Lumbar vertebrae

Function

Five bones built to carry weight

The lumbar region of the spine has five vertebrae (the bones stacked to form the spine), named L1 to L5¹. They carry more body weight than any other vertebrae, and have large, thick bodies as a result². Together they form a lordotic curve, the inward curve of the lower back¹. That curve develops as a child begins to stand and then walk².

Two views of the lower spine with the five lumbar vertebrae coloured orange. From the right side, with the front of the body to the right and the hip bone left out, the five bones are stacked in a curve that bows forward, with a pale disc between each pair. The last thoracic vertebra sits above them and the sacrum below, tilted backward. Each lumbar vertebra has a thick block-shaped body at the front and an arch with processes at the back. From behind, the five arches overlap like tiles down the midline, each with a transverse process sticking out on both sides, and the lowest vertebra rests on the sacrum beside the right hip bone. — The five lumbar vertebrae, from the right side and from behindRendered by Your Basic Trainer from the Open 3D Model by AnatomyTOOL, which builds on Z-Anatomy and BodyParts3D (Database Center for Life Science). (source, CC BY-SA 4.0). Rendered from the 3D model with only the last thoracic vertebra, the lumbar vertebrae and their discs, the sacrum and the right hip bone shown, the lumbar vertebrae coloured orange, and the hip bone left out of the side view

The parts of one vertebra

A typical vertebra has a body, an arch and seven processes (parts of a bone that stick out)².

The body is the front part, and it supports body weight². It is spongy bone holding red marrow, inside a thin outer layer of compact bone¹. It bears most of the load passed down the length of the spine¹.
The arch forms the back of the vertebra². It has a pedicle on each side, anchored to the back of the body, and a lamina on each side forming the roof². In lumbar vertebrae the pedicles are very strong, and the laminae are broad, short and strong⁷.
The transverse processes and the spinous process are short in lumbar vertebrae, and the blunt spinous process points backward².

The openings between arch and body line up to form the vertebral canal, the bony passage that protects the spinal cord².

Engraving of a single lumbar vertebra seen from its right side, with the front of the bone to the right. The large, drum-shaped body is on the right. A short, thick pedicle joins the body to the arch behind it. From the arch, a superior articular process points up, an inferior articular process points down, a transverse process points toward the viewer and a broad spinous process points backward. The body, pedicle, transverse process, spinous process and both articular processes are labelled. — A lumbar vertebra seen from the sideHenry Vandyke Carter, in Henry Gray, Anatomy of the Human Body (1918), Fig. 92. Public domain, via Wikimedia Commons. (source, Public domain)

Each vertebra has an upward-facing and a downward-facing articular process on each side². The upper pair of one vertebra joins the lower pair of the vertebra above, forming slightly movable joints². In a lumbar vertebra these processes are large, and they rise and hang from the points where pedicles and laminae meet^2,7. The joints they form are the facet joints¹. Facet joints keep the vertebrae aligned, limit how far they move and bear weight in certain positions¹.

The pars interarticularis is a small segment of bone between the upper and lower articular processes³. The lamina and the pedicle border it too⁸. This page calls it the pars.

L5 has the largest body and transverse processes of all the vertebrae¹. Its body is taller in front than behind¹. That shape helps form the angle where the lumbar spine meets the sacrum (the triangle-shaped bone at the base of the spine)¹. The lower articular processes of L5 join the upper articular processes of the sacrum².

Laboratory tests, some on bones from cadavers (bodies donated for study), and one account of spinal mechanics describe how the parts share load^9,10:

Compression. In cadaver lumbar joints, the facet joints resisted about 16 per cent of the compressive force in a posture simulating upright standing⁹. They resisted none in a posture simulating upright sitting⁹. Both figures were measured once sustained loading had reduced disc height⁹.
Forward shear. Shear is a force that pushes two surfaces to slide across each other. A 1976 account of spinal mechanics names three things that normally resist forward shear on a lumbar vertebra¹⁰. They are the outer ring of the disc, compression of the facet joints, and tension in the cortical bone (dense outer bone) of the pars¹⁰.
Strength of the pars. In tests of single vertebrae, a backward force was applied to the lower articular processes¹⁰. Two reports tested the pars of L4 and L5 vertebrae this way¹⁰. The mean force at failure was 1800 newtons (a unit of force) in one report and 3200 newtons in the other¹⁰.

Two studies used a finite element model to look at stress inside the pars^4,11. This is a computer model that splits a structure into many small pieces to work out the forces within it. In the model of an L4-L5 segment, pars stress was lowest under compression alone and high under extension (bending backward) and rotation⁴. In the model of the L3 to sacrum segment, the highest stresses in the L5 pars were at its lower front, in every loading mode¹¹.

Breakdown

A fatigue crack in the pars

A crack in the pars is called a pars defect or spondylolysis^12,13. A structured review found good evidence that, in children, it is an acquired fracture, on one side or on both¹⁴. A study of cricket fast bowlers describes lumbar stress fracture as a fatigue injury¹⁵. In that account, microdamage builds up and spreads across the arch faster than the bone can repair it¹⁵. A review describes repetitive loading as the cause, especially in activities needing spinal rotation and hyperextension (straightening a joint past its normal limit)³.

No pars defect was found in X-rays of 143 people who had never walked¹⁶. The authors took this to support the idea that the defect is a fatigue fracture arising from activities associated with walking¹⁶.

