Pedicle subtraction osteotomies (PSO) in the lumbar spine for sagittal deformities
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The narrow correlation between sagittal alignment parameters and clinical outcomes has been widely established, demonstrating that improper sagittal alignment is a clinical condition that is associated with increased pain and limitations in patients’ functional ability.
Lumbar pedicle subtraction osteotomy (PSO) is indicated in the treatment of large sagittal (more than 25° of rigid loss of lordosis) deformities of the lumbar spine or its combination with coronal deformity, especially when they are rigid. Indication should be based on careful assessment of the severity of symptoms, functional impairment, functional expectations of the patient, general clinical condition and surgical and anesthesiological team experience. Risk should be carefully assessed and discussed to obtain appropriate informed consent.
Surgical planning includes selection of the safest levels for the upper and lower instrumented vertebra, site of the osteotomy, modality of fixation, and, most importantly angular value of the correction goal (target lumbar lordosis). Failure to adequately obtain the necessary amount of sagittal correction is the most frequent cause of failure and reoperation.
PSO is a valuable surgical procedure in correction of severe hypolordosis (=relative kyphosis) in the lumbar spine. It is a demanding procedure for the surgeon, the anesthesiologist and the intensive care team. Although its complication rate is high, it has a substantial positive impact in the quality of life of patients, including the elderly.
KeywordsPedicle subtraction osteotomy Sagittal imbalance Adult deformity Lumbar kyphosis Lumbar spine
The narrow correlation between sagittal alignment parameters and clinical outcomes has been widely established in the last years, demonstrating that improper sagittal alignment is a clinical condition that is associated with increased pain and limitations in patients’ functional ability [1, 2, 3]. Recent classifications [4, 5] of adult deformity take into account sagittal alignment as a major factor determining outcomes. Recently, a validated, comprehensive classification of sagittal deformities has improved the comprehension of the deformity, showing how sagittal misalignment in any region of the spine has an influence on the rest of the spine, with reciprocal compensatory changes that express as deformity patterns (regional deformity plus typical compensatory changes for each type of deformity are a deformity pattern), and assisting the surgical planning .
Three major metric parameters that account for a better result have been extensively validated: lumbar lordosis/pelvic incidence mismatch (LL–PI), pelvic tilt (PT) and sagittal vertical axis (SVA) [7, 8]. The first of these parameters (PI) is actually the only morphologic indicator of alignment and its advantage is that it can be measured independently of the patient’s position (i.e., in the surgical decubitus position); the other parameters are positional parameters that present themselves as the result of the interaction between spinopelvic morphology (PI–LL, and thoracic kyphosis—TK—mainly) and muscular activity, and can only be measured with the patient standing in a full lateral spine film . Recent research has provided reliable methods to predict the positional parameters PT and SVA from patient’s age and morphological parameters (PI, LL, TK), thus permitting a calculation of the adequacy of alignment with the patient in decubitus. This finding is of paramount importance to intraoperatively predict the result of surgery in terms of alignment . Different surgical maneuvers can be used to correct abnormal sagittal alignment. Minor amounts of PI–LL mismatch can be surgically addressed with standard surgical maneuvers (cage positioning, partial arthrectomy, posterior compression, rod contouring), and generally these maneuvers can increase the lumbar lordosis by 5°–7° per instrumented level. In some instances of collapsing spine (a major PI–LL mismatch is present and the spine is still flexible in traction or fulcrum-extension films), these methods can also provide correct alignment. Conversely , in non-flexible spines with a major LL–PI mismatch, greater corrections are needed and osteotomies of the spine are the most frequently used method of correction. Posterior column osteotomies [12, 13] have a more limited power of correction, in the range of 8°–10° per osteotomy level. The combination of large anterior cages with posterior column shortening osteotomies can provide a greater correction (10°–15° per level). Tricolumnar pedicle subtraction osteotomies (PSO) give a greater correction at a single point of the spine, ranging from 20° to 35° per level.
Though both lumbar lordosis and thoracic kyphosis have an influence on final alignment, it has been demonstrated  that it is the amount of lumbar lordosis in relation to the pelvic incidence that is the most influential parameter for the prediction of the final standing alignment of the spine, being the amount of thoracic kyphosis less critical regarding the final alignment of the patient.
