Pedicle Subtraction Osteotomy

Transcription

1 Pedicle Subtraction Osteotomy Manish K. Singh, David M. Ibrahimi, Christopher I. Shaffrey, and Justin S. Smith Introduction Pedicle subtraction osteotomy (PSO) is a surgical procedure that can be used to help correct fixed sagittal plane deformities. These deformities may have a variety of etiologies, including degenerative, posttraumatic, neoplastic, infectious, metabolic, and congenital disorders [ 1 3 ]. The aging population and rising expectations for quality of life are leading to increasing numbers of patients seeking medical and surgical evaluation for symptomatic fixed sagittal deformity. With the advent of polysegmental three-column fixation with pedicle screws and advances in spinal instrumentation, posterior-only approaches for correcting kyphotic or kyphoscoliotic deformities have become feasible and more common in recent years, avoiding the need of anterior approaches [ 4 ]. The use of posterior-only osteotomies with pedicle screw instrumentation can provide significant alignment correction in all three dimensions, whereas greater pedicle screw pullout strength and stiffness limit the number of levels fused, allowing for preservation of more mobile spinal segment [ 4, 5 ]. Ponte or Smith-Petersen osteotomy (SPO), PSO, and vertebral column resection (VCR) are M.K. Singh, MD D.M. Ibrahimi, MD C.I. Shaffrey, MD J.S. Smith, MD, PhD (*) Department of Neurosurgery, University of Virginia, Charlottesville, VA, USA jss7f@virginia.edu the three major posterior osteotomy techniques used for correction of adult spinal deformity [ 1, 4, 6 ]. Each of these surgical options may be performed either in isolation or in combination, depending on the type, complexity, and severity of the deformity. PSO and VCR are both considered three-column osteotomies, as both involve osteotomy extending from the posterior column into the anterior column, whereas SPO involves removal of bone from the posterior column only [ 1, 4, 7, 8 ]. Thomasen first described PSO in 1985 [ 9 ]. He described the procedure in a series of 11 patients with ankylosing spondylitis who had severe disabling kyphosis which was corrected by wedge posterior osteotomy of the second lumbar vertebra. PSO is a powerful tool to restore lordosis in the thoracic, lumbar, and more recently the cervical spine [ 6, 10 ]. PSO has been addressed with various terminologies throughout the literature, including transpedicular wedge procedure, closing wedge osteotomy, and eggshell osteotomy [ 4 ]. It involves three-column osteotomy with transpedicular vertebral wedge resection extending from the posterior elements through the pedicles bilaterally into the vertebral body leaving the anterior cortex intact, which acts as a hinge for the closure of the wedge defect (Fig. 8.1a ) [1, 3, 4, 6, 11 ]. On closing the wedge, a substantial surface area is provided for osseous union from closure of the bony surfaces of the anterior, middle, and posterior columns. Wedge closure results in shortening of the posterior column without Y. Wang et al. (eds.), Spinal Osteotomy, DOI / _8, Springer Science+Business Media Dordrecht

2 90 M.K. Singh et al. a b Fig. 8.1 ( a, b ) Pedicle subtraction osteotomy: Threecolumn transpedicular vertebral wedge osteotomy, leaving the anterior cortex intact, which acts as a hinge for the closure of wedge defect. The osteotomy places two large cancellous surfaces together to achieve fusion and restore balance, and the two nerve roots exit through one large neural foramen lengthening the anterior column, thus providing maximal healing potential without stretching the major abdominal vessels and viscera anterior to the spine (Fig. 8.1b ) [1, 6 ]. After the posterior wedge has been closed, two nerve roots exit on each side at the vertebral level of the osteotomy through the newly formed neural foramina. Fixed sagittal deformity can be iatrogenic, with etiologies including multilevel laminectomy, fixation of thoracolumbar fractures, distal lumbar arthrodesis for degenerative lumbar disease, segmental fixation for correction of scoliosis, and previous arthrodesis with Harrington rods extending into the lumbar spine. It can be secondary to ankylosing spondylitis, multilevel disc degeneration, and kyphosis from multilevel compression fractures due to osteoporosis or trauma, or secondary to neoplastic, infectious, metabolic, and congenital disorders. Determination of whether the spinal deformity is fixed or flexible helps significantly with the surgical decision-making process. Assessment of curve flexibility and compensatory ability of adjacent segments of the spine helps in determining the surgical approach, number of spinal levels to be fused, and the need for corrective osteotomies. Deformities that demonstrate more than 30 % correction on bending radiographs are generally considered flexible and less likely to require osteotomies, whereas curves that demonstrate less than 30 % correction with bending are generally considered fixed or rigid deformities and are more likely to require osteotomies [ 12 ]. Many adult deformity patients have had previous fusion surgery, which may contribute to rigidity of the deformity and may increase the requirement for osteotomy in order to achieve sufficient alignment. Flexibility should be evaluated in all curves that are considered for surgical correction. Fixed sagittal deformity makes erect posture difficult without flexion of the knees and/or hyperextension of the hips. The sagittal vertical axis (SVA), which is the horizontal offset between the C7 plumb line and the posterior superior end plate of the S1 vertebrae, is often significantly positive in patients with fixed sagittal deformity. There is no clear consensus regarding normal SVA, but recently Schwab et al. in the Scoliosis Research Society (SRS) Schwab adult spine deformity classification considered SVA <4 cm as normal, SVA between 4 and 9.5 cm as moderately increased and SVA greater than 9.5 cm as severely increased [ 13 ]. The T1 spinopelvic inclination (SPI) and T-9 SPI are used as alternative measures of global sagittal alignment that rely on angular measurements rather than