Three studies looked at where and why the bone gives way^11,17,18:

Where the crack starts. Ten adolescents aged 11 to 17 had incomplete stress fractures of the pars¹¹. CT showed the fracture line only at the lower front of the pars in every case¹¹.
How thick the bone is. Vertebrae taken after death that had withstood repeated force without fracturing had a greater share of cortical bone in the pars than randomly selected ones¹⁷. The authors read this as suggesting that a thin pars may predispose to fatigue failure¹⁷.
What a one-sided crack does. In a finite element model with a left-sided L5 pars defect, stresses on the opposite side rose in every loading mode¹⁸. In the same study, of thirteen athletes under 20 with a one-sided defect, three had a stress fracture on the opposite side¹⁸.

Which vertebra, and at what age

CT scans of 2000 Japanese adults aged 20 to 92 were taken for reasons unrelated to low back pain⁵. A pars defect was found in 5.9%: 7.9% of males and 3.9% of females⁵. Of the 124 affected vertebrae, 112 were L5, and 26 had a defect on one side only⁵. A review also reports L5 as the level most often affected³. It attributes this to L5 lying where the mobile lumbar spine meets the relatively fixed sacrum³.

In an X-ray study of 500 unselected first-grade children, 4.4% had a pars defect at age six, rising to 6% in adulthood¹².

In CT scans of 188 adults aged 40 to 80 from the Framingham Heart Study, 11.5% had a pars defect¹⁹. Having one was not significantly associated with low back pain in the preceding 12 months¹⁹. The spondylolysis page covers who has symptoms and what follow-up studies found¹⁹.

Which sports

A literature review of pars defects in athletes of specific sports found the highest figures in these eight⁸:

Athletes with a pars defect in one review (%). Bars start at zero; the table below gives each value with its source.

Sport	Athletes with a pars defect in one review
Diving	35.38%⁸
Cricket	31.97%⁸
Baseball and softball	26.91%⁸
Rugby	22.22%⁸
Weightlifting	19.49%⁸
Sailing	17.18%⁸
Table tennis	15.63%⁸
Wrestling	14.74%⁸

One study followed 368 professional English cricket fast bowlers from 2010 to 2016, and 57 lumbar stress fractures were reported¹⁵. Of those fractures:

They were most common at L4 and L5, at the pars (77.2%) or the pedicle (22.8%)¹⁵.
93% were on the side opposite the bowling arm¹⁵.
74% were in bowlers under 25¹⁵.

In the same study, a peak week above 234 deliveries was associated with 11 times the odds of fracture, against fewer than 197¹⁵.

The vertebral body with age

In L3 vertebral bodies from 101 people aged 18 to 96, compressive strength fell with age twice as much as bone density did²⁰. A review of 62 articles found vertebral fractures on imaging in 18% to 26% of European women, depending on region²¹. In White North American women aged 50 or over, the figure was 20% to 24%²¹.

Recovery

Lumbar bone and loading

In eight power lifters, the bone mineral content of the L3 vertebra was extremely high⁶. It was closely correlated with the amount of weight lifted in training over the last year⁶.

On bone density scans, 23 cricket fast bowlers had more bone mineral on one side of L4: the side opposite the bowling arm²². The authors concluded that the lumbar spine responds to this one-sided, high-load activity by adding bone at the site of most strain²². They also reported that this adaptation was less marked in bowlers with a past lumbar stress fracture²². The bone page covers how bone tissue adapts in general²².

How these fractures heal

An MRI review covered 31 lumbar stress fractures in 20 elite male fast bowlers²³. Bone healing was seen in 25, a median of 203 days after the first scan²³.

A CT study followed 239 pars defects in 134 patients under 18 managed without surgery²⁴. The main factors affecting bony union were the level of the spine and how advanced the defect was²⁴. Another study grouped the defects of 37 patients under 18, who wore a hard brace, as early, progressive or terminal²⁵. Union was seen in 94% of early defects and none of the terminal ones²⁵. The spondylolysis page sets out healing by stage and what happened afterwards²⁵.

Connections

The five lumbar vertebrae make up the lumbar region of the spine¹. Each intervertebral disc keeps neighbouring vertebral bodies from touching bone to bone²⁶. Below, L5 joins the sacrum, which is part of the pelvis². The sacroiliac joint page covers how the sacrum meets the hip bones².

Several muscles attach to these bones²:

The psoas major, one of the hip flexors, arises from the transverse processes, bodies and discs of all the lumbar vertebrae⁷.
The quadratus lumborum attaches by four small tendons to the upper four lumbar vertebrae, at the tips of their transverse processes^7,27.
The lumbar fibres of the erector spinae arise from the L1 to L4 transverse processes and from other lumbar processes called the accessory processes²⁸.
The lumbar multifidus was found in dissection studies to divide into five bands, each arising from a lumbar spinous process²⁹.

Related exercise pages: good morning and deadlift. One study had four male nationally ranked weightlifters do good mornings with a typical training load³⁰. A model estimated mean peak compressive force between L5 and the sacrum at 6712.7 newtons³⁰. Among powerlifters at a national championship, a model-based analysis associated the sumo deadlift style with 8% lower L4/L5 shear force than the conventional style³¹.

Related stretch pages: child’s pose and knee-to-chest. One published description of child’s pose has the spine rounded³². In volunteers X-rayed sitting, flexing the hips widened the spaces between the lumbar spinous processes³³.