Cementless fixation of radial head implants is affected by implant stem geometry: an in vitro study.

Background: Metal radial head implants are widely used for the management of unreconstructible fractures. However, the effect of implant stem design on initial fixation and stability is unknown. This study determined the effects of radial head stem geometry on the initial stability of the uncemented implant. It was hypothesized that cortical contact and hence increased fixation would be achieved with an increased stem diameter, stem length and a tapered shape.

Methods: Eleven radii received five implants with differing stem geometries: short and long undersized, short and long optimally sized and short tapered. Inferiorly-directed compressive loads were applied successively to the anterior, posterior, medial and lateral edges of the implants. Maximum contralateral radial head "lift-off" was quantified by the distance between bone and implant surface markers.

Findings: The short and long undersized implants experienced greater micromotion with approximately 100 microm and 160 microm more lift-off respectively (P<0.02). The remaining optimally sized and tapered implants averaged 50 microm. There was greater motion for the undersized implants loaded at the lateral position (P0.005).

Interpretation: This study shows that filling the diameter of the medullary canal is more important than filling the length of the canal. The taper did not offer any additional advantages compared to the straight stem, suggesting that reaming of bone to accept the taper did not produce enhanced initial purchase. If rigid initial implant fixation is desired with an uncemented radial head implant, a canal-filling stem reduces initial implant micromotion to less than 50 microm which may enhance bone ingrowth.