Introduction: We developed a signal inducing bone cement for surgical interventions under MR guidance. This cement is based on conventional polymethylmeth-acrylate (PMMA), which is mixed with 0.9% saline solution and a contrast agent (CA), or with a hydroxyapatite based bone-filler (Ostim^®, aap Biomaterials, Germany). This signal inducing cement should allow bone filling procedures, like vertebro- and kyphoplasty, under MR guidance in an open Highfield MR Scanner. As we added the signal inducing substances (saline solution, CA, bone substitute) to the PMMA, we changed the biomechanical properties of the cement.

The purpose of this study was to evaluate the biomechanical properties of the signal inducing bone cement for vertebroplasty in a spine model.

Materials/Methods: We placed cadaveric vertebral bodies (n=18, of 4 lumbar spines) between the crosshead and baseplate of a universal testing machine (Zwick^®, Germany) and compressed to failure. Then, we injected cements into the broken vertebral bodies through a transpedicular approach on both sides, under image intensifier control. The so treated vertebral bodies were then tested again in the testing machine. We injected three cements: a conventional PMMA cement (BonOs^®, aap Biomaterials, Germany, 12g PMMA, 5 ml MMA), an NaCl-cement compound (3 ml 0.9% saline solution, 12g PMMA, 5 ml MMA) and a bone substitute-cement compound (3 ml Ostim^®, 12g PMMA, 5 ml MMA). As the CA amount is negligible (< 9μl), it was neglected for these tests. Each cement type was injected in 6 vertebral bodies.

We defined the initial strength (N) of the vertebral bodies as the load at failure, and the strength after treatment as the maximum load, which occurred within the first 6 mm of compression.

Results: The initial strength of the vertebral bodies (n=18) was 4179 N (SD 497 N). The strength after treatment was 7433 N (SD 503 N) for the conventional cements (n=6), 5900 N (SD 376) for the NaCl-cements (n=6), and 7000 N (SD 413 N) for the Ostim^®-cements (n=6).

Discussion: Although the PMMA cement is weakened by dilution with the signal inducing substances (saline solution, CA, bone substitute), the MRI-cements restored the initial strength of the vertebral bodies. The results suggest that these MRI-cements meet the biomechanical requirements for vertebroplasty, and can be used for MRI guided vertebroplasty.

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: Currently used small animal models of a critical size defect do not sufficiently simulate the biologically unreactive situation in an atrophic non-union. Furthermore, models using intramedullary nails are of little, and poorly standardised, biomechanical stability. This is a characteristic known to promote callus formation though, rather leading to a hypertrophic non-union.

The aim of this study was to establish an atrophic non-union model in the rat femur under well defined biomechanical conditions and with minimised interactions between the processes in the healing zone and the implant by using external fixation.

MATERIALS AND METHODS: 80 male Sprague Dawley rats were randomly divided into two groups (non-union vs. control). All animals received an osteotomy (app. 0.5 mm gap) of the left femur, stabilised with a custom made external fixator. In the non-union group the periosteum was cauterised 2mm distal and proximal of the osteotomy, and the bone marrow was removed. X-rays were performed once weekly. Animals were sacrificed at 14 or 56 days post-operation. At both time points the femurs of 16 animals of each group underwent histological/histomorphometrical and immunhis-tochemical analyses (PMMA or paraffin embedding). Additionally at 56 days 8 animals of each group were tested biomechanically. The maximum torsional failure moment and the torsional stiffness were determined in relation to the intact femur. Post-mortem x-rays were evaluated in a descriptive manner.

RESULTS: At 14 days the histology and radiology showed considerable mineralised periosteal callus in the control group, while the non-union group only showed very little periosteal callus, distant to the osteotomy. At 56 days the control group was completely, or at least partially, bridged by mineralised callus. The non-union group did not show a bridging of the osteotomy gap in any of the animals, moreover the bone ends were resorbed and the gap widened. The relative mean torsional stiffness was significantly larger (p< 0.001) in the control group compared to the non-union group (136.2±34.5% vs. 2.3±1.2%). In the non-union group no maximal torsional failure moment could be detected for the osteotomised femurs. In the control group it was 134.2±79.1%, relative to the intact femur.

DISCUSSION: The cauterisation of the periosteum and the removal of the bone marrow, in combination with a high stiffness of the external fixator may create an atrophic non-union under well defined biomechanical conditions and with minimised interactions between the healing zone and the implant. This model will allow better standardised investigations on the subject of atrophic non-unions.

Introduction: The Expert Tibia Nail was designed to address proximal, shaft, segmental and distal tibia fractures in one implant. Multiple locking options in various directions provide more stability and reduce the risk of secondary malalignment. Angle stable cancellous bone locking screws in the tibia head also improve fixation.

We evaluated this new implant in our series in a prospective, multicenter setting.

Methods: 190 patients were treated in 10 participating centers using the Expert Tibia Nail (Synthes). 127 patients suffered polytrauma, 58 presented as open fractures. Within the framework of the study 5 cases were proximal tibia fractures, 108 shaft fractures, 56 distal fractures, and 21 segmental fractures. These were followed-up postoperatively, after 3 months and one year and evaluated radiologically and clinically with regard to malalignment, union rate and complications.

Results: Non union occurred in 9 cases after one year of follow up (n=150). 20 patients showed delayed union. The rate of open and complex fractures was high in this group. Dynamisation was performed in 10 cases. Valgus/varus and recurvatum/antecurvatum malalignment of more than 5 degrees occurred in 13 cases. Stable reduction was achieved in 144 cases. In 4 complex fractures, initial reduction went into malalignment. 2 patients developed a deep infection after 3rd degree open fractures. 34 patients suffered from pain in the operated area. 6 screws broke during the follow-up.

Discussion: The Expert Tibia Nail proved to be an excellent tool to treat tibia fractures. Not only shaft fractures but also complex fractures in the proximal and distal metaphyseal area can be successfully stabilized due to advanced locking options and design of the nail. The rate of malalignment, non-union and complications was low.

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.