Comparative analysis of ovine and human spinal biomechanics

Prior to the use of spinal implants in clinical practice, implants undergo testing procedures such as biocompatibility and mechanical testing, finite element analysis and in-vitro testing in human and animal cadaveric spines. Then, implants are assessed in-vivo using animal models to identify surgical complications, and after euthanasia further in-vitro testing of implant stability is carried out. The ovine species is frequently used for these tests, as in-vitro and anatomical studies have identified the ovine and human spine to be similar. In-vivo ovine spinal kinematic data however are not known. The aim of the proposed study is to obtain comparative in-vivo thoracolumbar spinal kinematics and muscle function in sheep and humans obtaining diagnostic images of the human and sheep spine and relevant musculature, obtaining spinal kinematics and muscle activation data from sheep and humans and to analyse the integrative mechanisms of spinal kinematics and stabilization methods identifying variables significant for translational application from the ovine to human spine for future spinal implant research. The following hypotheses will be tested: a) Maximum and minimum angles and mean range of motion of the human thoracolumbar spine during a motion cycle at walk and run are more closely represented by the ovine spine during trot rather than walk, due to the similar diagonally reciprocal nature of the human walk and run and the quadrupedal trot. b) Sheep will demonstrate a muscle activation ratio of rectus abdominis to oblique externus closer to 1 during walk compared to the human during walk, as sheep require a greater need for stability against extension forces due to their spinal alignment. c) MRI assessed muscle cross-sectional area (CSA) and CSA to fatty infiltrate ratio are inversely correlated to degenerative spinal changes and age, but directly correlated to EMG in both the human and sheep. Two different age ranges of sheep (n=24) and human volunteers (n=24) will be examined using Magnetic Resonance Imaging (MRI), ultrasonography and radiography. Spinal kinematics and muscle activation data from sheep at stance, at walk and trot; and from the human during 4 point kneeling, bipedal stance and during walking and running will be obtained. Intra-individual and inter-individual species differences of movement and muscle activation patterns in the different measurement situations will be tested using Analysis of variance (ANOVA), and correlations between kinematic measurements, EMG activity and MRI appearance of the muscles will be documented. Availability of the biomechanics and muscular stabilization of the ovine spine will allow future in- vivo spinal implant studies to collect relevant data with similar methodology for enhanced translation to human medicine. The proposed study therefore provides the necessary baseline data required to bring forward a new dimension to the evaluation of spinal implant functionality in the future.

Before spinal implants come into clinical use, these are first tested in vitro and also in-vivo in animals, in order to identify the main complications of surgery; mainly post-mortem evaluations of the animals used are performed. Because in-vitro and anatomical studies have noted a similarity between the sheep spine and the human spinal column, sheep are often used for these studies. Prior to the beginning of the present study, the movement of the ovine spine had not been compared with the movement of the human spinal column, and therefore no basic data for transferring results in the sheep to the human had been established. Consequently, the present study collected data of comparative in vivo kinematics and muscle function of the thoracolumbar spine of sheep and humans, supported by imaging techniques. This allowed for a more data based comparative analysis of integrative mechanisms of spinal kinematics and spine stabilization with numerous parameters examined.First, the sheep were accustomed to the treadmill and in contrast to horses it could be established that sheep needed more than 5 treadmill training sessions to achieve a consistent movement on the treadmill. From the magnetic resonance images collected in sheep and in humans, the analysis of the lumbar extensor muscles showed that muscle fatty infiltration and muscle volumes were identified by different evaluators with excellent reliability except for the multifidii muscles, where the reliability was acceptable. In the human individuals studied, age and body mass index were identified as significant influences on the muscle fatty infiltration of the lumbar extensor muscles, but not of the lumbar flexor muscles. The different volumes and shapes of the lumbar muscles of humans and sheep showed the different bio-mechanical and functional requirements that each of the two species meet, this is of particular importance when using sheep as surgical models for the people. Comparison of muscle activity and active movements of the lumbar spine in humans and in sheep showed very significant differences. Also, the details of muscle fatty infiltrations in sheep showed, unlike in humans, much more such zones in the magnetic resonance examination, this a substantial limitation of direct transmission of results of sheep studies on humans.