Long-term study on the osteogenetic capability and mechanical behavior of a new resorbable biocomposite anchor in a canine model

Abstract

Background: Biodegradable suture anchors are commonly used for repairing torn rotator cuffs, but these biodegradable materials still suffer from low mechanical strength, poor osteointegration, and the generation of acidic degradation byproducts.

Method: The purpose of this study was to evaluate the long-term mechanical behavior and osteogenetic capabilities of a biocomposite anchor injection molded with 30% β-tricalcium phosphate microparticles blended with 70% poly (L-lactide-co-glycolide) (85/15). This study investigated in vitro degradation and in vivo bone formation in a canine model. The initial mechanical behavior, mechanical strength retention with degradation time, and degradation features were investigated.

Results: The results showed that the biocomposite anchor had sufficient initial mechanical stability confirmed by comparing the initial shear load on the anchor with the minimum shear load borne by an ankle fracture fixation screw, which is considered a worst-case implantation site for mechanical loading. The maximum shear load retention of the biocomposite anchor was 83% at 12 weeks, which is desirable, as it aligns with the rate of bone healing. The β-tricalcium phosphate fillers were evenly dispersed in the polymeric matrix and acted to slow the degradation rate and improve the mechanical strength of the anchor. The interface characteristics between the β-tricalcium phosphate particles and the polymeric matrix changed the degradation behavior of the biocomposite. Phosphate buffer saline was shown to diffuse through the interface into the biocomposite to inhibit the core accelerated degradation rate. In vivo, the addition of β-tricalcium phosphate induced new bone formation. The biocomposite material developed in this study demonstrated improved osteogenesis in comparison to a plain poly (L-lactide-co-glycolide) material. Neither anchor produced adverse tissue reactions, indicating that the biocomposite had favorable biocompatibility following long-term implantation.

Conclusion: In summary, the new biocomposite anchor presented in this study had favorable osteogenetic capability, mechanical property, and controlled degradation rate for bone fixation.

Translational potential of this article: The new biocomposite anchor had sufficient initial and long-term fixation stability and bone formation capability in the canine model. It is indicated that the new biocomposite anchor has a ​potential for orthopedic application.

Keywords: Anchor-bone interaction; Biocomposite anchor; Degradation rate; Mechanical property; Osteogenesis.

Figure 1

(A) Screw-shaped anchors for in vitro and in vivo studies. SEM image of the cross-section of the anchor showing the homogenous distribution of (B) β-TCP/PLGA, (C) phosphor, and (D) calcium.

Figure 2

(A–B) Plots of shear load versus displacement for both anchors at 0.14 weeks (24 ​h) and 26 weeks. (C) Change in maximum shear load with degradation time; (D) Bar chart of shear stiffness with degradation time; (E) Shear load retention with degradation time: #p ​< ​0.05 is considered to be statistically significant in t-test; *p ​> ​0.05 is considered to show no significant difference in t-test.

Figure 3

The DSC curves (A–B) and XRD spectrums (C–D) of both anchors with degradation time.

Figure 4

A) Mass retention and β-TCP content in both groups, (B) Change in molecular weight of both groups, (C) Diameter retention of both anchors, and (D) Images of dry anchors following in vitro degradation after 3, 12, 26, 52, 78,104 weeks.

Figure 5

SEM images of the surface and cross-section morphology of the plain PLGA and β-TCP/PLGA composite anchors. Note: N.A. ​= ​Not Available.

Figure 6

pH levels: (A) pH fluctuation zone of the solution containing plain PLGA anchors. (B) pH fluctuation zone of the solution containing β-TCP/PLGA composite anchors.

Figure 7

Digital images of β-TCP/PLGA composite and plain PLGA anchors stained with H&E and Masson at 52 weeks, 78 weeks, and 104 weeks. (A–L) show osteogenesis in the center of the anchors. (a–l) show osteogenesis at the screw interface. NB shows the new bone. Black arrows show central canals.

Figure 8

(A) relative osteogenetic quantity; (B) relative collagenous quantity; *p ​> ​0.05 is considered to show no significant difference in t-test.

Fig. A.1

A) Schematic representation of the shear test adapted from ASTM B769-11: A) before shear test (B) after shear test; (C) Fixture for the shear test in deionized water medium.