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      Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices

      research-article
      Author(s): , PhD 1 , , BA, BM, BCh, MA, FRCSEd, DM 3 , , PhD 2 ,
      Publication date (Electronic):
      Journal: Bone & Joint Research
      Keywords: Fracture healing, Strain, Boundary conditions

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          Abstract

          Objectives

          Secondary fracture healing is strongly influenced by the stiffness of the bone-fixator system. Biomechanical tests are extensively used to investigate stiffness and strength of fixation devices. The stiffness values reported in the literature for locked plating, however, vary by three orders of magnitude. The aim of this study was to examine the influence that the method of restraint and load application has on the stiffness produced, the strain distribution within the bone, and the stresses in the implant for locking plate constructs.

          Methods

          Synthetic composite bones were used to evaluate experimentally the influence of four different methods of loading and restraining specimens, all used in recent previous studies. Two plate types and three screw arrangements were also evaluated for each loading scenario. Computational models were also developed and validated using the experimental tests.

          Results

          The method of loading was found to affect the gap stiffness strongly (by up to six times) but also the magnitude of the plate stress and the location and magnitude of strains at the bone-screw interface.

          Conclusions

          This study demonstrates that the method of loading is responsible for much of the difference in reported stiffness values in the literature. It also shows that previous contradictory findings, such as the influence of working length and very large differences in failure loads, can be readily explained by the choice of loading condition.

          Cite this article: A. MacLeod, A. H. R. W. Simpson, P. Pankaj. Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices. Bone Joint Res 2018;7:111–120. DOI: 10.1302/2046-3758.71.BJR-2017-0074.R2.

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          Most cited references51

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          Inhibition of fracture healing.

          A Hamish R W Simpson, Benjamin Gaston (2007)
          This paper reviews the current literature concerning the main clinical factors which can impair the healing of fractures and makes recommendations on avoiding or minimising these in order to optimise the outcome for patients. The clinical implications are described.
          Article 0 comments Cited 92 times – based on 0 reviews     Review now
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            Biomechanical testing of the LCP--how can stability in locked internal fixators be controlled?

            Karl Stoffel, Ulrich Dieter, Gwidon Stachowiak … (2003)
            New plating techniques, such as non-contact plates, have been introduced in acknowledgment of the importance of biological factors in internal fixation. Knowledge of the fixation stability provided by these new plates is very limited and clarification is still necessary to determine how the mechanical stability, e.g. fracture motion, and the risk of implant failure can best be controlled. The results of a study based on in vitro experiments with composite bone cylinders and finite element analysis using the Locking Compression Plate (LCP) for diaphyseal fractures are presented and recommendations for clinical practice are given. Several factors were shown to influence stability both in compression and torsion. Axial stiffness and torsional rigidity was mainly influenced by the working length, e.g. the distance of the first screw to the fracture site. By omitting one screw hole on either side of the fracture, the construct became almost twice as flexible in both compression and torsion. The number of screws also significantly affected the stability, however, more than three screws per fragment did little to increase axial stiffness; nor did four screws increase torsional rigidity. The position of the third screw in the fragment significantly affected axial stiffness, but not torsional rigidity. The closer an additional screw is positioned towards the fracture gap, the stiffer the construct becomes under compression. The rigidity under torsional load was determined by the number of screws only. Another factor affecting construct stability was the distance of the plate to the bone. Increasing this distance resulted in decreased construct stability. Finally, a shorter plate with an equal number of screws caused a reduction in axial stiffness but not in torsional rigidity. Static compression tests showed that increasing the working length, e.g. omitting the screws immediately adjacent to the fracture on both sides, significantly diminished the load causing plastic deformation of the plate. If bone contact was not present at the fracture site due to comminution, a greater working length also led to earlier failure in dynamic loading tests. For simple fractures with a small fracture gap and bone contact under dynamic load, the number of cycles until failure was greater than one million for all tested constructs. Plate failures invariably occurred through the DCP hole where the highest von Mises stresses were found in the finite element analysis (FEA). This stress was reduced in constructions with bone contact by increasing the bridging length. On the other hand, additional screws increased the implant stress since higher loads were needed to achieve bone contact. Based on the present results, the following clinical recommendations can be made for the locked internal fixator in bridging technique as part of a minimally invasive percutaneous osteosynthesis (MIPO): for fractures of the lower extremity, two or three screws on either side of the fracture should be sufficient. For fractures of the humerus or forearm, three to four screws on either side should be used as rotational forces predominate in these bones. In simple fractures with a small interfragmentary gap, one or two holes should be omitted on each side of the fracture to initiate spontaneous fracture healing, including the generation of callus formations. In fractures with a large fracture gap such as comminuted fractures, we advise placement of the innermost screws as close as practicable to the fracture. Furthermore, the distance between the plate and the bone ought to be kept small and long plates should be used to provide sufficient axial stiffness.
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              Shear movement at the fracture site delays healing in a diaphyseal fracture model.

