Static and Dynamic Balance During Standing and Walking in Correlation With Quality of Life, in Primary Total Knee Replacement

Balance Master Testing

The Pro Balance Master system with software version 8 will be used in the study. The Pro Balance Master system is comprised of two 23cm × 46cm dual force plates connected at the midline of the anterioposterior axis by a pin joint. Each force plate is mounted on a pair of symmetrically positioned force transducers. The four transducers measure vertical ground reaction forces, which form the basis of subsequent calculations of center of pressure (COP) and center of gravity (COG) sway angels. All test data are acquired and then stored on a PC.

Subjects are instructed to remove their shoes and step onto the forceplates, which have to be covered with tracing paper. Their feet are positioned by aligning the lateral border of the foot with the appropriate height line marked on the forceplates (ie, short, 76 to 140 cm; medium, 141 to 165cm; or tail, 166 to 203cm). The medial maileoli are aligned with the transverse forceplate line and subjects adopted a comfortable amount of forefoot splay. The outline of their feet is traced onto paper to ensure the same placement in subsequent testing sessions. The visual display monitor is adjusted to the subject's eye level. Subjects are instructed not to move their feet and are asked to stand with their arms at their sides throughout the testing procedure. The standard test suite protocol will be administered, consisting of 6 tests, 3 of which are static and 3 dynamic. The static tests require the subject to look straight ahead while standing as still as possible, initially with eyes open, then with eyes closed, and finally focusing on the display monitor using visual feedback to maintain the position of a cursor (representing the subject's COG) within a centrally positioned target box. For each test, data relating to postural sway is recorded for a period of 20 seconds and the area of the sway is calculated and expressed as a percentage of the subject's theoretical limits of stability (LOS). Assuming that movement about the ankle (with a fixed foot position) resembles an inverted pendulum, these limits are defined as extending 6.25 ° anteriorly, 4.45 ° posteriorly, and 8.00 ° to each side, ~2 and delimit the area over which the COG can safely move without changing the base of support.

All three dynamic tests require subjects to shift their COG. The first dynamic test involves rhythmic weight shifting side to side to 50% of the LOS at 3-second then 2-second pacing. Subjects are instructed to match the timing and movement of a bail on the screen by shifting their body weight side to side to target lines denoted on the screen. The second test is similar to the first except movement was now in an anterioposterior direction. Data reflecting the average magnitude of the movement path (expressed as a percentage of the LOS) are produced from six trials at each pace for each movement direction. The absolute error relative to the targets (set at 50% of the LOS) is calculated by subtracting 50 from the score obtained (50% indicating perfect execution) and recording it in absolute terms.

The final dynamic test involved weight shifting to 8 targets positioned in an ellipse, the perimeter of which corresponded to 75% of the LOS. The subjects are required to follow a ball to each target as it is highlighted, and remain at that target for 3 seconds before returning to the central target (neutral).