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Some of the surgical options for spondylolisthesis that may be considered alone or in combination include:

• Direct repair of the pars
• Fusion surgery
• Decompression surgery
• Reduction of the isthmic deformity

Direct Repair of the Pars

This type of surgery is most successful in young adults with spondylolisthesis who have not had successful treatment with bracing and show no signs of degenerative disease. The typical candidate is one who has normal intervertebral disks and spinal stability. It involves debridement (surgical removal) of the bone fragments or bony growths and the use of instrumentation (wires or screws) to repair the pars. Good to excellent results have been reported in up to 80% of patients who undergo this procedure. This procedure is also effective for children with spondylolysis whose symptoms do not respond to conservative therapy.

Fusion Surgery

This type of surgery is also called arthrodesis. It is performed to stabilize two adjoining vertebrae at the slip level and prevent movement of the vertebrae. Unstable segments are immobilized by preventing motion across the facet joint and the intervertebral disk. Fusion may be done alone or in combination with decompression surgery with or without instrumentation for internal fixation. Although fusion is usually done between L5-S1, in patients with high grade spondylolisthesis, L4 may also need to be included in order to achieve a successful fusion.

In fusion surgery two or more vertebrae are fused together by placing a bone graft around the affected area which then forms a bridge between the two vertebrae and fuses them together. There are four types of bone grafts, namely:

• _ Autograft_ - the patients own bone, usually taken from the iliac crest (pelvis), is used for the graft. This is considered the gold standard for bone grafting and is used most often in fusion. Side effects of using this type of graft include infection or postoperative pain at the donor site (pelvis).
• Allograft - bone is taken from a cadaver. This can be used in place of an autograft or as a supplement to an autograft. Allografts can be obtained from a bone bank.
• Bone graft substitutes - products such as tricalcium phosphate or demineralized bone matrix can be used to assist or replace bone grafts for fusion. They can either be mixed with allograft bone or used to extend the graft and increase fusion rates.
• Bone morphogenic protein (BMP) is a protein that is mixed with bone graft in a putty form and promotes more bone growth and a higher fusion rate. Studies of its safety and efficacy are favorable so far. It is available commercially but is very expensive and not yet cost effective. Bone morphogenic protein continues to be actively investigated.

The graft slowly heals over several months and "welds" the vertebrae together. Fusion may involve the use of instrumentation (e.g. plates, screws, rods, cages) which creates an internal splint to stabilize the spine and vertebrae during healing as well as to correct spinal deformity.

Fusion surgery is the mainstay of surgical therapy for isthmic spondylolisthesis in children and adults, although opinions differ regarding the exact technique of choice.