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Using Imaging to Predict Bone Strength in Spinal Cord Patients
B Jenny Kiratli, PhD; Theresa Nauenberg, MS; Inder Perkash, MD
Introduction - Following spinal cord injury (SCI) resulting in paralysis, reduction in bone mass occurs below the level of the lesion. Increased incidence of long bone fractures also occurs with chronic paralysis, often resulting from minimal trauma such as stretching the muscles of the thigh. Most of these fractures occur in the femoral shaft and supracondylar region (above the knee). The objective of this project is to enhance the available assessment technology for prediction of risk for fracture, specific to lifestyle and clinical problems of the SCI population.
Approach - The state-of-the art method assesses `bone mineral density' (BMD) in the hip region. This is an accurate measurement of the total amount of bone in the region measured. However, this may be an inadequate tool for use in the SCI population because (i) fractures commonly occur in other regions of the leg than the hip and (ii) bone strength is not completely defined by the amount of bone in a given region. Bone integrity (i.e., resistance to fracture) depends on both structural and material properties. Although measured BMD is conventionally used to indicate bone strength and is often used clinically for estimation of fracture risk, consideration of geometric properties is expected to enhance the estimation of strength from bone mineral analysis
Methods - Bone mineral density was measured at the hip and at two distal sites of the femur (midshaft and 1/3 distal)(Figure 1) by the standard technology (dual energy x-ray absorptiometry). Concurrent radiographs were generated during these scans, and information about the geometry of the bone was obtained from these images. Calculations were then made of the combination of geometric and bone mass data to derive a "strength index" (SI). Validation of SI was accomplished on cadaver femurs in which equivalent measurements were made followed by mechanical testing. The predicted SI was highly correlated with measured breaking strength, indicating that our method was valid.
Figure 1. Bone mineral density was measured at the hip and at two distal
sites of the femur: midshaft and 1/3 distal.
Bone mineral measurements were then made on three subject groups: able- bodied controls, SCI individuals with no history of fractures, and SCI individuals with history of non-traumatic lower extremity fracture. The ages, weights, and femoral dimensions were similar between groups.
Findings - As expected, bone mineral values throughout the femur are significantly lower in SCI subjects compared with controls and further reduced in those SCI individuals with lower extremity fractures. This is consistent with previous studies in which only bone mass at the hip was measured. However, there was little difference in calculated strength between the SCI individuals with and without fractures. This result may indicate that most SCI individuals who have lost bone mass are, in fact, at increased risk for fracture but have not yet fractured.
Clinical Relevance - Better assessment of fracture risk in the SCI population is essential as this population survives to older ages and loses bone because of both immobilization and aging. An improved measurement tool will also facilitate monitoring of treatments and healing once a fracture has occurred as well as outcomes of intervention modalities. In addition to the bone strength assessment, we are currently working on an improved evaluation of physical stresses to which the body is subjected on a daily basis. With such information, we expect to be able to better understand why certain fractures occur, and thus further understand physical means to avoid them. Greater knowledge of the combination of applied physical stresses during specific activities and bone strength will allow more individualized prediction of risk and prevention of fractures.
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact
Jenny Kiratli.
