The effect of bone ingrowth depth on the tensile and shear strength of the implant-bone e-beam produced interface.

New technologies, such as selective electron beam melting, allow to create complex interface structures to enhance bone ingrowth in cementless implants. The efficacy of such structures can be tested in animal experiments. Although animal studies provide insight into the biological response of new structures, it remains unclear how ingrowth depth is related to interface strength. Theoretically, there could be a threshold of ingrowth, above which the interface strength does not further increase. To test the relationship between depth and strength we performed a finite element study on micro models with simulated uncoated and hydroxyapatite (HA) coated surfaces. We examined whether complete ingrowth is necessary to obtain a maximal interface strength. An increase in bone ingrowth depth did not always enhance the bone-implant interface strength. For the uncoated specimens a plateau was reached at 1,500 µm of ingrowth depth. For the specimens with a simulated HA coating, a bone ingrowth depth of 500 µm already yielded a substantial interface strength, and deeper ingrowth did not enhance the interface strength considerably. These findings may assist in optimizing interface morphology (its depth) and in judging the effect of bone ingrowth depth on interface strength.

Experimental versus computational analysis of micromotions at the implant-bone interface.

In total hip arthroplasty, micromotions at the implant-bone interface influence the long-term survival of the prosthesis. These micromotions are often measured using sensors that are fixed to the implant and bone at points that are remote from the interface. Given that the implant-bone system is not rigid, errors may be introduced. It is not possible to assess the magnitude of these errors with the currently available experimental methods. However, this problem can be investigated using the finite element method (FEM). The hypothesis that the actual interface micromotions differ from those measured in the experimental manner was tested using a case-specific FE model, validated against deflection experiments. The FE model was used to simulate an 'experimental' method to measure micromotions. This 'experimental' method was performed by mimicking the distance between the measurement points; the implant point was selected at the interface while the bony point was at the outer surface of bone. No correlation was found between the micromotions computed at the interface and when using remote reference points. Moreover, the magnitudes of micromotions computed with the latter method were considerably greater. By reducing the distance between the reference points the error decreased, but the correlation stayed unchanged. Care needs to be taken when interpreting the results of micromotion measurement systems that use bony reference points at a distance from the actual interface.

Balancing incompatible endoprosthetic design goals: a combined ingrowth and bone remodeling simulation.

In order to design a good cementless femoral implant many requirements need to be fulfilled. For instance, the range of micromotions at the bone-implant interface should not exceed a certain threshold and a good ratio between implant-bone stiffness that does not cause bone resorption, needs to be ensured. Stiff implants are known to evoke lower interface micromotions but at the same time they may cause extensive resorption of the surrounding bone. Composite stems with reduced stiffness give good remodeling results but implant flexibility is likely to evoke high micromotions proximally. Finding a good balance between these incompatible design goals is very challenging. The current study proposes a finite element methodology that employs subsequent ingrowth and remodeling simulations and can be of assistance when designing new implants. The results of our simulations for the Epoch stem were in a good agreement with the clinical data. The proposed implant design made of porous tantalum with an inner CoCrMo core performed slightly better with respect to the Epoch stem and considerably better with respect to a Ti alloy stem. Our combined ingrowth and remodeling simulation can be a useful tool when designing a new implant that well balances mentioned incompatible design goals.