Abstract

Purpose. To independently determine the mechanical properties of intramedullary nails (IMN), locking screws, and dynamic compression plates (DCP) from five commonly used osteosynthesis systems, and to assess inter-manufacturer variability in parameters relevant to safe early controlled axial loading (early weight-bearing) and improved accuracy of FEA modeling.

Methods. Osteosynthesis implants from five manufacturers (four are Chinese manufacturers and one is Indian) were selected. Five standard ten-hole DCPs, each from a different manufacturer, were tested in bending. Five IMNs from different manufacturers were evaluated under tensile loading. In addition, five locking screws from each manufacturer were tested in bending, and another five were tested in tension. A qualitative comparison was conducted to assess inter-manufacturer differences in the mechanical characteristics of these fixation devices.

Results. Between-manufacturer differences of up to approximately twofold were observed in the yield limit of plates. Intramedullary nails demonstrated considerably lower variability, with yield limits differing by approximately 10–15% across manufacturers and relatively consistent Young’s modulus values. Locking screws showed intermediate variability, with yield limit differences reaching up to 30%.

Conclusions. The identified inter-manufacturer differences in Yield limit, Ultimate strength, and Young`s modulus may substantially influence the boundaries of safe early controlled axial loading and the risk of mechanical failure of the fixation construct. The independently obtained experimental parameters are appropriate for use in FEA parameterization and for supporting the development of individualized loading protocols during post-fracture rehabilitation. Tested mechanical properties of the osteosynthesis implants comply with the manufacturing standards and are not below the required values.

Keywords Osteosynthesis implants, mechanical behaviour, fracture fixation, FEA

Introduction

The mechanical reliability of fracture fixation implants determines not only the stability of osteosynthesis but also the safety of early controlled axial loading. Although the precision of surgical technique remains a key factor in treatment success, the mechanical behavior of the “bone–fixator” construct defines stability during early mobilization and the risk of mechanical failure.
Previous biomechanical studies have demonstrated substantial variability in the stiffness and strength of fixation devices, driven by geometry, material properties, and loading methodology (Zhang S et al., 2022 [1]; Okazaki Y et al., 2020 [2]). Other investigations have highlighted the influence of bone elastic modulus and screw trajectory on fixation stability (Tsagkaris C et al., 2023 [3]) and the characteristics of stress distribution in numerical models (Li B et al., 2023 [4]). As noted by Aluede E et al., 2014 [5], even structurally similar plates from different manufacturers may differ considerably in yield strength, which directly affects clinical safety under loading.
The clinical consequences of mechanical failure remain highly relevant: the incidence of implant breakage after long-bone osteosynthesis reaches 5–9% (Gardner MJ et al., 2009 [6]), whereas excessive construct stiffness may reduce the mechanical stimulation necessary for callus formation (Windolf M et al., 2017 [7]). Achieving an optimal balance between strength and flexibility is therefore essential to ensure stability while providing sufficient mechanical stimulus for healing.
This variability underscores the need for independent bench testing to obtain experimental data for FEA modeling and to support the development of individualized loading protocols, particularly for widely used yet insufficiently standardized osteosynthesis systems.

Materials and Methods

Study Objects.
Five fixation systems were tested: Auxein, Jiangsu Jinlu, NX Medical (Changzhou Nanxiang Medical), Double Medical Technology, and Irene. All implants were made of titanium alloy Ti-6Al-4V Grade 5 (ISO 5832-3 / ASTM F136).

Sample Configuration.
– Intramedullary nails: Ø ≥ 9 mm, L ≥ 240 mm (n = 1 per manufacturer).
– Locking screws: 5 × 55 mm (n = 2 per manufacturer;).
– Plates: straight narrow locking plates (n = 1 per manufacturer).

Equipment and Calibration.
Mechanical bending testing and screw tension testing were performed using an electromechanical testing machine UTM-100 (SKTB IPMits, National Academy of Sciences of Ukraine; Fₘₐₓ = 100 kN) at 23 ± 1 °C. The load cell (class 0.5) and extensometer were pre-calibrated (verification certificate No. МХ/062/25, dated 21.02.2025); measurement traceability was confirmed by the corresponding certification documents. Tension testing of the intramedullary nails was conducted in the Testing laboratory of LLC SendLab (DSTU EN ISO 17025:2019 accreditation from 26 March 2022).

Tensile Test Protocol.

For screws, the displacement rate was v = 4.5 mm/min; for nails, testing followed ISO 6892-1 using displacement control. The following parameters were determined: σ₀.₂ (Rp₀.₂, 0.2% offset yield strength), σUTS (ultimate tensile strength; Rₘ), maximum load (Pmax), and elongation at failure (A).

