An engineering review of external fixators

Computational models of bone fracture healing and applications: a review

Fracture healing is a very complex physiological process involving multiple events at different temporal and spatial scales, such as cell migration and tissue differentiation, in which mechanical stimuli and biochemical factors assume key roles. With the continuous improvement of computer technology in recent years, computer models have provided excellent solutions for studying the complex process of bone healing. These models not only provide profound insights into the mechanisms of fracture healing, but also have important implications for clinical treatment strategies. In this review, we first provide an overview of research in the field of computational models of fracture healing based on CiteSpace software, followed by a summary of recent advances, and a discussion of the limitations of these models and future directions for improvement. Finally, we provide a systematic summary of the application of computational models of fracture healing in three areas: bone tissue engineering, fixator optimization and clinical treatment strategies. The application of computational models of bone healing in clinical treatment is immature, but an inevitable trend, and as these models become more refined, their role in guiding clinical treatment will become more prominent.

Mechanical Analysis of a Novel 3D-printed External Fixator Design Versus Industry-standard External Fixators

INTRODUCTION

External fixation is a critical component of orthopaedic fracture management and is used for various conditions, including trauma and pediatric orthopaedics. However, the availability and high cost of external fixation devices are a concern, especially in rural and developing countries. 3D printing technology has shown promise in reducing manufacturing costs and improving accessibility to external fixation devices. The purpose of this study was to evaluate the mechanical properties of a fully 3D-printed desktop external fixation device and compare the results with the mechanical properties of commonly used, clinically available external fixators.

METHODS

A fully 3D printable external fixator was designed and printed in polylactic acid at two infill densities, 20% and 100%. The mechanical properties of the 3D-printed external fixators and several commercially available fixators were tested according to applicable sections of the American Society for Testing and Materials F1541 standard protocol in axial, medial-lateral, and anterior-posterior orientations. The primary outcomes measured included failure load, safe load, rigidity, and yield load. The mean differences between experimental and control groups were calculated using one-way analysis of variance and Tukey tests.

RESULTS

The 20% infill 3D-printed construct showed poor performance compared with commercially available external fixators in all testing conditions and across most variables. The 100% infill 3D-printed construct was comparable with or superior to all commercially available devices in most testing conditions. The cost for printing a single 3D-printed 100% infill external fixator was $14.49 (United States Dollar).

DISCUSSION

This study demonstrates that a low-cost desktop 3D printer can create an entirely 3D-printed external fixator that resists clinically relevant forces similar to medical-grade industry-standard external fixators. Therefore, there is potential for customizable and low-cost external fixators to be manufactured with desktop 3D printing for use in remote areas and other resource-constrained environments for fracture care.

In-vitro external fixation pin-site model proof of concept: A novel approach to studying wound healing in transcutaneous implants

External fixation is an essential surgical technique for treating trauma, limb lengthening and deformity correction, however infection is common, with infection rates ranging from 4.5 to 100% of cases. Throughout the literature researchers and clinicians have highlighted a relationship between excessive movement of the pin and skin and an increase in the patient's risk of infection, however, currently no studies have addressed this role of pin-movement on pin-site wounds. This preliminary study describes a novel in vitro pin-site model, developed using a full-thickness human skin equivalent (HSE) model in conjunction with a bespoke mechanical system which simulates pin-movement. The effect of pin-movement on the wound healing response of the skin equivalents was assessed by measuring the expression of pro-inflammatory cytokines. Six human skin equivalent models were divided into three test groups: no pin as the control, static pin-site wound and dynamic pin-site wound (n = 3). On day 3 concentrations of IL-1α and IL-8 showed a significant increase compared to the control when a static fixation pin was implanted into the skin equivalent (p < 0.05) and (p < 0.005) respectively. Levels of IL-1α and IL-8 increased further in the dynamic sample compared to the static sample (p < 0.05) and (p < 0.0005). This study demonstrates for the first time the application of HSE model to study external-fixation pin-movement in vitro. The results of this study demonstrated pin-movement has a negative effect on soft-tissue wound-healing, supporting the anecdotal evidence reported in the literature, however further analysis of wound heading would be required to verify this hypothesis.

Pull-out resistance of connected K-wires for osteosynthesis: development of a numerical model

A predictive finite element model was developed to investigate the best configuration of a fixation pins system consisting of two K-wires inserted in a synthetic model (Sawbones®) at different angles and secured to a connecting rod. Two key parameters were considered to determine the best configuration delivering the higher pull-out strength and lower pull-out length: the diameter and insertion angle. Results show that as the diameter and insertion angle increased, the pull-out force increased, while the pull-out length decreased. Results are successfully compared with available experimental data in literature. This model can be used as an alternative to experimental study.

