An anatomical variation, anatomical variant, or anatomical variability is a presentation of body structure with morphological features different from those that are typically described in the majority of individuals. Anatomical variations are categorized into three types including morphometric (size or shape), consistency (present or absent), and spatial (proximal/distal or right/left). [1]
Variations are seen as normal in the sense that they are found consistently among different individuals, are mostly without symptoms, and are termed anatomical variations rather than abnormalities. [2]
Anatomical variations are mainly caused by genetics and may vary considerably between different populations. The rate of variation considerably differs between single organs, particularly in muscles. [2] Knowledge of anatomical variations is important in order to distinguish them from pathological conditions.
A very early paper published in 1898, presented anatomic variations to have a wide range and significance, [3] and before the use of X-ray technology, anatomic variations were mostly only found on cadaver studies. The use of imaging techniques have defined many such variations. [4]
Some variations are found in different species such as polydactyly, having more than the usual number of digits.
Kopsch gave a detailed listing of muscle variations. These included the absence of muscles; muscles that were doubled; muscles that were divided into two or more parts; an increase or decrease in the origin or insertion of the muscle; and the joining to adjacent organs. [2]
The palmaris longus muscle in the forearm is sometimes absent, as is the plantaris muscle in the leg. [5]
The sternalis muscle is a variant that lies in front of the pectoralis major and may show up on a mammogram. [6]
Usually there are five lumbar vertebrae but sometimes there are six, and sometimes there are four. [5]
A discoid meniscus is a rare thickened lateral meniscus in the knee joint that can sometimes be swollen and painful. [7]
Accessory small bones called ossicles may be mistaken for avulsion fractures.[ citation needed ]
A joint or articulation is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole. They are constructed to allow for different degrees and types of movement. Some joints, such as the knee, elbow, and shoulder, are self-lubricating, almost frictionless, and are able to withstand compression and maintain heavy loads while still executing smooth and precise movements. Other joints such as sutures between the bones of the skull permit very little movement in order to protect the brain and the sense organs. The connection between a tooth and the jawbone is also called a joint, and is described as a fibrous joint known as a gomphosis. Joints are classified both structurally and functionally.
In anatomy, a sesamoid bone is a bone embedded within a tendon or a muscle. Its name is derived from the Greek word for 'sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be present as a normal variant. The patella is the largest sesamoid bone in the body. Sesamoids act like pulleys, providing a smooth surface for tendons to slide over, increasing the tendon's ability to transmit muscular forces.
A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi to branch from the trachea at the carina are the right main bronchus and the left main bronchus. These are the widest bronchi, and enter the right lung, and the left lung at each hilum. The main bronchi branch into narrower secondary bronchi or lobar bronchi, and these branch into narrower tertiary bronchi or segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi. The bronchi, when too narrow to be supported by cartilage, are known as bronchioles. No gas exchange takes place in the bronchi.
The cremaster muscle is a paired structure made of thin layers of striated and smooth muscle that covers the testicles and the spermatic cords in human males. It consists of the lateral and medial parts. Cremaster is an involuntary muscle, responsible for the cremasteric reflex; a protective and physiologic superficial reflex of the testicles. The reflex raises and lowers the testicles in order to keep them protected. Along with the dartos muscle of the scrotum, it regulates testicular temperature, thus aiding the process of spermatogenesis.
A synovial bursa, usually simply bursa, is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of viscous synovial fluid. It provides a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement. Bursae are found around most major joints of the body.
The psoas major is a long fusiform muscle located in the lateral lumbar region between the vertebral column and the brim of the lesser pelvis. It joins the iliacus muscle to form the iliopsoas. In animals, this muscle is equivalent to the tenderloin.
The medial collateral ligament (MCL), also called the superficial medial collateral ligament (sMCL) or tibial collateral ligament (TCL), is one of the major ligaments of the knee. It is on the medial (inner) side of the knee joint and occurs in humans and other primates. Its primary function is to resist valgus forces on the knee.
A meniscus is a crescent-shaped fibrocartilaginous anatomical structure that, in contrast to an articular disc, only partly divides a joint cavity. In humans, they are present in the knee, wrist, acromioclavicular, sternoclavicular, and temporomandibular joints; in other animals they may be present in other joints.
The plantaris is one of the superficial muscles of the superficial posterior compartment of the leg, one of the fascial compartments of the leg.
The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of the clavicle, scapula, and coracoid. Some mammalian species have only the scapula.
The sternalismuscle is an anatomical variation that lies in front of the sternal end of the pectoralis major parallel to the margin of the sternum. The sternalis muscle may be a variation of the pectoralis major or of the rectus abdominis.
In the context of human evolution, human vestigiality involves those traits occurring in humans that have lost all or most of their original function through evolution. Although structures called vestigial often appear functionless, a vestigial structure may retain lesser functions or develop minor new ones. In some cases, structures once identified as vestigial simply had an unrecognized function. Vestigial organs are sometimes called rudimentary organs. Many human characteristics are also vestigial in other primates and related animals.
A tear of a meniscus is a rupturing of one or more of the fibrocartilage strips in the knee called menisci. When doctors and patients refer to "torn cartilage" in the knee, they actually may be referring to an injury to a meniscus at the top of one of the tibiae. Menisci can be torn during innocuous activities such as walking or squatting. They can also be torn by traumatic force encountered in sports or other forms of physical exertion. The traumatic action is most often a twisting movement at the knee while the leg is bent. In older adults, the meniscus can be damaged following prolonged 'wear and tear'. Especially acute injuries can lead to displaced tears which can cause mechanical symptoms such as clicking, catching, or locking during motion of the joint. The joint will be in pain when in use, but when there is no load, the pain goes away.
Extensor digitorum brevis manus is an extra or accessory muscle on the backside (dorsum) of the hand. It was first described by Albinus in 1758. The muscles lies in the fourth extensor compartment of the wrist, and is relatively rare. It has a prevalence of 4% in the general population according to a meta-analysis. This muscle is commonly misdiagnosed as a ganglion cyst, synovial nodule or cyst.
Discoid meniscus is a rare human anatomic variant that usually affects the lateral meniscus of the knee. Usually a person with this anomaly has no complaints; however, it may present as pain, swelling, or a snapping sound heard from the affected knee. Strong suggestive findings on magnetic resonance imaging includes a thickened meniscal body seen on more than two contiguous sagittal slices.
An accessory muscle is a relatively rare anatomical variation where duplication of a muscle may appear anywhere in the muscular system. Treatment is not indicated unless the accessory muscle interferes with normal function.
Anatomical terminology is a form of scientific terminology used by anatomists, zoologists, and health professionals such as doctors, physicians, and pharmacists.
The axillary arch is a variant of the latissimus dorsi muscle in humans. It is found as a slip of muscle or fascia extending between the latissimus dorsi muscle and the pectoralis major. There is considerable variation in the exact position of its origin and insertions as well as its blood and nerve supply. The arch may occur on one or both sides of the body. A meta-analysis revealed that the axillary arch had an overall prevalence of 5.3% of limbs.
Episternal ossicles are small bones that are sometimes present at the upper end of the chest bone. The prevalence of these ossicles is around 1.5%.