Evaluation of the soft tissue biocompatibility of MgCa0.8 and surgical steel 316L in vivo: a comparative study in rabbits

Abstract

Background: Recent studies have shown the potential suitability of magnesium alloys as biodegradable implants. The aim of the present study was to compare the soft tissue biocompatibility of MgCa0.8 and commonly used surgical steel in vivo.

Methods: A biodegradable magnesium calcium alloy (MgCa0.8) and surgical steel (S316L), as a control, were investigated. Screws of identical geometrical conformation were implanted into the tibiae of 40 rabbits for a postoperative follow up of two, four, six and eight weeks. The tibialis cranialis muscle was in direct vicinity of the screw head and thus embedded in paraffin and histologically and immunohistochemically assessed. Haematoxylin and eosin staining was performed to identify macrophages, giant cells and heterophil granulocytes as well as the extent of tissue fibrosis and necrosis. Mouse anti-CD79α and rat anti-CD3 monoclonal primary antibodies were used for B- and T-lymphocyte detection. Evaluation of all sections was performed by applying a semi-quantitative score.

Results: Clinically, both implant materials were tolerated well. Histology revealed that a layer of fibrous tissue had formed between implant and overlying muscle in MgCa0.8 and S316L, which was demarcated by a layer of synoviocyte-like cells at its interface to the implant. In MgCa0.8 implants cavities were detected within the fibrous tissue, which were surrounded by the same kind of cell type. The thickness of the fibrous layer and the amount of tissue necrosis and cellular infiltrations gradually decreased in S316L. In contrast, a decrease could only be noted in the first weeks of implantation in MgCa0.8, whereas parameters were increasing again at the end of the observation period. B-lymphocytes were found more often in MgCa0.8 indicating humoral immunity and the presence of soluble antigens. Conversely, S316L displayed a higher quantity of T-lymphocytes.

Conclusions: Moderate inflammation was detected in both implant materials and resolved to a minimum during the first weeks indicating comparable biocompatibility for MgCa0.8 and S316L. Thus, the application of MgCa0.8 as biodegradable implant material seems conceivable. Since the inflammatory parameters were re-increasing at the end of the observation period in MgCa0.8 it is important to observe the development of inflammation over a longer time period in addition to the present study.

Figure 1

MgCa0.8 bone screws, which were implanted into rabbit tibiae. The slotted screw head had diameter of 8.0 mm.

Figure 2

Explanted cranial tibial muscle of a rabbit after implantation of MgCa0.8 for 6 weeks. The former position of the screw head is marked by the black circle. To obtain cross sections of the central implantation site for histological and immunohistochemical studies, the muscle was cut into halves first, then one piece of the distal part was embedded in paraffin. The vertical black lines indicate which part of the muscle was used.

Figure 3

Radiographs of two rabbit tibiae four weeks post operation. (A) Medio-lateral projection of a rabbit tibia with an implanted MgCa0.8 screw. (B) Magnification corresponding to the white rectangle in (A). (C) Cranio-caudal projection of the same tibia. New bone (white arrows) and accumulation of gas (white triangles) are clearly observable at the implantation site of MgCa0.8. (D) Medio-lateral projection of a rabbit tibia with an implanted S316L screw. (E) Magnification corresponding to the white rectangle in (D). (E) Cranio-caudal projection of the same tibia. At the implantation site of S316L only new bone (white arrows) is detectable.

Figure 4

Image sections of cranial tibial muscle cross sections two weeks post operatively (H. E., 200×). (A) MgCa0.8 implant site and (B) S316L implant site, respectively. Note the fibrous tissue (ft) that formed between the cranial tibial muscle (m) and the implant site (i) and is surrounded by a layer of synoviocyte-like cells (sc) at its interface to the implant in both implant materials.

Figure 5

Boxplots of the histological evaluation of cranial tibial muscle cross sections after implantation of MgCa0.8 (light grey boxplots) and S316L (dark grey boxplots) for two, four, six and eight weeks (two weeks is represented as M2 for MgCa0.8 and as S2 for S316L, four weeks as M4 for MgCa0.8 and S4 for S316L, etc.). The boxplots are representing the amount of (A) periimplant fibrosis, (B) tissue cavities, (C) necrosis, (D) macrophages, (E) giant cells and (F) heterophil granulocytes. Non parametrical tests (Mann-Whitney-Tests) were calculated to determine differences between the implant materials. Stars represent significant difference, which was defined as p < 0.05.

Figure 6

Image section of an MgCa0.8 implant site two weeks postoperatively (H. E., 200×). Within the periimplant fibrous tissue (ft) a tissue cavity (tc) is clearly observable. The tissue cavity is surrounded by a synoviocyte-like cell layer (sc).

Figure 7

Image section of an S316L implant site two weeks postoperatively showing moderate necrosis and mild infiltration with macrophages within a moderately developed fibrous layer (H. E., 200×). The area of necrosis at the edge of the fibrous tissue is clearly observable. Macrophages are exemplarily represented by triangles.

Figure 8

Image section of the periimplant fibrous tissue of an MgCa0.8 screw four weeks postoperatively (H. E., 400×). Within the fibrous tissue mild infiltrations of giant cells (arrows) and macrophages (triangles) are detectable.

Figure 9

Image section of the periimplant fibrous tissue of an MgCa0.8 screw eight weeks postoperatively showing moderate infiltrations of heterophil granulocytes that are clearly observable due to their lobulated nucleus and red cytoplasmic granules (H. E., 400×).

Figure 10

Image sections of immunohistochemically stained cranial tibial muscle cross sections. (A) Periimplant fibrous tissue of an MgCa0.8 screw six weeks post operatively (CD79α, 400×). Arrows represent CD79α positive B-lymphocytes. (B) Periimplant fibrous tissue of an S316L screw two weeks postoperatively (CD3, 400×). Triangles indicate CD3 positive T-lymphocytes.

Figure 11

Boxplots of the immunohistochemical evaluation of cranial tibial muscle cross sections after implantation of MgCa0.8 (light grey boxplots) and S316L (dark grey boxplots) for two, four, six and eight weeks (two weeks is represented as M2 for MgCa0.8 and as S2 for S316L, four weeks as M4 for MgCa0.8 and S4 for S316L, etc.). The boxplots represent the amount of (A) B-lymphocytes and (B) T-lymphocytes. Non parametrical tests (Mann-Whitney-Tests) were calculated to determine differences between the implant materials. Stars represent significant difference, which was defined as p < 0.05.