Effect of shock wave therapy on longitudinal bone growth and growth plate cartilage

Abstract: Aim: The overall aim of this thesis was to investigate the potential for radial shock wave treatment (rSWT) to regulate growth plate chondrogenesis and longitudinal bone growth. It was based on the hypothesis that the effects are dose-dependent in a bimodal response pattern where low-energy rSWT will increase chondrocyte proliferation/hypertrophy, and thereby bone growth, while high frequency/energy rSWT will induce chondrocyte apoptosis and thereby, premature growth plate closure. Methods: To achieve this, we experimentally studied the role of rSWT in three different model systems: in vitro cultures of a mouse chondrocytic cell line, in ex vivo organ cultures of fetal rat metatarsal bones and human growth plate cartilage, as well as in vivo in rabbits. Results: A single session of high-energy rSWT was capable of locally promoting longitudinal bone growth in rat metatarsal bones cultured under normal physiological conditions. Detailed immunohistomorphometric analysis of sectioned growth plate cartilage revealed that this stimulatory effect was linked to augmented chondrocyte proliferation and hypertrophy, and decreased apoptosis. Furthermore, mechanistic studies of molecular markers governing growth plate chondrogenesis showed increased expression of PTHrP, GLI-1, NFkB, and IGF-1 in the bones exposed to rSWT (Paper I). Next, we investigated the role of rSWT in an ex vivo model of growth impairment where Hh signaling was blocked. Cultured fetal rat metatarsal bones were challenged to two different Hh inhibitors, vismodegib and GANT61, causing growth retardation, while when combined with a session of rSWT the inhibitory effects on growth plate chondrogenesis and bone growth were partially abrogated (Paper II). Thereafter, we performed in vivo studies in immature and adolescent rabbits and observed that high-energy rSWT increased the formation of chondrocyte columns and longitudinal bone growth, respectively (Paper III). Finally, mechanistic studies revealed that rSWT caused upregulation of chondrogenic genes when studied in a unique model of cultured human growth plate cartilage (SOX9, GLI-1, IHH, COL-X, and IGF-1) and also in the mouse ATDC5 chondrocytic cell line (Acan, Sox9 and, Col2a1) (Paper IV). Conclusion: Altogether, our observations, verified in preclinical and in vitro models, propose that high-energy radial shockwave treatment could potentially be used to stimulate growth plate chondrogenesis and longitudinal bone growth