Effect of Distal Fragment Length on Construct Stability in an Extra-articular Distal Tibial Fracture Model Fixed With Locked Intramedullary Nailing: A Biomechanical Study.

Introduction  Fractures of the distal tibia are complex injuries with high complication rates, which include delayed union, non-union, and wound complications like dehiscence and infection. The two commonly employed definite internal fixation modalities include locked intramedullary (IM) nailing and plating. There is controversy regarding the superiority of the fixation construct, although nailing is proven to be more biological and devoid of soft tissue complications. There is also no consensus regarding the minimum distance of the fracture from the tibial plafond that is amenable to nailing of the fracture. Hence, the present study is designed to evaluate the effect of distal fragment length relative to the total length of the tibia, which makes it stable enough for IM nailing to be effective.  Methods A prospective biomechanical study was performed using 28 fourth-generation composite tibial sawbones. Osteotomies were created at 12%, 15%, 20%, and 25% of the total tibial length (38 cm) from the distal articular surface, forming four experimental groups (A-D, n=7 each). All models were stabilized with 10 mm stainless-steel interlocking nails. Mechanical testing was conducted using a servo-hydraulic fatigue testing machine and included mediolateral (ML) and anteroposterior (AP) three-point bending, as well as cyclic axial loading. Outcome measures included bending stiffness, construct laxity (neutral zone), fracture gap angle, axial micromotion, and construct failure. Results  The bending stiffness of all constructs tended to be lower in the AP plane than in the ML plane. The neutral zone of all groups tended to be higher in the AP plane than in the ML plane. The peak fracture gap angle tended to be higher in the AP plane than in the ML plane. Group A (shortest distal fragment length) demonstrated significantly lower AP stiffness, higher AP neutral zone, and higher AP peak fracture gap angle as compared to group D (longest distal fragment length). Group A demonstrated significantly greater instability in the AP plane than Group D. No statistically significant difference was found in the stability parameters on medio-lateral three-point bending and axial compressive testing.  Conclusion The results of this biomechanical study show that comminuted extra-articular distal tibial fractures show significant instability in the sagittal plane when the length of the distal fragment is 12% of the total tibial length.