Adult & Pediatric Spine, The
3rd Edition

60
Reconstruction of the Osteoporotic Spine
Sigurd H. Berven
Serena S. Hu
Vertebral compression fractures are the most common clinical presentation of osteoporosis. These fractures may often be managed by percutaneous techniques of vertebral augmentation, including vertebroplasty and kyphoplasty, as discussed in Chapter 27. The patient with osteoporosis and progressive spinal deformity, instability, or insufficiency fractures with neurologic deficits cannot be managed effectively by techniques of percutaneous vertebral augmentation and may present a difficult challenge for reconstruction.
Instrumentation and internal fixation of the osteoporotic spine is measurably compromised by bone stock insufficiency. Strategies for management of deformity in the osteoporotic spine are now available and expand the ability of the surgeon to provide a stable and effective reconstruction. The purpose of this chapter is to identify osteoporosis as a significant comorbidity in the evaluation and treatment of patients who require spinal reconstruction, and to describe biomechanical and surgical problems associated with internal fixation of the osteoporotic spine. The information in this chapter is intended to guide decision making regarding preoperative evaluation and intraoperative management of the patient with osteoporosis requiring spinal reconstruction.
EPIDEMIOLOGY OF OSTEOPOROSIS
Osteoporosis is a skeletal disorder characterized by compromised bone strength leading to an increased risk of fracture (1). The pathophysiology of osteoporosis involves an uncoupling of the processes of bone resorption and bone formation, resulting in a microarchitectural deterioration of bony trabeculae and cortices (2,3 and 4). The definition of Albright and Reifenstein is descriptive, defining osteoporosis as the condition of having too little bone, but what bone there is, is normal (5). Osteoporosis is a systemic disease that is recognized as one of the major public health problems facing postmenopausal women and aging individuals of both genders (6). The World Health Organization has established diagnostic criteria for osteoporosis based on the results of dual x-ray absorptiometry (DEXA) scanning (7). Patients with a bone mineral density (BMD) between 1.0 and 2.5 standard deviations below peak BMD for gender are classified as having low bone mass. Patients with DEXA measurements less than 2.5 standard deviations below peak density for gender are classified as having osteoporosis (Fig. 1). Fractures and spinal deformity are important consequences of osteoporosis, and the lifetime prevalence of an insufficiency fracture is 40 to 75% in white women and 13% in white men (8). The prevalence of vertebral compression fractures may vary significantly depending on the criteria used to define a vertebral fracture; it also has been observed that less than one-third of vertebral compression fractures noted on radiographs come to medical attention (9). Osteoporosis of the spine is characterized by a high rate of nontraumatic vertebral fractures, disproportionate loss of trabecular bone within the vertebral bodies, and an association with spinal deformity and unstable scoliosis (10,11 and 12).
OSTEOPOROSIS AND SPINAL DEFORMITY
Osteoporosis has a significant impact on the etiology and clinical presentation of disorders of the spine. The association between BMD and spinal pathology is complex. Degenerative conditions of the spine, including facet arthropathy and intervertebral disc
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degeneration, appear to be associated with increased BMD in the axial and appendicular skeleton (13,14,15 and 16). In patients with more severe osteoporosis, however, facet arthropathy and disc degeneration may be correspondingly more severe (17). Spinal deformity with scoliosis and kyphosis is a characteristic feature of osteogenesis imperfecta, with most patients being affected (18). Deformity of the spine is influenced importantly by BMD, and the spectrum of deformity associated with osteoporosis is broad.
Fig. 1. Bone mineral density (BMD) changes with age, based on dual x-ray absorptiometry values.
Sagittal plane deformity resulting from vertebral compression fractures is the most characteristic manifestation of osteoporosis of the spine (19). The European Vertebral Osteoporosis Study Group described three types of vertebral deformity associated with osteoporotic compression fractures: crush, wedge, and biconcave (20). Crush and wedge fractures were most prevalent at the thoracic and thoracolumbar spine, whereas biconcave fractures were predominant at the lumbar spine. The prevalence of all types of osteoporotic compression fractures is higher in women than in men, and increases with age. A parallel epidemiologic pattern has also been demonstrated in a Japanese population (21). Prior back pain, loss of body height, and kyphotic deformity are the characteristic clinical presentation for vertebral compression fractures. The impact of compression fractures and progressive kyphosis on quality of life and on mortality has been well documented (22,23,24,25,26,27 and 28). BMD has a direct effect on the incidence of vertebral compression fractures (29,30,31 and 32). Patients with two or more vertebral compression fractures have a lower BMD than age-matched patients without fractures, and more severe fractures are observed in patients with lower bone density (33). When the incidence of vertebral fractures is adjusted for BMD, the gender differences observed in epidemiologic studies are eliminated, which denotes the primary importance of vertebral body strength rather than gender in determining fracture risk (34). In the child and adolescent, low BMD is directly associated with the development of kyphotic deformity. Children with idiopathic juvenile osteoporosis are characterized by kyphosis in all cases (35). Bradford has also demonstrated that osteoporosis is a significant factor in the development of Scheuermann’s kyphosis, further emphasizing a clear relationship between osteoporosis and sagittal plane deformity (36).
