Osteoporosis: Surgical Strategies

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CHAPTER 88 Osteoporosis

Surgical Strategies

Frank M. Phillips, MD, Michael J. Lee, MD

Osteoporosis is a systemic disease characterized by decreased bone mass and microarchitectural deterioration. The resulting decrease in bone mechanical strength typically manifests as fragility fractures, with about one half of osteoporotic fractures occurring in the spine. According to the National Osteoporosis Foundation, low bone mass currently affects more than 44 million Americans and frank osteoporosis affects 15% of postmenopausal white women and 35% of women older than 65 years of age. In addition, 50% of white women will sustain an osteoporotic fracture at some time during their lifetime. Men have a lower but still significant osteoporotic fracture risk, which peaks 10 years later than the risk in women.¹

The World Health Organization defines osteoporosis as the condition of having a bone mineral density more than 2.5 standard deviations below that of a young individual’s peak bone mass. Osteoclast-mediated osteoporosis is characterized by a rapid phase of bone loss, most commonly in postmenopausal women. It affects women six times more frequently than men. This form of osteoporosis involves primarily trabecular bone and is associated with vertebral and distal radial fractures. In contrast, senile osteoporosis, which is related to aging, is osteoblast mediated and characterized by hip and spine fractures. It affects women twice as frequently as men. With both types of osteoporosis, bone fractures may occur in the absence of trauma when the bone is unable to withstand physiologic loads.

The absolute number and the percentage of persons with osteoporosis are expected to increase because of the increasing longevity of the population, so today’s spine surgeon must appreciate the impact of osteoporosis on the management of spinal disorders in the elderly. Older patients desire to remain active and are reluctant to accept disability and deformity as an inevitable consequence of aging. These patient expectations coupled with advances in spinal surgical techniques have resulted in more spinal procedures being performed on the elderly. The spinal surgeon may be required to treat direct sequelae of osteoporosis in the form of painful spinal fractures or resultant deformity or to consider osteoporosis as it relates to spinal reconstructive surgery in the older patient. Regardless of any surgical decisions in the osteoporotic patient, the spine surgeon must ensure that the patient is being appropriately medically treated for osteoporosis.

Osteoporotic Vertebral Compression Fractures

Osteoporotic vertebral compression fractures are a leading cause of disability and morbidity in the elderly.²^–⁴ The consequences of these fractures include pain and, in many cases, progressive vertebral collapse with resultant spinal kyphosis. Osteoporotic vertebral compression fractures have been shown to adversely affect quality of life, physical function, mental health, and survival.⁵^–⁷ These effects are related to the severity of the spinal deformity and are, in part, independent of pain.^6,⁷

Nevertheless, the pain associated with acute vertebral compression fractures may be incapacitating. In a number of cases, the pain will subside over a period of weeks or months, although it is not uncommon for the pain to become chronic.⁷ Chronic pain after vertebral fracture most likely results from (1) incomplete vertebral healing with progressive bony collapse, (2) altered spine kinematics as a consequence of spinal deformity, or (3) the development of a pseudarthrosis at the involved vertebra. Chronic pain associated with vertebral compression fractures often leads to impaired quality of life and depression.^6,⁷

Kyphotic deformity in the osteoporotic spine may also create a biomechanical environment favoring additional fractures. The kyphotic deformity anteriorly shifts the patient’s center of gravity, creating a longer moment arm acted on by the center of gravity. This results in greater flexion-bending moments around the apex of the kyphosis, which will promote further increases in kyphotic angulation and additional fractures.^8,⁹ Clinical studies report that the risk of a new vertebral fracture in the first year after an incident vertebral compression fracture rises 5 to 25 times above baseline,¹⁰^–¹² with the vertebra adjacent to the previously fractured level at particular risk.¹³^–¹⁴ Prevention of progressive kyphotic deformity or correction of existing deformity may therefore be important both in reducing the adjacent level fracture risk and in preventing the consequences of spinal kyphosis such as impaired pulmonary function.

