19/07/2023
Osteoporosis
Diagnosis of osteoporosis is made by three methods:
Radiographic measurement of bone density
Laboratory biochemical markers
Bone specimen, with pathologic assessment
Of these three the best is radiographic bone density measurement. A variety of techniques are available, including single-photon absorptiometry, dual-photon absorptiometry, quantitative computed tomography (QCT) three dimensional volumentric assessment, dual x-ray absorptiometry (DEXA), and ultrasonography. Most often, site specific measurements are performed. The most common sites analyzed are those with greatest risk for fracture: hip, wrist, and vertebrae. The forearm and heel are more easily measured using single-photon absorptiometry, quantitative computed tomography, and quantitative ultrasonography, but these sites are typically unresponsive to therapy and give less information about response to therapy. Hip (femur) and vertebra can be easily measured by DEXA with an instrument dedicated to this task.
Additional techniques include DEXA-based trabecular bone score (TBS) to assess bone microarchitecture, FEA of computed tomography (CT) to estimate bone strength for fracture risk assessment, and magnetic resonance imaging (MRI) asssssing microarchitecture and bone tissue properties such as cortical porosity and marrow adipose tissue. (Martel et al, 2022)
A graphical display of a DEXA scan for the hip (femur) is shown below, comparing bone mineral density (BMD) to age and T-score (in standard deviations above or below the comparable healthy young adult woman's mean BMD). The asterisk representing a woman at age 48 is within the expected range for age. The circle marks the BMD for a woman age 60 and is concerning for greater bone loss from osteopenia (-1 to -2.5) but not yet osteoporosis. The X marks the BMD for a woman age 76 and is in the range of osteoporosis (> -2.5) with increased risk for fracture.
Laboratory testing may include serum creatinine, calcium, phosphorus, magnesium, 25-hydroxyvitamin D, alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Further possible tests when indicated include complete blood count, parathyroid hormone, thyroid-stimulating hormone, serum protein electrophoresis, and 24-hour urine calcium and cortisol. (Anam and Insogna, 2021)
The The U.S. Preventative Services Task Force (USPSTF) recommends screening for osteoporosis in women aged 65 years and older and in younger women whose fracture risk is equal to or greater than that of a 65-year-old white woman who has no additional risk factors. (USPSTF, 2017) The Fracture Risk Assessment Tool (FRAX) is available for use. (FRAX, 2017)
Increased risk for fracture correlates with decreasing bone density. Serial measurements over time can also give an indication of the rate of bone loss and prognosis (Bonnick and Shulman, 2006) (El Maghraoui and Roux, 2008).
Biochemical markers for bone turnover include bone alkaline phosphatase, osteocalcin in serum and deoxypyridinoline and pyridinoline in urine. (Bonnick and Shulman, 2006)
Alkaline phosphatase, which reflects osteoclast activity in bone, lacks sensitivity and specificity for osteoporosis, because it can be elevated or decreased with many diseases. It is increased with aging. Fractionating alkaline phosphatase for the fraction more specific to bone doesn't increase usefulness that much.
Osteocalcin, also known as bone gamma-carboxyglutamate. It is synthesized by osteoblasts and incorporated into the extracellular matrix of bone, but a small amount is released into the circulation, where it can be measured in serum. The levels of circulating osteocalcin correlate with bone mineralization, but are influenced by age, s*x, and seasonal variation. Laboratory methods also vary.
The bone resorption markers in urine are breakdown products of type I collagen and include pyridinium crosslinks known as pyridinoline and deoxypyridinoline. They reflect bone remodeling but not the status of bone mineral density.
Bone biopsy is not often utilized for assessment of bone density. This test has limited availability, and is most often performed as a research technique for analysis of treatment regimens for bone diseases. Bone biopsy involves double tetracycline labelling to determine appositional bone growth. Doses of tetracycline are given weeks apart, and the bone biopsy is embedded in a plastic compound, sliced thinly, and examined under fluorescent light, where the lines of tetracycline (which autofluoresce) will appear and appositional growth assessed. Osteomalacia, for example, has diminished appositional growth. (Malluche et al, 2007)
Consequences of Osteoporosis
Osteoporotic bone is normal in its composition of hydroxyapatite crystal on an osteoid matrix--there is just less bone. Less bone results in weakened bones that are more prone to fractures with trauma, even minor trauma. The areas most affected are:
Hip (femoral head and neck)
Wrist
Vertebrae
Hip fractures that occur, even with minor falls, can be disabling and confine an elderly person to a wheelchair. It is also possible to surgically put in a prosthetic hip joint. Wrist fractures are common with falls forward with arms extended to break the fall, but the wrist bones break too. Vertebral fractures are of the compressed variety and may be more subtle. Vertebral fractures may result in back pain. Another consequence is shortening or kyphosis (bending over) of the spine. This can lead to the appearance of a "hunched over" appearance that, if severe enough, can even compromise respiratory function because the thorax is reduced in size.
Persons suffering fractures are at greater risk for death, not directly from the fracture, but from the complications that come from hospitalization with immobilization, such as pulmonary thromboembolism and pneumonia.
Osteoporosis is so common that, on average, about 1 in 2 elderly Caucasian women will have had a fracture. In contrast, only about 1 in 40 men of similar age will have had a fracture. Men start out with a greater bone mass to begin with, so they have a greater reserve against loss. However, that is still a large number of men with osteoporosis. (Binkley, 2009)
Normal vertebral bone, gross.
