Prosthetic radial head stem pull-out as a mode of failure: a biomechanical study

Abstract

Purpose: Press-fit cementless radial head implant longevity relies on adequate bone ingrowth. Failed implant osseointegration remains a clinical concern and has been shown to lead to prosthetic failure. The purpose of this study was to test the hypothesis that implants with sufficient initial press-fit stability would be less likely to fail due to implant pull-out, as demonstrated by an increasing amount of energy required to remove the prosthesis from the canal.

Methods: Ten cadaveric radii were implanted with five sizes (6-10 mm in 1-mm increments) of grit-blasted, cementless radial head stems. A customised slap hammer was used to measure the energy required to remove each stem. Stem-bone micromotion was also measured.

Results: The suboptimally sized stem (Max - 1) (i.e. 1 mm undersized) required less energy (0.5 ± 0 J) to pull out than the optimally sized stem (Max) (1.7 ± 0.3 J) (p = 0.008). The optimally sized stem demonstrated greater initial stability (45 ± 7 μm) than the suboptimally sized stem (79 ± 12 μm) (p = 0.004).

Conclusions: This investigation demonstrates the importance of obtaining adequate press-fit stability for the prevention of radial head stem pull-out failure. These data add to the relatively scant knowledge in the literature regarding radial head biomechanics. The energy required to remove a prosthetic radial head ingrowth stem decreases in conjunction with diameter. The use of an inadequately sized stem increases the stem's micromotion as well as the risk of prosthetic loosening due to pull-out.

Fig. 1

The mean energy required for the insertion (left) and pull-out (right) of the prosthetic stems. Greater energy was required to both insert and remove the optimally sized stem as compared to the undersized stem. Undersized stems (Max − 1) required significantly less energy to insert (left) and also to pull out (right) than did the optimally sized stems (Max) or the oversized stems (Max + 1) that caused radial neck fracture. Data shown are the means ± standard error of the means (error bars). Lower-case letters indicate the results of repeated measures analysis. Columns with letters in common are not statistically different from one another (p ≤ 0.05)

Fig. 2

Stem micromotion values for the optimally sized (Max) and oversized implants (Max + 1) were significantly less than for the undersized implants (Max − 1) (p < 0.05). Data shown are the means ± standard error of the means (error bars). Lower-case letters indicate the results of repeated measures analysis. Columns with letters in common are not statistically different from one another (p < 0.05).

Fig. 3

A threshold effect was observed for the submaximally sized (max-1) stem. As the energy required for stem insertion (top) and removal (middle) increased, a corresponding decrease in micromotion (bottom) was observed. Each line represents individual specimens