Mice are increasingly used for fracture healing research because of the possibility to use transgenic animals to conduct research on the molecular level. Mice from both sexes can be used, however, there is no consensus in the literature if fracture healing differs between female and male mice. Therefore, the aim of the present study was to analyze the similarities and differences in endochondral fracture healing between female and male C57BL/6J mice, since this mouse strain is mainly used in bone research. For that purpose, 12-weeks-old female and male mice received a standardized femur midshaft osteotomy stabilized by an external fixator. Mice were euthanized 10 and 21 days after fracture and bone regeneration was analyzed by biomechanical testing, µCT analysis, histology, immunohistochemistry and gene expression analysis. At day 21, male mice displayed a significantly larger fracture callus than female mice accompanied by higher number of osteoclasts, higher tissue mineral density and absolute values of bone volume, whereas relative bone volume to tissue volume ratio did not differ between the groups. Biomechanical testing revealed significantly increased bending stiffness in both fractured and intact femurs from male vs. female mice, whereas relative bending stiffness of fractured femurs related to the intact femurs did not differ. 10 days after fracture, male mice display significantly more cartilage and less fibrous tissue area in the fracture callus than female mice, whereas bone area did not differ. On the molecular level, male mice displayed increased active β-catenin expression in the fracture callus, whereas estrogen receptor α (ERα) expression was reduced. In conclusion, male mice showed more prominent cartilaginous callus formation, increased mineralization and whole callus tissue formation, whereas functional outcome after fracture did not differ from female mice. This might be due either to the heavier weight of male mice or because of differences in molecular signaling pathways.

Histone modifications critically contribute to the epigenetic orchestration of bone development - in part by modifying accessibility of genes to transcription factors. Based on the previous finding that histone H2A deubiquitinase 2A-DUB/Mysm1 interacts with the p53-axis in hematopoiesis and tissue development, we here analyzed the molecular and cellular mechanisms of Mysm1-p53 interplay in bone development.

The bone phenotype of 4–5 week-old Mysm1-/- (MKO), Mysm1-/-p53-/- (DKO) and corresponding wildtype (WT) mice was determined using µCT and histology. Primary osteoblasts, mesenchymal stem cells (MSCs) and osteoclasts were isolated from long bones to assess cell proliferation, differentiation, apoptosis and activity. Statistics: one-way ANOVA, p<0.05.

MKO mice displayed an osteopenic bone phenotype compared to WT (BV/TV: 5.7±2.9 vs. 12.5±4.2, TbN: 1.3±0.6 vs. 2.7±0.7 1/mm, respectively), and these effects were abolished in DKO mice (BV/TV: 17.8±2.6, TbN: 3.7±0.4 1/mm). MKO mice compared to WT also showed both in vitro and in vivo disturbed osteoclast formation (in vitro: 1.5±1.2 vs. 9.9±1.8 OcN/mm2, in vivo OcN/BPm: 1.4±1.0 vs. 3.0±0.7 cells/mm, respectively) accompanied by increased apoptosis and DNA damage; additional p53 knockout attenuated these effects (7.8±1.8 OcN/mm2 and OcN/BPm: 2.2±1.0 cells/mm). Primary osteoblasts from both MKO and DKO mice showed decreased expression of the transcription factor Runx2 and of the osteogenic markers. ChIP-Seq analysis revealed direct binding of Mysm1 to Runx2 promoter regions in osteoblasts, implying that Mysm1 here regulates osteogenic differentiation. In contrast, MKO-MSCs differentiation did not differ from WT, but DKO-MSCs displayed a significantly increased expression of Alpl, Bglap and Runx2. The different effects of Mysm1-/- in MSCs and osteoblasts presumably resulted from the lower expression level of Mysm1 in MSCs in comparison to mature osteoblasts.

Thus, our data demonstrate that H2A deubiquitinase Mysm1 is essential for the epigenetic control of bone development via distinct mechanisms: 1) In osteoclasts, Mysm1 is involved in maturation of osteoclast progenitors and osteoclast survival. 2) In osteoblasts, Mysm1 directly controls Runx2 expression, thereby explaining osteopenic phenotype of MKO mice. 3) In MSCs, Mysm1 may play an inferior role due to low expression level. However, loss of p53 increases Runx2 expression during MSC differentiation, leading to normal bone formation in DKO mice.

