Introduction and Objective

Intramedullary nails are frequently used for treatment of unstable distal tibia fractures. However, insufficient fixation of the distal fragment could result in delayed healing, malunion or nonunion. The quality of fixation may be adversely affected by the design of both the nail and locking screws, as well as by the fracture pattern and bone density. Recently, a novel concept for angular stable nailing has been developed that maintains the principle of relative stability and introduces improvements expected to reduce nail toggling, screw migration and secondary loss of reduction. It incorporates polyether ether ketone (PEEK) inlays integrated in the distal and proximal canal portions of the nail for angular stable screw locking. The nail can be used with new standard locking screws and low-profile retaining locking screws, both designed to enhance cortical fixation. The low-profile screws are with threaded head, anchoring in the bone and increasing the surface contact area due to the head's increased diameter.

The objective of this study was to investigate the biomechanical competence of the novel angular stable intramedullary nail concept for treatment of unstable distal tibia fractures, compared with four other nail designs in an artificial bone model under dynamic loading.

Unstable distal tibia fractures are challenging injuries requiring surgical treatment. Intramedullary nails are frequently used; however, distal fragment fixation problems may arise, leading to delayed healing, malunion or nonunion. Recently, a novel angle-stable locking nail design has been developed that maintains the principle of relative construct stability, but introduces improvements expected to reduce nail toggling, screw migration and secondary loss of reduction, without the requirement for additional intraoperative procedures.

The aim of this study was to investigate the biomechanical competence of a novel angle-stable intramedullary nail concept for treatment of unstable distal tibia fractures, compared to a conventional nail in a human cadaveric model under dynamic loading.

Ten pairs of fresh-frozen human cadaveric tibiae with a simulated AO/OTA 42-A3.1 fracture were assigned to 2 groups for reamed intramedullary nailing using either a conventional (non-angle-stable) Expert Tibia Nail with 3 distal screws (Group 1) or the novel Tibia Nail Advanced system with 2 distal angle-stable locking low-profile screws (Group 2). The specimens were biomechanically tested under conditions including quasi-static and progressively increasing combined cyclic axial and torsional loading in internal rotation until failure of the bone-implant construct, with monitoring by means of motion tracking.

Initial axial construct stiffness, although being higher in Group 2, did not significantly differ between the 2 nail systems, p=0.29. In contrast, initial torsional construct stiffness was significantly higher in Group 2 compared to Group 1, p=0.04. Initial nail toggling of the distal tibia fragment in varus and flexion was lower in Group 2 compared to Group 1, being significant in flexion, p=0.91 and p=0.03, respectively. After 5000 cycles, interfragmentary movements in terms of varus, flexion, internal rotation, axial displacement and shear displacement at the fracture site were all lower in Group 2 compared to Group 1, with flexion and shear displacement being significant, p=0.14, p=0.04, p=0.25, p=0.11 and p=0.04, respectively. Cycles to failure until both interfragmentary 5° varus and 5° flexion were significantly higher in Group 2 compared to Group 1, p=0.04.

From a biomechanical perspective, the novel angle-stable intramedullary nail concept has the potential of achieving a higher initial axial and torsional relative stability and maintaining it with a better resistance towards loss of reduction under dynamic loading, while reducing the number of distal locking screws, compared to conventional locking in intramedullary nailed unstable distal tibia fractures.

Introduction: Standard treatment for distal tibia fractures is the fixation with locking compression plates. Locking plate fixation has revolutionized fracture treatment in the last decade and may be ideally suited for a bridging plate osteosynthesis. This technique allows some controlled axial fracture motion, what essential for secondary bone healing is. A disadvantage of the locking plate technique seems to be an unsymmetrical micro motion along the fracture gap. The micromotion at the far cortex side is much larger than at the near cortex side (near the plate). It is supposed to be that the fracture movement on the near cortex is too small.

To increase the motion at the near cortex side a new kind of screws has been developed. In this study we examined the micromotion using normal locking head screws versus the new dynamic locking head screws.

Materials and Methods: A simplified fracture model was created by connecting 2 plastic cylinders (POM C, EModul: 3.1GPa) with a standard 11-holes Locking Compression Plate (Synthes). The fracturegap (between the two cylinders) amounted 3mm. Three kinds of fracture models were constructed: The model of a transverse fracture, an oblique fracture and a spiral fracture. An axial load from 0N up to 200N was applied with a testing machine (Zwick). The motion of the fracture model was measured in three dimensions using the optical measurement system PONTOS 5M (GOM, Braunschweig, Germany). The accuracy of the optical measurement system was about 5 micrometers.

Results: A total of 72 measurements were compared. Using the new screw, axial stiffness was decreased for 16% and micromotion was up to 200 μm higher in comparison to the old screw.

Discussion: Using the new dynamic locking head screw it’s possible to increase interfragmentary motion up to 200μm on the near cortex side (plate side).

Navigation enables to precisely reproduce pre-operative planning. Most systems however do not tell us the target values of the planning parameters. As we operate more and more young patients with full or only slightly reduced mobility, kinematic properties of artificial hip joints become more important. The range of motion of artificial hip joints is limited, finaly resulting in impingement and thereby in wear and loosening.

This limited range of motion has to be devided under the different planes of movement and at the same time dislocation stability has to be considered. We have to search for the ideal compromise.

For this purpose a three-dimensional computer model has been developed. Input are the implant geometries and the implantation angles. As result the range of motion of the different planes of movement is calculated. Thus the surgeon knows the consequences of his implant choice and his pre-operative planning.

As a result of this investigation flexion proved to be the critical plane of movement, above all in full profile cups and if anteversion of the cup and the stem are not considered. Another critical plane of movement is external rotation.

Concerning implant design extralong heads as well as antidislocation rims proved to be problematic. The same problem we face in antidislocation cups with a closing angle of more than 180° of course.

Only by considering the kinematic results of an individual THR planning full benefit can be taken out of navigation.