Normal vertebral bone, gross.
Normal vertebral bone and marrow, low power microscopic.
Normal vertebral bone, polarized, medium power microscopic.
Vertebral bone with osteoporosis, gross.
Vertebral bone with osteoporosis and compressed fracture, gross.
Vertebral bone with osteoporosis, low power microscopic.
Femur with osteoporosis, radiograph.
Femoral neck fracture, radiographs.
Hip prosthesis, radiograph.
Prevention Strategies
The best long-term approach to osteoporosis is prevention. If children and young adults, particularly women, have a good diet and get plenty of exercise, then they will build up and maintain bone mass. This will provide a good reserve against bone loss later in life. Exercise places stress on bones that builds up bone mass, particularly skeletal loading from muscle contraction with weight training exercises. However, any exercise of any type is better than none at all, and exercise also provides benefits for prevention of cardiovascular diseases that are more common in the elderly. Athletes tend to have greater bone mass than non-athletes. Exercise in later life will help to re**rd the rate of bone loss. A healthy diet should include not only enough calcium and vitamin D, but also other nutrients, including a range of vitamins and minerals found in a diet that contains fruit and vegetables, as well as dairy products. Appropriate protein intake has an anabolic effect to build osteoid matrix. (NEJM, 2016)
Treatment
Persons with osteoporosis may benefit from an improved diet, including supplementation with vitamin D and calcium, and moderate exercise to help slow further bone loss. (NEJM, 2016) Exercise to improve muscle mass and mobility aids in fall prevention. (Anam and Insogna, 2021)
Most drug therapies work by decreasing bone resorbtion. At any given time, there is bone that has been resorbed but not replaced, and this accounts for about 5 to 10% of bone mass. By decreasing resorbtion of bone, a gain in bone density of 5 to 10% is possible, taking about 2 to 3 years. However, no drug therapy will restore bone mass to normal. Women past menopause with accelerated bone loss may benefit from hormonal therapy using estrogen with progesterone. The estrogen re**rds bone resorption and thus diminishes bone loss. This effect is most prominent in the first years after menopause, while risks from hormone replacement therapy increase. (Nelson et al, 2002)
One of the more common non-estrogen therapies is the use of bisphosphonates such as alendronate or risedronate that act an an inhibitor of osteoclastic activity. Bisphosphonates may be beneficial, particularly in women who cannot tolerate estrogen therapy. Bisphosphonaes are effective in inhibiting bone loss after menopause. In one study risedronate has shown effectiveness in reducing the risk of hip fracture among elderly women with osteoporosis. Short term adverse effects of bisphosphonate therapy include esophagitis, musculoskeletal pain, ocular inflammation, and hypocalcemia. Long term adverse effects include increased risk for esophageal cancer, osteonecrosis of the jaw, femoral fracture, and atrial fibrillation. (Anam and Insogna, 2021)
Raloxifene is a selective estrogen receptor modulator (SERM) that may also replace estrogen therapy. Raloxifene can act in concert with estrogen in bone to inhibit resorbtion and decrease the risk for fractures. Though raloxifene inhibits bone resorbtion, it does not have an anabolic effect. Additional potential benefits from raloxifene therapy include decreased risk for breast cancer, because raloxifene acts antagonistically to estrogen on the uterus. Conversely, raloxifene acts in concert with estrogen to protect against and reduce atherogenesis. (Anam and Insogna, 2021)
Teriparatide is a recombinant human parathyroid hormone administered by subcutaneous injection which binds to specific high-affinity cell-surface receptors in bone and kidney, similar to the 34 N-terminal amino acids of parathyroid hormone, and has the same physiological actions on bone and kidney. Daily administration of teriparatide stimulates new bone formation by promoting osteoblastic activity over osteoclastic activity, improving trabecular bone architectural remodelling and increasomg bone mass. (NEJM, 2016) (Anam and Insogna, 2021)
Denosumab is a human monoclonal antibody that binds to and inhibits the receptor activator of nuclear factor-kappaB ligand (RANKL) that is elaborated by osteoblasts. The RANKL interacting with RANK receptor expressed on osteoclasts is affected by this drug, leading to reduced osteoclast activation and survival, thus inhibiting bone resorbtion that helps increase bone mineral density. Thus, densosumab mimics osteoprotegrin that is reduced in osteoporosis. (NEJM, 2016)
Romosozumab is a humanized monoclonal antibody that binds to sclerositin, an antagonist of the Wnt signaling pathway that induces differentiation of mesenchymal stem cells to osteoblasts as well as promoting insulin-like growth factor I to enhance osteoblast function. Romosozumab diminishes bone loss while promoting bone formation to reduce risk for fractures. (Canalis, 2018)
Vitamin D Testing
There are too many vitamin D tests ordered. Evaluate risk factors for vitamin D deficiency to ensure very limited, targeted testing of only a few selected patients. Retesting after treatment should be performed only after three to six months. For the cost of testing you could go to the market and buy vitamin D and/or enroll in an exercise program.
Best advice: go outside ! If you go outside you will be in sunlight to generate vitamin D and get exercise to help maintain bone mass.
The FRAX® tool has been developed to evaluate fracture risk of patients. It is based on individual patient models that integrate the risks associated with clinical risk factors as well as bone mineral density (BMD) at the femoral neck.