Confirming clinical evidence, we recently demonstrated in a rodent model that a severe trauma which induces an acute systemic inflammation considerably impairs fracture healing. Interleukin-6 (IL-6) is a key cytokine in posttraumatic inflammation as its serum level correlates with injury severity and mortality. IL-6 signals are transmitted by the transmembrane glycoprotein 130 (gp130) via two distinct mechanisms: firstly, through classic signalling via the membrane-anchored IL-6 receptor and secondly, through trans-signalling using a soluble IL-6 receptor. Whereas IL-6 trans-signalling is considered a danger signal driving inflammation, classic signalling may mediate anti-inflammatory, pro-regenerative processes. The role of the two distinct pathways in bone healing has not yet been elucidated. Here, we studied the function of IL-6 in the pathophysiology of compromised bone healing induced by severe trauma.

Male C57BL/6J mice received an osteotomy of the right femur stabilized with an external fixator. Systemic inflammation was induced by additional blunt chest trauma (TxT) applied immediately after the osteotomy. Mice were injected with either fusion protein sgp130Fc, which selectively inhibits IL-6 trans-signalling, or a neutralizing anti-IL-6 antibody (IL-6 Ab), blocking both signalling pathways. Control mice received vehicle solution. Animals were euthanised 21 days after surgery. Fracture healing was analysed by biomechanical testing, μCT, and histomorphometry (n= 6–9; p=0.05; ANOVA/Fisher LSD post hoc).

Thoracic trauma significantly impaired fracture healing [bending stiffness (EI) −57%, p<0.00]. Treatment with sgp130Fc significantly attenuated bone regeneration as demonstrated by an increased EI (+110%, p<0.00) and a trend of augmented apparent Young”s modulus (+69%, p=0.13) compared to TxT control. Histomorphometric analysis could not detect differences in the amount of bone, confirming µCT results, but revealed a significantly decreased cartilage area after treatment with sgp130Fc (−76%, p=0.01). Inhibition of both signalling pathways with IL-6 Ab, however, did not have any effects.

In conclusion, severe trauma significantly impaired fracture healing, confirming previous studies. Treatment with sgp130Fc ameliorated the negative effects providing evidence that IL-6 trans-signalling triggers the excessive immune response after trauma impairing bone regeneration. Injection of IL-6 Ab did not improve fracture healing thereby implying that classic signalling may rather have beneficial effects.

Introduction

The medial patellofemoral ligament (MPFL) is the main stabilizer of the patella and therefore mostly reconstructed in the surgical correction of patellofemoral dislocation. Various biomechanical and clinical studies have been conducted on MPFL reconstruction, while the patellofemoral contact pressure (PFCP) which is indicated as one of the predictors of retropatellar osteoarthritis was neglected. Therefore, the aim of this study was to investigate how different MPFL reconstruction approaches affect PFCP.

Introduction

With processing age, meniscus degeneration occurs which is often associated with osteoarthritis. Existing data about the influence of degeneration on the biomechanical properties of the meniscus are still contradictory, or completely unknown regarding the hydraulic permeability. Thus, the aim of this study was to characterise the biomechanical properties and structural composition of the meniscal tissue depending on its degree of degeneration.

Complement C5a receptor 1 (C5aR1) has crucial functions in host defense against danger molecules, as does toll-like receptor 2 (TLR2). Both innate immunity receptors interact in immune cells in the context of infectious inflammatory diseases often associated with bone loss, such as periodontitis. C5aR1 plays an important role in bone, as it is expressed on bone cells and strongly upregulated due to bone injury. Importantly, C5aR1-ko mice are protected against arthritis and C5aR1 contributes to bone loss in periodontitis. In contrast, less data exist on the role of TLR2 on osteoblasts, however, it is known that TLR2 is expressed on osteoblasts and contributes to bacterial-induced bone resorption. The aim of this study was to evaluate the interaction of C5aR1 and TLR2 in osteoblasts, including intracellular signaling pathways and gene expression patterns.

Primary osteoblasts were isolated from 8–12 week-old WT mice and differentiated for 14 days. Osteoblasts were assessed for expression of C5aR1 and TLR2. Phosphorylation of mitogen-activated protein kinases (MAPK) in response to C5a and Pam3CSK4 (TLR2 agonist) was analyzed by immunoblotting. Gene expression profiling after 30 min and 4 h stimulation of C5a was performed by microarray and candidate genes were validated by quantitative Real-Time PCR (qRT-PCR). Immunoprecipitation was performed using a C5aR1-antibody and C5aR1 and TLR2 were subsequently detected by immunoblotting. Statistics: One way ANOVA p<0.05, n=4–6.