Nonsurgical Treatment

Traditionally, acute osteoporotic vertebral compression fractures have been treated nonsurgically except in rare cases of fractures associated with neurologic compromise or advanced spinal instability. Spinal surgery in the osteoporotic patient is fraught with complications related to the patient’s advanced age and frequent comorbidities and due to the difficulties in securing fixation in osteoporotic bone. Thus the treatment of most patients with painful vertebral compression fractures traditionally includes bed rest, analgesic medications, bracing, antiosteoporotic drugs, or some combination thereof.¹⁵^–¹⁸ Although these treatments appear to be reasonable, anti-inflammatory and narcotic medications are often poorly tolerated by the elderly and may predispose to confusion, increased risk for falling, and gastrointestinal side effects. Bed rest can lead to an overall physiologic deconditioning and acceleration of bone loss. In addition, bracing is typically poorly tolerated by older patients, is expensive, and may further restrict diaphragmatic excursion.

Vertebroplasty and Kyphoplasty

Orthopedic fracture care emphasizes restoring anatomy, correcting deformity, and preserving function. These goals have been largely ignored in the management of patients with osteoporotic spine fractures. Recently, minimally invasive procedures to address the pain and deformity associated with osteoporotic vertebral compression fractures have been developed. Vertebroplasty, involving the percutaneous fluoroscopically guided injection of polymethylmethacrylate (PMMA) directly into a fractured vertebral body, has been used to stabilize osteoporotic vertebral compression fractures. Substantial pain relief in a majority of patients treated with vertebroplasty has been reported.¹⁸^–²⁵ Kyphoplasty is a minimally invasive procedure that involves the percutaneous insertion of an inflatable bone tamp into a fractured vertebral body under fluoroscopic guidance. Inflation of the bone tamp will elevate the endplates, restoring the vertebral body back toward its original height while creating a cavity to be filled with bone void filler, most commonly PMMA. Results of kyphoplasty suggest significant pain relief, as well as the ability to improve height of the collapsed vertebral body and reduction of spinal kyphosis.^15,²⁵^–³²

When recommending vertebroplasty or kyphoplasty to treat a painful fracture, the vertebral compression fracture must be confirmed as the source of the patient’s back pain. This requires careful correlation of the patient’s history and clinical examination with radiographic documentation of an acute or nonhealed vertebral compression fracture. The physician should treat the symptomatic fracture(s) and should not indiscriminately treat multiple vertebral fractures seen on radiographic studies. Magnetic resonance imaging (MRI) is useful for detecting edema, which may indicate an acute vertebral fracture, and for helping to rule out malignancy or infection. Tumors resulting in vertebral compression fractures are usually associated with an ill-defined margin, enhancement, pedicle involvement, as well as a paravertebral soft tissue mass.³³ Sagittal MRI with short tau inversion recovery (STIR) sequences highlights the marrow edema changes associated with acute vertebral compression fractures and is useful in determining the acuity of a vertebral compression fracture.

Indications and Contraindications

Indications for vertebroplasty and kyphoplasty include (1) stabilization of painful or progressive osteoporotic and osteolytic vertebral compression fractures, (2) painful vertebra due to metastases or multiple myeloma, (3) Kummell disease, and (4) painful vertebral hemangioma. Contraindications include fractures that result in neurologic compromise, possess significant burst components, involve the posterior vertebral body wall, and have a morphology that restricts vertebral body access. Patients with localized spine infections, sepsis, bleeding diatheses, or cardiopulmonary compromise that precludes safely performing the procedure also should not be treated by vertebroplasty or kyphoplasty. We do not advocate performing kyphoplasty or vertebroplasty on more than three vertebral levels in one surgery because of the potential for deleterious cardiopulmonary effects related to cement, marrow, or fat embolization to the lungs.

Vertebroplasty is designed primarily to relieve pain, and the procedure may be considered when a painful osteoporotic vertebral fracture does not respond to a reasonable period of conservative care. In addition to providing pain relief, kyphoplasty is designed to reduce the fractured vertebra so that intervening before fracture healing offers the best chance of achieving optimal fracture reduction.³⁴ If significant kyphosis is already present at the time of presentation of an acute vertebral compression fracture, we consider kyphoplasty to improve sagittal alignment. In contrast, for patients with acute vertebral compression fractures and relatively minor degrees of vertebral collapse, an initial trial of nonsurgical care may be considered, during which serial radiographs are obtained. If the patient’s pain is incapacitating or does not respond to a period of nonsurgical care, kyphoplasty or vertebroplasty may be recommended. If progressive collapse of the vertebral body is observed during the trial of nonsurgical care, kyphoplasty is recommended. In a recent randomized controlled trial comparing kyphoplasty to nonoperative treatment, a significantly more rapid improvement in quality of life, function, mobility, and pain was observed in the kyphoplasty group.³⁵