Fig.1 Screw tensioning testing

a (Double Medical Technology)

b (Auxein)

Fig.2 Screws destruction during (a) tension testing and (b) bending testing

Bending Test Protocol.

Bending tests were performed using a three-point configuration with central loading and a support span of L = 105 mm for plates and L = 40 mm for screws (roller radii R₁ = 5 mm; indenter radius R₂ = 10 mm). Force is acting in the center of the specimen. The crosshead displacement rate was v = 11 mm/min, corresponding to quasi-static loading in displacement-control mode. The outer-fiber stress was calculated as

where W is the section modulus and M is the moment of the force and W is section modulus.

Moment was calculated by formula:

where P is the acting force and L is the half – span of the specimen.

For screws section modulus was calculated analytically according to the formula

where d = 4mm is minor diameter.

Thread was not considered for the strength capacity estimation of the screws. The section modulus for plates was determined based on the CT scan of the specimen. The resulting geometry was then processed in Simpleware software (Synopsis, USA) and subsequently converted to STL format for calculating cross-sectional properties in Inventor CAD software (Autodesk, USA). Bending tests were applied to plates and screws to evaluate stiffness and strength under quasi-static loading.

Fig.3 Bending testing of the screws

Fig.4 Bending testing of the plates

Comparison and Normalization.

Results were compared with ISO 5832-3:2021 specifications.

Plates — Bending

The Young’s modulus (E) ranged from 107.2 to 128.2 GPa, the 0.2% offset yield strength (σ₀.₂) from 867.7 to 1783.4 MPa, and the maximum bending stress (σmax) from 1094 to 2055.8 MPa. The maximum load (Pmax) varied between 1.02 and 1.92 kN. Elongation at failure (A) ranged from 6.6% to 8.3%. The inter-manufacturer variability in σ₀.₂ approached a twofold difference, whereas E remained relatively consistent (difference < 20%).

Table 1. Results of plates bending (n = 1 per manufacturer)

Параметр	Auxein	Jinlu	NX	Double Medical Technology	Irene
E (GPa)	107.2	109.6	126.9	128.2	115.7
σ₀.₂ (MPa)	1627	936	868	1783	1007
σmax (MPa)	2056	1144	1094	1993	1224
Pmax (kN)	1.59	1.27	1.02	1.92	1.31
A (%)	6.6	7.9	8.3	6.6	7.7

Fig. 5. Young’s modulus, yield strength, and elongation obtained from the bending tests of plates for each manufacturer.

Screws — Tension

The σ₀.₂ values ranged from 889 to 1121 MPa, σUTS from 1056 to 1303 MPa, elongation A from 13% to 26%, and Pmax from 14.6 to 16.8 kN. All values exceeded ISO 5832-3 minimum requirements (σ₀.₂ ≥ 795 MPa; σUTS ≥ 895 MPa).

Table 2. Results of screw tension (tension, n = 1 per manufacturer)

Параметр	Auxein	Jinlu	NX	Double Medical Technology	Irene
E (GPa)	107.2	109.6	126.9	128.2	115.7
σ₀.₂ (MPa)	1627	936	868	1783	1007
σmax (MPa)	2056	1144	1094	1993	1224
Pmax (kN)	1.59	1.27	1.02	1.92	1.31
A (%)	6.6	7.9	8.3	6.6	7.7

Fig. 6. Young’s modulus, yield strength, and elongation obtained from the tension tests of screws for each manufacturer.

Screws — Bending (Quasi-static)

Screw bending tests showed Rp₀.₂(bend) values between 1234 and 1761 MPa and σmax(bend) between 1634 and 2515 MPa. The maximum load (Pmax) ranged from 1253 to 1580 N (1.25–1.58 kN).

Table 2a. Results of screws bending (bending, n = 1 per manufacturer)

Параметр	Auxein	Jinlu	NX	Double Medical Technology	Irene
Rp₀.₂(bend) (MPa)	1234	1345	1334	1336	1761
σmax(bend) (MPa)	1634	1852	1851	1790	2515
Pmax (N)	1462	1253	1346	1447	1580

Fig. 7. Young’s modulus, yield strength, and elongation obtained from the bending tests of screws for each manufacturer.

Intramedullary Nails — Tension

For intramedullary nails, σ₀.₂ ranged from 883 to 1036 MPa, σUTS from 1003 to 1126 MPa, and elongation A from 4% to 13.5%.