3 8 Pedicle Subtraction Osteotomy distances. T1 SPI has been shown to correlate more accurately with health related quality of life (HRQOL) scores than the SVA, perhaps because SPI accounts for the relationship of the pelvis and lower extremities to the spine by measuring the offset of T1 from the pelvis in relation to the femoral heads and not the sacrum [ 14 ]. Recent studies have demonstrated the importance of pelvic parameters, including pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS), in assessment of sagittal spinal alignment [ 14, 15 ]. Progressive positive sagittal malalignment can lead to increased pelvic retroversion (increased PT) as a means of attempting to compensate for sagittal malalignment. Although increased PT may help to mitigate positive sagittal malalignment, abnormal pelvic retroversion can adversely affect ambulation and increase energy utilization, with resulting negative impact on HRQOL [ ]. Irrespective of the etiology, in patients with adult spinal deformity, there is a strong correlation between sagittal malalignment and pain and disability based on standardized measures of HRQOL [ 19 ]. In a recent study, Lafage et al. showed that T1-SPI had the strongest correlation with HRQOL followed by SVA and PT [ 14 ]. Correction of both SVA and PT has been reported to result in better outcome scores than correction of SVA alone [ 14, 15 ]. Schwab et al. recommended that the goal of surgical correction for sagittal malalignment, based on normative data and HRQOL-based postoperative outcomes, should be SVA <50 mm, T1 SPI <0, PT <20, and lumbar lordosis within 9 of PI [ 20 ]. The results of a study from Blondel et al. [ 21 ] that assessed the amount of sagittal correction needed for patients to perceive improvement in HRQOL scores were consistent with the proposed correction goals proposed by Schwab et al. for realignment surgery. The best HRQOL outcomes for adult spinal deformity patients with severe sagittal plane deformity were achieved with correction >120 mm of SVA or at least 66 % correction of the SVA [ 21 ]. Furthermore, they found that the greater the magnitude of correction of SVA, the better the improvement was in SRS-pain and ODI scores. The goal of surgery should be to have a complete or near complete 91 correction of sagittal malalignment if feasible; partial correction may in some cases increase the risks of instrumentation failure, proximal junctional kyphosis, and suboptimal improvement of HRQOL [ 20, 21 ]. Corrective surgery for fixed sagittal imbalance or kyphoscoliosis often involves osteotomies such as SPO, PSO, or VCR with transpedicular segmental instrumentation. The use of osteotomies to restore sagittal spinopelvic alignment in adults depends on the pathology, patient age, medical comorbidities, and experience of the surgeon. To effectively decide which osteotomy or combination of osteotomies to employ for deformity correction, it is important to have a working appreciation for the applications and limitations of all three key posterior osteotomies (SPO, PSO, and VCR) Smith-Petersen Osteotomy Smith-Petersen et al. first described SPO in 1945 for correction of fixed sagittal deformity in patients with rheumatoid arthritis [ 22 ]. It has been referred to as Chevron osteotomy, extension osteotomy, or Ponte osteotomy. SPO involves removal of the posterior bony elements including the bilateral facet joints, inferior portion of the lamina, and inferior portion of the spinous process, and removal of the posterior ligaments at the osteotomy level (Fig. 8.2a ) [4, 8, 23 ]. The osteotomy is closed with the axis of rotation at the posterior aspect of the disc space, which results in widening of the anterior disc space and disruption of the anterior longitudinal ligament and hence requires a mobile disc space (Fig. 8.2b ). It is usually not effective if the disk space is immobile, as in the case of bridging anterior osteophytes [ 8 ]. SPO produces shortening of the posterior column and lengthening of the anterior column, with attendant risks, including vascular, neurological, pseudarthrosis, or gastrointestinal complication [ 4, 8, 22 ]. One modification of the SPO procedure is to use an interbody graft or spacer in the middle or anterior third of the disk space and then to compress posteriorly using the graft as a fulcrum; this provides a greater degree of lordosis, with less risk of compromising the neural foramina [ 8 ].

4 92 M.K. Singh et al. a b Fig. 8.2 ( a, b ) Smith-Petersen osteotomy: Correction is obtained though disruption of the disc space and anterior longitudinal ligament, causing lengthening of the anterior column The amount of correction typically achieved with SPO is approximately 10 per level, or 1 of correction for each millimeter of bone removed [ 1, 4, 8, 22 ]. There have been reports of aggressive techniques of posterior extension osteotomy with anterior osteoclasis achieving more correction, but this is associated with higher risk of complications. SPO is typically indicated for cases with long, gradual sweeping deformity, with mobile intervertebral disc spaces, and no more than moderate elevation of SVA, features typically not associated with rigid deformities [ 24 ]. The classic indication for SPO is Scheuermann s kyphosis, but it can be used in the treatment of mild iatrogenic fixed sagittal imbalance (6 8 cm) [ 8, 24 ]. The role of the SPO is limited in fixed sagittal deformity as these osteotomies have limited effectiveness in sharp, angular kyphosis and are not effective in the presence of anterior bridging osteophytes or across previously fused segments due to lack disc mobility. In addition, SPO is not without risks, as anterior column lengthening puts vascular structures at risk, particularly in the elderly and in patients with ankylosing spondylitis, who may have calcified major vessels [ 4, 8 ]. There is also risk of gastrointestinal complication secondary to superior mesenteric artery syndrome. Neurological complications have been reported in as high as 30 % of cases, mainly radiculopathy secondary to compression of nerve roots in the foramen from closure of the SPO [ 4 ]. Other complications include, but are not limited to, pseudarthrosis, bleeding, durotomy, and pedicle fracture. Compared with three-column osteotomies, SPOs are usually associated with less operative time, blood loss, and rates of neurological complication [ 25 ]. In an attempt to cause less disruption of the anterior column, polysegmental osteotomy (Fig. 8.3a ) can be performed to correct sagittal deformity. Hehne et al. first described polysegmental lordosis osteotomy, with posterior osteotomy at each level, producing approximately 10 of correction per segment [ 26 ]. By closing multiple posterior osteotomies over multiple segments with transpedicular fixation, a more gradual correction of kyphosis is achieved with less disruption of the anterior longitudinal ligament (Fig. 8.3b ) [4, 8, 23 ]. This technique has similar limitations as the standard SPO for correction of rigid deformities including limited effectiveness in the setting of a previously fused level, bridging anterior osteophytes, or sharp kyphosis.