Techniques and Results

Vertebroplasty

Vertebroplasty may be performed in a radiology suite or operating room and is typically performed with the use of local anesthesia. The patient is positioned prone with the spine extended by chest and pelvic bolsters. Typically, an 11- to 13-gauge needle is advanced toward the center of the vertebral body using a transpedicular or extrapedicular approach and fluoroscopic guidance. If necessary, biopsy needles can be used to obtain samples before cement injection.^36,³⁷ PMMA, the bone cement most commonly used, is mixed with barium for fluoroscopic opacification. Whereas some physicians treat patients with intravenous antibiotics, some physicians add antibiotics to the cement mixture itself, especially when operating on immunocompromised patients.^25,^32,^36,³⁸ When the mixture attains the consistency of toothpaste, the cement is transferred to syringes or specially designed cement injection tools. Between 2 and 10 mL of cement is injected into the vertebral body under live, multidirectional fluoroscopy. Cement injection is stopped if extravertebral extravasation is detected. Ideally, the vertebral body is completely filled with cement, but pain relief has been reported when the anterior two thirds of the vertebra contains cement.³² The patient is not moved from the prone position until the cement has cured. Most patients rest supine under observation for at least 4 hours before discharge.

The mechanism of pain relief after vertebroplasty is not clear. One possible explanation is a mechanical immobilization of the fracture and support to the cortex by the cement.³⁹ Another theory suggests that the heat produced during PMMA polymerization causes deafferentation of the fractured vertebra.

Research on the outcome for vertebroplasty has suggested that most patients experience partial or complete pain relief within 72 hours of the procedure.¹⁸^–²⁵ ^,36 ^,40 ^,41 Overall, 60% to 100% of patients noted decreased pain after vertebroplasty with pain reduction maintained for months up to 10 years.^21,²⁴ In addition to decreased pain, improved functional levels and reduced analgesic medication requirements have been reported.^24,^36,⁴²^–⁴⁵ Published studies have noted a low complication rate for vertebroplasty, with most complications resulting from extravertebral cement leakage causing spinal cord injury, nerve root compression, or pulmonary embolism.^18,^19,^{21–25,36,40}^,41

The limitations of the vertebroplasty technique relate to the inability of the procedure to correct spinal deformity and the risk of extravertebral cement extravasation during injection. In certain instances, some degree of postural fracture reduction is achieved with vertebroplasty.⁴⁶^–⁴⁸ During vertebroplasty, the high-pressure injection of low viscosity cement directly into cancellous bone makes it difficult to control cement flow in the vertebral body. This creates an unpredictable risk of cement extravasation outside the vertebral body.³² In fact, extravertebral cement extravasation rates of up to 65% have been reported for vertebroplasty.⁴⁰ Although a low risk of clinically relevant complications results from cement extravasation, cases of fatal pulmonary embolism and paralysis have been described.⁴⁹^–⁵²

Kyphoplasty

Kyphoplasty involves the percutaneous placement and expansion of an inflatable bone tamp in a fractured vertebral body. The tamp elevates the depressed vertebral body endplate(s), thereby restoring vertebral body height, and also creates a cavity within the vertebral body that is then filled with bone void filler (Fig. 88–1).^15,²⁵^–³² The patient is positioned prone on a spinal frame. Fluoroscopy is used throughout the procedure, and we have found simultaneous, biplanar fluoroscopy to be advantageous. After needle positioning into the fractured vertebral body through a transpedicular or extrapedicular approach, a series of tools creates a working channel into the vertebral body. Once inserted through the cannula into the vertebral body, the balloon tamp is expanded using visual (fluoroscopy), volume, and pressure (digital manometer) controls. The inflation of the tamp continues until one of the following endpoints is reached: (1) fracture reduction is achieved, (2) the maximal balloon pressure or volume is reached, or (3) cortical wall contact occurs. The balloons are then deflated and removed. Thick cement can then be fed through the cannula under low pressure to fill the void created by the balloon tamp. The cement volume should approximate the volume of the intravertebral cavity. The patient is not moved from the prone position until the leftover cement has solidified.

FIGURE 88–1 A, During kyphoplasty the fractured vertebral body is percutaneously cannulated with an inflatable bone tamp. B, The tamp is inflated, which elevates the vertebral body endplates and restores vertebral body height. C, The tamp is withdrawn, leaving a cavity to be filled with bone void filler. D, Bone void filler is placed into the vertebral body under low pressure.

