Osteoporosis-related fractures: What they are and how they occur following spinal cord injury

Introduction

In 2017, approximately 22 million people around the world were living with a spinal cord injury (SCI), a rate increase of 14.21% in the last 10 years ( ). The better access to medical care resources such as emergency medical services and rehabilitation departments led to longer survival for this population ( ). Henceforth, many consequences were related to SCI such as neurogenic bladder, neurogenic bowel, neuropathic pain, spasticity, and osteoporosis ( ; ).

Osteoporosis affects the health of individuals with SCI because it increases the risk of fragility fractures with consequences related to fractures such as ulcers, immobility, depression and even death ( ; ; ). Osteoporosis after SCI is defined as an excessive bone resorption after SCI and bone fragility fractures are defined as fractures caused by a trauma that would be insufficient for a normal bone to fracture ( ; ) or by minimally loaded situations ( ; ).

The impact of fractures on the lives of individuals with SCI suggests that most patients will be hospitalized after the fracture and undergone to surgery, and complications as worse performance in activities of daily living and ambulation after the fracture are mostly described. These issues mean increased disability and costs for the health system ( ).

Special attention should be paid to the prevention of fractures, and appropriate treatments should be instigated to prevent excessive bone loss and futher investigations should be performed. Most of the physicians are not educated to measure risk factors for bone fragility fractures. Dual-energy X-ray absorptiometry (DEXA) should be performed in patients with SCI, likely before the occurrence of fractures. Bone remodeling markers could give us a clue of the resorption rate and treatments to prevent further loss should be consider.

Physiopathology

Bone demineralization after SCI occurs in a rate loss of approximately 4% per month in the first year ( ; ; ), primarily in the knees. Demineralization begins in the early days, peaks around the 10th to 16th weeks and stabilizes around the 16th to 24th months ( ; ; ). Recent studies suggest that the bone loss extends 3 to 8 years after SCI ( ). Previous studies demonstrated bone demineralization greater than 20% in the hip, 37% to 52% in the distal femur and 36% to 70% in proximal tibia after 1 to 3 years after SCI ( ; ; ).

Bone loss associated with SCI is related to immobility and metabolic changes ( ; ; ). Individuals with SCI exhibit primarily hypercalcemia, hypercalciuria, hyperparathyroidism, decreased levels of osteocalcin, and higher levels of sclerotin. The loss of bone mineral density is 2 to 4 times greater than the loss that occurs in an immobilized individual without SCI ( ).

Other related factors may contribute to this bone loss, such as vitamin D deficiency and the use of methylprednisolone, anti-convulsant drugs, and psychotropic substances. Previous studies discovered that 14% to 32% of individuals with SCI were deficient in vitamin D ( ; ). Even in tropical countries, vitamin D deficiency is a common health problem ( ).

Fractures

The consequence of osteoporosis in individuals with SCI is fracture due to bone fragility. It usually occurs in transfers and activities with minimal or no trauma, such as low-speed falls, torsional stress ( ; ). Studies ( ; ; ) detected a specific time between SCI and fracture was 9 to 10 years, average age of first fracture for women 50 + and for men 40 + years-old ( ). Fractures may occur in 25% to 46% of these individuals over their lifetimes ( ; ; ). Fractures are primarily related to torsional forces during transfer, passive mobilization, compressive forces, and falls ( ; ; ). It seems the most frequent fracture cause is due to falls from a wheelchair or even their own height while walking or standing ( ). This result is not surprising because falls are a common health problem in individuals with SCI, and the prevention of falls should be a priority. Another study found transfer as the main cause of fracture ( ).

The risk of fracture after SCI is different from the risk of osteoporosis ( ; ), individuals with tetraplegia have more osteoporosis than those with paraplegia, however, individuals with paraplegia have a higher frequency of fractures due to exposure to falls ( ; ; ). Most places of fractures are in the tibia and/or distal fibula, distal femur, or hip/proximal femur and proximal tibia ( ; ; ). It seems hip/proximal femur and distal tibia/fibula are as common as the distal femur ( ). However, individuals who are ambulatory and wheelchair dependent may have different areas of fractures. Ambulatory individuals fracture more often the distal tibia/fibula, and wheelchair-bound patients fractured the distal and proximal femur ( ). A second (15%) and third (2%) fracture are experienced by patients ( ).

The impact of fractures on the lives of people with SCI suggests that most of the patients are hospitalized after the fracture, and some had complications. Almost one-third of the participants reported worse performance in activities of daily living and ambulation after the fracture had healed. Individuals with osteoporosis-related fractures after SCI had several complications. Most individuals were hospitalized, and half of them underwent surgery ( ). These issues mean increased disability and costs for the health system.

Risk factors

Previous studies showed risk factors ( ; ; ) and the following ones were described associated with increased fracture criteria: AIS (ASIA Impairment Scale) A or B; age 40 + years old; SCI longer than 3 years; age at SCI of 16 years or less; three servings of coffee per day; smoking; women; family history of osteoporotic fractures; low bone mineral density; low weight Bone Mass Index (BMI) < 19 kg/cm ² ; alcohol intake greater than 30 g/day; paraplegia, and the use of corticosteroids ( ). Studies by Craven developed protocols conducive to treatment based on these risk factors. Other aspects related to fractures are 25 hydroxyvitamin D [25(OH)D] levels less than 20 ng/mL ( ).

Diagnosis

Dual-energy X-ray absorptiometry

Dual-energy X-ray absorptiometry (DEXA) for patients with SCI is a confirmatory test to predict the future risk of fracture and also serves for ongoing monitoring of bone health and to access bone loss ( ) by the bone mineral density (BMD) measurements. It is precise and safe ( ) since there were strong associations among different observers and it emits a very low level of radiation (0.1 mSV), that is, 1/10–1/30 less radiation than a chest X-ray. DEXA calculates the measurement of bone mineral density in g/cm ² and protocols for calculating bone mineral density for patients with SCI already exist. Some studies suggested that low BMD in the hip predicted fracture in individuals with SCI ( ) and other authors suggested that DEXA measurement of the distal femur and proximal tibia, with specific protocols for the evaluation of these sites ( ; ; ) and for knee fracture thresholds of 0.78 g/cm ² which a fracture may start to occur and fracture breakpoint of 0.49 g/cm ² ( ; Fig. 1 ).

The measurement of BMD of the knees is extremely important for monitoring bone loss, as the hip and spine do not show the highest losses of BMD after SCI and because osteoporosis is most severe below the level of the injury ( ; ). It can be even more useful in those patients where assessing the spine and hip is not possible, such as cases with spine surgeries or hip heterotopic ossifications. The limitations for measuring BMD in the knees are fractures and orthopedic surgeries. In cases of flexion deformities, the assessment of bone mineral density can be performed in the lateral scans ( Fig. 2 ).