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Proximal humeral fractures: how stiff should an implant be?

A comparative mechanical study with new implants in human specimens

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Abstract

Background

The objective of this study was to determine the in vitro characteristics of the clinically used and newly developed implants for the stabilization of proximal humeral fractures under static and cyclic loading. The goal was to optimize implant stiffness for fracture stabilization even in weak bone stock.

Methods

In a laboratory study using 35 fresh human humeri, the specimens were randomized into 5 groups, which included the clinically used humerus T-plate (HTP), the cross-screw osteosynthesis (CSO), the unreamed proximal humerus nail with spiral blade (UHN), the recently developed Synclaw Proximal Humerus Nail (Synclaw PHN) and the angle-stable Locking Compression Plate Proximal Humerus (LCP-PH). The implant stiffness was determined for three clinically relevant load cases: axial compression, torsion and varus bending. In addition, a cyclic varus-bending test was performed to determine the implant properties under cyclic loading.

Results

In contrast to a rather elastic and minimally invasive implant(LCP-PH), the conventionally designed ones (Synclaw PHN, CSO, HTP, UHN) showed rather high stiffness values under static loading. In cyclic loading, a strong decrease in stiffness (p<0.05) was found for the rigid implants HTP and UHN. In comparison with the other implants, only the elastic implant (LCP-PH) showed a significantly lower load reduction in a weak bone stock (17±6.2%).

Conclusion

The high initial stiffness of rigid implants led to an early loosening and failure of the implant-bone interface under cyclic loading. Implants with low stiffness and elastic characteristics, however, appear to minimize the peak stresses at the bone-implant interface, making them particularly suitable for fracture fixation in osteoporotic bone.

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Acknowledgements

We thank Prof. Reinhard Vock (†) from the Department of Forensic Medicine and Prof. Christian Wittekind from the Department of Pathology of the University of Leipzig, Germany, and Jan-Eric Hoffmann for support in mechanical testing. This study was supported by a grant of the Robert Mathys Foundation, Switzerland, and the Richard Maatz grant 1999 of the Gerhard Küntscher Society, Germany.

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Lill, H., Hepp, P., Korner, J. et al. Proximal humeral fractures: how stiff should an implant be?. Arch Orthop Trauma Surg 123, 74–81 (2003). https://doi.org/10.1007/s00402-002-0465-9

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  • DOI: https://doi.org/10.1007/s00402-002-0465-9

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