Osteoporosis and the Aging Spine: Diagnosis and Treatment

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12 Osteoporosis and the Aging Spine: Diagnosis and Treatment

Aasis Unnanuntana, Brian P. Gladnick, Joseph M. Lane

KEY POINTS

• Osteoporosis fractures markedly increase morbidity and mortality; therefore, strategies for diagnosis, prevention, and treatment of this condition are needed.
• Once a patient has been diagnosed with osteoporosis, a complete evaluation should be obtained. This includes a thorough medical history with particular attention paid to the risk of osteoporosis, a physical examination, and essential laboratory investigations. If secondary osteoporosis is suspected based on the clinical findings, further investigations are required directed at the secondary cause.
• All patients with osteoporosis should be tested for vitamin D deficiency. If inadequate, vitamin D supplementation is needed. The requirement for vitamin D intake in an adult is approximately 1000 to 2000 IU/day.
• Bisphosphonates are strong antiresorptive agents that have been shown to reduce both vertebral and nonvertebral fracture risks. Therefore they are the first-line drugs for the treatment of postmenopausal osteoporosis, unless contraindicated.
• Bisphosphonates might have an adverse effect on spinal fusion, whereas teriparatide facilitates fusion and increases the fusion mass. In addition, anabolic agents enhance fracture healing. We therefore recommend initiating an anabolic agent (PTH 1-34) when bone healing is required. If bisphosphonate therapy is considered, refrain 4 or more weeks before commencing therapy.

Osteoporosis is a systemic debilitating disease of the skeleton, characterized by significantly decreased bone mass in combination with the deterioration of bone microarchitecture. This process results in weakened bone with a great propensity for fracture with low-energy stress. As the average life expectancy and median age of the population rises, fractures secondary to underlying osteoporosis are becoming increasingly commonplace. More than 1.5 million osteoporotic fractures occur annually in the United States, the majority of which occur in the spine, hip, and wrist.1 Women are predominantly affected; a recent study estimates that as many as one in two women who are older than 50 years of age will suffer an osteoporotic fracture.2 These fractures can result in marked morbidity and mortality. For example, a single vertebral compression fracture in women is associated with a 1.2-fold increased age-adjusted mortality rate, and the presence of five fractures increases that risk to 2.3-fold.3 In addition, a vertebral fracture increases the risk of a second vertebral fracture by 5-fold, and a hip fracture by 2-fold. Among patients with osteoporotic hip fractures, only 25% of patients ever make a full recovery, while 20% die within the year secondary to complications. Thus, spine surgeons must be increasingly suspicious of this disease in certain patient demographics, achieve a firm understanding of the pathogenesis of osteoporotic bone and the conditions that result in bone fragility, and become familiar with the current strategies for diagnosis, prevention, and treatment of osteoporosis.

Physiology of Bone Remodeling and Bone Turnover

Bone is a dynamic, living tissue that continuously remodels itself throughout the lifetime of the patient. Bone homeostasis consists of three phases. The initial resorption phase is mediated primarily by osteoclasts, which are activated through the interaction of an osteoclast surface protein, receptor activator of nuclear factor-κB (RANK), with its ligand (RANKL). RANKL is primarily expressed by the osteoblast lineage and by stromal cells. During the reversal phase, osteoclasts become less numerous on the bony surfaces and are increasingly replaced by mononuclear cells. These mononuclear cells prepare the bony surface for the introduction of bone-forming osteoblasts and provide cytokine signaling, which stimulates the differentiation of osteoblasts and their subsequent migration to the surface of the bone. During the final formation phase, osteoblasts lay down newly formed woven bone to replace bone that had been previously resorbed.