The relationship between other spinal deformities and osteoporosis has not been well defined. Adult scoliosis, rotatory subluxation of the spine, and spondylolisthesis are spinal deformities for which the causative relationship is not as well defined as it is with sagittal deformities. Several reviews of scoliosis have concluded there is no causal relationship between scoliosis and osteoporosis in the elderly population (37,38,39,40,41 and 42). However, others have identified an association between osteoporosis and deformity in the adult spine (43,44,45,46 and 47). Healey and Lane estimated a prevalence of structural scoliosis in osteoporotic women of nearly 50%, with a predominance of lumbar and thoracolumbar curves (48). Curves were generally small, with only 10% measuring greater than 30 degrees. They also observed a high percentage of osteoporotic patients with lateral subluxation and anterolisthesis. Fractures occurred within the curve patterns, but did not contribute to the coronal plane deformity. Healy and Lane concluded that scoliosis in elderly women may be a sensitive marker for osteoporosis.
The presence of rotatory subluxation has been identified as a predictor of scoliosis morbidity, and it is a feature of unstable scoliosis and accelerated rate of deformity progression (10). Velis et al. identified osteoporosis as a distinct characteristic of patients with unstable scoliosis but found no association with stable scoliosis. Patients with unstable scoliosis had femoral neck density determined by dual-photon absorptiometry techniques averaging 26 to 48% lower than age-matched controls, whereas patients with stable scoliosis had a similar BMD at the femoral neck. These studies indicate a direct relationship between osteoporosis and rotatory subluxation of lumbar vertebra and unstable scoliosis, with a less reliable relationship between osteoporosis and stable scoliosis.
Healey and Lane also observed a higher prevalence of spondylolisthesis within the lumbar spine in patients with osteoporosis. The Study of Osteoporotic Fractures demonstrates that lumbar olisthesis is associated with higher BMD at L4-5, and lower BMD at L-5 to S-1 (49,50). Tabrizi reported a case of acquired spondylolisthesis related to de novo elongation of the L-5 pars interarticularis reportedly owing to osteoporosis, and this is consistent with one pathologic mechanism of spondylolisthesis (51,52). Spondylolisthesis occurring during childhood and adolescence is unrelated to osteoporosis, however (53,54).
Osteoporosis appears to be a characteristic of adolescents with scoliosis, and the association between scoliosis and osteoporosis in adults may be a long-term result of a compromised peak BMD in affected adolescents. Peak skeletal mass in young adulthood is a major determinant of adult bone density (10). Adolescents with scoliosis have lower BMD than age-adjusted controls (10,55,56,57,58 and 59). This difference is not a simple effect of bracing (60). In a longitudinal follow-up study, Cheng et al. demonstrated that compromised BMD in adolescents persists over time, and may lead to low peak bone mass in adulthood (61). These data suggest the association of spinal deformity, and osteoporosis may be an effect of compromised peak BMD during adolescence. In addition, osteoporosis in adulthood may predispose to collapsing of the spine, progressive kyphotic deformity, and unstable scoliosis, as well as rotatory subluxation of the spine (10).
A final category of deformity associated with osteoporosis involves postsurgical deformity. The patient with osteoporosis may be susceptible to significant and progressive deformity after surgery on limited segments of the spine. Natelson identified the potential hazards of laminectomy and facetectomy in a small group of patients with osteoporosis and compression fractures in the area treated, demonstrating progressive instability, deformity, and “disastrous results” (62). Progressive deformity after decompression without internal fixation or adjacent to
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fused segments of the spine is an important contemporary cause of deformity in the cervical, thoracic, and lumbar spine, and common indication for surgical revision (63,64,65,66,67,68,69 and 70). Osteoporosis is a comorbidity in these cases that presents a difficult challenge for effective reconstruction.
