JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 5(1994) 194-199 Tissue compatibility of polylactic acids in the skeletal site HIKOJIRO SAITOH, TAKASHI TAKATA tt, HIROMASA NIKAI tt, HIDEAKI SHINTANI Departments of Operative Dentistry, and ttOral Pathology, Hiroshima University School of Dentistry 1-2-3, Kasumi, Minami-Ku, 734 Hiroshima-City, Japan. SUONG-HYU HYON, YOSHITO IKADA Center of Biomedical Engineering, Kyoto University. 53, Kawara-cho, Syogoin, Sakyo-Ku, 606 Kyoto-City, Japan This study was undertaken to examine the osseous tissue compatibility of polylactic acid (PLA) with properties of bioabsorbability and plasticity. Two kinds of granular PLA with dif- ferent molecular weight (PLA-H: 28000 MW, PLA-L: 10600 MW) were implanted into rat tibiae and light microscopic sections were prepared at 1, 2, 4, 8, 16, and 24 weeks following surgery. The tissue compatibility of the PLAs was compared with that of hydroxyapatite (HAP) which is a non-absorbable biocompatible material. After implantation, both PLAs were gradually absorbed and replaced by marrow tissue at 24 weeks. PLA-H degraded slower than PLA-L. New bone formation was observed around each PLA by 4 weeks, and a vast amount of bone deposited on PLA-H at 16 weeks. Phagocytic reactions to the PLAs were noted, however, severe inflammation was not seen. HAP was not absorbed and bone surrounded it throughout the experiment. These results indicate good tissue compatibility of PLA as a bio- absorbable material at the skeletal site. 1. Introduction Polylactic acid (PLA) is a biomaterial of absorbable synthetic polymer type [1]. It easily undergoes hydro- lytic de-esterification in vivo. The degraded products of lactic acids are incorporated into metabolic path- ways and excreted as water and carbon dioxide, thus PLA does not remain in the body as a foreign mater- ial. PLA can be formed in various shapes by industrial processings. In addition, various mechanical strengths can be given to PLA according to the molecular weight, component and shape [2]. These properties make PLA an interesting material as a surgical device. At present, it is already in clinical use as a surgical suture. Moreover, PLA is expected to be used as a plate for fracturefixation [3], an artificial tendon [4], a blood vessel [5], and a delivery sub- stance for various drugs [6 8]. With regard to dental applications of PLA, it is thought to be a useful device in fixation of jaw fractures, filling of bone defects, and augmentation of the alveolar ridge. In these usages, PLA will achieve its object in the period required, then it will be absorbed and replaced by the proper tissues, and the second operation to remove the PLA will be eliminated. When PLA is used for the purposes described above, good tissue compatibility in the skeletal site is required. However, there are few reports on the os- seous tissue compatibility of PLA [9, 10]. Most stud- ies have only discussed the soft tissue compatibility, mechanical strength, and absorption rate of PLA as a surgical suture [1, 11, 12]. In this study, in order to investigate the usefulness of PLA as a surgical device in the skeletal site, we examined the osseous tissue compatibility ,of PLAs with different molecular weight. The tissue reactions of PLAs were compared with those of hydroxyapatite (HAP) which is known to be a non-absorbable im- plant material with good tissue compatibility in the skeletal site. 2. Materials and methods 2.1. Implant materials Two kinds of PLA with different molecular weight, and HAP were examined in this study. The abridged name, molecular weight, form, and size of each mater- ial are shown in Table I. TABLE I The implant materials used in this study Material Molecular Form Size of Abridged weight granule (gm) name Polylactic acid 28 000 Granular 100-400 PLA-H Polylactic acid 10 600 Granular 100-400 PLA-L Hydroxyapatite- Granular 297-500 HAP 1 94 0957 4530 © 1994 Chapman & Hall