Garfin and colleagues ⁵³ reported the initial multicenter experience with kyphoplasty to treat 2194 vertebral fractures in 1439 patients between 1998 and 2000. Ninety percent of patients reported significant pain relief within 2 weeks of the procedure.⁵³ In this large series of kyphoplasty procedures, a 0.2% per fracture complication rate was observed. Lieberman and colleagues³⁰ observed highly significant improvement in physical function, role physical, vitality, mental health, and social function scores of the SF-36 questionnaire after kyphoplasty. These authors reported five clinically insignificant cement leaks (8% overall).³⁰ Phillips and colleagues³⁴ reported on 29 patients treated with kyphoplasty. In this study, mean visual analog scale (VAS) pain scores decreased from 8.6 preoperatively to 2.6 one week postoperatively to 0.6 one year postoperatively. Cement leaks without apparent clinical consequence occurred in 6 of 61 vertebral levels with no cases of cement leakage into the spinal canal.³⁴

Kyphoplasty has the potential to improve spinal deformity by elevating the vertebral endplates before fixation. In an ex vivo study, Belkhoff and colleagues ²⁶ showed a 97% reversal of deformity with kyphoplasty compared with a 30% reversal with vertebroplasty. Lieberman and colleagues³⁰ reported vertebral height restoration in 70% of 70 fractured vertebrae treated with kyphoplasty. In those patients in whom the vertebral fractures were reduced by kyphoplasty, vertebral height was increased by a mean of 46.8%. Wong and colleagues³² and Garfin and colleagues²⁸ similarly noted increased vertebral body height after kyphoplasty. Theodorou and colleagues studied 15 patients (24 fractures) with osteoporotic vertebral compression fractures and reported a mean improvement in kyphosis of 62.4% ± 16.7%.³¹ Phillips and colleagues³⁴ reported that, in their early experience with kyphoplasty, local sagittal alignment was improved by a mean of 8.8 degrees for all fractures and 14.2 degrees in those fractures that were reduced by at least 5 degrees (Fig. 88–2). Voggenreiter, in a series of 39 patients, reported a kyphosis reduction of 6.5 degrees with intraoperative prone positioning and a subsequent improvement of 3.4 degrees with inflation of the balloon tamp. However, he did note a subsequent loss of 3.1 degrees on upright standing films post operatively.⁴⁷ Hulme and colleagues,⁵⁴ in their systematic literature review, reported the average angular correction with kyphoplasty to be 6.6 degrees.

FIGURE 88–2 A 73-year-old woman with an L1 vertebral compression fracture was treated with kyphoplasty 6 weeks after the injury occurred. The focal kyphosis was corrected from 16 degrees (A) to 5 degrees (B).

Three recent multistudy analyses have pooled the literature results and complications of kyphoplasty and vertebroplasty. In these analyses, clinical improvement was noted after both kyphoplasty and vertebroplasty.^54,⁵⁵ Kyphosis correction was noted more frequently in kyphoplasty than in vertebroplasty. Overall, complications were rare in both groups. Pooled results suggested the mortality and medical complications were comparable in both procedures.^55,⁵⁶ The rate of cement leakage, asymptomatic and symptomatic, was noted to be higher in vertebroplasty than in kyphoplasty in all three reviews. Lee and colleagues⁵⁶ reported the symptomatic cement leak rate to be 1.48% in vertebroplasty and 0.04% in kyphoplasty (P < 0.05). These differences are likely due to subtle differences between the techniques. In vertebroplasty, the cement is usually less viscous and injected under a higher pressure and subsequently more difficult to control.

Instrumentation of the Osteoporotic Spine

As reconstructive spine surgeries are performed on older patients with increasing frequency, the ability of the osteoporotic spine to support spinal implants or bone grafts must be considered. The selection of spinal instrumentation must take into account the fragility of osteoporotic bone, the stability of the spine, and the likely failure mechanisms of any applied instrumentation. The preoperative workup should include evaluation for the severity of osteoporosis, which might impact the surgeon’s choice of reconstruction techniques.

