The emerging utility of composite bone models in biomechanical studies of the hand and upper extremity

Anatomic variability of the human femur and its implications for the use of artificial bones in biomechanical testing

Aside from human bones, epoxy-based synthetic bones are regarded as the gold standard for biomechanical testing os osteosyntheses. There is a significant discrepancy in biomechanical testing between the determination of fracture stability due to implant treatment in experimental methods and their ability to predict the outcome of stability and fracture healing in a patient. One possible explanation for this disparity is the absence of population-specific variables such as age, gender, and ethnicity in artificial bone, which may influence the geometry and mechanical properties of bone. The goal of this review was to determine whether commercially available artificial bones adequately represent human anatomical variability for mechanical testing of femoral osteosyntheses. To summarize, the availability of suitable bone surrogates currently limits the validity of mechanical evaluations of implant-bone constructs. The currently available synthetic bones neither accurately reflect the local mechanical properties of human bone, nor adequately represent the necessary variability between various populations, limiting their generalized clinical relevance.

Evaluating the stability of external fixators following pelvic injury: A systematic review of biomechanical testing methods

INTRODUCTION

This systematic review aims to identify previously used techniques in biomechanics to assess pelvic instability following pelvic injury, focusing on external fixation constructs.

METHODS

A systematic literature search was conducted to include biomechanical studies and to exclude clinical trials.

RESULTS

Of an initial 4666 studies found, 38 met the inclusion criteria. 84% of the included studies were retrieved from PubMed, Scopus, and Web of Science. The studies analysed 106 postmortem specimens, 154 synthetic bones, and 103 computational models. Most specimens were male (97% synthetic, 70% postmortem specimens). Both the type of injury and the classification system employed varied across studies. About 82% of the injuries assessed were of type C. Two different fixators were tested for FFPII and type A injury, five for type B injury, and fifteen for type C injury. Large variability was observed for external fixation constructs concerning device type and configuration, pin size, and geometry. Biomechanical studies deployed various methods to assess injury displacement, deformation, stiffness, and motion. Thereby, loading protocols differed and inconsistent definitions of failure were determined. Measurement techniques applied in biomechanical test setups included strain gauges, force transducers, and motion tracking techniques.

DISCUSSION AND CONCLUSION

An ideal fixation method should be safe, stable, non-obstructive, and have low complication rates. Although biomechanical testing should ensure that the load applied during testing is representative of a physiological load, a high degree of variability was found in the current literature in both the loading and measurement equipment. The lack of a standardised test design for fixation constructs in pelvic injuries across the studies challenges comparisons between them. When interpreting the results of biomechanical studies, it seems crucial to consider the limitations in cross-study comparability, with implications on their applicability to the clinical setting.

A bending model for assessing relative stiffness and strength of orthopaedic fixation constructs

BACKGROUND

The purpose of this study is to develop a simple and reproducible bending model that is compatible with a wide range of orthopaedic fixation devices and 3D printed spacers.

METHODS

A robust 4-point bending model was constructed by securing sawbones blocks with different orthopaedic fixation device constructs. Stress strain curves derived from a fundamental mechanics model were used to assess the effect of bone density, type of hardware (staple vs intramedullary beam), the use of dynamic compression, orientation of staples (dorsal vs plantar), and the use of 3D printed titanium spacers.

FINDINGS

The high throughput 4-point bending model is simple enough that the methods can be easily repeated to assess a wide range of fixation methods, while complex enough to provide clinically relevant information.

INTERPRETATIONS

It is recommended that this model is used to assess a large initial set of fixation methods in direct and straightforward comparisons.

Biomechanical properties of artificial bones made by Sawbones: A review

Biomedical engineers and physicists frequently use human or animal bone for orthopaedic biomechanics research because they are excellent approximations of living bone. But, there are drawbacks to biological bone, like degradation over time, ethical concerns, high financial costs, inter-specimen variability, storage requirements, supplier sourcing, transportation rules, etc. Consequently, since the late 1980s, the Sawbones® company has been one of the world's largest suppliers of artificial bones for biomechanical testing that counteract many disadvantages of biological bone. There have been many published reports using these bone analogs for research on joint replacement, bone fracture fixation, spine surgery, etc. But, there exists no prior review paper on these artificial bones that gives a comprehensive and in-depth look at the numerical data of interest to biomedical engineers and physicists. Thus, this paper critically reviews 25 years of English-language studies on the biomechanical properties of these artificial bones that (a) characterized unknown or unreported values, (b) validated them against biological bone, and/or (c) optimized different design parameters. This survey of data, advantages, disadvantages, and knowledge gaps will hopefully be useful to biomedical engineers and physicists in developing mechanical testing protocols and computational finite element models.

