ePoster #1220 - 2013 ISAKOS Biennial Congress

Sit-To-Stand Symmetry Following Total Knee Arthroplasty With a Medial Pivot Knee

Sarah Reynolds, BKinH, MSc Candidate, Ottawa, ON, CANADA
Mario Lamontagne, PhD, Ottawa, ON, CANADA
Geoffrey F. Dervin, MD, Ottawa, ON, CANADA
University of Ottawa, Ottawa, Ontario, CANADA

FDA Status Not Applicable

Summary: The aim of this study was to compare the symmetry of ground reaction forces and joint moments exhibited by TKA patients with a Medial Pivot Knee during the sit-to-stand task; results showed that TKA participants had symmetrical ground reaction forces and joint moments, with indexes of symmetry similar to that of healthy, age-matched controls.

The ability to rise from a seated position is a key factor in maintaining an active, independent lifestyle and, therefore, an important measure in determining surgical success for individuals recovering from total knee arthroplasty (TKA). Previous studies have shown that TKA can successfully reduce knee pain and improve patient satisfaction. However, functional discrepancies remain. To address this disparity, the medial pivot (MP) knee is a recent prosthetic design modeled to reflect natural knee mechanics, based on a medial-stabilizing approach. While this newer prosthetic model is theorized to provide functional outcomes similar to that of a healthy knee, no previous study has inquired into its ability to provide such performance during activities of daily living, such as the sit-to-stand maneuver. The present study aims to examine how TKA patients with a MP knee perform this task, looking specifically at the degree of symmetry between the operated and non-operated limb. It is hypothesized that no significant differences will be observed between these groups, based on the assumption that the MP knee does in fact mimic natural knee mechanics. Individuals from the TKA group were recruited from the local hospital (n=5, age=61, BMI=29), having undergone unilateral total knee replacement with a Medial Pivot Knee (approx. 9 months post-op). Control participants (n=5, age=61, BMI=26) were recruited from the local community and were matched to TKA participants for age, sex and BMI. Approval was obtained from the university and hospital’s review boards and informed consent was provided by all participants prior to beginning the study. Motion analysis was conducted using a 10-camera Vicon system, and a seat set at 18 inches above the ground. Kinetic parameters were obtained using 2 Bertec force plates embedded within the floor in front of the seat. Each participant performed five successful trials moving from a seated to a standing position, with arms placed in front of the body. Kinetic measures were calculated using an inverse dynamics approach and were normalized for body mass. Each task was time normalized based on the initiation of seat-off and completion at a standing position. A symmetry index (S) was calculated using the following equation: S= [(Vno-Vo)/(Vno)+(Vo)]* 100 where Vno represents the variable for the non-operated limb and Vo represents the variable for the operated limb. This equation has been used in previous studies measuring lower limb symmetry for the sit-to-stand task, where S=10 indicates that the individual shows symmetry for the value being measured. Symmetry indexes were calculated for peak hip and knee extension moments, as well as for vertical ground reaction forces. Statistical analysis was performed using two-tailed t-tests (a=0.05) to compare symmetry indexes between the two groups. No significant differences were found between the TKA and Control groups in terms of BMI and age. During the sit-to-stand task, TKA participants had lower mean S values for peak knee extension moment (S=8.3) and higher values for peak hip extension moment (S=11.2), when compared to controls (S=15.9; S=6.0). These differences, however, were not found to be statistically significant. With respect to vertical ground reaction forces, TKA participants showed no significant differences for symmetry index values when compared to control participants, with a mean score of 6.0 and 7.3, respectively. These findings indicate that at approximately 9 months following surgery, TKA participants are able to rise from a seated position without the need to for one limb to overcompensate for the other. While these results are promising for surgical outcomes with a MP knee, it should be noted that there are some limitations to this study, such as the small sample size and the inclusion of males and females within the same testing group. The findings from this analysis represent preliminary results of a larger, ongoing research project and therefore there is a risk for type II error when comparing these initial, smaller groups. It would be of value for future research to examine whether gender and pre-operative levels of activity could influence performance on the sit-to-stand task, as well as investigating the role of muscle activity. The significance of this study is that it is the first to examine how individuals with a Medial Pivot knee perform a sit-stand maneuver. This unique focus allows for understanding of how prosthetic design may translate to functional outcomes at both lower limbs of individuals undergoing TKA and how this matches up to healthy, age-matched individuals. The authors would like to acknowledge Daniel Varin and Kyle Kemp for their ongoing help in conducting this research project.