Dynamization of fracture fixation is used clinically to improve the bone healing process. This study evaluated the effect of late dynamization on callus stiffness and size in a rat diaphyseal femoral osteotomy. The external unilateral fixator was dynamized by removal of the inner fixator bar, at three weeks (D3-group: n=8) or four weeks (D4-group: n=9) post-operation. Published data of a five week rigid (R-group: n=8) and flexible fixation group (F-group: n=8) were included for comparison. Preoperative and postoperative movements of the rats were measured using a motion detection system. After 5 weeks the rats were sacrificed and healing was evaluated by biomechanical and densitometric methods. By 34 days post-operation, rats from the four fixation groups had similar activity levels. There was no significant difference in flexural rigidity, callus volume or callus mineral density between the D3 and D4-groups. Both the D3-group and D4-group had significantly greater flexural rigidity (p< 0.01) and significantly lower callus total volume (p< 0.03) and callus bone volume (p< 0.03) compared to the F-group. There was no significant difference in flexural rigidity or callus mineral density between the dynamized groups compared to the R-group. However, the D3-group had less callus bone volume (p=0.06) compared to the R-group. The D4-group had significantly less callus bone volume (p=0.02) and less callus total volume (p=0.05) compared to the R-group. Late dynamization led to a stiffer callus with a smaller callus volume compared to continuously flexible fixation. The late dynamized groups had less callus volume than the continuously rigid group, but the stiffness and calcification and of the callus were similar. The late dynamized groups had undergone resorption processes, indicative of more advanced healing. Late dynamization enhanced fracture healing compared to the continuously rigid or flexible fixation.

Introduction: Dynamization is used to improve the healing process. The optimal time for dynamization however remains unknown. In this study we proved the hypothesis that an early dynamization will improve the fracture healing.

Material and Methods: Twenty-four rats underwent a diaphyseal femoral osteotomy, with a 1mm gap. The osteotomy was stabilized by either rigid (R-group; n=8) or flexible (F-group; n=8) external fixation. The dynamized group (D-group: n=8) had a rigid fixation for 1 week, and then a flexible fixation for the remaining 4 weeks. The flexible fixation design resulted in an axial stiffness of 10N/mm and the rigid fixation in 74N/mm. After 5 weeks, healing was evaluated by biomechanical, densitometric, and histological methods.

Results: The flexural rigidity was 47% higher in the R-group than in the F-group (p< 0.01). Also, the flexural rigidity was 45% higher in the R-group than in the D-group (p< 0.01) (Table 1). Mineralized callus tissue volume was 37% lower in the R-group than the D-group (p=0.002).

Conclusion: The hypothesis could not be supported, in that early dynamization did not improve healing compared to rigid or flexible fixation. The rigid fixation had a stiffer callus with smaller callus volume, and more calcified tissue in the whole callus. The rigid fixation had bridging in the gap more often, which explains the increased flexural rigidity measured. Dynamization utilized in previous studies allowed closure of the fracture gap and thereby enhanced the rate of healing, which was not the case in the present investigation.

Although IL-6 mRNA expression in rat is restricted to the first day post-fracture, the inflammatory phase, the protein has been observed later in the healing process, indicating additional roles. The importance of IL-6 was demonstrated by delayed healing in knockout mice through diminished osteoclast numbers, formation thereof being stimulated by IL-6. The aim of our study was to investigate with which cells this cytokine is associated and when during fracture healing.

A closed fracture of the lower right limb was created in rats. The tibia was obtained from six animals at each of 1, 3, 7, 14 and 28 days post-fracture, decalcified and prepared for standard immunohistochemistry with an IL-6-specific polyclonal antibody. The number and types of cells positively stained for IL-6 along the whole length of the periosteal callus on one surface and in the fracture was evaluated.

Mostly inflammatory cells were initially stained, becoming virtually absent by day 7 when this phase has normally ended. Within the immediate vicinity of the fracture where endochondrial ossification occurred, staining of chondrocytes was significant (69%) by day 7 when this cell was laying down cartilaginous tissue that was also calcified. Distally to the fracture where direct bone formation occurred through intra-membranous ossification by osteoblasts, staining of these cells was observed, peaking at day 14 (56%). As this bone started to take on the appearance of cortex and surviving embedded osteoblasts differentiated to osteocytes, the latter cells were stained, suggesting a role in remodelling. At the fracture as bone replaced the cartilaginous tissue and union occurred, staining of chondrocytes decreased, whereas local osteoblasts were positive.

IL-6 appears to play a role throughout fracture healing, in endochondrial and intra-membranous ossification. The level of staining of each cell type reflected the degree of their activity with respect to production of related tissue.