In anatomy, superficial is a directional term that indicates one structure is located more externally than another, or closer to the surface of the body.
The opposite of superficial is deep. For example, the spine is deep in the body, while the skin is superficial. The term superficial is a relative one. This means it is not limited to structures on the very outside of the body, such as the skin or eyes. Instead, it's all about what is located where relative to other structures.
And the use of this term is not limited to one type of body structure, either. It can equally refer to muscles, bones, organs, and more.
Let's go through a few examples of how this term is used when it comes to back pain.
Superficial Back Muscles
Anatomy geeks describe and understand back muscles in layers. The most superficial layer is a group of 4 collectively called the superficial layer of the extrinsic back muscles. (There's also an intermediate layer which some call the deep layer to the extrinsics, but we won't get into that here.)
The 4 superficial-most back muscles are Trapezius, latissimus dorsi, rhomboids (major and minor), and levator scapula. 1Note the highlighted triangular shape on the model in the image. That's the trapezius muscle, the most superficial of all back muscles. (FYI, the latissiumus dorsi, which is the 2nd most superficial back muscle is not highlighted in this image, but is located down from the trapezius.)
The beauty of a superficial back muscle is that it's right there beneath the surface under your skin. You can reach out and touch it, as long as you're fairly accurate about it. And because each of the extrinsics is pretty large, correctly locating any one of them by touching it through the skin is easily accomplished.
Of course back muscles plus other structures — spine-related and otherwise — don't stop at the superficial layer. Several more layers of back muscles live beneath the extrinsic group. We can say that any one of the extrinsic back muscles are (or the group as a whole is) "superficial to the" and then you can name the structure. For example, the trapezius muscle is superficial to the spinal column. Or the latissiumus dorsi is superficial to the kidney, etc.
Superficial Core Muscles
When you talk about your core muscles — those all important, back-protecting abs — the same idea can be applied. The most superficial ab muscle through most of its length is the rectus abdominus. This is the beautiful 6-packer you can see on body builders and fitness buffs who make it their business to sculpt and define their musculature.
But 5 other ab muscles are located deep to the rectus abdominus. They are two external obliques, two internal obliques, and one transverse abdominus. So we can say that the rectus abdominus is superficial to the external obliques, and the external obliques are superficial to the internal obliques, etc.
Superficial as Medicalese
One last consideration for our anatomical term. How might a spine surgeon use this during her day? Here's an example:
One common complication following spine surgery is surgical site infection or SSI. Jad Chahoud, author of the 2014 study "Surgical Site Infections Following Spine Surgery: Eliminating the Controversies in the Diagnosis" (published in Front Line Medicine) calls SSIs a "dreaded complication with significant morbidity and economic burden." He goes on to explain that SSIs can be superficial or deep. Doctors can tell an SSI is superficial when they can see drainage at the wound site, he explains.2
In case you're interested (for example, if you have a spine surgery planned) Chahoud says that most SSIs are caused by staph.2 He offers some insight on risk factors for SSI, as well. If you smoke, have diabetes, take steroids or need a transfusion your risk is higher. Once in the surgery, there are other types of risk factors. They include the degree of invasiveness of the procedure being done on you, the type of fusion (if you're having a fusion) and, whether or not something's being implanted.
These risk factors hold true for both superficial and deep SSIs. Superficial SSIs are generally treated with a combination of wound care (superficially) and antibiotics.