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Research Papers

Poly(Propylene Fumarate)–Hydroxyapatite Nanocomposite Can Be a Suitable Candidate for Cervical Cages

[+] Author and Article Information
Yong Teng

Department of Physiology and
Biomedical Engineering,
Mayo Clinic College of Medicine,
Rochester, MN 55905;
Department of Orthopedic Surgery,
Mayo Clinic College of Medicine,
Rochester, MN 55905;
Orthopedic Center,
General Hospital of Xinjiang
Military Region PLA,
Uygur Autonomous Region,
Xinjiang 830000, China
e-mail: orthtengyong@163.com

Hugo Giambini

Department of Orthopedic Surgery,
Mayo Clinic College of Medicine,
Rochester, MN 55905
e-mail: giambini.hugo@mayo.edu

Asghar Rezaei

Department of Physiology and
Biomedical Engineering,
Mayo Clinic College of Medicine,
Rochester, MN 55905
e-mail: rezaei.asghar@mayo.edu

Xifeng Liu

Department of Physiology and
Biomedical Engineering,
Mayo Clinic College of Medicine,
Rochester, MN 55905;
Department of Orthopedic Surgery,
Mayo Clinic College of Medicine,
Rochester, MN 55905
e-mail: liu.xifeng@mayo.edu

A. Lee Miller, II

Department of Orthopedic Surgery,
Mayo Clinic College of Medicine,
Rochester, MN 55905
e-mail: miller.alan@mayo.edu

Brian E. Waletzki

Department of Orthopedic Surgery,
Mayo Clinic College of Medicine,
Rochester, MN 55905
e-mail: waletzki.brian@mayo.edu

Lichun Lu

Department of Physiology and
Biomedical Engineering,
Mayo Clinic College of Medicine,
Rochester, MN 55905;
Department of Orthopedic Surgery,
Mayo Clinic College of Medicine,
Rochester, MN 55905
e-mail: lu.lichun@mayo.edu

1Corresponding author.

Manuscript received January 19, 2018; final manuscript received May 29, 2018; published online June 21, 2018. Assoc. Editor: James C. Iatridis.

J Biomech Eng 140(10), 101009 (Jun 21, 2018) (8 pages) Paper No: BIO-18-1038; doi: 10.1115/1.4040458 History: Received January 19, 2018; Revised May 29, 2018

A wide range of materials have been used for the development of intervertebral cages. Poly(propylene fumarate) (PPF) has been shown to be an excellent biomaterial with characteristics similar to trabecular bone. Hydroxyapatite (HA) has been shown to enhance biocompatibility and mechanical properties of PPF. The purpose of this study was to characterize the effect of PPF augmented with HA (PPF:HA) and evaluate the feasibility of this material for the development of cervical cages. PPF was synthesized and combined with HA at PPF:HA wt:wt ratios of 100:0, 80:20, 70:30, and 60:40. Molds were fabricated for testing PPF:HA bulk materials in compression, bending, tension, and hardness according to ASTM standards, and also for cage preparation. The cages were fabricated with and without holes and with porosity created by salt leaching. The samples as well as the cages were mechanically tested using a materials testing frame. All elastic moduli as well as the hardness increased significantly by adding HA to PPF (p < 0.0001). The 20 wt % HA increased the moduli significantly compared to pure PPF (p < 0.0001). Compressive stiffness of all cages also increased with the addition of HA. HA increased the failure load of the porous cages significantly (p = 0.0018) compared with nonporous cages. PPF:HA wt:wt ratio of 80:20 proved to be significantly stiffer and stronger than pure PPF. The current results suggest that this polymeric composite can be a suitable candidate material for intervertebral body cages.

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Figures

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Fig. 1

Flowchart describing the bulk material and cage characterization. This sequence of testing events was followed for each polymer group (PPF; PPF:HA (80:20); PPF:HA (70:30); and PPF:HA (60:40)). Bulk material was characterized by TGA and FTIR, and by ASTM standards for compression, tension, bending, and hardness. Nonporous (with/without holes) and porous (with holes) cages were tested in compression.

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Fig. 2

Mechanical testing setup for (a) compression (cylindrical samples of 6 mm in diameter and 24 mm in length), (b) tension (standard type IV dumbbell-shaped samples with an overall length 115 mm, length at the narrow section 33 mm, and thickness 3.2 mm), and (c) three-point bending (rectangular bars of length, width and thickness of 80 mm in length

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Fig. 3

(a) Thermal degradation and (b) FTIR spectra of the polymer groups

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Fig. 4

Scatter plot showing the moduli outcomes of the polymer groups under compression, tension, and bending (bar represents the median)

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Fig. 5

Scatter plot showing the failure strength of the four material groups in tension and bending (bar represents the median)

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Fig. 6

Scatter plot showing the hardness outcomes for the four polymer groups (PPF; PPF:HA (80:20); PPF:HA (70:30); and PPF:HA (60:40)). The Rockwell hardness numbers (M) are based on an indenter size of 6.35 mm in diameter, a fixed small load of 10 kg, and a large load (100 kg). Bar represents the median.

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Fig. 7

Design, fabrication, and mechanical testing of cervical interbody fusion cages with and without porosity made of pure PPF and HA-augmented PPF: (a) a mold designed and fabricated to develop cages with no holes; (b) a fusion cage made out of 30 wt % HA-augmented PPF; (c) sample cages (PPF, left; 20, 30, and 40 wt% HA-PPF). Threaded holes and screws were also implemented to mimic the attachments used during a routine surgical technique. These threaded screws connect the cage with the holder used to place the cage in position between the vertebral bodies. The screws were removed during mechanical testing; (d) PPF cage before compression; and (e) porous PPF with 40 wt %HA after failure.

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Fig. 8

Scatter plot showing the compressive stiffness outcomes for the cages: cages with no holes, cages with holes, and cages with holes and 50 wt % porosity (bar represents the median)

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