5 8 Pedicle Subtraction Osteotomy 93 a b Fig. 8.3 ( a, b ) Polysegmental posterior osteotomy: Correction is obtained through multilevel disc deformation, with only limited or no disruption of the disc space and anterior longitudinal ligament Pedicle Subtraction Osteotomy PSO is typically used to correct fixed sagittal deformity with sharp or angular kyphosis in patients with SVA >8 cm and is often applied for the treatment of acquired or iatrogenic flat back, ankylosing spondylitis, and for rigid deformities that lack anterior flexibility needed for SPO to be effective [ 4, 23, 24 ]. PSO can be technically challenging, with substantial blood loss from the epidural venous plexus and cancellous bone during the osteotomy, and it is more commonly performed below the conus in the lumbar region to reduce neurological complication from thecal sac manipulation and wedge closure, but can be performed in thoracic and cervical level if indicated but with increased risk [ 1, 10, 24, 27 ]. If it is performed in thoracic or cervical spine, extra precaution should be taken when working around the spinal cord and the thecal sac should not be retracted [ 24 ]. Resecting a portion of the rib bilaterally along with the costovertebral joints during PSO in the thoracic spine can help to facilitate a safe approach to the vertebral body laterally without thecal sac retraction. PSO tends to be avoided in distal lumbar vertebra because there are fewer fixation points available distally [ 24 ]. PSO is most commonly performed at L2 or L3, as these levels are generally below the conus and have a sufficient number of fixation points distally. Nevertheless, the vertebral level of the PSO should be guided by the type and location of the pathology. In patients with focal fixed-angle sagittal deformity, PSO is typically performed at the level of kyphosis; however in the absence of focal thoracolumbar kyphosis, lumbar PSO is the preferred option. In addition to providing greater correction of deformity, PSO has an advantage over SPO, as it does not require a mobile anterior segment or disc space and can be performed in patients with circumferential fusion across multiple segments, previous laminectomy, or area of rotation, although these features do potentially increase the technical difficulty and the risk of complications [ 24 ]. The extensive cancellous bony contact secondary to wedge osteotomy increases stability and reduces the risk of pseudarthrosis. Also, by performing an asymmetrical wedge resection

6 94 osteotomy correction of both sagittal and coronal plane deformity can be achieved. Extended PSO is a modification that involves extension of the bony resection cranially to include the rostral vertebral endplate with removal of the rostral intervertebral disc. The wedge osteotomy is closed over the vertebral endplate of the rostral vertebral body providing a greater degree of correction [ 3, 4, 7 ]. If necessary, an interbody spacer can be placed in the middle or anterior third of the disc space to act as a fulcrum to help prevent compromise of the neural foramina. The degree of deformity correction with PSO has been studied in multiple cadaveric, clinical, and radiographic studies. Li et al. obtained a mean of 36.4 correction with standard PSO and a mean of 48.5 correction with the modified extended PSO in a cadaveric study [ 4, 28 ]. Multiple clinical studies have shown deformity correction between 26.2 and 40.1, with an average correction of 32 with PSO [ 1, 2, 4, 11, 23, 24, 29, 30 ]. The expected correction for PSO in the thoracic spine is less than in the lumbar spine, likely due to the shorter heights of thoracic versus lumbar vertebral bodies Posterior Vertebral Column Resection MacLennan first described vertebral column resection in 1922 for treatment of severe scoliosis [ 31 ]. Suk et al. developed the posterior-only approach for VCR, with complete resection of one or more vertebral segments through a posterior- only approach, with the goal of achieving sagittal and coronal balance with shortening of the spinal column [ ]. This technique involves stabilization of the vertebral column with transpedicular screw fixation, followed by compete resection of all posterior elements at the level of the VCR, the vertebral body at the apex of deformity, and the adjacent rostral and caudal intervertebral discs [ 34 ]. In most cases, anterior fusion is performed via the posterior approach with structural support using an anterior cage that preserves anterior column height and enhances the degree of correction [ 4 ]. M.K. Singh et al. VCR is a complex procedure with significant risk of morbidity and neurological injury, especially when performed in the thoracic and upper lumbar region, as the spinal cord and conus may be vulnerable to injury during vertebral body resection, and they may be already compromised secondary to the deformity [ 31 ]. Therefore, VCR is generally reserved for severe, complex, rigid three-dimensional deformities that are not amenable to treatment with other techniques. It is primarily used for severe fixed sagittal deformity with severe coronal plane deformities, congenital kyphosis, hemivertebra, L5 spondyloptosis, and resection of spinal tumor [ 4, 6 ]. Suk et al. [ 34 ] reported correction of 61.9 in the coronal plane and 45.2 in the sagittal plane, whereas Lenke et al. [ 35, 36 ] in their two studies, noted major curve improvements of in scoliosis cases, in global kyphosis, in angular kyphosis and a combined in kyphoscoliosis cases. VCR remains the most powerful method for three-dimensional deformity correction, but this technique is associated with relatively higher risk of morbidity and neurological injury [ 25 ]. 8.2 Osteotomies and Complications (Table 8.1 ) Even though potential benefits of surgery for adult scoliosis patients have been shown in multiple studies [ ], these procedures are not benign and need careful planning and understanding of the risks and benefits by both patients and surgeons [ 40 ]. Smith et al. reviewed the SRS morbidity and mortality database for cases of thoracolumbar fixed sagittal plane deformity (FSPD) and analyzed the short-term complication rates in patients undergoing osteotomy, based on correction technique, surgical approach, surgeon experience, patient age, and history of prior surgery [ 25 ]. A total of 578 cases of FSPD were identified, of which 402 (70 %) were treated with an osteotomy, including 215 with PSO, 135 with SPO, 19 with anterior discectomy with corpectomy (ADC), 18 with VCR, and 15 with unspecified osteotomies. There were 170 complications (29.4 %) in 130 patients, and a mortality