Does Cement Augmentation of the Sacroiliac Screw Lead to Superior Biomechanical Results for Fixation of the Posterior Pelvic Ring? A Biomechanical Study

Background and Objectives: The stability of the pelvic ring mainly depends on the integrity of its posterior part. Percutaneous sacroiliac (SI) screws are widely implanted as standard of care treatment. The main risk factors for their fixation failure are related to vertical shear or transforaminal sacral fractures. The aim of this study was to compare the biomechanical performance of fixations using one (Group 1) or two (Group 2) standard SI screws versus one SI screw with bone cement augmentation (Group 3). Materials and Methods: Unstable fractures of the pelvic ring (AO/OTA 61-C1.3, FFP IIc) were simulated in 21 artificial pelvises by means of vertical osteotomies in the ipsilateral anterior and posterior pelvic ring. A supra-acetabular external fixator was applied to address the anterior fracture. All specimens were tested under progressively increasing cyclic loading until failure, with monitoring by means of motion tracking. Fracture site displacement and cycles to failure were evaluated. Results: Fracture displacement after 500 cycles was lowest in Group 3 (0.76 cm [0.30] (median [interquartile range, IQR])) followed by Group 1 (1.42 cm, [0.21]) and Group 2 (1.42 cm [1.66]), with significant differences between Groups 1 and 3, p = 0.04. Fracture displacement after 1000 cycles was significantly lower in Group 3 (1.15 cm [0.37]) compared to both Group 1 (2.19 cm [2.39]) and Group 2 (2.23 cm [3.65]), p ≤ 0.04. Cycles to failure (Group 1: 3930 ± 890 (mean ± standard deviation), Group 2: 3676 ± 348, Group 3: 3764 ± 645) did not differ significantly between the groups, p = 0.79. Conclusions: In our biomechanical setup cement augmentation of one SI screw resulted in significantly less displacement compared to the use of one or two SI screws. However, the number of cycles to failure was not significantly different between the groups. Cement augmentation of one SI screw seems to be a useful treatment option for posterior pelvic ring fixation, especially in osteoporotic bone.

Is Anterior Plating Superior to the Bilateral Use of Retrograde Transpubic Screws for Treatment of Straddle Pelvic Ring Fractures? A Biomechanical Investigation

Background: Fractures of the four anterior pubic rami are described as “straddle fractures”. The aim of this study was to compare biomechanical anterior plating (group 1) versus the bilateral use of retrograde transpubic screws (group 2). Methods: A straddle fracture was simulated in 16 artificial pelvises. All specimens were tested under progressively increasing cyclic loading, with monitoring by means of motion tracking. Results: Axial stiffness did not differ significantly between the groups, p = 0.88. Fracture displacement after 1000–4000 cycles was not significantly different between the groups, p ≥ 0.38; however, after 5000 cycles it was significantly less in the retrograde transpubic screw group compared to the anterior plating group, p = 0.04. No significantly different flexural rotations were detected between the groups, p ≥ 0.32. Moreover, no significant differences were detected between the groups with respect to their cycles to failure and failure loads, p = 0.14. Conclusion: The results of this biomechanical study reveal less fracture displacement in the retrograde transpubic screw group after long-term testing with no further significant difference between anterior plating and bilateral use of retrograde transpubic screws. While the open approach using anterior plating allows for better visualization of the fracture site and open reduction, the use of bilateral retrograde transpubic screws, splinting the fracture, presents a minimally invasive and biomechanically stable technique.

