Anke Bernstein

Nanocrystalline hydroxyapatite for bone repair: an animal study

Hydroxyapatite has become the most common material to replace bone or to guide its regeneration. Nanocrystalline hydroxyapatite suspension had been introduced in the clinical use recently under the assumption that small dimension of crystals could improve resorption. We studied the resorption and osteointegration of the nanocrystalline hydroxyapatite Ostim® in a rabbit model. The material was implanted either alone or in combination with autogenic or allogenic bone into distal rabbit femora. After survival time of 2, 4, 6, 8 and 12xa0weeks the implants had been evaluated by light and electron microscopy. We observed a direct bone contact as well as inclusion into soft tissue. But we could observe no or only marginal decay and no remarkable resorption in the vast majority of implants. In situ the nanocrystalline material mostly formed densely packed agglomerates which were preserved once included in bone or connective tissue. A serious side effect was the initiation of osteolysis in the femora far from the implantation site causing extended defects in the cortical bone.

Can bone healing in distraction osteogenesis be accelerated by local application of IGF-1 and TGF-β1?

Because complications of distraction osteogenesis are largely related to the long duration of therapy, increasing efforts were reached to shorten treatment by using osteoconductive replacement materials incorporating bioactive molecules such as IGF-1 and TGF-beta1. The controlled release of IGF-1 and TGF-beta1 from coated biodegradable poly(D,L-lactide) implants could stimulate fracture healing locally. We investigated the effect of locally applied IGF-1 and TGF-beta1 from IGF-1/TGF-beta1-enriched polylactide membranes on fracture healing in a sheep model of delayed callus formation. Twenty-eight sheep were used for this study. Callus distraction of 1 mm/day by means of a unilateral fixator was continued for 30 days. At the beginning of the subsequent consolidation phase, either growth factors were applied locally or the defect was packed with cancellous bone, or both. The groups treated with growth factors were compared to a control group. The consolidation phase lasted for 60 days and both tibiae were dissected for histological and histomorphometric analyses. This investigation found a reduced absolute callus area in the lengthening zone in all treatment groups. The two treatment groups that received a membrane coated with growth factors showed distinctly higher relative bone areas than the groups treated with an uncoated membrane or packing of the osteotomy defect with cancellous bone. The differences in bone areas were not statistically significant. Application of the growth factors accelerated bone healing and achieved results comparable with those of established treatment methods (packing with autologous cancellous bone). The best results were achieved with a combination of both methods.

Microporous Pure β–Tricalcium Phosphate Implants for Press-Fit Fixation of Anterior Cruciate Ligament Grafts: Strength and Healing in a Sheep Model

PURPOSEnA sheep study was conducted to test a press-fit technique using microporous pure beta-tricalcium phosphate (beta-TCP) dowels for fixation of the anterior cruciate ligament (ACL) graft.nnnMETHODSnMicroporous (5 mum) cylindrical plugs of beta-TCP (diameter, 7 mm; length, 25 mm) with interconnecting pores were used. The material featured a novel configuration of structure and surface geometry. Implants were tested by use of press-fit fixation of ACL grafts with and without bone blocks in 42 sheep over a period of 24 weeks. Biomechanical, radiologic, histologic, and immunohistochemical evaluations were performed.nnnRESULTSnIn load-to-failure tests at 6, 12, and 24 weeks after surgery, the intra-articular graft always failed, not the fixation. Grafts showed bony fixation in the tunnel at 6 weeks and primary healing at the junction of the tunnel and joint after 24 weeks. Tricalcium phosphate was resorbed and simultaneously replaced by bone. Remodeling was still incomplete at 24 weeks.nnnCONCLUSIONSnIn the sheep model microporous beta-TCP implants used with press-fit fixation of ACL grafts permit early functional rehabilitation. After 6 weeks, the graft is fixed by woven bone or bony integration. Implanted microporous tricalcium phosphate is resorbed and replaced by bone.nnnCLINICAL RELEVANCEnIn a sheep model we showed that primary healing of ACL grafts with resorption and bony replacement of the fixating implant can be achieved by means of press-fit fixation with pure beta-TCP.

Rehabilitation results following anterior cruciate ligament reconstruction using a hard brace compared to a fluid-filled soft brace.

