The Phineas Gage case made an important but indirect contribution to the development of brain surgery.  Although there had been operations for abscesses of the brain before 1885, it was in that year that the first brain surgery for the removal of a tumour was carried out.  What made this and later operations possible were aseptic methods of operating and knowledge of where some of the functions of the brain were localised.  It was to this latter that the Gage case contributed.

By about that time, in 1884, the American neurologist, M. Allan Starr, had collected the first large series of cases in which injury or damage to reasonably distinct areas of the brain could be related to particular symptoms.  His cases included a number in which there was injury to or tumours of the frontal lobes.  Starr's comparisons began with the frontal lobes and actually used Gage as a standard.  As he said:

"Lesions affecting the three frontal convolutions may be classed together. Ever since the occurrence of the famous American crowbar case it has been known that destruction of these lobes does not necessarily give rise to any symptoms.  That case is given in order to compare others with it."

For Starr the absence of symptoms simply meant no impairment in Gage's ability to use his senses and to move his muscles (e.g. no paralysis). But Starr granted that there were some mental symptoms.  In his summary he said that Gage's "disposition was altered, and ... he was irritable, easily excitable, and emotional."   He advocated using such mental changes to diagnose frontal tumours.

Nine years later, in 1894, Starr acted on his own diagnostic recommendation.  McBurney and he operated on a patient who had noticed himself gradually becoming dull in his thinking, generally weak, lazy, slow in mental activity, and unable to express his ideas reasonably quickly.  Although there were also physical symptoms, his was, they claimed, "the first case ... in which operative interference has been so directly dictated by the existence of mental symptoms." They drew a direct comparison with the mental changes shown by Gage in planning the site of the operation and removed a tumour from the patient's left frontal lobe.

In 1879, well before the McBurney and Starr operation, the Scots surgeon William Macewen diagnosed and operated for a tumour lying outside of the brain proper.  He thought it was pressing on the left frontal lobe mainly because the patient's mental symptoms included "obscuration of intelligence, slowness of comprehension, [and] want of mental vigour."  There is a very slight possibility that Macewen used knowledge of Gage in planning to operate on the frontal lobes.

However, it soon became clear that in only about half the cases of frontal tumour were there any 'mental symptoms' and only in a minority of those did the symptoms resemble Gage's.  Partly for that reason these symptoms ceased to be used for planning frontal operations.  At about the same time, in the early 1920's, Walter Dandy, the American brain surgeon, developed a more radical method of removing tumours.  He had found that about 60% of brain tumours could not be removed because they were not sufficiently differentiated from the tissue around them.  Dandy's new method removed the lobe containing the abnormal tissue.

Radical surgery like this was not performed often and was restricted to those patients who would otherwise have died from the effects of the tumours.  But, as cases accumulated, it was noted with more than a little surprise that the effects on the patient's behaviour of the removal of such large areas were minimal.  This lack of effect led Dandy to make a passing comparison with Gage.  By the early 1930's, a number of such radical frontal operations had been performed, and it is noticeable that Gage seems to be mentioned only once, again in passing, in the discussions of them.  (See also 'Gage and lobotomy' on this site).

What seems to have happened in the history of brain surgery is that 'mental symptoms,' supposedly resembling Gage's, were first used to diagnose frontal tumours.  Later, after that method was abandoned, Gage may have come into consideration when the operations for the resection of whole lobes were developed.  His surviving his injury may have reinforced the belief that large areas of the brain could be removed with relative impunity.  It seemed that the brain could be operated on without causing death or major impairment of psychological functions.

Further reading:

Macmillan, M. (2004). Localisation and William Macewen's early brain surgery. Part I: The controversy. Journal of the History of the Neurosciences, 13, 297-325

Macmillan, M. (2005). Localisation and William Macewen's early brain surgery. Part II:  The cases. Journal of the History of the Neurosciences, 14, 24-56.