Use of a novel maximum protrusion index to describe severe pectus carinatum

The absence of standardized criteria for grading pectus carinatum (PC) severity hampers clinical decision-making and insurance approvals. Although the Haller Index (HI) is frequently used, it underestimates deformity by measuring the distance to the posterior sternum rather than the point of maximal protrusion. This study introduces the Maximum Protrusion Index (MPI), a novel metric designed to more accurately reflect PC severity.

MPI is defined as the transverse chest diameter divided by the anterior-posterior distance from the vertebral column to the point of maximal chest wall protrusion (typically costal cartilage). We calculated and compared MPI and HI from CTs of 41 PC patients that underwent Ravitch and 110 controls with normal chest walls. Receiver Operating Characteristic (ROC) analysis was used to evaluate the diagnostic performance of both indices, and optimal thresholds were determined with calculated likelihood ratios (LR).

MPI measurements were more consistent than HI. At the accepted threshold of ≤2, HI failed to identify 36.6 % of surgical PC cases (sensitivity/specificity: 63.4 %/92.7 %). In contrast, MPI ≤2 identified all surgical patients (100 % sensitivity, 58.2 % specificity). The optimal HI diagnostic threshold was ≤1.975 (sensitivity/specificity = 63.4 %/97.3 %, LR+ = 23.25). The optimal MPI threshold was ≤1.745 (sensitivity/specificity = 85.4 %/98.2 %, LR+ = 46.95), reducing false positives at the expense of not identifying some PC.

The MPI is a standardized descriptive metric for PC that is more consistent and sensitive in identifying operative PC than the HI. We propose using MPI ≤2 as diagnostic criteria for severe PC when supported by physical exam findings. Further validation is warranted.