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Primary stability of the Fitmore® stem: biomechanical comparison

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Abstract

Purpose

After clinical introduction of the Fitmore® stem (Zimmer), we noticed the formation of cortical hypertrophies in a few cases. We questioned whether (1) the primary stability or (2) load transfer of the Fitmore® stem differs from other stems unassociated with the formation of hypertrophies. We compared the Fitmore® stem to the well-established CLS® stem.

Methods

Four Fitmore® and four CLS® stems were implanted in eight synthetic femurs. A cyclic torque around the stem axis and a mediolateral cyclic torque were applied. Micromotions between stems and femurs were measured to classify the specific rotational implant stability and to analyse the bending behaviour of the stem.

Results

No statistical differences were found between the two stem designs with respect to their rotational stability (p = 0.82). For both stems, a proximal fixation was found. However, for the mediolateral bending behavior, we observed a significantly (p < 0.01) higher flexibility of the CLS® stem compared to the Fitmore® stem.

Conclusion

Hip stem implantation may induce remodelling of the periprosthetic bone structure. Considering the proximal fixation of both stems, rotational stability of the Fitmore® stem might not be a plausible explanation for clinically observed formation of hypertrophies. However, bending results support our hypothesis that the CLS® stem presumably closely follows the bending of the bone, whereas the shorter Fitmore® stem acts more rigidly. Stem rigidity and flexibility needs to be considered, as they may influence the load transfer at the implant–bone interface and thus possibly affect bone remodelling processes.

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Authors and Affiliations

  1. Laboratory of Biomechanics and Implant Research, Department of Orthopaedics and Traumatology, University Hospital Heidelberg, Schlierbacher Landstrasse 200a, 69118, Heidelberg, Germany

    Wojciech Pepke, Jan Nadorf, Volker Ewerbeck, Marcus R. Streit, Stefan Kinkel, Tobias Gotterbarm, Michael W. Maier & J. Philippe Kretzer

Authors
  1. Wojciech Pepke
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  2. Jan Nadorf
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  3. Volker Ewerbeck
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  4. Marcus R. Streit
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  5. Stefan Kinkel
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  6. Tobias Gotterbarm
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  7. Michael W. Maier
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  8. J. Philippe Kretzer
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Corresponding author

Correspondence to Jan Nadorf.

Additional information

W. Pepke and J. Nadorf shared first-authorship: these authors contributed equally to the study.

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Pepke, W., Nadorf, J., Ewerbeck, V. et al. Primary stability of the Fitmore® stem: biomechanical comparison. International Orthopaedics (SICOT) 38, 483–488 (2014). https://doi.org/10.1007/s00264-013-2138-4

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  • DOI: https://doi.org/10.1007/s00264-013-2138-4

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