Bone remodeling is a complex process that is regulated both locally and systemically. As previously mentioned, RANKL/RANK interactions at the local level promote induction of osteoclast activity and subsequent remodeling. Conversely, osteoprotegerin (OPG) is a soluble receptor for RANKL that acts as an antagonist to decrease osteoclastic activation and thereby reduce the rate of bone resorption. Interestingly, there are a number of systemic signaling mechanisms that act through the RANKL/RANK/OPG pathway to regulate bone homeostasis.4 For example, parathyroid hormone (PTH) and the glucocorticoids both act to increase local expression of RANKL but decrease concomitant expression of OPG, resulting in a net increase in osteoclast activation and bone resorption. Alternatively, estrogens act to increase the local expression of OPG and decrease RANKL, which results in a net decrease in osteoclast activity and bone resorption (Figure 12-1). Derangement of these pathways can alter the delicate balance between bone resorption and bone formation, and may result in a net decrease of bone formation that contributes to the development of osteoporosis.

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FIGURE 12-1 Receptor activator of nuclear factor-kB (RANK) on the osteoclast precursor’s surface interacts with its ligand (RANKL), expressed by the osteoblasts and stromal cells, thereby activating the differentiation of the osteoclast precursor to activated osteoclast and subsequent bone resorption. Osteoprotegerin (OPG) is a soluble receptor for RANKL that competitively inhibits the RANKL-RANK interaction. Various systemic modulators (estrogen, parathyroid hormone [PTH], or glucocorticoids) influence the activation of the osteoclasts via their effects on the RANK/RANKL/OPG pathway. (+), Activation; (−), Inhibition.

Based upon the varying influences of bone resorption and formation, osteoporosis is subdivided into two categories: low-turnover and high-turnover osteoporosis. The low-turnover state describes a situation in which normal bone homeostasis is altered by decreased osteoblast activity; however, the osteoclast activity remains normal. Low bone mineral density (BMD) in this setting, therefore, is a result of reduced bone formation. Conversely, the high-turnover state is characterized by increased activity of both osteoblasts and osteoclasts. However, osteoclasts are activated to a greater extent. The bone remodeling process is shifted toward bone resorption, resulting in an imbalance of bone turnover that causes osteoporosis. High turnover osteoporosis is the most common form and appears at menopause, while low turnover osteoporosis occurs following drug interventions including chemotherapy, steroids, and prolonged bisphosphonate use.

Diagnosis of Osteoporosis

Although a good clinical understanding of osteoporosis takes into account the pathophysiology of bone remodeling, mineralization changes, and variable bone quality of the patient, the diagnosis of osteoporosis until recently has relied upon a single criterion: the bone mineral density. The current gold standard of measuring BMD is dual-energy x-ray absorptiometry (DXA), which uses an x-ray beam to calculate the patient’s BMD. The most preferred skeletal sites for evaluation of BMD are the spine and hip, because these two locations provide the best data for correlating low BMD with the risk of future fracture BMD is reported as the T-score, which is a measurement of how many standard deviations the patient’s bone density is below the mean of young, healthy individuals at their peak bone mass. Based on this T-score, the World Health Organization (WHO) developed a classification system to define osteoporosis (Table 12-1).

TABLE 12-1 WHO-Based Criteria for Diagnosis of Osteoporosis

T-Score Diagnosis
− 1.0 or above Normal bone
Below −1.0 to above −2.5 Osteopenia
− 2.5 or below Osteoporosis
− 2.5 or below with fracture Severe osteoporosis

Generally, osteoporosis is classified as either primary or secondary. Primary osteoporosis is further subdivided, based on its pathogenesis. Type I, or postmenopausal osteoporosis, is related to the abrupt decline of estrogen levels that occurs in menopausal women. Type II osteoporosis, known as senile or age-related osteoporosis, is due to the progressive decrease of BMD in both men and women that occurs with aging. Patients may suffer from both subtypes of primary osteoporosis.

