Evidence on the biomechanical characteristics of external locking plate fixation are still inadequate to support its clinical recommendations as an external locking plate. Our study showed higher stiffness of the conventional external fixator than that of the external locking plate fixator. In all models, the stiffness decreased as the distance between the plate/rod and the bone surface increased. To our best knowledge, this is the first finite element analysis of comparison between external locking plate fixator and conventional external fixator for proximal tibial fractures.
Ideal osteosynthesis involves the optimal balance between biology and mechanics that promotes fracture healing. The concept of biological fracture fixation implies preserving soft tissue and periosteal blood supply and achieving relative stability that promotes callus formation . Internal locking plate fixation can be too stiff to promote optimal fracture healing by callus formation or can cause inconsistent and asymmetric formation of the periosteal callus . Bottlang et al. introduced a modified internal locked plating technology, termed far cortical locking, in 2010 . In this technology, elastic fixation is achieved with cantilever bending of the far cortical locking screw shafts. The mechanism is similar to an external fixator that derives elasticity from fixation pin flexion. Compared with locked plating internal constructs, far cortical locking internal constructs form more callus by providing flexible fixation . External fixators also provide flexible fixation, although too flexible fixation can bring instability and nonunion.
Kloen et al. first use the locking compression plate as an external fixator and named this technique “supercutaneous plating” . External locking plate fixator is a low-profile external fixator with angular stable screw fixation, facilitating mobilization and providing more comfort and better aesthetics than does traditional bar-Schanz pin fixators. Zhang et al. evaluated the outcomes of one-stage external locking plate fixation in 116 tibial fractures . The mean fracture healing time was 12, 20, 14, and 24 weeks for proximal, shaft, distal, and multi-segmental tibial fractures, respectively . Luo et al. conducted a systematic review of 12 studies and reported that external locking plate fixation achieved satisfactory functional outcomes and union rate and low complication rate .
However, the few biomechanical studies that investigated the biomechanical aspects of external locking plate fixation were heterogeneous [8, 9, 10, 11]. Zhang et al. reported a finite element analysis of external locking plate fixation with contralateral femoral less invasive stabilization system (LISS) and different plate–bone distances (1, 10, 20, and 30 mm) in the distal tibial metaphyseal fracture . They concluded that the construct with a 30 mm plate–bone distance might be beneficial to induce callus formation. Further, more profound increases in stiffness were observed in the 1-, 10-, and 20-mm groups, indicating the potential of load shielding . Ma et al. conducted a finite element analysis to evaluate the biomechanical performance of external and internal locking plate fixation of the proximal tibial fractures with a LISS plate . They showed that compared to the internal locking plate model, axial stiffness was reduced by 84% for the external locking plate model with a 6-cm offset and by 94% for the external locking plate model with a 10-cm offset . In the clinical application of external fixation, the distance of the external fixator from the bone depends on the soft tissue swelling and the individual soft tissue thickness. In our study, increasing the distance of the plate or rod from the bone surface from 30 mm to 60 mm reduced uniformly the stiffness by more than 50% in all models. The stiffness of the ELP model with a 30-mm tibia plate offset was 57.42% higher than that of the ELP model with a 60-mm tibia plate offset. The stiffness of the EF-7 model with a 30-mm tibia plate offset was 50.45% higher than that of the EF-7 model with a 60-mm tibia plate offset. The stiffness of the EF-11 model with a 30-mm tibia plate offset was 58.03% higher than that of the EF-11 model with a 60-mm tibia plate offset.
In our study, the contact body between the locking screws and the bone; the locking screws and the locking plate; the Schanz pins and the bone; and the rod, clamps and the Schanz pins were set as tied constraints. With regard to tied constraints, the stiffness of the models was most affected by the moment of inertia of the plate or rod, which was 120.28 mm4 for the plate. The moment of inertia of the 7-mm rod and the 11-mm rod were 117.86 mm4 and 718.69 mm4, respectively. The moment of inertia of the 11-mm rod was 83.26% higher than that of the plate. The stiffness of the EF-11 model with a 30-mm tibia–rod offset was 70.44% higher than that of the ELP model with a 30-mm tibia–plate offset. The stiffness of the EF-7 model with a 30-mm tibia–rod offset was 10.52% higher than that of the ELP model with a 30-mm tibia–plate offset. The ELP model was more flexible than the EF-11 model due to its lower moment of inertia. Further, the stiffness of the ELP model can be improved by increasing the thickness of the lateral proximal tibial locking plate, which in turn leads to an increase in the moment of inertia of the plate.
Few limitations of the present study have to be considered. One limitation is that contact interfaces were tied constraints between the different fixator components and bone. Karunratanakul et al. showed that contact settings between the different fixator components are highly predictive of the external fixator stiffness . However, we compared the external locking plate fixator with the conventional external fixator under ideal contact settings because it is difficult to determine the real contact settings of the locking screw-plate and clamp-rod-Schanz screw without an experimental validation study. The second limitation is that two-dimensional CT images were obtained by scanning the composite tibia despite living bone. However, the use of a commercialize composite model (i.e. Sawbones) appears to be an acceptable practice to validate finite element models . The fourth-generation composite bones have average stiffnesses and strains that are in the range for natural bones . The third limitation of this study is that finite element analysis cannot evaluate the dynamic stability of models, which is important for understanding the effect of the fixator on fracture healing. Manipulation of the mechanical environment is important to optimize and accelerate fracture healing. One concept is reverse dynamization that postulates that the fracture should initially be stabilized with flexible fixation to promote cartilaginous callus formation . This should be the followed by more rigid fixation after adequate fracture callus formation to accelerate healing and the remodeling process . Further experimental fatigue tests with external locking plate fixator–composite tibia models and conventional external fixator–composite tibia models should be performed to determine the influence of locking screw–plate contact settings and clamp–rod–Schanz screw contact settings to the dynamic stability of each model.