The purpose of this study was to compare the clinical outcomes of rehabilitation after ACL reconstruction using a water-filled soft brace to those using a hard brace. The method used in this study was a prospective randomised clinical trial including 36 patients wearing a hard brace and 37 patients wearing a water-filled soft brace for 6 weeks after surgery. Preoperative and postoperative (seven examinations) clinical evaluation within a follow-up period of 1 year including effusion status, swelling and range of motion (ROM), IKDC 2000, KT1000 Arthrometer, Lysholm knee scoring scale and Tegner activity score. Mean values are presented with standard deviations. Data was analysed using descriptive statistics and Students t-test for unpaired samples. Significantly less effusion was found in the soft brace group from 5 days (p=0.002) to 12 weeks (p<0.024) postoperative. Hard brace patients presented with significantly more extension deficit from 5 days (p=0.036) to 12 months (p=0.014) postoperative but no significant difference was detected in complete ROM, laxity or thigh atrophy at any follow-up examination. Patients treated with a soft brace had significantly higher IKDC subjective ratings at 6 weeks (p=0.02) up to 12 months after operation (p=0.002) and rated significantly higher in Tegner activity score (p=0.004) and Lysholm knee scoring scale (p=0.006) 6 and 12 months (p<0.001 for both scores) postoperatively. The water-filled soft brace was superior regarding effusion, swelling, extension deficit and patient-measured midterm outcome. The soft brace presents a safe, easy-to-use and effective alternative to the hard brace.

Nanocristalline Hydroxyapatite for Bone Repair

The substitution of bone defects is of the main problems in contemporary orthopaedic surgery. The progress of skeletal surgery has opened new horizons of joint replacement, trauma management and skeletal repair in bone tumors. But loosening of joint protheses, tumor resection and reconstruction of traumatized bone needs to handle large size defects in the skeleton. Because bone grafts are limited for a number of reasons such as availability, risk of different viral infections, problems of storage or cost-intensive procedure of manufacturing, still remained a lack of adequate substitute materials to fill the bone defects. Bioactive ceramics became interesting and widely used as substitute materials for bone defects. An optimal substitute should replace the lacking tissue for a limited period, should be replaced step by step by new bone and should disappear without any remnants after doing its task. It has to be biocompatible, sterilizable, non mutagenic and non cancerogenic. However, the compact bioceramic implants used today mostly outlast in the bone. Their degradation behaviour in vivo is discussed controversially. It depends on a variety of characteristics such as sintering parameters, chemical composition, crystallinity and phase purity. Nanocrystalline hydroxyapatite had been described in the literature as interesting alternative to common compact or spongeous ceramic materials [1,2]. The present work describes the results of testing nanocrystalline hydroxyapatite in vivo in a rabbit animal model.

Inhibition of Mineralization by a Calcium Zirconium Phosphate Coating

Bioactive ceramics used as coating materials combine the conductive properties of a bioceramic with the mechanical stability of the metal implant. We studied a calcium zirconium phosphate-containing coating material, FA-CZP [Ca(5)(PO(4))(3)F, CaZr(4)(PO(4))(6)], that is relatively insoluble in the biological milieu. The reaction of bone to this material was investigated histologically and histomorphometrically in an animal trial. Cylindrical Ti6Al4V specimens that had been coated with FA-CZP by plasma spraying were implanted in the femoral condyles of rabbits. The implants were left in place for 2, 4, 6, 12, and 14 weeks. FA-CZP led to impaired mineralization of the newly formed bone at the interface. Noncalcified osteoid was found throughout the whole study period. The layer seemed to become thicker with time. The mineralization disorder is evidently caused by zirconium ions. The presence of zirconium in the osteoid in contact with the implant was demonstrated by means of two different staining methods.

In-Vitro Surface Reactions of Bioceramic Materials

Bioceramics used as coatings show different biocompatibility and bioactive behaviour in relation to their chemical and morphological composition. Hydroxyapatite of low crystallinity can be detected by XRD on the surface of different plasma sprayed bioceramic materials after treatment in SBF solution. Solubility and cell adhesion tests as well as test of toxicity on discs, however, did not show similar effects. Especially the crystallographic composition and also surface morphology of the material determined the release of ions into the solutions which could be responsible for cytotoxic reactions as well as for the ability of the hydroxyapatite formation. However, the reaction mechanisms which lead to a formation of apatite layers seems to be of different nature. Moreover, there is also the question if such an apatite formation in vitro can indicate the material`s ability to favour osteogeneses.

