In Traditional Chinese Medicine (TCM) it is said that the “kidneys control bones” (1). TCM theory suggests that many low back and knee pathologies can be attributed to the overall health of the kidneys and their “water element”. Is there an anatomical and/or physiological merit to these patterns observed by doctors of the ancient past? I believe so.
Shape Correspondence:
The kidneys are often described as being crescent or bean shaped. Intervertebral discs, the shock absorbers between each vertebrae in your spine, are anatomically described as being kidney or bean shaped in your lumbar region. The vertebral body of the lumbar vertebrae is also described as being kidney shaped which matches the shape of the intervertebral disc. Interestingly, thoracic vertebral bodies are heart shaped. The kidneys reside in the lumbar region while the heart resides in the thoracic region.
The meniscus, another shock absorber but this time for your knee, is crescent moon shaped and is occasionally referred to as being kidney shaped. The are 2 menisci, one medial (inner) and one lateral (outer) in each knee. The kidney organ is a yin organ which means its acupuncture channel traverses the inner part of the leg. Of the two menisci in each knee, it is the medial meniscus that appears more crescent/kidney shaped when compared to its lateral counterpart which matches the trajectory of the kidney channel.
Water Correspondence:
The meniscus is approximately 70% water (2). The kidneys are 79% water (3). The inner portion of your intervertebral disc, the nucleus pulposus, is 70-90% water at it’s peak and declines naturally with age. The outer portion of your disc, the annular fibrosus, is 60-70% water (4).
With normal aging, percentages of water found in tissues decline and are often represented by Kidney Yin Deficiency patterns in TCM. As such, structures with high water content (intervertebral discs, the meniscus, etc.) tend to degenerate. In addition, intervertebral disc degeneration may be ameliorated via a protein (BMP-7) that the kidney synthesizes in both embryonal and post-natal development (8).
Water content of both menisci is most concentrated at the posterior horns which are located at the inner and outer aspect along the back of the knee (5). In TCM, this follows the trajectory of the kidney and paired urinary bladder channels which belong to the water element. Talk about a coincidence!
Growth Correspondence:
According to TCM theory, the essence stored in the kidneys follows 7 year cycles in females and 8 year cycles in males (1). These cycles are milestones of growth, maturation, and decline that are commonly observed with age.
A female spine grows faster than a male spine until the age of 13 (6). At the age of 14, when the second 7 year cycle is completed, a female spine slows its growth and a male spine is now growing relatively faster until age 17. This explains why females appear taller and hit their growth spurts before males.
Bone Correspondence:
Again, with TCM theory, the overall health of a person's lower back and knees can be attributed to the kidneys which is based on observations of clinical patterns for thousands of years. Interestingly, in Asia, osteoarthritis of the lower back (25% of cases) and the knees (22% of cases) were the most common regions involved in arthritic cases in middle-aged and elderly populations (7).
When we hear the phrase "kidneys control bones" it may sound foreign or unfamiliar at first. Anatomy and physiology can bridge this gap. Although the physiology is quite complex, the kidney does have a strong influence over bone. The kidney influences bone development, remodeling, and repair by regulating calcium and phosphate homeostasis and producing cytokines as well as clearing bone regulators (8).
References:
G. Maciocia. The Foundation’s of Chinese Medicine: A Comprehensive Test, 3e.
E. Makris, et al. The knee meniscus: structure-function, pathophysiology, current repair techniques, and prospects for regeneration. Biomaterials. 2011 Oct; 32(30): 7411–7431.
N. Bogduk. Clinical Anatomy of the Lumbar Spine and Sacrum, 4e.
A. Fox, et al. The Basic Science of Human Knee Menisci: Structure, Composition, and Function. Ortho Surgery 2012.
Twomey and Taylor. Physical therapy of the low back, 2e.
X. Sun, et al. Osteoarthritis in the Middle-Aged and Elderly in China: Prevalence an Influencing Factors. Int J Environ Res Public Health, 2019
K. Wei, et al. Roles of the kidney in the formation, remodeling and repair of bone