Articular cartilage and meniscus reveal higher friction in swing phase than in stance phase under dynamic gait conditions

Abstract

Most previous studies investigated the remarkably low and complex friction properties of meniscus and cartilage under constant loading and motion conditions. However, both load and relative velocity within the knee joint vary considerably during physiological activities. Hence, the question arises how friction of both tissues is affected by physiological testing conditions occurring during gait. As friction properties are of major importance for meniscal replacement devices, the influence of these simulated physiological testing conditions was additionally tested for a potential meniscal implant biomaterial. Using a dynamic friction testing device, three different friction tests were conducted to investigate the influence of either just varying the motion conditions or the normal load and also to replicate the physiological gait conditions. It could be shown for the first time that the friction coefficient during swing phase was statistically higher than during stance phase when varying both loading and motion conditions according to the physiological gait pattern. Further, the friction properties of the exemplary biomaterial were also higher, when tested under dynamic gait parameters compared to static conditions, which may suggest that static conditions can underestimate the friction coefficient rather than reflecting the in vivo performance.

Figure 1

Dynamic friction testing device consisting of a dynamic materials testing machine (ElectroForce 5500, including a 1 DOF load cell, 200 N, accuracy class ≤ 1%, WMC-50-456, both BOSE/TA Instruments, New Castle, USA) equipped with a linear motor (linear stage VT-75, PI miCos GmbH, Eschbach, Germany), which was mounted on an additional aluminium frame (left). This frame was designed out of four linear guidance, an intermediate plate, a ball cushion (not shown in detail), the pin sample holder, a second load cell for measuring the resultant friction force F_F (3 DOF, maximum F_x,y = 20 N, maximum F_z = 50 N; accuracy class: 0.5%; ME Meßsysteme GmbH, Henningsdorf, Germany) (right) and additional counter weights (not shown).

Figure 2

Overview of the three friction test scenarios and the resultant applications of load (F_N) and motion: (motor) position and the approximated velocity in mm/s.

Figure 3

Comparison of the friction coefficients (median with raw data) for each material pairing (M, TC, S vs. FC, divided by column) obtained in the three different friction test scenarios (FT-I: F_N = const., v acc. to gait cycle, FT-II: F_N acc. to gait cycle, v = const., FT-III: F_N and v acc. to gait cycle, divided by rows). *p ≤ 0.05 with a minimum actual power of 70.1% (FT-II scaffold vs. femoral cartilage).

Figure 4

Comparison of the friction coefficients (µ_end) obtained for each material pairing: tibial cartilage (a), meniscus (b) and scaffold (c) each against femoral cartilage within the three different friction test scenarios (n = 8–10, mean ± standard deviation and raw data; ○ FT-I, *FT-II, • FT-III), *p ≤ 0.05 with a minimum actual power of 96.1% and 73.1% for the comparisons the meniscus and scaffold friction coefficient, respectively.

Figure 5

Comparison of the friction coefficients (µ_end) obtained within each friction test scenarios (○ FT-I: A, *FT-II: B, • FT-III: C) for the different material pairings (TC, M, S vs. FC; n = 8–10, mean ± standard deviation and raw data) *p ≤ 0.05 with an actual power of approximately 99%.