ANALYSIS OF THE SHOCK-ABSORBER EFFECT OF TWO MODELS OF LUMBAR DISC PROSTHESES

Abstract

Purpose: The objective of this study was to examine the shock absorption capacity of two currently marketed lumbar disc prostheses, a metal-polyethylene prosthesis and a metal-metal prosthesis. Shock absorption capacity, which could be a useful parameter for choosing between implants, has not been examined in the literature.

Material and methods: Two types of implants were tested: the Maverick prosthesis marketed by Medtronic, and the Prodic proposed by Spine Solution. Five implants of each type were tested. The disc prostheses were mounted on a testing device designed to analyse shock transmission by application of a constant force. Force captors were positioned on the upper and lower parts of the implant being tested. The force delivered and the force perceived on the opposite side of the implant were recorded simultaneously. The implant was submitted to a static loading force of 350 N to which was added a 100 N oscillating vibration force delivered at a frequency varying from 0 to 100 Hz. A supplementary 250 N shock was also applied every 10 s. The spectrum and frequency of each input and output were recorded. Vibration and transmission of the shock though the implant were defined as the ratio of the output over input spectra. Measurements were taken for all frequencies between 0 and 100 Hz. Phase deviation was calculated to characterise the shock absorption effect.

Results: The phase deviation between the input and the output signal was less than 10 for both prostheses. Under loaded oscillating vibration, shock transmission was greater than 99.8% for both implants. In the 1–100 Hz frequency interval, the difference in shock transmission was less than 0.3±0.1% between the two implants. More than 98% of the supplementary 250 N shocks were transmitted by both implants. The difference between the two implants was thus less than 0.8% and can be considered negligible since the machine’s test sensitivity was 0.5%.

Conclusion: The two implants tested exhibited the same capacity to absorb and transmit vibration and shocks. Shock absorption capacity was close to zero or at least less than the sensitivity threshold of the testing device. This degree of freedom is not sufficient to use shock absorption capacity as an argument for choosing between the two implants currently available.