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      Biomechanical Loading Comparison between Titanium and Bioactive Resorbable Screw Systems for Fixation of Intracapsular Condylar Head Fractures

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          Abstract

          Osteosynthesis resorbable materials made of uncalcined and unsintered hydroxyapatite (u-HA) particles, poly-L-lactide (PLLA), are bioresorbable, and these materials have feasible bioactive/osteoconductive capacities. However, their strength and stability for fixation in mandibular condylar head fractures remain unclear. This in vitro study aimed to assess the biomechanical strength of u-HA/PLLA screws after the internal fixation of condylar head fractures. To evaluate their biomechanical behavior, 32 hemimandible replicas were divided into eight groups, each consisting of single-screw and double-screw fixations with titanium or u-HA/PLLA screws. A linear load was applied as vertical and horizontal load to each group to simulate the muscular forces in condylar head fractures. Samples were examined for 0.5, 1, 2, and 3-mm displacement loads. Two screws were needed for stable fixation of the mandibular condylar head fracture during biomechanical evaluation. After screw fixation for condylar head fractures, the titanium screws model was slightly more resistant to vertical and horizontal movement with a load for a small displacement than the u-HA/PLLA screws model. There was no statistically significant difference with load for large displacements. The u-HA/PLLA screw has a low mechanical resistance under small displacement loading compared with titanium within the limits of the mandibular head fracture model study.

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          Bioresorbable devices made of forged composites of hydroxyapatite (HA) particles and poly-L-lactide (PLLA): Part I. Basic characteristics.

          Y Shikinami (1999)
          Compounds that had neither calcined nor sintered hydroxyapatite (u-HA) particles (particulate size 0.2-20 microns, averaging 3.0 microns, Ca/P = 1.69, and containing CO3(2-) uniformly distributed in a poly-L-lactide (PLLA, Mv: 400 KDa) matrix with a content of 20-50 wt% (with 10% increment) were reinforced into composites by a forging process, which was a unique compression molding, and were then machined on a lathe in order to produce practical radiopaque internal bone fixation devices having high mechanical strength which was maintained during bony union, total resorbability and bioactivity such as bone bonding capability and osteoconductivity. From the results of measurement of various mechanical properties, it was confirmed that the composites generally showed the highest mechanical strength among this type of reinforced bioceramic fibers or particles/bioresorbable polymer composite known to date. The bending strength (Sb) of about 270 MPa was far higher value than that for cortical bone, and the modulus (Eb) of 12 GPa was almost equivalent to that for cortical bone. In particular, the impact strength (Si) was extremely high at about two times the value (166 KJ/m2) of polycarbonate. The in vitro change in Sb, Mv (viscosity average molecular weight), Mw/Mn (molecular weight distribution) and crystallinity, and their relationship with each other was also examined by immersing samples in a phosphate buffer solution (PBS). An immediate decrease in the initial Mv could be found in composites with high u-HA contents (30-50 wt%), although a time-lag stage for degradation where the initial Mv hardly changes was apparent in cases of PLLA-only or in a composite with a low u-HA content (20 wt%). The Sb changed with corresponding decremental curves for the Mv and retained over 200 MPa for up to 24 weeks, the period of time necessary for full bony union, so that the composite satisfied initial mechanical strengths while maintaining them for as long as necessary for internal bone fixation devices. These results supported the idea that there is a difference in the degradation process such that PLLA alone required a period of time to achieve the possibility of hydrolysis into the inner side, whereas composites with high u-HA contents (30-50 wt%) immediately filled with water through to the inner side and were hydrolyzed homogeneously. Many hydroxyapatite crystals deposited and grew on the surface after 3-6 d and generously covered the surface with a fairly thick layer after 7 d of post-immersion in simulated body fluid (SBF) as evaluated by means of energy dispersive X-ray (EDX). This suggested the ability of the radiopaque composites to bond to bone. Since the composites were dense and had ultra-high strength, and the processability was so excellent, many kinds of fine and accurate screws, pins, plates, and other internal bone fixation devices for orthopedic, oral and maxillofacial, craniofacial, and plastic and reconstructive surgeries could be produced by machining treatment. These devices have potential applications for clinical use following the assessment of adaptation during in vivo studies.
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            Late degradation tissue response to poly(L-lactide) bone plates and screws.

            Maria Silva, J. Bergsma, F. R. Rozema … (1994)
            Patients with fractures of the zygomatic bone were treated with high molecular weight poly(L-lactic) acid (PLLA) bone plates and screws. Three years after implantation four patients returned to our department with a swelling at the site of implantation. At the recall of the remaining patients we found an identical type of swelling after the same implantation period. To investigate the nature of the tissue reaction, eight patients were reoperated for the removal of the swelling. The implantation period of the PLLA material varied from 3.3 to 5.7 years. Microscopic evaluation and molecular weight measurements were performed. The excised material showed remnants of degraded PLLA material surrounded by a dense fibrous capsule. Ultrastructural investigation showed crystal-like PLLA material internalized by various cells. The results of this investigation suggest that the PLLA material slowly degrades into particles with a high crystallinity. The intra- and extracellular degradation rate of these particles is very low. After 5.7 years of implantation, these particles were still not fully resorbed.
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              Bioresorbable devices made of forged composites of hydroxyapatite (HA) particles and poly L-lactide (PLLA). Part II: practical properties of miniscrews and miniplates.

