Clinical PaperThe modified percent depth: Another step toward quantifying severity of pectus excavatum without cross-sectional imaging

IntroductionCurrent approaches to quantifying the severity of pectus excavatum require internal measurements based on cross-sectional imaging. This study evaluated the modified percent depth (MPD), a novel index of severity that can be obtained with external measurements, potentially avoiding the need for cross-sectional imaging.MethodsPatients undergoing surgical repair of pectus excavatum (pectus group), and those undergoing cross-sectional imaging for unrelated reasons (control group), between 2010 and 2016 were included. The MPD of the deformity was calculated using external (i.e. skin surface to skin surface) measurements from the radiographic images. The same external measurements were taken using chest calipers on a subset of these patients in the outpatient clinic. The optimal threshold for MPD that defined severe pectus deformity was derived from receiver-operator characteristic (ROC) analysis. Sensitivity and specificity of the MPD was compared with that of the Haller Index (HI) and Correction Index (CI).ResultsThere were 92 children (49 pectus, 43 controls) included. The median MPD was 20.2% and 4.2% for pectus and control patients, respectively (p < 0.0001). An MPD cutoff of 10% optimally discriminated between severe pectus patients and controls by ROC analysis. An MPD of > 10% had 98% sensitivity and 98% specificity for severe pectus deformity. Sensitivity and specificity were respectively 93% and 93% for HI > 3.25, and 100% and 79% for CI > 10.ConclusionAn MPD > 10% performs slightly better than the HI and CI in distinguishing patients with severe pectus deformities. This novel measurement approach offers distinct advantages over existing indices, in that it does not require cross-sectional imaging and can be done using chest calipers in the office setting. Further studies with larger sample size are needed to verify reproducibility of the technique.Level of evidenceLevel II, Study of Diagnostic Test.