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.

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.

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/°).

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.