Secondary osteoporosis is defined by the presence of some preexisting disease process or other causative factor, which causes a secondary decline in BMD (Table 12-2). Forty-five percent of osteoporotic women and 66% of osteoporotic men have their osteoporosis secondary to some underlying condition. Therefore patients with secondary osteoporosis must be identified because definitive treatment of the underlying cause is necessary to prevent further bone loss, and thus lower the risk of fracture. In this regard, it is important to consider the patient’s BMD using the Z-score. The Z-score indicates how many standard deviations the patient’s BMD is below the expected value for his or her own age. The Z-score cannot be used to diagnose osteoporosis, but it is useful for screening the patient for secondary causes. A Z-score of −2.0 or lower should increase the index of suspicion that underlying medical problems, medications, or other factors may be responsible for the patient’s low BMD.6

TABLE 12-2 Causes of Secondary Osteoporosis5

Hormone Excess
• Parathyroid (primary or secondary)
• Thyroid
• Cortisol
Hormone Deficiency
• Estrogen (premenopausal or postmenopausal)
• Testosterone
Diseases
• Inflammation (rheumatoid arthritis, ulcerative colitis)
• Tumor or malignancy (multiple myeloma, lymphoma)
• Collagen vascular disease
• Renal osteodystrophy
• Others (liver diseases, immobilization)
Drugs
• Corticosteroids
• Thyroxine
• Alcohol
• Anticonvulsants (barbiturates, phenytoin)
• Anticoagulants (heparin, warfarin [Coumadin])
• Antimetabolites (methotrexate, cyclosporin)

Evaluation for Osteoporosis

Once diagnosed with osteoporosis, a complete medical history should be obtained with particular attention to the risk factors for osteoporosis. These include age of 65 years or older, a history of vertebral fracture or any fracture during childhood, a family history of hip fracture, low body weight (BMI < 21 or weight < 127 lb), cigarette smoking, and use of corticosteroids for more than 3 months.6 The physical examination should be performed particularly at the spine region. Height should be measured and compared with the greatest known height to determine height loss, which is an indicator of the presence of vertebral compression fractures. Balance and walking gait should be observed in each individual. The assessment of functional balance is performed by using the single limb stance test and the 6-minute walking test.

Screening for Osteoporosis with Bone Mineral Density Measurement

A number of risk factors for osteoporosis have been identified by the International Society for Clinical Densitometry (ISCD),7 and should be used to guide the screening process in a cost-effective manner. The current indications for BMD testing include any patient who is one or more of the following:

1. A female aged 65 or older
2. A postmenopausal female younger than 65 years who has clinical risk factors for fracture, such as low body mass index, prior fracture, or use of a high-risk medication
3. A woman during the menopausal transition with clinical risk factors for fracture
4. A male aged 70 years or older
5. A male younger than 70 years with clinical risk factors for fracture
6. An adult with a history of a fragility fracture
7. An adult with an illness known to cause bone loss or low BMD
8. An adult taking a medication known to cause bone loss or low BMD
9. Any patient being considered for pharmacologic treatment of bone loss
10. Any patient currently being treated for low BMD in order to monitor the treatment effect
11. Any patient not receiving therapy in whom evidence of bone loss would lead to pharmacologic treatment

In addition to these guidelines, it is important to take into account other factors that may increase a patient’s propensity for low BMD or fracture. Patients with poor general health, alcoholism, dementia, frailty, recent discontinuation of estrogen replacement therapy, or long-term history of estrogen deficiency should be considered for DXA scanning even if they do not fit the ISCD criteria.

Laboratory Investigations for Osteoporosis

Generally, laboratory investigations other than BMD measurement are not required for the diagnosis of osteoporosis. Some routine tests, however, should be performed to obtain baseline values as part of the initial workup. These include complete blood count with differential cell count, urinalysis, and blood chemistry profiles with serum calcium and phosphate. Some special laboratory tests are available to measure the balance between bone resorption and bone formation from serum and urine samples. These assays are called “bone markers.”