We showed that C5aR1 and TLR2 are expressed on osteoblasts and strongly upregulated during differentiation. Via immunoprecipitation, we could show that C5aR1 and TLR2 do physically interact in osteoblasts. We then examined if C5aR1 and TLR2, besides their physical interaction, also act via the same intracellular signaling pathways. Gene expression profiling upon C5a stimulation revealed that the top regulated pathways are related to MAPK and transforming growth factor beta (TGF-β). Respective genes, such as TGF-β (Tgfb1) and its receptor (Tgfbr) were found to be upregulated, and negative MAPK regulators were found to be downregulated, both by microarray analysis and qRT-PCR. Accordingly, we saw a C5aR1- and TLR2-dependent phosphorylation of p38 MAPK. Interestingly, this effect was enhanced and prolonged by costimulation of both receptors. An additive effect of C5aR1 and TLR2 was also seen regarding Cxcl10 levels, which were enhanced compared to C5aR1 or TLR2 stimulation alone.

This study shows that C5aR1 and TLR2 interact in osteoblasts, not only physically but also functionally, regarding downstream signaling and target genes. Those data strongly imply a synergistic interplay between the receptors, through which osteoblasts possibly contribute to inflammatory reactions affecting bone.

Cartilage injury is generally associated with cytokine release and accumulation of reactive oxygen species. These mediators trigger pathologic behaviour of the surviving chondrocytes, which respond by excessive expression of catabolic enzymes, such as matrix metalloproteinase 13 (MMP-13), reduced synthesis of type II collagen (COL2A1) and apoptosis. In the long run, these pathologic conditions can cause a posttraumatic osteoarthritis. With the objective to attenuate the progressive degradation of the extracellular matrix and, what is more, promote chondroanabolic processes, a multidirectional treatment of trauma-induced pathogenesis was tested for the first time. Therefore, we evaluated the combinations of one anabolic growth factor (IGF-1, FGF18 or BMP7) with the antioxidant N-acetyl cysteine (NAC) in a human ex vivo cartilage trauma model and compared the findings with the corresponding monotherapy. Human cartilage tissue was obtained with informed consent from donors undergoing knee joint replacement (n=24). Only macroscopically intact tissue was used to prepare explants. Cartilage explants were subjected to a blunt impact (0.59 J) by a drop-tower and treated by IGF-1 [100 ng/mL], FGF18 [200 ng/mL] or BMP7 [100 ng/mL] and/or NAC [2 mM] for 7 days. Following parameters were analysed: cell viability (live/dead staining), gene expression (qRT-PCR) as well as biosynthesis (ELISA) of type II collagen and MMP-13. For statistical analysisKruskal-Wallis or One-way ANOVA was used. All data were collected in the orthopedic research laboratory of the University of Ulm, Germany.

Trauma-induced cell death was completely prevented by NAC treatment and FGF18 or BMP7 to a large extent, respectively (p<0.0001). IGF-1 exhibited only poor cell protection. Combination of NAC and FGF18 or BMP7 did not result in enhanced effectiveness; however, IGF-1 significantly reduced NAC-mediated cell protection. While IGF-1 or BMP7 induced collagen type II gene expression (p=0.0069 and p<0.0001, respectively) and its biosynthesis (p<0.0001 and p=0.0131, respectively), NAC or FGF18 caused significant suppression of this matrix component (each p<0.001). Although COL2A1 mRNA was significantly increased by NAC plus IGF-1 (p<0.0001), biosynthesis of collagen type II was generally abolished after multidirectional treatment. Except for IGF-1, all tested therapeutics exhibited chondroprotective qualities, as demonstrated by attenuated MMP-13 expression and breakdown of type II collagen. In combination with IGF-1, NAC-mediated chondroprotection was reduced.

Overall, both chondroanabolic and antioxidative therapy had individual advantages. Since adverse interactions were found by simultaneous application of the therapeutics, a sequential approach might improve the efficacy. In support of this strategy current experiments showed that though cell and chondroprotective effects of NAC were maintained after withdrawal of the antioxidant, type II collagen expression recovered by time.

