pressure was not measured. Confirming this advantage and confirming the safety of the access point itself would both be helped by mapping the actual puncture site on the existing fluoroscopic and postoperative imaging against Brödel’s avascular plane and the mid-scapular line, rather than relying on the absence of complications in a small, single-center series.

The authors are to be commended for quantifying, for the first time, the anatomically ideal craniocaudal angle for superior calyceal puncture, and for demonstrating that a rib-constrained version of it is feasible, teachable and apparently safe in this initial series. The technique deserves further development and a larger, multicentric validation, and one in which the puncture site is documented against the mid-scapular line and the posterior segmental artery rather than inferred from outcomes alone. Rotating the C-arm craniocaudally solves one half of the upper-pole puncture problem; restoring even a modest rotation toward the surgeon, in keeping with the classic two-axis technique described by Ko et al.,[3] may be what is needed to solve the other.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.