Magnesium calcium alloys are promising candidates for an application as biodegradable osteosynthesis implants [1,2]. As the success of most internal fracture fixation techniques relies on safe anchorage of bone screws, there is necessity to investigate the holding power of biodegradable magnesium calcium alloy screws. Therefore, the aim of the present study was to compare the holding power of magnesium calcium alloy screws and commonly used surgical steel screws, as a control, by pull-out testing.

Magnesium calcium alloy screws with 0.8wt% calcium (MgCa0.8) and conventional surgical steel screws (S316L) of identical geometries (major diameter 4mm, core diameter 3mm, thread pitch 1mm) were implanted into both tibiae of 40 rabbits. The screws were placed into the lateral tibial cortex just proximal of the fibula insertion and tightened with a manual torque gauge (15cNm). For intended pull-out tests a 1.5mm thick silicone washer served as spacer between bone and screw head. Six animals with MgCa0.8 and four animals with S316L were followed up for 2, 4, 6 and 8 weeks, respectively. Thereafter the rabbits were sacrificed. Both tibiae were explanted, adherent soft tissue and new bone was carefully dissected around the screw head. Pull-out tests were carried out with an MTS 858 MiniBionix at a rate of 0.1mm/sec until failure of the screw or the bone. For each trial the maximum pull-out force [N] was determined. Statistical analysis was performed (ANOVA, Student's t-test).

Both implant materials were tolerated well. Radiographically, new bone was detected at the implantation site of MgCa0.8 and S316L, which was carefully removed to perform pull-out trials. Furthermore, periimplant accumulations of gas were radiographically detected in MgCa0.8. The pull-out force of MgCa0.8 and S316L did not significantly differ (p = 0.121) after two weeks. From 6 weeks on the pull-out force of MgCa0.8 decreased resulting in significantly lower pull-out values after 8 weeks. Contrary, S316L pull-out force increased throughout the follow up. Thus, S316L showed significantly higher pull-out values than MgCa0.8 after 4, 6 and 8 weeks (p<0.001).

MgCa0.8 showed good biocompatibility and pull-out values comparable to S316L in the first weeks of implantation. Thus, its application as biodegradable osteosynthesis implant is conceivable. Further studies are necessary to investigate whether the reduced holding power of MgCa0.8 is sufficient for secure fracture fixation. In addition, not only solitary screws, but also screw-plate-combinations should be examined over a longer time period.

Degradable implants made of magnesium alloys as osteosynthesis material for weight-bearing bone are at present a main research area. With regards to biocompatibility, a MA with 0.8 wt.

% Calcium (MgCa(0.8)) has been shown to possess advantageous qualities. Long-term investigations in animal models however, showed that the degradation rate of this magnesium alloy was relatively rapid and therefore the mechanical properties decreased early during the implantation period. An implant for osteosynthesis in weight-bearing bones however needs to exhibit adequate stability during the first few weeks of fracture healing. This cannot sufficiently be assured by the MgCa(0.8) alloy. It has been suggested in the literature, that the degradation rate of MA could be reduced using a fluoride coating. Therefore it was the aim of this study to investigate, whether the coating of degradable MA MgCa(0.8) implants with magnesium fluoride layer leads to decreased degradation rate and in consequence to an improvement of the mechanical properties using an animal model.

Extruded pins (2.5 mm x 25 mm) of MgCa(0.8) were produced. Twenty of these pins were coated with a fluoride layer by submerging the implants in a bath with 40% hydrofluoric acid. With this procedure, the pins were covered with a thin (150–200μm thickness) MgF2 layer. Coated and uncoated pins were intramedullary implanted into both tibiae of ten New Zealand White Rabbits. Three and six months after surgery five animals of each group were euthanized and the tibiae were explanted for further analysis. Micro-computed tomography (μCT) and scanning electron microscopy (SEM) were performed of the explanted pins. In order to investigate changes of the mechanical properties, 3-point bending tests were carried out with MgCa(0.8) pins at the initial state and with the explanted pins, with and without the fluoride layer at both times. In addition, the mass loss of the pins was determined. To evaluate the degradation process of the MgCa(0.8) pins with the MgF2 layer, micrographs and element analyses (EDX) were accomplished after the three point bending tests.

During the investigation period, the rabbits showed no signs of lameness or pain. The MgCa(0.8) alloy and the MgCa(0.8) alloy with the MgF2 layer showed significant differences regarding the mechanical properties in dependence of the implantation duration. Generally, the mechanical resistance decreases with increasing implantation time. The 3-point bending test showed, that the values of maximal force of the coated MgCa(0.8) implants after three month implantation duration were lower than those of the uncoated implants. After an implantation duration of six months, the values of maximal force of the implants coated with MgF2 were higher than those of the uncoated implants. Regarding the implant mass, the coated and uncoated MgCa(0.8) implants showed a loss of mass during the implantation period. The mass loss of the coated implants was only slightly lower. This difference was minor after three months and more obviously after six months. With μCT new endosteal bone formation could be seen close to all implants. A decrease of the cross section dimension could be demonstrated with μCT and SEM and changes of the surfaces due to pitting corrosion could be demonstrated in both the coated and uncoated MgCa(0.8) implants on the whole length, which was more obvious after six months. The micrographs showed corroded surfaces but not preferred corrosion on the grain boundaries. The element analysis showed a degradation layer on the implant surface, which was more bulky on implants after six month implantation duration. The mapping shows, that the fluoride molecules are clearly visible after three and six months around the margin of the implant.