Bone markers can be classified into two groups: bone formation and bone resorption markers. Bone specific alkaline phosphatase and serum osteocalcin are both produced when bone is formed and can be used as markers for bone formation. On the other hand, during bone resorption, human collagen is broken down and released into the bloodstream and subsequently secreted into the urine. By assaying the amount of collagen breakdown products, such as the cross-linked amino-terminal telopeptides of bone collagen (NTx) in urine or blood, the rate that bone is being resorbed can be determined (Figure 12-2). Measurement of bone markers is helpful for following a patient’s response to treatment over time. Therefore it is advisable to get a baseline value as part of the initial workup. The goal of treatment of osteoporosis is to keep the NTx levels close to the normal range for premenopausal women.

image

FIGURE 12-2 The normal structure of a single human collagen molecule is shown. The collagen molecule is a triple helix consisting of two pro-α1 chains (red) and a single pro-α2 chain (blue). During osteoclast-mediated bone resorption, the bone collagen molecule is degraded, producing an amino-terminal (N)-telopeptide, a carboxy-terminal (C)-telopeptide, and a central region of intact triple helix. Cross-linked N-telopeptides (NTx) and cross-linked C-telopeptides (CTx) are specific for bone and achieve a measurable concentration in blood and urine. When osteoclast activity increases, bone is resorbed and more collagen is degraded, which coincides with an increased measured concentration of NTx and CTx.

Vitamin D deficiency is very common among elderly, with a prevalence of approximately 50%; however, many patients are asymptomatic. In addition, serum calcium and phosphate levels in this group of patients may not necessarily be abnormal. All older individuals, therefore, should be tested for vitamin D deficiency by measuring levels of 25-hydroxy vitamin D. If low, adequate vitamin D supplementation is encouraged. Values below 20 ng/mL are associated with poor muscle function as well as mineralization defects.

Evaluation for Secondary Osteoporosis

When secondary osteoporosis is suspected on the basis of clinical findings or because the patient is relatively young and presented with fragility fracture, specific tests should be considered to evaluate contributing causes that may require additional medical attention. These include basic laboratory investigation of a complete blood count with differential, erythrocyte sedimentation rate, serum calcium and phosphate level, liver function tests, thyroid-stimulating hormone level, testosterone level in men, and a serum protein electrophoresis if myeloma is considered (Table 12-3). When abnormalities are detected, the patient should be referred to a specialist for further evaluation and specific treatment.

Table 12-3 Laboratory Investigations for Secondary Osteoporosis

Medical Diseases Diagnostic Study
Endocrine

• Hyperparathyroidism
• Hyperthyroidism
• Hypogonadism
• Diabetes

serum calcium, serum phosphate, parathyroid hormone level
TSH, T3, free T4
LH, FSH, estrogen, testosterone (men)
blood glucose

GI Disorders

• Crohn disease, Ulcerative colitis
CBC, ESR, CRP, serum albumin, colonoscopy Liver Disease

• Primary biliary cirrhosis, Chronic active hepatitis
Liver function test, antimitochondrial antibody, antibody for hepatitis A,B and C Bone Marrow Disorders

• Multiple Myeloma, Leukemia, Lymphoma
CBC with differential, serum calcium, serum protein electrophoresis Collagen Vascular Disease

• Osteogenesis imperfecta, Ehlers-Danlos syndrome, Marfan syndrome
Genetic testing for collagen defects Others

• Rheumatoid arthritis
• Renal Failure
CBC, ESR, CRP, rheumatoid factor
BUN, creatinine

TSH = thyroid stimulating hormone; LH = luteinizing hormone; FSH = follicle stimulating hormone; CBC = complete blood count; ESR = erythrocyte sedimentation rate; CRP = c-reactive protein; BUN = blood urea nitrogen

Assess for Risk of Falls and Fractures

Certain comorbidities associated with the aging population, such as unsteady gait, use of sedative or hypnotic medications, and impaired visual or neuromuscular function may predispose a patient to falls. By identifying patients at particularly high risk for falls early in the course of treatment, it is possible to prevent a subsequent fracture. It is well recognized that the fracture rate is highest among patients with osteoporotic bones (T-score −2.5 or below). However, a much larger proportion of patients reside in the range of osteopenia (below −1.0 to above −2.5). Consequently, more total fractures occur in this osteopenic group (55% of hip fractures). To adjust for the disparity, a new vehicle called the Fracture Risk Assessment Tool (FRAX) has been developed that adds additional risk factors to the calculation, and offers a better assessment of fracture risk than DXA scanning alone. This instrument calculates the patient’s 10-year fracture risk based on age, sex, weight, height, previous fracture, parent with fractured hip, current smoking, use of glucocorticoids, presence of rheumatoid arthritis, secondary osteoporosis, alcohol use (>3 drinks/day), and BMD at the femoral neck area.

