A morphological analysis of calcium hydroxylapatite and poly-l-lactic acid biostimulator particles

Abstract

Injectable fillers, pivotal in aesthetic medicine, have evolved significantly with recent trends favoring biostimulators like calcium hydroxylapatite (CaHA-CMC; Radiesse, Merz Aesthetics, Raleigh, NC) and poly-l-lactic acid (PLLA; Sculptra Aesthetics, Galderma, Dallas, TX). This study aims to compare the particle morphology of these two injectables and examine its potential clinical implications. Utilizing advanced light and scanning electron microscopy techniques, the physical characteristics of CaHA-CMC and PLLA particles were analyzed, including shape, size, circularity, roundness, aspect ratio, and quantity of phagocytosable particles. The findings reveal several morphological contrasts: CaHA-CMC particles exhibited a smooth, homogenous, spherical morphology with diameters predominantly ranging between 20 and 45 µm, while PLLA particles varied considerably in shape and size, appearing as micro flakes ranging from 2 to 150 µm in major axis length. The circularity and roundness of CaHA-CMC particles were significantly higher compared to PLLA, indicating a more uniform shape. Aspect ratio analysis further underscored these differences, with CaHA-CMC particles showing a closer resemblance to circles, unlike the more oblong PLLA particles. Quantification of the phagocytosable content of both injectables revealed a higher percentage of phagocytosable particles in PLLA. These morphological distinctions may influence the tissue response to each treatment. CaHA-CMC's uniform, spherical particles may result in reduced inflammatory cell recruitment, whereas PLLA's heterogeneous particle morphology may evoke a more pronounced inflammatory response.

Keywords: CaHA; PLLA; calcium hydroxylapatite; poly‐l‐lactic acid; radiesse; sculptra.

FIGURE 1

An example of the workflow for image analysis using ImageJ's biovoxxel plugin. (A) Standard brightfield image of calcium hydroxylapatite (CaHA) particles prior to analysis. (B) Shows the particles after applying a binary mask. (C) Shows the post watershed colorimetric heat map with pink showing perfect circularity. (D) Brightfield images of poly‐l‐lactic acid (PLLA) (E) after binary conversion and (F) after colorimetric shape mapping.

FIGURE 2

Scanning electron microscopy (SEM) images of calcium hydroxylapatite (CaHA‐CMC) (A and B) and PLLA (C and D) particles.

FIGURE 3

Brightfield images of calcium hydroxylapatite (CaHA‐CMC) (A and C) and poly‐l‐lactic acid (PLLA) (B and D) particles.

FIGURE 4

Circularity of calcium hydroxylapatite (CaHA‐CMC) and poly‐l‐lactic acid (PLLA) particles. (A) The quantification of circularity graphically illustrated, with a perfect circle measuring at 100% circularity. (B) Box‐and‐whisker plots of CaHA‐CMC and PLLA particle circularity and their (C) frequency distributions illustrating significantly different average circularities and significantly different frequency distributions.

FIGURE 5

Roundness of calcium hydroxylapatite (CaHA‐CMC) and poly‐l‐lactic acid (PLLA) particles. (A) The quantification of roundness graphically illustrated, with a perfect circle measuring at 100% round. (B) Box‐and‐whisker plots of CaHA‐CMC and PLLA particle circularity and their (C) frequency distributions illustrating significantly different average roundness and significantly different frequency distributions.

FIGURE 6

Aspect ratios of calcium hydroxylapatite (CaHA‐CMC) and poly‐l‐lactic acid (PLLA) particles. (A) The quantification of aspect ratio graphically illustrated, with a perfect circle measuring at 1.0 and increasing with axial elongation. (B) Box‐and‐whisker plots of CaHA‐CMC and PLLA particle aspect ratios and their (C) frequency distributions illustrating significantly different average aspect ratios and significantly different frequency distributions.

FIGURE 7

Differences in phagocytosable material in samples of calcium hydroxylapatite (CaHA‐CMC) and poly‐L‐lactic acid (PLLA) particles. (A) The ratio of phagocytosable particles (<20 µmin major axis) relative to total particles. (B) The percentage of total particles with major axes under 20 µmin length.

FIGURE 8

(A) Summary of particle geometry in inflammasome (IL‐1β production) and cytotoxicity. Reprinted from Baranov et al. (B) Cytokine levels in M1 macrophages after 24 h incubation with calcium hydroxylapatite (CaHA‐CMC) or PLLA identified with the human inflammation array. Adapted from Nowag et al.