Multiscale analyses of the bone-implant interface

Abstract

Implants placed with high insertion torque (IT) typically exhibit primary stability, which enables early loading. Whether high IT has a negative impact on peri-implant bone health, however, remains to be determined. The purpose of this study was to ascertain how peri-implant bone responds to strains and stresses created when implants are placed with low and high IT. Titanium micro-implants were inserted into murine femurs with low and high IT using torque values that were scaled to approximate those used to place clinically sized implants. Torque created in peri-implant tissues a distribution and magnitude of strains, which were calculated through finite element modeling. Stiffness tests quantified primary and secondary implant stability. At multiple time points, molecular, cellular, and histomorphometric analyses were performed to quantitatively determine the effect of high and low strains on apoptosis, mineralization, resorption, and collagen matrix deposition in peri-implant bone. Preparation of an osteotomy results in a narrow zone of dead and dying osteocytes in peri-implant bone that is not significantly enlarged in response to implants placed with low IT. Placing implants with high IT more than doubles this zone of dead and dying osteocytes. As a result, peri-implant bone develops micro-fractures, bone resorption is increased, and bone formation is decreased. Using high IT to place an implant creates high interfacial stress and strain that are associated with damage to peri-implant bone and therefore should be avoided to best preserve the viability of this tissue.

Keywords: biological process; dental implantation; finite element analysis; orthodontic anchorage techniques; osseointegration; prosthesis implantation.

Figure 1.

Characterization of peri-implant responses to low and high insertional torque (IT). (A) DAPI/TUNEL staining of the osteotomy site before implant insertion, in which osteocyte nuclei stain blue and cells undergoing programmed cell death stain green for TUNEL (arrows). (B) Dead osteocytes are indicated with red arrowheads. A dotted white line demarcates the zone of cell death. (C) Schematic of the osteotomy relative to the implant’s external diameter. Asterisks mark gap between the peri-implant bone and implant. Runx2 immunostaining on representative tissue sections on PID3 in (D) no IT, (E) low IT, and (F) high IT cases. PCNA immunostaining on representative tissue sections on PID3 in (G) no IT, (H) low IT, and (I) high IT cases. Quantification on PID3 and PID7 in different IT environments of (J) Runx2 expression and (K) PCNA expression. im, implant; b, bone; f, fibrous tissue; IT, insertional torque; PID, postimplant day. Scale bar, 50 μm; ^*P < 0.05. Quantification of Runx2 and PCNA expression are represented as mean ± SEM (n = 4). Differences were analyzed by Wilcoxon test.

Figure 2.

Relationships among IT, strain, and ostecyte death. (A) Three-dimensional finite element analyses showing the implant engaged in the bone disc and the resulting principal compressive strains. (B) Representative sagittal section stained with Pentachrome showing orientation of the implant in bone. (C) In low IT cases, contour plots of principal compressive strain fields and (D) Stress-Opticon (photoelastic) stresses. (E) In high IT cases, contour plots of principal compressive strain fields and (F) Stress-Opticon (photoelastic) stresses. (G) Dotted lines represent strain magnitudes measured from the implant crest radially outward into the cortical bone; solid lines represent the strain magnitudes measured from either the edge of the implant hole (C) or the bone-implant interface (E) outward into the cortical bone; compressive strain magnitudes are shown as positive values. Abbreviations as in Figure 1. Scale bar, 50 μm.

Figure 3.

High IT causes microfracturing and increased osteocyte cell death. Peri-implant bone, imaged after removal of an implant placed with (A) low IT or (B) high IT. White bar, 140 µm. Representative Pentachrome-stained sections on PID3, where the implant was placed with (C) low IT or (D) high IT. Arrows indicate microcracks. Representative Fuchsin-strained sections on PID3, where the implant was placed with (E) low IT or (F) high IT. DAPI^+ve nuclei and TUNEL^+ve osteocytes on PID3, around implants placed with (G) low IT or (H) high IT. Co-stained images are superimposed on differential interference contrast images. DAPI and TUNEL staining on PID7, where the implant was placed with (I) low IT or (J) high IT. (K) Quantification of the zone of TUNEL^+ve and DAPI^+ve cells on PID1 in cases where an osteotomy was prepared (white bar), in low IT cases (red bars), and in high IT cases (blue bars). Histograms are represented as mean ± SEM (n = 4). Differences between high and low IT groups were analyzed by Wilcoxon test. Abbreviations as in Figure 1. Scale bar, 50 μm; ^*P < 0.05.

Figure 4.

Insertional torque affects peri-implant bone remodeling and implant stability. (A) Representative tartrate-resistant acid phosphatase (TRAP)–strained tissue sections on PID7, where the implant was placed with low IT; the gap between the implant and the bone is filled with fibrous tissue (asterisk). (B) Representative TRAP-strained sections on PID7, where the implant was placed with high IT; dotted line identifies the bone cut by the implant thread. Representative ALP–strained sections on PID7, where the implant was placed with (C) low IT or (D) high IT. (E) Representative Pentachrome-strained sections on PID7, where the implant was placed with low IT; new bone formation is indicated with a dotted yellow line. Pentachrome stain distinguishes native mature (yellow-orange) bone from newer (blue-green) bone. (F) Equivalent section from a case where the implant was placed with high IT. (G) Representative tissue sections from PID17, stained with Picrosirius red and visualized under polarized light from a case where the implant was placed with low IT; the arrow indicates mature, linearly arranged, yellow-orange collagen fibers. (H) Representative tissue sections from PID17, from a case where the implant was placed with high IT. Peri-implant TRAP activity on representative tissue sections from (I) low and (J) high IT groups on PID17. (K) Design of implant-bone model for measuring stiffness. (L) The bone-implant stiffness test procedure where the mouse femur is clamped to a solid support. The load is applied perpendicular to the long axis of the implant. (M) Representative load and displacement plots as a function of time (s). (N) Bone-implant stiffness following placement of implants with low and high IT, measured on PID1 and PID7 (n = 5 for each condition; ^*P < 0.05, ^**P < 0.01). Abbreviations as in Fig. 1. Scale bars, 50 μm.