7 8 Pedicle Subtraction Osteotomy Table 8.1 Examples of complications of lumbar pedicle subtraction osteotomy Early complications Transient or permanent neurological deficit Excessive blood loss/ coagulopathy Dural tear Postoperative respiratory distress Wound infection Deep vein thrombosis Incomplete correction Upper extremity compartment syndrome Brachial plexus injury Myocardial infarction Fluid overload Great vessel injury Ileus Late complications Pseudarthrosis Junctional kyphosis Rod fracture Breakdown of the L5-S1 disc distal to long posterior fusion Prominent iliac screws 95 rate of 0.5 %. The more common complications included durotomy (5.9 %), wound infection (3.8 %), neurological deficit (3.8 %), implant failure (1.7 %), wound hematoma (1.6 %), epidural hematoma (1.4 %), and pulmonary embolism (1 %). The overall complication rates associated with cases including an osteotomy were significantly higher than those not requiring osteotomy (34.8 % vs. 17 %), and this remained significant after adjusting for patient age, surgeon experience and history of prior surgery. There was a progressive increase in complication rates with more aggressive osteotomy, from no osteotomy (17.0 %), to SPO (28.1 %), to PSO (39.1 %), to VCR (61.1 %). In addition, there was a higher rate of complication in patients who underwent revision procedures (24.5 % versus 18.2 %). These risks should be carefully considered in patient counseling and surgical planning. To what degree a patient needs to be corrected and how much he or she can tolerate varies with each individual. Evaluation of this risk benefit balance should be personalized, with careful discussion with the patient. When considering surgical treatment, one should carefully consider patient age, overall health of the patient, severity of the symptoms, impact on patient quality of life, and willingness of the patient to accept the risk of surgery. A recent multicenter study has looked into the risk factors for major perioperative complication in adult spinal deformity surgery and found significantly higher rates of complications were associated with staged and combined anterior posterior surgeries [ 43 ]. Although these complex surgical procedures have been shown on average to be significantly beneficial in treating these patients, surgery is not necessarily indicated for every patient with sagittal deformity, and nonoperative methods should generally be the first line of treatment. 8.3 PSO Indications and Contraindications The main indication for PSO is fixed sagittal imbalance that needs more than 30 correction of lumbar lordosis [ 11 ]. Many of these patients have a history of previous fusion and a rigid flat back that requires three-column osteotomy for alignment correction. Patients with sharp angular kyphosis, ankylosing spondylitis with cervicothoracic kyphosis or thoracolumbar kyphosis, and progressive untreated idiopathic scoliosis may require PSO for adequate correction of deformity. PSO is a technically demanding procedure, with relatively longer operative time and blood loss in comparison with SPO. When considering PSO, the patient s overall health should be properly assessed to evaluate if he or she can tolerate the procedure. Patients with fixed sagittal deformity with severe coronal plane deformity may benefit more with a VCR. Similarly, patients with a gradual curve and an SVA <8 cm, or requiring only 10 to 20 of lordosis correction may be better treated with one or more SPOs, which are relatively less morbid. 8.4 Preoperative Clinical Assessment All patients with spinal deformity should have a global assessment of the spine with long cassette (36-in.) standing posteroanterior (PA) and lateral

8 96 radiographs, with the hips and knees fully extended, arms flexed and hands positioned along the clavicle, head facing forward, and feet placed shoulder width apart [ 44 ]. The pelvis and hip joints should be included in the film to demonstrate the true extent of the deformity. Changes in the joint angle of the lower extremity may change the sagittal alignment; hence, it is important to follow strict standard protocols for patient X-ray positioning. The lateral radiographs should include C2 to the pelvis along with femoral head. The sagittal parameters, including SVA, regional spinal alignment (lumbar lordosis and thoracic kyphosis), and pelvic parameters, including pelvic incidence and pelvic tilt, should be measured. On the PA view, the margins of the rib cage and the pelvis along with the femoral heads should be clearly visible. Coronal curves and coronal imbalance should be measured. Visualization of the ribs aids in identifying associated rib cage deformity and helps in the assessment of vertebral body rotation at the apex of the coronal curve. These radiographs can be complemented with additional studies including flexion/extension views, side bending views, supine full-length views, and use of traction or bolster radiographs to assess flexibility. Flexibility should be evaluated in all curves that are considered for surgical correction, as it helps in predicting the magnitude of curve correction that can be achieved, the number of spinal levels needed to be fused, and the need of osteotomy [ 12 ]. Advanced imaging studies, including computed tomography (CT), CT myelograms, and/or magnetic resonance imaging (MRI), are typically required to evaluate the bony anatomy, spinal canal, neural elements, vascular anatomy, and discs. Preoperative MRI and/or CT myelogram can be particularly important in patients with neurological symptoms or deficits. CT myelogram can be particularly useful in the setting of previous instrumentation, since it is often less affected by imaging artifact compared with MRI. Dual energy X-ray absorptiometry (DEXA) scans provide valuable information regarding bone density, and patients should be treated for osteoporosis prior to surgery if feasible. Patients with kyphoscoliosis with increased SVA but low PT (inability to compensate) should be evaluated for hip or lower extremity pathology, such as hip deformity or hip flexion contracture before considering surgical correction, as failure to diagnose these pathologies can lead to poor postoperative outcome. These patients are usually referred for preoperative physical therapy for better spinopelvic alignment prior to surgical fusion. The Thomas test is helpful in the assessment of hip flexure contracture [ 15 ]. 8.5 Preoperative Surgical Planning M.K. Singh et al Surgical Approach and Goal Osteotomy used to correct fixed sagittal plane deformity depends on the severity and flexibility of the deformity, and whether the kyphosis is long, sweeping, and gradual, versus short and angular. The PSO has the advantage of producing substantial correction of about 30 at a single level, without lengthening the anterior column, does not require the disc space to be mobile, and provides a large area of three columns of bone contact for successful bony fusion. The goal of surgery is to achieve complete or near complete correction of sagittal spinopelvic malalignment while minimizing perioperative and postoperative complications. It has been shown in multiple studies that correction of both SVA and PT has better postoperative clinical outcome than does correction of SVA alone [ 14, 15 ]. Schwab et al. have proposed that the ideal goals of surgical correction should be SVA <50 mm, T1 SPI <0, PT <20, and LL within 9 of PI [ 20 ]. Preoperative planning includes determining the extent of osteotomy required at the vertebral level to achieve the predicted level of correction and determining the ideal level for the PSO. Smith et al. performed a comparative analysis using a multicenter PSO database and the available mathematical formulas used to predict optimal postoperative SVA (<5 cm) [ 45 ]. They concluded that mathematical formulas that do not include pelvic parameters and changes in the unfused spinal segment (reciprocal changes) are

