Isthmic Reconstruction with Pedicle Screws and a V-Shaped Rod in Spondylolysis without Degenerative Disk Disease

Philippe Gillet

Spondylolysis is a fatigue fracture thought to be the result of repetitive microtrauma of the pars interarticularis or isthmus of the posterior arch of lower lumbar vertebrae. Although spondylolysis can lead to back pain and referred leg pain, true sciatica, or even neurologic deficit and progressive deformity of the spine in the sagittal plane, it is often well tolerated during a patient’s lifetime. Surgery is needed by a very restricted number of patients. Spondylolisthesis defines an anteroposterior displacement of the pathologic vertebra in regard of the lower vertebrae. It can be the result of the pars interarticularis defect, but spondylolisthesis can also occur in degenerative disorders of the spine because of dysplasia of posterior facet joints and after severe trauma. In spondylolisthesis or spondylolysis, many patients presenting with thigh pain or even leg pain do not really suffer from root entrapment; the pain can be referred pain. Even true radicular pain does not mean that a true decompression of the nerve root must be performed. Root pain can be initiated by local inflammatory conditions due to excessive motion of the mobile segment; a stabilization procedure without decompression may lead to disappearance of radicular symptoms as well as back pain and muscle contracture. True entrapment of nerve roots can, however, exist in severe vertebral slippage and severe disk narrowing deforming the neural foramen or in the case of associated herniated disks. When spondylolisthesis is due to dysplasia of the lumbosacral facets or to degenerative conditions, multiple root compromise can occur and may be severe because the whole vertebra including the posterior arch slips forward, causing central canal stenosis in addition to foraminal stenosis.

These different pathologic conditions may lead to intractable pain and functional deficit requiring surgical treatment. The type of surgery should be tailored to each individual situation and should be as least aggressive as possible. Depending on the anatomical status, an isolated fusion, a fusion with correction of the deformity, a combined fusion, and decompression must be considered. Fusion is generally performed across a mobile segment (arthrodesis) but may also be considered at the level of the pars itself, without sacrifice of spinal mobility.

GUIDELINES FOR SURGICAL TREATMENT

The following sections summarize the author’s current guidelines for surgical treatment to be considered in the various pathologic situations associated with spondylolysis or spondylolisthesis:

Spondylolysis Without Associated Disk Disease and Without Spondylolisthesis

A pars defect reconstruction is advised whenever possible to avoid loss of mobile segments and increased stress on adjacent structures. In cases where a L5 spondylolysis is present with an intact L5-S1 disk but with degenerative disk changes at the L4-L5 level, isthmic reconstruction of L5 may have been considered to avoid L4-S1 fusion. The reconstruction is performed in such cases in the hope that the L5 spondylolysis is the main cause of back pain; results have been satisfactory in our hands but inferior to those in the isolated spondylolysis cases. Such a therapeutic option should be considered with caution and only in carefully selected cases.

Spondylolysis with Associated Disk Disease and Grade 0 or 1 Spondylolisthesis

A posterolateral in situ fusion with or without posterior instrumentation is the classical procedure; reduction of a Grade 1 slip is optional. If the nerve root is entrapped, resection of the posterior arch should be performed and, if needed, removal of a herniated disk.

In heavy patients who anticipate engaging in strenuous work, if disk material has been removed or if the disk space is high, a posterior lumbar interbody fusion (PLIF) with posterior instrumentation is a recommended option to avoid secondary kyphosis due to progressive narrowing of the disk space.

Spondylolysis with Associated Disk Disease and Grade 2 or 3 Spondylolisthesis

A PLIF with partial or total reduction of the slip and moreover restoration of an adequate lumbosacral lordosis combined with posterior instrumentation is recommended. If the disk space is very narrow, if there is no significant loss of lumbosacral lordosis, and if no neurologic symptoms are present, a posterolateral in situ fusion with posterior instrumentation would be an option, especially in Grade 2 slips.

