Assessing the Variability of the Twelfth Rib as a Landmark for Percutaneous Nephrolithotomy Using Computed Tomography

Jake A Tempo,1 Georgina M Williams,1 Iain K Robertson,2,3 Darren J Rama,1 Claire EA Pascoe,1 Richard J Cetti1 1Department of Urology, Launceston General Hospital, Launceston, Tasmania, Australia; 2College of Health and Medicine, University of Tasmania, Hobart, Tasmania, Australia; 3Clifford Craig Foundation, Launceston General Hospital, Launceston, Tasmania, AustraliaCorrespondence: Jake A Tempo, Department of Urology, Launceston General Hospital, 3/182 Hawdon Street, Heidelberg, VIC, Launceston, Tasmania, 3084, Australia, Tel +61448438937, Email jake.tempo@outlook.comBackground: The variability in length of the twelfth rib is mentioned in the literature but has never been formally studied. The highly variable rib length provides a challenge for urologists seeking a consistent landmark for Percutaneous Nephrolithotomy (PCNL). We set out to determine whether the twelfth rib is safe to use as a consistent landmark for surgery.Methods: Single centre, cross-sectional retrospective observational study and analysis of CT scans of 100 adults who had imaging between 23rd March and 12th April 2020. The distance from the mid-sagittal line to the twelfth rib tip in the axial plane was measured as was the distance from the twelfth rib tip to the kidney, spleen and liver.Results: Length from the mid-sagittal line to the right twelfth rib tip varied from 46 (percentile 95% CI 40 to 57) to 136mm (percentile 95% CI 133 to 138). On the left, the distances varied from 55 (percentile 95% CI 50 to 64) to 134mm (percentile 95% CI 131 to 135). Twenty-three percent of people had an organ lying between the tip of the twelfth rib and the kidney on the right and 11% on the left.Conclusion: The twelfth rib is highly variable in length. Similar variability was recorded in the distance from the tip to intra-abdominal organs. Due to the frequency of organs lying between the tip of the rib and the kidney it should not be used as a landmark for accessing the kidney without prior knowledge of an individual patient’s anatomy as seen on imaging.Keywords: percutaneous nephrolithotomy, rib, anatomical marker