Bone Remodeling

Introduction

Wolff's Law
- bone remodels in response to mechanical stress
Piezoelectic charges
- bone remodels is response to electric charges
- compression side is electronegative and stimulates osteoblast formation
- tension side is electropostive and stimulates osteoclasts
Hueter-Volkmann Law
- theory that bone remodels in small packets of cells known as Basic Multicellular Units (BMUs)
- theory suggest that mechanical forces influence longitudinal growth
- compressive forces inhibit growth
- may play role in scoliosis

Remodeling Mechanism

Cortical bone
- remodels by osteoclastic tunneling (cutting cone)
  - osteoclastic resorption > layering of osteoblasts > layering of lamellae > cement line laid down
  - osteoclast make up head of cutting cone, followed by capillaries and then osteoblasts which lay down the osteoid to fill the cutting cone
- cortical bone continues to change over time
  - cortical area decreases as age increases
    - linked to increase fracture risk
  - medullary canal volume increases as age increases
Cancellous bone remodels by
- osteoclastic resorption
- osteoblastic deposition of layers of lamellae

TAG

(OBQ10.16) The femur radiograph of a healthy 25-year-old female is compared to the femur radiograph of a healthy 85-year-old female. Which of the following best describes the 25-year-old's femur? Topic

Review Topic

1. Increased cortical thickness and a smaller medullary canal volume
2. Decreased cortical thickness and a larger medullary canal volume
3. Equivalent cortical thickness and medullary canal volume
4. Increased cortical thickness and larger medullary canal volume
5. Decreased cortical thickness and a smaller medullary canal volume

PREFERRED RESPONSE ▶

DISCUSSION: As the human body ages the cortical thickness/area decreases and subsequently the diameter/volume of the medullary canal increases. Therefore, a young healthy 25-year-old woman should have thicker cortices and a smaller medullary canal volume than her counterpart at 85 years. Decreases in cortical bone area's are linked to increase fracture risk.

Stein et al studied femurs from subjects aged 21-92 years of age. They found that a reduction in cortical area was seen in older specimens and this change was more pronounced in female specimens.

Wrong Answers:
Answer 2: Decreased cortical thickness and a larger medullary canal volume describes the bone characteristic of an elderly patient.
Answer 3: Bone mass decreased with aging and is not constant.
Answer 4. The volume of bone remains constant with aging, so you can not have both an increased cortical thickness and larger medullary canal volume.
Answer 5: The volume of bone remains constant with aging, so you can not have a decreased in both cortical thickness and medullary canal volume.

REFERENCES:

1. Miller JD, McCreadie BR, Alford AI, Hankenson KD, Goldstein SA. Form and function of bone. In: Einhorn TA, O'Keefe RJ, Buckwalter JA, eds. Orthopaedic Basic Science: Foundations of Clinical Practice, 3rd ed. Rosemont, IL: American Academy of Orthopaedic Surgeons; 2007:129-159.

2. Stein MS, Thomas CD, Feik SA, Wark JD, Clement JG. Bone size and mechanics at the femoral diaphysis across age and sex. J Biomech. 1998 Dec;31(12):1101-10. PMID:9882042 (Link to Abstract)

Question Authors:

Jan Szatkowski MD, Patrick McCulloch MD, Dave Marcu MD,

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TAG

(OBQ04.231) A 25-year-old male sustains an transverse humeral shaft fracture and undergoes open reduction and internal fixation with rigid compression plating. What kind of bone healing would be expected with this type of fracture fixation? Topic

Review Topic

1. Primary bone healing through haversian remodeling
2. Secondary healing through callus formation
3. Primary healing through callus formation
4. Endochondral ossification
5. Secondary healing through osteonal cutting cones

PREFERRED RESPONSE ▶

DISCUSSION: Fractures and osteotomies that are stabilized with rigid compression plating undergo primary bone healing, also known as haversian remodeling. Absolute stability constructs, such as a compression plate, allow for bone healing without visible callus formation. Healing occurs via extension of clusters of osteoclasts (known as osteonal cutting cones) across the fracture site, along with osteoblasts depositing new bone and blood vessels to re-establish the haversian system.

Seconday bone healing occurs when fractures heal through callus formation. Relative stability constructs, such as an intramedullary nail, allow for some motion at the fracture site which leads to healing through a cartilage scaffold (endochondral ossification).

Illustration A demonstrates a transverse fracture stabilized with a compression plate. Illustration B shows a femoral shaft fracture that has healed through callus formation.

Incorrect Answers:
Answer 2: Secondary healing through callus formation occurs when fractures heal with some motion at the fracture site; for example intramedullary nailing of a diaphyseal femur fracture
Answer3: Primary healing is defined by the absence of visible callus
Answer 4: Endochondral ossification is bone generation or healing through a cartilage scaffold
Answer 5: Secondary healing does not occur through osteonal cutting cones as there not enough stability at the fracture site

Illustrations: A

REFERENCES:

1. Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System. Rosemont, IL, American Academy of Orthopaedics Surgeons, 2000, pp 371-399

2. Koval KJ (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 19-29.

Question Authors:

Brian Weatherford MD, Patrick McCulloch MD, Dave Marcu MD,

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4.0

Average 4.0 of 10 Ratings

Textbooks

Review of Orthopaedics, 6th Edition, Mark D. Miller MD, Stephen R. Thompson MBBS MEd FRCSC, Jennifer Hart MPAS PA-C ATC, an imprint of Elsevier, Philadelphia, Copyright 2012
AAOS Comprehensive Orthopaedic Review, Jay R. Leiberman. Published by American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2009
Orthopaedic Knowledge Update 10, John M Flyn. Published by American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2011
Hoppenfeld SP. Surgical Exposures in Orthopaedics: The Anatomic Approach. Lipponcott, Williams, and Wilkins, Philadelphia, PA, Copyright 2009
Orthopaedic In-training Examination (OITE) Questions 2004-2012, American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2004-2012
Self-Assessment Examination (SAE) Questions 2004-2012, American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2004-2012
Miller JD, McCreadie BR, Alford AI, Hankenson KD, Goldstein SA. Form and function of bone. In: Einhorn TA, O'Keefe RJ, Buckwalter JA, eds. Orthopaedic Basic Science: Foundations of Clinical Practice, 3rd ed. Rosemont, IL: American Academy of Orthopaedic Surgeons; 2007:129-159.

Undefined

Buckwalter JA, Einhorn TA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System. Rosemont, IL, American Academy of Orthopaedics Surgeons, 2000, pp 371-399
Koval KJ (ed): Orthopaedic Knowledge Update 7. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2002, pp 19-29.
Stein MS, Thomas CD, Feik SA, Wark JD, Clement JG. Bone size and mechanics at the femoral diaphysis across age and sex. J Biomech. 1998 Dec;31(12):1101-10. PMID:9882042 (Link to Abstract)

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