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Hip and Knee Arthroplasty - Surface Hippy Guide to Hip Resurfacing

Hip and Knee Arthroplasty - Surface Hippy Guide to Hip Resurfacing

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Observed Component Years: The cumulative number of years that a procedure is at risk of being<br />

revised. This is calculated for each procedure as the number of days from the date of the primary<br />

procedure until either the date of revision, date of death or end of study (31/12/2007) whichever happens<br />

first. This is then divided by 365.25 <strong>to</strong> get the number of ‘component years’. Each primary procedure<br />

then contributes this calculated number of component years <strong>to</strong> the overall observed component years for a<br />

particular category.<br />

For example<br />

1. A primary <strong>to</strong>tal hip procedure performed on 1/1/2007 was revised on 1/7/2007. Therefore, the<br />

number of days that this procedure is at risk of being revised is 183 days. This patient then<br />

contributes 0.5 (183/365.25) component years <strong>to</strong> the overall number of observed component<br />

years for the <strong>to</strong>tal hip procedure category.<br />

2. A patient with a primary procedure on 1/1/2007 died without being revised on 1/4/2007. This<br />

individual has 0.25 component years.<br />

3. A primary procedure on 1/1/2007 <strong>and</strong> has not been revised. This individual has 1 component year<br />

(as observation time is censored at 31/12/2007).<br />

Survival Curve: A plot of the proportion of subjects who have not yet experienced a defined event (for<br />

example death, revision of prosthesis) versus time. The Kaplan-Meier method is the one most commonly<br />

used. The curve takes account of subjects whose ultimate survival time is not known, a phenomenon called<br />

“censoring”. The survival estimate at each time is accompanied by a confidence interval based on the<br />

method of Greenwood. An interval is interpretable only at the time for which it was estimated <strong>and</strong> the<br />

sequence of intervals (depicted as shading on the Kaplan-Meier curve) cannot be used <strong>to</strong> judge the<br />

significance of any perceived difference over the entire time course.<br />

196

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