Petit triangle (synonym: lumbar triangle, lumbar trigonum) – site of the posterior abdominal wall, bounded from below by the iliac crest, medially along the edge of the latissimus muscle of the back, transversely with the outer oblique muscle of the abdomen; place of liberation of lumbar hernia.
The lumbar triangle can refer to either the inferior lumbar (Petit) triangle, which lies superficially, or the superior lumbar (Grynfeltt) triangle, which is deep and superior to the inferior triangle. Of the two, the superior triangle is the more consistently found in cadavers, and is more commonly the site of herniation; however, the inferior lumbar triangle is often simply called the lumbar triangle, perhaps owing to its more superficial location and ease in demonstration.
The lumbar triangle is a well-defined triangular space in the posterolateral lumbar region. Also known as the inferior lumbar triangle, its boundaries are: inferior, the iliac crest; anteromedial: latissimus dorsi muscle; posterolateral: posterior border of the external oblique muscle. The triangle has a superior apex, and the floor of the triangle is the internal oblique muscle.
The triangle is named after Jean-Louis Petit (1674 – 1750), a French Surgeon who is said to have been an anatomy teacher at an early age and became a surgeon when he was only eighteen!
The lumbar triangle is an area that is not as thick as the rest of the abdominal wall and as such it is a site of potential weakness that can lead to a lumbar hernia, also known as Petit’s hernia.
Inferior lumbar (Petit) triangle
The margins of the inferior lumbar (Petit’s) triangle are composed of the iliac crest inferiorly and the margins of two muscles – latissimus dorsi (posteriorly) and external abdominal oblique (anteriorly). The floor of the inferior lumbar triangle is the internal abdominal oblique muscle. The fact that herniations occasionally occur here is of clinical importance.
Superior lumbar (Grynfeltt-Lesshaft) triangle
The superior lumbar (Grynfeltt-Lesshaft) triangle is formed medially by the quadratus lumborum muscle, laterally by the internal abdominal oblique muscle, and superiorly by the 12th rib. The floor of the superior lumbar triangle is the transversalis fascia and its roof is the external abdominal oblique muscle.
Hernia of Petit’s triangle
A lumbar hernia may occur anywhere in the lumbar region which is bounded above by the 12th rib, below by the crest of the ilium, in front by a line drawn vertically downward from the anterior extremity of the 12th rib to the crest of the ilium, and behind by the vertebral column and the erector spinae muscles. The two main areas of lumbar herniation are the superior lumbar triangle (Grynfeltt-Lesshaft) and the inferior lumbar triangle (Petit). Goodman and Speese,15 in dissections of 76 cadavers, found the superior lumbar triangle present in greater than 93 per cent of the specimens. To expose the “triangle,” which sometimes takes a quadrilateral, deltoid, trapezoid, or polyhedral shape,37 one must retract both the latissimus dorsi and the serratus posterior inferior muscles. If a triangle is present, it is inverted, the base being formed by the lower border of the 12th rib and the portions of the serratus posterior inferior. The anterior border is formed by the internal oblique and the posterior border is the quadratus lumborum; these borders are easily remembered if the area is thought of as the lumbocosto-abdominal triangle.27 The floor of the triangle is the transversalis fascia which is a portion of the fusions of the lumbodorsal fascia which continues anteriorly as the aponeurosis of the transversus abdominis muscle and posteriorly splits into three layers which include the quadratus lumborum and the sacrospinalis. The size and shape of the space depend upon the development of the bordering muscle masses, the length and position of the 12th rib and the position of its muscle attachments, and the position of attachment of the overlying latissimus dorsi. Weak points in the superior lumbar triangle are immediately beneath the 12th rib where the transversalis fascia is not covered by the external oblique and where it is perforated by the 12th dorsal intercostal neurovascular bundle.27 The inferior lumbar triangle is.normally present in adults, occasionally present in children. It is usually triangular shaped with the base being the iliac crest. The posterior border is the free edge of the latissimus dorsi and the anterior border is the external oblique. The musculofascial floor is much stronger than that of the suSubmitted for publication February 26, 1970. perior lumbar triangle, being composed of the lumbodorsal fascia with underlying internal oblique and transversus abdominis. The superior lumbar triangle is larger and more constant than the inferior lumbar triangle, possibly accounting for the greater frequency of hernias in this area.
Lumbar hernias are rare; scattered reports of hernias of both superior and inferior lumbar triangles have appeared in both the English and foreign literature since the collection of 186 cases in 1948 reported by Watson,37 the total now being about 220. Hernia of the superior lumbar triangle is most commonly associated with either straining or direct trauma in the lumbar region. The diagnosis is relatively easy if there is a reducible mass beneath the 12th rib which transmits a cough impulse. An additional case of spontaneous hernia of the superior lumbar triangle with a method of successful repair is reported.
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