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Clin Orthop Relat Res. 2014 Jul;472(7):2231-6. doi: 10.1007/s11999-014-3577-y. Epub 2014 Mar 25.

Loosely implanted cementless stems may become rotationally stable after loading.

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  • 1Orthopaedic Research Laboratory, Departments of Orthopaedic Surgery & Biomedical Engineering, Virginia Commonwealth University, PO Box 980694, Richmond, VA, 23298-0694, USA.

Abstract

BACKGROUND:

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.

QUESTIONS/PURPOSES:

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.

METHODS:

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 2 N-m at 0 N, 250 N, and 500 N vertical load in 25° adduction simulating single-legged stance. Subsidence was measured as vertical movement during the toggle tests. Then at 500 N initial vertical load, femoral stems were externally rotated to failure. The construct stiffness between 5 and 40 N-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.

RESULTS:

Loose tapered cementless stems were less stable (ie, more toggle) than well-fixed at 0 N of load (p < 0.0001), but no difference was detectable in toggle between loose and well-fixed stems at 250 N (p = 0.7019) and 500 N (p = 0.9970). Loose tapered cementless stems showed significant subsidence at 250 N (p < 0.0001) and 500 N (p < 0.0001), which was not found in the well-fixed stems at 250 N (p = 0.8813) and 500 N (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.

CONCLUSIONS:

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.

CLINICAL RELEVANCE:

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.

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