To the OP:
Although I would not discourage you from pursuing a profession in Dentistry, but I do have to further caution you about the potential of further damaging your back during your 4 years of dental education and on forth during your private practice years.
I personally have no back problems (that I'm aware of) and here are my experiences after only 1.25 years of dental education under my belt:
1) First-year second semester: in Operative course when we do all those restoration preps for amalgams and composites, since these lab courses are always 4 hours in the afternoons three times a week, I frequently get sore backs and light headaches after these sim lab courses after 5 PM when I get home. At Nova, we have specific lectures on correct posture and ergonomics and YES, the faculty does drills it in our heads. Even after all that WARNING, training, and/or reinforcement, you will always find yourself slipping from that "perfect" sitting position and find yourself slauching over the manniquin someway or somehow. Sometimes EVEN with loupes on!
I guess sitting right just takes some getting used to, but be prepared to have those long afternoons where you find yourself stretching and twisting your back in attempts to untie some of those knots. This is where a nice masseuse can come in!
2) First-year second semester AND Second-year first semester: Periodontology clinic. We have a 4-hour long prophy clinical session scheduled for actual patients. Believe me, after these four hour sessions, my back is also sore and "knotted". Man, am I glad that I am not a Hygienist!
I guess what I'm trying to say is, even for a normal back, back soreness will be expected in dental school. Now, if you already have a "bad" back, I would hate to see you agitate it further! Maybe you won't, but most likely you will.
I look around in the sim lab during any lab practical (exams on manniquins), I can always expect at least 10 classmates will sit the wrong way or leaning over improperly. This is a given, even with all that warning in lectures and from faculty. I'm sure all current dental student can attest to this.
Also, many of our Adjunct Faculties are from private practices on Disability because of their back. I would say, if there were to be one negative aspect that comes with the profession of dentistry, it would be the potential to injure your back.
I wish you the best of luck in your application process to your favorite dental school. My best advice for you, work out and stay fit (build a strong lower back - hyperextension exercise is great for this purpose), stretch out often, and have your SO massage you as much as possible.
Any other thoughts from current dental students?