Why are some tummy tuck lines so straight while others are like a zig-zag or not even? Is it the surgeon or the person's shape?

There was a time when Dr. Pauline Regnault taught tummy tuck techniques and introduced the W incision. Despite what Dr. Regnault saw as advantages, the aesthetics of the scar leave much to be desired. Dr. Regnault felt that putting the scar in the pubic hair (1975) kept the mons from being pulled up.  She also felt that the higher incision in the midline made it possible in all cases to remove the belly button donor site hole without having to close it as a small vertical. Her design was intended to put more tension laterally (which I think is a very good idea) and so equalize the midline and lateral tension. The upper flap usually has a longer skin edge than the lower, so the shape of the W made the length of the lower skin edge more equal to the upper, thus decreasing skin folds from the length discrepancy. Regnault P, Abdominoplasty by the W technique. Plast Reconstr Surg 1975; 55: 265. 
Despite these supposed advantages, times have changed. Less tension is present in the midline with advancement of the flap with progressive tension sutures, and with careful measurement and redraping, the upper and lower flaps can usually be brought  together with  any excess length of the upper worked in laterally.
Women today often do not have pubic hair, so the hairline is not as important as a smooth, pleasing and low hidden scar that does not rise higher than a small bikini. 

Thank you for this excellent question.  The more tummy tucks a surgeon does, in general, the neater the scars are.  However, there is not a single plastic surgeon around who does tummy tucks who hasn't had an uneven scar.  Sometimes it's the surgeon, sometimes it's the patient's anatomy.  For instance, uneven excess loose skin on the anterior thighs can often make the ultimate scar asymmetric because as you pull the abdomen down, you pull the anterior thigh up.   In my practice, I try to have the scar 2 finger breadths above the vagina, across the pelvic area, and then follow the crease between the abdomen and thigh.  I intentionally put tension between the mons pubis area and the abdominal flap to pull the vulva upward, decreasing or eliminating the stress incontinence.  To accomplish this, I flex the surgical table to 30 degrees to create this tension.  It is my belief, along with many other plastic surgeons, that the scar should be low and thin.  A beautiful scar above the bikini or panty line is still a scar.  The scar your plastic surgeon creates often can make or break the surgery.  However, a well hidden scar trumps a beautiful scar any day.

I make my incisions for a tummy tuck based on how patients wear there underwear.  If a patient's underwear tends to be straight across, I'll make my incision straight across.  I'll use a "W" type incision when patients like to wear their underwear real low.

You are quite perceptive.  Some surgeons use a w shaped pattern which I think looks messy.  I use a gentle smile shaped incision that I think heals much more neatly.  It is worth discussing this issue with the surgeon a head of time so that there is agreement on this issue.

Most tummy tuck incisions are curvilinear with a smooth line, based on the surgeon's design and how much tension is applied to the closure. A patient's anatomy does factor into this design somewhat. Consult with a few plastic surgeon's to determine if you are a good candidate or not for this procedure.Best wishes,Dr.Bruno

Thank you so much for sharing with us your question. 
 Every Surgeon has his own technique, protocol and before surgery he/shes communicate the patient and let them know approximately how the scar it will be. The heal, body type, skin, Surgeon technique, etc, are different and that are variable to keep it mind in the healing process.
That is very important to know your Surgeon previews surgery and share with her/him your concerns, question before have your Surgeon performed. Always choose a Board Certified plastic Surgeon. 
Regards,
Dr. Quiroz!

Dear FourtyandFabulous,
both, surgery technique and patient's factors like shape and size of patient, skin quality and elasticity will have an effect on the final results. I always try to place the incision as low as possible in order to allow the patient to hide the scar under bikini or underwear. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Thank you for sharing your excellent question.  Ultimately it is a combination of factors including the area of skin excess, physician training, patient healing, and the type of undergarments a patient wears.    Hope this helps.

A combination.   Scars are largely a function of the patient’s ability to heal. I create the best possible scar which is flat all the way across and is not a smiley face. This way the ends of the scar never rise above low cut clothing.  In addition, though healing is never entirely predictable, many patients have commented that the scar is not only low but the limbs are perfectly symmetric and straight.  

You correct on both accounts.  The incision shape is influenced by the surgeon's personal techniques as well as the tension needed to close it properly based on the individual patient.  I encourage you to discuss this concern with your surgeon so you understand what to expect after surgery.