Purpose The aim of this study was to compare the initial biomechanical characteristics of the angle-stable locking system for intramedullary nails using the new biodegradable sleeve with conventional locking in the treatment of unstable distal tibial fractures. Methods Eight pairs of fresh, frozen porcine tibiae were used for this study. The expert tibial nail (Synthes) was equipped with either conventional locking screws (CL) or the angle-stable locking system (AS). This system consists of a special ASLS screw with a biodegradable sleeve. For this investigation distal tibias (5.5 cm) were used and the nails were locked with three screws in both groups. Biomechanical testing included non-destructive torsional and axial loading. Results The AS group showed a significantly higher torsional stiffness (70%) compared to the CL group. The range of motion was 0.5 times smaller for the AS constructs. The neutral zone was eight times higher in the CL group (p<0.001). In axial loading the AS group also showed a 10% higher axial stiffness and a 12% lower range of motion (p<0.001). Conclusion The angle-stable locking system (ASLS) using a special screw and sleeve locking for intramedullary nails provides a significantly higher primary stability. The differences determined in this study may have clinical relevance particularly for torsional loads. For the new biodegradable angle-stable sleeve we found a comparable stability to the PEEK-based sleeve system. This system has the potential to decrease complications such as secondary loss of reduction and mal-/non-union.
BackgroundIn the last years intramedullary nailing has become the treatment of choice for most displaced diaphyseal tibia fractures. In contrast intramedullary nailing of distal tibia fractures is accompanied by problems like decreased biomechanical stability. Nevertheless the indications for intramedullary nailing have been extended to include even more distal fractures. The purpose of this study was to compare long-term mechanical characteristics of angle-stable versus conventional locked intramedullary nails in the treatment of unstable distal tibia fractures. Therefore, the effect of time on the mechanical properties of biodegradable sleeves was assessed.Methods8 pairs of fresh, frozen porcine tibiae were used. The expert tibial nail (Synthes) was equipped with either three conventional locking screws (CL) or the angle-stable locking system (AS), consisting of a special ASLS screw and a biodegradable sleeve. Biomechanical testing included torsional and axial loading at different time-points over 12 weeks.ResultsThe AS group showed a significantly higher torsional stiffness at all time-points (at least 60%) compared to the CL group (p < 0.001). The neutral zone was at least 5 times higher in the CL group (p < 0.001). The mean axial stiffness was maximum 10% higher (week 6) in the angle-stable locked group compared to the conventional group. There was no significant change of the torsional mechanical characteristics over the 12 weeks in both groups (p > 0.05). For axial stiffness and range of motion significant differences were found in the AS group.ConclusionsThe angle-stable locking system (ASLS) with the biodegradable sleeve provides significantly higher long-term stability. Especially the differences determined under torsional loading in this study may have clinical relevance. The ASLS permits the potential to decrease complications like secondary loss of reduction and mal-/non-union.
In the development of new implants biomechanical testing is essential. Since human bones vary markedly in density and geometry their suitability for biomechanical testing is limited. In contrast artificial bones are of great uniformity and therefore appropriate for biomechanical testing. However, the applied artificial bones have to be proved as comparable to human bone. An anatomical shaped artificial bone representing the distal human femur was created by foaming polyurethane. To get a bone model with properties of osteoporotic bone a foam density of 150 kg/m3 was used. The biomechanical properties of our artificial bones were evaluated against eight mildly osteoporotic fresh frozen human femora by mechanical testing. At the artificial bones all tested parameters showed a very small variation. In contrast significant correlation between bone mass density and tested parameters was found for the human bones. The artificial bones reached 39% of the compression strength and 41% of the screw pullout force of the human bone. In indentation testing the artificial bones reached 27% (cancellous) and 59% (cortical) respectively of the human bones strength. Regarding Shore hardness artificial bone and human bone showed comparable results for the cortical layer and at the cancellous layer the artificial bone reached 57% of human bones hardness. Our described method for customizing of artificial bones regarding their shape and bone stock quality provides suitable results. In relation to the as mildly osteoporotic classified human bones we assume that the biomechanical properties matching to serve osteoporotic bone.
The treatment of osteoporotic distal femur fractures is still an unsolved problem of trauma surgery. The poor bone stock often leads to secondary loss of reduction and implant failure. Therefore, the development of new implants and their biomechanical testing is essential. In a previous study, we developed and initially characterized an artificial osteoporotic bone model of the distal femur. This follow-up study was performed to characterize this model in a biomechanical comparison. We investigated two different artificial bones: five foam cortical shell (Sawbones) and 10 custom-made artificial femoral condyles. Additionally, eight human femora were used for comparison. For biomechanical testing, two intramedullary nails (distal femur nail (DFN) and supracondylar nail (SCN)) were cyclically axial loaded in an AO 33 C2 unstable distal femoral fracture model. In our testing, the artificial bone showed a decrease in the axial stiffness of 27% for the SCN and 28% for the DFN compared to the human results. Also the number of cycles for a deformation of 2.5 mm was reduced by 55% (SCN) and 62% (DFN). This decrease was homogenous and caused by the relative high bone mineral density of the human specimen used. The modes of failure showed no difference between the artificial and human bones. Our customized artificial bone provides suitable results. In relation to the human bones classified as mildly osteoporotic, we assume that the biomechanical properties match to serve as an osteoporotic bone. Yet, we suggest to check transferability of the results with human material.
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