Mechanical and Biological Advantages of a Tri-Oval Implant Design

Abstract

Of all geometric shapes, a tri-oval one may be the strongest because of its capacity to bear large loads with neither rotation nor deformation. Here, we modified the external shape of a dental implant from circular to tri-oval, aiming to create a combination of high strain and low strain peri-implant environment that would ensure both primary implant stability and rapid osseointegration, respectively. Using in vivo mouse models, we tested the effects of this geometric alteration on implant survival and osseointegration over time. The maxima regions of tri-oval implants provided superior primary stability without increasing insertion torque. The minima regions of tri-oval implants presented low compressive strain and significantly less osteocyte apoptosis, which led to minimal bone resorption compared to the round implants. The rate of new bone accrual was also faster around the tri-oval implants. We further subjected both round and tri-oval implants to occlusal loading immediately after placement. In contrast to the round implants that exhibited a significant dip in stability that eventually led to their failure, the tri-oval implants maintained their stability throughout the osseointegration period. Collectively, these multiscale biomechanical analyses demonstrated the superior in vivo performance of the tri-oval implant design.

Keywords: alveolar bone remodeling/regeneration; biomechanics; bone biology; dental implant; finite element analysis (FEA); osseointegration.

Figure 1

Tri-oval implants placed in type III bone with the same insertion torque exhibit higher primary stability as compared to conventional round implants. (A) Maxillary first molars (M1) were extracted from skeletally mature (8-week-old) male mice. (B) Intraoral photos of extraction socket (white arrow) and (C) Healed extraction site (black arrow). (D) Representative micro-CT imaging and (E) Representative aniline blue staining of the healed extraction socket on PED28. (F) Quantification of mean bone mineral density (BMD) on PED28, where the BMD of the healed extraction site was equivalent to surrounding pristine alveolar bone. (G) Osteotomies (0.45 mm dia.; pink arrow) were produced in the healed extraction sites using dental drill. (H) Representative micro-CT image of the prepared osteotomy site. (I) Geometries of the round and (J) tri-oval implants in cross-section. (K) Implant placement surgery. (L) Implants were positioned at the height of the gingiva. (M) In vitro IT testing and (N) In vivo IT testing where the white arrow indicates a round implant; blue arrow indicates a tri-oval implant. (O) Quantification of in vivo IT for round (white) and tri-oval (blue) implants. (P) Lateral stability testing of round and tri-oval implants (arrows) in the mouse maxillae; a stepper motor laterally displaces the implant a known amount while the force to do so is measured by a transducer. (Q) Tri-oval implants are significantly more stable than round implants at the time of insertion. Abbreviations: M1, maxillary first molar; M2, maxillary second molar; M3, maxillary third molar; hES, healed extraction site; PED, post-extraction day; imp, implant; IT, insertion torque. Scale bars = 500 µm.

Figure 2

Compared to a round implant, the minima of a tri-oval implant are associated with significantly lower strains and a significantly smaller zone of osteocyte death. (A) FE modeling of round (left) and tri-oval (right) implants in bone, using CAD files of the actual implants used in vivo. In a transverse plane, the threads of each type of implant (blue) penetrate the bone, which is modeled as a solid material. (B) The calculated bone-implant contact area due to thread penetration. (C) Formulation of a FE model of laterally-loaded implant in bone. (D) Peri-implant strains surrounding laterally-loaded round and tri-oval implants in the sagittal plane. (E) Peri-implant strains arising from initial misfit of the round and tri-oval implants as seen in the transverse plane; only the maxima of the tri-oval implant penetrate the bone. (F) DAPI staining of interfacial bone surrounding a representative round implant and (G) a representative tri-oval implant; white arrow denotes a circumferential osteocyte-free zone and dotted white line demarcates the osteotomy edge. (F’, G’) TUNEL staining on adjacent tissue sections. Quantification of the distribution of (H) viable and (I) apoptotic osteocytes as a function of distance from implant. Abbreviations: imp, implant; PID, post-implant day. Scale bars = 50 µm.

Figure 3

Tri-oval implants exhibits less bone resorption but more robust mineralization. (A) TRAP staining of interfacial tissues surrounding a representative round implant on PID3. (B) TRAP staining of the minima region around a tri-oval implant on PID3. (C) TRAP staining was quantified around the entire circumference of round and tri-oval implants. (D) Lateral stiffness test of round and tri-oval implants on PID0 and 3. (E) ALP staining of interfacial tissues surrounding a representative round and (F) a tri-oval implant on PID10, quantified in (G). (H) TRAP staining of interfacial tissues surrounding a representative round and (I) a tri-oval implant on PID10, quantified in (J). (K) Aniline blue staining of interfacial tissues surrounding a representative round and (L) a tri-oval implant on PID20; quantified in (M). Abbreviations as previously stated. Scale bars = 50 µm.

Figure 4

Stability over time as the function of implant geometry. (A) Schematic of an occlusal, or functional implant. (B) Quantification of lateral stability of sub-occlusal round and tri-oval implants at different timepoints. Aniline blue-stained tissue sections from PID20 through an (C,C’) occlusal round implant and (D,D’) an occlusal tri-oval implant. (E) Quantification of lateral stability of occlusal round and tri-oval implants on PID20. (F) In round occlusal implants, FE modeling of peri-implant strain on PID3 and (G) picrosirius-red stained tissues from PID20. (H) In tri-oval occlusal implants, FE modeling of peri-implant strain on PID3 and (I) picrosirius red-stained tissues from PID20. Abbreviations: op, occlusal plane; imp, implant; fe, fibrous encapsulation. Scale bars = 50 µm.