Mechanical Comparison of a Novel Hybrid and Commercial Dorsal Double Plating for Distal Radius Fracture: In Vitro Fatigue Four-Point Bending and Biomechanical Testing

This study compares the absolute and relative stabilities of a novel hybrid dorsal double plating (HDDP) to the often-used dorsal double plating (DDP) under distal radius fracture. The “Y” shape profile with 1.6 mm HDDP thickness was obtained by combining weighted topology optimization and finite element (FE) analysis and fabricated using Ti6Al4V alloy to perform the experimental tests. Static and fatigue four-point bending testing for HDDP and straight L-plate DDP was carried out to obtain the corresponding proof load, strength, and stiffness and the endurance limit (passed at 1 × 10⁶ load cycles) based on the ASTM F382 testing protocol. Biomechanical fatigue tests were performed for HDDP and commercial DDP systems fixed on the composite Sawbone under physiological loads with axial loading, bending, and torsion to understand the relative stability in a standardized AO OTA 2R3A3.1 fracture model. The static four-point bending results showed that the corresponding average proof load values for HDDP and DDPs were 109.22 N and 47.36 N, that the bending strengths were 1911.29 N/mm and 1183.93 N/mm, and that the bending stiffnesses were 42.85 N/mm and 4.85 N/mm, respectively. The proof load, bending strength and bending stiffness of the HDDPs were all significantly higher than those of DDPs. The HDDP failure patterns were found around the fourth locking screw hole from the proximal site, while slight plate bending deformations without breaks were found for DDP. The endurance limit was 76.50 N (equal to torque 1338.75 N/mm) for HDDP and 37.89 N (equal to torque 947.20 N/mm) for DDP. The biomechanical fatigue test indicated that displacements under axial load, bending, and torsion showed no significant differences between the HDDP and DDP groups. This study concluded that the mechanical strength and endurance limit of the HDDP was superior to a commercial DDP straight plate in the four-point bending test. The stabilities on the artificial radius fractured system were equivalent for novel HDDP and commercial DDP under physiological loads in biomechanical fatigue tests.

Treating Hand Trauma in Low-Resource Setting: A Challenge for Low- and Middle-Income Countries

Hand trauma surgical treatment and perioperative therapy are often lacking in low- and middle-income countries resulting in high rates of patient morbidity following injury. Providing education through a multifaceted approach including in-person teaching, written resources, videos, and Internet and social media platforms and facilitating skill acquisition through simulation permits local providers to gain expertise in hand trauma care and thus benefits patients. This article outlines challenges faced by low- and middle-income countries in caring for hand trauma patients and possible implementable solutions.

Additively manufactured implant components for imaging validation studies

Radiographic imaging is the current standard for evaluating postoperative joint replacements. Prior to application, such imaging methods need to be validated to determine the lower limits of performance under ideal conditions, using either a phantom or cadaver setup. Conventionally manufactured orthopedic implants for use in such studies are not always accessible and may be cost-prohibitive to purchase. We propose the use of additively manufactured implants as a cheaper, more accessible alternative for use in radiographic imaging validation studies. Bias and repeatability were compared between conventionally manufactured and additively manufactured reverse total shoulder implant sets under a standard model-based radiostereometric analysis phantom study environment. Measurements were compared using the humeral stem or glenosphere model relative to reference bone beads, and the humeral stem relative to the glenosphere model to measure implant relative displacement. Compared to the conventionally manufactured implants, the additively manufactured implants had less bias along the internal-external rotation axis (p < 0.001), but greater bias along the abduction-adduction and flexion-extension rotation axes (p = 0.005, 0.011). Additively manufactured implants had greater repeatability along the internal-external rotation axis (p < 0.001), but worse repeatability along the medial-lateral translation axis (p = 0.001) and the abduction-adduction rotation axis (p < 0.001). Differences were on the orders of 0.01 mm and 0.5°. For the purpose of validating two-dimensional-three-dimensional radiographic imaging techniques of orthopedic implants, additively manufactured implants can be used in place of conventionally manufactured implants, assuming they are fabricated to the manufacturer's specifications. Observed differences were within the errors of the measurement technique and not clinically meaningful.

Effect of different orientations of screw fixation for radial head fractures: a biomechanical comparison

Background

Screw fixation is a common method used for the treatment of Mason type II radial head fractures. The purpose of our study was to evaluate the mechanical properties of three different screw orientations used for fixation of Mason type II radial head fractures.