Spondylolysis with Associated Disk Disease and Grade 4 Spondylolisthesis or Spondyloptosis

A posterior reduction with a PLIF, or a combined anterior and posterior approach with anterior lumbar interbody fusion (ALIF) and posterolateral fusion and posterioror instrumentation should be considered, remembering that correction of the lumbosacral kyphosis is more important than correction of the slip. If true spondyloptosis is present, the Gaines procedure may be an option.

Lysis at the Level of the Pedicle

If a unilateral pedicle lysis associated with contralateral spondylolysis is present or if a bilateral pedicle lysis is present, the only option is an interbody fusion since a posterolateral bone graft will not stabilize the vertebral body and the motion segment; a PLIF or ALIF would be our procedure of choice.

Dysplasic Spondylolisthesis

A thorough posterior midline and lateral decompression must be performed, keeping in mind that severe narrowing of the spinal canal may be present with severe compromise
of the cauda equina. Further injury of the nerve roots must be avoided during the intracanalar use of surgical instruments. Reduction and fusion are performed according to the abovementioned rules.

Degenerative Spondylolisthesis

In symptomatic degenerative spondylolisthesis, spinal stenosis is the rule, and the main or the sole indication for surgical treatment is often the neurologic deficit. In the event of primary severe instability or postlaminectomy instability, a posterolateral fusion, instrumented or not, a PLIF or a lateral retroperitoneal ALIF should be considered (since degenerative spondylolisthesis often occurs at L3-L4 or L4-L5). The aim of the fusion in this specific indication is more often to avoid iatrogenic secondary increase of the spondylolisthesis than to treat back pain.

Details on these different techniques have been described elsewhere (5).

Pars Repair or Isthmic Reconstruction

A small number of patients presenting with isolated pars interarticularis defect suffer from chronic disabling low back pain, sometimes radiating to the thighs without true sciatica. These symptoms can be accounted for by hypermobility of the loose posterior arch with stimulation of the defect tissue, which seems rich in nociceptive nerve endings, and the relative instability of the vertebral body, which induces excessive stress to the underlying disk.

Removal of the pathologic soft tissue and bone grafting of the defect to restore the stabilizing role of the posterior arch is a logical form of treatment in this small group of patients. “Isthmic reconstruction” or “direct repair of the pars interarticularis” has been proposed by different surgeons since 1968.

Kimura (8) used an isolated bone graft followed by postoperative bed rest and a cast. Most procedures include some sort of internal fixation in order to improve the fusion rate and promote more rapid return to active life, if possible without external support. Buck (2) used a screw across the pars. Nicole and Scott (12) described a technique using the passage of wires under the laminae and transverse processes. Salib and Pettine (16) and Songer and Rovin (17) similarly proposed wiring the posterior arch but fixing the cable on pedicle screws. Morsher et al. (11) designed a special screw-hook construct. Louis (10) proposed bone grafting of the pars associated with temporary fixation of the lumbosacral junction with his butterfly plate. We reported on an original technique using a V-shaped rod and pedicle screws, associated with direct bone grafting of the pars defect (4).

PATIENT SELECTION

Since the beginning of our experience with the original V rod technique starting in 1992, patients have been selected to be candidates for pars repair when they meet the following criteria. They have to suffer from chronic disabling pain, located in the low back, possibly radiating to the thighs, which has been resistant to conservative treatment for at least 1 year and incompatible with comfortable everyday life, work, or sports. The absence of degenerative disk disease has to be confirmed by a normal T2 weighted magnetic resonance imaging (MRI) scan. If there is still doubt after the MRI, a provocative discogram is performed at the suspect spinal level and there must be absence of pain during injection of the dye or saline. We did not consider patients presenting with spondylolisthesis from
Grade 1 on to be candidates for this conservative surgery. However, in slight spondylolisthesis, the disk is not necessarily pathologic and pars repair has been performed in such cases by other authors (10). Increased pain upon hyperextension of the lumbosacral spine was considered a positive provocative sign. Infiltration of the pars defect with saline or lidocaine was not used in this study but may be recommended for optimal selection. Litigation or compensation cases are considered relative contraindications as in all other surgical procedures. Smoking may favor nonunion but is not considered a specific contraindication against the choice of a pars interarticularis repair technique instead of an intervertebral fusion procedure.