For this reason, PSO in the lumbar spine is one of the most frequent procedures in the treatment of severe sagittal misalignment of the spine in adults.
In this article, we will discuss the indications, decision rules and surgical technique of PSO in the lumbar spine for sagittal deformities.
Indications for PSO
Indications for PSO
Objective (measurable: X-rays, other imaging)
• Proven progression of the curve
• Sagittal or sagittal and coronal deformity
• Presence of lumbar hypolordosis
• Spine stiffness in functional X-rays (supine position in extension over fulcrum, traction films)
• If amount of correction needed is ≥25°
• Amount of thoracic kyphosis when stiff or flexible
• Osteoporosis (bone density measurement), old fractures, pending new fractures, etc.
• Spinal canal narrowing: central, recesal, foraminal (MRI and/or Myelo-CT)
Subjective (history and clinical findings)
• Severity of symptoms
Back pain (when upright, walking, in bed, with physical activity)
Radicular pain (constant, only when upright standing, or when sitting or lying)
Claudication symptomatology: walking distance
• Combined with neurological signs
Sensory disturbances, motor deficit, pos. Lasègue sign
• Clinical balance: amount of leaning forward: need for rollator or crutches
• Morbidity status (pulmonary, cardiac, renal thromboembolic disorder, allergies, overweight, previous spine and other surgery
• Expectations of the patient
Location of the PSO and extent of fixation for correction and its maintenance
Location of the OT
Principle: at the level of the apex of the kyphosis or in case of lumbar kyphosis (hypolordosis) at L4 (best effect since 60 % of lordosis is between L4 and S1)
Extension of fixation
Principle: two above and two below the OT i.e. OT at L3 fixation min. L1—L5 or OT at L4, fixation min. L2—sacrum/pelvis
Above or below, but not at the TH-L junction: L2—S1/pelvis or TH 10—S1/pelvis
Never stop at the apex of the thoracic kyphosis (Th4-8): end at Th2—S1/pelvis
In rigid thoracic kyphosis without the potential of compensation: Th2—S1 (plus rigorous posterior release)
SVA >6 cm: long fixation necessary
Parkinson disease and malalignment: long fixation
In osteoporosis do not stop at thoracic kyphosis (prophylactic vertebral body augmentation: end vertebra with screws and two vertebra above)
After decancellation has been completed in the posterior aspect of the vertebra extending from lateral to lateral wall, retraction of the dural sac medially permits exposure of the posterior wall, which can be resected with Kerrisons. The most medial, thinned posterior wall can then be impacted anteriorly under the dural sac with angled bone tamps. After doing this, resection of the lateral walls is performed to complete the osteotomy . Several maneuvers have been described to close the osteotomy. The authors’ preferred method is reverse breaking of the table, as it does not put any stress on the instrumentation. Other methods include a cantilever maneuver with the contoured rod (in this case the rod should be solidly fixed at least to three solid anchors below the osteotomy and pushed to the anchors above the osteotomy). Repeated contouring of the rod, extremely sharp contouring or use of rod benders that make notches in the rod is likely to decrease the resistance of rod to cyclic loading and should be avoided. Additionally, the choice of rod material and diameter has an influence on the ability to bend it, its stiffness and its resistance to cyclic load. Manual force is exerted on the rod until it is securely fixed to two or three anchors above the osteotomy. This strategy reduces the risk of screw pullout, which is possible if a single screw is loaded. In difficult cases, a sublaminar-band-assisted technique for osteotomy closure can be used . During closure of the osteotomy attention is paid to potential impingement of the dural sac or roots by bony edges or ligament remnants. Some dural kinking is common and can be resolved by sufficient undercutting of the laminae. A C-arm lateral view with two parallel images at S1 and L1 is used to measure the final lordosis. If the general condition of the patient is not critical, any under-correction must be addressed to obtain the planned alignment (generally expressed as magnitude of lumbar lordosis). Ideally, posterior interlaminar contact at the end of the closure of the osteotomy should be achieved. This increases the stability and the chance of obtaining a solid fusion. After closure of the osteotomy evoked potentials are run to detect damage to neural structures. Meticulous preparation of a posterior fusion bed along the instrumented area, plus intertransverse fusion bed around the osteotomy is essential to obtain fusion. Grafting is performed with local bone, eventually supplemented with iliac crest, homologous bone or other bone substitutes.
