Background Decreased shock absorbing of the cartilage between the femur-tibia is assumed in patients with knee osteoarthritis (OA), and the impulse response method was considered to apply the evaluation of initial degeneration for cartilage.

Objectives The purposes of the present study were 1) to establish the impulse response method deciding the appropriate location for input and output impulse to assess cartilage degeneration in the knee joint, and 2) to determine the differences between young and elderly women using this method.

Methods 1) Ten young women (age: 20 ± 0 years) participated. Input value was measured by an input device attached to the distal end of the load cell. Output value was measured by a one-axis accelerometer. Input locations were the medial epicondyle and the medial condyle. Output points were distal 5 cm, 10 cm and 15 cm from the knee joint space. The input-output ratio of ten times was calculated to six combinations of input points and output points. The intraclass correlation coefficient (ICC) was calculated by re-measurement several days before.

2) Two groups of 26 elderly (age: 69 ± 6 years) and young (age: 21 ± 1 years) women each were recruited. The elderly group included those who matched the knee osteoarthritis classification criteria by clinical findings. The outcome measures were the input-output ratio (unit: ×9.8 m/s²/N) and the attenuation rate that was the ratio of a second amplitude for the first amplitude, the frequency, and the power spectrum. The power spectrum was obtained by Fast Fourier Transform and each 10 Hz ranging from 1 to 100 Hz was used as the final analysis [1,2].

Results 1) ICCs of output location at 15 cm point with two input locations were greater than 0.9. The medial condyle (22.7±3.1 [N]) was significantly greater than compared to the medial epicondyle (18.4±2.8 [N], p<0.001).

2) The input-output ratio was 0.11 ± 0.03 in the young group and 0.11 ± 0.04 in the elderly group; there were no significant differences in the input-output ratio. The attenuation rates of the young and elderly groups were 0.38 ± 0.14, and 0.36 ± 0.19, respectively, and there were no significant differences in the attenuation rate of the two groups. The frequencies (unit: Hz) were 68.7 ± 13.1, and 77.2 ± 25.2, respectively, and there were no significant differences in the frequency of the two groups. The power spectrum (unit: (×9.8 m/s²)²) of the young group (1.21×10^–4±1.06×10^–8) was significantly greater than the value of the elderly group (2.97×10^–4±1.61×10^–7) at 21–30 Hz (p=0.036). Additionally, the power spectrum of 21–30 Hz was correlated to knee pain (r=0.39, p=0.049).

Conclusions The location of the medial condyle was appropriate in the input point, and 15 cm point distal from the knee joint space was a suitable location as the output point based on the present findings.

In 21–30 Hz, the impulse absorbing by the cartilage in elderly women was less than compared to the values in young women. The impulse response method of the present study could be applied to assess early knee osteoarthritis.

1. Fulcher GR, et al. BMC Musculoskelet Disord 2009; doi: 10.1186/1471-2474-10-61.

2. Tanaka N, et al. J Musculoskelet Res 2011; 14: 1–9.

Disclosure of Interest None declared