PREOPERATIVE EVALUATION
The prevalence of osteoporosis in postmenopausal white women is estimated to be 16 to 30% (71,72 and 73). Prevalence in African-American men and women is lower but remains an important consideration in preoperative evaluation of patients. The measurement of osteoporosis requires a quantitative radiographic system. DEXA scanning is the standardized test based on the definitions of the World Health Organization. Other techniques for quantifying BMD include quantitative computed tomography (CT), nuclear scanning, radiographic absorptiometry, and ultrasonography (74). Plain radiographs or CT scans without quantification have poor sensitivity and specificity for the evaluation of osteoporosis (75). Although osteoporosis is a systemic condition, DEXA scan results may vary significantly depending on the anatomic site of measurement. DEXA measurements may be made centrally at the spine or hip, or peripherally at the calcaneus or distal or proximal radius. Measurement of BMD in the spine is limited by concurrent conditions, including calcification of the aorta, sclerosis of the vertebral end-plates, calcification of the intervertebral discs, osteophyte formation, and facet hypertrophy (76,77,78 and 79). Similarly, the accuracy of DEXA measurements on the lateral view of the spine is compromised by the superimposition of ribs, iliac crest, and, in scoliosis, overlap of vertebral bodies (80). Measurement at the spine and hip increases the accuracy and the precision of the DEXA measurement (74). The use of bone densitometry is significantly limited in children and adolescents who have not reached peak BMD (81). Anticipation of compromised BMD in chronically ill children and in children with compromised mobility and activity is important in planning surgical intervention.
It is also important to determine if osteoporosis is primary or secondary. Some risk factors may be modified and others cannot (3). Idiopathic, or primary osteoporosis, accounts for more than 95% of the cases of osteoporosis in the elderly population (2,4,82). The Study of Osteoporotic Fractures identified important risk factors for osteoporotic insufficiency fractures including postmenopausal white women older than 50 years; history of a fracture after age 40 years; history of a fracture of the hip, wrist, or spine in a first-degree relative; and current cigarette smoking (83).
Primary osteoporosis also is clearly related to a genetic predisposition and factors that are not modifiable, including white race, poor health, and comorbidities (84). Secondary osteoporosis is caused by pathologies that may be modifiable, including endocrine disturbances (e.g., hyperparathyroidism, hyperthyroidism, hypercortisolism, estrogen withdrawal); renal failure; and drug use, including heparin, prednisone, alcohol, cigarette smoking, and excessive vitamin A. Patients with secondary osteoporosis may have a measurable improvement in BMD with treatment (85–88.) Because their condition may be improved, the identification of secondary osteoporosis is of value in the preoperative evaluation and treatment of the patient who is being considered for spinal reconstruction.
The value of preoperative treatment of the patient with osteoporosis with antiresorptive agents has not been demonstrated to reduce rates of hardware failure or improve rates of arthrodesis. Antiresorptive agents approved for the treatment of osteoporosis include bisphosphonates, calcitonin, estrogen, and selective estrogen receptor modulators. Bae et al. demonstrated that alendronate positively affects the process of spinal fusion at low doses in a rabbit model, although this effect is inhibitory at higher doses (89). In fracture healing models, bisphosphonates have been shown to have an inhibitory effect on callus remodeling and the process of healing (90). However, these effects may not compromise the overall strength and mechanical properties of the healed bone (91,92 and 93). In a clinical trial of clodronate of Colles’ fractures, Adolphson et al. demonstrated increased BMD in patients treated compared with placebo (94). Intermittent parathyroid hormone has a potent anabolic effect in the treatment of osteoporosis (95). Animal studies have demonstrated that intermittent parathyroid hormone may have a positive effect on fracture healing by enhancing callus formation, increasing production of bone matrix proteins, and enhancing osteoclastogenesis during the phase of callus remodeling (96,97). The resultant effect is an increase in callus mechanical strength. In a study of screw fixation strength and intermittent parathyroid hormone administration in rats, Skripitz et al. demonstrated enhanced strength of the bone-implant interface with the treated group (98). Despite these data, the role of pharmacologic therapies in the perioperative management of patients with osteoporosis remains to be established.
SURGICAL STRATEGIES: INTERNAL FIXATION AND THE OSTEOPOROTIC SPINE
Fractures in patients with osteoporosis most commonly involve the spine. Other areas commonly affected by insufficiency fractures include the peritrochanteric region, pelvis, and distal radius (9). Osteoporosis is recognized as an important risk factor for the etiology of these fractures, and a significant factor in compromising the stability of internal fixation of the skeleton (99,100,101,102,103,104,105,106 and 107). In a study of hip fractures, osteoporosis was identified as an independent predictor of nonunion and implant failure, regardless of the implant used (108). Similarly, osteoporosis is a predictor of implant failure in the spine. Complications of internal fixation of the osteoporotic spine are well known, and include implant pullout and failure, adjacent segment fractures, nonunion with secondary implant fracture, and progressive deformity (109,110 and 111). Therefore, establishing stable internal fixation in osteoporotic bone is a challenge in many areas of skeletal reconstruction.