9 8 Pedicle Subtraction Osteotomy 97 less reliable predictors of postoperative alignment and outcome. Ondra et al. [ ] used a trigonometric method to predict the angle of correction required from a PSO to achieve optimal sagittal alignment, but the formula did not incorporate the pelvic parameters in the planning and failed to predict the extra correction needed in patients with increased PT. Kim et al. proposed another formula based on sagittal Cobb angle, recommending that the difference between LL and TK be a minimum of 20 (i.e., LL TK + 20 ) [ 49 ]. They also did not include pelvic parameters and reciprocal changes in the unfused segments. Rose et al. modified this formula, adding PI, and concluded that PI and TK can predict the LL necessary to correct sagittal alignment with the formula PI + TK + LL 45 [ 50 ]. The least amount of LL needed to regain sagittal alignment is LL 45 TK PI, but this formula did not integrate the amount of PT needed to maintain upright posture; hence, patients with higher PT were at risk of being undercorrected based on this formula. Schwab et al. proposed the formula LL = PI ± 9 for patients with decreased LL but otherwise reasonable spinal contour [ 20, 51 ]. This formula was also limited by the exclusion of PT. By predicting both postoperative PT and SVA with their formula, Lafage et al. have tried to overcome these shortcomings [ 52, 53 ]. The expected PT is dependent on LL and PI and is calculated by: PT = PI 0.52 (Maximal lumbar lordosis) 0.19 (Maximal thoracic kyphosis), whereas SVA is calculated by another formula: SVA = PI 5.13 (Maximal lumbar lordosis) 4.45 PT 2.09 (Maximal thoracic kyphosis) (Age). Smith et al. in their analysis of five formulas found that the formulas by Lafage et al. were able to predict both poor and good SVA correction with good accuracy and had the best total prediction accuracy [ 45 ]. The preoperative plan should be altered if poor outcome is predicted [ 52, 53 ]. The inclusion of spinopelvic parameters and the integration of patient age into the calculation were credited for the improved accuracy of their formula. The decision regarding where to place the PSO depends on the site of the deformity. A patient with loss of lumbar lordosis, without thoracolumbar kyphosis, is treated with PSO below the level of conus medullaris to reduce the risk of spinal cord injury from manipulation, retraction, or wedge closure. It is usually performed at L2 or L3, and occasionally at L4, as the apex of lumbar lordosis in a sagittally aligned spine is usually at the L3 L4 disc space. If there is coexisting flexible thoracolumbar kyphosis, a PSO can be performed in the lumbar region and the fusion extended proximally in the thoracic spine with or without multilevel SPO. If there is coexisting rigid thoracolumbar kyphosis, then the PSO is usually performed at the apex of kyphosis. A PSO can be performed in the thoracic or cervical spine if indicated, but with potential increased risk of neural injury. Lafage et al. in a multicenter clinical study found that changes in the regional spinal curvatures (thoracic kyphosis and lumbar lordosis) correlated with PSO degree of resection but not with the level of PSO [ 54 ]. The multicenter data indicated a consistent range of correction afforded through one level PSO in the lumbar region, with mean correction of 24 at L1 and L2, 25 at L3, and 22 at L4, which is less correction in comparison to other reports from a single center [ 1, 24, 30, 54 ]. This finding demonstrates that greater degrees of realignment may not be possible universally in all cases and that during surgical planning, a singlelevel PSO with multilevel SPO or an extended PSO should be considered for greater correction. Lafage et al. also reported in their multicenter study, that parameters associated with spinal global alignment (SVA and T1/T9 SPI) did not correlate with vertebral level of PSO, which contradicts previous reports that suggested that the vertebral level of PSO has significant impact on the degree of correction of SVA. However, previous reports did report correlation between PSO level and PT. These findings support the clinical application of their predictive formulas mentioned earlier, for planning realignment surgery. The lack of correlation between SVA and PSO level could be related to the pelvis acting as a principal regulator for subtle changes in the lumbar alignment (PSO at L1 versus L4) and needs to be further investigated [ 54 ]. Several studies have shown the importance of pelvic parameters in the assessment of sagittal

10 98 spinal alignment and correction of both SVA and PT for better outcomes. Schwab et al. in their multicenter retrospective consecutive PSO case series of 99 patients evaluated the causes of failed spinopelvic alignment following PSO and reported a 23 % realignment failure rate [ 55 ]. Patients with failed PSO had significantly larger preoperative spinopelvic deformity, including larger preoperative SVA, larger PT, larger PI, and greater PI-LL mismatch. Deformities associated with larger spinopelvic malalignment must be recognized during surgical planning, as these patients should receive larger osteotomies or additional corrective procedures to avoid under correction [ 50, 55 ]. Another important factor to be considered during surgical planning for correction of sagittal malalignment is reciprocal changes. Reciprocal changes are compensatory changes that occur in the unfused spine segments and are more prone to develop when major corrections of spinal deformity are performed over fewer spinal segments [ 56 ]. Lafage et al. have reported negative impact of increased reciprocal thoracic kyphosis on postoperative SVA after lumbar PSO [ 57 ]. In 34 patients who underwent lumbar PSO with upper instrumented vertebra below T10, the mean PSO resection was 26, lumbar lordosis increased from 20 to 49, SVA improved from 14 to 4 cm, and PT improved from 33 to 25. The mean increase in thoracic kyphosis was 13 within the unfused thoracic segment, but was unchanged in 11 patients, while 5 patients had favorable reciprocal changes and 18 patients had unfavorable changes. In 6 of the 18 patients with unfavorable changes, this was due to junctional failure (defined as a 10 or more increase of kyphosis over 2 vertebrae above the fusion). They found that unfavorable reciprocal changes were associated with greater preoperative sagittal malalignment, higher PI, higher preoperative PT, and greater age. Furthermore, it has also been shown that there are reciprocal changes in LL after correction of TK [ 56, 58 ]. Another important regional parameter that is often overlooked is cervical lordosis and cervical sagittal alignment. Smith et al. reported that patients with sagittal plane deformity may have increased cervical lordosis to maintain horizontal gaze and surgical correction of the sagittal alignment, results in reciprocal change with relaxation of cervical hyperlordosis [ 59 ]. Whether or not fusion is performed across the lumbosacral junction is an important decision for the overall success of the surgery. The status of the lower lumbar spine discs, the level of curve, or lumbosacral fractional curve determines whether instrumentation and fusion is warranted across the lumbosacral curve. However, patients with lumbar degenerative disorders in whom the lumbosacral junction is not included are at higher risk of breakdown in the region and worsening of sagittal malalignment. If the osteotomy is at L4 or below or if the patient is osteoporotic with poor sacral fixation, then extension of instrumentation to the pelvis using iliac screws is recommended [ 3 ] Patient Positioning M.K. Singh et al. Prone positioning for posterior spinal reconstructive surgery needs appropriate padding of all bony prominences, as patients are often required to be prone for an extended period of time. We typically use a radiolucent, open-frame Jackson spine table, which allows the abdomen to float freely, thereby decreasing venous pressure, and accentuates lumbar lordosis with hip extension [ 60, 61 ]. The chest pad and the hip/thigh pad should be far enough apart to facilitate lordosis correction with closure of the PSO. The arms should be properly positioned and padded to prevent plexus and other nerve injuries. Care should be taken to ensure that the face and eyes are free of pressure throughout the surgery. The patient should be placed in a slight reverse Trendelenberg position to help prevent ischemic optic neuropathy. A severely kyphotic patient, when positioned prone may not fit properly on the table and additional padding for the chest and pelvis may be required to properly accommodate the patient on the table. Some of these patients may warrant placement in Gardner- Wells tongs with gentle axial traction (10 15 lb) in order to minimize pressure on the face and eyes during the surgery. Countertraction with femoral pins may be considered in patients with