In polytrauma patients invasive surgeries can potentiate the posttraumatic systemic inflammation thus increasing the risk of multi organ dysfunction. Therefore, fractures are initially treated by external fixators, which later are replaced by intramedullary nails. We showed that a severe trauma impaired the healing of fractures stabilized by external fixation. Here we studied, whether the conversion to an intramedullary nail increases posttraumatic inflammation and leads to further impairment of healing.

44 rats received a femur osteotomy stabilized by an external fixator (FixEx). Half of the rats underwent a thoracic trauma (TXT) at the same time. After 4 days the fixator was replaced by an intramedullary nail (IMN) in half of the rats of each group. The rats were killed after 40 and 47 days. C5a serum levels were measured 0, 6, 24, and 72h after the 1st as well as the 2nd surgery. The calli were evaluated by three-point-bending test, μCT and histomorphometry.

The TXT significantly increased serum C5a levels after the 2nd surgical intervention. After 40 days the switch from FixEx to IMN significantly decreased bending stiffness in rats with and without TXT. After 47 days flexural rigidity in rats subjected to conversion was significantly decreased compared to rats treated only with a FixEx, particularly in combination with TXT.

This study showed that after a severe trauma the conversion of the fixation could provoke a second hit and contribute to delayed fracture healing.

Introduction

The fracture healing outcome is often evaluated via ex vivo testing of the fracture callus. However, there is only a small time window, where the callus stiffness is significantly different, i.e. a delayed fracture healing might be undetected if the time point of sacrifice is improper. The aim of this study was to develop an in vivo monitoring concept, which allows determining the fracture callus stiffness in vivo over the whole healing time in rats.

Secondary fracture healing processes are strongly influenced by interfragmentary motion. Shear movement is assumed to be more critical than axial movement, however experimental results are controversial. Numerical fracture healing models allow to simulate the fracture healing process with variation of single input parameters and under comparable normalized mechanical conditions. Therefore, a direct comparison of different in vivo scenarios is possible. The aim of this study was to simulate fracture healing under several axial and shear movement scenarios and compare their respective time to heal. We hypothesize that shear movement is always more critical than axial loading. For the presented study, we used a corroborated numerical model for fracture healing in sheep. Numerous variations of the movement amplitude, the fracture gap size and the musculoskeletal loads were simulated for comparable axial compressive and shear load cases. In all simulated cases, axial compressive load had less inhibitory influences on the healing process than shear load. Therefore, shear loading is more critical for the fracture healing outcome in general. Thus, our findings suggest osteosynthesis implants to be optimized to limit shear movements under musculoskeletal loading.

There is evidence that fracture healing is delayed in severely injured patients. We recently demonstrated that a blunt chest trauma, which induced posttraumatic systemic inflammation, considerably impaired fracture healing in rats. Because the complement anaphylatoxin C5a is an important trigger of systemic inflammation, we tested the hypothesis, whether the impairment of fracture healing observed after a severe trauma resulted from systemically activated complement.

16 male Wistar rats received a thoracic trauma and a femur osteotomy stabilized by an external fixator. Immediately and 12 h after the trauma, half of the animals received a C5aR-antagonist to prevent the C5a-dependent systemic inflammation. Control rats received a nonsense peptide, which does not provoke any biological effect. The animals were killed after 35 days and the calli were analyzed by three point bending testing, μCT and histomorphometry. Statistics: Mann-Whitney U test, level of significance to p<0.05.

The treatment with the C5aR-antagonist increased flexural rigidity significantly by 55%, improved bony bridging of the fracture gap and led to a slightly larger and qualitatively improved callus as evaluated by μCT and histological measurements.

This study shows, that the immunomodulation by a C5aR-antagonist significantly reduced the deleterious effects of a thoracic trauma on fracture healing. C5a could possibly represent a target to prevent delayed bone healing in patients with severe trauma.

There is evidence that fracture healing is impaired in patients with chronic immune disorders the reasons remaining unclear so far. To further elucidate the role of the immune system in bone healing, this study investigated the hypothesis that fracture healing would be considerably disturbed in a mouse model with severe defects of the innate as well as adaptive immune system.

Immune deficient Nod-scidIL2Rγ^null and immune competent BALBcByJ mice were used (12 weeks, male, each n=24). The mice received a femur osteotomy stabilized by an external fixator and were sacrificed at d 21, 28, and 35. The calli were evaluated by three-point-bending testing, μCT and histomorphometry.