In the spine, implants serve to provide stability where there is a fracture, deformity, or intrinsically unstable biologic environment. The use of implants in the spine for the correction and maintenance of correction of spinal deformity was introduced by Paul Harrington. He studied the anatomy of the spine and the pathology of scoliosis in approximately 3,000 patients with poliomyelitis and began using an implantable device in 1949 (112). The original Harrington device included a prestressed distraction rod using a ratchet system and hooks, a threaded compression rod with hooks, and a sacral bar for pelvic fixation. The device was designed to achieve correction through distraction of the concavity and compression of the convexity of the curve. He proposed using the device with or without a Hibbs midline fusion to supplement the instrumentation. Harrington recognized that different
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portions of the vertebra had variable strength as points of fixation for distraction and compression hooks, varying from 5 lb of force application before fracture at the tip of the transverse process to 300 lb or greater at the sublaminar region and pars interarticularis. He reported instrument failure in 58% of his early cases, and instrument dislocation in up to 10% of cases. In 1973, Eduardo Luque recognized an unacceptably high rate of complications using Harrington spinal instrumentation in “soft, young, postpoliomyelitic bone” (113). This concern led him to introduce a method of spinal segmental instrumentation using sublaminar wires in addition to hook fixation, recognizing that “the more points utilized to apply corrective forces, the less corrective force was required at a given point.” The Luque method of segmental instrumentation provides a transverse force at every vertebra by applying a moment arm of correction in a plane that is perpendicular to the plane of the Harrington distraction/compression system (114). A direct comparison of the two methods demonstrated a failure pattern at the metal-bone interface in the Harrington system, and this did not occur with segmental fixation (115).
Fig. 2 Depiction of a photomicrograph of bone mineral density in normal and osteoporotic bone.
The use of transpedicular fixation in the spine was first reported by Boucher in 1959 (116). He described the use of long screws through the facet joints and into the pedicles as a technique to avoid an unacceptably high incidence of failure in lumbar spine fusion. Roy-Camille introduced the use of metal plates with transpedicular fixation to gain segmental fixation in spinal fusions (117). The screw and plate construct extended the use of segmental fixation to applications including treatment of fractures, malunions, and tumors, as well as deformity (118).
Contemporary instrumentation systems for the management of spinal deformity may involve combinations of fixation strategies, using devices to maximize strength of fixation at the bone-metal interface, and to maximize corrective power of the implant on the spine. A clear benefit for the use of transpedicular segmental fixation compared with wires and hooks remains to be demonstrated in clinical outcomes (119). McAfee et al. demonstrated that successful arthrodesis of the spine is enhanced by rigid internal fixation. However, they also observed that rigid internal fixation of the spine also led to device-related osteoporosis (i.e., stress shielding) of the vertebra. They concluded the improved mechanical properties of spinal instrumentation on spinal arthrodesis more than compensates for the occurrence of device-related osteoporosis in the spine. The mechanical properties of fixation in the osteoporotic spine depend importantly on the relationship between the bone and the implant.
OSTEOPOROSIS AND THE BONE-IMPLANT INTERFACE
In the osteoporotic spine, it is particularly important to recognize the relationship between implant position and performance relative to BMD. BMD is directly related to the mechanical properties of the surgical construct, and it has measurable effects on insertional torque and pullout strength of the implant (120). The relationship between load to failure of a bone-implant construct and BMD has been studied for thoracolumbar implants by Coe et al., and this work lends important insight into the use of various implants in fixation of the osteoporotic spine (121). Those authors demonstrated transpedicular screws and spinous process wires had a load to failure when subjected to posteriorly directed tensile forces that correlated with BMD. In contrast, laminar hooks demonstrated a load to failure that was significantly higher than pedicle screws or spinous process wires, and this load to failure was not compromised by diminished BMD. The authors concluded that the relative strength of laminar fixation compared with pedicle screw fixation in the osteoporotic spine is related to the reduced rate of bone turnover in cortical bone compared with cancellous bone.
