Adult degenerative scoliosis(ADS) is a prevalent issue in the elder population which defined as a spinal deformity with a curve over 10° measured by Cobb method in a skeletally mature individual [21, 22]. ADS prevalence is about 6% in people older than 50 years old, the average age of people suffering from ADS sought for medical care is in their sixties. Importantly, the curve is in potential progression with an average of 3.3° every year.
In the progression of adult degenerative scoliosis, the spinal structure is changing. In detail, the disc and the facet joint degeneration leads to an asymmetric loading of the spinal segment. Consequently, an asymmetric deformity, for instance, scoliosis or kyphosis occurs. And then at the facet joint (spondylarthritis) and at the vertebral endplates (spondylosis), osteophytes are formed. This change makes a contribution to the increasing narrowing of the spinal canal together with the hypertrophy and calcification of the ligamentum flavum and joint capsules, resulting in central and recessed spinal stenosis.
There are two ways to treat adult degenerative scoliosis: medicine and surgery. Pain relief drugs are used to decrease the symptom. Additionally, surgery is the final way to treat adult degenerative scoliosis. However, there is also disputed for long-segment or short segment treatment for adult degenerative scoliosis[23, 26]. Long segment internal fixation treatment perfectly restores the sagittal balance of human body, however, proximal junctional kyphosis (PJK) is one of the major complications due to decreased bone mineral density in these patients[27, 28]. Even multi-use of anti-osteoporosis drugs does not receive demanded outcome. In other hand, even though short segment internal fixation treatment temporary relieves pain, the body force line remains unbalanced, resulting in progressed adult degenerative scoliosis in long term.
It’s widely accepted that osteoporosis accelerates curve progression. In addition, larger curves tend to progress faster than small curves in adult degenerative scoliosis patients for biomechanical reasons.
Osteoporosis is characterized by a decrease in bone density and abnormal microstructure, which leads to reduced bone strength and increased fracture risk. The population of osteoporosis is around 200 million over the world. Additionally, the morbidity of osteoporosis is around 60% in the population with age over 60. With increasing age, the incidence of osteoporosis and its related diseases, including fracture and kyphosis, occurs. While osteoporosis is often considered a disease common for women, it is also recognized as an important health problem for men.
Osteoporosis is classified into primary and secondary type. Importantly, exercise effectively prevents osteoporosis. In osteoporosis environment, biochemical pathways are activated and inflammatory catabolic molecules, such as tumor necrosis factor(TNF), interlukin (IL) and A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) are induced[32, 33]. These induced molecules mediate bone absorption, resulting in severer situation of osteoporosis. However, appropriate mechanical load improves the situation of osteoporosis. The biomechanical load transduced biochemical signals help bone mass deposit. Notably, in the progression of osteoporosis, bone mass losses gradually, the existed bone no longer supports its loads, leading to bone-associated degenerative diseases.
Previous studies demonstrate bone is indeed in dynamic change after maturation[7, 8, 19, 35–37]. For instance, in the knee joint, tibial platform in medial part exhibits a lower height than that in lateral side with aging. Moreover, upper fibula shows larger curve in the elderly population. Furthermore, femoral neck rake angle and collodiaphyseal angle is decreased with aging. In contrast, acetabular rake is positive associated with age. This finding reveals the reason why elderly individual demonstrates extorsion of the lower extremities. Interestingly, lumbar structure is also found to be changed with aging. In detail, lumbar lordosis is increased with aging, indicating the spinal curve is larger and larger in lifetime. Additionally, the transverse section angle is decreased in aged population, suggesting more incidence of spinal stenosis.
Even the finding demonstrates some bones, such as tibia, fibula, femur and lumbar vertebrae are in dynamic change with aging, the involved mechanism still remains unclear. Taken the fact that bone mass is gradually decreased after its peak at the age of 35, and bone mineral density is the key for maintaining bone strength and mass, herein we found the dynamic change of lumbar structure was associated with bone mineral density. Angle between upper and lower endplate (AULE) and sagittal section angle presents the sagittal curve and it’s found to be associated with bone mineral density. Importantly, this association is linearity. Furthermore, this BMD-associate lumbar dynamic change is gender-independent. The phenomena is also true in clinic. The osteoporosis patients usually exhibit bended spine, showing the increased curve in the progression of osteoporosis.
With aging, bone loss gradually processes, leading to osteopenia or osteoporosis. The bone mineral density is also decreased in this progression, leading to reduced strength and microfractures. Consequently, the bone can not support sustained weight, resulting in the shape change to adapt to the mechanical loads. Present study indicates this hypothesis is true in lumbar structure.
Collectively, lumbar structure is in dynamic change in whole life. Bone mineral density’s change initiates bone shape’s change. Additionally, bone mineral density associated change is gender independent. Anti-osteoporosis treatment might be potential for kyphosis and other bone-related degenerative diseases.