Elasticity per Thickness Index for Breast Sonoelastography: Quantitative Analysis of Implant- and Clinically Related Peri-Implant Capsular Formation

Background: Peri-implant capsular formation remains a multifactorial process influenced by implant characteristics and patient-specific tissue responses. Ultrasound with shear-wave elastography offers evaluation of the biomechanical properties of both the capsule and surrounding breast tissues. Elastographic assessment does not distinguish intrinsic stiffness from the amount of fibrotic tissue. We introduced the Elasticity per Thickness Index (ETI) as an exploratory measure of capsular stiffness related to capsule thickness. Methods: This prospective study assessed peri-implant capsules and adjacent breast tissue compartments using B-mode ultrasound and shear-wave elastography over more than two years of follow-up. Capsular stiffness, thickness, and the exploratory Elasticity per Thickness Index (ETI; kPa/mm) were compared across implant characteristics, and their associations with clinical variables were analyzed. Results: Significant postoperative increases in stiffness were found across multiple native tissue compartments. Breast tissues remained on average stiffer than preoperatively throughout follow-up. Implant type or shape (F and M series, L3, MP EU, HP EU; anatomical vs. round) showed no association with stiffness changes, although round implants tended to yield higher fascial stiffness (p = 0.080). The ETI was independent of implant shape (p = 0.430). Higher-profile implants showed a nominally higher ETI than medium-profile implants (39.1 ± 13.4 vs. 31.9 ± 13.4 kPa/mm; unadjusted p = 0.050), which should be interpreted as an exploratory finding. The ETI was not significantly associated with implant profile after accounting for bilateral clustering (p = 0.455). Capsular stiffness was inversely associated with time from surgery to assessment (Rs = −0.329, p = 0.013), although this cross-sectional association does not demonstrate longitudinal softening. Conclusions: At the postoperative assessment, the peri-implant capsule showed a mean stiffness of 15.52 ± 3.81 kPa. The ETI may provide a complementary thickness-related description of capsular stiffness, although its reliability and clinical relevance require further validation. No robust association between implant profile and the ETI was identified after accounting for bilateral clustering.