In addition to BMD, trabecular orientation is an important factor in the strength of the bone-implant construct (122). The structural characteristics of the osteoporotic spine differ significantly from those of the normal spine, accounting for important difference in properties of internal fixation. The overall strength and stiffness of an individual vertebral body is defined mainly by trabecular bone. Osteoporosis greatly affects trabecular bone, as the large surface area exposed leads to increased bone turnover (84). With a decrease in bone mass comes a decrease in the size and the number of trabeculae (Fig. 2). In addition, osteoporosis causes a decreased connectivity of the trabecular bone. Connectivity refers to the number of connections between individual trabeculae, which creates a lattice-like structure. In normal bone, the microstructure resembles a plate, whereas osteoporotic bone resembles a collection of narrow, unconnected bars. These unconnected bars are much weaker in axial compression and shear, resulting in a decrease in vertebral body strength in compression (123).
The apparent density of bone is defined as mass per bulk volume and is an indicator of the porosity of a structure. Carter and
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Hayes modeled bone as a two-phase porous structure, and they determined that in normal and osteoporotic bone, the compressive strength of bone (trabecular or cortical) is proportional to the square of the apparent density, and the compressive modulus of elasticity is proportional to the cube of the apparent density (124). Hirano et al. examined the regional BMD of the pedicle measured by peripheral quantitative CT in normal and osteoporotic cadaveric specimens (125). They found that the BMD increased from the inner core of trabecular bone to the outer cortical shell in all specimens. The osteoporotic specimens had decreased BMD in all layers, and their cortices were significantly thinner. Those investigators also found that the pedicle rather than the vertebral body is responsible for 80% of the bone-screw interface stiffness, and 60% of the pullout strength of 6.25-mm diameter pedicle screws.
Characteristics of the implant may have an important effect on the bone-screw interface in a spinal construct. The integrity of the bone-screw interface is key to the maintenance of rigidity in spinal instrumentation systems. Factors that influence the bone-screw interface include the geometry of the screw, the elastic modulus of the bone, and the “quality of fit” of the screw. The interface is compromised in the osteoporotic patient, leading to micromotion and subsequent loss of fixation. The mechanism of screw failure is usually a combination of toggling in the sagittal plane and axial pullout (126,127,128 and 129). Zindrick et al. showed that when screws fail in the osteoporotic spine, they do so by toggling in the sagittal plane. This toggling can be minimized by contact with the cortical bone of the pedicle. The BMD at the tip of the screw within the vertebral body was found to affect the toggling less than the pedicle contact, owing to the fact that the motion occurs about the isthmus of the pedicle. The same study also noted that toggling was minimized by purchase of the anterior cortex of the vertebral body, mainly because the fulcrum then moves anteriorly. The associated risks to the anterior vascular and visceral structures are significant, however, and this technique is not recommended except at the sacrum below the vascular bifurcation.
The geometry of the screw has an important influence on the strength of the bone-screw interface. Pedicle screws have been designed to optimize the bone-screw interface. Factors such as length of threaded portion, ratio of inner to outer diameter (i.e., thread depth), and pitch greatly affect the pullout strength. DeCoster et al. looked at various screw design parameters and their effects on pullout strength (130). They found that pullout strength was linearly related to major diameter; it was also related to the ratio of major to minor diameter, although the increase was not as great. An increase in pitch also increased pullout strength, in some cases to a greater extent than an increase in major diameter. To maximize pullout strength and minimize the risk of cutout and pedicle fracture, pedicle screw major diameter should be 70 to 80% of the pedicle diameter (131). However, screws larger than 80% of the outer diameter (medial/lateral) of the pedicle may increase the risk of pedicle fracture (131,132). Cook et al. compared the pullout strengths of self-tapping pedicle screws with an expansile type of pedicle screw. The latter has a fin on the anterior tip that increases the screw diameter by 2 mm, much like drywall bolts. Pullout strengths in normal and low BMD groups were increased significantly, by 30% and 50%, respectively.
Pullout strength of pedicle screws has been found to be inversely proportional to BMD by a number of authors (126,132,133,134 and 135). Augmentation of osteoporotic bone with polymethylmethacrylate (PMMA) enhances the effective modulus of elasticity of bone and the apparent density of the bone surrounding the screw. PMMA augmentation has been shown to increase pullout strength of pedicle screws between 49% and 162% compared with nonaugmented pedicle screws. Zindrick et al. reported that pressurization of cement increases the pullout strength by 96%, compared with nonpressurized cement, which restored mechanical pullout to levels for normal bone (129).
