L5 Partial Pedicle Subtraction Osteotomy in High Pelvic Incidence Patients: A New Way to Manage Lordosis

The first pedicle subtraction osteotomy (PSO) was described by Thomasen in 1985.1 Since then, various types of spinal osteotomies have been described.2–6 All these techniques may have different impact on the quantity of lordosis they can provide to restore a correct sagittal balance.7 More recently, Schwab et al8 have summarized it in a graduate classification. A PSO is a demanding procedure consisting in a posterior wedge resection of the vertebral body at the level of the pedicles to achieve shortening of the posterior column.9 Common indications for PSO are patients with fixed sagittal imbalance, including lumbar degenerative disorders, flat back syndrome, posttraumatic kyphosis, and ankylosing spondylitis.9 Currently, it is globally accepted that sagittal spinopelvic alignment is mandatory for an optimal management of lumbar fusions to avoid complications such as proximal junctional failure.10,11 The PSO is most often performed in L3 or L4 allowing a distal fixation in L5 and the pelvis.9 Lafage et al12 advocated L3 PSO as safer than L4 and L5, because of less neurological risk. On the contrary, in a recent publication Alzakri et al13 presented advantages of PSO localization at L5. According to the Roussouly classification and its evolution in degenerative spine, restoring an adequate shape and apex of lordosis is mandatory.14,15 Lordosis segmentation described by Berthonnaud et al16 emphasized the importance of the distribution of lordosis in the lumbar area as well as the direct equivalence of the lower arc of lordosis with the sacral slope (SS) (Fig. 1). Diebo et al17 recently published the interest of a more caudal level of osteotomy to improve the postoperative pelvic tilt. As a consequence, for the degenerative lumbar spine, it seems more logical to focus the restoration of lordosis to the lower arc. Owing to greater efficiency of PSO at the very distal level L5, a standardize complete L5 PSO is not always necessary. On the one hand, a small angle (15 to 20 degrees) of reduction may often be sufficient for rebalancing the spine. In contrast, at L5 the PSO is much more demanding than in the levels above, and the high complication rate like the L5 root compression could be a hindrance for the surgeon. To take advantage of the distal localization, but to minimize the neurological risk, we propose a limited PSO.

F1FIGURE 1:

The geometrical illustration showing the different spinopelvic angles. The reciprocity between sacral slope and the lower arch of lordosis is illustrated.

The purpose of this study is to describe the technique of this partial L5 PSO to manage lordosis in high pelvic incidence (PI) patients with fixed sagittal imbalance.

MATERIALS AND METHODS

Institutional review board approval was not required for this technical description with retrospective review of the first few cases.

The surgical technique for PSO in the lumbar area has been well-described.5 We describe here the partial L5 PSO specificities.

The patient is placed in prone position under general anesthesia on a specific carbon table for spine surgery with adequate padding. The pelvic pads should be placed very caudally to allow a maximum extension in the lumbar area.

A standard posterior midline approach is performed. Laterally, the exposure needs to be large enough to have access to the transverse processes and the lateral walls of the L5 vertebra. As a stiff distal fixation is mandatory below the osteotomy’s level we recommend bilateral sacral plates with S1 and S2 screws in primary cases and additional iliac fixation in revision cases. The advantage of sacral plate fixation is the greater lever arm between lumbar screws and the sacral fixation by displacing the sacral anchor to lower than S1. Effectively, one of the mechanical problems of L5 PSO is the L4 and S1 screws proximity that can avoid closing the osteotomy.

Cranial to osteotomy, a classic instrumentation using pedicle screws is performed on at least the 2 adjacent levels. We use a lateral loading, monoaxial pedicle screw system (Colorado Spinal System; Medtronic Inc.).

