The physis (growth plate) undergoes a myriad of different external forces during growth. Sustained and cyclic mechanical loads modify a number of different physiological factors in the physis which control longitudinal and latitudinal growth. Many types of loading are unknown in terms of their effects on growth regulation. In addition, little has been studied about the effect of physical, and chemical factors on the physis with respect to time. Thus, methods to experiment on the effect of these factors are largely underdeveloped. Monitoring said factors while viewing isolated regions at a high resolution is also a major challenge that has yet to be overcome. In order to fully understand how the physis reacts and adapts to these forces, researchers must be able to load the tissue in a similar way during experimentation, modulate the conditions that the physis is subjected to as well as view and quantify changes to the physis. Clinicians need easy-to-implement treatments for severe physeal injuries that have a higher probability of resulting in permanent deformities.

The physis is a disk of cartilage near the end of a bone that is sandwiched between the epiphyseal cartilage (near the joint) and the metaphysis (away from the joint)^8,9. It is divided into 4-5 zones starting from the epiphyseal cartilage: the resting/germinal zone, proliferative zone, columnar zone, hypertrophic zone, and calcification zone ^8,9. The hypertrophic zone has the least amount of supporting structures ⁸. Physis is especially affected by shear forces ^8,9_.. There is some debate to how the physis receives blood. Some sources say that the metaphyseal and epiphyseal arteries provide blood to the physis whereas others state only the metaphyseal artery provides blood ^8,9,14. The physis and the metaphysis are polar and this polarity determines whether the physis generates bone toward the epiphysis side or the metaphysis side ¹⁴. The polarity also determines from which side the physis receives blood (on the same side that the physis begins generating bone) ¹⁴.

Transport of oxygen, nutrients, signaling molecules, etc. occurs through chondro-osseous junctions.¹⁹ In Figure 1, the left image¹⁹ shows transport routes critical to chondrocyte development while the right image¹⁹ shows hypothesized transport patterns within the growth plate, with cartilage permissivity coded by dark/light shading for lower and higher permissivity respectively. Metaphyseal blood vessels invade the epiphyseal growth plate²⁰, where arterial blood flows from the metaphysis to epiphysis and regular vascular branches/merges are found.²⁴

At the epiphyseal growth plate, cartilage is calcified, degraded, and then replaced by bone tissue. Calcification involves matrix vesicles secreted by the chondrocytes at a specific stage.¹⁶ Linear growth is split into proliferative and hypertrophic phases, which are regulated by autocrine/paracrine feedback loops.¹⁷ Paracrine communication occurs at a permissive region between chondrocytes and cells in the surrounding connective tissues.¹⁹ Microenvironment regulates chondrocyte differentiation into either articular growth or growth plate cartilage. In a study performed by M. Chau et al, it was found that hypertrophic differentiation was inhibited in growth plate cartilage that was transplanted to an articular surface ²². Insulin like growth factor I (IGF-I) is used to induce production of the matrix that is vital for cartilage upkeep from chondrocyte ⁷. TGF-β is a class of molecules that include bone morphogenetic proteins (BMP) and growth and differentiation factors (GDF)⁷. BMPs can start cartilage formation, regulate how the cartilage is formed, and are dynamic in their function ⁷. GDF-5 can increase the amount of chondrocytes and cartilage during development ¹⁰. BMP-2 affects resting zone chondrocytes and is responsible for “chondrocyte proliferation, differentiation, and matrix production” (Erickson et al.) ¹¹. Which reaction comes from the BMP-2 stimulus depends on where in the cell cycle the chondrocyte is ¹¹. Use of basic fibroblast growth factor (bFGF) can cause cartilage to grow. bFGF differentiates chondrocytes and induces them to generate more matrix for cartilage upkeep ¹². IGF-1 and bFGF have effects on both growth and morphology in chondrocytes and act synergistically when used together.¹⁵ GH can promote growth plate chondrogenesis and longitudinal bone growth directly at the growth plate, even when the local effects of IGF-1, 2 are prevented.²³ This is shown in Figure 2, where knockout mice (no IGF receptors) that were treated with GH showed substantially more growth than mice that were untreated or treated with IGF-1²³. IGF-1 and GHR were found to be reduced during food restriction, but increased during following periods of catch-up growth.¹⁸ According to a study performed by J. Piao et al, expression of IHH and downstream genes were reduced when the Sirt6 gene was knocked down, which is indicative of a direct linkage between Sirt6 and IHH since IHH regulates growth of adjacent proliferative chondrocytes.²¹ This implies that Sirtuin 6 regulates growth plate chondrocyte differentiation and proliferation.²¹

According to a literature review performed by I. Villemure, longitudinal growth in the growth plate is controlled by modifying the number of growth plate chondrocytes in each zone, the rate of proliferation, the amount of hypertrophy and the amount of controlled synthesis and degradation of matrix throughout the growth plate.²⁵ Aforementioned variables can be tweaked to change growth under sustained or cyclical mechanical load. The relationship between static compressive loading and bone growth modulation is known to be²⁵:

G = G_m(1+B(s-s_m)) with B = 1.71 MPa^-1

Where G (um/day) is the actual growth, G_m (um/day) is the mean baseline growth (unaltered stress), s the actual stress on growth plate (compressive negative) and s_m the mean prevailing (baseline) stress on growth plate

Additionally, little is known about the effects of time-varying changes in volume, water content, osmolarity of matrix, etc. on differentiation, maturation and metabolic activity of chondrocytes. The effects of torsion and shear forces have yet to be quantified as well.²⁵ Villemure goes on to state that future work should distinguish between changes in growth regulation that are a result of different processes.²⁵

Presently, there is very limited research on physeal cartilage bioreactors. There is a scarcity of literature pertaining to the the actual form and function of a physeal cartilage bioreactor. However, there was literature pertaining to a bioreactor for epiphyseal cartilage that could be utilized for the construction of a novel physeal cartilage bioreactor. In the epiphyseal cartilage bioreactor, chondrocytes were removed from epiphyseal cartilage of human fetus ¹³. The chondrocytes were then seeded into polyglycolic acid (PGA) scaffold. The scaffold was then stitched to more scaffold containing osteoblasts and placed in recirculation column bioreactors ¹³. Through the use of this method, cartilage quality was increased and the cartilage and bone bonded strongly when compared to the control ¹³. Cartilage quality based on magnitude of collagen change and percent glycosaminoglycan (GAG) concentration change compared to control ¹³.