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AN2.1-6 | General features of bones & Joints — Part 3
CLINICAL PEARL
The first carpometacarpal (thumb) joint is the only true saddle joint in the body. Its unique shape allows opposition — the single most important movement that distinguishes human hand function. When this joint develops arthritis (basal thumb OA, common in women over 50), patients lose the ability to grip, pinch, and open jars — a devastating functional loss from a tiny joint.
SELF-CHECK
At the atlantoaxial joint, the atlas rotates around the dens of the axis. What type of synovial joint is this?
A. Hinge joint
B. Ball-and-socket joint
C. Pivot joint
D. Condyloid joint
Reveal Answer
Answer: C. Pivot joint
The atlantoaxial joint is a pivot (trochoid) joint — it allows rotation around a single vertical axis (the dens). This is the joint that allows you to shake your head 'no'. About 50% of cervical rotation occurs here.
Nerve Supply of Joints — Hilton's Law (AN2.6)
Hilton's Law (1863): A nerve that supplies a muscle acting on a joint also supplies the joint itself and the skin over the insertion of that muscle.
Practical application: You do not need to memorise the nerve supply of every joint individually. Instead, identify the muscles that move the joint, recall the nerves that supply those muscles, and those same nerves will supply the joint.
Example — Knee joint:
• Muscles acting on the knee: quadriceps (femoral nerve), hamstrings (sciatic/tibial nerve), sartorius (femoral nerve), gracilis (obturator nerve).
• Therefore, the knee joint is supplied by: femoral, sciatic (tibial and common peroneal), and obturator nerves.
• The skin over the knee: femoral nerve territory (anterior) and sciatic territory (posterior).
Why this matters clinically:
• Joint pain may be referred to the skin territory of the same nerve (hip pathology can present as knee pain — obturator nerve supplies both).
• When performing a nerve block for joint surgery, you must block ALL nerves that supply the joint per Hilton's Law, not just one.
SELF-CHECK
A child presents with hip joint disease but complains of pain in the knee. Using Hilton's Law, which nerve explains this referred pain pattern?
A. Femoral nerve
B. Sciatic nerve
C. Obturator nerve
D. Superior gluteal nerve
Reveal Answer
Answer: C. Obturator nerve
The obturator nerve supplies the hip joint AND sends an articular branch to the knee joint. Therefore, hip pathology can refer pain to the knee via the obturator nerve. This is a classic clinical trap — always examine the hip in a child presenting with knee pain!