Osseointegration in porous coated knee arthroplasty. The influence of component coating type in sheep.
Although cementless knee arthroplasty is a commonly performed procedure, to date very little was known about the process of osseointegration of knee arthroplasty components. Using a knee prosthesis that was specially designed for the sheep stifle joint, this process of osseointegration could be studied in vivo, together with its effects on clinical and functional performance, its influence on mechanical fixation, and its influence on component stability or migration over time. Additionally, the osseointegration capacity of a newly developed cast mesh porous coating could be examined. The rationale for this newly developed coating was to provide a surface texture with theoretically superior osseointegration capacity, by offering a larger and better controlled pore size, with higher ingrowth area compared to conventional bead type coatings. In summary, the conclusions that are drawn from this work are the following: 1. The degree of osseointegration of knee arthroplasty components is not correlated with clinical and functional performance. Knee arthroplasty components with fibrous integration can function as well as osseointegrated components at least during the first years after implantation. This explains the occasional reports in the literature of post mortem retrieved, well functioning knee arthroplasty components, with purely fibrous integration on histomorphometric analysis. 2. Fibrous integration of tibial knee arthroplasty components, however, leads to less mechanical fixation strength of these components. Osseointegrated components are much more strongly fixed to the underlying bone. This difference in mechanical fixation strength is detectable under physiologic loads. 3. Fibrous integration of tibial knee arthroplasty components leads to increased migration, becoming apparent after 1 year with radiostereometric analysis (RSA). Osseointegrated components are significantly more stable over time. 4. Fibrous integration is less desirable, since it leads to mechanically less rigidly fixed implants, and subsequently to migration over time. On the long-term, fibrous integration might therefore lead to loosening. 5. RSA is an effective tool to assess migration of knee arthroplasty components. The RSA migration of an uncemented component is also an indicator of its degree of osseointegration and its mechanical fixation strength, since RSA migration is correlated with these two parameters. RSA is therefore especially useful during the first postoperative years, since increased migration indicates fibrous integration and low mechanical fixation strength, suggesting an increased risk for subsequent loosening at a later stage. Patients with increased early component migration on RSA might therefore be advised to impose specific restrictions on their knee arthroplasty. 6. Osseointegration is not routinely achieved in conventional porous coated tibial knee arthroplasty components. The development of a theoretically superior cast mesh coating did not lead to a significantly higher degree of osseointegration. It is suggested that the lack of sufficiently small interface gaps and the lack of sufficiently small interface micromotion--two known prerequisites for reliable osseointegration to occur--are the main reasons inhibiting osseointegration, both in this animal model and in the human situation. 7. Significant osseointegration, however, does occur in the newly developed cast mesh coated components, when they are additionally coated with a vacuum plasma sprayed 50 microns hydroxyapatite layer. The newly developed cast mesh coating is ideally suited for such an additional hydroxyapatite coating. Remarkably high percentages of both bone ingrowth and ongrowth can be seen with these hydroxyapatite coated cast mesh coatings, higher than so far reported for current tibial knee arthroplasty components. These osseointegrated hydroxyapatite coated cast mesh components show significantly higher mechanical fixation strength and lower RSA migratio