L5 pedicles are catheterized. The spinous processes of L4 and L5 are first removed. Then we perform a L4-L5 total bilateral facetectomy to open the foramens. The transverse processes are separated from the pedicle at their basis. The interlaminar space is opened by removing the ligamentum flavum and resection of the upper laminar part of L5 and the lower laminar part of L4. The medial wall of each pedicle is identified in the canal and the dura and the L5 root are gently retracted. Subsequently, the upper half of the pedicle is removed using an osteotome. The lower part remains and the L5-S1 foramens remain intact. Next, the vertebra is weakened by crushing the cancellous bone inside the vertebral body in the axis of the pedicle. The anterior cortex of L5 must be respected to avoid vascular complications anteriorly (Fig. 2). In case of flexible L4-L5 segment above, a discectomy is performed to increase fusion rate either by the same posterior approach or secondary by an anterior or lateral approach.

F2FIGURE 2:

The drawing showing the resection of the upper part of the L5 pedicle. Before (A) and after (B) resection.

One 5.5-mm titanium rod on each side is used. The contouring of the rod is key to close the osteotomy and to create an adequate lordosis shape. A very acute angulation in the rod in the middle between the L4 and sacral fixation is mandatory to obtain the desired lower arc of lordosis, and thus the desired degree of SS. The upper arc of the lordosis needs to be more progressive to be accorded to the lower of kyphosis above (Fig. 3).

F3FIGURE 3:

The drawing showing the shape modification in the sagittal plane during the procedure: preoperative, peroperative, and postoperative view.

Then, the system is tightened from the sacrum to the top. The osteotomy is closed using the progressive tightening and bilateral compression between L4 screws and the sacral fixation (Fig. 4). The space available for the exiting nerve roots are checked at the end of the maneuver.

F4FIGURE 4:

The drawing showing the surgeon’s view before and after the procedure.

RESULTS

We present 2 different cases reviewed retrospectively to illustrate this new way to manage lordosis. The described technique is feasible either in first intention or in a revision surgery.

First Intention Case

A 45-year-old woman with a Roussouly type 4 lordosis shape and a lumbosacral transitional vertebra (lumbarization) without mobility in the last lumbar disk. She complained about low back pain with posterior facets’ arthritis. She did not respond to a conservative treatment. Our plan was to modify the lumbosacral anatomy by performing a small PSO in L5 (Fig. 5, Table 1).

F5FIGURE 5:

First intention case with lumbarization. Preoperative (A) and postoperative (B) full spine radiographs.

TABLE 1 - Corresponding Radiographic Parameters (First Intention Case) Preoperative L5 PSO Postoperative L5 PSO PI (deg.) 87 87 PT (deg.) 27 21 SS (deg.) 60 66 TK (deg.) 46 38 Total LL (deg.) 91 79 L4-S1 LL (% total LL) 41 (45) 49 (62) SVA (mm) −17 12 Apex L2-L3 disk L3 vertebra Inflexion point T11-T12 T11-T12 Roussouly shape Balanced type 4 Balanced type 4

LL indicates lumbar lordosis; PI, pelvic incidence; PSO, pedicle subtraction osteotomy; PT, pelvic tilt; SS, sacral slope; SVA, sagittal vertical axis; TK, thoracic kyphosis.


Revision Case

A 59-year-old woman with a Roussouly type 4 lordosis shape. She underwent a previous L4-L5 posterior fusion in another hospital. After 6 year of poor functional outcome she was still complaining about low back pain. The plan was to perform a small L5 PSO to restore a balanced spine and keep the maximum lordosis in the lower arc with extension to the sacrum (Fig. 6, Table 2).

F6FIGURE 6:

Revision case with short fusion. Preoperative (A) and postoperative (B) full spine radiographs.

TABLE 2 - Corresponding Radiographic Parameters (Revision Case) Preoperative L5 PSO Postoperative L5 PSO PI (deg.) 77 77 PT (deg.) 18 17 SS (deg.) 59 60 TK (deg.) 37 40 Total LL (deg.) 75 75 L4-S1 LL (% total LL) 44 (59) 58 (77) SVA (mm) 59 36 Apex L2-L3 disk L3-L4 disk Inflexion point T11-T12 T11-T12 Roussouly shape Unbalanced type 4 Balanced type 4

LL indicates lumbar lordosis; PI, pelvic incidence; PSO, pedicle subtraction osteotomy; PT, pelvic tilt; SS, sacral slope; SVA, sagittal vertical axis; TK, thoracic kyphosis.


