Finite element analysis of weightbath hydrotraction treatment of degenerated lumbar spine segments in elastic phase

3D finite element models of human lumbar functional spinal units (FSU) were used for numerical analysis of weightbath hydrotraction therapy (WHT) applied for treating degenerative diseases of the lumbar spine. Five grades of age-related degeneration were modeled by material properties. Tensile material parameters of discs were obtained by parameter identification based on in vivo measured elongations of lumbar segments during regular WHT, compressive material constants were obtained from the literature. It has been proved numerically that young adults of 40–45 years have the most deformable and vulnerable discs, while the stability of segments increases with further aging. The reasons were found by analyzing the separated contrasting effects of decreasing incompressibility and increasing hardening of nucleus, yielding non-monotonous functions of stresses and deformations in terms of aging and degeneration. WHT consists of indirect and direct traction phases. Discs show a bilinear material behaviour with higher resistance in indirect and smaller in direct traction phase. Consequently, although the direct traction load is only 6% of the indirect one, direct traction deformations are 15–90% of the indirect ones, depending on the grade of degeneration. Moreover, the ratio of direct stress relaxation remains equally about 6–8% only. Consequently, direct traction controlled by extra lead weights influences mostly the deformations being responsible for the nerve release; while the stress relaxation is influenced mainly by the indirect traction load coming from the removal of the compressive body weight and muscle forces in the water. A mildly degenerated disc in WHT shows 0.15 mm direct, 0.45 mm indirect and 0.6 mm total extension; 0.2 mm direct, 0.6 mm indirect and 0.8 mm total posterior contraction. A severely degenerated disc exhibits 0.05 mm direct, 0.05 mm indirect and 0.1 mm total extension; 0.05 mm direct, 0.25 mm indirect and 0.3 mm total posterior contraction. These deformations are related to the instant elastic phase of WHT that are doubled during the creep period of the treatment. The beneficial clinical impacts of WHT are still evident even 3 months later.