Each risk factor is weighted differently in the calculation depending on its importance. For example, in the absence of BMD measurements, a prior history of fracture is considerably more predictive of a patient’s future fracture risk than is a history of smoking, and is weighted as such in the FRAX calculation (Table 12-4). As the patient accrues more risk factors, the FRAX score increases in a predictable manner.8

TABLE 12-4 Ten-Year Probability for Osteoporotic Fracture Based on FRAX Calculations8

Clinical Risk Factors∗ 10-Year Fracture Probability (%)†
None 8.6
Current smoker 9.2
Alcohol (≥3 drinks/day) 10.4
Rheumatoid arthritis 11.7
Oral glucocorticoids 13.7
Parent with history of hip fracture 16.0
Previous fragility fracture 16.4

* Assume in a female patient, aged 65 years, with BMI = 25 kg/m2.

† Probabilities are given in absence of a known T-score.

Treatment in Osteoporosis

Nonpharmacologic Treatment

A multidisciplinary approach is critically important in the management of osteoporosis.9 Nonpharmacologic treatment is used concurrently with pharmacologic therapy to optimize fracture risk reduction. Thus, every patient should be considered for nonpharmacologic management. Commonly used nonpharmacologic treatments include, but are not limited to, calcium and vitamin D supplementation, fall prevention, and balance and exercise programs.

Calcium and Vitamin D Supplementation

Calcium and vitamin D supplementation is the cornerstone of all treatment modalities for osteoporosis. Literature clearly shows that adequate calcium and vitamin D intake reduces the risk of fractures. For optimal treatment, adequate calcium intake of 1000 to 1500 mg/day should be maintained in all patients on any type of treatment. To maximize the absorption of calcium across the small bowels, no more than 500 to 600 mg of elemental calcium should be taken at any given time. Among all calcium formulations, calcium citrate is the preferred form. Calcium citrate binds to oxalate, reducing its intestinal absorption, and citrate in urine inhibits crystal formation, thus reducing the incidence of kidney stones. In addition, calcium citrate does not require low pH for salt dissociation; therefore the absorption of this calcium formulation is reliable in patients taking H2 blockers or proton pump inhibitors.

The current recommended dosages of vitamin D3 from the Institute of Medicine are 200 to 600 IU/day. However, many experts consider these recommendations to be too low and suggest that the minimum adult intake should be 1000 to 2000 IU/day. The appropriate amount of vitamin D intake should be evaluated by monitoring 25-hydroxy vitamin D level and serum PTH. Patients with vitamin D insufficiency will have low levels of 25-hydroxy vitamin D and elevated serum PTH from secondary hyperparathyroidism. At the Hospital for Special Surgery, 45% of patients undergoing elective surgery and 66% of patients presenting with fractures have vitamin D insufficiency. Higher doses of vitamin D are required in these patients to replenish depleted total body stores. Fifty thousand international units of vitamin D2 can be taken orally once a week or every other week for 6 to 8 weeks, followed by a maintenance dose of vitamin D3 of 1000 to 2000 IU/day. Toxicity is rare even if a dosage of 10,000 IU/day is given for up to 5 months.

Pharmacologic Treatment

Osteoporosis has been divided into two categories, high-turnover and low-turnover osteoporosis. The pharmacologic agents currently available are commonly divided into antiresorptive and anabolic groups. Antiresorptive agents have been developed to address the high-turnover state. These include estrogen, selective estrogen receptor modulators (SERMs), calcitonin, and bisphosphonates. The anabolic agent, parathyroid hormone, provides active building of bone mass and has been suggested to treat the low-turnover state. Both antiresorptive and anabolic agents have demonstrated antifracture efficacy in many studies (Table 12-5).

