Investigating the biomechanical function of the plate-type external fixator in the treatment of tibial fractures: a biomechanical study

Locking compression plate as a sequential external fixator following the distraction phase for the treatment of tibial bone defects caused by fracture-related infection: experiences from 22 cases

BACKGROUND

The purpose of this study was to report the clinical and psychological outcomes of using a locking compression plate (LCP) as a sequential external fixator following the distraction phase in the treatment of tibial bone defects caused by fracture-related infection (FRI).

METHODS

We retrospectively analyzed the clinical records and consecutive X-ray images of patients with tibial bone defects who were treated with an LCP as a sequential external fixator following the distraction phase, between June 2017 and December 2022. The ASAMI criteria were applied to assess the bone and functional outcomes, and postoperative complications were evaluated by using the Paley classification. The SCL-90-R questionnaire was used to evaluate patients' psychological symptoms, documented and compared at Time 1 (before bone transport), Time 2 (after distraction phase), and Time 3(final follow-up). Statistical significance was set at P < 0.05.

RESULTS

This study included 22 participants with a mean age of 37.72 ± 9.65 years, comprising 17 males (77.2%) and 5 females (22.7%). The mean postoperative follow-up time was 29 ± 2.65 months. The mean number of previous surgical interventions per patient was 5.22 ± 1.26. Twenty-two patients with tibial bone defects caused by FRI were successfully treated using an LCP as a sequential external fixator following the distraction phase, with a mean bone union time of 9.95 ± 1.52 months. Bone union was achieved in all cases (100%) without the use of bone grafts at the docking sites. Following the completion of distraction, the Ilizarov apparatus or monorail fixator was retained for an additional 2.20 ± 0.53 weeks before being exchanged for the external locking compression plate (ELCP). The mean external fixation time (EFT) was 12.29 ± 1.67 months, with a mean external fixation index (EFI) of 1.83 ± 0.22 month/cm (Table 2). At the final follow-up, bone and functional outcomes were evaluated using the ASAMI criteria. Bone outcomes included 10 excellent and 12 good results, while functional outcomes included 12 excellent, 9 good, and 1 fair result. Statistically significant differences in psychological impacts were observed among the three time points (Time 1 vs. Time 2, P = 0.034; Time 2 vs. Time 3, P = 0.020; Time 1 vs. Time 3, P = 0.012). Complications were observed in 6 patients (27.2%), including joint stiffness (n = 3), pin-track infection (n = 3), and refracture (n = 1). All complications were successfully managed.

CONCLUSION

LCP used as a sequential external fixator following the distraction phase is an effective method for treating massive tibial bone defects caused by FRI and is also suitable for patients with scars and poor tissue conditions resulting from multiple previous debridement. Furthermore, this combined technique could be more beneficial in alleviating psychological burdens, supporting patients' engagement in rehabilitation, and facilitating a return to normal life.

CLINICAL TRIAL NUMBER

Not applicable.

Biomechanical comparison of three different surgical methods in the surgical treatment of distal tibial metaphyseal fractures. An animal model study

BACKGROUND

The aim of this study is to investigate mechanical properties of minimally invasive plate osteosynthesis (MIPO), supracutaneousplating (SP), and unilateral external fixators (UEF) which can be performed for open tibial fractures.

METHODS

An unstable diaphysial tibia fracture was created in 60 fresh sheep tibia specimens by performing an osteotomy at the middle of bones. Specimens were divided into 3 groups. Specimens underwent fracture fixation with a standard MIPO technique, im-planting the plate 15 mm from the bone for SP group. Unilateral uniplanar external fixators were achieved for UEF group. First, thirty specimens (10 specimen for each group) were loaded vertically along the tibial axis to 1800 N. Second, other 30 preperated bones were used for cyclical loading to avoid metal fatigue. For dynamic tests, a 350 N force was applied for 10,000 cycles.

