Intuitive Surgical Inc. shares dropped in the extended session Thursday after the surgical robot maker's quarterly results fell short of Wall Street estimates. Intuitive Surgical shares fell 7.3% after hours, following a 0.6% gain to close the regular session at $528.06. The company reported first-quarter net income of $306.5 million, or $2.56 a share, compared with $287.6 million, or $2.44 a share, in the year-ago period. Adjusted earnings were $2.61 a share. Revenue rose to $973.7 million from $847.5 million in the year-ago quarter. Analysts surveyed by FactSet had forecast earnings of $2.70 a share on revenue of $975.3 million.