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Young Investigator Award 1999 Runner-Up

1999 Young Investigator Research Award Runner-Up: Effect of Augmentation on the Mechanics of Vertebral Wedge Fractures

Wilson, David R. DPhil*; Myers, Elizabeth R. PhD*; Mathis, John M. MD†; Scribner, Robert M. BSc‡; Conta, Jonathan A. BSc*; Reiley, Mark A. MD§; Talmadge, Karen D. PhD‡; Hayes, Wilson C. PhD*

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Abstract

Study Design. The effect of cement augmentation of wedge-fractured vertebral bodies on spine segment compliance was studied in 16 cadaver specimens.

Objectives. 1) To assess the mechanical effects of cement augmentation of vertebral wedge fractures. 2) To determine whether a new reduction/injection procedure has the same mechanical effects as the established direct injection procedure.

Summary of Background Data. Although wedge fractures cause pain and disability in hundreds of thousands of people, few effective treatments are available. Clinical studies have shown that cement augmentation, a new procedure, effectively relieves pain and restores mobility in patients suffering from weak or fractured vertebrae. However, only a few studies have examined the mechanics of vertebral augmentation.

Methods. A wedge fracture was created in the middle vertebra of 16 three-vertebra cadaver spine segments. Neutral and full-load compliance of each fractured spine segment in flexion/extension and lateral bending were assessed by measuring the relative rotation of the vertebral bodies in response to applied moments. Eight of the fractured vertebral bodies were then augmented using direct injection, while the remaining eight fractured vertebral bodies were augmented using a combined reduction/injection procedure. Compliance of the augmented segments was then assessed.

Results. Augmentation significantly reduced the neutral compliance (reduction of 25% ± 23%) (mean ± standard deviation) and the full-load compliance (reduction of 23% ± 20%) in flexion/extension (P < 0.005). Augmentation also significantly reduced the neutral compliance (reduction of 34% ± 20%) and the full-load compliance (reduction of 26% ± 17%) in lateral bending (P < 0.0001). No significant difference was found between the two procedures for compliance reduction.

Conclusions. Augmentation of wedge fractures using both direct injection and reduction/injection reduces spine segment compliance significantly.

Vertebral compression fractures affect about 500,000 individuals annually in the U.S. 4 Compression fractures are characterized by a loss of height in the anterior, posterior, or central region of the vertebral body that is usually evident on a lateral or anterior/posterior (A/P) radiograph. 15 These fractures occur when the load transmitted by a vertebra exceeds its failure load. 21 It is widely accepted that the increased incidence of spinal wedge fractures in the elderly is directly associated with osteoporotic weakening of the bone that leaves it unable to support the loads of daily living. Recent studies estimate the prevalence of vertebral fractures in postmenopausal women to be 25%, and suggest that the prevalence of fractures in men may approach that in women of the same age group. 18 Fracture incidence rises dramatically with age, 18,19 with at least one fracture being the rule rather than the exception in octogenerians. The primary complication is acute pain: 84% of patients with radiographic evidence of a compression fracture reported associated back pain. 4 Other complications include kyphotic deformity, transient ileus or urinary retention, and, rarely, cord compression. 15 The pain, discomfort, and deformity associated with compression fractures often lead to significant physical, psychological, and functional impairments and frequently have a substantial impact on quality of life. 9,16

Few effective, low-risk interventions are available for the treatment of vertebral compression fractures. Most research efforts have addressed the prevention of the underlying conditions that lead to compression fractures rather than their direct treatment. Attempts to reduce the risk of fracture have involved therapeutic strategies for strengthening bone including exercise, vitamin, and mineral supplements and antiresorptive agents such as estrogen, calcitonin, and bisphosphonates. 14 Once the fracture has occurred, however, treatment options are limited. Physical therapy, including strengthening of the paraspinal musculature and patient training to reduce the risk of fracture and pain during specific activities, is often recommended. 9,26 Surgery, which is complicated by osteoporosis, 10 is not currently indicated unless neurologic function is at risk or the deformity is substantial. 1 Bracing and analgesics may relieve pain but because immobilization can lead to further bone loss, bracing may actually increase the risk of further fractures. 2 Many compression fractures currently go untreated due to the limitations of current approaches.

Cement augmentation of the vertebral body, a relatively new procedure, has been shown to relieve the pain and loss of mobility associated with weak and fractured vertebrae. 5,6,7,8,11,12,28 The procedure, sometimes known as vertebroplasty, has been used in France since 1984 and was first performed in the U.S. in 1995. 11 In this technique, acrylic cement is injected under pressure through a needle placed percutaneously through the pedicle into the anterior vertebral body. Several different vertebral augmentation techniques have been developed and shown to reduce pain when used to treat vertebral angiomas, 7,8 metastases, 5,7,8,12,28 and compression fractures due to osteoporotic weakening. 6,7,8,11 Pain relief was found to be sustained, although most studies report mean follow-up times shorter than 2 years. Augmentation was found to prevent further collapse of the treated vertebral bodies. 5 Treatment of as many as seven vertebrae in a single individual has been reported. 17

It has been suggested that augmentation relieves pain by reducing the relative motion between fractured fragments of bone. 12 However, few studies have examined the effect of augmentation on vertebral mechanics. Mermelstein et al20 showed that cement augmentation of thoracolumbar burst fractures stabilized with short-segment pedicle screw instrumentation increased spine segment stiffness in flexion/extension. However, this study did not examine the effect of augmentation on the uninstrumented segment. Galibert and Deramond showed that augmentation increased the axial stiffness of a small number of cadaver vertebral bodies. 7 Bostrom and Lane and Schildhauer et al found that augmentation increased the compressive force required to collapse cadaver vertebral bodies. 1,25 These studies are limited because only compressive loads were studied. For a more complete mechanical assessment of augmentation, relative motion of spine segment vertebrae in response to applied “pure” anterior/posterior and lateral bending moments must be determined.

A new method for augmenting fractured vertebrae, which includes a fracture reduction step, is emerging. In this reduction/injection technique, 1 an inflatable bone tamp (Kyphon Inc., Santa Clara, CA) is introduced percutaneously into the fractured vertebral body and expanded in an effort to compress cancellous bone to create a void. The tamp is then removed and the vertebral body is filled with acrylic cement. One potential advantage of this approach is that the cement can be introduced into the vertebral body at a lower pressure than it would be during direct injection, which may reduce the risk of cement leakage. However, it is not clear whether this procedure will have the same mechanical effect as direct injection.

The aim of the current study was to assess the mechanical effect of cement augmentation using both direct injection and reduction/injection on the fractured vertebral body. Compliance, the rotation of one vertebra in the segment relative to another in response to an applied load, was determined and compared to address two research questions:

1. In thoracolumbar spine segments in which a wedge fracture has been created, does augmentation of the fractured vertebral body with polymethylmethacrylate (PMMA) cement reduce segment compliance?

2. Does the reduction/injection technique of vertebral body augmentation reduce spine segment compliance as effectively as the direct injection technique?

These questions were studied by performing biomechanical tests on cadaver spine segments.

© 2000 Lippincott Williams & Wilkins, Inc.

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