Instrumentation is currently being applied throughout the spine in elderly, osteoporotic patients. Cervical surgery to address degenerative pathologic processes in the elderly most often involve decompression and fusion. Stabilization is most typically achieved with anterior plate constructs, although posterior fixation is being increasingly used to supplement anterior reconstructions. In the thoracolumbar spine, anterior instrumentation composed of plate or rod-screw constructs may be useful in the management of fractures, tumors, or deformity. Posterior thoracic and lumbar pedicle screw or hook fixation is widely applied in the treatment of spinal deformity or to enhance spinal fusion. In all of these applications, osteoporotic bone presents unique challenges to the spinal surgeon.

Cervical Instrumentation

In the subaxial spine, anterior cervical instrumentation is frequently used to maintain alignment and improve fusion rates after anterior bone grafting. Few clinical reports specifically addressing anterior fixation in the osteoporotic spine have been published. Biomechanical studies of anterior cervical plating have shown a direct relationship between bone mineral density (BMD), screw insertion torque, and the axial force generated at the plate-screw junction.⁵⁷ Because low BMD predisposed to plate pullout, Zink⁵⁷ suggested that stable anterior cervical fixation with 3.5-mm screws cannot be obtained with a BMD below 150 mg/mL. The risk of anterior cervical plate failure in osteoporotic bone is reduced by increasing screw length and placing bicortical screws.⁵⁷ The desire to increase stability with bicortical cervical screw fixation should be tempered by the neurologic risk associated with screw penetration of the posterior vertebral body cortex.

Anterior cervical plate-screw constructs spanning multiple levels have been reported to have a high rate of clinical failure, which usually resulted from graft-plate dislodgement.⁵⁸ Biomechanical studies have confirmed that excessive screw-vertebra motion caused by fatigue at the lower end of a multiple-level corpectomy construct may explain the clinically observed failures at the caudal end of long anterior cervical plate constructs.⁵⁹ This research also suggested that longer screws, larger diameter screws, and supplemental posterior fixation would potentially decrease screw-vertebra failure.⁵⁹ These findings seem to be particularly relevant in osteoporotic bone where impaired bone quality would predispose to excessive screw-vertebra motion and failure. DiAngelo and colleagues⁶⁰ have reported that although multiple-level cervical plating increases construct stiffness after corpectomy, the anterior plate results in the graft being excessively loaded in extension. Because these loads may exceed the adjacent endplate yield strength, pistoning and strut settling into the adjacent vertebral body may occur.⁶⁰ This problem is accentuated in the osteoporotic spine, where the adjacent vertebral endplates are less able to resist graft settling. When using anterior instrumentation over multiple levels in the osteoporotic cervical spine, the surgeon should attempt to use as many points of fixation as the construct will allow. If the surgeon is concerned that anterior cervical screw purchase is compromised in osteoporotic bone, it may be advisable to proceed with segmental posterior cervical instrumentation to stabilize the anterior arthrodesis.

Dens fractures are the most common condition of the upper cervical spine requiring surgery in the elderly. This population is predisposed to dens fractures by osteoporosis of the dens coupled with the increased rigidity of the subaxial spine as a consequence of age-related degenerative spondylosis.⁶¹ Halo immobilization of these injuries has been recommended; however, this is poorly tolerated in the elderly and is associated with a high incidence of nonunion and complications.^62,⁶³ Dens fractures in this age group are frequently treated surgically using anterior or posterior approaches. For example, acceptable clinical results have been reported for the widely used technique of posterior C1-2 fusion using either wiring or screw fixation.^64,⁶⁵ Andersson and colleagues⁶⁶ reported an unacceptable rate of anterior screw fixation failure and nonunion in eight elderly patients with dens fractures. In contrast, Berlemann and Schwarzenbach⁶⁷ reported a low failure rate and successful healing of the fracture in 16 of 19 elderly patients with dens fractures treated with anterior screw fixation.

Posterior Spinal Instrumentation

Posterior instrumentation is most commonly applied to the osteoporotic spine in an effort to stabilize the spine and promote fusion after decompression of neural elements. In this situation, the anterior column is typically intact and no frank instability exists, so posterior instrumentation alone is usually adequate. Surgery primarily for deformity correction in the elderly is challenging and is infrequently indicated. Posterior instrumentation may be used to correct spinal deformity; however, if the deforming forces exceed the stability of the implant-bone interface, posterior instrumentation failure will occur typically by screw pullout or toggling. In elderly, osteoporotic patients, accepting a lesser degree of deformity correction when feasible may be prudent to reduce risks of instrumentation failure. Anterior release and structural support may also reduce the deforming forces on posterior instrumentation, but this strategy should be weighed against the added morbidity of an anterior approach in the older patient.

