Influence of Synthetic Bone Substitutes on the Anchorage Behavior of Open-Porous Acetabular Cup

Abstract

Background: The development in implants such as acetabular cups using additive manufacturing techniques is playing an increasingly important role in the healthcare industry. Method: This study compared the primary stability of four selectively laser-melted press-fit cups (Ti6Al4V) with open-porous, load-bearing structural elements on the surface. The aim was to assess whether the material of the artificial bone stock affects the primary stability of the acetabular cup. The surface structures consist of repeated open-porous, load-bearing elements orthogonal to the acetabular surface. Experimental pull-out and lever-out tests were performed on exact-fit and press-fit cups to evaluate the primary stability of the cups in different synthetic bone substitutes. The acetabular components were placed in three different commercially available synthetic materials (ROHACELL-IGF 110, SikaBlock M330, Sawbones Solid Rigid). Results & conclusions: Within the scope of the study, it was possible to show the differences in fixation strength between the tested acetabular cups depending on their design, the structural elements used, and the different bone substitute material. In addition, functional correlations could be found which provide a qualitative reference to the material density of the bone stock and the press-fit volume of the acetabular cups.

Comparison of Test Setups for the Experimental Evaluation of the Primary Fixation Stability of Acetabular Cups

“…A visual examination of the bone cavity surfaces after tilting with Method 1 and Method 2 revealed that there were clear traces in the equatorial anchoring zone with Method 1, as described for the other cup types [ 16 , 21 , 37 ]. These traces corresponded to the plastic deformation of the foam and were generated by inserting the acetabular cup.…”

“…The micro surface roughness of the acetabular cup used in this study promotes the frictional connection between the acetabular cavity and the implant, which is required to generate the primary fixation stability. However, the influence of different surface structures, such as porous coatings [ 32 ], different rough blasting [ 20 , 37 , 42 ], and varying macrostructures [ 16 ] on the primary stability was not within the scope of the present study. The macroscopic barb-like surface structure of the acetabular cup used in this study led to an interlocking of the acetabular cup and the reamed cavity in the implant–bone interface [ 1 ].…”

“…Method 1 indicated significant differences between the cavities with increasing defect extent only for 30 pcf foam density. Other studies determined moments with the lever-out method ranging from 1.13 to 55.5 Nm [ 1 , 2 , 16 , 17 , 18 , 19 , 20 , 21 , 28 , 30 , 35 , 36 , 57 , 58 ], whereby the moments determined with Method 1 (lever-out method) in our study ranged from 2.72 ± 0.29 Nm (M1_15_D2) to 49.08 ± 1.50 Nm (M1_30_Intact), which were in good agreement with the literature. Conversely, the tilting moments determined with Method 2 (edge-load method) ranged from 41.40 ± 1.05 Nm (M2_15_D2) to 112.86 ± 5.29 Nm (M2_30_D1).…”

“…A large area of frictional contact between the implant and the bone promotes osseointegration of the implant and leads to good long-term stability [ 12 , 13 , 14 ]. The frictional contact is generated in the equatorial region of the acetabular cup [ 15 , 16 ]. In order to ensure the best possible bone ingrowth of cementless acetabular cups, the primary stability of press-fit cups has been experimentally investigated and characterized in recent decades.…”

“…The material properties of human pelvic bone commonly show high inter-specimen variability between different individuals. Due to the reasons of reproducibility and comparability of the experimental results and the limited availability of human donors, polymer foams are predominantly used as bone substitute materials [ 1 , 2 , 16 , 19 , 22 , 23 , 25 , 27 , 30 , 35 , 36 , 37 , 38 ]. In the lever-out method, the center of rotation (CoR) at which the acetabular cup is levered out of the artificial bone block is located in the interface between the acetabular cup and the acetabular cavity in the loading direction.…”