Hernias may be broadly divided into two main groups, depending on whether they develop in the upper abdomen or in the groin (see the image below), and each group contains multiple types.
Groin hernias include the following:
Indirect inguinal hernia - Bounded by the inguinal (Hesselbach) triangle, an indirect inguinal hernia passes through the internal inguinal ring; it is the most common hernia type and is more commonly seen in males
Direct inguinal hernia - This type is similarly bounded by the inguinal triangle, but it passes directly through the muscular and fascial wall of the abdomen; it carries a minimal risk of incarceration
Femoral hernia - Originating below the inguinal ligament, a femoral hernia passes through the transversalis fascia and through the femoral canal; it presents a high risk of incarceration 
Abdominal hernias include the following:
Umbilical hernia - This type (see the image below) is seen traversing the fibromuscular ring at the umbilicus; commonly seen in infants, it usually resolves by the age of 5 years; repair is indicated when an umbilical hernia develops in older children or adults, is larger than 2 cm, or is incarcerated 
Epigastric hernia - This type is a midline hernia that passes through the linea alba
Spigelian hernia - This rare type is located at the lateral edge of the rectus abdominis and passes through the semilunar line
Before surgical repair of a hernia, manual reduction can return the tissue to its original compartment.  Reduction benefits the patient by mitigating associated symptoms, avoiding adverse outcomes such as strangulation, and permitting elective surgical repair, which has lower morbidity than emergency repair. [5, 6] The most common hernias amenable to reduction are described in this article.
The presence of a nonstrangulated hernia is an indication for manual reduction.  Although an incarcerated hernia can be strangulated without the usual signs and symptoms of strangulation, reduction should be performed for most incarcerated hernias when clinical evidence of strangulation is not present.
Although strangulation can be missed,  one prospective study showed that clinicians are usually correct in deciding when to reduce an incarcerated hernia and when to defer reduction of a strangulated hernia.  In addition, harmful outcomes to attempted reduction are unlikely in these unrecognized strangulated hernias.
Manual reduction is contraindicated in strangulated hernias. In such cases, nasogastric suction, fluid replacement, and antibiotic therapy can be started.
If the diagnosis of strangulated hernia is missed and manual reduction is performed, necrotic bowel may be introduced into the abdomen. This could result in clinical deterioration and could necessitate urgent reduction in the operating room. 
For the purposes of manual reduction, hernias are best classified into the following three groups  :
Easily reducible hernia
This classification also helps direct treatment.
Easily reducible hernia
If a hernia is easily reducible, the abdominal contents can easily be returned to their original compartment. Reduction not only allows symptomatic relief for patients but also reduces the risk of future incarceration.  Although reduction helps alleviate patient's symptoms, elective surgical repair is usually warranted for long-term management.
In some cases, nonoperative treatment may suffice for asymptomatic patients with easily reducible hernias. A large prospective trial suggested that in patients who are minimally symptomatic, nonoperative treatment can produce outcomes similar to those experienced by minimally symptomatic patients who undergo surgical repair. 
An incarcerated hernia cannot easily be returned to its original compartment. The overlying skin should appear to be normal, the contents should not be tense, and bowel sounds can sometimes be heard. The incarcerated tissue may be bowel, omentum, or other abdominal contents. A smaller aperture of herniation and adhesions can precipitate incarceration. An incarcerated hernia can often be reduced manually, especially with sufficient anesthesia. [9, 13]
Obstruction is a concern; a hernia is one of the three most common causes of obstruction. In addition to causing signs of obstruction, an obstructed hernia has a more tense appearance than a nonobstructed hernia, and radiographs may show bowel shadows at the site of herniation. (See the image below.)
A strangulated hernia (see the image below) is a surgical emergency in which the blood supply to the herniated tissue is compromised. Strangulation stems from herniated bowel contents passing through a restrictive opening that eventually reduces venous return and leads to increased tissue edema, which further compromises circulation and stops the arterial supply.
Such a hernia may be signaled in the early stages by severe pain and by tenderness, induration, and erythema over the herniation site. As tissue necrosis ensues, findings may include leukocytosis, decreased bowel sounds, abdominal distention, and a patient who appears to be toxic, dehydrated, and febrile. Mortality is high, and treatment should be initiated immediately.