11 8 Pedicle Subtraction Osteotomy significant pelvic obliquity including neuromuscular deformities. However, since femoral countertraction causes spinal distraction, it should be avoided in patients undergoing VCR and patients with angular deformity, as it may increase the risk spinal cord injury. Intraoperative fluoroscopy can be used to localize the levels after positioning and after subperiosteal exposure, as well as to evaluate for change in spinal alignment after positioning [ 62 ]. Notably some patients may have significant improvement in lumbar lordosis after positioning, which in some cases may alter the surgical plan Neurophysiological Monitoring New neurologic deficits, including nerve root injury, cauda equina injury, and spinal cord injury, are inherent risks and among the most feared complication of spine surgery for correction of deformity [ 30, ]. There have been multiple studies that have demonstrated that neurophysiologic monitoring can assist in early detection and possible prevention of new neurologic deficit [ 6 8, 67, 68 ]. We routinely use somatosensory-evoked potentials (SSEP), transcranial motor-evoked potentials (tcmeps), and free-running electromyography (EMG). The combined monitoring of SSEP and tcmeps helps to decrease false-negative rates. Transcranial MEPs are very sensitive to any changes in spinal cord blood flow, either due to hypotension or a vascular insult [ 9 ]. Also changes in tcmeps are detected earlier than changes in SSEP, hence facilitating more rapid identification of impending spinal cord injury [ 9, 10 ]. Neurophysiological monitoring of the upper extremities helps to monitor for brachial plexus compromise secondary to positioning and also serves as a control for comparison with lower extremity monitoring Anesthetic Considerations Medications that effect neurophysiological monitoring should be avoided. Total intravenous anesthesia (TIVA) is often preferred for 99 electrophysiological monitoring, and Propofol is often the agent of choice for induction of general anesthesia in PSO cases. Proper venous access should be maintained, preferably with central venous line. An arterial line for blood pressure monitoring is typically used. A mean arterial pressure may be maintained greater than mmhg to help ensure adequate perfusion of the spinal cord, especially during the osteotomy and closure of the osteotomy. PSO often involves significant blood loss; we routinely use two cell savers to decrease the need for blood transfusion. Consideration should be given to the use of antifibrinolytic agents such as tranexamic acid; we routinely use tranexamic acid, unless contraindicated, in PSO cases to minimize blood loss. Antifibrinolytic agents, including tranexamic acid, slow plasmin- mediated fibrin clot dissolution, but are not true procoagulating agents [ 69, 70 ]. Plasmin formation is not inhibited, but instead it prevents plasmin antifibrinolytic complex from binding to and degrading fibrinogen and fibrin monomers [ 69, 71 ]. Antifibrinolytic agents have been shown to significantly reduce perioperative blood loss and postoperative hemovac output in posterior spinal fusion in adolescent idiopathic scoliosis [ 72 74]. Blood coagulation factors are closely monitored and replaced as required during the surgery. For cases involving blood loss in excess of two liters, patients may need fresh frozen plasma in addition to blood transfusion to replace coagulation factors. The replacement of coagulation factors should be based on estimated blood loss rather than laboratory values of increased prothrombin time [ 2, 3 ]. Rotational thromboelastometry (ROTEM) is a whole blood point of test used to assess the patient s coagulation status, that rapidly detects systemic changes of in vivo coagulation earlier than standard coagulation tests and it might be a helpful device in guiding early transfusion and correction of perioperative coagulopathy secondary to excessive blood loss [ 75 77]. Postoperative blindness is extremely rare complication of anesthesia, with an incidence of % and is more likely to occur after major surgery such as cardiopulmonary bypass procedures or spine surgery [ 78 ]. The incidence is higher

12 100 in spinal surgery to 0.2 %, and patient factors associated with postoperative blindness include hypertension, diabetes, smoking, peripheral vascular disease, carotid artery disease, glaucoma, obesity and optic nerve vascular anomalies [ 78 ]. 8.6 Surgical Techniques (Table 8.2 ) Proper preoperative surgical planning, with regard to defining the surgical goals and plan for instrumentation, facilitates smooth and sequential progression of the procedure. Careful planning is important, as it helps to improve the efficiency of the surgical procedure, thereby decreasing the operative time and blood loss, which in turn helps to reduce the risks of perioperative complications. Table 8.2 Key steps of pedicle subtraction osteotomy surgical technique Proper posterior exposure of spine Placement of pedicle fixation points Perform laminectomy through pars interarticularis at the level above and below the PSO Skeletonize the pedicle from the posterior osseous attachment, exposing the nerve root cephalad and caudad to the pedicle Decancellate and resect the pedicle bilaterally flush with the vertebral body, protecting the thecal sac and nerve root Create and maintain a subperiosteal plane between the lateral vertebral body wall and the soft tissue Perform vertebral wedge-shaped decancellation and osteotomy with wider resection posteriorly and focal point anteriorly without breaching the anterior cortex Develop the epidural plane between the posterior vertebral cortex and anterior thecal sac Remove the posterior cortical wall and make sure to enlarge the canal centrally and remove any bony fragment that may impinge on the exiting nerve root Wedge osteotomy closure with sequential compression and cantilever maneuver for symmetrical complete closure of osteotomy Make sure there is no compression of thecal sac, nerve root, or any changes in the neurophysiological monitoring Posterolateral decortication and placement of harvested bone graft M.K. Singh et al. A meticulous exposure with good hemostasis should be performed, as many of these patients have had multiple previous surgeries. The exposure depends on the nature and complexity of the deformity and is usually extensive, often involving both thoracic and lumbar regions. The interspinous and supraspinous ligaments, especially at the proximal/cephalad end should be preserved during exposure, as this may help in preventing later development of proximal junctional kyphosis [ 79 ]. After the exposure, instrumentation is next placed, which typically includes pedicle screws but may also include hooks. We prefer to place all fixation points including pedicle and iliac screws before starting the osteotomy. Generally, at least two levels of fixation above and below the planned level of the PSO are needed, but in most deformity cases multilevel posterior segmental fixation points are used for correction of deformity. Careful attention should be paid to the anatomical variation of the pedicle size, angulation, and rotation, as coronal plane deformity and previous fusion mass may distort the normal anatomical landmarks. Attention is next turned to performing laminectomies and posterior decompression. A partial laminectomy that extends through the pars interarticularis bilaterally at the level superior to the level of PSO and complete laminectomy that extends through the pars interarticularis bilaterally at the level of the PSO, with bilateral facetectomies is performed. Central decompression is completed with excision of the ligamentum flavum. In cases of revision in which laminectomy has been performed at the level of the planned PSO, care should be taken to remove scar tissue overlying the thecal sac, as this tissue may buckle during osteotomy closure and compromise neural structures. In essence, the initial stage of a PSO includes performing an SPO both cephalad and caudad to the pedicle for the PSO level. The bone surrounding the pedicle is then completely removed including the transverse process, exposing the cephalad and caudad nerve roots. The pedicles are isolated and separated from all osseous attachments other than the vertebral body (Fig. 8.4 ). The medial wall of each pedicle is then carefully delineated and the thecal sac and exiting