Table 12-5 Antifracture Efficacy of Agents for Treatment of Osteoporosis

image

Antiresorptive Agents

Estrogen is an antiosteoporotic agent that has been shown to increase bone mass and thus decrease the risk of vertebral and hip fracture by approximately 30% to 40% as compared with patients taking placebo. Estrogen, however, has been found to increase rates of stroke and deep vein thrombosis, whereas combined estrogen and progesterone therapy is associated with increased risks of cardiovascular disease, breast cancer, dementia, and gallbladder disease. As a consequence, estrogen is mainly used in the early postmenopausal period to treat postmenopausal syndrome and then lowered to the least effective dose to control symptoms. Because of the risks of estrogen formulations, this precludes their use as primary agents in the treatment of osteoporosis.

SERMs are a class of agents that bind to estrogen receptors. They have a significant effect on breast tissue and bone cells; however, they act as antagonists in the other receptor sites. Of the SERMs currently being used for clinical settings, only raloxifene has been approved for the prevention and treatment of osteoporosis. Early data suggest that raloxifene decreases the risk of breast cancer by 70%, which made raloxifene a preferred agent in this particular indication. Although raloxifene has been shown to reduce the risk of vertebral fracture, there was no significant reduction in the overall risk of nonvertebral fracture. In addition, by stimulating estrogen receptors, raloxifene increases the risk of pulmonary emboli and thrombophlebitis and may cause profound postmenopausal symptoms. Therefore clinicians must weigh the benefits of the reduced risks of vertebral fracture and invasive breast cancer against the increased risks of venous thromboembolism and fatal stroke when considering this agent for osteoporosis management.

Calcitonin is available as both a parenteral injection and nasal spray. The intranasal spray is the most commonly used formulation because of its superior compliance and ease of use. Calcitonin reduces the risk of vertebral fracture, but there is only a modest increase in BMD. Additionally, calcitonin treatment shows no benefit for reducing the risk of hip and other nonvertebral fractures. There are some data suggesting the analgesic effect of calcitonin. Although there is a hypothesis that calcitonin-induced analgesia may be mediated by increased beta-endorphins and may directly affect pain receptors in the central nervous system, the exact mechanism is still unknown. Therefore the current indication for calcitonin treatment is for alleviating painful vertebral compression fractures. It should be discontinued as soon as pain has been controlled, because other pharmacologic agents are much more effective at preventing future fractures.

Bisphosphonates are a class of antiresorptive agents that have been shown to be extremely efficacious in high-turnover osteoporosis. Currently, four bisphosphonates have been approved by the U.S. Food and Drug Administration for the treatment of posmenopausal osteoporosis: alendronate (Fosamax), risedronate (Actonel), ibandronate (Boniva) and zoledronic acid (Reclast).10 These drugs differ in their potency, mode of administration, and dosing schedules. Alendronate and risedronate are administered orally, whereas zoledronic acid is administered by intravenous injection. Ibandronate is available in both oral and intravenous formulations (Table 12-6).

Table 12-6 Bisphosphonates Recommended to Treat Osteoporosis

image

The mechanism of action of bisphosphonate involves interposition of a nondegradable drug barrier between osteoclasts and Howship’s lacunae, thus interfering with resorption. The drug is then ingested by osteoclasts and disrupts the cellular membrane mevalonate synthesis pathway, leading to premature osteoclast death. Bisphosphonates slow the bone turnover rate, which has been reported to be decreased within 6 weeks with the oral formulations and within 3 days with the intravenous formulations. All oral bisphosphonates reduce the risk of both vertebral and nonvertebral fractures. The intravenous zoledronic acid appears to be effective in increasing bone mass and decreasing both the vertebral and nonvertebral fracture risk. In addition, when given within 3 months of an acute hip fracture, zoledronic acid lowers mortality by more than 20% and does not interfere with fracture healing.