              Y Shikinami, M Okuno (2001)
              Miniscrews and miniplates made of forged composites composed of raw hydroxyapatite (u-HA) particles (particle size 0.2-20 microm, averaging 3.0 microm, Ca/p = 1.69 and containing CO3(2-)) and a poly L-lactide (PLLA, Mv: about 180 kDa, containing residual 0.05 wt% lactide) with osteological bioactivity such as direct bonding to bone and osteoconductivity, total resorbability and radiopacity were examined for various mechanical properties in order to evaluate their usefulness for cranio-, oral and maxillo-facial as well as plastic and reconstructive surgeries with PLLA-only or titanium devices. The composites containing u-HA particles at 30wt% for miniscrews and 40wt% for miniplates were selected based on total mechanical strengths and bioactivity, respectively. It was found that the composite devices generally had slightly different mechanical properties than forged PLLA-only devices of which strengths are ranked the highest among the reinforced PLLA-only ones that having been used in many clinical cases to date, in spite of their approximate 2 or 3 times lower absolute strengths than those of titanium ones. However, a remarkable distinction that makes the composite miniplates stand above the titanium ones was confirmed on their fatigue resistance to alternate bendings such that they retained 70% of their initial strength even after 60 times without revealing any damage, whereas the metallic devices fully broke off at only 8 times. This behavior was similar to that of forged PLLA-only devices but is unique as composites made of organic polymers divided by inorganic particles. In addition, profile plates such as L-, T-, X, T, C-, Mesh-, Box-, and Barhole types which were processed by forging twice exhibited nearly directional isotropy in strength and could be deformed in situ at ordinary temperatures to adjust their shapes along the surface undulations of the skull, mandible, maxilla, zygomatic bone and the like without thermoforming and did not return to their original shapes inside an alive body due to the high PLLA's Tg (65 degrees C) over an alive body temperature (37 degrees C). Since it had already been confirmed in previous papers that these stiff and tough composites have the osteological bioactivity which is missing from both PLLA-only and titanium ones, and radiopacity which is wanting in PLLA-only ones, these various small and thin screws and plates have conclusively less objectionable practicality for use in oral-maxillo and craniofacial as well as plastic and reconstructive surgeries.
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                Author and article information

                Journal
                Journal ID (nlm-ta): Materials (Basel)
                Journal ID (iso-abbrev): Materials (Basel)
                Journal ID (publisher-id): materials
                Title: Materials
                Publisher: MDPI
                ISSN (Electronic): 1996-1944
                Publication date (Electronic): 15 July 2020
                Publication date Collection: July 2020
                Volume: 13
                Issue: 14
                Electronic Location Identifier: 3153
                Affiliations
                [1 ]Department of Oral and Maxillofacial Surgery, Kagawa Prefectural Central Hospital, 1-2-1, Asahi-machi, Takamatsu, Kagawa 760-8557, Japan; furukiy@ 123456ma.pikara.ne.jp
                [2 ]Department of Orthopaedic Surgery, Kagawa Prefectural Central Hospital, Takamatsu, Kagawa 60-0065, Japan; norio-yamamoto@ 123456umin.ac.jp
                [3 ]Department of Oral Pathology and Medicine, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama 700-8530, Japan; pir19btp@ 123456okayama-u.ac.jp (K.N.); gmd422094@ 123456s.okayama-u.ac.jp (K.T.); de18018@ 123456s.okayama-u.ac.jp (H.K.); jin@ 123456okayama-u.ac.jp (H.N.)
                [4 ]Department of Oral and Maxillofacial Surgery, Shimane University Faculty of Medicine, Shimane 693-0021, Japan; tkanno@ 123456med.shimane-u.ac.jp
                Author notes
                [* ]Correspondence: gouwan19@ 123456gmail.com ; Tel.: +81-87-811-3333; Fax: +81-87-802-1188
                Author information
                https://orcid.org/0000-0001-7986-2735
                Article
                Publisher ID: materials-13-03153
                DOI: 10.3390/ma13143153
                PMC ID: 7411721
                PubMed ID: 32679803
                SO-VID: 14cb331e-2396-43ab-9a5c-b15b48db7b99
                Copyright © © 2020 by the authors.
                License:

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                Date received : 03 June 2020
                Date accepted : 08 July 2020
                Categories
                Subject: Article

                Keywords: titanium screw,bioactive resorbable screw,biomechanical loading evaluation,fracture fixation,intracapsular condylar head fracture
                Data availability:
                Keywords: titanium screw, bioactive resorbable screw, biomechanical loading evaluation, fracture fixation, intracapsular condylar head fracture

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