Introduction: Cementless press-fit cups are the most widely used acetabular implants in total hip arthroplasty today. Their primary and secondary stability is largely determined by the design and choice of surface coating. Porous titanium coatings are used for the majority of cementless acetabular implants. However, an additional hydroxyapatite (HA) layer has been advocated for superior bone ongrowth. We studied the effect of additional HA coating on early micromotion of a porous titanium plasmasprayed cup with a flattened pole. A secondary objective was to compare the extent of micromotion of this well established cementless cup to data of other press-fit cups.

Methods: A total of 44 female patients (45 hips) undergoing total hip arthroplasty for osteoarthritis consented to participate in this prospective, IRB approved study. They were randomized for either a press-fit cup (EP-FIT PLUS^®, Plus Orthopedics AG, Switzerland) with a titanium plasma sprayed surface (Ti-group) or with additional HA coating (HA -group). All cups were used with the same combination of stem, PE liner and ceramic head. Model-based radiostereometry (MBRSA) was used to measure translation and rotation immediately postoperative, at 6 weeks, 3, 6, and 12 months. Statistical analysis of migration was performed utilizing one-sided Mann-Whitney tests and ANOVA.

Results: At one year, mean translation in the HA-group (Ti-group) along the medial-lateral (x), proximal-distal (y) and anterior-posterior (z) axes was −0.01 (0.07), 0.08 (0.09), and 0.03 (−0.06) mm, respectively. Mean rotation around the x-axis (anterior-posterior tilt) was −0.19 (−0.16), the y-axis (anteversion-retroversion) was −0.10 (−0.19), and the zaxis (adduction-abduction) was 0.23 (−0.05) degrees. Our hypothesis that translation and rotation would be different in the two groups was rejected (p< 0.00) for all dimensions except for rotation about the z-axis (p=0.10). The was no evidence for different migration patterns throughout the examined time points. All patients had excellent clinical outcome with a mean Harris Hip Score of 95.4 (HA-group) and 95.3 (Ti-group) (p=0.10). Plain radiographs of the cups showed good osseointegration.

Discussion: With excellent primary stability in both the Ti-group and the HA-group, we conclude, that HA-coating does not significantly increase stability of this flattened pole press-fit cup during the first postoperative year. We were able to show that the early stability of this cup is well comparable to that of the more frequently used hemispherical cups with initial stability being one prerequisite for long-term success. RSA measurements after 2 years will be conducted to confirm the current findings and the cup is also being studied in a long-term observation.