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.

The high risk and the associated high mortality of secondary, contralateral hip fractures [1,2] could justify internal, invasive prophylactic reinforcement of the osteoporotic proximal femur to avoid these injuries in case of a low energy fall. Previous studies have demonstrated high potential of augmentation approaches [3,4,5], but to date there has no ideal solution been found. The development of optimized reinforcement strategies can be aided with validated computer simulation tools that can be used to evaluate new ideas.

A validated non-linear finite element (FE) simulation tool was used here to predict the yield and fracture load of twelve osteoporotic or osteopenic proximal femora in sideways fall based on high resolution CT images. Various augmentation strategies using bone cement or novel metal implants were developed, optimized and virtually performed on the bone models. The relative strengthening compared to the non-augmented state was evaluated using case-specific FE analyses.

Strengthening effect of the cement-based augmentation was linearly proportional to cement volume and was significantly affected by cement location. With the clinically acceptable 12.6 ± 1.2 ml volume and optimized location of the cement cloud, compared to the non-augmented state, 71 ± 26% (42 – 134%) and 217 ± 166% (83 – 509%) increase in yield force and energy was reached, respectively. These were significantly higher than previously published experimental results using the “central” cement location [5], which could be well predicted by our FE models. The optimized metal implant could provide even higher strengthening effect: 140 ± 39% (76 – 194%) increase in yield force and +357 ± 177% (132 – 691%) increase in yield energy. However, for metal implants, a higher risk of subcapital fractures was indicated. For both cement and metal, the originally weaker bones were strengthened exponentially more compared to the stronger ones.

The ideal solution for prophylactic augmentation should provide an appropriate balance between the requirements of being clinically feasible, ethically acceptable and mechanically sufficient. Even with the optimized location, the cement-based approach may not provide enough strengthening effect and adequate reproducibility of the identified optimal cement cloud position may not be achieved clinically. While the metal implant based strategy appears to be able to deliver the required strengthening effect, the ethical acceptance of this more invasive option is questionable. Further development is therefore required to identify the ideal, clinically relevant augmentation strategy. This may involve new cement materials, less invasive metal implants, or a combination of both. The FE simulation approach presented here could help to screen the potential ideas and highlight promising candidates for experimental evaluation.