13 8 Pedicle Subtraction Osteotomy 101 Fig. 8.4 Isolation of pedicle from all osseous attachment except the vertebral body. The thecal sac is protected with the nerve root retractor. Complete pars to pars bony removal is done before pedicle removal Fig. 8.5 Vertebral body decancellation is performed through the stump of the pedicle with a curette or drill or osteotome. The lateral and posterior wall is preserved till the intervertebral osteotomy is completed nerve roots are protected with a nerve root retractor. The next step is to decancellate the pedicle using curettes or a drill. The nerve root superior to the pedicle is usually sufficiently far from the pedicle to not be at high risk of injury during pedicle resection, but the inferior nerve root and the thecal sac must be carefully retracted and protected. Following decancellation, the cortical walls of the pedicles are then resected flush with the vertebral bodies using a Leksell rongeur. Hemostatic agents and cottonoids can help to control epidural bleeding during isolation and resection of the pedicle. It is important to be certain that the pedicles are resected flush to the vertebral body, as any osseous remnant may impinge on the nerve roots when the wedge osteotomy is closed. Subsequently, the plane between the lateral aspects of the vertebral body and the adjacent soft tissues is developed. This can be achieved by carefully passing a small Cobb elevator in the subperiosteal plane along the lateral vertebral body wall, sweeping from cephalad to caudal reflecting the soft tissue away from the lateral vertebral wall. Care should be taken to avoid injury to the segmental vessel, the exiting nerve roots and the sympathetic chain. If a segmental artery is compromised, hemostasis can be achieved with bipolar coagulation or hemostatic agent. Defining the lateral aspects of the vertebral body is important, since this provides visualization to help protect the adjacent structures and provides visualization of the bony anatomy including the anterior depth and lateral aspects of the vertebral body. The plane along the lateral aspects of the vertebral body is maintained during the osteotomy with the use of a sponge or one of many specially designed retractors. Vertebral body decancellation is performed next in a preplanned wedge-shaped fashion. This may be achieved using primarily using an osteotome or with a combination of a high-speed drill, curettes, and pituitary rongeurs (Fig. 8.5 ). During the decancellation and osteotomy, it is important to maintain proper orientation, with wider resection posteriorly that leads to a focal point anteriorly. Fluoroscopic guidance can be helpful in maintaining a proper orientation and in fashioning an appropriate osteotomy wedge. The decancellation and osteotomy are typically completed on one side before performing the same procedure on the opposite side. We use an L-shaped osteotome to make a cut at the inferior and medial margin of the pedicle, followed by a straight

14 102 M.K. Singh et al. Fig. 8.6 Resection of the lateral vertebral wall is performed after the lateral soft tissue is reflected subperiosteally with a Cobb elevator. Care should be taken to protect the exiting superior nerve root and sympathetic chain when removing the lateral wall. Once the osteotomy is completed on one side, temporary rods should be placed before starting osteotomy on the opposite side to prevent premature closure or translation osteotome to cut the lateral cortex of the vertebral body. The initial bone wedge is removed and further medial decancellation of the vertebral body is performed in a symmetrical fashion using curette, drill, and/or pituitary rongeur from lateral to medial and ventral to the posterior vertebral wall. It is important to ensure that a wedge shape with an anterior apex is maintained, otherwise closure of the defect may simply produce column shortening, rather than impacting the sagittal alignment. The osteotomy wedge must extend laterally to include the lateral cortical walls of the vertebral body, since residual cortical wall can prevent closure of the osteotomy (Fig. 8.6 ). In order to maximize the potential osteotomy angle, the vertebral body removal should begin below the pedicle stump inferiorly and extend to be just below the cephalad end plate superiorly. The anterior cortex is persevered, including at the apex of the wedge, to act as a hinge and to avoid translation at the time of closure of the osteotomy. In addition, preserving the anterior cortex helps to minimize risk to the great vessels and potentially other structures. During the osteotomy, the thecal sac and nerve roots are protected with nerve root retractors. Gentle traction may be applied with the retractor on the thecal sac if the level of osteotomy is below the level of conus. Bleeding is usually brisk during this procedure and is usually controlled with application of surface hemostatic agents, packing with cottonoids/ sponges, and by working to finish the osteotomy in an organized and efficient way. After completion of the osteotomy on one side, it should be packed to control bleeding and a temporary rod should be placed across the osteotomy on that side to help prevent premature, uncontrolled closure of the osteotomy. The temporary rods also help to prevent translation of the spine in patients with long fusion above or below the osteotomy [ 2, 3 ]. The osteotomy should then be completed on the opposite side in a similar fashion, and the decancellation continues until one side connects with the other. Working underneath the posterior vertebral cortex, the cortex is thinned as much as possible by thoroughly removing the cancellous bone behind the posterior vertebral cortical wall. Once the posterior wall is sufficiently thin, the epidural space between the posterior vertebral wall and the anterior dura should be carefully developed using a Woodson elevator in order to free any potential adhesions between the dura and bone in order to prevent an anterior dural tear. The risk of durotomy and spinal fluid leak is high, as many of these patients have had one or more previous surgeries, resulting in scar tissue that can disrupt normal tissue planes. Epidural bleeding is controlled with judicious use of bipolar cautery and hemostatic agents (Fig. 8.7 ). Once the posterior wall of the vertebral body is sufficiently thin, a plane has been developed between the thecal sac and the posterior vertebral wall, and the desired wedge resection including the lateral vertebral walls is complete, the posterior cortical wall of the vertebral body is carefully pushed into the cavity created from the wedge resection with a Woodson elevator, reverse angled curette or one of many specially designed posterior vertebral wall impactors (Fig. 8.7 ). The fractured posterior cortex is then removed using

