Introduction: Rabbits are a well-established animal model for orthopaedic research

and the tibia is commonly used for investigations of fracture repair with different implant materials

Therefore, a new method for the direct measurement of forces in the rabbit tibia was developed. The aim of this study was to determine maximal forces during weight bearing in the rabbit for future implementation into FEM-simulation.

Animals and Methods: An external ring fixation was attached to the left tibiae of 5 rabbits and an ostectomy followed. Force sensors were included into the collateral rods to incur the emerging forces completely. On each side, a measurement amplifier was applied to transfer the collected data telemetrically. During the study, the animals were weighted and x-rays were taken regularly. Measurements started 8 days postoperatively and were repeated 8 times until day 50 post-op. The rabbits were placed in a run and animated to move while the forces were registered. Force peaks were filtered from the collected data of each measurement as absolute values and relative to the animals’ weight (force-weight ratio/FWR).

Results: All included animals tolerated the external fixa-tion well and no clinical intolerances occurred. Beginning of callus formation was detected radiographically about 3 weeks post-op and all fixations could be removed 12–14 weeks after application without any permanent detriments. The maximal force amounted to 6950 g and 172 % FWR in animal 4 during the first recording. Means of the 5 maximal values for each measurement were located between 55 % FWR and 152 % FWR for the first measurement, converged to approx. 80 % FWR during the second recording 3 days later and descended to 20–40 % FWR until the end of the experiment.

Discussion: Aim of this study was to determine maximal forces during weight bearing in a rabbit model. Our model for in-vivo monitoring of these forces was practicable and provided profound data. The highest values occurred during the first or second recording. That coincides with the radiographic detection of callus after 3 weeks. Therefore, reliable measurements have to be carried out during the first 2 weeks postoperatively. Detected values show that the rabbit tibia is strained with up to 170 % of the body weight, which is the compressive force an implant in a weight bearing bone has to be able to bear. Future research will focus on the in-vivo monitoring of bending and torsion forces and the implementation of these data into FEM-simulation.

Although osteochondral grafting techniques have nearly been perfected, donor site morbidity still causes concern. A synthetic β-tricalcium phospate cement was used in the attempt to obtain a primary closure of such osteochondral defects, while supplying a scaffold for tissue ingrowth.

Twenty merino sheep underwent an osteochondral grafting procedure. The paste-like β-TCP cement was used to fill the ensuing cylindrical, full-thickness defect. Animals were sacrificed after 3 or 6 months.

The macroscopic observations revealed neither osteophytes nor synovial proliferation, while demonstrating coverage of the defect with cartilage-like tissue. After 6 months, all defects were covered with a ”neo-cartilage” and the congruity of the joint surface was restored in 6 of 10 animals. A surface depression was found in the remaining cases. A demarkation of the defect border at the interface with the original cartilage could only be seen in 2 instances. The x-rays of the retrieved distal femurs revealed only traces of the dense β-TCP particles. Microradiographs demonstrated the incorporation of the implant. Fluorescent staining showed continuous bone ingrowth. Histologically, masses of unabsorbed TCP were irregularly distributed through-out the defect. Newly formed bone had filled much of the defect. The histological evaluation confirmed that the surface of the cement was covered with a cartilage-like tissue.

This study showed, that the newly developed in-situ self-hardening resorbable β-tricalcium phosphate cement is easy to handle, hardens in a clinical-type setting, is bioactive and resorbable. Its osteoconductive effect lead to a restoration of biomechanically stable bone and allows for a normal remodeling process. Biomaterials made of β-TCP promise to play a role as a biodegradable scaffold, allowing osteo-blast ingrowth and cartilagenous resurfacing, while being fully resorbed during the process. The cement may also be used to deliver bioactive agents and cells for defect repair in the near future.

Among the wide variety of bone substitutes presently available, pure β-tricalcium phosphate ceramics have become available (Biosorb®; Aesculap, Tuttlingen). During the first 12 months of a prospective clinical trial, Biosorb® products were implanted in 21 patients. The ceramics were used in a variety of clinical settings, ranging from pelvic osteotomies in children (n=9), to filling of bone cysts or osseous defects (n=4), to dorsal spondylodesis (n=6), as well as for the grafting of pseudarthroses (n=2). Average follow-up period was 13 (6–18) months.

The β-TCP granules, when used as part of a composite graft in combination with autologous bone, were completely resorbed after an average period of 14 weeks, while the cubes required 12 to 15 months. The more massive wedges have shown only a decrease in size and radio density. Due to the ability of the cubes and wedges to bear loads of up to 30 MPa, they were successfully implanted during pelvic osteotomies to augment or completely replace the bicortical grafts. Complications or foreign body reactions were not noted. The osseointegration was found to be favorable for all forms.

In light of the problems associated with autologous and allogeneic grafts, the use of synthetic bone substitutes will continue to increase. The combination of complete resorption, lack of risk of infection, and load sharing ability make the β-tricalcium phosphate implants a valuable addition to the spectrum of bone replacement products presently available. Their use in pediatric orthopedics could help avoid donor site morbidity including contour changes or growth disturbances, while providing a more stable graft. During the first phase of a prospective clinical trial, we have come to the conclusion, that the β-tricalcium phosphate ceramics represent a real alternative to other bone substitutes.