gms | German Medical Science

Objectives: The treatment of distal tibia shaft fractures using intramedullary nailing is still discussed controversially. Very distal fractures with short fragments require stable fixation to prevent malunions. The recent introduction of angular stable locking options for intramedullary nails pledges to provide increased mechanical stability. This study compared intramedullary nails with and without the angular stable locking options for the treatment of very distal tibia fractures. The hypothesis was that angular stability of interlocking screws in intramedullary nailing will increase construct stiffness and reduce fracture gap movement and fatigue failure compared to conventional locking.

Methods: Biomechanical experiments were performed on 24 human cadaveric tibiae divided in three groups: conventional locking with 8 mm diameter nail, conventional locking with 10 mm diameter nail and angular stable locked nail with 8 mm diameter nail. After osteotomy of the intersection of the distal 4/5 to 5/5 of the tibia, stiffness of the implant-bone-construct and movement of the fragments were mechanically tested firstly under static axial loading and then under torsion. In a second step cyclic fatigue loading was applied by a stepwise increasing load protocol with combined axial and torsional loading. As outcome parameters construct-stiffness, fracture gap movement and the range of motion at the mechanical zero (clearance) were calculated. Statistical comparison was based on ANOVA with Bonferroni post hoc analysis and Kaplan Meier survival analysis.

Results and Conclusion: Axial stiffness of the 10 mm nail was about 50% larger compared to both 8 mm nail constructs independent of the type of locking mode (p < 0.01). Accordingly interfragmentary motion was smallest for the 10mm nail constructs (p = 0.3). No differences in axial performance were found between angular stable and conventional locking neither under static nor under cyclic testing conditions (p>0.5). The interfragmentary motion under torsional load resulted from twisting of the nail construct and from clearance of the screws within the nail. Angular stability significantly decreased the clearance by more than 50% compared to both conventionally locked constructs (p=0.03). However, due to the larger diameter of the nail, the total interfragmentary motion was still smallest for the 10 mm nail construct (p < 0.01). Although the 10 mm nail constructs survived slightly longer under cyclic loading the differences between groups were minor and not statistically significant (p = 0.4).

Our hypothesis that angular stable interlocking of intramedullary nails would improve mechanical performance of distal tibia fracture fixation was not confirmed in a clinically relevant loading scenario. Minor mechanical advantages provided by angular stability of the locking screws are likely not clinically relevant.