Biodegradation of Different Calcium Phosphate Coatings

Introduction Calcium phosphate ceramic coatings are bioactive coatings that have been shown to conduct bone apposition by direct bone bonding. The cell and tissue level mechanisms that underlie the osteoconductivity are not known. The biodegradation of calcium phosphate biomaterials is assumed to take place by solution – drive and cell – mediated processes ([1-4]). The factors concerning the biodegradation have not been completely elucidated. The chemical composition, physical characteristics and crystal structures play an important role in the biological behaviour of calcium phosphates. In addition, the biodegradation may be influenced by the experimental conditions: experimental models, implantation sites, and animal species. The physical factors include form (particulate or bulk), porosity, surface area, and crystallinity (crystal size, crystal perfection grain size). Factors tending to increase rate or extent of biodegradation include increases in porosity, reductions in crystal size, increase in number of crystal imperfections, and decreases in grain size. The chemical factors depend on composition and ionic substitution in materials. The biological factors include pH drops due to cell mediated factors, osteolytic infection and disease, degree of bone contact, type of bone, animal species, age sex, hormone levels, and genetic predisposition. The Ca-P coatings are readily resorbed in a physiological environment in contrast to hydroxyapatite ceramic coatings. This in vitro study using rabbits as the animal model determined the degradation rate of the ceramic by histomorphometry and analyzed the bony reaction to ceramic coated pins.

Hydroxyapatite Coating Improves Bone Integration and Interface Strength of Polymer Implants in Bone

Many attempts had been made to improve the durability of artificial joint replacement and other orthopaedic implants by approaching the mechanical properties of bone and artificial material. The most joint prostheses used today are manufactured of metal alloys based on cobalt, chromium or titanium. The mechanical stiffness of these materials is much higher than that of natural bone resulting in adverse effects such as local overloading on one hand or stress shielding phenomena with the lack of adequate mechanical load on the other. Both mechanisms contribute to earl loosening and failure of implants. Polymer materials may deliver mechanical properties very similar to bone and their mechanical behaviour may be modified in a wide range during the process of manufacturing. First attempts to lower the stiffness of the implant material and to gain the stiffness range of natural bone were made in the seventies by R. Matthys with his concept of “isoelastic hip prosthesis”. In this prosthesis the femoral stem was manufactured of polyacetal, a thermoplastic polymer with very good biocompatibility and elastic properties which are much nearer to bone than common metal alloys. While the prosthesis showed good results during the mechanical testing the clinical use in vivo became a disaster. Shortly after implantation polyacetal was degraded in the body and broke down under the immense loading of the human hip joint. Later attempts to use polymer materials alone for load bearing implants also failed in clinical practice over a long time because the mechanical interlocking between bone and implant was not sufficient for the biological demand. To make the outstanding properties of polymer materials useable for load bearing implants they are backed with metal alloys (as polyethylene for hip joint cups) until the presence. Only recent developments of polymer science succeeded in the use of polymers for loaded implants. One of the most interesting materials seems to be the polyetheretherketone (PEEK) which is successfully used for spinal fusion cages [2] and computerdesigned individual implants for defect reconstruction in the skull [4] meanwhile. A pre-clinical study of a new anatomically shaped flexible acetabular cup reported satisfactory results recently [3].

In Vitro and In Vivo Behaviour of a Ceramic with Interconnected Macroporosity

Bioactive ceramics such as β-tricalcium phosphate (β-TCP) promote and enhance biological fixation. Ceramics with a porous interconnected structure are suited for facilitation of bony ingrowth. An interconnected pore system with pore diameters in excess of 100 µm is required for cell penetration, tissue ingrowth, vascularization and nutrient delivery to the centre of the regenerating tissue. Human osteoblasts were cultured on the surface of a ceramic. In an in-vivo study, β-TCP samples with a porous interconnected structure were implanted into the femur of sheep and then investigated 6 weeks after operation. Histological analysis was performed on the area surrounding the implant. An indentation test was performed to complete failure of the bone/ceramic compound. Linear load, peak load and stiffness were recorded. All cylinders were found to be biocompatible and osteoconductive. Bone was more abundant in the outer ring than in the rest of the cylinder. The ceramic/bone compound was of low mechanical grade.