RESULTS

In compression testing (vertical loading up to 1800 N) of the three fixation instruments; construct stiffness was highest in MIPO group when compared with SP and UEF groups. While the stiffness of the MIPO group was similar to SP group, it was statistically higher than UEF group (P=0.08 and P=0.002, respectively). SP group was significantly stiffer than UEF group (P=0.0021). The mean peak load was highest in SP group and lowest in UEF group. The peak load in SP group was similar to the MIPO group, it was statistically higher than the UEF group (P=0.743 and P=0.002, respectively).

CONCLUSION

Based on the biomechanical properties from this in vitro animal model study, SP technique was biomechanically stronger than UEF and has similar biomechanical properties with MIPO in terms of axial loading.

The Influence of Sagittal Pin Angulation on the Stiffness and Pull-Out Strength of a Monolateral Fixator Construct

Monolateral pin-to-bar-clamp fixators are commonly used to stabilize acute extremity injuries. Certain rules regarding frame geometry have been established that affect construct stability. The influence of sagittal pin angulation on construct stiffness and strength has not been investigated. The purpose of this biomechanical study was to demonstrate the effect of a pin angulation in the monolateral fixator using a composite cylinder model. Three groups of composite cylinder models with a fracture gap were loaded with different mounting variants of monolateral pin-to-bar-clamp fixators. In the first group, the pins were set parallel to each other and perpendicular to the specimen. In the second group, both pins were set convergent each in an angle of 15° to the specimen. In the third group, the pins were set each 15° divergent. The strength of the constructions was tested using a mechanical testing machine. This was followed by a cyclic loading test to produce pin loosening. A pull-out test was then performed to evaluate the strength of each construct at the pin-bone interface. Initial stiffness analyses showed that the converging configuration was the stiffest, while the diverging configuration was the least stiff. The parallel mounting showed an intermediate stiffness. There was a significantly higher resistance to pull-out force in the diverging pin configuration compared to the converging pin configuration. There was no significant difference in the pull-out strength of the parallel pins compared to the angled pin pairs. Convergent mounting of pin pairs increases the stiffness of a monolateral fixator, whereas a divergent mounting weakens it. Regarding the strength of the pin-bone interface, the divergent pin configuration appears to provide greater resistance to pull-out force than the convergent one. The results of this pilot study should be important for the doctrine of fixator mounting as well as for fixator component design.

A finite element study of a fractured tibia treated with a unilateral external fixator: The effects of the number of pins and cortical thickness

INTRODUCTION

In the early stage of fracture fixation, the aim of a unilateral external fixator (UEF) to stimulate healing and maintain stability may be suppressed by using inadequate number of pins. Cortical thinning due to age or osteoporosis endangers a successful fracture fixation.

MATERIALS AND METHODS

This study evaluates the initial strength and stability of the fracture fixation and tissue differentiation under the influences of variable cortical thickness (5 mm to 1 mm) and variable number of pins (1 to 4 in each bone fragment). A finite element program was utilised to develop 20 three-dimensional models of simplified diaphyseal tibia with fracture callus fixed with UEF. A mechano-regulation code based on the deviatoric strain theory was written and applied to simulate tissue differentiation. The values of von Mises stress, interfragmentary strain (IFS), and fibrocartilage index (FCI) were evaluated.

RESULTS

Cortical thinning from 5 mm to 1 mm increased IFS and FCI by an average of 30.3% and 18.7%, respectively, and resulted in higher stresses in the UEF and bone. Using 1 pin in each bone fragment produced excessive IFS in the models with 1 mm, 2 mm and 3 mm cortical thickness. Inserting the second pin into the bone fragment could considerably reduce the IFS and fibrocartilaginous tissue formation in the fracture site and improve load transmission to the fixator. Whereas inserting the fourth pin could minimally affect the mechano-biological environment of healing.

CONCLUSIONS

This study suggests that initial instability due to cortical thinning can be efficiently alleviated by adding the number of pins up to 3 in a UEF; additionally, it may improve the knowledge about applying UEFs adequately stable, whilst promoting inclination toward endochondral ossification, simultaneously.

An engineering review of external fixators

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Mechanical stability plays a key role in the effectiveness of external fixators.

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Strength and stiffness are the main factors which contributes towards stability.

