Biomechanical evaluation of three different configurations of external fixators for treating distal third tibia fracture: Finite element analysis in axial, bending and torsion load

Highlights

• Assessing a novel concept of self-locking bars construct which able toenhance stability by using the same number of elements.

• Analyzing the stability of construct of external fixators via finite element analysis.

• The double cross self-locking construct (Model 3) had superior performance toprovide stability after tibia fracture.

Abstract

External fixators have been widely used in treating open fractures and have produced excellent outcomes, as they could successfully heal bones. The stability of external fixators lies greatly in their construction. Factors that associated with the stability of the external fixators includes stress, displacement, and relative micromotion. Three-dimensional (3D) models of bone and external fixators were constructed by using 3D modelling software, namely Materialise and SolidWorks, respectively. Three different configurations of external fixators namely Model 1, Model 2, and Model 3 were analysed. Three load cases were simulated to assess the abovementioned factors at the bone, specifically at the fracture site and at the external fixator. Findings showed that the double-cross configuration (Model 3) was the most promising in axial, bending, and torsion load cases as compared to the other two configurations. The no-cross configuration (Model 1) had the highest risk of complication due to high stress, relative micromotion, and displacement in the bending and torsion load cases. On the other hand, the single-cross configuration (Model 2) had the highest risk of complication when applied with axial load. In conclusion, the double-cross locking construct (Model 3) showed the biggest potential to be a new option for medical surgeons in treating patients associated with bone fracture. This new double-cross locking construct showed superior biomechanical stability as compared to single-cross and no-cross configurations in the axial, bending, and torsion load cases.

Introduction

External fixation is one of the common techniques used for treating severe trauma and open fractures. The use of external fixators is to enable wound care, allowing additional plastic surgery to be carried out and perform good visualisation of the fracture site by using radiography without compromising on bone loss at the fracture site. Therefore, this treatment is a widely chosen treatment method amongst medical practitioners to treat severe trauma and open fractures[1]. Furthermore, the external fixator device is mainly used to avoid vital anatomical structures, allow access to the injured area, and fulfil the mechanical demand for stability[2]. Besides, an important element of biomechanics is rigidity of the construct in which it should be able to sustain fracture reductions and allow weight bearing. Various types of external fixators have been considered by medical practitioners in which the configurations depend on the patient's condition, fracture classification, surgical experiences as well as the availability of component such as rods and pins[3]. For treating simple fractures such as spiral and oblique fractures, the unilateral construct is normally preferred by medical surgeons[4]. On the other hand, multiplanar configuration is the favourable option for treating more complex fractures such as comminuted and pilon fractures[5]. However, these surgical strategies depend on many considerations. Either a simple or complex configuration, all related literatures state that external fixator devices can facilitate the bone healing process during intervention period[5].

The stability of external fixators is one of the important measurements to ensure the success of bone healing as well as avoid complications after surgery. In terms of biomechanical features, important parameters such as stress and displacement are believed to influence the achievement of complete bone union by ensuring the success of bone healing[[6], [7], [8]]. The existence of intermittent stress would trigger mesenchymal cell to differentiate of which the proliferation of this cell may lead to callus formation[9,10] and this is the initial step for bone healing process. The formation of callus requires good blood supply and small amounts of stress[9]. However, if the implantation of the external fixators induces high stress, it could interrupt the bone formation and lead to undesired outcomes such as delayed bone union or, in the worst case, bone non-union[9,11]. A high magnitude of stress is estimated to form long cells of connective tissue and later result in non-union of the bone at the fracture site[10,12]. Besides, an excessively rigid fixation ensures the bone to remain in the right place, but could delay the healing process[13]. On the other hand, large displacements could affect the volume and quality of the calluses and bone bridging formation for a complete and faster bone union[7]. Several factors could contribute to the high displacement incidence, which include the angle of pin[14], improper pin placement[[15], [16], [17]], size of pins [18] as well as the distance of pin to the fracture gap[19]. Different angles of pin could result in direction changes of the load exerted to the external fixators[14]. Ideally, pins should be inserted perpendicularly into the bone, while the fixator should be in a straight position, either parallel in a similar plane with the bone or parallel with other planes[20]. However, in reality, surgeons face some difficulties in order to place the pin in a perpendicular position with the bone. As a consequence, the stability of external fixators may be affected, resulting in a large fixators displacement. In another point of view, improper pin placement could lead to increased risk of complications due to pin infections and loosening which recorded about half (50%) of the cases[[15], [16], [17]]. Technically, the repetition of higher displacement magnitude could result in the formation of voids at the pin-bone interface, which later could cause pin loosening. Numerous methods were used to conduct orthopaedic biomechanics research such as animal testing (in vivo), physical modelling, using a cadaveric (in vitro), and also computational simulation via finite element analysis (FEA). However, each method has its advantages and disadvantages[21]. The FEA is a computational method that could be repeated and sustained. This method is able to provide a biomechanical evaluation and prognosis for various diseases and types of injury, various implant fixations, and surgical techniques by tailoring finite element settings such as material properties and boundary conditions [22]. This method has been used for orthopaedic research by constructing three-dimensional (3D) models of the bone and external fixators and converting them into finite elements in which simulated physiological loads are applied to analyse and predict the outcome of surgery[23].

In this present study, a new double-cross configuration of the external fixator was compared with two other configurations of external fixators (single-cross and no-cross), which were commonly used by orthopaedic surgeons for treating tibia fracture. This newly practiced configuration has been used by the orthopaedic surgeon, (N.B.W.) in the Department of Orthopaedics and Traumatology, Hospital Universiti Kebangsaan Malaysia, Kuala Lumpur and Hospital Segamat, Johor. This configuration showed a promising outcome, since it could achieve bone union within six months. The main goal of this study is to conduct in silico study to assess the biomechanical stability of new double-cross delta external fixator configuration as compared to the two conventional configurations. Therefore, several parameters were considered, such as stress, displacement, and relative micromotion at the fracture site and fixators. Three load cases were considered in the analyses which were axial, bending, and torsion loads. This study aims to provide an additional option for medical practitioners to choose at suitable external fixator construct before a surgery can be conducted. Besides, researchers and engineers could also benefit from this study in terms of new knowledge and can strategise for further improvement.