SURGICAL TECHNIQUE AND POSTOPERATIVE CARE

The technique described here is for a L5 pars repair. Anatomical landmarks should be adapted accordingly for L4 and L3 levels.

The patient can be positioned on any operating table the surgeon is familiar with; however, the sagittal alignment of the lumbosacral spine is important. Access to the isthmic defect is easier with the patient in slight kyphosis, but placement of the instrumentation in this position may lead to impingement between the instrumentation and the underlying vertebra when the patient stands. We place the patient prone on a Hall frame with the hips flexed no more than 20 degrees to 30 degrees in order to avoid disturbing the lordosis of the lumbar spine. Alternatively, the surgeon may choose to put the patient on an operating table that allows moving the lower limbs during surgery in order to perform the cleaning and decortication of the defect as well as the graft placement easily in kyphosis and the instrumentation in lordosis.

The lumbosacral area is approached through a midline incision from the L3-L4 level down to the S1-S2 level. Subperiosteal dissection is performed laterally down to the origin of the transverse processes at the L5 level—far enough to properly locate the entry point of the pedicular screws, but not farther than the articular processes at the L5-S1 level. Care is taken to preserve the capsuloligamentous structures of the L4-L5 and L5-S1 facet joints. However, to prevent stress being put on the isthmus of L5 by the overlying inferior L4 facets during spine extension, which could possibly lead to recurrence of the spondylolysis, 2 or 3 mm of the distal aspect of these facets is removed with an osteotome, as described by Louis (8), taking care to remove as little capsular structure as possible. The soft tissue situated in the pars defect is removed using rongeurs; if the preoperative MRI shows no root impingement (which is most often the case in these patients), a very thin layer of soft tissue is preserved at the bottom of the defect to avoid migration of the bone graft in the foramen. The sides of the defect, the upper half of the L5 laminae and the lateral, extra-articular aspect of the upper L5 articular process joint are exposed down to bleeding bone. Lumbar pedicular screws 35 mm long and 5 mm in diameter are inserted in the L5 pedicles, avoiding violating the L4-L5 joint. An autologous iliac bone graft is harvested and trimmed to be fitted in the defects. To prevent inadvertent anterior migration of the bone graft, a posterior fin is trimmed to rest on the border of the laminae (Fig. 18.1). The fusion mass is further augmented by putting an onlay graft on top of the posterior aspect of the L5 laminae and on the lateral extra-articular aspect of the L5 upper articular processes (Fig. 18.2). The pedicle screws are placed with their grooves oriented 30 degrees to 45 degrees to the longitudinal axis of the patient. A 6-mm diameter rod, usually 8-10 cm in length, is bent in a V shape and put under the L5 spinous process after the L5-S1 interspinous ligament has been removed. The rod is firmly fixed against the spinous process and the laminae, offering the possibility of both
compressing the graft in the defect and stabilizing the posterior arch. Sometimes, a slight bending of the rod is necessary in the sagittal plane to fit it properly against the posterior arch of the vertebra and in the grooves of the open pedicle screws. Finally, the locking bolts are firmly tightened against the rod to fix it to the pedicle screws while, using a compression clamp, some compression is applied between the apex of the bent rod and the screws to stabilize the graft. Care is taken to avoid any impingement between the rod and the superior aspect of the S1 spinous process during extension of the spine; sometimes, a few millimeters of bone must be removed from the base of the S1 spinous process (Fig. 18.3).

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