Detection of biomaterial in vivo microenvironment pH (μe-pH) and its effect on bone defect regeneration under unbalanced bone remodling condition

Abstract

In scenario of osteoporotic fracture, significantly higher activity of osteoclasts than osteoblasts may lead to continuous loss of bone in fracture/defect site. The impaired bone regeneration efficiency is the major barrier that influences endosseous implants to get a better performance, and this substantially increases the risk of a second fracture, non-union and aseptic implant loosening. Currently, there are no clinically approved biomaterials specifically tailored for applications in osteoporotic bones, and it is a challenging topic for material scientists to design proper orthopaedic biomaterials with biological functions for osteoporotic patients.

The key issue for developing such biomaterials is to re-establish normal bone regeneration at the fracture site. According to the literatures, acid-base equilibrium is one of the most important factors that influence behaviours of bone cells. Therefore, microenvironment pH (μe-pH), which is influenced by implants biodegradation, may play a crucial role in guiding the localized bone regeneration. We then propose to reconstruct the regeneration balance by controlling the μe-pH through the application of biodegradable materials.

The aims of this study include: 1. Establish a method for in vivo μe-pH detection; 2. Evaluate the effect of μe-pH on early-stage bone regeneration process; 3. Reveal the mechanisms by examining osteoclasts behavior in response to the change of μe-pH.

The measurement of in vivo μe-pH was realized by using the pH microelectrode. Alkaline biodegradable materials generated an in vivo μe-pH which was higher than the normal physiological value, in particular, at the initial stage. The preliminary results indicated that osteoclasts may play important roles in the early-stage of defect healing process. Therefore, in order to further study the osteoclasts behaviors in response to the elevated μe-pH in a bone marrow microenvironment, a boneimplant interaction mouse model and a borosilicate glass system (with μe-pH gradient) with same type of ions and similar composition were developed. Based on our in vitro data, osteoclasts differentiation and pit-formation activity were significantly suppressed when RANKL-stimulated RAW264.7 cells were cultured in different glasses extracts which were adjusted to higher pH conditions (pH 7.59-8.02). Furthermore, the abnormal osteoclastogenesis potential of bone marrow cells in mouse after hindlimb unloading treatment could also be balanced by the elevated culture media pH (pH 7.62-7.84). In vivo, significantly suppressed osteoclasts activity together with a thicker new bone on materials surface were observed for glasses with higher e-pHs. Further assessments by using RT-PCR and immunostaining indicated that the decreased activity of matrix-degrading proteases (e.g. cathepsin K) may be one of the reasons for the suppressed osteoclasts activity under higher μe-pH conditions.

In conclusion, the impaired regeneration process under osteoporotic or immobilized conditions may be ameliorated by adjusting the materials to generate a weakly-alkaline microenvironment. And the e-pH is an important and accessible factor which should be taken into consideration in the development of orthopaedic biomaterials, in particular for repair of osteoporotic bone fracture /defect.