Consideration of load distribution and native soft tissue anatomy when choosing an animal bony model

We read the article “Mechanical comparison of application of locking intramedullary nail to locking compression plate in the ovine metatarsus” with great interest (1). In an era of rapidly expanding interest in preclinical animal fracture modeling to test an array of prosthetic implant options, orthobiologic adjuncts, fracture fixation modalities, and treatments for nonunion- or infection-derived bony defects, selection of the optimal animal host and fracture site is imperative. Steward et al. have eloquently demonstrated a greater construct stiffness for intramedullary nailing (IMN) of the ovine metatarsus when compared to lateral locking compression plating (LCP) constructs. As may be expected with the high polar moment of inertia on nail construct, IMN showed less variance in strain under torsional loading. Perhaps less intuitively, IMN also showed less strain than LCP in 4-point bending. We find these results to be of great interest and relevance for further investigation in critical-size osteotomy defects in the ovine metatarsus. As these authors demonstrate, when testing therapeutics such as the latest orthobiologic derivative or evaluating risk facture contribution to poor bony healing, researchers must fully understand the strain environment of a particular site of interest. This is just one factor which can impact the results of the experimental model. The purpose of this editorial is to delve further into the stress-strain relationship in animal fracture modeling. We aim to call attention to careful consideration of the specific characteristics of a chosen animal bony model, appreciating the importance of the load distribution of the particular bone in question and the native surrounding soft tissue anatomy.

To discuss how the bony healing environment is impacted by strain, it is imperative to understand the underlying principles of bony growth and remodeling. Strain refers to a force placed upon a bone, while stress refers to the elastic force that resists the intermolecular stretching (2). Wolff’s law, first described in 1892, states that “Every change in the form and function of bone or of their function alone is followed by certain definite changes in their internal architecture, and equally definite alteration in their external conformation, in accordance with mathematical laws.” (3). Put simply, bones grow and remodel in response to the forces from their mechanical environment. Research from 19^th century anatomists elucidated a balance between strength of bone and its weight, achieved by self-regulating deposition and absorption mechanisms (4). Wolff’s contribution to these concepts was the proposal that trabecular bone formed during periods of growth in the same orientation as the stressor on bone (5). While Wolff intended this as a means of explaining the development of the proximal femur during ontogeny, it has expanded to become a more generalized rule foundational to the field of orthopaedics (5) His simplistic concept, while still highly referenced the orthopaedic literature, has been revised and nuanced by later researchers to incorporate the diverse array of factors that contribute to bone modeling and remodeling.

Bone modeling describes the shaping of bones by independent actions of osteoclasts and osteoblasts (6). Thus, remodeling refers to the sequential actions of osteoclasts and osteoblasts in turning over small packets of bone, referred to as bone remodeling units (BRU) or bone metabolic units (BMU) (5) While trabecular bone and cortical bone are modeled and remodeled by the same physiologic processes, the mechanisms that regulate each are not the same and are remarkably complex (5). When a bone is exposed to a certain kind of strain, there are four possible outcomes as described by Pearson (5). The first outcome is a lack of response or change by the bone, either because the signal was not strong enough or because of the presence of an inhibitory stimulus on response (2,5). Modeling may occur either through bone formation by osteoblasts or reportion by osteoclasts. Finally, bone may turn over through remodeling of BMUs/BRUs (5). Generally, bone deposition occurs in response to strain to increase bone strength and resistance to deformation; however, this is nuanced and varies based on age, sex, and other host factors (7,8). Other studies have also asserted that below normal or low levels of strain can lead to bone resorption (9,10). Despite the pervasive oversimplification of Wolff’s law ignoring the cellular and hormonal factors which are critical to osteoblastic regulation, this principal can serve as a useful scaffold for understanding the macro effects of strain on bone modeling and remodeling.

To further understand the complex relationship of strain and mechanical stress to bony remodeling, various animal models are utilized depending on specific experimental characteristics. Rats are historically the most common in vivo bony models followed by mice, rabbits, and dogs (11). Based upon the human bone type and the specific forces a researcher is attempting to investigate, different animal models may be more or less appropriate. For example, when modeling trabecular bone, comparative studies have demonstrated that human bone has much lower fracture stress value than other animals, although pig and dog models appear to be the most similar in this regard (12). In contrast, when modeling longitudinal fractures in long bones, comparative studies have found that canine bones have the highest fracture toughness followed sequentially by human, baboon, rabbit, and bovine bone. Human cadaveric femurs in particular were found to be most similar in fracture toughness and bone mineral density to the femurs of rabbit and baboon models (13). While the priority of many researchers is to utilize an animal model where bone composition and load bearing is most similar to the human clinical scenario of interest, there are other important considerations. Factors such as cost, size of skeletal system for fixation, time to healing, and surrounding soft tissue anatomy all impact model decision (11).

