Anatomic planes and axes

Basic concepts and terminology

Anatomy as a science studies form, development, and structure of the organism, inextricably linking them with functions and environmental influences. By comparison, physiology focuses on the processes themselves within organs and systems.

To navigate the three-dimensional space of the human body without error, a standard coordinate system is used:

• Three planes: sagittal, frontal, and horizontal.
• Three axes: sagittal, frontal, and vertical.

Paired terms are used to describe the relative position of structures:

• Medial (closer to the midline) and lateral (further from the center).
• Proximal (closer to the trunk) and distal (further from it).
• Ventral (anterior) and dorsal (posterior).

Imaging planes in MRI diagnostics

In modern diagnostic radiology (e.g., cardiac MRI), classical anatomic planes are projected onto the main axes of the body. This allows for a detailed study of the morphology of organs, vessels, and the pericardium (normally less than 4 mm thick, yielding a low signal on T1-weighted images).

• Transverse (axial) plane: Analogous to the horizontal plane. Ideal for studying the great vessels: ascending and descending thoracic aorta, superior and inferior venae cavae, pulmonary trunk, and pulmonary veins. The free wall of the right ventricle, right ventricular outflow tract (RVOT), and mediastinum are clearly visualized on axial slices. Quantitative evaluation of right ventricular volume by planimetry on these slices is considered the most accurate.
• Coronal (frontal) plane: Demonstrates the relationship between the heart and great vessels. The anatomic picture in this projection visually resembles a classical chest X-ray.
• Sagittal plane: Optimal for assessing the right ventricular outflow tract in relation to the pulmonary valve, as well as the junction of the right atrium with the venae cavae.

Clinical significance of axes in orthopedics

Understanding normal axes is critically important when evaluating injuries, such as proximal tibia fractures. On two-projection radiographs (anteroposterior and lateral), physicians analyze deviations from the norm:

• In the frontal plane: Normally, the knee joint line forms an angle of 87° (85–90° is acceptable) with the anatomic axis of the tibia. This creates a slight physiological varus. The width of the tibial articular surface (plateau) is normally 1 mm wider than the articular surface of the distal femur.
• In the sagittal plane: The angle between the joint line and the bone axis is 81° (77–84°). The physiological posterior slope of the articular surface is about 10° from the horizontal line.

In trauma, under the influence of deforming forces, these reference angles are disrupted, resulting in varus or valgus deformity and an altered joint line slope. Restoring the correct limb axis and normal load distribution is the primary biomechanical objective when planning surgeries. When conservative treatment of stage I–II gonarthrosis is ineffective, corrective osteotomies (valgus, varus, flexion, or rotational) are used.

Relationship between body structure and regulation

Body form and structure are individual and determined by constitutional type. A simple clinical method of assessment is measuring the subcostal angle:

Regardless of constitution, the vital activity of the organism is controlled by a unified neurohumoral regulation. Here, the humoral system (hormones and mediators) acts as primary, and the nervous system (impulses) as secondary. At the same time, nervous regulation works faster and completely subjugates the humoral component.

Class and exam questions

What else to study on this topic

• MRI anatomy of the cardiac chambers: right and left ventricles, atria
• Structure of the valvular apparatus: atrioventricular and semilunar valves
• MRI sequence modes: "dark blood" and "bright blood"
• Radiological evaluation of proximal tibia fractures
• Indications and biomechanics of corrective periarticular osteotomies

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.