Bone healing response in cyclically loaded implants: Comparing zero, one, and two loading sessions per day

Abstract

When bone implants are loaded, they are inevitably subjected to displacement relative to bone. Such micromotion generates stress/strain states at the interface that can cause beneficial or detrimental sequels. The objective of this study is to better understand the mechanobiology of bone healing at the tissue-implant interface during repeated loading. Machined screw shaped Ti implants were placed in rat tibiae in a hole slightly bigger than the implant diameter. Implants were held stable by a specially-designed bone plate that permits controlled loading. Three loading regimens were applied, (a) zero loading, (b) one daily loading session of 60 cycles with an axial force of 1.5 N/cycle for 7 days, and (c) two such daily sessions with the same axial force also for 7 days. Finite element analysis was used to characterize the mechanobiological conditions produced by the loading sessions. After 7 days, the implants with surrounding interfacial tissue were harvested and processed for histological, histomorphometric and DNA microarray analyses. Histomorphometric analyses revealed that the group subjected to repeated loading sessions exhibited a significant decrease in bone-implant contact and increase in bone-implant distance, as compared to unloaded implants and those subjected to only one loading session. Gene expression profiles differed during osseointegration between all groups mainly with respect to inflammatory and unidentified gene categories. The results indicate that increasing the daily cyclic loading of implants induces deleterious changes in the bone healing response, most likely due to the accumulation of tissue damage and associated inflammatory reaction at the bone-implant interface.

Keywords: Bone; Gene expression; Histomorphometry; Implant; Loading; Micromotion.

Fig. 1

Photograph of the implant and micromotion system for rat tibiae (A), Mark-10 Force Gauge loading component (B), the implant and motion device adapted in situ in the proximal tibia metaphysis. (C–F) A cap protects the implant from accidental external forces (D,F).

Fig. 2

In the FE model, the bone site is idealized as being cylindrical with a 0.6 mm-thick cortical bone having trabecular bone underneath (A). However, the “gap” interface (B, blue shading in right figure) – produced by the 2 mm diameter drill – surrounds the 1.7 mm-diameter implant; the mechanical properties of this gap region can be altered to explore the influence of healing. The top surface of the implant is loaded as per the protocols described. The sides and base of the FE model are constrained.

Fig. 3

Light microscope images of decalcified sections, stained with HE, from the unloaded (A), micromotion 1× (B) and micromotion 2× (C) groups at 7 days post-surgery. Histological observations revealed that new bone forms around the implant in all groups, including between the implants threads. However, signs of disruption of bone healing at the bone/implant interface were noticed in all the animals from the Micromotion 2× group.

Fig. 4

Light microscope images of decalcified sections, stained toluidine blue showing cartilage formation in a single micromotion 2× rat.

Fig. 5

Histomorphometry of bone formation in Unloaded, Micromotion 1× and Micromotion 2× groups at 7 days post-surgery. The Micromotion 2× group showed overall lower bone formation in the interface implant/bone in the BFAo area (A), lower bone implant contact (BIC, B) and larger bone–implant distance (BID, C).

Fig. 6

Pie charts presenting the percentage distribution of biological process ontologies identified for statistically significant genes (p < 0.05) differentially upregulated (A, C, E) and downregulated (B, D, F) between Micromotion 1× group vs. Unloaded group (A, B), Micromotion 2× group vs. Unloaded group (C, D), Micromotion 2× group vs. Micromotion 1× group (E, F) at day 7 post-surgery.

Fig. 7

When the implant is axially loaded immediately after implantation, a fibrin clot in the gap interface has a low modulus (~ 0.05 MPa). The loading causes axially-downward micromotion (negative z-direction) of about 93 μm (A below). In turn, this micro-motion creates strain in the interface (B, below): Principal compressive strains (and tensile strains, not shown) reach high magnitudes (> 30%) near the implant's apex, and even higher values (> 60%) near the tips of the threads (black arrows). However, in between the tips of the threads and farther away from the apex (green arrows), stains are more moderate (~ 10%) and permissible for the initial stages of bone healing, e.g., collagen formation.

Fig. 8

In regions in the gap where compressive strains are small to moderate, bone healing can occur, which has the effect of increasing the gap's properties (e.g., elastic modulus). In turn, that will stiffen the gap, thereby decreasing implant micromotion (under the same applied force level) as well as the strain. However, the degree of bone regeneration in the gap depends on a “race” between local damage accumulation in locations with initially high strain vs. bone regeneration in locations with low/moderate strain (as explained in the text). Note that the color bars denoting strain magnitude are different A–C; strains are much larger in A than in B and C because the modulus values for the gap region increase from 0.05 MPa, 1 MPa, and 5 MPa in A, B, and C respectively. For the same magnitude of axial force on the implant (D), the axial micromotion of the implant decreases substantially as the elastic modulus of the interfacial gap increases. Likewise, the magnitude of the peak strains in the gap decreases with increasing modulus of the interfacial gap.