POSTERIOR LUMBAR FUSION

Posterior Lumbar Fusion with Instrumentation

Posterior lumbar fusion with instrumentation is performed to eliminate motion or instability of vertebrae in the lower back. Instrumentation provides temporary stability of the spine until a bridge of bone connects the vertebrae together (fusion).

The decision to perform Posterior Lumbar Fusion is based on three factors: Patient symptoms, findings on physical exam, and radiographic abnormalities.

A. PATIENT SYMPTOMS

The patient's symptoms are generally categorized into three patterns of pain. Axial pain is that which occurs along the spine and the overlying musculature. Referred pain is that which occurs along the buttocks or pelvic region. Radicular pain is that which extends partially or completely along the length of the leg.

Axial pain is generally vague and is typically similar irrespective of the underlying problem. Referred pain into the pelvis or buttocks is also generally vaguely defined. These patterns, which may be due to irritation of minor nerves along the surface of the disc or joints, can be difficult to differentiate from patterns of pain due to soft tissue inflammation such as ligamentous injuries or tendonitis. Segmental instability (abnormal shifting of joints or vertebrae) in the lumbar spine can also produce similar patterns of referred pain. Radicular pain typically conforms to a specific nerve root distribution or course. A radicular pattern of pain is most easily identified as specifically related to nerve root impingement and, as such is generally most accurately correlated with a specific abnormality or area within the spine.

B. PHYSICAL FINDINGS

Physical findings can generally be divided into four groups:

1) Limitation of motion, 2) the presence of tenderness, 3) neurological deficits and 4) deformity or instability. While limited range of motion in the lumbar spine is frequently present, it is a very nonspecific finding and does not reflect any particular abnormality, which would suggest an indication for performance of a posterior lumbar fusion. However, occasionally certain movements or positions may recreate back or leg symptoms and these patterns may correlate with radiographic abnormalities to suggest a source for the pain. Tenderness is also frequently present and may be diffusely noted along the musculature, joints, and is usually present over the sciatic notch in patients with complaints of sciatic nerve irritation. The sciatic notch is located in the lower buttock and is the point of origin of the sciatic nerve from the pelvis. Tension signs, such as the straight leg-raising maneuver, are another method of eliciting tenderness in the patient. These techniques involve stretching the affected nerve and recreating the symptomatic leg pain. In lumbar disc abnormalities, these generally include the femoral stretch test, which elicits pain along the femoral nerve, and the straight leg raise, which elicits pain along the course of the sciatic nerve. Commonly, these maneuvers may be negative in conditions requiring lumbar fusion or may only produce back pain rather than pain along the leg.

Neurological deficits are the most specific indicator of nerve root compression. Neurological deficits include loss of a specific reflex, loss of sensation in a specific area correlating with a particular nerve, or loss of strength in a muscle or muscles conforming to a particular nerve. Neurologic deficits may or may not be present depending on the degree of neural compression present, usually dependent on the degree of instability present.

Curvature of the spine, (scoliosis) or instability (such as spondylolisthesis) may be apparent. These conditions may predispose a patient to any of the pain patterns. Symptoms of radicular pain may be present, but generally are less severe than axial pain in patients undergoing posterior lumbar fusion. In patients who demonstrate greater radicular pain than axial or referred pain, consideration should be given to performing a more minimal surgery such as decompression alone. Radiographic factors assessing stability determine the relative need for fusion. Candidates for posterior lumbar fusion do not necessarily have positive tension signs, although frequently these maneuvers will produce severe lower back pain.

In contrast to more minimal procedures, posterior lumbar fusion is generally not considered until a patient has failed to respond to at least six months to one-year of non-operative treatment. Frequently, these patients have pursued more minimal surgery such as microdiscectomy or decompression and have had persistent complaints of severe back pain in the presence of spinal deformity or instability. Non-operative treatment preceding posterior lumbar fusion would include activity restrictions, physical therapy or chiropractic treatment, narcotic analgesics, non-steroidal anti-inflammatory medication, epidural and facet cortisone injections, and brace wear. One exception, which indicates a need for more urgent or emergent posterior lumbar fusion, would be the presence of a progressive neurological deficit associated with significant spinal instability.

C. RADIOGRAPHIC ABNORMALITIES

Radiographic indications for posterior lumbar fusion would include spinal deformity or segmental instability. Advanced disc or facet degeneration is considered by some to be an indication for posterior lumbar fusion as well.

Spinal deformity can appear in several patterns. Scoliosis is curvative from side to side. Kyphosis is severe curvature of the spine from front to back resulting in abnormal bending forward. Instability can also appear in several patterns. Spondylolisthesis is the slippage forward of one vertebrae. Retrolisthesis is the slippage backwards and olisthesis the slippage sideways of a vertebrae. Spinal curvature or slippage can result in compression or pinching of nerve roots and cause pain, both back or leg.

