|Summary||Among the various techniques to reconstruct or enlarge a deficient alveolar ridge, the concept of guided bone regeneration (GBR) has become a predictable and well-documented surgical approach. At present, autogenous bone grafts are preferably combined with barrier membranes. Using synthetic biodegradable bone substitute materials, however, is advantageous, since it avoids second-site surgery for autograft harvesting. A bone substitute for alveolar ridge augmentation must be rapidly resorbable and should undergo complete substitution by newly formed functional bone tissue in view of placing dental implants in such augmented sites. Compared to the bone substitutes which are currently clinically available, there is a significant need for bone substitutes which degrade more rapidly, but still stimulate osteogenesis at the same time. This has led to the development of novel, bioactive, rapidly resorbable glassy crystalline calcium-alkali-orthophosphate materials.
This in vivo study evaluates the effect of two novel bioactive particulate calcium-alkali-phosphate bone substitute materials as compared to the currently clinically used material β-tricalcium phosphate (β-TCP) on bone regeneration and expression of osteogenic markers after implantation in the sheep mandible. This was in addition to examining the biodegradability. Test materials were two glassy crystalline calcium-alkali-orthophosphates: first, a material with a crystalline phase Ca2KNa(PO4)2 and with a small amorphous portion containing magnesium potassium phosphate (material denominated GB14) and second, a material with a novel crystalline phase Ca10[K/Na](PO4)7 (material denominated 352i). These materials (grain size 300-350 µm) were implanted in the sheep mandible for 1, 4 , 12 and 24 weeks to regenerate membrane protected critical size defects and were compared to β-TCP particles of the same grain size as well as autogenous bone chips. Empty defects, in which collagen sponges were utilized for stabilization of the blood clot, served as control. In this study a total of 34 sheep were used. In order to generate chronicle defects, the premolars and one molar were extracted bilaterally in the sheep mandible and critical-size defects 14 mm in width and 1 cm in height were created. After 8 weeks a reentry was performed, and the defects were augmented with the various bone grafting materials and covered with a titanium membrane.
At implant retrieval the tissue samples were fixed in an alcohol based fixative. The specimens were then embedded in a resin which facilitated performing immunohisto-chemical analysis on hard tissue sections. 50 µm-sections were cut in a buccal - lingual direction using a Leitz 1600 sawing microtome. Sections were then deacrylized and immunohistochemical staining was performed using primary antibodies specific to collagen type I, alkaline phosphatase, osteocalcin, bone sialoprotein, osteopontin and osteonectin in combination with the DAKO EnVision+TM Dual link System Peroxidase. Mayer's haematoxylin was used as a counterstain. Semi-quantitative analysis of the sections was performed. A scoring system quantified the amount of staining observed using light microscopy. A score of (+++), (++) and (+) corresponded to strong, moderate or mild, whereas a score of (0) correlated with no staining. Furthermore, histomorphometrical evaluation of the sections was performed. The bone area fraction, the particle area fraction as well as the bone-particle contact and the osteoid-particle contact was measured using a light microscope in combination with a digital camera and SIS Analysis software (Olympus, Germany). Additional specimens were prepared to scanning electron microscopy.
Of the various grafting materials studied, GB14 had the greatest stimulatory effect on bone formation and expression of osteogenic markers and displayed the best bone bonding behaviour, while exhibiting the highest biodegradability. This was true for all time points studied. At 24 weeks, GB 14 even facilitated greater bone formation (82.3%) than the autogenous bone chips (75.6%), which is a remarkable result, since autogenous bone is generally regarded as the gold standard. The material 352i showed poor bone bonding behaviour and the lowest biodegradability thereby yielding a disadvantageous result. The TCP particles displayed a lower biodegradability than the GB14 particles. Both bone bonding behaviour and biodegrability increased at the later time points. Overall, TCP displayed good osteoconductivity and facilitated regeneration of the critical-size defects, which is in agreement with other studies reported in the literature.
The diagram displaying the results for the osteogenic marker expression shows that the curve for GB 14 has a similar shape compared to that for autogenous bone. Hence, it can be concluded that the process of new bone formation with GB14 is comparable to that with autogenous bone chips with respect to molecular mechanisms and temporal pattern, i.e. time-dependent course of the respective diagram. Therefore, GB14 can be regarded as a promising bone substitute material for pre-implantology alveolar ridge augmentation. In order to take this bone grafting material to the clinical arena, controlled clinical studies need to be performed. Application for CE approval is currenty underway.