Abstract
INTRODUCTION
Total Knee Arthroplasty (TKA) is a durable procedure which allows most patients to achieve a satisfactory functional level, but there can be instability under stressful conditions. Instability is one cause of early revision, often due to misalignment or inadequate ligament balancing. Persistent instability may cause elevated polyethylene wear. Lower levels of instability may cause patient discomfort with certain stressful activities. Hence quantifying instability may have an important role in the functional evaluation of TKA. Several previous studies showed that accelerometers have advantages in kinematic studies including low cost, ease of application, and application to any activity. The aim of this study was to demonstrate the use of an accelerometer attached to the anterior of the tibia, as an evaluation of knee stability of TKA patients. It was postulated that accelerations between TKAs and normal controls will be different, which could indicate abnormal TKA kinematics involving instability, especially for high intensity activities.
METHODS & MATERIALS
We tested 38 TKA knees in 27 patients, in the age range of 50–80 years, with a minimum follow up of 6 months; and 25 knees in 16 shoulder patients, who had no known knee pathology as age-matched controls. A tri-axial accelerometer was firmly attached to the anterior proximal tibia to measure 3-axis accelerations with a sample rate of 100 Hz. Four activities were tested;
Starting with the test leg, walk 3 steps then come to a sudden stop
Take one step forward with the non-tested leg and make a tight 90∗∗∗∗∗ turn towards the non- tested knee direction
Sit down for 3–4 seconds then stand back up
Step up on a 7″ inches high box with the test leg, followed by the non-test leg. Then step down from the box with the test leg, followed by the non-test leg.
During the activities, the patients responded to a questionnaire on instability and pain for each activity. For each test at the time of foot impact, there was a high/low peak acceleration, the peak-to-valley being taken as the indicator. The mean total magnitude of the acceleration was compared between the TKA and control groups in the anterior-posterior direction using the Student's t-test. Statistical significance was at p-value < 0.05.
RESULTS
Significant differences were seen between TKR and normal controls for stepping down, and for turning. Significance was close for a sudden stop. From the instability questionnaire, 15 knees had pain and 13 knees felt unstable, most of the pain and instability (13 and 11 respectively) while performing stepping up and down activity. This was followed by the sudden stop activity which had 8 painful and 6 unstable knees
DISCUSSION
The significant differences between TKR and normal control knees indicated that TKR did not restore normal kinetics, which could be due to TKR design, persistent muscle weakness or other factors.
ACKNOWLEDGEMENTS
Funded by New York University Medical School – NYU-Polytechnic Seed Grant Program.
