Migration of two different cementless hip arthroplasty stems in combination with two different heads: a biomechanical in vitro study

The aim of the present in vitro study was to evaluate migrational characteristics of cementless primary hip arthroplasty stems in combination with a Ø 50 mm head (hemiarthroplasty) and a Ø 28 mm head in and with a polyethylene cup (total hip arthroplasty) in fresh-frozen human specimens. Two different types (Endo SL^®, FMT^®) were implanted into seven pairs of fresh-frozen human femoral specimens. The implanted stems were combined with a Ø 50 mm head (hemiarthroplasty) on one side and with a polyethylene cup and a 28 mm head (total hip arthroplasty) on the other side. Dynamic mechanical loading was applied for 50,000 cycles while recording relative motions between stem and bone stock using a 3-D motion analysis system. The Endo SL^® stem showed a significantly higher amplitude of relative motion in all translational and rotational components regardless of the head used. In both stem types a strong tendency for higher axial migration with the Ø 50 mm head in comparison to the THA head was found. The lowest axial migration was found in the FMT^® stem in combination with the small head and a PE cup. The highest axial migration was found in the Endo SL^® stem combined with the Ø 50 mm head. Our results indicate that the head might play an important role for axial migration of cementless stems.