A Novel Nanostructured Surface on Titanium Implants Increases Osseointegration in a Sheep Model

Abstract

Background: A nanostructured titanium surface that promotes antimicrobial activity and osseointegration would provide the opportunity to create medical implants that can prevent orthopaedic infection and improve bone integration. Although nanostructured surfaces can exhibit antimicrobial activity, it is not known whether these surfaces are safe and conducive to osseointegration.

Questions/purposes: Using a sheep animal model, we sought to determine whether the bony integration of medical-grade, titanium, porous-coated implants with a unique nanostructured surface modification (alkaline heat treatment [AHT]) previously shown to kill bacteria was better than that for a clinically accepted control surface of porous-coated titanium covered with hydroxyapatite (PCHA) after 12 weeks in vivo. The null hypothesis was that there would be no difference between implants with respect to the primary outcomes: interfacial shear strength and percent intersection surface (the percentage of implant surface with bone contact, as defined by a micro-CT protocol), and the secondary outcomes: stiffness, peak load, energy to failure, and micro-CT (bone volume/total volume [BV/TV], trabecular thickness [Tb.Th], and trabecular number [Tb.N]) and histomorphometric (bone-implant contact [BIC]) parameters.

Methods: Implants of each material (alkaline heat-treated and hydroxyapatite-coated titanium) were surgically inserted into femoral and tibial metaphyseal cancellous bone (16 per implant type; interference fit) and in tibial cortices at three diaphyseal locations (24 per implant type; line-to-line fit) in eight skeletally mature sheep. At 12 weeks postoperatively, bones were excised to assess osseointegration of AHT and PCHA implants via biomechanical push-through tests, micro-CT, and histomorphometry. Bone composition and remodeling patterns in adult sheep are similar to that of humans, and this model enables comparison of implants with ex vivo outcomes that are not permissible with humans. Comparisons of primary and secondary outcomes were undertaken with linear mixed-effects models that were developed for the cortical and cancellous groups separately and that included a random effect of animals, covariates to adjust for preoperative bodyweight, and implant location (left/right limb, femoral/tibial cancellous, cortical diaphyseal region, and medial/lateral cortex) as appropriate. Significance was set at an alpha of 0.05.

Results: The estimated marginal mean interfacial shear strength for cancellous bone, adjusted for covariates, was 1.6 MPa greater for AHT implants (9.3 MPa) than for PCHA implants (7.7 MPa) (95% CI 0.5 to 2.8; p = 0.006). Similarly, the estimated marginal mean interfacial shear strength for cortical bone, adjusted for covariates, was 6.6 MPa greater for AHT implants (25.5 MPa) than for PCHA implants (18.9 MPa) (95% CI 5.0 to 8.1; p < 0.001). No difference in the implant-bone percent intersection surface was detected for cancellous sites (cancellous AHT 55.1% and PCHA 58.7%; adjusted difference of estimated marginal mean -3.6% [95% CI -8.1% to 0.9%]; p = 0.11). In cortical bone, the estimated marginal mean percent intersection surface at the medial site, adjusted for covariates, was 11.8% higher for AHT implants (58.1%) than for PCHA (46.2% [95% CI 7.1% to 16.6%]; p < 0.001) and was not different at the lateral site (AHT 75.8% and PCHA 74.9%; adjusted difference of estimated marginal mean 0.9% [95% CI -3.8% to 5.7%]; p = 0.70).

Conclusion: These data suggest there is stronger integration of bone on the AHT surface than on the PCHA surface at 12 weeks postimplantation in this sheep model.

Clinical relevance: Given that the AHT implants formed a more robust interface with cortical and cancellous bone than the PCHA implants, a clinical noninferiority study using hip stems with identical geometries can now be performed to compare the same surfaces used in this study. The results of this preclinical study provide an ethical baseline to proceed with such a clinical study given the potential of the alkaline heat-treated surface to reduce periprosthetic joint infection and enhance implant osseointegration.

Fig. 1

An overview of the study design, with implant and specimen numbers for each process, is shown.

