The term “mediolateral” is a fundamental directional descriptor in anatomy, used to precisely locate structures and define the path of motion within the human body. Understanding this concept is necessary for interpreting medical reports, exercise instructions, or biomechanical analyses. Anatomical language is standardized, ensuring that every reference point is consistent by always referring back to the standard anatomical position. This precision allows for clear communication about where a structure is situated or the orientation of a movement or imaging view.
Defining the Mediolateral Direction
The word “mediolateral” combines the opposing directional terms “medial” and “lateral.” This concept relies on the imaginary vertical midline that divides the body into equal right and left halves. Medial describes a position or movement closer to this central midline. For example, the sternum (breastbone) is medial to the shoulder joints because it lies closer to the center of the chest.
Lateral describes a position or direction farther away from the midline, toward the sides of the body. The ears are lateral to the nose, as they are positioned toward the outer edges of the head. Therefore, “mediolateral” describes the entire side-to-side dimension or axis, spanning from the center of the body outward.
In biomechanics, this mediolateral axis is also known as the frontal axis or transverse axis, defined by a line passing horizontally from left to right through the body. This axis is the imaginary pin around which movement occurs in the sagittal plane, such as the flexion and extension of the elbow joint.
The Role of the Mediolateral Axis in Stability and Movement
In biomechanics, the mediolateral axis is particularly significant because it defines the direction of movement in the frontal plane, which is where side-to-side motions like abduction and adduction occur. Furthermore, a mediolateral axis is the axis of rotation for flexion and extension movements, such as walking, where the hip and knee joints rotate about this side-to-side line. The stability of the body during physical activity is often analyzed along this side-to-side dimension.
Maintaining stability while standing or walking relies heavily on controlling the body’s center of mass (COM) within the mediolateral boundaries of the base of support. During walking, for instance, the body must constantly manage mediolateral sway, which is the slight, side-to-side oscillation of the COM. Biomechanical analysis of gait involves measuring the mediolateral Margin of Stability (MOS), which quantifies the distance between the COM’s dynamic state and the lateral edges of where the feet contact the ground.
Controlling this mediolateral stability is accomplished through precise foot placement and the generation of joint moments, particularly by the hip abductor muscles. These muscles must generate a counterbalancing force to prevent the upper body from accelerating too far medially (inward) during the single-support phase of walking. Researchers often analyze the mediolateral components of movement to understand differences in walking patterns between healthy individuals and those with movement impairments, such as children with cerebral palsy.
Mediolateral Applications in Clinical and Diagnostic Settings
In clinical practice, the term “mediolateral” defines the precise angle or orientation of a diagnostic procedure or surgical approach. This ensures that a specific area of the body is consistently viewed or accessed for comparison and treatment.
A common example is in diagnostic imaging, particularly mammography, where two standard views are taken. One standard view is the “mediolateral oblique (MLO) view,” which is an angled side-view of the breast. This description indicates that the X-ray beam passes from the medial side of the breast (near the midline) toward the lateral side (the armpit side) at an oblique angle. The MLO view is crucial because it allows for the maximum amount of breast tissue to be included in the image, especially the tissue near the chest wall.
The term is also used to describe surgical incisions or approaches to joints, such as a “mediolateral approach” to the knee or ankle. This indicates that the surgeon will access the joint by cutting along a path that runs from a medial starting point to a lateral ending point, or vice versa, to minimize disruption to nearby nerves or blood vessels. Using this specific directional language helps ensure that medical records and surgical plans are unambiguous and reproducible.