              Peter Augat, Johannes Burger, Sandra Schorlemmer … (2003)
              This study tested the hypothesis that interfragmentary axial movement of transverse diaphyseal osteotomies would result in improved fracture healing compared to interfragmentary shear movement. Ten skeletally mature merino sheep underwent a middiaphyseal osteotomy of the right tibia, stabilized by external fixation with an interfragmentary gap of 3 mm. A custom made external fixator allowed either pure axial (n=5) or pure shear movement (n=5) of 1.5 mm amplitude during locomotion by the animals. The movement of the osteotomy gap was monitored weekly in two sheep by an extensometer temporarily attached to the fixator. After 8 weeks the sheep were killed, and healing of the osteotomies was evaluated by radiography, biomechanical testing, and undecalcified histology. Shear movement considerably delayed the healing of diaphyseal osteotomies. Bridging of the osteotomy fragments occurred in all osteotomies in the axial group (100%), while in the shear group only three osteotomies (60%) were partially bridged. Peripheral callus formation in the shear group was reduced by 36% compared to the axial group (p<0.05). In the axial group bone formation was considerably larger at the peripheral callus and in between the osteotomy gaps but not in the intramedullary area. The larger peripheral callus and excess in bone tissue at the level of the gap resulted in a more than three times larger mechanical rigidity for the axial than for the shear group (p<0.05). In summary, fixation that allows excessive shear movement significantly delayed the healing of diaphyseal osteotomies compared to healing under axial movement of the same magnitude.
              Article 0 comments Cited 75 times – based on 0 reviews     Review now
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                Author and article information

                Contributors
                A. MacLeod: Role: Research Associate
                A. H. R. W. Simpson: Role: George Harrison Law Professor of Orthopaedic Surgery
                P. Pankaj: Role: Professor of Computational Biomechanics
                Journal
                Journal ID (nlm-ta): Bone Joint Res
                Title: Bone & Joint Research
                ISSN (Electronic): 2046-3758
                Publication date Collection: January 2018
                Publication date (Electronic): 8 February 2018
                Volume: 7
                Issue: 1
                Pages: 111-120
                Affiliations
                [1 ]The University of Edinburgh
                [2 ]The University of Edinburgh, School of Engineering, Institute for Bioengineering, Faraday Building, Edinburgh EH9 3DW, UK
                [3 ]Department of Orthopaedic Surgery, University of Edinburgh, Chancellor’s Building, 49 Little France Crescent, Old Dalkeith Road, Edinburgh EH16 4SB, UK
                Author notes
                [*]P. Pankaj; email: pankaj@ 123456ed.ac.uk
                Article
                Publisher ID: 10.1302_2046-3758.71.BJR-2017-0074.R2
                DOI: 10.1302/2046-3758.71.BJR-2017-0074.R2
                PMC ID: 5805837
                PubMed ID: 29363522
                SO-VID: ad704c87-ba51-4ea9-b38d-baf727058310
                Copyright © © 2018 MacLeod et al.
                License:

                This is an open-access article distributed under the terms of the Creative Commons Attributions licence (CC-BY-NC), which permits unrestricted use, distribution, and reproduction in any medium, but not for commercial gain, provided the original author and source are credited.

                History
                Categories
                Subject: Biomechanics
                Subject: 10
                Subject: Fracture Healing
                Subject: Strain
                Subject: Boundary Conditions

                Keywords: fracture healing,strain,boundary conditions
                Data availability:
                Keywords: fracture healing, strain, boundary conditions

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