Effect of callus development on the deformation of external fixation frames.

PURPOSE: We designed a sensor that measures the bending moments at the articulations and the torque of the rod of a Hoffmann II® external fixation. We considered the effect of the callus formation in the stabilisation of a "fracture-fixation system." METHODS: Four Hoffmann II® frame configurations were mechanically tested. Two carbon fibre tubes represent the bone fragments (length 180 mm, outer diameter 25 mm, inner diameter 19 mm). The callus is represented by the interposition of springs of different rigidity (10-405 N/mm) in the fracture gap between the tubes. RESULTS: The deformation of the frame is in inverse proportion to the stiffness of the callus; the slope of the curve drops rapidly during early development of the callus, to reach a plateau after some 50 % of recovery of the normal mechanical characteristics of the bone. This simulation supports the theoretical approach, i.e. the external frame resists larger stresses at the start of the fracture healing. Over a callus stiffness of some 200 N/mm the pattern of the curves remains similar, regardless of the frame configuration. CONCLUSION: An optimisation of the frame is possible, adapted to the actual mechanical situation of the callus. A monitoring system is deemed reliable after making sure that the elementary components behave the same way in the clinical condition as in the laboratory. In an experimental set up we confirmed its reliability in a clinical-like situation.

External fixation design evolution enhances biomechanical frame performance.

BACKGROUND: External fixation has become a quick and easy application for fracture stabilisation of the extremities and/or pelvis to maintain the reduction and provide stability while sparing the soft tissues. Over the last years, enhanced construct stiffness has become an essential requirement to preserve fracture reduction, particularly in active and overweight patients. This study was performed to determine whether the advancement of design features enhances the external fixation construct stiffness. The stiffness of the recently developed Hoffmann 3 external fixation system was determined and its characteristics compared with the widely clinically accepted Hoffmann II MRI fixation system. METHODS: A synthetic fracture model was used. Two carbon tubes with a fracture gap of 20 mm were appropriate to determine the stiffness of three different configurations: the basic frame configuration (group H 3, representing Hoffmann 3 with a rod diameter of 11 mm) using a double rod construction with 6 mm Apex pins, was compared with the Hoffmann II MRI fixation system using two 8.0 mm diameter rods with 6 mm (group H II-6 mm) and 5 mm (group H II-5 mm) Apex pins. Each group was tested five times under anterior-posterior bending (N/mm), medio-lateral bending (N/mm) and axial torsion loading directions (Nm/deg). The stiffness results of each construct were compared statistically. RESULTS: The basic frame construct (group H 3) showed consistently higher stiffness properties compared with the other configurations. The anterior-posterior-bending loads resulted in a mean value of 31 N/mm, which was significantly higher compared with the other groups (p=0.008) at 16 N/mm. The medio-lateral-bending test revealed a mean stiffness of 59 N/mm in the H3 group, compared with 43 N/mm in the H II-6 group and 31 N/mm in the H II-5 group. The axial torsion measurements of the Hoffmann 3 group yielded significantly higher results (1.03 Nm/°) compared with group H II-6 (0.61 Nm/°) and group H II-5 (0.56 Nm/°). CONCLUSIONS: The Hoffmann 3 construct showed the highest stiffness properties under bending and torsion loads. The enhanced stiffness of the Hoffmann 3 device may be helpful in maintaining fracture reduction and soft tissue compromise. This investigation showed the advancement of Hoffmann design features may be effective in enhancing frame stiffness.