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Anatomy and Metabolism of Bone

mnemonic:Some Lovers Try Positions That They Can't Handle

proximal rowdistal row
ScaphoidTrapezium
LunateTrapezoid
TriquetrumCapitate
PisiformHamate

Scaphoid!!navigator!!

[scaphion, Greek = boat]

  • The largest bone of proximal carpal row!
    • Function: acts as an intercalated segment between lunate proximally + trapezium and trapezoid distally
    • Division: proximal third; middle third with waist; distal third with tuberosity on palmar surface
    • Blood supply: branches of radial artery enter bone near midportion / waist dorsally → perfusion of mid to proximal 70–80% of scaphoid + proximal pole supplied by end artery in retrograde fashion

Lunate!!navigator!!

[luna, Latin = moon]

= moon-shaped configuration on LAT view

  • Parts: body, volar pole, dorsal pole
  • Function: acts as keystone of proximal carpal row
  • Blood supply: single vessel (20%); 2 nonarticular nutrient arteries with consistent intraosseous anastomosis (80%)

Carpal Bone Blood Supply!!navigator!!

  • Blood vessels frequently enter distal half of bone putting proximal bone at risk for avascular necrosis!
Single arterial supplyscaphoid, capitate, lunate (in 20%)
Two nutrient arteries without intraosseous anastomosistrapezoid, hamate
Two nutrient arteries with intraosseous anastomosistrapezium, triquetrum, pisiform, lunate (in 80%)

Ulnar Variance!!navigator!!

= HULTEN VARIANCE = RADIOULNAR INDEX

= relative lengths of distal articular surfaces of radius and ulna

Definition:

  • neutral = both surfaces at same level = equal length of ulna + radius
  • positive = ulnar surface distal to radial surface = long ulna
    • Gymnasts have a predilection for a positive ulnar variance!

    Risk:
    1. ulnar impaction (→ chondral fibrillation, chondromalacia, degenerative arthritis)
    2. ligamentous and carpal disturbances (→ TFCC tear, focal chondral lunate injury)
  • negative = ulnar surface proximal to radial surface = short ulna
    • Most children aged 12–16 years have a negative ulnar variance!

Effect of wrist position:

  1. increase of ulnar variance
    1. maximum forearm pronation
    2. firm grip
  2. decrease of ulnar variance
    1. maximum forearm supination
    2. cessation of grip

Radiographic standard view of unloaded wrist: posteroanterior, neutral forearm rotation, elbow flexed 90°, shoulder abducted 90°

Guyon Canal!!navigator!!

[Jean Casimir Félix Guyon (1831–1920), 1st French chair in urology at University of Paris]

= canalis nervi ulnaris = loge de Guyon (French)

= small superficial tunnel-like structure at base of hypothenar

  • Floor: depression between pisiform + hook of hamate
  • Roof: volar carpal ligament and pisohamate ligament; retinaculum flexorum manus; flexor carpi ulnaris m.
  • Contents: ulnar nerve with bifurcation into superficial and deep branches; ulnar artery
  • Clinical significance: site for compression injury by anomalous muscle, ganglion, hamate fracture

Important Stabilizing Wrist Ligaments!!navigator!!

  • Important for carpal stability!
  1. Intrinsic = between carpal bones
    1. Scapholunate lig.
    2. Lunotriquetral lig.
  2. Extrinsic = between carpal + metacarpal bones or between carpal bones + radius / ulna
    1. Palmar: carpal stability
      1. Radial collateral lig.
      2. Radiolunotriquetral lig. (RLTL): preventing ulnar translation
      3. Radioscaphocapitate lig. (RSCL): keeps scaphoid in position
    2. Dorsal radiocarpal ligg.: prevent perilunate instability + volar intercalated segment instability
    3. Dorsal intercarpal ligg.: prevent perilunate instability + dorsal intercalated segment instability

Outline