Objective: To conduct a biomechanical study on composite synthetic tibia bone models under physiologic torsional loading conditions to evaluate the relationship between fracture obliquity and locking screws configuration of distal metaphyseal tibial fracture model fixed by intramedullary (IM) nail fixation.

Material and Method: The biomechanics of six different locking screws configurations in an IM nail used to fix various distal metaphyseal tibial fracture obliquities were evaluated. The Sawbones were osteotomized following reaming of the medullary canal 2 mm over the respective nail diameter. Specimens were tested in internal and external rotation at 0.25 degrees/second to 7 Nm followed by a hold at 7 Nm for two seconds. The testing was conducted under two compressive loading conditions, 20 N and 500 N, to determine if a constant compressive load affects interfragmentary motion during torsion. The influence of screw configurations on construct stability was investigated at fracture obliquities ranging from 0° to 60° including a spiral obliquity (spiral configuration and obliquity of 45° by definition).

Results: During both internal and external rotation, significant differences were observed in interfragmentary motion between the two compressive loading levels (20 N and 500 N). For internal rotation testing, there are significant differences in interfragmentary displacement for fracture obliquities less than or equal to 40° when the degree of interlocking is diminished. For obliquities between 0° and 30°, which could be classified as a transverse and short oblique fracture pattern, significant rotatory changes become present as the degree of interlocking is decreased. Similar results are found in external rotation, however, stability appears to be compromised with obliquities of 50° or less.

Conclusion: Our study examined interfragmentary motion during pure compressive loading to determine the optimal interlocking pattern for distal metaphyseal fractures of various obliquities. The appropriate fixation configuration should be selected to apply adequate fracture stabilization for all physiologic loading modalities. Although increasing fracture obliquities provide increased stability during torsional loading, stability is adversely affected for increasing fracture obliquity at greater than 30° and certainly by 40° of fracture obliquity during pure compression. The data confirmed that not just the interlocking pattern but also the fracture configuration and compressive load level contribute to overall construct stability when physiologic torsional loads were applied. Interfragmentary displacement was significantly increased, in compression loading mode, when a progressive fracture obliquity degree was tested.

Keywords: Distal metaphyseal tibial fracture, Intramedullary nail, Biomechanic, Proximal interlocking screw, Distal interlocking screw, Fracture obliquity, Screw configuration