Biomechanics of a short stem: In vitro primary stability and stress shielding of a conservative cementless hip stem

Abstract

Short stem prostheses provide conservative surgery and favorable metaphyseal load transmission. However, clinical longterm results are lacking. Therefore, in vitro trials can be used to predict bone-implant performance. In this in vitro study, primary stability and stress shielding of a new cementless short stem implant was evaluated in comparison to a straight stem using nine pairs of human cadaver femurs. Primary stability, including reversible micromotion and irreversible migration, was assessed in a hip simulator. Furthermore, changes in the pattern of cortical strain were evaluated. The short stem was more resistant to reversible micromotion and irreversible migration into retroversion. Axial stability was similar, with mean reversible micromotions of 9 mm for the short stem and 7 mm for the straight stem. Proximal load transmission was more physiological with the short stem, though both implants could not avoid stress shielding in Gruen zones 1 and 7. Primary stability of the short stem prosthesis was not negatively influenced compared to the straight shaft. Furthermore, proximal femoral strain pattern was more physiological after insertion of the short stem prosthesis. ß

Trade-off between stress shielding and initial stability on an anatomical cementless stem shortening: in-vitro biomechanical study

“…This finding is consistent with those of previous biomechanical studies examining different types of femoral stems, such as anatomic, straight, custom made, short, and low modulus stems [7,16,[29][30][31][32]. The distal part of the anatomical stem is likely to support the load because of a proximal-distal mismatch (tight distal fit and fill), which may lead to proximal bone resorption and cortical hypertrophy [33].…”

“…Second, our test did not take into consideration dynamic loading cycle. The initial stability describes the reversible implant-bone micromotion during dynamic loading and early irreversible migration [16,34]. The presented relative axial displacements and rotations would include the elastic and irreversible motion.…”

“…The equivalent load where this moment was reached was calculated for each bone. In intact femur, the equivalent load was 1600 N [16]. In operated femurs, the equivalent load ranged from 1455 to 2111 N according to the altered head position after stem insertion.…”

“…Several reports indicate that short stems have good clinical performance [8,[10][11][12], reduced intraoperative complications [13], and stable fixation in osteoporotic bone [14,15]. Biomechanical studies of short stems have demonstrated a sufficient initial stability and an improved proximal load transfer [6,7,16]. These reports would help to expand the indications for short femoral stems; however, the abovementioned advantages of short stems are controversial.…”

Tibial revision knee arthroplasty with metaphyseal sleeves: The effect of stems on implant fixation and bone flexibility

“…Both variables can be measured precisely with experiments. For example, some biomechanical studies measured implant related stress with strain gauges [23–26] or implant-bone relative micromotions with inductive sensors [23, 26, 27]. However, implant fixation and bone flexibility of tibial revision knee implant modularity with regards to using sleeve and stem fixation individually and in combination has not yet been analyzed.…”