Increased long-term failure risk associated with excessively thin cement mantle in cemented hip arthroplasty. A comparative in vitro study

BACKGROUND: There is no consensus on possible detrimental effects of reduced thickness of the cement mantle surrounding hip stems. This work originated from the suspect that a successful design would yield inferior survival when implanted with a lower cement thickness. The scope was to assess with a reproducible in vitro experiment if a thinner cement mantle could cause a reduction of the long-term implant fatigue resistance. METHODS: A comparative in vitro study was designed based on the same commercial stem (Centrament, Aesculap), implanted with the recommended (2-3mm) and reduced (1-2mm) cement thickness. Tests were carried out simulating 24 years of activity of active hip patients. A multifaceted approach was taken: inducible and permanent micromotions were recorded throughout the test; cement micro-cracks were quantified using dye-penetrants and statistically analyzed; crack surface and stem-cement fretting damage was investigate under Scanning Electron Microscopy. FINDINGS: The same stem, when implanted with a thinner cement mantle, tended to migrate significantly more (up to 0.3mm), with a tendency to loosen over time (increasing migration rate over time). After cyclic testing the cement mantle of the thinner specimens showed significantly more and longer cracks than the standard implants, with loose cement chips and signs of stem-cement fretting. INTERPRETATION: All these results confirmed that: (i) the stem under investigation had a good performance (in comparison with similar published work) when implanted with the recommended thickness; (ii) micromotion, cement cracking and fretting damage significantly increased when reduced cement mantle was used. Excessively thin cement mantle can result in critical conditions even for designs, which, in general, show low complications.