Methods

We sawed 24 medium-frequency fourth-generation Synbone radial bones to simulate unstable radial head fractures, which we then fixed with three different screw orientations. Implants were tested under axial load by the tension-torsion composite test system. If the implant-radial constructs did not fail after the axial load test, an axial failure load was added to the remaining constructs.

Results

The stiffness of the divergent group was the highest of the three orientations, and this group had statistically significant difference from the other two groups (p < 0.05). However, there was no statistically significant difference between the convergence group and the parallel group (p > 0.05). When the displacement reached 2 mm, the load of the divergent screw was still larger than the other two groups (p < 0.05).

Conclusions

The divergent screw orientation was the most stable and had the greatest control of Mason type II fractures of these three groups. Therefore, it can be better applied in clinical settings.

Sawbones laboratory in orthopedic surgical training

Sawbones are artificial bones designed to simulate the bone architecture, as well as the bone’s physical properties. The incorporation of sawbones simulation laboratories in many orthopedic training programs has provided the residents with flexibility in learning and scheduling that align with their working hour limitations. This review paper deliberates the organization of sawbones simulation in orthopedic surgical training to enhance trainee’s future learning. In addition, it explores the implications of sawbones simulation in orthopedic surgical teaching and evaluation. It scrutinizes the suitability of practicing on sawbones at the simulation laboratory to improve orthopedic trainee’s learning. This will be followed with recommendations for future enhancement of sawbones simulation-based learning in orthopedic surgical training.

Biomechanical Comparison of 2 Methods of Intramedullary K-Wire Fixation of Transverse Metacarpal Shaft Fractures

PURPOSE

To determine the relative importance of intramedullary wire (IMW) diameter and IMW number in conferring stability to a metacarpal fracture fixation construct. Our research hypothesis was that the stiffness of IMW fixation for metacarpal shaft fractures using a single 1.6-mm-diameter (0.062-in) wire would be greater than three 0.8-mm-diameter (0.031-in) wires.

METHODS

Our study compared the biomechanical stiffness between one 1.6-mm K-wire and three 0.8-mm K-wires in a composite, fourth-generation, biomechanical metacarpal construct under cantilever testing to treat transverse metacarpal shaft fractures. Six composite bone-wire constructs were tested in each group using constant-rate, nondestructive testing. Stiffness (load/displacement) was measured for each construct.

RESULTS

All constructs demonstrated a linear load-displacement relationship. Wires were all tested in their elastic zone. The mean stiffness of the 1-wire construct was 3.20 N/mm and the mean stiffness of the 3-wire construct was 0.76 N/mm. These differences were statistically significant with a large effect size.

CONCLUSIONS

The stiffness of IMW fixation for metacarpal shaft fractures using a single 1.6-mm-diameter wire was significantly greater than using three 0.8-mm-diameter wires.

CLINICAL RELEVANCE

When IMW fixation is clinically indicated for the treatment of metacarpal fractures, the increased stiffness of a single large-diameter construct provides more stability in the plane of finger flexion-extension.

Mechanical advantages of a truss-structure-based fracture fixation system--a novel fracture fixation device "PinFix"

A small, light, ball-joint device called PinFix, which can instantly convert a simple percutaneous cross pin fracture fixation system into a rigid external fracture fixation system based on truss structure, was developed. The purpose of this study was to compare the mechanical load and breaking strength of this truss-structure-based fixation system to that of the conventionally used external cantilever structure-based fixation system. Three types of mechanical loading tests, axial, bending, and torsion, were performed on an artificial fractured bone treated with either three-dimensional PinFix fixation, two-dimensional PinFix fixation, or conventional external fixation. The three- and two-dimensional PinFix fixations showed significantly more stiffness than conventional fixation on all three loading tests. Finite element analysis was next performed to calculate the stress distribution of the parts in PinFix and in the conventional fixator. The applied stress to the rod and connectors of PinFix was much less than that of the conventional external fixator. These results reflected the physical characteristic of truss structure in which applied load is converted to pure tension or compression forces along the members of the PinFix. In conclusion, PinFix is a simple fracture fixation system that has a truss-structure with a high rigidity.