Functional Anatomy of the Lumbar Spine and Abdominal Wall

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Introduction

The lower back typically includes five vertebrae in the lumbar region and supports much of the weight of the upper body. The spaces between the vertebrae are maintained by intervertebral discs that act like shock absorbers throughout the spinal column to cushion the bones as the body moves. Ligaments hold the vertebrae in place, and tendons attach the muscles to the spinal column. The skeletal support of the abdominal wall includes the vertebral column and lower ribs. It contains the skin, fascia, and muscle which provide coverage for the organs, generate inter-abdominal pressure, and move the vertebral column. This article discusses the key anatomical structures of the lumbar vertebra and abdominal wall, including the bony structures, articulations, ligaments, muscles, nerves and the vascular supply.

Key Terms

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right, and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduce friction between the moving parts of the body joints. A bursa is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. For example, the closed pack position for the interphalangeal joints is full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane.Sideway movements (abduction/adduction) occur in the frontal plane. The transverse plane movements are rotational (internal and external rotation).

Lumbar Spine Structure

Lumbar vertebrae

L1-L5 are known as lumbar vertebrae. However 10–17.4% of adults presents with spinal abnormality of having the 6th lumbar vertebra. This abnormality may influence the saggital parameters including a diminished sacral tilt and more vertical sacrum. The presence of additional lumbar vertebra can be associated with a decrease in the number of thoracic vertebrae (individuals with eleven thoracic vertebrae).

The lumbar vertebrae are located between the 12th thoracic vertebra and the sacrum bone. They are the largest vertebrae of the human body. Each lumbar vertebra contains the following structural elements:

• Vertebral body
  • kidney-shaped
  • wider from side to side than from front to back
  • the posterior aspect of the vertebral body changes from concave to convex
• Vertebral foramen
  • triangular in shape
  • larger than in the thoracic vertebrae but smaller than in the cervical vertebrae
• Two pedicles
  • posterior element of the lumbar vertebra
  • attach to the cranial half of the vertebral body
  • becoming shorter, broader and more lateral from L1 - L5
• Two laminae
  • flat and broad
  • two pedicles and two laminae are forming the vertebral arch
  • centrally connects to spinous process
• Spinous process
  • projects perpendicularly from the body
  • short and sturdy
  • describe as "hatchet-shaped"lumbar spine spinous process
  • point of muscle and ligaments attachement
• Two transverse processes
  • located on the posterior surface
  • project laterally on each side of the vertebra
  • long and slender
  • points of muscle and ligaments attachement
  • L1-L3 projecting horizontally
  • L4-L5 projecting upward
• Four articular processes
  • two superior articular facets and two inferior articular facets
  • the point where two facets are joining is called the zygapophyseal joints

Lumbar Lordosis

The natural curve for the lumbar spine is lordosis. It defines lumbar spine alignment in saggital plane, however it is the pelvis that is "the cornerstone of spinal sagittal alignment". The curve of the sacrum determines the lumbar curve.

The lumbar spine sagittal alignment offers two main benefits during locomotion:

• Allows the body's centre of mass to be positioned above the hip, knee, and ankle joints
• Minimises the muscular force used for postural control

Assessment of the lumbar spine alignment must include the analysis of the lower limbs alignment. The results from the knee flexion angle measurement and pelvic shift can help identify the compensatory mechanisms as a consequences of the sagittal plane malalignment.

Abdominal Wall Structure

The abdominal wall has a skeletal support made of the vertebral column and lower ribs posteriorly, and connects to the thoracic cage at the superior end and pelvic bones inferiorly. It is defined by the following landmarks: xiphoid process and costal cartilages of the 7th–10th ribs outline the superior border, the umbilicus in the middle, and inguinal ligament, pubic crest and pubic symphysis define the inferior border.

You can read more about pelvis anatomy here.

Bones and Articulations of the Lumbar Spine

Lumbar spine has two types of joints located between each lumbar vertebra: the intervertebral disc joint and zygapophyseal joint or the facet joint. The intervertebral disc joint is formed between the vertebral bodies and is separated by the intervertebral disc. The zygapophyseal joints are formed between the articular processes of each lumbar vertebra.

Bones	Articulations	Characteristics	Key palpation points
Vertebral body	Intervertebral disc joint	This joint is formed between the inferior surface of one vertebral body and the superior surface of the vertebral body that lies below (for example, the inferior surface of L1 and the superior surface of L2). An intervertebral disc separates the vertebral bodies.	To find each segment of the lumbar spine, position the patient prone. The location of the spinal processes will determine the location of each segment. You can palpate the spinal processes in the centre of the patient's back. Start with finding T12 by palpating the lowest rib of the patient and follow your finger along the border of the rib towards the centre of the spine.T12 spinal process is in line with 12th rib that inserts into the spinal segment. Below T12 you can palpate L1 and continue along the spinal process of L2,L3,L4. When starting your palpation from the distal end of the lumbar spine you must consider the following caution: the spinal process of L4 "is usually determined by the position of the highest point on both sides of PSIS", and the accuracy of palpation is only 36%.
Articular processes	Zygapophyseal joint (Facet joint)	Located on the left and right sides of the spine Links one vertical segment to the other. For example, a facet joint between L3 and L4 will connect L3 to L4. The superior articular surface is vertical and concave, and the inferior is vertically convex. The primary role includes the stabilization of the spinal motion segment.C Contribute to axial compressive load transmission on the spine	The facet joint CANNOT be directly palpated due to overlying soft tissue. When you move your fingers two to three centimetres in the lateral direction from the spinal process, you will be on the muscles overlying the facet joint.

