Standard and line‐to‐line cementation of a polished short hip stem: Long‐term in vitro implant stability

The current trend is toward shorter hip stems. While there is a general agreement on the need for a cement mantle thicker than 2 mm, some surgeons prefer line‐to‐line cementation, where the mantle has only the thickness provided by the cement‐bone interdigitation. The aim of this study was to assess if a relatively short, polished hip stem designed for a standard cementation can also be cemented line‐to‐line without increasing the risk of long‐term loosening. Composite femurs with specific open‐cell foam to allow cement‐bone interdigitation were used. A validated in‐vitro biomechanical cyclic test replicating long‐term physiological loading was applied to femurs where the same stem was implanted with the Standard‐mantle (optimal stem size) and Line‐to‐line (same rasp, one‐size larger stem). Implant‐bone motions were measured during the test. Inducible micromotions never exceeded 10 μm for both implant types (differences statistically not‐significant). Permanent migrations ranged 50–300 μm for both implant types (differences statistically not‐significant). While in the standard‐mantle specimens there was a pronounced trend toward stabilization, line‐to‐line had less tendency to stabilize. The cement cracks were observed after the test by means of dye penetrants: The line‐to‐line specimens included the same cracks of the standard‐mantle (but in the line‐to‐line specimens they were longer), and some additional cracks. The micromotions and cement damage were consistent with those observed in‐vitro and clinically for stable stems, confirming that none of the specimens became dramatically loose. However, it seems that for this relatively short polished stem, standard‐mantle cementation is preferable, as it results in less micromotion and less cement cracking.