This is not an exhaustive overview of the muscles of the arm and forearm, but it demonstrates some of the tricky relationships that often catch people up. Some of the key points of compression are also noted and the muscles are coloured per their innervation. One thing not mentioned in the doodle that is important to notice is the way that the aponeurosis of the biceps veers medially (ulnarly) this is why it is also a supinator and not just an elbow flexor.
The branches of the three main terminal branches of the brachial plexus can be difficult to remember. Even worse is trying to remember where all of those pesky compression points are and why it is that you get some symptoms with some and not others.
This diagram attempts to clarify the branches of the radial, median, and ulnar nerves and where they can get squished along the way. There are of course, slight anatomic variations, but this is a good starting point. I’ve even included where the famed Martin-Gruber anastomosis and the Riche-Cannieu anastomosis are, since they can make an otherwise (totally not) straightforward examination of a median or ulnar nerve palsy more muddied since both carry motor fibers between the two nerves.
Most interestingly is John Struthers, whose namesake structures compress the median nerve as a ligament and the ulnar nerve as an arcade.
I’ve drawn the brachial plexus before showing more of its anatomical relationships (which is actually why the trunks and cords are named as they are). As I’m gearing up studying, I created this more schematic diagram of the plexus, including the distal targets (mostly the muscles but some sensory too).
Hopefully this will help you figure out “where is the lesion?” when you are faced with a brachial plexus question on your exams (and in life) as well.
I’ve also included a printable version for your printing and pasting-up-to-the-wall-to-passively-absorb pleasure.
Long thoracic: serratus anterior
Dorsal scapular: rhomboids, levator scapulae
Suprascapular: supraspinatus, infraspinatus, sensory to the AC & GH joints
Nerve to subclavius: subclavius
Lateral pectoral: pec major (clavicular head), sensation to pec
Superior subscapular: subscapularis (upper part)
Thoracodorsal (aka middle subscapular): lat dorsi
Inferior subscapular: subscapularis (lower part), teres major
Medial pectoral: pec minor, pec major (sternocostal head)
Medial cutaneous n. of arm: sensory to medial surface of arm (tiny area)
Medial cutaneous n. of forearm (antebrachial cutaneous): sensory to skin over biceps and medial forearm
Tumescent solution is also called “Klein’s Solution” after the physician who characterized the recipe and the use of it.
It’s called “tumescent” because it makes things tumescent, which is a fancy word for swollen. Tumescent is a dilute solution of lidocaine, epinephrine, and sodium bicarbonate that is injected in the subcutaneous tissue (fat). The epinephrine is the most important ingredient as it causes vasoconstriction, this means that the blood loss that could be a big problem for large procedures like burn surgery and liposuction becomes much less of a big deal.
The other interesting thing is that since fat is relatively avascular compared to other tissues, the “safe amount” of tumescent is much higher than what is normally stated for injections of lidocaine or epinephrine.
For example, it was reported by Klein that the toxic dose of lidocaine for tumescent solution is 35 mg/kg of body weight.
There are a few different recipes for tumescent anesthesia, the one presented in the doodle is the one first outlined by Klein, some use more or less lidocaine or epinephrine.
The carpal bone ossify aka turn into bone aka magically become visible on an x-ray in a predictable order.
The easiest way to remember is that it starts at the capitate (smack dab in the middle) and then goes in a ulnarly-directed spiral. I was going to say “clockwise” or “counter-clockwise” but that would depend on which side of which hand you were looking at. So capitate, followed by hamate and then down to triquetrum and so on. Except for the pisiform, being a sesamoid bone it gets left behind and only develops years later.
I included the distal radius and distal ulna in there for good measure.
I know I could have been fancier with changing the length of the metacarpals or their growth plates, but it was more fun to make the animated gif.
The scaphoid shift test aka midcarpal shift test is a variation of the Watson Test for scaphoid instability. A positive test can be caused by scapholunate ligament laxity or injury.
The Watson test evaluates scaphoid instability as the wrist is moved from radial to ulnar deviation (it’s not an “active” test)
To do the scaphoid shift test (as described by Lane in 1993)
For being such a small anatomic location, people find it very difficult to describe where on the hand or digits things are actually happening when there is an injury.
I think part of it stems back to medical school when we are taught that the digits all have numbers, the thumb is D1, index D2 and so forth. The problem comes when people say “the 3rd finger” and all of the sudden one has no idea whether they are talking about the long finger (D3) or the ring finger (D4 but then, the thumb doesn’t count as a finger, does it?)
This is why it’s always best to call digits by their names, this even goes for metacarpals. It is totally OK, and generally less confusing to call a bone the index finger metacarpal.
The same goes for which side of the hand the problem is on. There is no lateral or medial side to the hand. One could argue that it’s how someone is in anatomical position, so obviously the small finger side is medial, unfortunately very few people walk around in anatomic position and it’s their thumbs that point to the body.
So best to describe side by two things that stay put regardless of how someone has their hands in space: the radius and the ulna.
Finally for the top and bottom (or is it back and front) of the hands: use the terms DORSAL (where the nails are) and VOLAR (or palmar)
Scaphoid fractures are very common but due to its weird blood supply, the scaphoid is prone to not healing well (review the anatomy of the scaphoid in this doodle). This is why fractures of the scaphoid and even SUSPECTED fractures of the scaphoid are treated very conservatively.
Even if you’re suspicious of a fracture but don’t see one on x-ray, that’s enough to subject someone to a cast for 2 weeks and then bring them back to re-x-ray.
This doodle goes through the basic algorithm for treating scaphoid fractures centred around a timeline to show how long the treatment course can be. There are of course nuances to the management, so take a person’s work and hobbies and handedness into consideration. Also, don’t be afraid to consult your friendly hand/wrist specialist.
The scaphoid bone is one of the eight carpal bones of the wrist (you can check out this doodle for a refresher).
The scaphoid is the most commonly fractured carpal bone, accounting for almost 70% of fractures. It tends to be young males who break their scaphoid this is both an anatomical thing: younger kids get ligament injuries and older folks break their distal radius and a lifestyle thing: falling on outstretched hands (skateboarding, snowboarding) or throwing a punch both place a lot of force across the scaphoid leading to fractures.
The bad thing about scaphoid fractures is that the blood supply (from a branch of the radial artery) comes from distal to proximal. Since most fractures happen at the waist of the scaphoid the likelihood of having poor blood supply to the fracture site is quite high. It doesn’t help matters that around 80% of the scaphoid is articular surface (joint surface), so if it doesn’t heel well, it can lead to problems with arthritis of the wrist later on.
Scaphoid fractures present with a pretty classic story and the person is usually swollen and bruised and will have tenderness in their “snuffbox.” So even if the x-ray doesn’t show a fracture, it’s best to treat with a cast for comfort and safety and then recheck them in 2 week’s time (this will be discussed in a separate post).
The Z-Plasty is one of the most fundamental local flaps. It’s a variation of a transposition flap (meaning simply that it was rotated into a defect right next to it).
The trick is that all three limbs need to be equal and that the angles should be equal.
If the angle between the central limb and the lateral limb is 60°, then there should be an increase of the central limb by 75% (ex: 2cm -> 3.5cm)*
Since the Z-Plasty lengthens and changes the line of tension, it is great for releasing scar contractures.
* If you want you can measure the doodle, it’s pretty close to a 75% increase which I found really cool (in that I created it by rotating the flaps). MATHMAGIC!