particles. Good attachment of the CaP coating could be expected from the evaluation of the interface strength measurements [56–58]. The authors did not report on osteoclastic resorption of the coating. Moreover, they considered the long-term stability of the calcium phosphate coating as a minor problem because the implant stability was maintained by bone ingrowth into the porous layer. The primary purpose of the coating was to provide an early boost in the initial healing response (i.e., to promote osteoconductivity).
Another study [59] with 0.1, 1.0, and 4.0 mm thick radiofrequency (RF) magnetron-sputtered CaP coatings on roughened titanium implants described the disappearance of the CaP thin coatings depending on the thickness—the 0.1 and 1.0 mm thick coatings having been dissolved after 1 week and the presence of other phases after a short 30 seconds, with heat treatment of 4258 to 4758C with crystalline and amorphous phases and OCP. The 4 mm thick coatings showed apatite and amorphous phases after heat treatment. The latter coatings were still found partially after 8 and 12 weeks. The surroundings of the implanted disks in the back of rabbits displayed an inflammatory response after 1 week, with a decline in the number of inflammatory cells and develop-ment of a fibrous capsule after 8 and 12 weeks [59]. How the coating was lost was not mentioned, and especially there is no information on the question of chemical dissolution of the CaP coating or the interference with resorbing cells (i.e., osteoclasts or mineraloclasts). The mechanism of mineral absorption or resorption connected with the activity of cells was described for hot isostatic pressed HA that was implanted into the femur of rabbits [60].
The incorporation of silicate into HA has been shown [54] to increase significantly the rate of bone apposition to HA bioceramic implants. This was proven in an experiment in which 0.8 or 1.5 wt% Si-HA particles were implanted into the femoral condyles of 2- to 3-year-old Texcel X Continental sheep and explanted after 6 and 12 weeks. Controls did not contain Si in the HA. TEM revealed focal dissolution of the material in the order of 1.5 wt%
SiHA > 0.8 wt% Si-HA > pure HA , suggesting that silicate ions increase the solubility of the HA. This increased solubility and the corresponding defects increase the apposition of bone to these bioceramics. It seems that this principle has not yet been introduced to new thin CaP coatings.
Pure titanium rods were coated 2 mm thick with HA in an electrochemical process [61]. They were implanted in the femur medullary cavity of Sprague-Dawley rats and biomechanically tested versus uncoated titanium wire [61]. The coated rods revealed a more than threefold increase in shear strength. This corresponded in some respect to the 2.6-fold increase of bone–implant contact.
The follow-up intervals of the experiment were only 7, 28, and 56 days after implantation. Therefore, the long-term results of the thin coating, thickness 2 mm, are still awaited, considering this experiment.
There are arguments in favor of thin CaP coatings based on an experiment in which pulsed laser deposition (PLD) of HA was compared with plasma spray-ing (PS) [62]. The titanium rods were grit-blasted or additionally HA coated using PLD or PS. The rods were implanted in the tibias of rabbits and harvested
24 weeks after implantation with the surrounding bone. HA-PLD showed the highest percentage of bone contact and bone linear density and the lowest lacunae contact and lacunae linear density. This result speaks for the usage of the HA-PLD technique for thin CaP coatings.
It can be expected that with more time the coating is better absorbed and that a more or less uncoated metallic implant subsists. The advantage of thin coating would be early bone apposition or bonding to the metal surface. At a later stage, when the coating has been absorbed, the positive effect would be minimal if not zero.
Although the following investigation [63] did not use a thin coating (the coating was 50–75 mm), it is mentioned here because it describes the influence of crystallinity and solubility on bone bonding and soft tissue at the interface. The different crystallinity (75% vs. 95%) of plasma-sprayed HA coatings at the surface of dental implant cylinders (4 mm diameter, 10 mm length) was investi-gated in the mandible of beagle dogs after 3 and 15 weeks [63]. The crystallinity of the plasma-sprayed coating on titanium alloy was increased by a hydrothermal treatment of 75% to 95%. Statistically, there was no difference between the two coatings regarding the pullout force at failure after 3 and 15 weeks after implan-tation. The extent of bone apposition was also statistically not different between the two coatings, being approximately 40% after 3 weeks and 90% after 15 weeks. Histology showed in both types of coatings either bone apposition with bonding or soft tissue with some macrophages. Phagocytosed HA particles could be identified in some macrophages. This was interpreted as proof of in vivo degradation of the coating. This fact implies that after a prolonged time the coating should have disappeared irrespective of the thickness.
In another comparative study, rectangular Ti6Al4V implants with square cross sections and an oblique diameter of 4.6 mm were also inserted in the mandible of beagle dogs. They were either (1) polished or (2) coated with collagen; (3) coated with mineralized collagen; (4) coated with collagen and HA; or (5) coated with HA using a special procedure in which there was a change between electrochemical deposition of HA and the growing of an anodic oxide layer [64]. The thickness of the coatings was < 5 mm. After 1 and 3 months, the implants were retrieved and analyzed mainly using morphometric methods.
The best result concerning the development of bone in the cortical and cancellous bone areas was obtained with the pure HA coating (type 5), and the least result was observed with pure Ti6Al4V. Bone began to develop at the corners of the rectangular implants in the round bore hole; then the planes between the corners were covered by bone; and finally the rest of the hole filled up with bone and Havers’ systems. This pattern of bone development demonstrates the influence of the implantation bed for the outcome of bone healing. In other words, the bridging of the gap between the bore hole and the implant surface takes time;
and the smaller the gap, the shorter is the time for bone growth to achieve fixation of the implant. There is another important result of this experiment. It can be postulated that the introduction of collagen (type 2), mineralized collagen (type 3), and collagen and HA (type 4) inhibited bone development. Apparently,
the tissue reaction did not profit from the collagen that was introduced into the coating of the implant. This should be a warning for all those investigators who intend to increase bone healing by adding substances perhaps at the wrong time, at the wrong place, or with the wrong concentration.
