Development of an Artificial Vertebral Body Using Biomaterial, Hydroxyapatite/collagen Composite

Relevance to Muscloskeletal Condition: Although the use of autografts in anterior interbody fusion of the cervical spine is the gold standard, complications and morbidity rates are very high. Also, allografts present numerous other problems including a low bone fusion rate as well as the possibility of disease transmission. Therefore, artificial bone substitutes such as ceramics and polymethylmethacrylates may be more suitable to increase the bone fusion rate. Introduction: Hydroxyapatite/Collagen (HAp/Col) composites having a bone-like nanostructure and chemical composition were synthesized according to a simultaneous titration method under controlled pH and temperature. The HAp/Col composites were subjected to pressure dehydration, and shaped into implants for anterior fusion of the cervical spine. This study was designed to develop an artificial vertebra system using the novel implant. Materials and Methods: A novel biomaterial, hydroxyapatite/type I collagen (HAp/Col) composite in which the c-axes of HAp nanocrystals were aligned along the collagen molecules, was prepared under a cold isostatic pressure of 200 MPa. The 3-point bending strength of the composite was found to be 39.5± 0.88 MPa, and the Young's modulus 2.5± 0.38 Gpa. The composite materials were cut in cylindrical implants of 5x5x10 mm. In order to prevent dislocation of an implant, PLLA plates were prepared (7x15x2mm, 3-point bending strength of 120MPa) having a screw hole at both ends. Anterior fusion at the C2-C3 or C3-C4 vertebrae was carried out on 4 beagle dogs with the implants containing rhBMP-2 (400 μg/ml), and 8 dogs with the implants containing vehicle only. In 2 dogs of the rhBMP(+) group and 5 dogs of the rhBMP(-) group the implant was fixed with a PLLA plate and 2 titanium screws. Periosteum of the anterior vertebrae was completely removed. X-ray images were prepared weekly after operation. Radiographic analysis of bone union was performed, and to evaluate disc space collapse, disk space height measured immediately after operation and at the time when the implants were removed. The implants were taken out from 2 dogs in the rhBMP(+):plate(-) group after 13 weeks and 2 dogs in the rhBMP(+):plate(+) group after 24 weeks. The implants were taken out from the rhBMP(-):plate(+) group after 13 weeks (2 dogs) and 24 weeks (3 dogs), and 3 dogs in the rhBMP(-):plate(-) group after 13 weeks. The specimens were prepared for light microscopic (LM) observation with HE, toluidine blue and Villanueva stain, and for transmission electron microscopic (TEM) observation. To compare the ability of bone conduction and to evaluate stability of the implants, the remaining gap between the newly formed bone bridging superior and inferior ends of vertebrae adjacent to the implant was determined using LM-photographs. Results: In the non-rhBMP-treated group, the interface between the implants and recipient bone became unclear after 12 weeks surgery, although callus formation was poorly visible on the X-ray images. After this period, a faint bone-bridging-like shadow could be observed. Displacement of the implant did not occur in both the plate-fixed and non-plate-fixed groups throughout the experimental period. In the rhBMP-treated group, callus formation appeared already after 2 weeks, while a thick new bone coverage adjacent to the implant, i.e. anterior bone bridging, could be observed after 10-12 weeks. Absorption of the newly formed bone did not occur; rather bone maturation continued until 24 weeks after operation. The decrease in disk space height was smaller in the group with plate-fixation, as compared to the non-platefixation group. The decrease was smallest in the rhBMP-treated group and independent of plate-fixation. Histological findings revealed that the larger parts of the composite materials were absorbed as early as 13 weeks and transformation into bone started from the site near the recipient bone, with non-calcified soft tissue remaining extensively in the disk space. Howship's lacunae-like structures were formed at the bone site, and cartilage column formation observed in the interface between the implant and newly formed bone. The maturation process of the cells advanced from the implant site (immature chondrocytes) towards the recipient site (mature chondrocytes). The PLLA plates showed no degradation until 24 weeks after surgery. In the rhBMP-treated group newly formed bone elongated from both superior and inferior ends of the recipient bone after 13 weeks, and thick new bone coverage adjacent to the implant, i.e. completed anterior bone bridging, was observed after 24 weeks. Cartilage column formation was found more prominent in the rhBMP-treated group as compared to the non-rhBMP-treated group. There was no difference in gap length between the PLLA plate-fixed and non-plate-fixed groups, although the remaining gap length in the rhBMP-treated group was significantly shorter as compared to the non-rhBMP-treated group (independent of plate-fixation).