The terms 'spine' and 'vertebral column' in descriptions of conditions or procedures are often used loosely in clinical discourse but can relate to three different general anatomical concepts:

421060004 |Structure of vertebral column (body structure)| (synonym: Spinal column, ‘Backbone’, Spine)

• includes the bones of the spine and associated joints and ligaments

289959001 |Musculoskeletal structure of spine (body structure)|

• includes the vertebral column plus muscles and tendons associated with the spine

1141981001 |Structure of vertebral column region (body structure)|

• includes the musculoskeletal structure of spine plus spinal canal, meninges, spinal cord, roots and ganglia and immediate soft tissue including adjacent vascular structures where specified.

The last concept has been allocated the following definition: This volume includes the spinal column, its spaces and contents, notably the spinal canal, spinal meninges and spinal cord. It also includes the muscles associated with the spine within (and including) the anterior and posterior thoracolumbar fascia and its equivalent nuchal fascia in the neck. This region also includes the spinal ventral (anterior) and dorsal (posterior) roots, the dorsal root (spinal) ganglions and the spinal nerve trunks.

To emphasize the different volumes, cross-sections are shown below of (Figure 1) the vertebral column in yellow, and (Figure 2) the larger vertebral column region in amber color.

From the preceding descriptions, it is clear that Vertebral column region is a broader concept for 'spine', and it should be used when a procedure or condition could involve not only the bony component of the vertebral column, but also the spinal cord, nerve root, muscle, bone, or joint of spine.

For example,

• MRI of thoracic spine (procedure) is modeled with 1141986006 |Structure of thoracic vertebral column region (body structure)|.

• Pain in sacrum (finding) is interpreted as being related to the more general notion of 1144746008 |Structure of sacral vertebral column region (body structure)|.

When a procedure or disorder is exclusively related to musculoskeletal structures of the spine, the 'musculoskeletal structure of spine' should be used.

For example,

• Cervical traction (procedure), Manipulation of the cervical spine (procedure), and Rotational deformity of cervical spine (finding) are all modeled with 297166009 |Structure of musculoskeletal system of cervical spine (body structure)|.

Where a procedure or condition only effects the bone, joint, or ligament component (and not the muscles or tendons directly) the ‘vertebral column variant’ is used for modeling.

For example,

• Benign neoplasm of lumbar vertebral column (disorder) and Kyphoplasty of fracture of lumbar spine using fluoroscopic guidance (procedure) are both modeled with 122496007 |Structure of lumbar vertebral column (body structure)|.

The sacrum is considered equivalent to the sacral vertebral column, as it is composed of the bone structure of the sacrum plus the joint structure of sacrococcygeal junction of spine.

The spine is traditionally divided into the following regions:

• Cervical

• Thoracic

• Lumbar

• Sacral

The anatomy of these individual segments are structured in accordance with the description above.

The regions and joints between these identified segments are referred to as cervicothoracic, thoracolumbar, lumbosacral and sacrococcygeal. The meaning of these words are subject in common parlance and some literature to be ambiguous.

For example,

• ‘Thoracolumbar’ sometimes refers to the thoracic spine and lumbar spine, or alternatively, it is used to express the junction between the thoracic spine and lumbar spine.

To avoid false assumptions, the junction of spinal segments in SNOMED CT have been made explicit by including the word junction in descriptions. This avoids potential misinterpretation as to whether a word such as ‘thoracolumbar' relates to both spinal segments or just the adjacent volume.

For example,

• The notion of 1145014005 |Structure of thoracolumbar junction of vertebral column (body structure)| is only used for modeling when it is explicitly stated in the target concept e.g. 281907005 |Fracture dislocation of thoracolumbar junction (disorder)|.

By contrast, in the circumstance where a dependent concept relates to the combination of two segments, e.g. 702487007 |CT of thoracolumbar spine (procedure), (FSN Computed tomography of thoracic and lumbar spine), the concept is modeled with two axioms, namely Structure of lumbar vertebral column region and Structure of thoracic vertebral column region.

The volume or extent of the junctional zones themselves are not defined consistently in the literature but most commonly relate to the junction between two segments and one vertebra above and below. So the convention used in the SNOMED CT anatomy hierarchy follows this guidance.