The modeling of femur and tibial fracture and fixation has been most extensively studied, with mice and rats being the primary models for investigation. Early studies demonstrated that in closed fracture models with 3-point bending, mice femurs healed through endochondral ossification pathways with fracture bridging, and therefore were appropriate models for fracture healing (14). Mice have also been utilized for open fracture modeling through transverse osteotomy. Through open fracture modeling, researchers can ensure high quality intramedullary and external fixation which can be challenging in the smaller bony anatomy of mice and rats (15).

For distal radius fracture (DRF) and plating, canines have been utilized due to the similarities between the human orientation of abductor pollicis longus and extensor carpi radialis at the distal radius to more closely replicate in vivo modeling of tendon injury after DRF plating (16). Canine models have also been utilized for modeling of pediatric flexible IMN and K-wire fixation. Canine models offer a similar size profile to that seen in the pediatric patient population and therefore researchers are closely able to mimic real life bony fixation placement (17).

With regard to modeling the bones of the hand, a bony environment with lower load bearing is essential. In recent years, pigs have been utilized frequently as biomechanical models of metacarpal fracture and fixation. Comparative studies demonstrate that cross sections of non-weightbearing second and fifth metacarpals of commercial pigs are comparable to the central diaphyseal shafts of human proximal phalanges (18). More specifically, when cadaver proximal phalanges were measured for their circumference and cross-sectional area, they were no significant differences as compared to porcine metacarpals (18). Due to these similarities, pig metatarsals have been utilized to examine the biomechanical properties and efficacy of K-wire fixation, plate and screw, flexible wire loops, and twisted wire in phalangeal fracture models (19,20). Aside from porcine models, rabbits have also been utilized for metacarpal fracture in vivo investigation. Alignment and motion of diaphyseal metacarpal fractures have been examined in New Zealand white rabbits due to the similarities of forepaw muscle tendon and bony anatomy to that of human hands (19). Rabbits also have cortical bone with primary osteon morphology that mirrors human cortical bone (20).

Rodent models tend to differ from humans in their external anatomy and load bearing environments, but the current variety of transgenic models developed to mimic human disease states makes them useful investigative tools. For example, researchers have developed genetically modified mice and rats without matrix metalloproteinase 9 and bone morphogenetic protein 2 in order to assess nonunion fracture healing (21). Additionally, mice have provided researchers the opportunity to model diseases such as diabetes and osteoporosis with cost efficiency and large cohorts (22,23). As a final consideration for orthopaedic animal modeling, the trialing of total joint prostheses has more often occurred in larger mammals as opposed to the models discussed in this editorial which primarily center around bony fracture. Goel et al. examined the femurs of sheep, canines, and baboons in order to determine the best experimental model for hip implants. They found that baboons and dogs were most similar in external anatomy and cancellous bone area and distribution, and were therefore the most appropriate model organisms (24).

