There were many researches regarding that the stability of fracture broken end directly affects the healing of bone fracture(25–28). While during the first phase of Masquelet, the bone defects filling with PMMA spacer are fixed with internal plate, intramedullary nail, external fixator, casting, or nothing(17, 19–21). Each fixed mode could result in a different mechanical environment, subsequently have significant influence on bone repair. No study has been done yet to evaluate the effect of PMMA spacer stability on subsequently bone formation in the membrane cavity. This was the first study to investigate the influencing of topical mechanical stability of fracture broken end on the subsequently formation of Masquelet membrane. In order to explore the specific mechanisms, two fixation variables, rigid fixation offered by plates and relative fixation by suturing, were selected for further research.
The rabbit models of bone defects were commonly used in laboratory experiments, and stainless-steel internal plate was usually applied for the fixation of bone defect ends(9, 29, 30). Through X-ray measurement postoperatively we confirmed that the Masquelet models with plate internal fixation was successfully made. The X-ray results suggested no significant movement between the fracture ends, even in the control group without PMMA and plate. Due to the integrity of ulna and interosseous membrane, there was no visible displacement of fracture segments(19). And due to the supporting of PMMA in defects, and suture tension of fascia, PMMA in Non-fixation group was in relatively stable pattern with little distortions(30). Still, the jiggle of fracture fragments was existed theoretically, which might have impacts on subsequently membrane formation.
Through gross histology we found that a translucent and membrane-like fibrotic capsule was formed around PMMA, and that connective tissue formed in the bone defects in the control group, which is consistent with the literature reported(15, 19). H&E and Alizarin red staining results suggested that the intensive fibrous, cell-rich, calcium-positive, and vascularized membrane around PMMA would be more conducive to osteogenic activity.
Content of proliferation was assessed by detection the percentage of Ki67 positive cells. Ki67 is a core protein seen in proliferating cells, which exhibits good morphological properties of cell proliferation(31). The proliferation was slightly higher in the induced membranes compared to the control groups, and a slightly increased proliferative activity was observed in membranes of Fixation group in comparison to that of Non-fixation group, suggesting that the mechanical stability offered by plates was beneficial to cell proliferation in the induced membrane. The results was corresponding to previous research that the cell proliferation was relatively active in the membrane(15, 19). Besides, our research found that a qualitative trend toward increased proliferation was observed in PMMA groups between 4 weeks and 6 weeks.
The Col I was selected for osteogenesis qualitatively. And the expression levels of ALP and RUNX2 were used for the quantitative analysis of osteogenic activity, of which ALP is an early sign of osteoblast differentiation and maturation, and RUNX2 plays a central role in coordinating multiple signals involved in osteoblast activity(32). By comparing results between PMMA groups and the control group, osteogenic activity of the induced membranes was obviously expressed during the whole observation period. Similar animal results were obtained from previous studies. It is well-known that the different mechanical stabilities around bone defects lead to different callus responses and osteoblast activities(33). And current clinical findings indicate that appropriate fixation of the bone defect is desirable in Masquelet technique, with the use of internal fixator, external fixator, or plaster cast(1, 3, 7). Still, the exact mechanism remains unknown. In our research, an increasing trend in the protein expression of Runx2 and ALP was observed in Fixation group, in comparison to Non-fixation group. These results revealed that the rigid fixation provided by internal plate could further promote osteogenesis, which implying that the use of fixation in Masquelet technique has potential clinical benefits.
The detection of CD31 positive cells represents the formation of new vessels. And the vascularity of the membrane was quantitatively evaluated by detecting the level of VEGF and TGF-β1(16, 34). Previous researches have reported that the tube-like vascular structure is formed in the induced membrane around PMMA, and the highly vascular membrane has many similarities to the periosteum(13, 35). The present study indicated that CD31 positive cells were relatively high in the induced membranes, and the higher level of CD31 expression signals greater capacity of vascularization. And the relative expression levels of VEGF and TGF-β1 in membrane around PMMA were higher than those in connective tissue of the control group. Besides, significant differences were found between Fixation and Non-fixation group, suggesting that the rigid fixation provided by internal plate could further promote vascularization too.
Exploring the influencing of topical mechanical stability on subsequently membrane formation could help to further the clinical applications. Further experiments and clinical research to verify their effectiveness would help to further understand the exact generating and regulatory mechanism in Masquelet technique and help to improve clinical outcomes. And the next step in this research would be to further validate if rigid fixation benefits bone defect repairing in the second stage of Masquelet.