THE EFFECT OF THE GROWTH PLATE ON MEASUREMENTS OF RAT-TAIL VERTEBRAL BIOMECHANICAL BEHAVIOUR

Abstract

Rodents are often used as preclinical models for investigating the biomechanical consequences of spinal pathologies and interventions. Growth plates are present within rat vertebrae throughout life and may alter the vertebral biomechanics. This study investigates the biomechanical response of rat-tail vertebrae to axial compressive loading using μCT imaging and image registration to spatially resolve strain fields.

The sixth caudal vertebrae of eight immunocompromised (rnu/rnu) rats were μCT scanned (17.5 ×17.5×17.5μm/pixel) in both loaded (27N-32N axial compression) and unloaded configurations. Image registration was used to calculate strain and displacement fields in the bone due to the applied load by finding a spatial mapping between the two scans. Strain was resolved to varying spatial resolutions; high strain gradient regions, such as the growth plates, were analyzed to higher spatial resolutions.

Axial strains calculated by image registration ranged from 2% in tension to 16% in compression with an average axial strain of 1.6% in compression. In seven rats the majority of the strain measured within the vertebrae was concentrated in the growth plate. Very soft growth plates in three specimens resulted in maximum axial strains from 10–16% in compression. The remaining four rats with strain concentrations in the growth plate had maximum axial strains ranging from 2.2%–3.2%. Centrally located strain concentrations of lower magnitudes and more limited spatial extent were observed in the trabecular bone.

The majority of the strain within the rat vertebrae was absorbed by the growth plates. The amount of strain within the growth plate is important to consider when interpreting biomechanical data on rat vertebrae. Load application to rodent vertebrae will first compress the growth plate and only following compression of this structure cause significant development of displacement and strains within the trabecular and cortical bone. This insight into the biomechanical response of rat vertebrae is apparent through the application of image registration to analyse vertebral body behaviour; such information would not be evident in analysing preclinical whole vertebral body response using finite element modeling or experimental testing protocols.