In conclusion, preclinical animal fracture modeling has become a topic of substantial interest in an era of enhanced understanding of the complex array of factors which impact bone healing. Wolff’s law, while incomplete in its consideration of the complex interplay governing bony remodeling, forms the basis for our understanding of the stress response of bone. The choice of animal model along with the specific bone of study presents a unique set of factors such as local stress-strain environment, load distribution, and native surrounding soft tissue anatomy which can be creatively utilized to simulate human conditions.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Translational Medicine. The article did not undergo external peer review.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-23-1513/coif). ATA receives consulting fees from QpixSolutions. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Steward SKT, Brodin E, Gadomski B, et al. Mechanical comparison of application of locking intramedullary nail to locking compression plate in the ovine metatarsus. Ann Transl Med 2023;11:191. [Crossref] [PubMed]
2. Frost HM. Wolff's Law and bone's structural adaptations to mechanical usage: an overview for clinicians. Angle Orthod 1994;64:175-88. [PubMed]
3. Wolff J. The Law of Bone Remodelling. Springer Berlin Heidelberg; 1986.
4. Martin RB, Burr DB, Sharkey NA, et al. Skeletal Tissue Mechanics. New York: Springer; 1998.
5. Pearson OM, Lieberman DE. The aging of Wolff's "law": ontogeny and responses to mechanical loading in cortical bone. Am J Phys Anthropol 2004;63-99. [Crossref] [PubMed]
6. Langdahl B, Ferrari S, Dempster DW. Bone modeling and remodeling: potential as therapeutic targets for the treatment of osteoporosis. Ther Adv Musculoskelet Dis 2016;8:225-35. [Crossref] [PubMed]
7. Bertram JE, Biewener AA. Bone curvature: sacrificing strength for load predictability? J Theor Biol 1988;131:75-92. [Crossref] [PubMed]
8. Bätge B, Feydt A, Gebken J, et al. Age-related differences in the expression of receptors for TGF-beta in human osteoblast-like cells in vitro. Exp Clin Endocrinol Diabetes 2000;108:311-5. [Crossref] [PubMed]
9. Baldwin KM, White TP, Arnaud SB, et al. Musculoskeletal adaptations to weightlessness and development of effective countermeasures. Med Sci Sports Exerc 1996;28:1247-53. [Crossref] [PubMed]
10. Collet P, Uebelhart D, Vico L, et al. Effects of 1- and 6-month spaceflight on bone mass and biochemistry in two humans. Bone 1997;20:547-51. [Crossref] [PubMed]
11. Mills LA, Simpson AH. In vivo models of bone repair. J Bone Joint Surg Br 2012;94:865-74. [Crossref] [PubMed]
12. Aerssens J, Boonen S, Lowet G, et al. Interspecies differences in bone composition, density, and quality: potential implications for in vivo bone research. Endocrinology 1998;139:663-70. [Crossref] [PubMed]
13. Wang X, Mabrey JD, Agrawal CM. An interspecies comparison of bone fracture properties. Biomed Mater Eng 1998;8:1-9. [PubMed]
14. Manigrasso MB, O'Connor JP. Characterization of a closed femur fracture model in mice. J Orthop Trauma 2004;18:687-95. [Crossref] [PubMed]
15. Johnson JP, Born CT, Thomas N, et al. Development of a novel murine femur fracture and fixation model. J Orthop 2020;17:162-7. [Crossref] [PubMed]
16. Sinicropi SM, Su BW, Raia FJ, et al. The effects of implant composition on extensor tenosynovitis in a canine distal radius fracture model. J Hand Surg Am 2005;30:300-7. [Crossref] [PubMed]
17. Battle J, Carmichael KD, Morris RP. Biomechanical comparison of flexible intramedullary nailing versus crossed Kirschner wire fixation in a canine model of pediatric forearm fractures. J Pediatr Orthop B 2006;15:370-5. [Crossref] [PubMed]
18. Massengill JB, Alexander H, Parson JR, et al. Mechanical analysis of Kirschner wire fixation in a phalangeal model. J Hand Surg Am 1979;4:351-6. [Crossref] [PubMed]
19. Ochman S, Doht S, Paletta J, et al. Comparison between locking and non-locking plates for fixation of metacarpal fractures in an animal model. J Hand Surg Am 2010;35:597-603. [Crossref] [PubMed]
20. Feehan LM, Tang CS, Oxland TR. Early controlled passive motion improves early fracture alignment and structural properties in a closed extra-articular metacarpal fracture in a rabbit model. J Hand Surg Am 2007;32:200-8. [Crossref] [PubMed]
21. Hixon KR, Miller AN. Animal models of impaired long bone healing and tissue engineering- and cell-based in vivo interventions. J Orthop Res 2022;40:767-78. [Crossref] [PubMed]
22. Gooch HL, Hale JE, Fujioka H, et al. Alterations of cartilage and collagen expression during fracture healing in experimental diabetes. Connect Tissue Res 2000;41:81-91. [Crossref] [PubMed]
23. Gaston MS, Simpson AH. Inhibition of fracture healing. J Bone Joint Surg Br 2007;89:1553-60. [Crossref] [PubMed]
24. Goel VK, Drinker H, Panjabi MM, et al. Selection of an animal model for implant fixation studies: anatomical aspects. Yale J Biol Med 1982;55:113-22. [PubMed]