Overview of Bone Quality

Educational Objectives

The goal of this program is to improve management of fractures. After hearing and assimilating this program, the clinician will be better able to:

1. Discuss elements of bone quality such as collagen cross-linking and the lamellar structure of bone itself.

Summary

Epidemiology of low-trauma fractures: rate of fractures per 1000 increases with decreasing bone mineral density (BMD); largest number of fractures occurs among individuals with T-score between -1.5 and -2.0 despite lower rate, because many more people fall into that range; suggests BMD failing to capture some important factor related to risk for fracture

Bone quality: in speaker’s view, encompasses all contributors to mechanical performance of bone that defy measurement; therefore, “moving target”; components change over time with changes in ability to measure different parameters; definition of exclusion

Trabecular bone score (TBS): uses image from dual-energy X-ray absorptiometry of lumbar spine to assess uniformity of BMD; uniform BMD indicates stronger bone than patchy BMD; speaker posits that uniformity of trabecular structure no longer part of “bone quality” because of ability to measure it; TBS approved by Food and Drug Administration; shown in multiple studies to predict fracture independently of BMD; fracture risk assessment tool allows inclusion of TBS when calculating score

Mechanics: speaker’s laboratory performs 3-point bending test on mice; bone supported on both ends, and middle pushed down; applied force, amount of bending, and point of breaking measured; 2 different mechanical interventions are occurring; top of bone compressed with downward bending, and bottom of bone tensed and stretched; different materials contribute to strength during tension and compression

Bone tissue: mixture of protein (mostly type 1 collagen) and mineral (mostly apatite); protein provides tensile strength; mineral provides compressive strength

Load-displacement curve: first portion (elastic region) is straight; bone can return to original size and shape essentially without damage when pressure released during first portion of curve; permanent damage begins to occur at yield point; pressure beyond yield point injures bone so that bone does not return to normal state; bone continues to resist after reaching maximum load until failure; plastic deformation part of curve characterizes difference between brittle and ductile components; ductile component has large plastic deformation; brittle bone breaks soon after reaching maximum load; overmineralized bone brittle; slope of elastic region indicates stiffness of bone; different materials have different mechanical properties; bone is composite

Protein: problem with protein leads to weak bone; most elements of protein part of bone currently considered part of bone quality

Collagen: structural protein found in bone, skin, connective tissue, and tendons; composed of 3 individually synthesized strands that assemble in endoplasmic reticulum; modifications in helical portion essential for proper folding to occur; many diseases affect bone through point mutations in genes encoding production or modification of type 1 collagen; type 1 collagen self-assembles into fibrils with periodic spacing and regular organization of molecules; cross-links important for determination of strength; more cross-links lead to increased tensile strength; maturation of initial cross-links can proceed nonenzymatically; mature cross-links stronger than immature ones; extent and maturity of cross-links not clinically measurable, so elements of bone quality; mutations can cause improper assembly and overmineralization of collagen

Bone: appears as collections of fibrils at microscopic level, with precipitations of mineral enmeshed within matrix of protein; hierarchically organized (collagen to fibrils to fibers to lamellae of osteons); lamellae have interface with neighbors; problem at lowest level of organization cascades up through hierarchical assembly; lamellar structure observed in cortical and trabecular bone; layers oriented in different directions

Lamellar structure: provides structural basis for dissipating force of crack and preventing complete penetration; interfaces between lamellae protect layers, deflect cracks, separate layers slightly, and absorb force; force dissipated instead of propagated; contributes to plastic portion of curve; most forces in long bones transverse to lamellar organization of Haversian system in cortical bone (transverse across long bones)

Woven bone: lacking cement lines; may be dense but mechanically impaired; cannot stop propagation of fracture through bone

Atypical fractures: atypical fractures related to tension and failure of protein in bone

Readings

Fonseca H et al: Bone quality: the determinants of bone strength and fragility. Sports Med 2014 Jan;44(1):37-53; Hunt HB et al: Bone quality assessment techniques: geometric, compositional, and mechanical characterization from macroscale to nanoscale. Clin Rev Bone Miner Metab 2016 Sep;14(3):133-49; Licata A: Bone density vs bone quality: what’s a clinician to do? Cleve Clin J Med 2009 Jun;76(6):331-6.

Disclosures

For this program, members of the faculty and planning committee reported nothing to disclose.

Acknowledgements

Dr. Blank was recorded at the Wisconsin Osteoporosis Symposium: The Bare Bones of Osteoporosis Care, held June 2, 2017, in Madison, WI, and presented by the University of Wisconsin-Madison Interprofessional Continuing Education Partnership. For information about upcoming CME opportunities, please visit https://www.iscd.org/event/wisconsin-osteoporosis-symposium-bare-bones-osteoporosis-care/.  The Audio Digest Foundation thanks the speakers and the sponsors for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 0 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 0 CE contact hours.

Lecture ID:

OR411802

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.