Loosely Implanted Cementless Stems May Become Rotationally Stable After Loading

Abstract

Experimental studies have suggested that initial micromotion of cementless components may lead to failure of osteointegration. Roentgen stereophotogrammetric analyses have shown durable implant fixation can be achieved long-term even when initial instability exists, as evidenced by subsidence. However improved implant stability as a result of subsidence, before osteointegration, has not been shown biomechanically. We asked whether insertionally loose cementless tapered femoral stems show (1) less rotational stability (more toggle); (2) more subsidence; and (3) reduced ability to resist torsion (lower initial construct stiffness), lower torque at failure, and greater rotation to failure in comparison to well-fixed cementless tapered femoral stems. Ten matched pairs of cadaveric femurs were implanted with well-fixed and loose cementless tapered stems. The loose stem construct was obtained by appropriately broaching the femur but afterwards inserting a stem one size smaller than that broached. Femoral stem rotational stability of implanted femurs was tested by measuring the angular rotation (ie, toggle) required to produce a torque of 2N-m at 0N, 250N, and 500N vertical load in 25° adduction simulating single-legged stance. Subsidence was measured as vertical movement during the toggle tests. Then at 500N initial vertical load, femoral stems were externally rotated to failure. The construct stiffness between 5 and 40N-m was determined to assess ability to resist torsion. The torque and rotation to failure were recorded to compare failure characteristics. Groups were compared using mixed model ANOVA followed by Tukey-Kramer post hoc pairwise comparison for toggle and subsidence tests and by Student's paired t-tests for stiffness, torque at failure, and rotation to failure tests. Loose tapered cementless stems were less stable (ie, more toggle) than well-fixed at 0N of load (p<0.0001), but no difference was detectable in toggle between loose and well-fixed stems at 250N (p=0.7019) and 500N (p=0.9970). Loose tapered cementless stems showed significant subsidence at 250N (p<0.0001) and 500N (p<0.0001), which was not found in the well-fixed stems at 250N (p=0.8813) and 500N (p=0.1621). Torsional stiffness was lower for loose stems as compared with well-fixed stems (p=0.0033). No difference in torque at failure (p=0.7568) or rotation to failure (p=0.2629) was detected between loose and well-fixed stems. In this study, we observed that insertionally loose cementless stems have the ability to subside and become rotationally stable with loading. They did not exhibit a lower torque or rotation to failure in comparison to well-fixed stems when under simulated single-legged stance. Secondary rotational stabilization may prevent insertionally loose tapered stems from producing a stress pattern that predisposes to early postoperative periprosthetic fracture around loose cemented stems.