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 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.
- Capitate: 1-3 months
- Hamate: 2-4 months
- Distal radius: 1 year
- Triquetrum: 2-3 years
- Lunate: 2-4 years
- Scaphoid: 4-6 years
- Trapezium: 4-6 years
- Trapezoid: 4-6 years
- Distal ulna: 5-6 years
- Pisiform: 8-12 years
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)
- Use the same hand as the patient’s affected hand (suspicious of a right scaphoid problem? Use your right hand to test)
- Place your hand on the patient’s so that your thumb is over the volar surface of the scaphoid tubercle (the distal pole). Don’t apply any pressure (remember this area is probably at least a little sore and you want to remain friends for now)
- Gently move the wrist through ulnar/radial deviation (you can be fancy and consider this your Watson Test) and flexion/extension to relax the patient
- With the patient’s wrist in neutral extension and neutral (or slight radial deviation), forcefully and quickly push the scaphoid tubercle in the dorsal direction
- At this point, the patient is likely no longer your friend
- Note the degree of shift, any crepitus or clunk, and pain evoked.
- Remember to compare this to the opposite wrist
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?)
Which finger (digit?!) is which?
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.
- Thumb = D1
- Index = D2
- Long = D3
- Ring = D4
- Small = D5
Which side of the hand?
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.
- Thumb side = RADIAL
- Small finger side = ULNAR
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).
People can be whiners sometimes. Their hand will be in a cast for some break and you’ll take it off and they will say, “my hand is stiiiiifffff”
It’s not just them, the mechanics of their hand is working against them and if the cast wasn’t positioned properly, it can make matters much worse as far as stiffness is concerned. This is why when a hand or wrist is being casted or splinted, care is taken to put it in the position that will minimize stiffness.
The “safe position” is also known as the intrinsic plus position as it favours the weaker motions of MCP flexion and IP extension that are difficult to recover.
Wrist: The weight of your hand, gravity and resting muscle tension all work together to pull the wrist into flexion. When the wrist is flexed, there is more tension on the extrinsic extensor muscles and they pull the MCP joints into extension. The extrinsic flexors are stronger than the extensors and pull the IP joints into flexion. Taking the tension off the extensors limits their pull across the MCP joints.
The position of flexed wrist, extended MCP joints and flexed IP joints is known as intrinsic minus.
Metacarpal Phalangeal (MCP) Joint: These joints are a little funny due to the collateral ligaments on either side. These ligaments pass slightly above the axis of rotation of the joint, this means that when the joint is flexed, they’re at their longest and when the joint is extended, they’re at their shortest. This is due to the famed “CAM EFFECT.” Though often quoted, you have to wonder, what is a cam*? This website explains it well.
* This does not apply to all those people who remember basic mechanical principles or were trained in something more hands-on than neuroscience
Interphalangeal (IP) Joints: The ligaments around the IP joints are at maximum stretch when they are fully extended (aka 0 degrees)
There are a whole lot of wrist/finger extensors trying to fit in the wrist and anatomically these are divided into 6 compartments.
- First compartment – it’s this that is affected in de Quervain tenosynovitis
- APL (abductor pollicis longus): attaches to 1st MC
- EPB (extensor pollicis brevis): attaches to base of proximal phalanx
- Second compartment
- ECRB (extensor carpi radialis brevis): attaches to 3rd MC
- ECRL (extensor carpis radialis longus): attaches to 2nd MC
- Third compartment
- EPL (extensor pollicis longus): passes around Lister’s tubercle of radius and inserts on distal phalanx of thumb (extends thumb IPJ)
- Fourth compartment – the posterior interosseus nerve lies on the floor of this compartment
- EDC (extensor digitorum communis): no direct attachment to phalanx, attaches to the extensor expansions
- EIP (extensor indicis proprius): lies ulnar to 1st EDC tendon)
- Fifth compartment
- EDM (extensor digiti minimi): attaches to extensor expansion of little finger
- Sixth compartment
- ECU (extensor carpi ulnaris): attaches to base of 5th MC
- Zone I: over the DIP (this is where mallet finger injuries occur)
- Zone II: middle phalanx
- Zone III: over the PIP
- Zone IV: proximal phalanx
- Zone V: over the MCP
- Zone VI: dorsum of hand/metacarpals
- Zone VII: over the extensor retinaculum/carpals
- Zone VIII: proximal wrist
- This is the connections of fascia between the EDC tendons and why you can’t stick your ring finger up alone, as it prevents independent movement.
- It can also lead to confusion about whether an extensor tendon has been cut as the juncture tendinum transmits MCP joint extension even if a tendon is cut (as long as it’s cut distal to the JT)
- But it’s also helpful as it prevents the cut tendon from retracting up into the forearm
Thumb fractures, and by this I mean 1st metacarpal fractures, have a couple of distinct patterns that are different from the other metacarpals.
Type I: Bennett Fracture
- This fracture is intra-articular on the ulnar side of the first metacarpal, basically making a little triangle
- It’s that little ulnar fragment that stays attached to the trapezium by the virtue of the volar ligament
- The distal aspect of the metacarpal gets supinated and dislocated radially no thanks to the adductor pollicis
- The fragment gets pulled proximally by the abductor pollicis brevis and abductor pollicis longus
Type II: Rolando Fracture
- You can think of this fracture as a really busted up Bennett’s (comminuted). It is also intra-articular and usually makes a Y or T shape
- These kind generally heal poorly but thankfully are fairly rare
Type III: Other extra-articular fractures
- This is basically any other 1st metacarpal fracture (all the extra-articular ones)
- They are the most common, but don’t have fancy names, just lame ones like “transverse“, “oblique“, etc.
- These really only exist in paediatrics and involve the proximal physis (growth plate)
Treatment: it’s best to treat Bennett and Rolando Fractures with thumb spica splints and then refer them to your friendly neighbourhood plastic surgeon or orthopaedic surgeon as they might need pinning or an open reduction.
This wasn’t even going to be an education post, but I got guilted into it. The original is 18×24″, oil on canvas.
The hand exam
- Inspection: You can use the acronym SEADS: swelling, erythema, atrophy, deformity, skin changes. Remember: you should be able to see their whole arm up to the elbows
- Palpation: You can use the acronym TEST CA: tenderness, effusion, swelling, temperature, crepitations, atrophy
- Range of Motion and Power: Do active, if it’s not full you can then check passive.
- Special Tests: fancy things for carpal tunnel like Phalen’s and Tinel’s