MEDIAL PIVOT IMPLANTS OFFER IMPROVED KNEE STABILITY DURING A STAIR ASCENT TASK COMPARED WITH POSTERIOR-STABILISED IMPLANTS

Introduction Patients undergoing a total knee arthroplasty (TKA) are now living longer and partaking in more active lifestyles. They expect a high level of post-operative function and long term durability of their implant. Using electromyography (EMG) analysis helps further explain biomechanical findings by giving insight as to what is occurring at the level of the muscles. Normal biomechanics are not restored post-TKA as patients have reduced knee flexion and weakened quadriceps muscles compared to their healthy peers. Purpose The purpose of this study was to compare muscle activation in TKA patients who received a medial pivot (MP) or posterior stabilized (PS) implant to those of healthy controls (CTRL) during a stair ascent task. Methods A total of 12 patients were assigned to either a MP or PS TKA operated by the same surgeon. Approximately 9 months following surgery, the 12 patients along with 6 CTRL patients completed an EMG analysis during a stair ascent task (Table 1). Wireless EMG electrodes were placed on 4 muscles: vastus medialis (VM), vastus lateralis (VL), biceps femoris long head (BF), and semimembranosus (SM) muscles. All participants completed maximal voluntary contractions (MVICs) during knee flexion and extension while seated with the knee flexed at 60°. Following the MVICs, participants completed 5 trials of a 3-step stair ascent task. TKA patients were instructed to make the first step onto the staircase with their operated limb. EMG data were processed in Matlab. Peak muscle activity (PeakLE EMG) and total muscle activity (iEMG) from each muscle was obtained during stance phase. Data were averaged between left and right limbs for the CTRL group and compared to the operated limb of TKA groups. Non-parametric Kruskal Wallace ANOVA tests were used to test for statistical significance between groups and Wilcoxon rank sum tests were used to identify differences with α=0.05. Results Both TKA groups had significantly greater PeakLE EMG of the quadriceps muscles compared to the CTRL group (Figure 1). PeakLE EMG of the BF and VM muscles were significantly greater in the PS group compared to both CTRL and MP groups. The PS group had significantly greater iEMG of the BF, VL and VM muscles compared to the CTRL group (Figure 2) whereas the PS group had significantly greater iEMG of the SM and VM muscles compared to the MP group. Conclusion The MP group had lower PeakLE EMG for both VM and BF muscles compared to the PS group, indicating that the MP group activates these muscles less to achieve the stair ascent task. BF muscle stabilizes the knee roll-back motion while the VM muscle extends the leg to clear the steps. iEMG for the VM and BF muscles were greater for the PS group indicating that they have to activate their muscles longer and to a greater extent in order to stabilize the joint. This increased stability in MP implants is achieved through the concave aspect on the medial tibial plateau for the femoral condyle to pivot in. This will reduce implant wear, prolonging implant longevity.