Preclinical assessment of the long-term endurance of cemented hip stems. Part 1: Effect of daily activities - a comparison of two load histories

Abstract

The loads during daily activities contribute to fixation failure of cemented hip stems. In-vitro preclinical testing so far has consisted of simulating one or two conditions. Only a small percentage of hip implants fail, with a higher failure rate in most active patients. The goal was to define a procedure to assess the long-term effect of the lifestyle of a reasonably active patient on implant micromotions. Thus, a cyclic load of constant amplitude is unsuitable. All activities inducing high loads were included, to replicate the most critical scenario in terms of fatigue. The following motor tasks were simulated: stair climbing and descending, car entry and exit, bathtub entry and exit, and stumbling. An in-vitro simulation running for 15 days was able to replicate the load peaks occurring in 24 years of patient activity. Inducible micromotion and permanent migration were monitored. The load history was successfully applied to two different designs with known clinical performance, yielding significantly different micromotions for the two types. Results from the present load history were compared against a simpler profile including only stair climbing. Results showed that the new load profile is more sensitive to differences and can more easily discriminate between different designs. Part 2 of this work describes a further validation against retrieved implants.

Interface Micromotion of Uncemented Femoral Components from Postmortem Retrieved Total Hip Replacements

“…These systems are particularly useful for fatigue studies to quantify migration and inducible motion of the stem, but are not capable of measuring motion at the interface. Drilling through the bone at different locations along the construct and attaching extensometers or LVDT’s provides more information regarding spatial distribution of micromotion [17–20]. However the measurements assume that the bone is a rigid body relative to the moving implant.…”

The influence of stem length and fixation on initial femoral component stability in revision total knee replacement

“…This human bone analogue has been widely used to assess stability of stemmed femoral components used in total hip replacement. 12-14 A number of studies have shown the composite femur to be a suitable substitute that replicates the strength and material properties of bone adequately while permitting higher levels of repeatability than their biological equivalent for smaller sample sizes due to the standardised nature of their geometry. 15 Three different femoral components were investigated in this study; a posterior-stabilising (PS), a total-stabilising (TS) implant with short stem (12 mm × 50 mm) and a TS implant with long offset stem (19 mm × 150 mm stem with 4 mm lateral offset), all from the Triathlon series (Stryker, Newbury, United Kingdom) (Fig.…”

Mechanical testing of bones: the positive synergy of finite–element models andin vitroexperiments

“…Moreover, as strain is described by a tensor, and displacement by a vector, strain or displacement measurements according to a given direction (which is determined by the sensor itself) are affected by alignment. Therefore, if a transducer is randomly misplaced or misaligned, the readout will suffer from an unpredictable error Cristofolini et al , 2007b). …”

“…Different motor tasks need to be simulated to represent the physiological range of loading configurations (O'Connor et al 1996;Heller et al 2001;Cristofolini et al 2007b). In general, such in vitro simulations involve a more complex set-up, often including the action of relevant muscle groups (Cristofolini et al 1995;Duda et al 1998;Cristofolini & Viceconti 1999b;Szivek et al 2000;Stolk et al 2001;Britton et al 2003).…”