Wolff's law

Wolff's law, developed by the German anatomist and surgeon Julius Wolff (1836–1902) in the 19th century, states that bone in a healthy person or animal will adapt to the loads under which it is placed.^[1] If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading.^[2][3] The internal architecture of the trabeculae undergoes adaptive changes, followed by secondary changes to the external cortical portion of the bone,^[4] perhaps becoming thicker as a result. The inverse is true as well: if the loading on a bone decreases, the bone will become less dense and weaker due to the lack of the stimulus required for continued remodeling.^[5] This reduction in bone density (osteopenia) is known as stress shielding and can occur as a result of a hip replacement (or other prosthesis).^[6] The normal stress on a bone is shielded from that bone by being placed on a prosthetic implant.

Mechanotransduction

The remodeling of bone in response to loading is achieved via mechanotransduction, a process through which forces or other mechanical signals are converted to biochemical signals in cellular signaling.^[7] Mechanotransduction leading to bone remodeling involve the steps of mechanocoupling, biochemical coupling, signal transmission, and cell response.^[8] The specific effects on bone structure depends on the duration, magnitude and rate of loading, and it has been found that only cyclic loading can induce bone formation.^[8] When loaded, fluid flows away from areas of high compressive loading in the bone matrix.^[9] Osteocytes are the most abundant cells in bone and are also the most sensitive to such fluid flow caused by mechanical loading.^[7] Upon sensing a load, osteocytes regulate bone remodeling by signaling to other cells with signaling molecules or direct contact.^[10] Additionally, osteoprogenitor cells, which may differentiate into osteoblasts or osteoclasts, are also mechanosensors and may differentiate one way or another depending on the loading condition.^[10]

Computational models suggest that mechanical feedback loops can stably regulate bone remodeling by reorienting trabeculae in the direction of the mechanical loads.^[11]

Associated laws

• In relation to soft tissue, Davis' Law explains how soft tissue remolds itself according to imposed demands.
• Refinement of Wolff's Law: Utah-Paradigm of Bone physiology (Mechanostat Theorem) by Harold Frost.^[12]

Examples

Tennis players often use one arm more than the other

• The racquet-holding arm bones of tennis players become much stronger than those of the other arm. Their bodies have strengthened the bones in their racquet-holding arm since it is routinely placed under higher than normal stresses. The most critical loads on a tennis player's arms occur during the serve. There are four main phases of a tennis serve and the highest loads occur during external shoulder rotation and ball impact. The combination of high load and arm rotation result in a twisted bone density profile.^[13]
• Weightlifters often display increases in bone density in response to their training.^[14]
• The deforming effects of torticollis on craniofacial development in children.^[15]
• An Anterior-Posterior Pelvic X-ray of an amputee. The right hip is much more dense with bone than the left.^[16]

See also

• Functional matrix hypothesis
• Osteogenic loading

References

1. Anahad O'Connor (October 18, 2010). "The Claim: After Being Broken, Bones Can Become Even Stronger". New York Times. Retrieved 2010-10-19. This concept — that bone adapts to pressure, or a lack of it — is known as Wolff’s law. ... there is no evidence that a bone that breaks will heal to be stronger than it was before.
2. Frost, HM (1994). "Wolff's Law and bone's structural adaptations to mechanical usage: an overview for clinicians". The Angle Orthodontist 64 (3): 175–188. doi:10.1043/0003-3219(1994)064<0175:WLABSA>2.0.CO;2. PMID 8060014.
3. Ruff, Christopher; Holt, Brigitte; Trinkaus, Erik (April 2006). "Who's afraid of the big bad Wolff?: "Wolff's law" and bone functional adaptation". American Journal of Physical Anthropology 129 (4): 484–498. doi:10.1002/ajpa.20371. Retrieved 2 March 2015.
4. Stedman's Medical Dictionary
5. Wolff J. "The Law of Bone Remodeling". Berlin Heidelberg New York: Springer, 1986 (translation of the German 1892 edition)
6. ., M.I.Z. Ridzwan; ., Solehuddin Shuib; ., A.Y. Hassan; ., A.A. Shokri; ., M.N. Mohamad Ibrahim (1 March 2007). "Problem of Stress Shielding and Improvement to the Hip Implant Designs: A Review". Journal of Medical Sciences(Faisalabad) 7 (3): 460–467. doi:10.3923/jms.2007.460.467.
7. Huang, Chenyu; Rei Ogawa (October 2010). "Mechanotransduction in bone repair and regeneration". FASEB J. 24.
8. Duncan, RL; CH Turner (November 1995). "Mechanotransduction and the functional response of bone to mechanical strain". Calcified Tissue International 57 (5): 344–358. doi:10.1007/bf00302070.
9. Turner, CH; MR Forwood; MW Otter (1994). "Mechanotransduction in bone: do bone cells act as sensors of fluid flow?". FASEB J. 8 (11).
10. Chen, Jan-Hung; Chao Liu; Lidan You; Craig A Simmons (2010). "Boning up on Wolff’s Law: Mechanical regulation of the cells that make and maintain bone". Journal of Biomechanics 43: 108–118. doi:10.1016/j.jbiomech.2009.09.016.
11. Huiskes, Rik; Ruimerman, Ronald; van Lenthe, G. Harry; Janssen, Jan D. (8 June 2000). "Effects of mechanical forces on maintenance and adaptation of form in trabecular bone". Nature 405 (6787): 704–706. doi:10.1038/35015116. Retrieved 2 March 2015.
12. Frost, HM (2003). "Bone's mechanostat: a 2003 update". The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology 275 (2): 1081–1101. doi:10.1002/ar.a.10119. PMID 14613308.
13. Taylor RE; Zheng c; Jackson RP; Doll JC; Chen JC; Holzbar KR; Besier T; Kuhl E. "The phenomenon of twisted growth: humeral torsion in dominant arms of high performance tennis players.". Comput Methods Biomech Biomed Engin. Retrieved 27 Feb 2013.
14. Mayo Clinic Staff (2010). "Strength training: Get stronger, leaner, healthier". Mayo Foundation for Education and Medical Research. Retrieved 19 October 2012.
15. Oppenheimer, AJ; Tong, L; Buchman, SR (Nov 2008). "Craniofacial Bone Grafting: Wolff's Law Revisited.". Craniomaxillofacial trauma & reconstruction 1 (1): 49–61. doi:10.1055/s-0028-1098963. PMC 3052728. PMID 22110789.
16. http://www.health1stchiropractic.com/wolffs-law-degenerative-joint-disease-health/

• Das Gesetz der Transformation der Knochen - 1892. Reprint: Pro Business, Berlin 2010, ISBN 978-3-86805-648-8.
• "The Classic: On the Inner Architecture of Bones and its Importance for Bone Growth". Clin Orthop Rel Res. 468 (4): 1056–1065. Apr 2010. doi:10.1007/s11999-010-1239-2.

External links

• Julius Wolff Institut, Charité - Universitätsmedizin Berlin, main research areas are the regeneration and biomechanics of the musculoskeletal system and the improvement of joint replacement.