15 8 Pedicle Subtraction Osteotomy 103 Fig. 8.7 The thin posterior wall is fractured into the cavity formed by the decancellation of the vertebral body with specially designed posterior vertebral body impactor Fig. 8.8 Before closing the wedge, superior and inferior lamina are further resected and nerve root completely decompressed. The wedge should be inspected to confirm that it extends from inferior to the pedicle to just below the end plate superiorly and connects one side to the other, with no residual bony element pituitary rongeurs. Careful attention and orientation is needed for the wedge osteotomy to be symmetrical in order to result in symmetric closure and the desired sagittal correction. However, in cases of kyphoscoliotic deformity, an asymmetrical osteotomy can be performed and should be well planned preoperatively. By resecting a larger wedge on the convex side rather than the concave side of the vertebral body, coronal plane correction can be achieved during wedge closure [ 3, 4, 7 ]. It should be noted that all bone removed during the osteotomy should be preserved to be used as graft for posterolateral arthrodesis. The next step is wedge closure, which can be accomplished by compression with manual pressure or cantilevering the spine. Before closing the osteotomy, a Kerrison rongeur may be used to enlarge the central canal and remove any bone fragments or residual elements of the pedicles that may impinge on the exiting nerves, as the two adjacent foramina are now combined into one foramen with two exiting nerve roots on each side (Fig. 8.8 ). The temporary rod-screw construct is loosened, but not removed, as it can provide a guide during the closure of the wedge resection (Figs. 8.9 and 8.10 ). The neural elements are carefully observed for any compression from bony or ligamentous structures during the closure. If the osteotomy is closing adequately with no changes in the neurophysiological monitoring, then a permanent contoured rod spanning the entire segment is placed with caps applied loosely over the screws on the side contralateral to the temporary rod. A compressor is placed along the heads of pedicle across the osteotomy on each side and gently compressed, thereby further closing the osteotomy defect (Fig ). The temporary rod on the opposite side is then replaced with a contoured permanent rod and again compressed across the osteotomy level before final tightening of the screw caps. The thecal sac and nerve root should be inspected again for any bony compression or excessive buckling of thecal sac, in which case additional bony resection should be performed. If the osteotomy does not completely close, then intervening residual bone fragments,

16 104 M.K. Singh et al. Figs. 8.9 and 8.10 The screws over the temporary rods are loosened to allow for the wedge closure, but are not removed as they provide a guide during the closure. Compressive forces are applied to close the osteotomy, note the two exiting nerve roots through the single large newly created foramina Fig The bony margins are properly opposed, the thecal sac is inspected for any compression or excessive buckling, and following which a permanent contoured rod is placed on the opposite side of temporary rod and secured over the screws to hold the wedge closure. Then the temporary rod over the opposite site is replaced with a permanent contoured rod inadequate rod contouring, or subluxation should be considered. Residual bony fragments can be removed with a curette or rongeur. In situ benders may be used to further contour the rod. A recent report from Smith et al. documented high rates of rod fracture across PSO sites; as a result, some surgeons routinely place a satellite rod on one or both sides across the PSO level using side connectors [ 80, 81 ]. Subluxation may occur during closure; most commonly posterior subluxation of the proximal elements over the distal elements and it should be addressed for proper closure and anatomical alignment and to help prevent spinal cord or cauda equina compromise. Reduction screws placed at caudal adjacent level can be used to reduce subluxation with rod reduction into the screw head [ 13 ]. If there is concerning change in the neurophysiological monitoring during the closure, it should be stopped and reversed. Further decompression posteriorly and/or laterally may be necessary, depending on whether the changes are in the MEPs or EMGs, respectively, before attempting to reclose the osteotomy wedge. Neuromonitoring changes may also be secondary to subluxation causing impingement of the thecal sac. Neurophysiological monitoring

17 8 Pedicle Subtraction Osteotomy 105 a b c d Fig Case example of an L3 extended pedicle subtraction osteotomy (PSO). A 72-year-old woman with a history of T8-S1 instrumented arthrodesis presented with sagittal imbalance and gross pseudarthrosis at L5-S1 with large halos surrounding the S1 screws and significant L5 radiculopathy. Shown are full-length standing preoperative anteroposterior (AP; a ) and lateral ( b ) radiographs and postoperative AP ( c ) and lateral ( d ) radiographs. Revision surgery included removal of prior instrumentation, replacement of instrumentation from T3-ilium, L3 extended PSO, and L5-S1 transforaminal lumbar interbody fusion (TLIF). Vertical white lines represent C7 plumb lines and the arrow indicates the level of the PSO should be continued until the end of surgery, even if there were no changes in monitoring during the closure, to monitor for any delayed neurological compromise. The procedure is completed with posterolateral decortication using a high-speed drill, followed by placement of harvested bone graft from the osteotomy and iliac crest for posterolateral fusion. The use of osteobiologic agents may enhance the fusion. We usually use two subfascial drains and close the wound in a standard multilayer fashion. Modification of the PSO technique may increase the amount of deformity correction. An extended PSO involves including rostral disc space in the osteotomy and closing the wedge onto the end plate of the adjacent vertebral body (Fig ). For an extended PSO, an interbody device can be placed anteriorly into the osteotomy defect to serve as a fulcrum and help prevent compromise of the neural foramina. 8.7 Postoperative Care Postoperatively patients are monitored in an intensive care unit. Patients may be left intubated overnight if required secondary to prolonged intubation, fluid shifts, and blood loss. If the patient is left intubated, we routinely lighten the sedation postoperatively to get a good a detailed neurological exam and intermittently lighten the sedation to check the neurologic exam throughout the evening following surgery. Extubation should be planned as soon as the patient meets the criteria for extubation. We typically leave the subfascial drains in place until output is <30 cc per 8-h shift for two consecutive shifts [ 82 ]. Postoperative hemoglobin, hematocrit, and coagulation profile should be monitored closely and blood products transfused as needed. Sequential compression devices are used to reduce the risk of