DISCUSSION

When dealing with fixed spinal deformity, the surgeon has to master the different techniques of osteotomy. For several years, the tendency was to perform PSOs in L3 or L4 to restore the missing amount of lordosis. More recently, the restoration of an adequate shape of lordosis, as described by Roussouly seems to become the best way to manage sagittal malalignment.15 The lower an osteotomy is performed, the more efficient it is for correction of lower arc of lordosis and restoration of physiological SS.

Owing to the strong effect of the L5 PSO, a small angle of PSO is sufficient to restore sagittal balance. The lower the osteotomy, the lower the apex of the lordosis could be placed. As a consequence, the adequate shape could be restored according to PI after identification of which Roussouly type the patient was before degeneration.14

Others have debated L5 PSO due to the bone resection close to the lumbosacral junction with potential instability and root trepanning in the L5-S1 foramens. This is the reason why we proposed a technique of partial pedicular resection, avoiding the possibility of L5-S1 foramen closure. Furthermore, the posterior bone saving allows a better posterior bone contact after closing to prevent nonunion.

A similar idea has already been developed around the thoracolumbar junction to find an alternative technique in-between a limited Smith-Peterson Osteotomy and a classic PSO. Kim et al18 published a mean correction of 18.8 degrees with this technique. In posttraumatic thoracolumbar kyphosis Gao et al19 reached a mean correction of 34 degrees comparable to a standard PSO. More recently, Bourghli et al20 have modified the Schwab classification describing a partial osteotomy in L4. They recommend to only preserve the lower third of the pedicle and the disk above. The authors found the same advantages about the exiting root protection and bony contact after closure. The description of a modified partial PSO in the lumbosacral junction and its effectiveness on lordosis shape has never been described to date.

We have identified some difficulties related to this partial osteotomy. The L4 foramen is a bit narrowed by the closure but the upper part of this foramen and the L4 nerve root was never compressed in our patients. The only radicular complication we had was an L5 nerve root compression in the lateral recessus at the beginning of our experience. Postoperative magnetic resonance imaging showed that this was caused by the bulging of the ligamentum flavum after closure of the osteotomy. This compression could be avoided by a large flavectomy during the posterior interlaminar decompression. On the first 10 procedures, we did not have any dural tear or other neurological complications. Another important concern in this technique is the distal fixation because of the absence of L5 pedicle screws as compared with more proximal PSOs. Here it is mandatory to have a strong fixation on the pelvis. We recommend, at least, the use of sacral plates with S1 convergent and S2 divergent screws. The best is to add iliac screws for more rigidity. To prevent mechanical complications, we recommend performing additional anterior interbody procedures (oblique/anterior lumbar interbody fusion). The short follow-up is not long enough to conclude about bony union and mechanical complications but we did not have early mechanical failure. We did not had infection for those first few cases.

The mean blood loss was 1500 mL (range, 1000 to 1800 mL) and the mean operative time was 200 minutes (range, 180 to 260 min). All those procedures were performed by the same expert surgeon with an operative time similar to a classic PSO but less blood loss.

This technical note reported the first few experiences of L5 partial PSO. A systematic clinical study has been designed to confirm the efficacy of this technique both on lordosis shape and on patient’s outcome.

In conclusion, L5 partial PSO can be considered a safe and reliable lordosing technique in patients with fixed sagittal imbalance. It is a new technique to manage the distribution of lordosis in the lumbar spine in high PI patients. A partial resection of the pedicle is often sufficient to give enough lordosis to balance the spine. A less aggressive partial PSO in L5 level could be more efficient on the sagittal profile than a complete L3 or L4 PSO. However, further radiologic and clinical long-term evaluations are needed.

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