The aim of this study was to examine the therapeutic potential of locally transplanted MSCs or osteoprogenitor cells (OPCs) in delayed unions. Autologous MSCs were cultured in DMEM or osteogenic medium. A femoral osteotomy was created in rats and stabilized with an external fixator. Except for the Control-group (C-group), a delayed union was induced by cauterization of the periosteum and bone marrow removal. After 2 days, these animals received an injection of DMEM in the gap containing MSCs (MSC-group), OPCs (OPC-group) or no cells (Sham-group). Histomorphometrical analysis showed significant differences in the fraction of mineralized bone, cartilage and connective tissue between the C- and the Sham-group after 2 (p=0.001) and 8 weeks (p≤0.009). After 2 weeks, the MSC- and OPC-groups developed a larger cartilage fraction (each p=0.019) compared to the Sham-group. Biomechanical testing after 8 weeks demonstrated a significantly lower torsional stiffness (p=0.001) in the Sham-group compared to the C-group. Both the MSC and OPC groups showed a higher torsional stiffness than the Sham-group with statistically significant differences (p< 0.002) in the OPC-group. Locally applied MSCs and OPCs slightly improved the healing in this model. The MSCs were less effective compared to the OPCs. The less than expected healing improvement of both cell treatments may be related to an unfavourable microenvironment at the application time. An explanation for the superior outcome of the OPCs might be that the OPCs may be protected by macroscopically visible matrix at the transplantation time point.

*Winner of ISFR Young Investigator Award

Purpose: The aim of this study was to compare the temporal expression pattern of factors related to cartilage and bone formation and endochondral ossification during standard and delayed bone healing for a more in-depth understanding of the molecular basis of disturbed bone healing and to elucidate suitable timing for substitution of factors to stimulate the healing process.

Methods: A tibial osteotomy was performed in two groups of sheep (n=30 each) and stabilized with either a rigid external fixator leading to standard healing or with a mechanically critical one leading to delayed healing. Hematoma/callus tissue was harvested 4, 7, 14, 21 and 42 days postop. qPCR was employed to determine the expression patterns of BMPs and other molecules.

Results: Gene expressions of BMP2, BMP4, BMP7, Noggin, MMP9 and MMP13 were distinctly lower in the delayed compared to the standard healing group at several time points from day 14, whilst no differential gene expression of Coll II and Coll X was found between both groups. Among the BMPs, BMP7 showed the most markedly differential expression. The first evident difference in BMP7 expression between both groups was found at day 14 suggesting that exogen substitution in the context of a therapeutic approach should be postponed. The differential expression pattern of both MMP9 and MMP13 suggests that there might be a failure or delay in endochondral ossification in delayed bone healing.

Conclusion: Downregulation in gene expression of osteogenic BMPs and cartilage matrix degrading MMPs may account for a considerable delay of bone healing.

Introduction: The formation of new blood vessels is a prerequisite for bone healing. CYR61 (CCN1), an extracellular matrix-associated signaling protein, is a potent stimulator of angiogenesis and mesenchymal stem cell expansion and differentiation. A recent study showed that CYR61 is expressed during fracture healing and suggested that CYR61 plays a significant role in cartilage and bone formation. The hypothesis of the present study was that decreased fixation stability, which leads to a delay in healing, would lead to reduced CYR61 protein expression in fracture callus. The aim of the study was to quantitatively analyze CYR61 protein expression, vascularization, and tissue differentiation in the osteotomy gap and relate this to the mechanical fixation stability during the course of healing.

Materials and Methods: A mid-shaft osteotomy of the tibia was performed in two groups of sheep and stabilized with either a rigid or semirigid external fixator, each allowing different amounts of interfragmentary movement. The sheep were sacrificed at 2, 3, 6, and 9 weeks postoperatively. The tibiae were tested biomechanically and histological sections from the callus were analyzed immunohistochemically with regard to CYR61 protein expression and vascularization.

Results: Expression of CYR61 protein was upregulated at the early phase of fracture healing (2 weeks) and decreased over the healing time. Decreased fixation stability was associated with a reduced upregulation of the CYR61 protein expression and a reduced vascularization at 2 weeks leading to slower healing. The maximum cartilage callus fraction in both groups was reached at 3 weeks. However, the semirigid fixator group showed a significantly lower CYR61 immunoreactivity in cartilage than the rigid fixator group at this time point.

Discussion: The fraction of cartilage in the semirigid fixator group was not replaced by bone as quickly as in the rigid fixator group leading to an inferior histological and mechanical callus quality at 6 weeks and therefore to slower healing. The results supply further evidence that CYR61 may serve as an important regulator of bone healing.

Introduction: External fixators are frequently used for stabilization and treatment of problematic fractures. Pintrack infections cause complications such as osteomyelitis and loss of stability of osteosynthesis. It remains unclear, whether pintrack infection provokes pin loosening, or loss of the pin stability results in pintrack infections. The aim of this study was to investigate the correlation between the mechanical stability of pins, the incidence of pin track infections and the osseus anchorage of pins.

Methods: 27 sheep received an external fixator with 6 Schanz screws (pins). Pin insertion and extraction torque was measured and radiographs were taken postoperatively and postmortem. The daily pin care routine included scoring the pin entries. The animals were sacrificed after 3, 6 and 9 weeks. The pins were analysed microbiologically (incidence of infection: ≥103 colony forming units (CFU)) and histologically. The x-rays and histological slices were scored.

Results: Histomorphometrically, the largest callus area and the highest mineralised bone-fraction was found at 6 weeks. The bone density of the periosteal callus increased over time. The cortical bone density decreased over time.

The relative extraction torque increased over the course of healing (3w: 53.4% ± 7.1%, w: 61.8% ± 20.7%; 9w: 84.0% ± 44.9% of insertion torque).

5% of 108 pins showed clinical signs of infection; the microbiologically detected infection rate was 3 times higher.

Radiologically, 22% of 104 pins showed osteolysis in the cortex (entry or exit), 4% showed medium osteolysis and 2% revealed severe osteolysis along the complete pintrack. An accumulation of pintrack infections (75 %) and the incidence of osteolyses (47% of all osteolyses) were found at 6 weeks. The most severe osteolyses were found at 9 weeks. The histological grading revealed the best results for the earliest time point compared to the latter ones.

Discussion: This study reveals an increasing stability of osseous pin-anchorage over the course of healing. As the cortical bone density decreased over time, the increased anchorage-stability can only be explained satisfactorily by the increase of the periosteal callus bone density. The periosteal callus, biggest at 6 weeks, determines the magnitude of extraction force. In contrast to clinical studies, a very low percentage of pintrack infections was observed in this study. However, the microbiologically affirmed infection rate was three times higher than clinically ascertained. The evidence of osteolyses was twice as high as the microbiological infection-rate. The low infection-rate could not prevent evidence of cortical lyses. No correlation was found between infections, osteolyses and pin loosening. Callus formation seems to be essential in pin fixation and should be supported to prevent pin loosening. Targeting only cortical anchorage may fail to avoid pin loosening in external fixation.