The flexural rigidity of the callus did not significantly differ between both genotypes after 21 and 28 days but was significantly lower in Nod-scidIL2Rγ^null mice after 35 days (31%). The maximum moment of inertia was significantly increased after 21 days (by 34%), and the callus cross section area after 21, 28 and 35 days in Nod-scidIL2Rγ^null mice. BV/TV of the callus of Nod-scidIL2Rγ^null mice was significantly decreased after 28 and 35 days (by 32% and 41%). The histological evaluation showed a significantly enhanced amount of cartilage in the fracture gap of Nod-scidIL2Rγ^null mice.

These data indicate an only moderate delay in fracture healing in Nod-scidIL2Rγ^null mice suffering on severe defects in innate and adaptive immune response.

Since osteoimmunology is gaining increasingly interest and evidence for involvement of complement in bone biology was found, the role of complement in bone biology and fracture healing was evaluated.

After characterizing the bone phenotype, a fracture healing experiment with C3- and C5- deficient mice was performed. After osteotomy of the right femur and external fixation, healing was analyzed after 1, 3, 7 and 21 days. Bone characterization revealed a reduced number of osteoclasts in C5-deficient animals with a significantly reduced resorption activity. While bone mineral density was significantly higher in complement-deficient strains, stiffness was significantly reduced. After 21 days of fracture healing, C5-deficient animals showed reduced stiffness and a smaller callus volume compared to controls. Interestingly, C3- more than C5-deficient animals showed reduced bone formation. Altogether, bone phenotype of complement-deficient animals resembles a mild form of osteopetrosis.

This might be due to the resorption defect seen in C5-deficient mice. A reason for the minor involvement of C3-deficient mice compared to the C5-deficient animals could be the cross-talk between the coagulation cascade with side activation of complement factor C5 by thrombin.

These results indicate for the first time an essential role of complement in bone biology and fracture healing. Future studies should focus on the molecular basis of complement involvement and the osteoclastic resorption defect.

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.

Introduction: Lateral callus distraction can be used to treat bone defects and increase bone diameter. However, this requires longitudinal splitting of the bone, which can be avoided by a new method.

Material and Methods: Twenty sheep were operated at the medial site of the tibia. After drilling 1 mm holes into the medullary cavity, a titanium plate with a hydroxyapatite coating was attached to the bone surface using a custom-made device

After 10 days this device was distracted 0.3 mm, twice a day until a distance of 6 mm was achieved (n=10). In the control group (n=10), the titanium plate was adjusted to a distance of 6 mm. All sheep received fluorescence labeling. 10 weeks p. o., bone formation underneath the titanium plate was investigated using pQCT, x-ray and histomorphology.

Results: In the distraction group there was 5 times more newly formed bone. > Bone columns bridging the space between the cortex and titanium plate were found. In the first 30 days, the bone formation was significantly enhanced in the distraction group as indicated by the fluorescent labeling

Conclusion: A strain induced bone formation is not only possible between two bony surfaces created by an osteotomy, but also between a bony surface and an appropriately designed implant. This new method allows for bone apposition in a large number of bones, which could not be properly treated in the past.

Introduction: Metaphyseal fracture healing is seldom studied and models used so far do not control the biomechanical conditions in the healing area. The aim of this study was to develop a new standardized and biomechanically adjustable osteotomy gap in an animal model.

Material and Methods: A pilot study with two sheep was performed. An incomplete osteotomy was created in the retro patellar region of the distal femur. The compression forces of the patella lead to a cyclic reversible reduction of the 3 mm osteotomy gap size. This inter-fragmentary movement (IM) can be limited by a steel implant fitted into the proximal osteotomy gap. After 8 weeks the sheep were sacrificed and the healing studied by x-ray and histology.

Results: The osteotomy gap, which allowed 0.3 mm IM showed woven bone bridging in the whole gap. The sheep with 1 mm IM showed only partial bone formation but mainly fibrous tissue and fibrous cartilage. There was no external callus formation.

Discussion: This new model allows the study of metaphyseal bone healing under various biomechanical conditions. Even though metaphyseal healing is different from diaphysical callus healing, IM appears to have a similar effect, leading to direct bone formation under stable condition and enchondral ossification under high IM.

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.