Calcium Phosphate Nanoclusters Reduce Orthodontic Relapse

A single local injection modulates PDL remodeling and supports bone microarchitecture

A single local injection modulates PDL remodeling and supports bone microarchitecture

Orthodontic relapse remains one of the most persistent challenges after active treatment. Even with retainers, teeth can drift toward their pretreatment positions, increasing the risk of functional issues, oral disease, and the need for retreatment. A recent open access study in Biomaterials reports that a single local injection of high concentration polymer induced liquid precursor calcium phosphate nanoclusters, termed HC-PILP, reduced post treatment relapse in a rat model by influencing early periodontal ligament remodeling and later trabecular bone microarchitecture.

Why relapse happens in phases

Relapse is often discussed as a mechanical problem, but the biology matters. Early relapse is strongly linked to rapid periodontal ligament, PDL, remodeling, while later relapse is associated with compromised bone quality that gradually recovers over time. Existing strategies, including fixed or removable retainers and surgical approaches such as fiberotomies, can be limited by compliance, invasiveness, or variable evidence. This creates a need for biocompatible, locally delivered approaches that target periodontal biology rather than relying solely on mechanics.

The material approach: high concentration PILP nanoclusters

PILP systems use polyanionic polymers to chelate calcium and phosphate ions, helping stabilize a hydrated amorphous phase and prevent premature apatite precipitation. In this study, the authors formulated HC-PILP nanoclusters using high molecular weight PASP, 14 kDa, and PAA, 450 kDa, to maintain high calcium and phosphate concentrations in a stable solution without early precipitation. A central design goal was to support periodontal repair while minimizing the risk of ectopic mineralization in the non mineralized PDL, which could lead to ankylosis.

Figure 1. Schematic of HC-PILP synthesis and pH adjustment.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

Nanocluster characterization and collagen mineralization control

Cryo-EM and DLS measurements indicated nanoclusters on the order of a few nanometers, with additional analysis supporting a non crystalline, stabilized calcium phosphate state under the reported conditions. The authors then tested whether HC-PILP would mineralize collagen type I, a key concern when working near the tooth PDL bone interface. In vitro, HC-PILP did not produce evidence of extrafibrillar or intrafibrillar mineralization in pre fibrillated collagen type I across 7 and 14 day incubations, and cell based mineralization assays did not show a material driven increase beyond expected controls.

Figure 2. Nanocluster size and structural characterization, collagen mineralization assays, and rheological profiling.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

A single local injection reduced relapse in vivo

To test translational relevance, the authors used a rat orthodontic tooth movement model, applying a closed coil spring for 28 days. On day 28, they delivered a single 30 μL injection to the distopalatal surface of the maxillary first molar, then removed the appliance and monitored relapse for 24 days. HC-PILP significantly reduced relapse compared with PBS and polymer only controls across measured relapse time points, with stronger separation at later stages. The μCT analyses did not show major differences in bone density metrics at the reported endpoint, but trabecular microarchitecture improved over time after HC-PILP treatment, and critically, the PDL space remained non mineralized in the scans.

Figure 3. In vivo experimental timeline and relapse outcomes after a single injection of HC-PILP versus controls.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

Early stage effect: altered PDL collagen remodeling

A notable finding was that relapse reduction was not explained solely by early bone changes. Histology and polarized picrosirius red staining indicated that HC-PILP altered collagen remodeling in the PDL during early relapse, particularly on the compression side. Collagen signal recovery was reduced at early relapse time points and later returned toward baseline by day 24, suggesting a transient modulation of PDL remodeling dynamics rather than permanent suppression.

Figure 4. Picrosirius red polarized light imaging and quantification of PDL collagen remodeling during relapse.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

Mechanistic insight: collagen fibrillogenesis and protein secondary structure

To probe how HC-PILP might influence PDL collagen organization, the authors tested collagen fibrillogenesis in vitro in the presence of HC-PILP and polymer only controls. Fibril formation kinetics shifted in a dose dependent manner. FTIR analysis suggested changes in the Amide I region consistent with altered collagen secondary structure in higher dose conditions. TEM imaging further indicated aggregation behavior during fibrillogenesis under high dose exposure, without evidence of mineral crystallinity in SAED. These findings support a mechanism in which HC-PILP interacts with collagen assembly and remodeling processes, which could be relevant to the early relapse phase where PDL collagen recovery is strongly implicated.

Figure 5. Fibroblast gene expression and collagen fibrillogenesis kinetics under HC-PILP and polymer only conditions.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

This figure provides two complementary views. Fibroblast gene expression showed limited changes under the reported conditions, while collagen fibrillogenesis kinetics shifted with HC-PILP in a dose dependent manner.

Figure 6. FTIR Amide region analysis indicating shifts consistent with altered collagen structure under selected conditions.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

This FTIR result supports the interpretation that higher dose conditions were associated with changes in collagen secondary structure signatures in the Amide I region.

Figure 7. TEM images showing aggregation behavior during collagen fibrillogenesis with HC-PILP and polymer controls.
Figure note, small text: Reproduced or adapted from Cuylear et al., Biomaterials 2025, 315:122965, Open Access, DOI: 10.1016/j.biomaterials.2024.122965.

TEM and SAED provide morphology level context, showing aggregation behavior under high dose exposure while lacking evidence of mineral crystallinity in the reported fibrillogenesis setup.

What this could mean for clinical translation

This work suggests a materials based strategy to reduce relapse using a single local administration, with dual phase effects: early modulation of PDL collagen remodeling and later improvements in trabecular microarchitecture. The approach is notable because it does not rely on pharmacologic morphogens, and it places emphasis on avoiding ectopic mineralization in the PDL. Future studies will need to address longer follow up, dose optimization, tissue residence and distribution, tooth mobility outcomes, and broader biological variables, including sex as a biological factor, to better map translational potential.

Source:

Cuylear DL, Fu ML, Chau JC, Bulkley D, Kharbikar B, Kazakia GJ, Jheon AH, Habelitz S, Kapila SD, Desai TA. Calcium phosphate nanoclusters modify periodontium remodeling and minimize orthodontic relapse. Biomaterials. 2025;315:122965. DOI: 10.1016/j.biomaterials.2024.122965. Open Access.