For example,

1145014005 |Structure of thoracolumbar junction of vertebral column (body structure)| includes the following concepts in its class:

• 66794005 |Bone structure of L1 (body structure)|

• 23215003 |Bone structure of T12 (body structure)|

• 714833001 |Structure of intervertebral syndesmosis of T12 and L1 (body structure)|

In addition, a further convention is required as to where segmental junctional joints belong, i.e. either to the cephalic or caudal segment. For instance, does the T12/L1 facet joint relate to the Thoracic spine joint structure or/and Lumbar spine joint structure?

The convention used (based on common criteria of spinal injuries) is that the junctional joints are included with the cephalic segment. So, in the case of the T12/L1 facet joint, it is included in the class of Thoracic spine joint structure (body structure).

Similarly, 8454000 |Lumbar spine joint structure (body structure)| subsumes:

• Intervertebral L5-S1 disc

• Structure of lumbosacral joint

• Structure of facet joint between L5 and S1

Skeletal system, bony skeleton

The skeletal system (systema skeletal in Nomina Anatomica) includes bones and cartilage. The bony skeleton includes bones only. The vertebral column is part of the skeletal system and includes the intervertebral discs (fibrocartilage). Individual vertebrae are part of the bony skeleton.

In ordinary usage, bone combines the meanings bone organ and bone tissue.

The 5 anatomical concepts related to bone are:

1. 3138006 |Bone (tissue) structure (body structure)|. Tissue type that makes up bones; a quantity of regular connective tissue consisting of osteocytes and related cells, the intercellular matrix of which is ossified; or any part thereof.

2. 90780006 |Entire bone (organ) (body structure)|. Individual bones, e.g., femur, tibia, ulna, scaphoid, lunate. An organ with cavitated parts; consists primarily of compact (cortical) and cancellous bone surrounding bone marrow cavities; also includes periosteum, endosteum (and, according to FMA, articular cartilage).

3. 118966000 |Skeletal system subdivision (body structure)|. Groups of bones, e.g., spine, skull, bony pelvis.

4. 128530007 |Entire bony skeleton (body structure)|. Pars ossea systematis skeletalis, bone part of the skeletal system.

5. 113192009 |Skeletal system structure (body structure)|. Entire skeletal system, including bones and cartilage.

Bone (tissue) is part of entire bone (organ); entire bone (organ) is part of skeletal system subdivision (system); skeletal system subdivision (system) is part of entire bony skeleton (body structure); and entire bony skeleton (body structure) is part of skeletal system structure (body structure). We can use Entire bone (system) to define aggregate concepts that involve bones.

Bone organs are composed primarily of bone tissue, but there are some non-ossified parts. In particular, periosteum is clearly a part of a bone organ, but is not ossified tissue.

Bone marrow is contained within the marrow cavity, but it is not part of the bone organ. The (empty) marrow cavity is part of the bone organ. The bone marrow structure (body structure) is not a subtype of Bone structure (body structure).

Clinically, marrow disorders are not usually considered bone disorders, nor are marrow procedures considered bone procedures.

For example,

• Bone marrow disorders are not musculoskeletal disorders, but bone disorders are musculoskeletal disorders. Bone marrow transplants are not considered types of bone transplant.

  • 60168000 |Osteomyelitis (disorder)| is not the same as 44462005 |Osteitis (disorder)|.

To differentiate marrow, vessels, nerves, and periosteum from the actual hard tissue of bones, we differentiate structure of tibia from 12611008 |Bone structure of tibia (body structure)|. The bone marrow and other soft tissues of the tibia can then be categorized separately from the hard tissues. Bone marrow diseases are not considered musculoskeletal diseases, so bone marrow structures should not be placed in the bone (tissue) structure hierarchy.

ICD does not use the standard anatomical definition of long bone. For instance, Benign neoplasms of long bones are distinguished from benign neoplasms of short bones ; the bones of the hand are considered short bones. The anatomical definition of long bone cites the proportional relationship between length and width (length >> width). It is clear that metacarpals, metatarsals, and phalanges are included in the anatomical definition of long bone.