Developments in circular external fixators: A review

Circular external fixators (CEFs) are successfully used in orthopedics owing to their highly favorable stiffness characteristics which promote distraction osteogenesis. Although there are different designs of external fixators, how these features produce optimal biomechanics through structural and component designs is not well known. Therefore, the aim of this study was to conduct a review on CEFs following the PRISMA statement. A search for relevant research articles was performed on Scopus and PubMed databases providing the related keywords. Furthermore, a patent search was conducted on the Google Patent database. 126 records were found to be eligible for the review. Different designs of CEFs were summarized and tabulated based on their specific features. A bibliometric analysis was also performed on the eligible research papers. Based on the findings, the developments of CEFs in terms of materials, automation, adjustment methods, component designs, wire-clamping, and performance evaluation have been extensively discussed. The trends of the CEF design and future directions are also discussed in this review. Significant research gaps include a lack of consideration towards ease of assembly, effective wire-clamping methods, and CEFs embedded with online patient-monitoring systems, among others. An apparent lack of research interest from low-middle and low-income countries was also identified.

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.

Circular External Fixator Removal in the Outpatient Clinic Using Regional Anaesthesia: A Pilot Study of a Novel Approach

Introduction

External fixator (EF) devices are commonly used in the management of complex skeletal trauma, as well as in elective limb reconstruction surgery for the management of congenital and acquired pathology. The subsequent removal of an EF is commonly performed under general anaesthesia in an operating theatre. This practice is resource-intensive and limits the amount of time available for other surgical cases in the operating theatre. We aimed to assess the use of regional anaesthesia as an alternative method of analgesia to facilitate the EF removal in an outpatient setting.

Design and methods

This prospective case series evaluated the first 50 consecutive cases of EF removal in the outpatient clinic between 10/06/22 and 03/02/23. Regional anaesthesia using ultrasound-guided blockade of peripheral nerves was administered using 1% lidocaine due to its rapid onset and short half-life. Patients were assessed for additional analgesia requirements and then were asked to evaluate their experience and perceived pain using the visual analogue scale (VAS).

Results

Fifty patients were included in the study. The mean age was 46.8 years (range 21-85 years). About 54% of the patients were male patients (N = 27). Post-procedure, all patients indicated positive satisfaction ratings, each participant responded as either 'satisfied' (N = 6), 'very satisfied' (N = 24) or 'highly satisfied' (N = 20). In addition, 90% of the participants reported that they would opt for this method of EF removal again in future. The VAS for pain immediately following completion of the procedure was low, with a mean score of 0.36 (range 0-4), where a score of 0 = 'No pain', and 10 = 'worst pain possible'. The median score was 0.

Conclusion

We present the first description of outpatient EF removal using regional anaesthesia, with a prospective case series of 50 fully conscious patients from whom the EF was removed. This novel technique is likely to be cost-effective, reproducible, and safe. This technique reduces the burden of EF removal from an operating list and also improves the patient's experience when compared with other forms of conscious sedation. By eliminating the use of Entonox and methoxyflurane for sedation and analgesia, this technique also demonstrates a method of improving environmental sustainability.

How to cite this article

Williams LM, Stamps G, Peak H, et al. Circular External Fixator Removal in the Outpatient Clinic Using Regional Anaesthesia: A Pilot Study of A Novel Approach. Strategies Trauma Limb Reconstr 2023;18(1):7-11.

Pullout strength of connected pins in hand surgery: Effect of pin diameter and insertion angle

OBJECTIVE

The purpose of this study was to examine the effect of insertion angles on the pullout strength of connected pins using a synthetic model simulating a hand bone.

MATERIAL AND METHODS

The material consisted of Sawbones® (20 mm×20 mm × 60 mm), fixation pins secured to a connecting rod much like an external fixator, an electric drill (speed 1,290 rpm) and a tensile testing machine. The Sawbones® were drilled with different pin diameters (1.2 mm, 1.5 mm, and 1.8 mm) and insertion angles (100°, 110° and 120°). A vertical displacement of 1 mm/min was applied until the pins were extracted (maximum force).

RESULTS

The pullout strength increased with the insertion angle of the connected pins. It also increased with their diameter. Regardless of the pin diameter, the load-displacement curve during the pullout test had 4 sections (peak 1, ascending slope, peak 2, descending slope) that corresponded to the combined frictional force and contact force between the pins and Sawbones®.

DISCUSSION

Our study findings showed that, theoretically, for wrist or hand fractures treated with connected pins, the larger the diameter and insertion angle, the better the mechanical holding power of the pins.

LEVEL OF EVIDENCE

I, experimental study.