Anatomy of the Hand:
The hand is an underratedly complex part of the body. Engineers and doctors together have struggled for years trying to find a way to replicate the same fluid and controlled motion in the form of a bionic hand for patients who have lost their own. While there have recently been advancements in technology that have made their efforts more fruitful, there are still many roadblocks that lie in the way. Many different features are essential for making this ability possible, and to fully appreciate the situation, it is important that one understands the anatomy of the hand.
With 27 bones, extensor and flexor tendons, tendon sheaths, pulleys, muscles, nerves, an artery and blood vessels, the hand is an incredibly complex part of the body. What makes this appendage even more impressive is the tight space in which all of these elements operate in together. In order to best treat your condition, you really need to know what things look like inside the hand, and how these various pieces interact with one another to make motion possible.
While both flexor and extensor tendons control movement to the hand, this site primarily serves those who have questions regarding a flexor tendon injury. Therefore, all we'll say about the extensor tendons is that they connect the phalanges to the extensor muscles (on the dorsal side of the forearm) and run on the dorsal side of the hand. These tendons allow us to open our fists and extend our fingers.
Now let’s talk about why you’re probably here, the flexor tendons. To best explain how these tendons work, we’ll first consider all the fingers and then discuss the thumb after. The flexor tendons connect the forearm muscles to the fingers, running from the muscles through the carpal tunnel in the wrist up the palmar side of the hand all the way to each of the fingers. Each finger is connected to the muscles by two tendons, the flexor profundus (FP) and the flexor superficialis (FS). The FP connects to and controls the motion of the distal phalange (the most distant bone in each finger) and the FS connects to and controls the motion of the middle phalange (the next bone down). There are eight tendons total controlling finger motion. As these tendons travel up the hand, each one goes through several ligament pulleys in their individual finger.
We waited on the thumb because it works pretty differently. If you lacerated your tendons at the wrist, it is likely you did not get the tendons in your thumb because they connect to the thenar muscles in the palm for movement (the flexor policis longus and the flexor policis brevis). As mentioned before, they are powered by the median nerve, so if your median nerve was lacerated or damaged in your injury, you have probably noticed thumb impairment as well.
The muscles controlling thumb movement are powered by innervation from the median nerve. This nerve travels down the arm and through the carpal tunnel in the wrist to the radial side of the hand, connecting to the thenar muscles and also providing sensation to the thumb, index, and middle fingers as well as half of the ring finger (the side adjacent to the middle finger). The ulnar nerve similarly travels down the arm but instead of going through the carpal tunnel, it makes its way to the hand on the ulnar side through what’s called, Guyon’s canal. This nerve provides innervation to the hypothenar muscles controlling the pinky as well as sensation to the pinky and the remaining half of the ring finger.
The Muscles and Nerves:
Let’s start with the basics. The hand is divided in four ways: the front side of the hand is naturally referred to as, the palmar side, as that is where your palm is. The back of your hand is called, the dorsal side. Now, picture your hand flat on a table. The half from your middle finger to your thumb is called the radial side, and the middle finger to your pinky the ulnar side. Try to remember this because it will be important for referencing injury points later.
As you also know, there is no movement for your fingers without both the muscles for providing the raw power, and the tendons for connecting those muscles to the bones in your fingers called, the phalanges. There are different kinds of phalanges in the hand but for now we will not worry about that. Instead, let’s focus on the muscles. Most of the muscles that control finger movement run between the elbow and the wrist, although there are also multiple muscles within the hand that provide more accurate and controlled motion for the thumb and pinky. These muscles are:
The abductor policis brevis which abducts the thumb, meaning that it moves the thumb away from the hand.
The flexor policis brevis which flexes (or bends) the thumb downward.
The adductor policis brevis which adducts or moves the thumb toward the midline of the hand - just the opposite of the abductor policis brevis.
The opponens policis brevis which moves the thumb to face inward toward the hand and allows you to touch your pinky (that curling motion that you would need to properly hold a drink).
The hypothenar muscles which control pinky movement and are ordered in a similar way to the thenar muscles that control the thumb.