How Old Bones Fracture [Lines Numbered for Reference]

  1 

  2 Old bones fracture more easily than

  3 young ones, even under small amounts

  4 of stress.

  5 This is partly because old bones lose

  6 density, but partly because they also

  7 change their structure.

  8 

  9 Many human bones contain two different

 10 internal structures.

 11 Some central tissue is quite solid

 12 (cortical bone).

 13 Other tissue, however, contains a

 14 cellular foam or open-celled lattice

 15 of collagen (trabecular bone).

 16 Trabecular bone occurs in

 17    * individual parts of the spine

 18      (vertebrae),

 19    * the femur (the long bone that joins

 20      the knee to the hip),

 21    * the tibia (the larger of the two

 22      bones connecting the knee to the

 23      ankle),

 24    * the humerus (the long bone linking

 25      the shoulder to the elbow), and

 26    * the iliac crest (the widest part of

 27      the hip bone).

 28 

 29 Trabeculae are like sponges.

 30 They absorb loads from the joints just as

 31 Styrofoam absorbs impacts in packages.

 32 

 33 All bone renews itself regularly.

 34 The human body continuously removes

 35 old bone (resorption) and replaces it

 36 with new bone (formation).

 37 The average bone turnover rate is

 38 about 6 years.

 39 

 40 In menopausal women, however, this

 41 turnover process becomes unbalanced.

 42 More bone is resorbed that is formed,

 43 so bone density decreases (osteoporosis).

 44 Medical treatments for osteoporosis

 45 usually focus on stopping resorption

 46 of old bone while still allowing new

 47 bone to fill in.

 48 Reducing turnover in this way leaves

 49 bone more massive yet also more brittle.

 50 

 51 Three-dimensional computed-tomography

 52 images reveal a structural change in

 53 human trabecular bone with age.

 54 In young bone, the trabeculae form a web

 55 of short, stubby struts.

 56 In older bone, on the other hand, these

 57 trabeculae are mostly long, slender

 58 columns.

 59 

 60 Fracture (failure) in younger bones

 61 usually happens when stress overcomes

 62 the strength of the bone tissue.

 63 Fracture in older bones happens at

 64 much lower stress levels.

 65 The long, thin trabeculae in old bones

 66 are unstable.

 67 Hence, when compressed, they buckle at

 68 far lower stresses than bone tissue

 69 normally withstands.

 70 

 71 Bone resorption forms small pits on the

 72 surface of the trabeculae.

 73 As bone turnover increases, the number

 74 of pits increases too.

 75 

 76 An empty aluminum can requires much

 77 stress to crush it axially.

 78 But with a small dent in its side wall

 79 the can buckles easily.

 80 In the same way, trabecular pits are

 81 themselves enough to destabilize

 82 bone lattice.

 83 In old human bone, 60% of trabeculae

 84 are vulnerable to buckling just because

 85 of their pits.