In order to accommodate the differences between anatomical definitions and classifications, SNOMED CT has anatomical groupings that correspond to the ICD groupings. Scapula, humerus, radius, or ulna, and long bone of thigh or lower leg are used as the sites for grouper concepts that match ICD definitions and groupings.

The sternum is considered a bone organ. The manubrium, body, and xiphoid are parts of the sternum, classed as zones in the FMA.

Even though teeth are supported by the maxillary or mandibular bone, they are not part of the 70925003 |Bone structure of maxilla (body structure)| or 91609006 |Bone structure of mandible (body structure)|. Teeth are part of the 4335006 |Upper jaw region structure (body structure)| and 48077000 |Lower jaw region structure (body structure)|.

In many diseases and procedures, reference is made to areas of the body that may ambiguously imply either a joint or a region surrounding the joint. Some common ones are:

272691005 | Bone structure of shoulder girdle (body structure)| This concept is used to define diseases and procedures affecting bones in the shoulder region, i.e. proximal humerus, scapula, and clavicle. It is not a bone, but a bone structure, and is part of the shoulder region.

27949001 |Intertarsal joint structure (body structure)|; SYN: Tarsal joint: This structure is part of a group of bones forming the tarsus or tarsal joint (ankle). The 27162001 |Talocalcaneonavicular joint structure (body structure)| is the articulation between the talus (one of the seven bones of the ankle joint) and the other bones of the tarsus, and is what is meant by the rarely-used term talotarsal joint. The talocalcaneal joint is a synonym for the 127863007 |Subtalar joint structure (body structure)|. Dislocations of the subtalar joint usually involve the 127864001 |Structure of talonavicular joint (body structure)|. The subtalar and talonavicular joints constitute the talocalcaneonavicular joint.

The meaning of the words arm and leg may be misinterpreted.

• Arm may refer to the upper limb, but it may also refer to the upper part of the arm.

• Leg may refer to the lower limb, but it may also refer to the lower part of the leg.

• In common usage, leg is a synonym of lower extremity, and arm is a synonym of upper extremity.

In SNOMED CT,

• 53120007 |Upper limb structure (body structure)| includes the shoulder, upper arm, forearm, wrist, and hand

• Upper arm is a synonym to 40983000 |Structure of upper extremity between shoulder and elbow (body structure)|.

• 61685007 |Lower limb structure (body structure)| includes the hip, thigh, lower leg, ankle and foot.

The word limb appears in the FSN of the body structure, while the word extremity appears as a synonym. Therefore, when constructing an FSN for a new clinical finding concept, this precedent should be followed:

• FSN: 61685007 |Lower limb structure (body structure)|

• PT: Lower limb structure

• Synonym: Lower extremity

Additional descriptions of leg and arm are permitted for concepts whose FSNs refer to lower limb and upper limb respectively.

The shoulder region is part of the upper limb, and the hip region is part of the lower limb. This follows the general pattern used in the Foundation Model of Anatomy (FMA).

The FMA defines:

• upper limb as the free upper limb and the pectoral girdle (of which the shoulder region is part). SNOMED CT has the concept 896766000 |Structure of free upper limb (body structure)|.

• lower limb as the free lower limb and pelvic girdle (of which the hip region is part). SNOMED CT has the concept 699617006 |Structure of free lower limb (body structure)|, i.e. the lower limb not including the pelvic girdle.

The axilla is bound by the upper limb laterally and the thorax medially. It may be viewed as not strictly part of the upper limb or the thorax or it may be views as part of both. 91470000 |Axillary region structure (body structure)| is defined in SNOMED CT as being both an upper limb structure and a thoracic structure.

A muscle may be considered an entire functional unit, including attachments to the skeletal system, or merely the contractile part of this unit. In clinical use, muscle is the contractile part only. The FMA definition implies that tendons should be considered part of their corresponding muscles, rather than organs in their own right. SNOMED CT models |Tendon structure| as a subtype of |Skeletal muscle and/or tendon structure (body structure)|. Muscle and tendon are two separate anatomical entities.

• For example,

  • 61352006 |Structure of achilles tendon (body structure)| is not a 53451005 |Triceps surae muscle structure (body structure)| (gastrocnemius and/or soleus) muscle structure.

When modeling muscle categories according to their functions, assume they mean the function of the entire muscle, unless stated otherwise.