Compressed nerves can be decompressed by removal of bone. The decision to fuse is usually based on whether the instability or deformity is likely to progress or cause recurring symptoms. For example scoliosis greater than 30 in the lumbar spine has been shown more likely to progress following decompression. Lumbar spondylolisthesis has been shown to be more likely to progress after decompression when both sides are decompressed, more than 50 of the facet is removed, when disc space height is preserved, when the facet joints are oriented parallel to the slippage, and when mobility is noted on flexion-extension x-rays.

When these factors are present, or decompression is necessary that will destabilize the spine, fusion should be performed. Fusion may also be advantageous when symptoms of back pain appear related to deformity or instability. This benefit has to be weighed against the loss of motion and the potential for adjacent areas to become painful when degenerative changes are subjected to increased stress.

II. SURGICAL TECHNIQUE

The posterior spinal fusion can be achieved through either a midline or bilateral paraspinous approach. More commonly, the midline exposure is performed. A longitudinal incision is made over the spinous processes spanning one to two levels both above and below the levels to be fused. The incision is extended through the subcutaneous tissue to the dorsal fascia. The fascia is incised in the midline and the musculature is elevated from the surface of the underlying bone. The muscles are elevated bilaterally to the tips of the transverse process and retractors are inserted.

The appropriate levels are verified by x-ray. At this point decompression of the spinal canal and neural elements at one or more levels is carried out. Decompression is performed as described in the operative technique for laminectomy. 1

Following decompression, instrumentation can be attached to the vertebra to provide stability, correct deformity, and maintain a particular alignment until fusion develops. These implants usually consist of hooks or screws attached to bone and connected by rods or plates. Typically, these implants are titanium and rigidly maintain spinal position until the vertebra fuse together. Fusion permanently stabilizes these vertebrae and assumes the stress upon the instrumentation. More recently, instrumentation systems have been developed which allow flexibility. These systems employee a flexible rod connecting pedicles screws. The technique of insertion is nearly identical to rigid systems although no effort is extended to fuse the vertebra together.

Instrumentation is performed under fluoroscopic visualization. The pedicles are identified and marked . A probe is passed into the vertebral body through the pedicle under fluoroscopic guidance. Screw threads are created in the channel with a tap . A screw of the appropriate length is inserted and advanced until flush . The contralateral screw is inserted in an identical fashion , then progressing to the next level until all screws had been placed .

A rod is cut at the appropriate length and contoured into lordosis. Although the mechanism differs among instrumentation systems, the rod is fastened to the screws , in this case using locking nuts , and securely tightened . This results in a rigid construct stabilizing the vertebra instrumented .

Although the instrumentation provides immediate stability to the spine, the instrumentation will loosen or fail if fusion of the vertebra does not develop. A fusion occurs when bone connects one vertebra to another. Bone fusion replaces the instrumentation and its role of stabilizing the vertebra. Surgical fusion is achieved in two steps. First, the surface of the bones to fuse are removed or scored(decortication).

Second bone graft or a substitute is placed over the decorticated surface This results in the growth of a bone bridge connecting the vertebra. Bone graft can include bone removed during decompression, bone taken from the pelvis(iliac autograft), allograft(cadaver bone), demineralized bone matrix(processed allograft), or synthetics(coral calcium). Regardless of the bone graft used, it is placed over the decorticated transverse processes, and lamina if they remain following decompression. Bone may also be placed between the facet joint after resection of the cartilage.

Following decompression, placement of bone graft, insertion and tightening of the instrumentation, the procedure is completed. The elevated muscle is approximated in the midline and effort to obliterate the cavity overlying the defect created by the decompression and prominence of the instrumentation. The majority of closure strength comes from sutures through the dorsal fascia which is closed as a separate layer. The subcutaneous fat is also sutured as a separate layer but offers little strength. The skin can be closed with staples or suture. Commonly, one or two drains are placed between the layer's closed and connected to a reservoir under section. This evacuates any postoperative bleeding or drainage which might result in neural compression or more extensive scarring.

III. POST-OPERATIVE COURSE

Postoperatively, patients are fitted with altemteafsaereri jarthosis for approximately 2-3 months. This is a two-part hard plastic brace, which is secured by velcro straps. Patients are advised to remain in the brace except when lying flat in bed.