Table 3. Results of Intramedullary Nails tension (n = 1 per manufacturer)

Параметр	Auxein	Jinlu	NX	Double Medical Technology	Irene
Rp₀.₂(bend) (MPa)	1234	1345	1334	1336	1761
σmax(bend) (MPa)	1634	1852	1851	1790	2515
Pmax (N)	1462	1253	1346	1447	1580

Fig. 8. Young’s modulus, yield strength, and elongation obtained from the tensile tests of Intramedullary Nails for each manufacturer.

Discussion

The results demonstrate significant inter-manufacturer variability in mechanical properties of osteosynthesis implants. Differences in Yield limit, Ultimate strength, and Young`s modulus reflect underlying heterogeneity in manufacturing processes and may influence the permissible limits of early controlled loading during rehabilitation.

Across all implant types, moderate variability in stiffness (E) and substantial variability in σ₀.₂ were observed, indicating the influence of design and manufacturing factors. All devices demonstrated values consistent with Ti-6Al-4V Grade 5; however, the inter-manufacturer σ₀.₂ spread—up to ~2-fold for plates and 10–15% for nails—may meaningfully affect safety thresholds in FEA simulations and early weight-bearing clinical protocols.

Fixation failure rates after long-bone osteosynthesis approximate 5–9% [6–9]. Insufficient mechanical strength or excessive stiffness may lead to implant breakage or delayed union. An optimal balance between rigidity and flexibility is required to maintain fragment stability while preserving strain levels necessary for callus formation [7,10].
Although this study does not aim to rank manufacturers, it clearly demonstrates that structurally similar implant systems may exhibit clinically meaningful differences relevant to early loading decisions.

Integrating experimentally determined mechanical parameters into FEA models reduces uncertainty, enhances prediction accuracy for implant failure risk, and supports rational planning of early weight-bearing in clinical practice. Independent bench testing represents a valuable component of orthopedic implant evaluation and individualized rehabilitation planning, though it does not replace formal regulatory certification.

Ethical Considerations

This study did not involve biological specimens and did not require ethics committee approval.

Conflict of Interest

The authors declare no conflict of interest.

Funding

The study was conducted within the “ComeBackMobility” project. Implant manufacturers had no role in study design, testing, data analysis, or interpretation.

References

Zhang S, Li Y, Chen T, et al. Experimental testing of fracture fixation plates: a review. Proc Inst Mech Eng H. 2022;236(10):1205–1220. doi:10.1177/09544119221108540.
Okazaki Y, Gotoh E, Fujishiro T, et al. Mechanical performance of metallic bone screws. Materials (Basel). 2020;13(21):4862. doi:10.3390/ma13214862.
Tsagkaris C, Stathopoulos I, Vlachos P, et al. Bone density optimized pedicle screw insertion: A computational and experimental study. Front Bioeng Biotechnol. 2023;11:1270522. doi:10.3389/fbioe.2023.1270522.
Li B, Ding Z, Shen J, et al. Biomechanical comparison of intramedullary nails and plates in a sheep model. Ann Transl Med. 2023;11(10):367. doi:10.21037/atm-23-1879.
Aluede E, McDonald E, Jergesen H, et al. Mechanical behaviour of low-cost dynamic compression plates correlates with manufacturing quality standards. Int Orthop. 2014;38:141–147. doi:10.1007/s00264-013-2148-2.
Gardner MJ, Evans JM, Dunbar RP. Failure of fracture plate fixation. J Am Acad Orthop Surg. 2009;17(4):247–257.
Windolf M, Braunsberger P, Rüden C, et al. Systematic review of biomechanical studies on locking plates: How variable are stiffness and strength? Clin Biomech (Bristol, Avon). 2017;48:45–56. doi:10.1016/j.clinbiomech.2017.07.009.
Bhat AK, Rao SK, Bhaskaranand K. Mechanical failure in intramedullary interlocking nails. J Orthop Surg (Hong Kong). 2006;14(2):138–141.
Lenz M, Windolf M. Biomechanical concepts for fracture fixation: Variability and clinical translation. Injury. 2018;49(Suppl 1):S29–S35. doi:10.1016/j.injury.2018.11.042.
Frost HM. Orthopaedic biomechanics: The law of bone adaptation. Springfield, IL: Charles C Thomas Publisher; 1973.
ISO 6892-1:2019. Metallic materials — Tensile testing — Part 1: Method of test at room temperature. International Organization for Standardization, Geneva.
ISO 5832-3:2021. Implants for surgery — Metallic materials — Part 3: Wrought titanium 6-aluminium 4-vanadium alloy. International Organization for Standardization, Geneva.
ASTM F382, ASTM F543, ASTM F1264. Standard test methods for metallic bone plates, screws and intramedullary fixation devices. ASTM International, West Conshohocken, PA.