Podoplanin Immunopositive Lymphatic Vessels at the Implant Interface in a Rat Model of Osteoporotic Fractures

Abstract

Insertion of bone substitution materials accelerates healing of osteoporotic fractures. Biodegradable materials are preferred for application in osteoporotic patients to avoid a second surgery for implant replacement. Degraded implant fragments are often absorbed by macrophages that are removed from the fracture side via passage through veins or lymphatic vessels. We investigated if lymphatic vessels occur in osteoporotic bone defects and whether they are regulated by the use of different materials. To address this issue osteoporosis was induced in rats using the classical method of bilateral ovariectomy and additional calcium and vitamin deficient diet. In addition, wedge-shaped defects of 3, 4, or 5 mm were generated in the distal metaphyseal area of femur via osteotomy. The 4 mm defects were subsequently used for implantation studies where bone substitution materials of calcium phosphate cement, composites of collagen and silica, and iron foams with interconnecting pores were inserted. Different materials were partly additionally functionalized by strontium or bisphosphonate whose positive effects in osteoporosis treatment are well known. The lymphatic vessels were identified by immunohistochemistry using an antibody against podoplanin. Podoplanin immunopositive lymphatic vessels were detected in the granulation tissue filling the fracture gap, surrounding the implant and growing into the iron foam through its interconnected pores. Significant more lymphatic capillaries were counted at the implant interface of composite, strontium and bisphosphonate functionalized iron foam. A significant increase was also observed in the number of lymphatics situated in the pores of strontium coated iron foam. In conclusion, our results indicate the occurrence of lymphatic vessels in osteoporotic bone. Our results show that lymphatic vessels are localized at the implant interface and in the fracture gap where they might be involved in the removal of lymphocytes, macrophages, debris and the implants degradation products. Therefore the lymphatic vessels are involved in implant integration and fracture healing.

Effects of adding resorbable chitosan microspheres to calcium phosphate cements for bone regeneration

“…In addition effective osteoconductivity and osteoinductivity provide an interconnected porous network that facilitates cellular infiltration and nutrient and waste transport [1]. CPCs have good biocompatibility, are self-setting and possess osteoconduction properties [2][3][4][5]. Specifically, CPCs can be molded or injected and set in situ to closely adapt to complex shapes caused by bone defects [6][7][8].…”

Animal models for studying metaphyseal bone fracture healing

“…Not surprisingly, osteoanabolic treatments support healing of both metaphyseal and diaphyseal fractures. This was shown for PTH, which increases osteoblast differentiation and bone formation independent of the fracture location (Bernhardsson and Aspenberg, 2018;Li et al, 2017;Nozaka et al, 2008). Also, the activation of the crucial osteoanabolic Wnt/β-catenin signalling pathway, by antagonising the Wnt inhibitor Dkk1, accelerates both metaphyseal and diaphyseal fracture healing (Agholme et al, 2011;Jin et al, 2015).…”

Bone graft substitutes and bone morphogenetic proteins for osteoporotic fractures: what is the evidence?

“…Despite improvements in the treatment of osteoporosis, osteoporotic fractures remain challenging to treat. Osteoporotic fractures have an impaired ability to heal (1,2), and often require more time to heal (3)(4)(5)(6). Since osteoporotic bone is less likely to heal on its own and the degree of comminution is generally high, patients often require surgery to repair the Bone grafts can be used to stabilize the implants and provide a scaffold for ingrowth of new bone.…”