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Now, That’s What I Call High-Yield: Musculoskeletal, Part 1

Go ahead – tell an orthopedist that the musculoskeletal questions on Step 1 aren’t very high-yield. They will bench press you faster than you can say ORIF. While it may seem that the musculoskeletal system (MSK) isn’t as elegantly intertwined as cardiovascular, renal, and pulmonary, it’s literally what makes us move. It is within MSK that we have incredible high-yield topics, including common menaces like osteoarthritis and back pain, fractures, and the poster child of first year anatomy, the glorious brachial plexus! Give this subject the attention it deserves. Because a lot of the material stands alone without overlap into other systems, many topics will need a lot of devoted effort.

Upper extremity dermatomes/muscles (8) – Knowledge of muscles and their functions is nothing more than straight up anatomy. And just as it did during your anatomy course, this material needs over-and-over rote memorization.

The full body dermatomal map is just something you will have to know. The thorax is pretty straight forward – snap a line at the T4 nipples and T10 umbilicus, draw some lines of demarcation in between, and you’ve got it put together. It’s these appendages that make things tricky…

For the upper extremity, your landmark is C7 as the middle finger. In anatomic position (i.e., thumbs lateral), think cranial as you course laterally (C6 for the thumb, C5 for the lateral forearm, and C4 atop the shoulder), and caudal as you trace up the arm medially (C8 at the pinky, T1 for the medial forearm, and T2 for the medial upper arm). This little bit of information will get you very far.

Brachial Plexus (10) – I’ve got one thing to say: Take 30-60 minutes, watch some youtube videos, know how to reconstruct the brachial plexus as a schematic drawing, and commit it to memory inside and out. It’s a relatively large investment to make, but will paid guaranteed dividends in the long run. Don’t stop at the plexus itself, familiarize yourself with the distal branches, their anatomical paths, the muscles that they innervate, and the result of neuropathy/nerve injury. At the absolute minimum, knowledge of the branches (musculocutaneous, median, ulnar, axillary, radial) is of the utmost importance.

Lower extremity physical exam (7) – In today’s “let’s-talk-after-you-get-an-MRI” world, becoming a master physical examiner is slightly less important (don’t let your professors hear that!). For Step 1 studying, however, knowledge of these signs will definitely score you some points. The knee, despite its cockamamie design of “two balls (condyles) on a slanted tabletop (tibia),” is held together in a tight, neat little capsule with the help of a number of ligaments. There should not be any appreciable “give” in any range of motion, other than classic flexion and extension. If laxity is detected in the joint, figure out which ligament is letting its guard down, and there’s your pathology. For example, if the tibia is sliding forward, the ACL isn’t doing its job. There’s your pathology. If a medially applied force opens up the lateral aspect of the capsule, than the lateral collateral ligament is lax, and is the one to blame.

Lower extremity dermatomes/nerves/muscles (8) – So many nerves, so little time. At the top of your list is the almighty femoral nerve, composed of L2-4. It controls your quadriceps and is responsible for sensation in the anterior thigh. It’s distal branch, the saphenous nerve, takes care of sensation for the medial leg.

The sciatic nerve (L4-S3) has a motor component of the posterior thigh (hamstrings). Injury often occurs from a herniated disc (sciatica), which often occurs at the L4-5 or L5-S1 disc space. The sciatic nerve gives rise to two branches, the common peroneal and tibial. One of my favorite mnemonics of all time, TIPPED, is at play here. Tibial nerve Inverts and Plantarflexes, Peroneal nerve Everts and Dorsiflexes. The peroneal nerve (also known as fibular nerve), courses around the fibular neck, a point susceptible to injury and compression.

For the dermatomal map, your midpoint is the L3 (rhymes with) knee. More proximally L2 hits the upper thigh and L1 covers the groin. Distally, L5 is an important landmark for herniated discs; lesions here affect the big toe. For S1, move laterally to the pinky toe. S2-4 are found in the perineal area.

Common childhood MSK conditions (8.5) – Common things are common and will show up on the test. Luckily, history and patient demographic can sort out most of your pediatric MSK conditions, scoring you some easy points. Congenital hip dysplasia is going to noticed at birth, with abnormal physical exam findings in the Ortolani and Barlow maneuvers. Legg-Calve-Perthes disease is an avascular necrosis that is usually seen in 5-7 year old males.

Slipped capital femoral epiphysis (SCFE, phonetically “skiffy”) conveniently self-describes its pathology, and is classically seen in an obese tween (~11-13 years old).

For Osgood-Schlatter disease, a repeated-use injury resulting in tibial tubercle avulsion, look no further than a teenage cross-country runner.

That should help trim the fat away the meat, and concludes Part 1. Musculoskeletal, Part 2 revolves all around JOINTS.

Looking for more high-yield USMLE study materials? For starters, here’s high-yield cardiology and high-yield pathology.

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