Electron beam melting is a relatively new technique capable of producing complex 3-dimensional geometries [21
]. With this technique many possible surface characteristics can be engineered thus aiding in the development of an optimal surface structure for bone ingrowth for cementless prostheses. In this study two controls were selected; a titanium plasma sprayed and a sandblasted specimen. The titanium plasma spray coating was selected as this coating is already widely used on femoral stems. Coated stems produce good medium- to long-term survival rates [23
] and they perform better than uncoated cementless stems [26
]. The plain sandblasted control was chosen to determine the effect of the E-beam technology itself.
Some limitations of the present study should be acknowledged. First of all, the frictional properties in contact with cortical bone were determined, whereas a prosthetic surface is also in contact with trabecular bone. The friction coefficient of a metal surface on cortical bone is generally lower than the same specimen on trabecular bone [27
]. Therefore, the reported friction values are probably in the lower range of frictional values that occur around a hip prosthesis. Secondly, no chemical surface analysis was performed. Thirdly, the results for bone ingrowth showed a large variability, partly due to the rather large tolerance of the E-beam technology and differences in implantation location. Although not statistically significant it is likely that in between goat differences, overall biological variability and unavoidable differences in surgical precision (i.e. the quality of the press fit implantation) will affect this.
In this study the frictional properties of the surfaces were determined as these values have a direct effect on the stability potential of the metal surface relative to the bone surface. Obviously, the friction is influenced by the roughness of the metal structure and we considered quantifying the roughness of the investigated surfaces. However, it appeared to be impossible to define the roughness of the new surface structures accurately, because of the three dimensional character of the new surface structures.
The friction coefficients of the new E-beam specimens were significantly higher compared to the sandblasted specimens and are in the same range of the titanium plasma sprayed control. Since implants with a titanium plasma sprayed surface have a high survival rate (indicating sufficient initial stability) [23
], it can be deduced that the two new surface structures can also provide sufficient initial stability (albeit that this will also largely depend on prosthetic shape). Several other studies have reported the frictional properties of coatings for orthopedic implants (Table ) [9
]. Although the values of the friction coefficient may depend to some extent on the testing method which vary amongst the different studies, comparison to our results indicate that the new surface structures result in relatively high friction coefficients.
Frictional properties of coatings for orthopedic implants
In this study three different methods were used to quantify bone ingrowth; all have advantages and disadvantages with regard to the interpretation of the results. For example, bone ingrowth depth is restricted by pore depth and measuring direct bone–implant contact is the only method which enables comparison of porous and solid specimens.
The amount of direct bone–implant contact of the E-beam produced surface structures appeared to be comparable to the titanium plasma sprayed control. Hence, the new surface structures have the potential to be successful surface structures for orthopedic implants.
The cubic structure showed greater bone ingrowth depth compared to the wave structure. On the contrary, the wave structure showed better results for bone area percentage. This difference in outcome can be explained by differences in the structure of the coating. The cubic specimen has a high porosity and large pores deep inside the core material, which were too deep for the bone to reach in the limited postoperative time (6 weeks) of the study. Consequently bone area percentage of the cubic specimen was less than the wave specimen.
Furthermore, the importance of pore size and porosity influencing bone ingrowth is supported by the differences in bone ingrowth between the 3-dimensional surface structures and the rough E-beam control. Although made of the same material and manufactured using the same methods, the bone ingrowth of the new surface structures (large pores, high porosity) was greater compared to the sandblasted (plain) E-beam control.
Although it is clear that pore size affects bone ingrowth, the optimal pore size has yet to be determined. Bobyn et al. [32
] considered 100–400 μm as the optimum pore size range for bone ingrowth, but revealed no significant differences in bone ingrowth between pore sizes in this range and larger pores at 12 weeks after implantation. Similar results were found by Fisher et al. [33
]. Bobyn et al. [15
] showed that the extent of ingrowth of implants with pores of 710 μm was significantly greater compared to those with pores of 550 μm at 4 and 16 weeks after implantation. This indicates that 400 μm is not the maximum pore size to enhance bone ingrowth.
One setback of a larger pore size and a higher porosity is the length of time it takes for full integration of bone into the implant. Hing et al. [34
] showed that the volume of bone ingrowth and the bone–implant contact of specimens with high porosities (80%) increases gradually from 5, 13 and 26 weeks after implantation. Bobyn et al. [15
] demonstrated the same effect for pore sizes of 430 and 650 μm. These observations concur with the results of our study showing that the ultimate bone ingrowth is not accomplished for the new E-beam structures in the 6-week study period. Based on the current results bone ingrowth is likely to continue after the 6-week study period and will further anchor the implant to the bone. However, one can expect that ingrowth beyond a certain depth does not enhance the strength of the bone–implant interface, similar as seen for the cement–bone interface [35
One of the few studies on E-beam engineered surfaces in the orthopedic literature is reported by Ponader et al. [22
] They demonstrated a reduced proliferation of human fetal osteoblasts on porous E-beam produced surfaces, compared to smooth and unprocessed E-beam surfaces. However, in that same study, SEM analysis demonstrated that the cells attached and spread well on all surfaces [22
]. This indicates that the E-beam produced material itself does not hamper cell viability and proliferation, but that this may be influenced by the micro and macro geometrical characteristics. In contrast to the study of Ponader et al. [22
], the E-beam produced surface structures used in this study were superior compared to a sandblasted E-beam produced specimen.
Several explanations can clarify this difference; differences in finishing surface treatment, testing methods and surface characteristics. With respect to the finishing treatment, an additional step was added in the surface structures tested in this study. Before biological cleaning and sterilization a sandblasting step was performed in order to remove all residual powder particles. Regarding the method of testing, it is questionable whether the examination of pre-osteoblastic cell proliferation of Ponader et al. [22
] provides valid data for bone ingrowth of prosthetic components. In this process stroma cells probably play a more prominent role [36
]. Concerning surface characteristics, two out of the three surface structures tested by Ponader et al. [22
] had smaller pores than the structures used in this study. The pore size of the third surface structure was comparable. Additionally, the porosity of these structures was lower as well. The superiority of our structures suggests that a large pore size and high porosity enhance bone ingrowth and support the importance of surface characteristics influencing bone ingrowth.
In conclusion, the newly developed surface structures engineered in this study provide sufficient friction at the bone–implant interface thus achieving initial stability. The ultimate bone ingrowth was not accomplished in this study, due to the high porosity and large average pore size of the new E-beam surface structures. However, the bone ingrowth into the new surface structures appears to be comparable to more conventionally made surfaces of clinically successful implants at 6 weeks after surgery.
The results of this study are promising as bone ingrowth is likely to continue after the 6 weeks allotted for this study. Testing of bone ingrowth for an extended period is necessary to support our hypothesis that the new surface structures can provide improved fixation properties compared to conventionally made surfaces.