The next experiment deals with the surface preparation and coating of ceria stabilized tetragonal zirconia alumina ceramics (Ce-TZP) [65]. The surface of the 15 10 2 mm plates was (1) uncoated and polished; (2) acid-treated and heated to corrode the Ce-TZP particles to create a microporous surface struc-ture; (3) coated with submicron thick HA; (4) coated with 4 mm thick HA using a biomimetic procedure with simulated body fluid (SBF). There were clear morphological differences in the structure of the four dissimilarly prepared surfaces. The plates were implanted in the proximal tibia of rabbits and exam-ined after 4, 8, and 16 weeks postoperatively. The detachment test provided best results for surfaces of groups (3) and (4)—i.e., failure loads around 25 N. Back-scattered SEM showed bone bonding to the implant surfaces. At 16 weeks, remains of the coating could not be detected by high resolution back-scattered SEM in group (4). The intended function of the thin coating was to provide early fixation and bonding of mineralized tissue to the surface of the implant.
Plasmapore1-coated Ti6Al4V cylinders were additionally coated by orga-noapatite using a wet chemical technique to cover the pore surfaces. The procedures for organoapatite coating of the cylinders were described in detail by Hwang et al. [66]. The objectives of the study were to find out whether thin coating with organoapatite improves bone development at porous Ti6Al4V cylinders. Controls were implant cylinders without organoapatite coating.
Following DIN 47868, the maximum surface roughness RTwas 109 mm (SD 8.1 mm), and the thickness of the porous structure was 75 mm. The thickness of the organoapatite coating was in the order of 250 A˚ [66]. The surface structure of the coated implant cylinder is depicted in Fig. 4.8. The cylinders
Fig. 4.8 Plasmapore1-coated and additionally organoapatite-coated titanium cylinder. SEM after sterilization and before implantation. Roughness according to DIN 4768 Rt 109 mm (SD 8.1 mm)
(length 8 mm, diameter 4 mm) with organoapatite coated or uncoated surfaces were implanted into the distal femur epiphysis of Chinchilla rabbits for 7, 28, and 84 days. Figure 4.9a,b shows the implantation site in the distal femur epiphysis of Chinchilla rabbits. Bone trabecules in the area were oriented as in Fig. 4.9c. The histology of undecalcified sawn sections, prepared as in [2], reveals fibrous tissue with some macrophages near the surface of the implants 7 days after implantation (Fig. 4.10). However, after 28 and 84 days, trabecular bone had developed up to the porous, organoapatite coated or uncoated circumference of the cylinders (Figs. 4.11, 4.12). Interestingly, the flat nonpor-ous uncoated dorsal plane of the cylinders was covered with a plate of bone (Fig. 4.11b) and not with trabecular bone as at the circumference of the cylinder (Figs. 4.11a,b, 12). Morphometry of the tissues at the implant interface (e.g., bone, osteoid, chondroid) and soft tissue allow more detailed interpretation of the findings. Most important is the increased percentage of bone at the circum-ference of organoapatite-coated implants versus uncoated implants after Fig. 4.9 a Implantation site in the distal femur epiphysis of a Chinchilla rabbit below the patella sliding plane with an implant in a radiographic view. b Freshly cut surface in the frontal direction through the implantation site with an implant in situ. There is trabecular bone below the joint cartilage with fat and hematopoietic tissue in red in the metaphysis of the femur. c Undecalcified, formaldehyde-fixed, methylmethacrylate-embedded, sawn section of the distal femur of a Chinchilla rabbit stained with von Kossa & Fuchsin to demonstrate the mineralized trabecular bone structure and bone marrow as well as cartilage and ligament (See Color Insert)
Fig. 4.10 a Plasmapore1 and organoapatite-coated implant 7 days after insertion into the distal femur epiphysis of a Chinchilla rabbit with young fibrous tissue and a few mononuclear cells and macrophages (M). Sawn section. Von Kossa & Paragon stain. Bar = 100 mm.
b Macrophages at the nanometer-thin organoapatite coating on top of the Plasmapore1 titanium coating 7 days after implantation. Sawn section. Von Kossa & Paragon stain. Bar = 50 mm (See Color Insert)
Fig. 4.11 a Attachment of one bone trabecule to the organoapatite and Plasmapore1-coated periphery of the implant cylinder 84 days after implantation. In the surrounding area is a thin fibrous layer with only a few macrophages and fibroblasts and bone marrow. Sawn section.
Von Kossa & Paragon stain. Bar = 200 mm. b Trabecular bone attaches to the Plasmapore1 and organoapatite-coated periphery of the implant cylinder but not at the flat, uncoated dorsal part of the cylinder (arrow) 84 days after implantation in the distal femur epiphysis.
Sawn section. Von Kossa & Paragon stain. Bar = 1000 mm (See Color Insert)
28 days (Table 4.3). After 84 days there is no difference between bone attached to coated or uncoated cylinders. We assume that this outcome can be connected with the degradation and absorption of the coating. The result demonstrates a short-term benefit for bone attachment to an implant surface covered by a thin organoapatite coating. Comparable results were described for thick coatings with OCP and biphasic calcium phosphate 12 weeks after implantation into the femurs of goats [67].