Lumbar Spine Kinematics

Lumbar spine is considered a three-joint complex or the articular triad. The zygapophyseal joint or facet joints and intervertebral disk joints belong to the spinal motion segment, which, when work together, allow for spinal motion and protect the spine from excessive movements.

The following is the characteristic of the spinal motion segment:

• The intervertebral disk joints transmit mostly vertical compressive loads
• The facet joints guide and stabilise the motion segment
• In the erect standing position and erect sitting posture, spine load is also transmitted through the facet joints

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Intervertebral disc joint	Cartilaginous joint, symphysis	A single joint: structurally fused but allows very limited, multidirectional movements All lumbar intervertebral disc joints and zygapophyseal joints combined together: Saggital Frontal Transverse	Single joint: Translating, tilting, rocking and compressing movements All lumbar intervertebral disc joints and zygapophyseal joints combined together: Flexion/Extension Lateral flexion Rotation	All lumbar intervertebral disc joints and zygapophyseal joints combined together: Flexion 60 degrees Extension 25 degrees Lateral flexion 20-30 degrees Rotation 1-2 degrees	Full extension	Half way between flexion and extension
Zygapophyseal joint	Synovial plane joint	Facilitate a multiplanar and multidirectional movement of the spine Enable lumbar spine extension, lateral flexion, and rotation in the saggital transverse and frontal planes.	Flexion/Extension Lateral flexion Rotation	All lumbar intervertebral disc joints and zygapophyseal joints combined together: Flexion:60 degrees Extension 25 degrees Lateral flexion 20-30 degrees Rotation 1-2 degrees	Full extension	Half way between flexion and extension

Lumbar Spine Ligaments

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Anterior longitudinal ligament	The anterior portion of the vertebral body at the base of the skull	The anterior portion of the vertebral body at the sacrum	Limit extension of the vertebral column Reinforce the intervertebral disc
Posterior longitudinal ligament	The body of the C2	Posterior surface of the sacrum.	Limits flexion of the vertebral column Reinforces the intervertebral disc
Ligamentum flavum: Series of short ligaments that connects the laminae of each vertebra. There are two ligamenta flava at each vertebra. Each ligament is divided into: The medial portion: passes to the back of the next lower lamina and across the gap between the adjacent vertebrae fusing with the interspinous ligament The lateral portion: passes in front of the facet joint, attaches to the anterior aspect of the inferior and superior articular processes and forms the anterior capsule. Most lateral fibres extend beyond the superior articular process to the pedicle below.	Lower half of the anterior surface of the lamina above	Posterior surface and upper margin of the lamina below	Assists with lumbar spine flexion and extension by accommodating a limited distancing of the vertebral laminae during flexion and recoiling and returning the laminae to normal during lumbar spine extension. Compresses the intervertebral discs.
Intertransverse ligaments	Transverse processes of the vertebra above	Transverse processes of the vertebra below	Contributes to the stability of the lumbar spine Provides the opposite side resistance during lateral flexion	To palpate the transverse process, the patient is sitting. The transverse processes of the lumbar spine are most likely located directly in line with spinous process of the corresponding vertebra. Start with palpating the spinous process of the L1 vertebra and move directly lateral to find the spinal process of L1
Supraspinous ligament	Tip of spinous processes of the vertebra above from seventh cervical vertebra to the third or fourth lumbar vertebra	Tip of spinous processes of the vertebra below	Prevents the separation of the spinous processes during forward flexion thus limiting lumbar spine flexion	You can palpate the spinous processes in the centre of the patient's back. Start with finding T12 by palpating the patient's lowest rib and following your finger along the border of the rib towards the centre of the spine.T12 spinous process is in line with the 12th rib that inserts into the spinal segment. Below T12, you can palpate L1 and continue along the spinous process of L2, L3, and L4.
Interspinous ligament	Spinous processes of the vertebra above between the ligamenta flava anteriorly and the supraspinous ligament posteriorly	Spinous processes of the vertebra below between the ligamenta flava anteriorly and the supraspinous ligament posteriorly	Limits forward flexion of the vertebral column.

Muscles of the Lumbar Spine

Muscle	Origin	Insertion	Innervation	Action
Psoas major	Vertebral bodies of T12-L4, intervertebral discs between T12-L4, transverse processes of L1-L5 vertebrae	Lesser trochanter of femur	Anterior rami of spinal nerves L1-L3	Lateral flexion of the trunk Contributes to lumbar spine flexion when acting bilaterally and when the insertion point is fixed.
Psoas minor	Vertebral bodies of T12 to L1	Iliopubic eminence	Anterior ramus of spinal nerve L1	Weak trunk flexor. Can be absent in 40% of people.
Quadratus lumborum

Muscles of the Abdominal Wall

add text here relating to the differential diagnosis of this condition

Resources

1. Inoue N, Orías AAE, Segami K. Biomechanics of the Lumbar Facet Joint. Spine Surg Relat Res. 2019 Apr 26;4(1):1-7.