With the results of this study it could be demonstrated, that the coating of the MgCa(0.8) implants with a flouride layer did not have a positive influence on the mechanical properties and the degradation rate of the implant in the bone. This leads to the conclusion that MgF2-coated MgCa(0.8) implants are also not suitable for osteosynthesis in weightbearing bones.

Introduction: Rabbits are a well-established animal model for orthopaedic research

and the tibia is commonly used for investigations of fracture repair with different implant materials

Therefore, a new method for the direct measurement of forces in the rabbit tibia was developed. The aim of this study was to determine maximal forces during weight bearing in the rabbit for future implementation into FEM-simulation.

Animals and Methods: An external ring fixation was attached to the left tibiae of 5 rabbits and an ostectomy followed. Force sensors were included into the collateral rods to incur the emerging forces completely. On each side, a measurement amplifier was applied to transfer the collected data telemetrically. During the study, the animals were weighted and x-rays were taken regularly. Measurements started 8 days postoperatively and were repeated 8 times until day 50 post-op. The rabbits were placed in a run and animated to move while the forces were registered. Force peaks were filtered from the collected data of each measurement as absolute values and relative to the animals’ weight (force-weight ratio/FWR).

Results: All included animals tolerated the external fixa-tion well and no clinical intolerances occurred. Beginning of callus formation was detected radiographically about 3 weeks post-op and all fixations could be removed 12–14 weeks after application without any permanent detriments. The maximal force amounted to 6950 g and 172 % FWR in animal 4 during the first recording. Means of the 5 maximal values for each measurement were located between 55 % FWR and 152 % FWR for the first measurement, converged to approx. 80 % FWR during the second recording 3 days later and descended to 20–40 % FWR until the end of the experiment.

Discussion: Aim of this study was to determine maximal forces during weight bearing in a rabbit model. Our model for in-vivo monitoring of these forces was practicable and provided profound data. The highest values occurred during the first or second recording. That coincides with the radiographic detection of callus after 3 weeks. Therefore, reliable measurements have to be carried out during the first 2 weeks postoperatively. Detected values show that the rabbit tibia is strained with up to 170 % of the body weight, which is the compressive force an implant in a weight bearing bone has to be able to bear. Future research will focus on the in-vivo monitoring of bending and torsion forces and the implementation of these data into FEM-simulation.

Introduction: Fractures in long bones are frequently managed with intramedullary implants, plates ore external fixators. X-ray images are normally used to determine the point of full weight bearing and implant removal. Plain radiographs give only poor information about the mechanical properties of the healing callus. Several quantitative Methods: like QCT and DEXA provide information about the density of the new bone, but the mechanical properties remain unknown. For direct monitoring of the mechanical properties of the healing callus a 4-point-stiffness device for small animals was constructed. This devise is used to detect the influence of degradable implants on bone healing. Long term aim is to develop “smart” implants that degrade during healing and speed up the healing process.

Materials and Methods: An uniplanar, bilateral external fixator was mounted on the tibiae of New Zealand White Rabbits after osteotomy and introduction of different degradable, intramedullar implants. The 4-point-bending measurement unit was temporarily fixed to record deflection with a non-contact displacement transducer. Load cells were instrumented to record the stepwise load increase (25g). The max. bending moment was only 0.14 Nm to avoid bending of the implant. Additional μ-CT analysis was conducted on the stiffness measurement days to quantify bone healing. After the in-vivo tests the stiffness measurement device was validated with ex-vivo measurements of bone models in a Material Test System (MTS).

Results: The bending stiffness unit showed a high precision with a standard deviation of 5.55E-04 N/μm and a mean deviation error of all models of 1.74E-04 N/μm. We found a significant non-linear correlation between the measured stiffness and the diameter of the models (p< 0.05, r2=0.96). Furthermore a significant correlation between the stiffness device and the MTS in vitro was shown (r2=0.96, p< 0.005). A significant correlation between the data of the bending stiffness device and the MTS was found for all animals (r2=0.64, p< 0.01). μ-CT analysis showed an increase in callus formation and density during the increase in bending stiffness.

Discussion: In this study a precise measurement unit to mirror the mechanical properties of healing bone is presented. The device was successfully tested in an in-vivo model of fracture healing. The healing of callus around different degradable implants can be monitored to develop implants that degrade during fracture healing to avoid stress shielding or implant removal. Not only data about the healing bone can be gatherd with the μ-CT analysis, but also processes around the implants can be well monitored to evaluate degradation and quality of the implants.