Posterior spinal instrumentation may be achieved with plates or rods attached to the spine by means of wire, hooks, or pedicle screws. Coe and colleagues ⁶⁸ studied the modes of failure of these anchors in osteoporotic thoracic spines. Wire fixation failed by cutting through the bone posteriorly. Sublaminar hooks typically failed by pulling through the lamina posteriorly, although 30% of the failures occurred at the pedicle or the pedicle-body junction. Pedicle screws typically failed by screw pullout. Overall, sublaminar hooks showed superior stability compared with wire or pedicle screw constructs. These findings were supported by Butler and colleagues,⁶⁹ who reported that the performance of hook fixation in the thoracic spine was not adversely affected by osteoporosis, whereas sublaminar wire cutout increased with decreasing BMD.

In current clinical practice, the large majority of posterior instrumentation spinal surgeries performed involve lumbar pedicle screw instrumentation. In the osteoporotic spine, the weak link in the instrumentation construct is the implant-bone interface. Most instrumentation failures involve screw loosening and pullout, which may lead to failure of fusion or the development of recurrent or de novo deformity. Posterior thoracolumbar instrumentation failure has been shown to correlate with BMD.^68,^70,⁷¹ Screw pullout and also cutout through the adjacent endplate with cyclical flexion-extension loading are directly related to BMD and may occur even at physiologic loads in the osteoporotic spine.^68,^70,⁷¹ In a biomechanical study, Soshi and colleagues⁷⁰ concluded that pedicle screw fixation should be avoided in patients with a BMD less than 0.3 g/cm².

At the time of pedicle screw insertion, the surgeon may recognize poor screw purchase in osteoporotic bone because of the low insertion torque required to advance the screw. Insertion torque not only correlates with BMD and screw pullout but also predicts early screw failure.⁷²^–⁷⁴ If poor screw purchase is recognized intraoperatively, the surgeon should attempt to salvage the situation rather than rely on inadequate fixation to achieve the goals of instrumentation.

The surgeon may consider increasing the length or diameter of the pedicle screw in an attempt to improve the screw purchase in bone. Increasing screw length does increase screw pullout strength, although this effect may be less pronounced in osteoporotic bone.^75,⁷⁶ The inability to accurately gauge the anterior vertebral body cortex intraoperatively may affect the surgeon’s ability to safely place longer screws because screws extending beyond the anterior vertebral body may predispose to vascular injury. In the sacrum, optimal screw purchase is achieved by directing the screws toward the disc space anteriorly or through the sacral promontory. Increasing screw diameter will also increase pullout strength.^75,⁷⁷^–⁷⁹ Kiner and colleagues⁸⁰ reported that 8-mm diameter screws demonstrated superior pullout strength than 6-mm screws with cement augmentation. However, the dimensions of the pedicle being cannulated may limit the screw diameter. In the osteoporotic spine, when the screw diameter exceeds 70% of the pedicle diameter, a high risk of pedicle fracture is created.⁸¹

In addition to appropriate screw size, optimal screw trajectory can enhance fixation in the osteoporotic spine. In the sagittal plane, screw placement so that the threads engage subchondral bone may provide superior fixation than thread engagement of cancellous bone centrally within the body. In the axial plane, medialization of the screw vector and triangulation of bilateral screws have been demonstrated to increase pullout strength.⁸² Bilateral triangulated pedicle screws allow the screws to, in effect, hold all of the bone between the screws rather than just the bone within the threads of the individual screws. Ruland and colleagues⁸² suggested that for triangulated screws to fail simultaneously, a transverse fracture through the vertebral body at the level of the tips of the pedicle screw had to occur. Triangulation of pedicle screws is limited by the bony anatomy. Lower lumbar levels generally allow for more medialization of screws, whereas upper lumbar and thoracic pedicle vectors allow for less.

