During the 1991-92 season, West Indies played mainly one-day cricket, taking part in tournaments in Sharjah--where Ambrose took seven wickets, including an analysis of five for 53--and Australia, and took part in the World Cup in Australia and New Zealand. In this tournament, Ambrose took seven wickets in seven games at an average of 33.57 and was the seventh most economical bowler among those who played more than one game. West Indies finished sixth in the qualifying table and failed to reach the semi-finals. Ambrose returned home to play twice for the Leeward Islands in January 1992.  In April 1992, South Africa toured West Indies for the first time, and played their first Test match for 22 years. Ambrose played in all three ODIs, all of which were won by West Indies. The Test match was the first time West Indies bowled under a new playing regulation which permitted only one bouncer per over; this seemed to affect the home bowlers, but Ambrose took two for 47 from 36 overs. South Africa began the final day of the match requiring 79 runs to win with just two batsmen out, but Ambrose and Courtney Walsh took the last eight wickets for 26 runs to bowl West Indies to a 52-run win. On a difficult pitch for batting, the ball bounced unevenly, and both bowlers concentrated on accuracy. Ambrose took six for 34 in the second innings, and was named joint man of the match; in just over 60 overs, he took eight for 81 in the match.  Returning to play for Northamptonshire, he was less effective. Hampered by a knee injury, which necessitated surgery after the English season, and suffering from many dropped catches, he took 50 first-class wickets at an average of 26.14, but his performance compared unfavourably with other bowlers on the team. He was more effective in the NatWest Trophy, a one-day competition that Northamptonshire won that season, in which he conceded fewer than two runs per over across five games.

Answer this question "Why was curtly less effective?" by extracting the answer from the text above.
knee injury, which necessitated surgery after the English season,