What's the difference between behind the muscle and in front of the muscle implants? (Photo)

Thank you for the question. There are pros and cons to the placement of breast implants in the “sub muscular” position versus the "sub glandular position”.    I will try to outline some of the differences here I think it is in the best interests of most patients seeking breast augmentation surgery to have implants placed in the “dual plane” or sub muscular position.  This positioning allows for more complete coverage of the breast implants leading to generally more natural feel/look  of the implants in the long-term. This position will also decrease the potential for rippling and/or palpability  of the implants (which may increase with time, weight loss, and/or post-pregnancy changes). The submuscular positioning  also tends to interfere with mammography less so than breast implants in the sub glandular position. The incidence of breast implant encapsulation (capsular  contraction)  is also decreased with implants placed in the sub muscular position. On the other hand,  sub glandular breast implant positioning does not have the potential downside of “animation deformity” ( movement/ distortion of the breast implants  seen with flexion of the  pectoralis major muscle)  that can be seen with breast implants placed in the sub muscular position.    Best wishes.

Implants can be placed over the muscle or under the muscle. If it is placed over the muscle, it is called a subglandular placement. This is because the only tissue covering the implant will be the woman's breast gland. The other option is to place the implant under the muscle (submuscular). The muscle referred to in this case is a muscle called the pectoralis muscle. It is the main muscle of the chest and is located directly beneath the breast gland. These options are shown in the diagram on this page. 
The most common placement is submuscular. In order for the implant to fit under the muscle, and for the appearance to be aesthetically pleasing, some muscle fibers must be detached from the inferior ("bottom") of the breast and from the corner of the middle portion of the muscle ("inferior-medial"). The remaining attachments of the muscle are strong enough to allow full function of the muscle. Once the muscle fibers are detached to allow the implant to fit under it, the muscle will move upward slightly. You can see this in the diagram. The bottom of the muscle is no longer located at the bottom of the breast. Instead, the bottom of the muscle is located near the middle or lower 1/3 of the implant. Thus, the upper portion of the implant is covered by the muscle and the lower half or 1/3 of the implant will not be covered by muscle. The lower part of the implant will be covered by breast tissue and gland only. This is why a submuscular placement is also called a "dual plane." It's a "dual" plane because two kinds of tissue are covering it. 
Placing the implant under the muscle has several advantages. The muscle provides more coverage and cushion over the implant. This creates smoother contours, a more natural look, and prevents rippling or edges from being seen. Another advantage is that a submuscular placement has been shown to decrease the risk of capsular contracture. To learn more about breast augmentation, you can read “A Comprehensive Guide to Breast Augmentation."

The decision whether to place breast implants in front of or behind the pectoralis major muscle is an individualized one, and is dictated by the specific circumstances of a patient’s situation.
Advantages of placing an implant in front of the muscle include a faster operative and recovery time, including a reduced waiting time for the breast to assume its final shape.  This subglandular placement can be quite helpful in treating breast ptosis in some situations. Additionally, in women who are fitness models and bodybuilders, subglandular (in front of the muscle) placement of the breast implant may minimize distortion when the patient fires her pectoralis major muscle.  
Drawbacks of subglandular placement of breast implants include an increased incidence of rippling (visibility of the contour of the breast implant through the skin); an increased incidence of capsular contracture (hardening of the breast due to scar tissue), and a faster rate of stretching and descent of breast tissue (ptosis) over time.  
It is important to note that many of these potential complications can be minimized or eliminated by choosing the appropriate type and size implant for the right patient.  In certain situations, subglandular placement of the breast implant is the best answer.
Advantages of submuscular placement of a breast implant include a decreased incidence of rippling; a decreased incidence of capsular contracture; and a slower rate of stretching of the breast tissues and breast tissues.  
Patients often ask surgeons which technique of performing a surgery is “better” in the abstract, but this is the wrong framing of the question.  The question you really want answered  is, given your aesthetic goals, anatomy, and clinical situation, “what is the best technique for me?”