The adverse effects of oral bisphosphonates are their inherent toxicity to epithelial cells lining the gastrointestinal tract. The results are irritation of the esophagus, acid eructation, nausea, and heartburn. Patients with a significant background of gastrointestinal intolerance, therefore, are not suitable for prescribing oral formulations. However, clinicians may improve patients’ tolerance by giving oral bisphosphonates in small doses and slowly increasing the dosage until full dose is achieved. This will lead to better compliance even with individuals who have a past history of dyspepsia. The other potential complication is osteonecrosis of the jaw. Osteonecrosis of the jaw is defined as exposed bone in the musculofacial region that fails to heal within 8 weeks after identification by a health care provider. The group of patients who carry the greatest risk for developing this complication are those with multiple myeloma or metastatic carcinoma of the skeleton who are being treated with relatively high doses of intravenous bisphosphonates. Therefore, before starting bisphosphonate treatment, especially with an intravenous formulation, patients should complete any dental work and should establish meticulous oral hygiene. In addition, intravenous bisphosphonates can cause a release of cytokines from osteoclasts and result in flulike symptoms. These include fever, muscle pain, headache, and bone pain. Most of the symptoms resolve within 3 days. Co-administration of diphenhydramine (Benadryl) and acetaminophen (Tylenol) can minimize these side effects.

The sequelae of long-term bisphosphonates on bone metabolism remain unclear. There is a hypothesis that prolonged treatment with bisphosphonates may lead to adynamic, fragile bone. In this state of oversuppression, microfractures generated through the wear and tear of normal daily life begin to accumulate and coalesce, leading to insufficiency fractures. Accumulation of microdamage is associated with a reduction in bone toughness. Many studies report low-energy subtrochanteric or mid shaft fractures after prolonged treatment with alendronate. This type of fracture is caused by minimal or no trauma and is characterized by (1) simple or transverse fracture, (2) breaking of the cortex on one side, and (3) hypertrophied diaphyseal cortices, Currently, the recommendation for bisphosphonate therapy is a 5-year period of treatment. Once the BMD is in its plateau and the urine level of NTx is in therapeutic range (20-40 nmol bone collagen equivalents/mmol of creatinine), changes in treatment, such as a rest period from bisphosphonates or the use of an anabolic agent, should be considered.

Anabolic Agents

Teriparatide, parathyroid hormone (1–34), is the only anabolic agent available in the United States for the treatment of postmenopausal osteoporosis. It is administered subcutaneously once a day. When given intermittently, teriparatide can lead up to a 13% increase in bone mass over 2 years of therapy. This increase is greater than that achieved with bisphosphonate therapy. The risk of fracture was reduced by 65% and 53% for vertebral and nonvertebral fracture, respectively. Other benefits of teriparatide have been proposed. Many studies reported the possible benefits of teriparatide on fracture healing. Callus formation was accelerated by the early stimulation of proliferation and differentiation of osteoprogenitor cells and increases in production of bone matrix proteins. Teriparatide should be considered in the following conditions:

1. Patients with low-turnover osteoporosis
2. Patients who have been on bisphosphonates and still have fragility fracture
3. Patients with declining bone densities
4. Premenopausal women with osteoporosis

Contraindications include active Paget’s disease of bone, metastatic cancer in the skeleton, history of skeletal irradiation, and children with open epiphyses. The adverse reactions associated with teriparatide are nausea, headache, dizziness, leg cramps, swelling, pain, weakness, erythema around the injection site, and elevation of serum calcium. There is a concern regarding osteosarcoma due to evidence showing that rodents, exposed to prolonged high doses of teriparatide, developed osteosarcoma. Therefore teriparatide should be discontinued after 2 years of treatment. After that, bisphosphonate therapy should be initiated to maintain its results.

