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Orthopedic and Rehabilitation Engineering

Orthopedic and Rehabilitation Engineering - Poster Session E

Poster AA2 - Frictional properties of New Zealand white rabbit knees with and without the infrapatellar fat pad

Saturday, October 26, 2024
10:00 AM - 11:00 AM EST
Location: Exhibit Hall E, F & G

Introduction:  The infrapatellar fat pad (IFP) is an adipose tissue structure located posterior to the patellar tendon and distal to the patella. Blunt force impact to the knee can cause inflammation of the fat pad, which has a rich supply of blood and nerves. It can be a major source of anterior knee pain. The IFP is believed to provide protection of the underlying bones, ligaments, tendons, and nerves from impact forces. The IFP can also become pinched between bones and aggravated by repetitive movement causing impingement syndrome (Hoffa’s syndrome). This study investigated whether the IFP plays a role in knee friction during passive knee flexion-extension movements. New Zealand white rabbit cadaver knees were tested using a passive pendulum friction testing device under a joint load of 15N (about 40% of BW) at a flexion angle of 130°. If the IFP acts as a shock absorber, we might expect that removing it would decrease the ability of the knee to dampen motion during a passive flexion-extension swing. On the other hand, if the IFP aids in joint lubrication by helping to distribute synovial fluid, we might expect removal of the IFP to increase the coefficient of friction as measured in a pendulum friction test. Our null hypothesis was that we would find no difference in joint friction between knees with an intact IFP and knees with no IFP.

Materials and

Methods:  Twelve knees from six (12-18 month) female New Zealand white rabbits were prepared. All soft tissue was removed except for the surrounding ligaments and the joint capsule. The tibia was held fixed at 30° to the horizontal and the femur was attached to the pendulum apparatus, allowing the knee joint to serve as the pivot flexion-extension axis for the pendulum. The oscillation frequency of the pendulum was approximately 0.7 Hz. An iPhone (14 Pro Max) and the Phyphox app were used to collect gyroscopic data which were exported to MATLAB.
Each knee was first tested intact (Control). Then, an incision was made parallel and medial to the patello-femoral ligament, extending from the distal pole of the patella down to the tibial tuberosity. The fat pad was located, and palpated with forceps, and the incision closed with a single suture knot (Sham). The IFP was extracted following testing, and the knee retested without the fat pad (NFP). Each test was done five times for a total of fifteen tests per knee.
A Fast Fourier Transform (FFT) was used to convert the angular velocity-time data into the frequency domain. The FFT data was normalized, and a cutoff frequency was determined by identifying the first peak before high frequencies dominated. This cutoff frequency was used to design a second-order high-pass Butterworth filter to remove low-frequency noise and minimize gyroscopic drift. Subsequently, cumulative trapezoidal integration was performed to obtain angular position data.

Results, Conclusions, and Discussions: The knee friction coefficient (μ) was calculated by averaging amplitude decay over all pendulum cycles starting from a peak amplitude of 10° until equilibrium. Cyclic decaying amplitudes between 10° and equilibrium were calculated using Stanton’s boundary friction model (μ = ∆θL/4r), where ∆θ = mean angular change in flexion-extension, L = distance between pendulum center of mass and the flexion-extension axis, r = radius of the femoral condyle. The mean (range) knee flexion angle at rest was 130° (range 118° – 146°).
Coefficients of friction were analyzed (JASP 0.8.13) by one-way repeated measures ANOVA with three factors (Control, Sham, NFP) and two planned contrasts (Sham-NFP and Control-Sham). Slight differences were observed (P = 0.015) between the means (SD) of the coefficients of friction for the Control = 0.023 (0.003), Sham = 0.022 (0.004), and NFP = 0.021 (0.003) groups. However, the power of the study was low (0.25), and the effect size was small (ω² = 0.052 < 0.06). No differences could be seen between Control vs. Sham (P = 0.219), and the evidence for a difference between Sham vs. NFP was weak (P = 0.068).
Therefore, we conclude that removing the IFP did not result in a meaningful change in the coefficient of friction as determined by Stanton’s equation when the knee motion was analyzed over a flexion-extension range of 120° –140°.
Many investigators have used the pendulum method to extract both the boundary friction coefficient (as in our study) and the viscous coefficient of friction. Future work should, therefore, include analyzing the data with a viscous friction model. In addition, because the results of this study were obtained over a range of motion in which the knees were closer to full extension when the patellar tendon and retinaculum are relatively lax, further testing is needed over smaller flexion angles where the IFP is more likely to be placed in compression due to tension in the patellar retinaculum and patellar tendon (ligament) tissues.

Acknowledgements (Optional):  Research reported in this abstract was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under grant number R15AR082624. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.