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

Abstract

Objectives: Orthopaedic surgeons use stems in revision knee surgery to obtain stability when metaphyseal bone is missing. No consensus exists regarding stem size or method of fixation. This in vitro study investigated the influence of stem length and method of fixation on the pattern and level of relative motion at the bone-implant interface at a range of functional flexion angles.

Methods: A custom test rig using differential variable reluctance transducers (DVRTs) was developed to record all translational and rotational motions at the bone-implant interface. Composite femurs were used. These were secured to permit variation in flexion angle from 0° to 90°. Cyclic loads were applied through a tibial component based on three peaks corresponding to 0°, 10° and 20° flexion from a normal walking cycle. Three different femoral components were investigated in this study for cementless and cemented interface conditions.

Results: Relative motions were found to increase with flexion angle. Stemmed implants reduced relative motions in comparison to stemless implants for uncemented constructs. Relative motions for cemented implants were reduced to one-third of their equivalent uncemented constructs.

Conclusions: Stems are not necessary for cemented implants when the metaphyseal bone is intact. Short cemented femoral stems confer as much stability as long uncemented stems.

Keywords: Cemented; Femoral component micromotion; Influence of stems; Multiple flexion angles; Uncemented.

Figs. 1a - 1b

Figure 1a – three-dimensional drawing of the micromotion measurement setup. Figure 1b – schematic drawing of sensor arrangement and reference point used for coordinate transformations (DVRT, differential variable reluctance transducer).

Figs. 1a - 1b

Figure 1a – three-dimensional drawing of the micromotion measurement setup. Figure 1b – schematic drawing of sensor arrangement and reference point used for coordinate transformations (DVRT, differential variable reluctance transducer).

Fig. 2

Photographs of the implants investigated, showing a posterior-stabilised (PS) implant (top row), a total-stabilised (TS) implant with short stem (middle row) and a TS implant with long offset stem (bottom row).

Fig. 3

Equations used to determine the relative motion of the implant reference point based on individual sensor displacements.

Fig. 4

Diagrams showing the comparison of translational (first column) and rotational (second column) relative motions for: a posterior-stabilised (PS) implanted femur (top row), a total-stabilised (TS) implanted femur with a short stem (middle row) and a TS implanted femur with a long offset stem (bottom row) for both cemented and uncemented cases. Where u, v and w are relative translational motions in the directions of x, y and z, respectively, and θ_x, θ_y and θ_z are relative rotations about the axes of x, y and z, respectively.

Fig. 5

Bar chart showing the overall magnitude of relative motions for the three flexion angles investigated for cemented and uncemented implants (PS, posterior-stabilised; TS, total-stabilised)..