ORIGINAL ARTICLE Angular Stability Potentially Permits Fewer Locking Screws Compared With Conventional Locking in Intramedullary Nailed Distal Tibia Fractures: A Biomechanical Study Boyko Gueorguiev, PhD,* Ben Ockert, MD,*† Karsten Schwieger, PhD,* Dirk Wa ¨hnert, MD,* Matthew Lawson-Smith, MD,‡§ Markus Windolf, MSc,* and Karl Stoffel, MD, PhD‡§ Objectives: To compare mechanical stability of angle-stable locking construct with four screws with conventional five screw locking in intramedullary nailed distal tibia fractures under cyclic loading. Methods: Ten pairs of fresh-frozen human cadaveric tibiae were intramedullary nailed and assigned to either an angle-stable locking construct consisting of four screws or conventional five-screw locking. After simulating an unstable distal two-fragmental 42-A3.1 fracture, the specimens were mechanically tested under quasistatic and cyclic sinusoidal axial and torsional loading. Results: Bending stiffness of the angle-stable and the conventional fixation was 644.3 N/° and 416.5 N/°, respectively (P = 0.075, power 0.434). Torsional stiffness of the angle-stable locking (1.91 Nm/°) was significantly higher compared with the conventional one (1.13 Nm/°; P = 0.001, power 0.981). Torsional play of the angle- stable fixation (0.08°) was significantly smaller compared with the conventional one (0.46°; P = 0.002, power 0.965). The angle-stable locking revealed significantly less torsional deformation in the frac- ture gap after one cycle (0.74°) than the conventional one (1.75°; P = 0.005, power 0.915) and also after 1000 cycles (angle-stable: 1.56°; conventional: 2.51°; P = 0.042, power 0.562). Modes of failure were fracture of the distal fragment, loosening of distal locking screws, nail breakage, and their combination, equally distributed between the groups (P = 0.325). Conclusions: Both the angle-stable locking technique using four screws and conventional locking consisting of five screws showed high biomechanical properties. Hence, angle-stable locking reflects a potential to maintain fixation stability while reducing the number of locking screws compared with conventional locking in intramedul- lary nailed unstable distal tibia fractures. Key Words: distal tibia fracture, unstable fracture, intramedullary nail, angle-stable locking, biomechanical study (J Orthop Trauma 2011;25:340–346) INTRODUCTION Intramedullary nailing has become the treatment of choice for most displaced diaphyseal tibia fractures, because it provides high mechanical stability and little soft tissue damage. 1–7 As a result of the development of newly designed implants with the ability of multiplanar locking in varying directions, indication for tibia nailing has extended including extra-articular fractures of the proximal and distal one third of the tibia. 8–16 However, mechanically stable fixation becomes chal- lenging with the fracture extending more distally. 17 The difference in size between the implant diameter and the metaphyseal diameter results in small nail–cortex contact and the diminished cortical bone support of the distal tibia limits the construct stability. 18 Consequently, fractures of the distal one third of the tibia treated with intramedullary nailing frequently result in varus, valgus, or torsional deformities and nonunions. 19–23 To improve the construct stability of intramedullary nailed distal tibia fractures, recently, angle-stable interlocking screws encased by a sleeve made of polyetheretherketone have been introduced. However, until now, this novel locking technique has only been mechanically tested under quasistatic axial loading. 24 Besides postoperative deformities and nonunion, intra- medullary nailing of tibia fractures is generally perceived as having a low incidence of complications. Nonetheless, neurologic and vascular injuries are attributed to placement of locking screws because they are in close proximity to relevant anatomic structures. 25–29 Achieving a highly bio- mechanical stable construct consisting of fewer locking screws Accepted for publication November 15, 2010. From *AO Research Institute Davos, AO Foundation, Davos, Switzerland; †Department of Trauma and Orthopaedic Surgery–Campus Innenstadt, University of Munich, Munich, Germany; ‡Orthopaedic Surgery, Kant- onsspital Graubu ¨nden, Chur, Switzerland; and §Department of Ortho- paedic Surgery, Fremantle Hospital, Fremantle, Australia. Boyko Gueorguiev and Ben Ockert contributed equally to this work. Synthes GmbH, Solothurn, Switzerland, is acknowledged for providing the implants. The authors are not compensated and there are no other institutional subsidies, corporate affiliations, or funding sources supporting this work unless clearly documented and disclosed. No external funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Neverthe- less, the results of this article may benefit the commercial interests of the manufacturer of the implants, although this was not the aim of the study. Reprints: Boyko Gueorguiev, PhD, AO Research Institute Davos, Clavade- lerstrasse 8, 7270 Davos, Switzerland (e-mail: boyko.gueorguiev@ aofoundation.org) Copyright Ó 2011 by Lippincott Williams & Wilkins 340 | www.jorthotrauma.com J Orthop Trauma  Volume 25, Number 6, June 2011