Human Histology and Persistence of Various Injectable Filler Substances for Soft Tissue Augmentation.

Soft tissue augmentation has become a popular aesthetic procedure worldwide. However, many filler substances lack sufficient clinical evidence to support their safety and effectiveness. While some fillers are well-documented in the U.S., others are used in other regions without rigorous testing. This gap motivated researchers to investigate the biocompatibility and persistence of various injectable fillers. Prior research has shown that different materials can elicit distinct immune responses. Still, no prior work had resolved the long-term tissue interactions of all commercially available substances. This uncertainty drove the need for a comparative histological study. The study aimed to assess how the human body reacts to these substances over time. Researchers focused on both resorbable and nonresorbable fillers. The goal was to determine their safety and longevity in the skin.

The study aimed to evaluate the biocompatibility and tissue persistence of ten injectable filler substances. Each filler was tested for its clinical and histological behavior over a nine-month period. The researchers sought to determine how the body reacts to each material and whether any long-term adverse effects occur. The substances included collagen, hyaluronic acid, PMMA, and others. The study focused on both resorbable and nonresorbable fillers. Researchers injected small volumes of each substance into the forearm of one of the authors. The test sites were monitored for visible reactions and then excised at specific intervals. The purpose was to document the immune response and material degradation. The findings were intended to inform clinical decisions about filler safety and effectiveness.

The researchers selected ten commercially available filler substances for testing. Each substance was injected in a 0.1 cc volume into the volar forearm of one of the authors. The injection sites were monitored for clinical reactions and tissue persistence. At 1, 3, 6, and 9 months, the test sites were surgically excised for histological analysis. The samples were graded based on foreign body reaction classifications. Researchers examined the presence of macrophages, giant cells, and fibrous capsules. They also noted the degree of material resorption and inflammation. The study compared the histological outcomes of each filler. The authors used a standardized grading system to assess tissue response. The methodology allowed for a direct comparison of the body's reaction to each substance.

Collagen (Zyplast) was phagocytosed by the body at 6 months. Hyaluronic acid (Restylane) was absorbed at 9 months. PMMA microspheres (Artecoll) were encapsulated with connective tissue and macrophages. Silicone oil (PMS 350) caused a chronic foreign body reaction despite being clinically inconspicuous. Polylactic acid microspheres (New-Fill) induced a mild inflammatory response and vanished at 4 months. Dextran microspheres (Reviderm intra) caused a pronounced reaction and disappeared at 6 months. Polymethylacrylate particles (Dermalive) had the lowest cellular reaction but were gone at 6 months. Polyacrylamide (Aquamid) remained palpable for the entire 9-month period. Histologically, it was retained in fibrous capsules. Calcium hydroxylapatite (Radiance FN) showed minimal reaction but was absorbed by 12 months. Polyvinylhydroxide microspheres (Evolution) were well-tolerated and slowly diminished over 9 months.

The study found that host defense mechanisms react differently to various filler materials. Both resorbable and nonresorbable substances appeared to be safe in the short term. However, all fillers exhibited some undesirable side effects. The mechanism behind late inflammation or granuloma formation remains unclear. Early histological findings do not reliably predict late reactions. The results suggest that long-term monitoring is necessary for all filler types. The authors emphasized the need for more research on late immune responses. They noted that current histological assessments are insufficient for predicting future complications. The findings support the importance of continued clinical observation. The study highlights the variability in tissue response to different materials. Researchers recommend caution when using fillers with unknown long-term effects. The results underscore the need for standardized safety evaluations.