Postoperatively the patient is usually kept in intensive care for 24 h. After 48–72 h, they can sit and stand up. Long, full spine standing X-rays are obtained as soon as the patient can stand without assistance to assess the final alignment. A thoracolumbar orthosis, to decrease cyclic load on the rods, worn for 4 months during the daytime is appropriate if tolerated.
A satisfactory postoperative alignment should include a SVA less than 5 cm from the posterosuperior corner of the sacrum and a PT not exceeding 21°, as these thresholds have demonstrated to be related to improved outcomes. Postoperative alignment resulting in imbalance has shown to be associated with increased risk of reoperation and poorer clinical outcomes in terms of VAS and ODI score reduction, and can be a risk factor for implant failure .
A thoracolumbar orthosis worn for 4 months during the daytime is appropriate, if tolerated to decrease cyclic load on the rods and can protect to some extent from the risk of rod failure. Our rehabilitation protocol consists mainly in instruction for independent ambulation and isometric exercises for the abdominal and back muscles in supine position, and change of position from standing to sitting and laying. No attempt is made to increase the range of motion of the spine in the postoperative period.
Two key issues for clinicians treating adult spine deformity (ASD) are: complications and beneficial outcome for patients after surgery.
The fate of the distal discs—when the lower instrumented vertebra is L5—is a concern for surgeons. Cho et al.  compared fusion ending at L5 versus ending at S1 in adults with scoliosis. Blood loss and operative complications were similar between the two groups. Correction of the lumbar lordosis was significantly better in the S1 group. After 2 years of follow-up, 58 % of the patients in the L5 group developed significant degenerative disc disease at L5-S1; half of them were symptomatic. The authors recommend that fusion to S1 is preferable to fusion to L5, even in patients with a normal L5-S1 segment of motion.
Mechanical and general complications are frequent in surgery for ASD. In 2011, the Scoliosis Research Society (SRS) published a large study including nearly 5,000 cases of surgery for adult scoliosis reported in a society database . They found an overall complication rate of 13.4 %, without significant differences due to age group (over and below 60 years) or etiology (idiopathic vs degenerative). Higher complication rate was observed in revisions, anterior and posterior approaches and osteotomies. The reported mortality rate was 0.3 %.
A different study from the SRS with focus on patients with rigid sagittal deformity undergoing surgery presents different data . This study includes 578 patients. Twenty-nine percent of them had complications, and the rate was higher in cases requiring osteotomies (34.8 %). There was an increase in complication rates from no osteotomy (17.0 %), to SPO (28.1 %), to PSO (39.1 %), to VCR (61.1 %). Raw mortality was 0.5 %, and it was not related to the type of osteotomy or even patients without osteotomies. In this study, patient’s age was not related with different risk of complications.
A systematic review of the literature on complications for ASD surgery in patients older than 60 years  found a complication rate of 38 % and 0.85 % of mortality. The same study found substantial improvements in outcomes measures in these patients, with average ODI improvement of 24 % and VAS improvement of 5.2 points in a ten-point scale. There was no specific information regarding procedures with osteotomies. Smith et al.  in a multicenter study that retrospectively reviewed a prospectively built database with 2-year follow-up of patients operated for ASD reported a complication rate that increased with age group from 17 % (below 45 years) to 42 % (45–64 years), and 71 % (older than 65 years). The clinical outcomes measured with ODI and VAS scores were similar across age groups, but the baseline status was worse as age increased. Thus, older patients had higher complication rates but also higher clinical benefit as shown by these outcomes measures. There were no deaths reported in this series of 453 patients.
PSO is a valuable surgical procedure in correction of severe hypolordosis in the lumbar spine. It is a demanding procedure for the surgeon, the anesthesiologist and the rehabilitation team. Key factors of success are correct indication (considering the clinical impact of the deformity, the general condition of the patient, the degree of the deformity and the experience of the hospital team), appropriate surgical planning with correct calculation of the correction needed, diligent execution of the procedure and high-level anesthesiological and medical care. In spite of high complication rates, it has a substantial positive impact in the quality of life of patients, including the elderly.
Some excellent surgical videos describing different modalities of the PSO technique have been published in the ESJ’s OOT film collection (www.oot-esj.com) and we recommend to view the films [24, 30, 31, 32].
Conflict of interest
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