The use of PMMA in the spine does have significant potential drawbacks. PMMA polymerization is an exothermic reaction, which can lead to local necrosis; it gives off toxic monomers on polymerization; it is not resorbed, leading to potential problems with further bone loss in revision spinal surgery; and extravasation into the spinal canal and the vascular system has been reported (136a and 136c). Hydroxyapatite composite resin cements and bioresorbable polymers may be a substitute for PMMA in the spine. Although most of the materials investigated have mechanical properties that are inferior to those of PMMA, the material properties are certainly superior to those of osteoporotic bone. These substances have, in general, enhanced the bone-screw interface as manifested by pullout strength, albeit not as much as PMMA. However, they have the advantage of not causing local toxicity by heat or chemicals, and they are resorbed with time, avoiding the problems caused by cement removal. The property of bioresorbability also introduces the potential complication of late loosening of fixation. In vivo studies are in progress with some of these materials in animal and human models. For example, Lotz et al. examined screws augmented with carbonated apatite in vitro in a human cadaveric model (128). They found an increase in pullout strength of 68%, and the peak pullout strength was linearly related to BMD. They then looked at a rigid beam on elastic foundation analysis, and found that the cement increased stiffness by 30% within the vertebral body and 74% within the pedicle. Goodman et al. used Norian Skeletal Repair System (SRS) to augment screws in the femur and found higher load to failure and less screw sliding when cement was used (137). Heini et al. performed vertebroplasty in a cadaveric model with PMMA and brushite (CaPO4) cement (138). The PMMA significantly increased stiffness in osteoporotics only, whereas the brushite increased stiffness in all specimens by 120%. Both substances significantly increased the loads to failure. They did caution that overstiffening may lead to adjacent segment fracture and did note cement leakage with high volumes. Eriksson et al. tested PMMA versus Norian SRS in foam blocks of different densities (139). Femoral neck implants were pulled out of the augmented blocks. They found that PMMA significantly increased pullout strength for all densities; the SRS increase was less pronounced and most obvious in the low-density blocks. They also noted that the PMMA failed at the cement-bone interface, whereas the SRS failed at the screw-cement interface. Ignatius et al. compared a bioabsorbable polymer versus PMMA in bovine vertebral bodies and human femur (140). They found that both substances significantly increased pullout strength as well as insertion torque and found that polymer augmentation was most effective in low-density specimens. They also demonstrated a linear correlation between pullout strength and BMD, but only in the nonaugmented specimens, indicating that augmentation decreased the adverse effects of osteoporosis. Moore et al. compared PMMA
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to CaPO4 cement augmentation in human cadaveric vertebrae (141). Both substances significantly increased pullout strengths of various screw types (147% for PMMA, 102% for CaPO4). Wittenberg et al. compared PMMA and polyglycol fumarate augmentation in human and bovine vertebra (128). They found that both substances increased pullout strength significantly, but that only PMMA significantly increased transverse bending stiffness. Yerby et al. used hydroxyapatite cement to rescue failed pedicle screws in human lumbar vertebrae (142). They found that augmentation of 6.0-mm and 7.0-mm failed screws increased pullout strength more than 300%. In a clinical review of experience using calcium apatite cement to augment anteriorly-placed vertebral screws and posteriorly placed pedicle screws, Wuisman et al. reported cement leakage in four of forty-eight augmented dorsal screws, with no complications reported (143). Jang et al. demonstrated stronger stabilization and facilitation of short segment instrumentation using PMMA augmentation for fixation in metastatic disease (144). Metastatic involvement of the vertebral column presents challenges that are similar to osteoporosis (Fig. 3) Overall, augmentation of osteoporotic bone with a bone filler material significantly increases the rigidity of the bone-screw interface, and improves the efficacy of instrumentation in the osteoporotic spine. The long-term effect of implant loosening between PMMA and a resorbable material remains to be demonstrated in clinical study.
Fig. 3 Thirty-eight-year-old man with multiple myeloma and vertebral column insufficiency. Reconstruction of the spine involved posterior segmental fixation with vertebral augmentation using polymethylmethacrylate at each level of the fusion.
The “quality of fit” is a final important consideration in optimization of the bone-screw interface. The “strength” of a pedicle screw lies in its ability to transfer stresses to the bony
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shell of the pedicle, which is cortical bone and able to bear higher loads than trabecular bone. Therefore, a large-diameter pedicle screw should increase the stability of the bone-screw interface. Fatigue failure (cycles to failure) of an individual pedicle screw is related to the ratio of the screw diameter to the pedicle diameter, again implicating the importance of the “quality of fit” (131). In the osteoporotic patient, the pedicle is affected as well as the trabecular bone of the vertebral body, although to a lesser degree (84,127). The BMD of the entire pedicle (i.e., cortical and subcortical bone) is decreased, increasing the risk of pedicle fracture or screw cutout during track preparation and screw insertion. Modified methods of screw track preparation and screw insertion may be useful in the osteoporotic spine. Untapped and undertapped tracks have been shown to enhance pullout strengths in vitro (132,139,145). Halvorson et al. looked at different insertion techniques as well as supplemental fixation techniques in a human cadaveric model, and found that in osteoporotic specimens, not tapping and undertapping increased the pullout strength for 6.5-mm diameter screws. It is interesting that no significant difference was noted with these techniques in spines with normal BMD. Enhancing the amount of bone between the screws by triangulating has also been shown to be effective in stabilizing the bone-screw interface. Barber and colleagues tested screw configurations of varying degrees of convergence and found that 30 degrees of convergence resulted in significantly increased resistance to pullout and sustained higher loads than parallel screws (146) (Fig. 4). Rudland et al. compared the pullout strengths of a variety of instrumentation configurations: laminar hooks, single screws, and triangulated screws connected via a transverse plate (147). They found that the pullout strengths of the triangulated screws were significantly higher than any other group, and postulated that the mass of bone between the screws was more important than the amount of bone in contact with the screw threads.
