The flexor pollicis brevis is a muscle from the intrinsic group of the hand, which is part of the thenar eminence. It consists of two muscular heads or bellies, one superficial and one deep. Its main function is to collaborate in the opposition movement of the thumb, or pincer, which is a fundamental and characteristic function of the human hand.
Both the structure and the innervation of this muscle is the subject of research and controversy to this day. Some authors consider that it does not have a deep portion, but rather that this belly is part of another muscle in the hand, the adductor pollicis. This discrepancy is based on the fact that there are cases in which some of the muscle portions are absent.
Short flexor of the thumb. Public Domain, Injuries to the flexor pollicis brevis can be evidenced on physical examination using special clinical maneuvers. Damage to this muscle generally requires surgical repair.
Location and origin
The flexor brevis muscle of the thumb is part of the thenar eminence along with three other muscles. This muscle group is located at the base of the thumb and is responsible for flexing and moving the thumb inward, to perform the opposition movement of the thumb or pincer.
It is located medial and inferior to the abductor pollicis brevis muscle and above the opponent of the thumb.
By Anatomist90 - Own work, CC BY-SA 3.0, It consists of two muscular bellies called superficial and deep, which have different origins in the carpal bones.
The superficial portion originates from the lateral projection of the trapezius bone. Sometimes a muscle bundle is found that is anchored to the fibrous lamina of the flexor retinaculum.
Mechanism of muscular movement of the human hand. By Internet Archive Book Images - https://www.flickr.com/photos/internetarchivebookimages/14759425146/Source book page: https://archive.org/stream/appliedanatomyk00bowe/appliedanatomyk00bowe#page/n155/mode/1up, No restrictions, The origin of the deep portion is in two of the carpal bones. It reaches the lateral aspect of the trapezoid and the medial aspect of the great one. This belly is closely related to the flexor longus muscle of the thumb.
Distally it ends up joining with the superficial fascicle to form a single muscle that ends in the thumb.
Insertion
Both the superficial and deep portions become a tendinous structure. After traveling part of the way along the flexor pollicis longus, the deep fascicle ends up joining the superficial one.
Forming a single tendon, the flexor pollicis brevis attaches to the base of the proximal phalanx of the thumb and to the sesamoid bone that is embedded in the ligaments that are part of that joint.
Irrigation and innervation
The vascular supply of the flexor brevis of the thumb is ensured by the radial artery, superficial endings and some branches that emit from the superficial palmar arch, which is a complex vascular network that forms the radial and ulnar arteries.
Arteries of the hand. - Own work, CC0, In terms of innervation, this is different for the superficial and deep belly, which is why it has been the subject of study, research and controversy.
Scientific studies carried out measuring the nerve impulses of the muscle show that the superficial bundle receives motor innervation through the median nerve.
For its part, the deep belly is innervated by the deep branch of the ulnar or ulnar nerve.
In a rare anatomical variation, only the ulnar nerve is responsible for innervating both portions of the flexor pollicis brevis. The so-called dual innervation has also been observed.
In these cases, the median and ulnar nerves join at one point, forming an arch that gives motor neurological branches and innervates the muscle.
Features
The flexor brevis muscle of the thumb is responsible for the flexion movements of the thumb both at its base and at its interphalangeal joint.
Its contraction brings the thumb inward and forward. With this movement, it collaborates with the opposing thumb muscle and the adductor muscle to perform one of the main functions of the human hand, which is to bring the thumb closer to the rest of the fingers.
This is known as the opposite thumb or pincer, and it is a fundamental characteristic that differentiates the hand of men from that of primates, since they are unable to perform this movement.
Movement of the index finger and human thumb. By Almécija, Moyà-Solà & Alba - Almécija S, Moyà-Solà S, Alba DM (2010) Early Origin for Human-Like Precision Grasping: A Comparative Study of Pollical Distal Phalanges in Fossil Hominins. PLoS ONE 5 (7): e11727. doi: 10.1371 / journal.pone.0011727, CC BY 3.0,
Injuries
The flexor pollicis brevis tendon can be affected mainly by trauma that causes its laceration or complete disinsertion. Inflammation of the tendon, or tendinitis, is also a frequent reason for consultation.
Neuritis and trauma affecting both the ulnar and median nerves have repercussions on the motor function of the muscle.
Diagnosis of a flexor brevis injury is made first through clinical evaluation. The location of the trauma presented by the patient provides the physician with an important idea for the diagnostic approach.
The way to examine this muscle is to isolate it to see if the patient can adduct the finger. The doctor should gently press the rest of the fingers of the hand, in order to lock them. Thus, the patient is asked to bring the thumb inward.
If the individual is able to perform the movement without difficulty, it is assumed that the tendon has not suffered any injury. Otherwise, when there is great difficulty or the movement cannot be performed, there is a high probability that the tendon is injured or detached.
Neurological function should also be evaluated to check the integrity of the ulnar and median nerves. The fastest and most efficient way is through the evaluation of nerve impulses with electromyography. But there is also physical evidence in case the study is not available.
Electrostimulation of the ulnar nerve. By Paul Anthony Stewart - Own work, CC BY-SA 4.0, To evaluate the median nerve, the back of the patient's hand is placed on a flat surface and asked to raise the thumb. In the case of the ulnar nerve, the patient must take a sheet of paper performing the pincer movement.
Treatment
Once the diagnosis has been established, the appropriate treatment is administered.
In the event that there is a wound, basic care must be taken. If the tendon suffered a partial cut or detachment due to the trauma, the treatment is surgical.
In the case of tendinitis, treatment is clinical and includes immobilization of the finger with a splint, either plaster or aluminum. In this way, the tendon is given rest for a few weeks.
Upper limb orthosis. By N16tran - Own work, CC BY-SA 3.0, In both cases, therapy includes oral pain relievers, rest, and immobilization.
References
- Caetano, E. B; Nakamichi, Y; Alves de Andrade, R; Sawada, M. M; Nakasone, M. T; Vieira, L. A; Sabongi, RG (2017). Flexor Pollicis Brevis Muscle. Anatomical Study and Clinical Implications. The open orthopedics journal. Taken from: ncbi.nlm.nih.gov
- Day, M. H; Napier, JR (1961). The two heads of flexor pollicis brevis. Journal of anatomy. Taken from: ncbi.nlm.nih.gov
- Guzmán, R. A; Bralic Echeverria; M. P; Cordero Garayar, J. (2013). Location of the Innervation Zone of the Short Flexor Thumb Muscle in Healthy Individuals of Both Sexes. International Journal of Morphology. Taken from: scielo.conicyt.cl
- Delgado, AD; Alcántara, T. (2001). Hand injuries in the emergency room. Journal of Integral Medicine. Taken from: elsevier.es
- Pacheco-López, RC (2017). Acute repair of the flexor tendons. Ibero-Latin American Plastic Surgery. Taken from: scielo.isciii.es