While four teams prepare to battle for a spot in Super Bowl XL on Sunday, the other 28 teams are busy preparing for next season. And the Chicago Bears are a team with high hopes for 2006.

After all, the Bears went 11-5 this season despite scoring a little more than 16 points a game. But the offense showed a spark after quarterback Rex Grossman returned from a broken leg at the end of the season, and now the Bears can devote the entire off-season to improving on that side of the ball. Grossman will have the off-season to learn more about the system, and the team may bring in a crafty veteran to compete for the starting job. All the Bears need to do is improve their offense, and they’ll be a leading Super Bowl contender.

There’s only one problem with this plan: The defense isn’t going to be this good again in 2006.

That idea would sound ridiculous to a Bears fan. Chicago’s defense is young and most of its players are signed to long-term contracts. But great defenses are historically very temporary. There’s no way around the fact that offense is more consistent from year to year than defense is, and teams cannot count on great defenses to stay great.

Consider the top dozen defenses from 1990 through 2004, as judged by fewest points allowed. These 12 teams averaged 12.3 wins and allowed an average of just 202 points. Four of them won Super Bowls.

The following season, half of these teams went 8-8 or worse. The same 12 teams averaged 8.8 wins and allowed an average of 293 points. Tampa Bay went from a Super Bowl title in 2002 to 7-9 in 2003.The Giants went from a Super Bowl title in 1990 to 8-8 in 1991.And the team that Chicago fans don’t want to think about, the 2001 Bears, went from 13-3 to a dismal 4-12 in 2002.

Only the 1991 New Orleans Saints won more games or allowed fewer points the following season. The 1991-92 Saints are the last team to lead the league in either points or yards allowed for two straight seasons. No team has managed this accomplishment since the modern salary cap rules took effect in 1994.

There are plenty of reasons to explain this trend. Turnover numbers can swing wildly from year to year, particularly fumble recoveries. Players leave as free agents – not just star players, but role players who can make more money and see more playing time as starters elsewhere. And defensive players suffer more injuries and are harder to replace when hurt. A team that escapes injury one year will rarely escape it again the following year.

Losing one key player can destroy a team’s total performance by starting a domino effect. Other players are forced to cover for the injured star by learning new roles, while situational players are forced to play every down, trying to utilize skills they don’t really have. Losing role players in free agency often doesn’t seem like a big deal on the surface, but it hurts team depth, making it even harder to withstand big injuries.

Injuries in the front seven also have a greater effect than injuries in the secondary. Take the New England Patriots as an example. They weathered the loss of both starting cornerbacks in 2004, but their defense faltered this year without defensive lineman Richard Seymour and linebacker Tedy Bruschi. When they returned at midseason, the Patriots defense returned to a high level of play.

The Bears probably will not suffer from free-agent defections. No starters are going to be unrestricted free agents, and the only restricted free agent is linebacker Hunter Hillenmeyer. While another team might swoop in to try to lure him away with a big contract, the Bears have the right to match any offer.

But the Chicago defense had very few injuries this year – the only starter to miss significant time was safety Mike Brown. It would defy probability for the Bears to make it through another season without a significant injury in the front seven.

For the worst-case scenario, the Bears need only look back one year, to the 2004 Buffalo Bills. Last year, the Bills began 0-4, but turned things around with a 9-3 run and fell just short of the playoffs. The turnaround was keyed by the defense, which was the best in the league from October forward.

In fact, the Buffalo defense may have been better than the current Bears defense. In their final 12 games, the 2004 Bills allowed 245 net yards per game, and 4.0 yards per play. This year’s Bears, even if we don’t count the final game in which Minnesota ran up the score on Chicago’s second stringers, allowed 275 net yards per game and 4.3 yards per play.

Like the Bears, the 2004 Bills largely escaped defensive injuries. But in 2005, their best player, linebacker Takeo Spikes, went down with a torn Achilles in the third game of the season. Buffalo had already lost space-filling defensive tackle Pat Williams to Minnesota in free agency. Even though the other nine defensive starters were the same as the previous year, Buffalo allowed 344 net yards per game, and dropped from 9-7 to 5-11.

Nothing says the Bears will suffer a similar injury or go through a similar collapse. But it is virtually guaranteed that their defense will give up more yards and points next year. If the Bears work on their offense and assume the defense will just be the same in 2006, they may be in for a nasty surprise.

Mr. Schatz is the editor in chief of FootballOutsiders.com.