Fig. 4. Drawing of convergent versus parallel pedicle screws.
OTHER STRATEGIES
The use of laminar hooks or interbody cages in combination with transpedicular fixation may significantly improve construct rigidity. Halvorson et al. demonstrated that the addition of two laminar hooks, one at the level of and one above the pedicle screw, serves to stabilize the construct and minimize micromotion at the bone-screw interface (132). Hasegawa and colleagues examined the fixation of a screw alone, and one coupled with a same-level laminar hook in a nondestructive manner (148). They found that the hook significantly increased the stiffness of the bone-screw interface. In addition, they found similar improvements in the normal BMD and low BMD groups, indicating that supplemental fixation should be beneficial in all patients. At the cephalad extent of a long fusion, the upper thoracic spine presents a challenge for adequate fixation. Hooks, wires, pedicle screws, or combinations may be used effectively. Butler et al. demonstrated that a pedicle hook claw is superior to sublaminar wires alone in strength to failure using osteoporotic spine (149). Hilibrand et al. demonstrated that a supralaminar hook may restore the fixation strength when added to a compromised pedicle screw (150). Figure 5 demonstrates the case of a 69-year-old woman with hook failure at the cephalad portion of a long fusion. The patient was revised with a combination construct using pedicle screws and wires with good result.
The relationship between osteoporosis and pseudarthrosis has not been defined, and osteoporosis is not identified as an independent risk factor for pseudarthrosis (151,152). Osteoprogenitor cells from osteoporotic bone marrow are less in number, however, and function with a reduced capacity compared with cells derived from normal bone marrow (153,154,155,156 and 157). Rodriguez et al. demonstrated that mesenchymal stem cells from osteoporotic women have a lower growth rate than control marrow stem cells, respond to growth signals less potently, and have a compromised capacity to differentiate into an osteoblastic lineage (158). These data may provide a physiologic basis for compromised bone formation in the osteoporotic spine.
Stable arthrodesis is more reliable with the use of combined anterior and posterior surgery (159,160 and 161). Hasegawa et al. studied the biomechanics of anterior interbody devices in a cadaveric model (148). The authors demonstrated that larger cages transferred load borne by the body to the cortical shell, sparing the weaker cancellous bone, and resulting in higher maximum loads before fracture. Jost and colleagues examined the compressive strengths and load/displacement curves of a variety of interbody fusion cages, and noted that low BMD specimens failed at lower loads and exhibited less resistance to compressive loading compared with normal BMD specimens (162).
The addition of anterior column support clearly contributes to the overall stability of a spinal reconstruction instrumentation strategy. Circumferential fusion of the spine is also useful in improving the stability of fixation in the osteoporotic spine in the perioperative period and in the late postoperative period by reducing the rate of pseudarthrosis. Anterior column support also introduces risk of significant morbidity as discussed later, and this risk must be balanced against the benefits in determining an optimal surgical strategy. Osteoporosis has a measurable effect on the efficacy of implants in the anterior and posterior columns of the spine. The addition of laminar and anterior interbody fixation improves the overall rigidity of the construct in osteoporotic bone.
In summary, internal fixation of the osteoporotic spine is a difficult surgical challenge. Techniques for optimizing construct rigidity include implant combinations with multiple sites of fixation, screw augmentation, and circumferential arthrodesis. Sublaminar hooks and wires have mechanical characteristics, including pullout strength, that are relatively preserved even in the presence of osteoporosis.
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However, in the patient with previous laminectomy and in the lumbar spine, pedicle screws with augmentation may provide more reliable fixation. The goals of deformity correction in the osteoporotic spine may be less than those in a patient with normal BMD, as compromise of the bone-implant interface may prevent the safe use of some corrective maneuvers.
