Effects of mechanical load/stress on bone growth

Leg length discrepancy (LLD) is a condition characterised by a difference in bone length, where one leg is shorter than the other. Surgical treatments for LLD are often associated with complications such as pain at the surgical site, infection, and delayed bone union. Thus, there is an ultimate need to establish noninvasive approaches to treat LLD. This thesis explores the use of mechanical loading as a potential noninvasive method to treat LLD.

Study I focused on the creation of a portable, computer-controlled microloading device capable of delivering precise mechanical loading to small bone organs and animals, such as mice. Using this device, we tested the direct effects of mechanical forces on rat embryonic femur and metatarsal bones cultured ex vivo. These results revealed that mechanical loading at 0.4N significantly decreased growth in metatarsal bones (by approximately 1 mm) while significantly increasing growth in femurs (by approximately 4 mm). These findings suggest that the impact of mechanical forces on bone growth appears to be influenced by both the size and unique traits of the bones.

Study II investigated the transcriptomic responses of adolescent growth plate cartilage to mechanical forces. RNA sequencing identified 15 significantly regulated genes and 6 associated signalling pathways in growth plate samples treated with mechanical loading. This represents the first report of the transcriptomic effects of mechanical forces on adolescent growth plate cartilage, filling a critical gap in our understanding of the interaction between human growth plate biology and mechanical loading.

Study III extended these findings to young mice, where mechanical loading was applied to the joints of one hindlimb (both female and male, 4-week-old and 8-week-old). Mechanical loading significantly increased femur length, with the most pronounced effects observed in 4-week-old mice of both sexes. Furthermore, we identified PTGS2 (prostaglandin- endoperoxide synthase 2) as a key gene involved in the bone-lengthening effects of mechanical loading. PTGS2 expression was significantly elevated in the CD73+ and PTHrP+ skeletal stem cell niches of the growth plate in treated legs. Pharmacological inhibition of PTGS2 abolished the bone-lengthening effect, confirming its critical role. Furthermore, mechanical loading significantly increased both PTGS2 expression and the size of ex vivo cultured human growth plate cartilage.

In conclusion, these findings indicate that mechanical loading offers a promising noninvasive treatment strategy to enhance bone growth and correct LLD in patients, paving the way for future clinical applications.

List of scientific papers

I. Zhengpei Zhang, Farasat Zaman, Tobia Sebastiano Nava, Tim R J Aeppli, Elena M Gutierrez-Farewik, Artem Kulachenko, Lars Sävendahl. Micromechanical Loading Studies in Ex Vivo Cultured Embryonic Rat Bones Enabled by a Newly Developed Portable Loading Device. Ann Biomed Eng. 2023 Oct;51(10):2229-2236. https://doi.org/10.1007/s10439-023-03258-2

II. Zhengpei Zhang, Nageswara Rao Boggavarapu, Laila Sara Arroyo Muhr, Ainhoa Garcia-Serrango, Tim R J Aeppli, Tobia Sebastiano Nava, Yunhan Zhao, Elena M Gutierrez-Farewik, Artem Kulachenko, Lars Sävendahl #, Farasat Zaman #. Genomic Effects of Biomechanical Loading in Adolescent Human Growth Plate Cartilage: A Pilot Study. Cartilage. 2024 Dec 10:19476035241302954. https://doi.org/10.1177/19476035241302954

III. Zhengpei Zhang #, Tim R J Aeppli #, Nageswara Rao Boggavarapu2, Laila Sara Arroyo Muhr, Yunhan, Zhao, Eva Pontén, Elena M Gutierrez-Farewik, Artem Kulachenko, Lars Sävendahl #, Farasat Zaman #. Mechanical Stimulation: A Non-invasive Treatment Strategy for Leg Length Discrepancy. [Manuscript]

# Denotes equal last-author contribution