Fig. 2

The position of implants in the sheep femoral and tibial cancellous and cortical bone is shown.

Fig. 3

A-C SEM images of alkaline heat-treated implants preimplantation at magnifications of (A) 200×, (B) 10,000×, and (C) 25,000×, are shown. Spikes are visible on the surface of the implant preimplantation (white arrows).

Fig. 4

(A) Photograph of sheep implants before implantation, from left to right: porous-coated titanium before alkaline heat treatment, after alkaline heat treatment, and PCHA; and SEM images demonstrating the nanotopographies and microtopographies of the same implants: (B) porous-coated titanium (before alkaline heat treatment), (C) alkaline heat-treated, and (D) PCHA. The SEM analysis demonstrates the change in nanosurface morphology on the porous plasma-sprayed coating before and after alkaline heat treatment processing, where the implant maintained the underlying microporous structure because of plasma-sprayed titanium. In contrast, the PCHA implant surface was characterized by a microcrystalline topography because of the application of hydroxyapatite on the plasma-sprayed titanium coating.

Fig. 5

A-C SEM images of alkaline heat-treated implants 12 weeks postoperatively at magnifications of (A) 150×, (B) 8500×, and (C) 34,000× are shown. Spikes are visible on the surface of the implant 12 weeks after implantation (white arrows); osseous material (black arrows) is evident and intimately associated with these spikes (open white arrows).

Fig. 6

Representative 3D-rendered reconstructions of micro-CT images of bone (blue) and representative two-dimensional toluidine blue–stained sections of bone (blue stain) surrounding AHT (left) and PCHA (right) implants in the cancellous and cortical bone regions are shown.

Fig. 7

A-B Descriptive statistics for interfacial shear strength for AHT and PCHA implants at the cancellous femoral and tibial locations are shown. In the box and whisker plots, the box’s center line indicates the median value, the box’s limits indicate the first (25th percentile) and third (75th percentile) quartiles, and the whiskers indicate the minimum and maximum values. The p values refer to the results of the corresponding linear mixed model.

Fig. 8

A-C Descriptive statistics for percent intersection surface (i.S/TS; i.S – intersection surface [mm²], TS – total surface [mm²]) of bone on AHT and PCHA implants for (A) cancellous bone in the femur and tibia and for each location in the (B) medial and (C) lateral tibial cortex are shown. In the box and whisker plots, the box’s center line indicates the median value, the box’s limits indicate the first (25th percentile) and third (75th percentile) quartiles, and whiskers indicate the minimum and maximum values. The p values refer to the results of the corresponding linear mixed model.

Fig. 9

A-F Descriptive statistics for mechanical testing outcomes for AHT and PCHA implants at the cancellous femoral and tibial locations and cortical locations are shown: (A) stiffness - cancellous, (B) stiffness - cortical, (C) peak load - cancellous, (D) peak load – cortical, (E) energy to failure - cancellous, and (F) energy to failure - cortical. In the box and whisker plots, the box’s center line indicates the median value, the box’s limits indicate the first (25th percentile) and third (75th percentile) quartiles, and the whiskers indicate the minimum and maximum values. The p values refer to the results of the corresponding linear mixed model.

Fig. 10

Descriptive statistics for bone-implant contact for AHT and PCHA implants from cancellous femoral and tibial locations are shown. In the box and whisker plot, the box’s center line indicates the median value, the box’s limits indicate the first (25th percentile) and third (75th percentile) quartiles, and the whiskers indicate the minimum and maximum values. The p value refers to the result of the corresponding linear mixed model.

Fig. 11

A-C Descriptive statistics for femoral and tibial (A) bone volume/tissue volume (BV/TV, %), (B) trabecular thickness (Tb.Th, mm), and (C) trabecular number (Tb.N, mm^-1) for cancellous bone surrounding AHT and PCHA implants are shown. In the box and whisker plots, the box center line indicates the median value, box limits indicate the first (25th percentile) and third (75th percentile) quartiles, and the whiskers indicate the minimum and maximum values. The p values refer to the results of the corresponding linear mixed model.