Fig. 5 Failure of fixation at the cephalad end of the construct. Revision with transpedicular fixation and sublaminar wiring.
OUTCOMES AND COMPLICATIONS OF SURGERY ON THE OSTEOPOROTIC SPINE
Surgical strategies in the spinal reconstructive surgery require attention to the physiologic status and comorbidities of the patient as well as to the biomechanics of internal fixation. Implant failure, adjacent segment decompensation, and complications related to medical comorbidities in the elderly or functionally compromised patient population are of particular concern in treating patients with osteoporosis. Implant failure may be avoided by optimization of the rigidity of internal fixation, as outlined previously. Progressive kyphosis above or below a region of fixation is an important consideration in the osteoporotic spine, and attention to fusion of the spine above the area of maximal kyphosis in the thoracic spine or into the area of lordosis in the lumbar spine protects the spine from progressive deformity in the adjacent segments. Termination of the implant construct with a rigid claw construct or screw fixation rather than wires prevents settling of the end vertebrae and progressive kyphosis. The addition of anterior column reconstruction clearly contributes to the stability of a spinal column reconstruction. However, complication rates for combined anterior and posterior surgery are clearly higher than posterior-only surgery (163,164,165 and 166).
The rate of complications in surgery on the spine is age dependent, and this further contributes to complex considerations in choosing a strategy for reconstruction of the osteoporotic spine. In an age-related assessment of outcome in spinal deformity surgery, Takahashi et al. reported significantly less correction of deformity and inadequate restoration of lumbar lordosis in patients older than 50 years (167). The overall complication rate was 29%, and was unrelated to age. The authors identified factors that contribute to potential complications and surgical difficulty in elderly patients, including curve rigidity, osteoporosis, and comorbidities. In a study of perioperative complications after anterior spine surgery for deformity, McDonnell et al. reported that patients between 61 and 80 years of age had a significantly higher rate of perioperative complications than younger cohorts, with pulmonary complications identified as the most common (168). There also is an increased risk in the elderly for cardiac morbidity and pancreatitis in the postoperative period (169,170). Age and comorbidities were identified as independent predictors of hospital stay, operative time, and intraoperative blood loss in patients undergoing lumbar decompression with segmental instrumentation (171). Kostuik et al. identified age and preoperative American Society of Anesthesiologists score as important predictors of minor and major complications in adult spinal deformity surgery, reporting an overall complication rate of 57% and major complications in 27% of patients. They concluded that age is an important preoperative consideration for surgical planning and avoidance of complications (172).
Given all of the surgical challenges of complex rigid deformities, osteoporosis, and an increased risk for intraoperative and perioperative complications, the surgeon may consider options including decompression alone or in situ arthrodesis. Nonoperative options, including activity modification and physical therapy to improve endurance, may be shared with the patient as alternatives to reconstruction of the spine. Sharing knowledge of complication rates and risks of surgical intervention empowers the patient to participate in an informed choice regarding his
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or her care, and contributes to the patient’s overall satisfaction with management.
When a patient presents with deformity, vertebral body fracture, pain, or neurologic involvement, the physician must decide if there are any risk factors for osteoporosis. If yes, the condition should be confirmed by DEXA and secondary causes evaluated by the appropriate laboratory tests. If a secondary cause is confirmed, it should probably be treated radically.
Once the basic spine problem is evaluated (e.g., deformity, fracture, pain), nonoperative treatment should first be attempted. If symptoms continue, surgical assessment should proceed including an assessment of comorbidities.
If surgery is felt to be possible, and instrumentation is necessary, several strategies should be followed. For pedicle screws, 70 to 80% of the pedicle should be filled by the screw’“best fit.” The vertebral body should be penetrated without going through the anterior cortex, and the screws should not be tapped. If secure fixation cannot be obtained, cement should be considered.
CONCLUSION
Spinal reconstruction in the setting of osteoporosis is an important and contemporary topic for the spinal deformity physician. The mean age of the population is increasing in nearly all industrialized countries. In the year 2000, 12.6% of the U.S. population was older than 65 years (173). By 2030, 20% of the U.S. population will be older than 65 years. In correlation with the increase in life expectancy and increased population age, the activity level and health care expectations of older people may increase (174,175). The mission of health care providers in contributing to the health of the population includes consideration of physical, mental, and social well being (176). Reconstruction of the spine is an important intervention for the population of patients with spinal instability, progressive deformity, and pain related to vertebral column insufficiency. The development of safe and effective techniques for reconstruction of the osteoporotic spine offers the spinal deformity physician the opportunity to contribute to health in this growing patient population.
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