Collagen fiber orientation near dental implants in human bone: do their organization reflect differences in loading?

This study is the first to investigate the collagen fiber orientation in human bone around titanium dental implants after loading. Birefringence under circularly polarized light (CPL) and scanning electron microscopy (SEM) investigation were used to assess both collagen orientation and density. Twenty osseointegrated dental implants were retrieved from the mandible bone of 10 patients and were used in the present study. The implants were retrieved after 6 months of function. The birefringence measurements were performed on digitized images using both the peri-implant bone (PB) and the alveolar bone (AB) retrieved at the moment of implant placing. All images detected at 100x were measured using a software image analysis. The results showed that 32.96% (3.208 +/- 0.435 mm(2)) of the PB area was composed of transverse collagen fibers while only 19.70% (1.957 +/- 0.253 mm(2)) was composed of longitudinally collagen fibers. In the AB 26.99% (2.620 +/- 0.520 mm(2)) of the examined area was composed of transverse collagen fibers while 22.25% (2.160 +/- 0.320 mm(2)) was composed of longitudinally collagen fibers. The CPL measurements of the birefringence indicated that the difference in area between the two collagen fibers orientations was statistically significant (p = 0.03) for both AB and PB. In PB the transverse collagen fibers were significantly increased (p < 0.01) than in AB; also the longitudinal collagen fibers differ significantly in the two groups (p = 0.02). The transverse collagen fibers in PB are more present under the lower flank of the threads where the load acts with a compression vectors, while outside the tip of the implant threads the collagen fibers run more longitudinally due to the loads that act with a tensile vectors. SEM observations showed, in the first collagen layers facing the implant surface, a random direction of the collagen fibers, while in the areas away from the implant surface the collagen fibers were well oriented and run parallel. In conclusion, in these loaded dental implants, the bone adjacent to the first two implant threads shows a significant increase of the amount of transverse collagen fibers that were mainly associated with the lower flank of the threads where compressive load acts. The longitudinal collagen fibers also differs significantly from those found in the AB.