Undertapping the pedicle in an osteoporotic spine can also enhance screw purchase. By tapping the pedicle with a smaller-diameter tap than the screw, cancellous bone is conserved and can be compacted around the screw threads, providing additional purchase. Although studies have shown that this technique may not be advantageous in patients with normal bone density,⁸³ numerous authors have reported increased insertional torque and pullout resistance when undertapping by 1 mm in the osteoporotic spine.^84,⁸⁵

Another strategy to improve stability of the pedicle screw construct in osteoporotic bone is to distribute forces by increasing the number of fixation points to the spine by including additional levels in the construct. The advantages of this approach must be weighed against the risks and morbidity associated with the additional-level surgery, as well as the potential long-term consequences of a fusion spanning additional levels. The surgeon may also augment the pedicle screw construct with offset sublaminar hooks, which are well suited for use in the osteoporotic spine because they rely on the relatively unaffected cortical laminar bone for fixation.^68,⁸⁶ Biomechanical studies have supported the ability of supplemental sublaminar hooks to increase the rigidity and pullout strength of pedicle screw constructs.^87,⁸⁸

The bone-screw interface may also be improved by injecting PMMA bone cement into the pedicle around the pedicle screw. A twofold to threefold increase in screw pullout has been demonstrated with the use of PMMA injected into the vertebral body through a cannulated pedicle.^70,⁷⁶ Possible risks of this technique include cement extravasation outside the vertebra, with potential for leakage into the spinal canal or neural foramina. Other cements such as hydroxyapatite cement, calcium phosphate, and carbonated apatite have also been shown to enhance the screw-bone interface and increase pedicle screw pullout strength.^79,^89,⁹⁰ The disadvantage of using non-PMMA cement is that it often requires 4 to 24 hours to reach maximal stiffness for screw fixation augmentation, whereas PMMA achieves immediate stiffness. In the correction of deformity with various forces applied through the screw, use of non-PMMA cement augmentation and its delayed setting may result in higher risk for immediate fixation failure. Moore and colleagues⁹⁰ reported that the failure modes seen with PMMA and calcium phosphate cement differed in pullout tests. With PMMA augmentation, pedicle fracture occurred at or near the junction with the vertebral body in 80% (25 of 30) of the samples. In contrast, failure of calcium phosphate augmentation occurred at the cement-screw interface in 80% (24 of 30) of the samples. Cement augmentation of screws has been used in patients with osteoporosis and metastatic spinal tumors undergoing spinal instrumentation with acceptable clinical results and low rates of instrumentation failure.⁹¹^–⁹⁴

Novel variations of instrumentation have been developed to enhance pedicle screw fixation. Cannulated screws have been developed with fenestrations toward the tip of the screw. After screw placement, cement is injected into the vertebral body through the screw. Becker and colleagues ⁹⁵ reported that the use of cement augmented, cannulated, fenestrated pedicle screws in cadavers significantly improved pullout strength when compared with standard screw placement. However, the authors noted that cement augmentation with the fenestrated screw resulted in a more posterior final location of the cement within the vertebral body as compared with standard cement augmentation techniques. In 2 of the 10 specimens there was extravasation posteriorly. This occurrence is likely secondary to location of the perforations within the screw.

Alternatively, expandable pedicle screws have been proposed as an alternative technique to enhancing fixation. With these screws, the distal two thirds of the screw is split lengthwise by two perpendicular slots to form four anterior fins when expanded. An expansion peg (a smaller-gauge screw) is threaded into the inner core of the pedicle screw. As the expansion peg advances into the slotted portion of the screw, it spreads and opens up the slotted tip of the screw, creating fins. Withdrawal of the expansion peg collapses the fins, allowing for removal of screw. Ngu and colleagues ⁹⁶ tested expandable pedicle screws and found that while expandable pedicle screws resulted in superior fixation compared with initial standard screw purchase, they were inferior to PMMA augmented pedicle screw fixation. Cook and colleagues⁹⁷ reported a twofold increase in pullout strength with the expandable pedicle screw and a 250% increase with cement augmentation. In a clinical study of 145 patients who received expandable screw fixation, 21 of these patients were osteoporotic. Expandable screw breakage occurred in 3% of all patients studied, with osteoporotic patients demonstrating a higher screw breakage rate of 5% (1/21 cases, 5/97 screws). Broken expandable screws were difficult to remove. Screw breakage most frequently occurred at the level of the prongs.