Pharmacologic Agents and Spinal Fusion

There are no clinical studies that evaluate pharmacologic agents for osteoporosis in the setting of vertebral fractures or spinal fusion. Bisphosphonates increase fracture callus size during endochondral repair but cause delay in maturation. Some animal studies have demonstrated that bisphosphonates delayed fusion. Therefore until clinical studies are available, we recommend that bisphosphonates should be started a few weeks after spinal fusion or vertebral fracture to reduce the possible adverse effects to the early biological processes of fracture healing. Teriparatide in all animal studies facilitated fracture healing. In both rat and rabbit models of spinal fusion, the administration of teriparatide speeded fusion, and increased the fusion mass. Based on these animal studies, teriparatide offers superior biology for spinal indications, when compared to bisphosphonates and a control group that only took calcium and vitamin D. However, the application of this agent in patients still awaits evidence-based studies.

Future Directions

The available pharmacologic agents used to treat osteoporosis have their own limitations at some degrees. Understanding of cellular mechanism regulating bone formation and remodeling is critically important to developing new agents which will reduce the adverse effects or improve the outcome of treatment. Denosumab, a human monoclonal antibody against RANKL, has been shown in preclinical trials to increase bone mineral density and decrease bone resorption in postmenopausal women with osteoporosis. It is now awaiting approval for entry into the market. Cathepsin K inhibitors are another group of antiresorptive agents that were designed to reduce the activity of cathepsin K (a powerful osteoclast protease). Theoretically, this agent can limit the enzymatic degradation of bone matrix proteins.

Strontium ranelate is an agent that has both antiresorptive and anabolic action. Treatment of postmenopausal osteoporotic women with strontium ranelate has been shown to decrease fracture risk and increase bone mineral density. The long-term effects, however, remain unknown. The development of new formulations of teriparatide (noninjectable forms) or development of alternative parathyroid hormone analogs that possess longer half-life, leading to less frequent dosing, are also under investigation.

Summary

In general, the program for spinal care in elderly patients, particularly with osteoporosis/osteopenia, should include a thorough medical history and physical examination, appropriate diagnostic tests, maintenance of adequate nutrition including calcium, vitamin D, and the use of appropriate osteoporotic therapeutic agents. In the setting of spinal fusion and fracture healing, an anabolic agent such as parathyroid hormone may be advantageous in the early steps to enhance appropriate biological responses. Patients with osteoporosis require a multidisciplinary approach.

References

1. Keen R.W. Burden of osteoporosis and fractures. Curr. Osteoporos. Rep.. 2003;1:66-70.

2. Lane J.M., Gardner M.J., Lin J.T., van der Meulen M.C., Myers E. The aging spine: new technologies and therapeutics for the osteoporotic spine. Eur Spine. J. 2003;12(Suppl. 2):S147-S154.

3. Lin J.T., Lane J.M. Osteoporosis: a review. Clin. Orthop. Relat. Res. 2004 Aug;425:126-134.

4. Hadjidakis D.J: Androulakis II, Bone remodeling, Ann. N. Y. Acad. Sci, 1092, 385-396, 2006.

5. Stein E., Shane E. Secondary osteoporosis. Endocrinol Metab. Clin. North Am. 2003;32:115-134.

6. Kaufman J.D., Cummings S.R. Osteoporosis and prevention of fractures: practical approaches for orthopaedic surgeons. Instr. Course Lect. 2002;51:559-565.

7. International Society for Clinical Densitometry. 2007 Official Positions & Pediatric Positions of the International Society for Clinical Densitometry. Available at http://www.iscd.org/Visitors/pdfs/ISCD2007OfficialPositions-Combined-AdultandPediatric.pdf. Accessed October 23, 2008.

8. Kanis J.A., Johnell O., Oden A., Johansson H., McCloskey E. FRAX and the assessment of fracture probability in men and women from the UK. Osteoporos. Int. 2008;19:385-397.

9. Bouxsein M.L., Kaufman J., Tosi L., Cummings S., Lane J., Johnell O. Recommendations for optimal care of the fragility fracture patient to reduce the risk of future fracture. J. Am. Acad. Orthop. Surg. 2004;12:385-395.

10. Gehrig L., Lane J., O’Connor M.I. Osteoporosis: management and treatment strategies for orthopaedic surgeons. J. Bone Joint Surg. Am. 2008;90:1362-1374.

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