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Modified configurations of linear, circular and hybrid fixators are investigated.

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Light weight composite materials are gradually replacing traditional metallic alloys.

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Existing research gaps in further optimizing external fixators are identified.

External Fixators are a common technique used to treat a variety of issues related to bones, predominantly due to its non-intrusive nature and versatility in terms of form and materials. While it is mainly used to treat open fractures, its other uses include limb lengthening, deformity correction, bone grafting, compression of non-unions and stabilization of dislocations. Its earliest use dates as far back as 400 BCE and has undergone significant improvements, focusing on both customization and optimization. These two aspects highlight the significance of complementing the orthopaedic requirements with engineering knowledge and its applications. Hence, this review paper aims to conduct an examination of recent developments of external fixators with a special focus on its structure, the usage of materials and biomechanical investigations using experimental and numerical techniques. The paper presents the existing level of engineering knowledge with regards to these aspects and identifies research gaps, which can improve the quality of the commonly used external fixators.

Optimization of a Patient-Specific External Fixation Device for Lower Limb Injuries

The use of external fixation devices is considered a valuable approach for the treatment of bone fractures, providing proper alignment to fractured fragments and maintaining fracture stability during the healing process. The need for external fixation devices has increased due to an aging population and increased trauma incidents. The design and fabrication of external fixations are major challenges since the shape and size of the defect vary, as well as the geometry of the human limb. This requires fully personalized external fixators to improve its fit and functionality. This paper presents a methodology to design personalized lightweight external fixator devices for additive manufacturing. This methodology comprises data acquisition, Computer tomography (CT) imaging analysis and processing, Computer Aided Design (CAD) modelling and two methods (imposed predefined patterns and topology optimization) to reduce the weight of the device. Finite element analysis with full factorial design of experiments were used to determine the optimal combination of designs (topology optimization and predefined patterns), materials (polylactic acid, acrylonitrile butadiene styrene, and polyamide) and thickness (3, 4, 5 and 6 mm) to maximize the strength and stiffness of the fixator, while minimizing its weight. The optimal parameters were found to correspond to an external fixator device optimized by topology optimization, made in polylactic acid with 4 mm thickness.

Biomechanical study of a low-cost external fixator for diaphyseal fractures of long bones

Background

External fixation improves open fracture management in emerging countries. However, sophisticated models are often expensive and unavailable. We assessed the biomechanical properties of a low-cost external fixation system in comparison with the Hoffmann® 3 system, as a reference.

Methods

Transversal, oblique, and comminuted fractures were created in the diaphysis of tibia sawbones. Six external fixators were tested in three modes of loading—axial compression, medio-lateral (ML) bending, and torsion—in order to determine construction stiffness. The fixator construct implies two uniplanar (UUEF1, UUEF2) depending the pin-rods fixation system and two biplanar (UBEF1, UBEF2) designs based on different bar to bar connections. The designed low-cost fixators were compared to a Hoffmann® 3 fixator single rod (H3-SR) and double rod (H3-DR). Twenty-seven constructs were stabilized with UUEF1, UUEF2, and H3-SR (nine constructs each). Nine constructs were stabilized with UBEF1, UBEF2, and H3-DR (three constructs each).

Results

UUEF2 was significantly stiffer than H3-SR (p < 0.001) in axial compression for oblique fractures and UUEF1 was significantly stiffer than H3-SR (p = 0.009) in ML bending for transversal fractures. Both UUEFs were significantly stiffer than H3-SR in axial compression and torsion (p < 0.05), and inferior to H3-SR in ML bending, for comminuted fractures. In the same fracture pattern, UBEFs were significantly stiffer than H3-DR (p = 0.001) in axial compression and torsion, while only UBEF1 was significantly stiffer than H3-DR in ML bending (p = 0.013).

Conclusions

The results demonstrated that the stiffness of the UUEF and UBEF device compares to the reference fixator and may be helpful in maintaining fracture reduction. Fatigue testing and clinical assessment must be conducted to ensure that the objective of bone healing is achievable with such low-cost devices.