expansion of the surrounding tissue, this effect is transient.9
The cosmetically relevant mechanism of action (MOA) of
PLLA involves the initiation of a desired subclinical inflammatory
tissue response to the polylactides.8 This inflammatory
response leads to encapsulation of the microparticles, stimulation
of host collagen production, and fibroplasia.10 Over
time, the PLLA degrades, the inflammatory response wanes,
and host collagen production increases (Figure 3),1 generating
new volume and structural support in a gradual, progressive
manner.1,8,11,12 Due to the prolonged nature of its activity, the
cosmetic benefits of PLLA can last for several years.13,14 It
should be noted that the prolonged activity of PLLA is also a
key consideration in the avoidance of overcorrection with its
use in soft tissue augmentation.1
The MOA of PLLA contrasts with the MOA of products that directly
augment tissue volume. However, neocollagenesis is not unique
to PLLA. Even hyaluronic acid has been shown to stimulate collagen
production,15 although at a level lower than that seen with
PLLA. Both injectable calcium hydroxylapatite (CaHA) and, as
previously mentioned, PMMA, act primarily through the stimulation
of collagen production.16,17 Compared with PLLA, the scaffold
provided by CaHA microspheres is degraded relatively quickly
