Comparison of elastic stable intramedullary nailing versus retrograde screw fixation for pubic ramus fractures-a biomechanical study.

Background: Percutaneous minimally invasive retrograde screw fixation is a widely accepted treatment strategy for patients with superior ramus fractures and has shown good biomechanical stability compared to plating. Recently, elastic stable intramedullary nailing (ESIN) devices have been proposed as an alternative in the treatment of superior ramus fractures. However, biomechanical studies on this new treatment are lacking. Thus, the aim of this study was to compare the biomechanical stability of ESIN in pubic ramus fractures versus retrograde screw fixation.

Methods: Standardized pubic ramus fractures (Nakatani type II) were created in fresh-frozen paired hemipelves. Fractures were either stabilized with a 6.5 mm cannulated screw (n = 4) or a 3.5 mm Stainless Steel Elastic Nail System (n = 4). In a validated setup, a cyclic loading protocol was applied with increasing axial force (1500 cycles, 250-750 N). Outcome parameters were fracture mobility over time, fracture displacement and construct survival. Descriptive and opto-metric methods were used to describe the mode of failure.

Results: Amongst all tested hemipelves (n = 8), no construct failure was observed. There was no significant difference in mean vertical fracture displacement between the groups (ESIN 0.07 mm, SD 0.12 versus screw 0.04 mm, SD 0.05; p = 0.773). After 500 cycles at 250 N, mean vertical fracture displacement was 0.09 mm (SD 0.16) in the ESIN group and 0.03 mm (SD 0.04) in the screw group (p = 0.773). After subsequent 500 cycles at 500 N in the vertical plane, mean fracture displacement increased to 0.35 mm (SD 0.31) in the ESIN group and to 0.14 mm (SD 0.17) in the screw group (p = 0.281). With a maximum load of 750 N, after 500 cycles, mean fracture displacement was 0.58 mm (SD 0.51) in the ESIN group and 0.31 mm (SD 0.26) in the screw group (p = 0.376). There was no difference between the implants regarding the accumulated fracture movement over time (ESIN 494 mm*cycles, SD 385 versus screw 220 mm*cycles, SD 210; p = 0.259).

Conclusions: In this in-vitro biomechanical study, fixation of superior ramus fracture using ESIN was not different in construct survival, relative motion to fracture, and fracture displacement when compared to retrograde screw fixation.