Failure of posterior instrumentation in the osteoporotic spine usually occurs through loss of screw fixation by screw toggling, loosening, and eventually pulling out. This may result in a relatively large void around the loose screw that can preclude reusing the same pedicle for revision screw fixation. If revision instrumentation is required, all previously mentioned strategies for enhancing posterior fixation should be considered. In addition, strong consideration should be given to including anterior column structural support and fusion as part of the revision strategy. The anterior vertebral endplates provide a wide surface area that is advantageous for promoting fusion and also for improving the performance of structural struts.^98,⁹⁹ Anterior column support will also help reduce flexion-bending moments on the posteriorly placed instrumentation, thereby reducing risks of instrumentation failure.

Anterior Thoracolumbar Instrumentation

Anterior approaches to the thoracolumbar spine may be indicated in elderly patients with neurologic deficit resulting from anterior pathologic processes such as fracture or tumor, for anterior release with deformity, and to allow for shorter segment fixation. Anterior arthrodesis allows reconstruction of both the anterior and middle spinal columns, as well as placement of bone graft under compression, which provides a favorable mechanical environment for fusion.⁹⁸^–⁹⁹

Anterior plate or rod-screw fixation may be used for anterior column reconstruction at the thoracolumbar junction and in the upper lumbar spine. These implants are customarily applied to the lateral aspect of the vertebral body and are less suited for use in the lower lumbar spine because the local vascular anatomy and the pelvis may impede the surgeon’s ability to apply instrumentation. Anterior instrumentation is designed to be load sharing and is typically used in combination with a longitudinally orientated bone graft, strut, or cage to reinforce the deficient anterior column. The use of anterior instrumentation may allow for fusion of fewer mobile segments than might be required with posterior fixation. In osteoporotic patients, achieving adequate fixation with anterior instrumentation is challenging and, in many instances in which anterior column reconstruction is required, combining an anteriorly placed strut with posterior multiple-level segmental fixation may provide a more reliable construct (Fig. 88–3).

FIGURE 88–3 A, A 72-year-old woman sustained an L1 vertebral compression fracture with a minor degree of superior endplate depression. She was treated in a brace, but on repeat radiographic evaluation (B and C) at 6 weeks the fracture had continued to collapse, resulting in severe kyphotic deformity. The patient complained of unrelenting pain and her appearance. D and E, Nine months later she underwent anteroposterior reconstruction using a vertical cage anteriorly and pedicular fixation, with good restoration of sagittal alignment and relief of pain.

In osteoporotic bone, anterior construct failure typically occurs with implant loosening or with subsidence of the intervertebral strut-implant into the cancellous bone of the adjacent vertebral body, which may lead to failure of fusion and recurrence of deformity. Also, intravertebral screws may loosen if fixation is poor, as is common in osteoporotic bone. Lim and colleagues ¹⁰⁰ correlated anterior screw insertion torque with BMD and correlated BMD with pullout strength of bicortical Kaneda screws placed in the vertebral body. Fixation can be improved by obtaining bicortical purchase of the vertebral body with wide-diameter screws.¹⁰¹ Care must be taken not to penetrate an unfused disc space with the screws. PMMA or biodegradable cement augmentation can also be used to improve screw pullout strength.^102,¹⁰³ The risk of construct failure resulting from the settling of the longitudinal strut or cage into the adjacent vertebral bodies may be reduced by maintaining the integrity of the vertebral endplates during their preparation.¹⁰⁴ In addition, small-diameter struts or cages should be avoided because these tend to cut into the vertebral endplates and piston into the vertebral body. If the stability of the anterior reconstruction is questionable, the construct should be supplemented with posterior instrumentation.

When anterior structural support is required, it is particularly important in the osteoporotic spine to consider the strut material and strut design. Commonly used struts include autograft and allograft bone and metallic and carbon devices. In the osteoporotic spine, the vertebral endplates are weak, so the strut may readily piston into the vertebral body. To reduce this risk, high-modulus, stiff devices should be avoided. Instead, the modulus of elasticity of the strut should be matched with that of the patient’s bone. Autologous corticocancellous bone may best accomplish this goal. In addition, the strut should have a wide and uniform surface area resting on the adjacent endplates to avoid concentrating loads at the periphery of the devices. Strut devices with sharp, thin edges tend to cut into the adjacent osteoporotic vertebral body and should be avoided. Furthermore, the possibility of construct failure may make metallic devices less desirable because of the potential for damaging adjacent vessels or viscera if they loosen and migrate. When performing anterior column reconstruction in the osteoporotic spine, we prefer to supplement the anterior strut with posterior multiple-level segmental fixation.