In an aging society, abnormal posture with gait disorder is one of the critical issues, which results in the decreasing of activity of daily living (ADL) and quality of life (QOL). The abnormal posture has been basically considered to occurr from the adult spinal deformity (ASD) with the trunk muscle weakness and spondylotic change. Spinal instrumentation has been applied to correct the spinal malalignment operatively; however, the indication and target of the alignment in surgical procedures are still controversial. Because the body balance and posture are thought to be an inverted pendulum, which starts from the feet as a fulcrum, connects to lower extremities and spine, and ends in head, all these factors involving spine may affect the varying of the posture with age. The hip-spine syndrome has been defined as the abnormal posture and the low back pain with hip disorders. On the contrary, total hip replacement could have a potential to repair hip disorders and improve the posture. It still needs more discussion about the operative approach in the hip-spine syndrome. Similar to that, the knee-spine syndrome has been discussed as a reason of the abnormal posture.

Osteoarthritis of the knee is one of the common diseases for the middle age and older people. Because of the limitation of the knee range of motion and the malalignment with the joint degeneration, many surgical procedures have been developed, that is, total knee arthroplasty (TKA), unicompartmental arthroplasty (UKA), and around knee osteotomies (AKO). All techniques show good clinical outcomes; however, the proper use of each procedure is still unclear. Although the targets of the alignments in lower extremities are different in each procedure, the optimal alignment and posture in elder people are still under debate.

With these backgrounds, in this special issue, we would like to focus on the posture deformity, the correction of the malalignment with surgical procedures, and what is thought to be the best alignment for the lower extremities and spine.

Potential topics include but are not limited to the following:

• Principles of adult spinal deformity (ASD)
• Principles of hip-spine syndrome
• Principles of knee-spine syndrome
• Various concepts of alignments in hip and knee arthroplasties
• Target of alignments and clinical results in pelvis, hip joint, and around knee osteotomies (AKO)
• Effect of knee surgeries on abnormal posture