Patients are generally maintained at bedrest the day of surgery. They are typically advanced to ambulation on the day after surgery. Patients are usually advanced to a regular diet on a gradual basis and IV fluids are discontinued when they are able to accept liquids. It is not uncommon for patients to develop an ileus, or a decrease in bowel activity, for several days. This may delay progression of their diet. Antibiotics are given pre-operatively and for 24 hours post-operatively. If the patient is independent with ambulation, able to tolerate a regular diet, and afebrile and able to void, they are generally discharged 4-6 days following surgery. Average length of stay for 115 patients has been 5.5 days with range 3-10.

Patients are advised at the time of discharge to avoid activities such as bending, lifting, and vigorous twisting. They are instructed on body mechanics or techniques in sitting, standing, and transferring out of bed etc. Patients are typically prescribed pain medication to be taken by mouth as needed and occasionally anti-inflammatory medication for residual nerve root swelling and irritation.

Patients are generally advised to refrain from getting the incision wet for three days post-operatively. At that time they may shower. It is generally advisable to avoid submerging the incision in a tub or pool for at least one week.

Follow-up examinations are typically conducted at one week, one month, three months, 6 months and 1 year post-operatively.

Strict limitation of bending and lifting is continued for 2-4 months, with a gradual decrease in restrictions by 4-6 months. Patients are usually able to return to sedentary activities within the first month. At approximately 2-3 months post-operatively patients are referred for physical therapy and their brace discontinued. This includes a graduated course of lower extremity and lumbar flexibility, strengthening, and instructions on body mechanics and postural alignment. Patients are generally advised to increase their recreational and daily activities commensurate with their progress at physical therapy.

IV. OUTCOMES

A. BENEFITS

Appropriately selected patients, as previously described, can generally expect approximately an 80% likelihood of improvement, if not relief, of the majority of their back and leg pain.

Considering all patients, all diagnosis posterior lumbar fusion with instrumentation has yielded successful results in 78% of patients (90 of 116). A significant variance has been noted between patients covered by workmen's compensation 52% (13 of 25) and the success rate among all other patients 85% (77 of 91).

Success rates have also been noted to vary dependent on the diagnosis. Patients undergoing PLFI for spondylolisthesis were more likely to have successful results 85% (70 of 82) then those with other diagnosis 62% (21 of 34). Separation of the patients with spondylolisthesis by the specific etiology revealed a 75% (6 of 8) success rate among patients with a post Laminectomy spondylolisthesis, and a 78% (25 of 32) success rate in patients with an isthmic spondylolisthesis. The best results were identified in patients with a degenerative spondylolisthesis 93% (39 of 42).

Relief of leg pain is common, unless there is damage or severe scarring of the nerve. Back pain relief may be limited by the coexistence of adjacent abnormalities or degeneration.

B. COMPLICATIONS

Operative risks include infection, bleeding, neurological injury, instrument failure, pseudoarthrosis, deep vein thrombosis.

Post-operative infection rates for all surgical procedures performed vary on a national basis between 1 and 2%. Infection rates of posterior lumbar fusion with instrumentation's have been reported as high as 7%. Four deep infections have developed in 115 post lumbar fusion with instrumentation 3.4%. Two responded to antibiotics and wound drainage only with development of a solid fusion. Two required removal of instrumentation after which one fused and the other required anterior fusion for pseudoarthrosis.

Bleeding associated with posterior lumbar fusion is usually significant. Patients are requested to arrange for 2 units of donated blood. This is transfused in approximately 50% of cases. Use of cell saver equipment and auto transfusion was discontinued several years ago due to the minimal amount of blood usually collected.

Neurological injury is an unlikely operative complication. The exact incidence of permanent neurological injury is unknown although it is probably on the order of less than .4%. Most frequently, post-operative neurological deficits were present pre-operatively. There have been no instances of neurological injury with posterior lumbar fusion with instrumentation in our experience.

Pseudoarthrosis is the failure of the bone graft to fuse to the vertebra. This results in motion, which can produce pain. Pseudoarthrosis has been proven in 7 patients 6.1%. In three patients re-operation for revision of a painful pseudoarthrosis was performed.

Instrumentation failure is uncommon. In two patients breakage of a screw was noted. One developed a solid fusion, the other was revised operatively.

Deep vein thrombosis is the development of a blood clot in the leg, thigh or pelvis. This usually results in pain, swelling, and tenderness of the affected extremity. The greatest risk is that the clot might dislodge and travel to the lung where it might obstruct blood flow producing a potentially life threatening condition. No DVT's have been identified after posterior lumbar fusion.

Reoperation was performed in 13 patients for a total of 15 reoperations. Surgical indication were pseudoarthrosis in 3 patients, removal of instrumentation in 6 patients for pain presumed due to soft tissue irritation, and drainage of infected wounds in four patients with subsequent removal of implants in two.