Long term pain in the low back when not due to pressure on a nerve root, may require lumbar spinal fusion. This refers to a procedure that results in the permanent loss of movement between any two or more adjacent lumbar vertebrae (spine bone segments in the low back), thereby reducing back pain. There are several procedures that may accomplish a fusion.

• Posterior fusion: When the procedure is carried out through an incision in the back.
• Anterior fusion: When carried out from the front of the patient.
• Interbody fusion: First the disk between two vertebrae is removed and then bone is laid down in the space created between the two vertebral bodies (the interspace).
• Instrumented fusion: A fusion is referred to being instrumented when an appliance is used to accomplish or support the fusion

Anatomy  

• The lumbar spine and pelvis, the low back, supports your entire body. The lumbar spine allows you to bend forward and backward and to twist at the waist. The normal lumbar spine is composed of five building blocks called vertebrae that sit on the sacrum, which is the back part of the pelvic bone (Fig. 1a). Each vertebra is constructed of a body, lamina, and pedicles, which surround an opening, the spinal canal. (Fig. 1b)

Figure 1a	Figure 1b

• Each vertebra has a section of bone that extends backwards from the middle of the lamina called a spinous process, Passing through the spinal canal are the nerve roots which emerge from the end of the spinal cord and form the nerves to the legs. (All the roots together in the lumbar spinal canal are called the cauda equinae.) Each root exits the spinal canal through a ‘hole’ in the side of the canal formed by two adjacent vertebrae called a foramen. (Fig. 1c)
• The roots float in fluid and are contained within a fibrous sac called the dura (Fig. 1d)

Figure 1c	Figure 1d

• Separating any two vertebral bodies is a soft elastic material called a disk. In total, there are five lumbar disks comprised of two parts, a soft center called the nucleus and a tough outer band called the annulus (Fig. 1e)

Figure 1e

• On each side of the back of the spinal canal and linking one vertebra to the next are a series of small joints called facets. Extending outward from the area of the pedicle and facet and acting as an anchor for some of the short muscles between vertebrae, is a short segment of bone called the transverse process.(Fig. 1b)
• Normally, there is motion between the adjacent vertebrae and occurs through the disk, facets and ligaments

Pathology

With advancing age, injury or surgical removal of the disk, several changes occur in the bone, disk, joints and ligaments of the lumbar spine that can produce back pain as a result of a nerve root being compressed.

• Bones: With aging, bones tend to lose water and become less dense, a condition called spondylolsis. These degenerative changes near the disk may cause an overgrowth of bone producing bony spurs (osteophytes) that can extend into the foramina and compress the exiting nerve roots.
• Disk: The disks also lose water and shrinks thus narrowing the disk space. As the disk becomes thinner the space between the vertebrae likewise narrows thereby narrowing the foramina, resulting in nerve root compression and pain.
• Joints: Along with the other degenerative changes, there can develop a degenerative arthritis of the facet joints that causes the joints to enlarge. Sometimes the lining of the joint (synovium) enlarges or becomes like a cyst. This combination of events results in narrowing of the spinal canal and increasing stiffness of the spine. If the joints degenerate such that one vertebra slips over the one below (called spondylolesthesis) the spine may become unstable resulting in pain. It can also cause narrowing of the spinal canal requiring Decompressive Laminectomy Ligaments: With advancing age the ligaments tend to stretch and thicken. This may cause instability between vertebrae, as well as result in pressure on the spinal canal and foramina.

Making the diagnosis

Despite long standing pain not helped by pain medication, physical therapy and support garments, the physician may find few abnormalities on examination.

• The reflexes on tapping the knee or ankle may be slowed.
• There may be a mild weakness of the legs.
• The response to a pin or touch may be less than normal.

Special tests are necessary in making the correct diagnosis. These are:

• X-ray of the lumbar spine:The x-rays may show collapse of the disk spaces with narrowing of the foramina, bony spurs, scoliosis, slippage of one vertebra over another (spondylolesthesis) and degenerative changes in the facet joints. X-rays taken while the patient bends forward and back may demonstrate increased motion between vertebrae revealing instability(Fig. 2).

Figure 2

• Lumbar spine X-rays with patient bent forwards (flexion) and backwards (extension). Note (arrows) that on flexion there is forward slippage (spondylolesthesis) of the third lumbar vertebra over the fourth lumbar vertebra
• CT (computerized tomography) scan: This test images the lumbar spine by X-ray as a series of cross-sections where the changes in the bone and joints can be evaluated and spinal canal and foraminal narrowing can be seen. Frequently this is combined with a myelogram. (Fig. 4)

  Figure 4

   
• Lumbar myelogram with CT scans of same patient as in figure 2. Note that the dye placed in the spinal canal is seen at the mid L3 level (arrow) but not at the level of the disk space (disk). This is also seen on the CT scans taken at these indicated levels. The small dots seen within the dye at L3 on the CT scan are nerve roots
• MRI (magnetic resonance imaging) scan: This test images the lumbar spine though the use of a magnetic field and radar waves. The soft tissues such as the ligaments are imaged better than bone. This test can also determine the degree of spinal canal and foraminal narrowing, disk collapse, the presence of spondylolesthesis, and bony changes in the body of the vertebrae caused by disk degeneration (modic changes) (Fig. 3)

Figure 3

• MRI of the same patient as in figure 2. Note the narrowing (stenosis) of the spinal canal at the level of the L3/4 disk space (arrow)
• Lumbar myelogram:This test involves the injection of an X-ray contrast material containing iodine into the fluid that surrounds the cauda equinae (all the nerve roots together). Regular X-rays as well as a CT scan are then obtained. This test usually gives the clearest picture of the degree of spinal narrowing and the number of spinal levels involved. (Fig. 4)

Figure 4

• Lumbar myelogram with CT scans of same patient as in figure 2. Note that the dye placed in the spinal canal is seen at the mid L3 level (arrow) but not at the level of the disk space (disk). This is also seen on the CT scans taken at these indicated levels. The small dots seen within the dye at L3 on the CT scan are nerve roots
• Surgery is usually not the first treatment of choice
• Pain:Anti-inflammatory medication such as aspirin, ibuprofen, acetaminophen and naproxen may be of benefit.
• Physical therapy: Exercises for strengthening and stretching the back and abdominal muscles may help.
• Epidural cortisone injection: Cortisone is injected into the spine between the bone and the dura. This sometimes may reduce inflammation and reduce pain. Injections of steroids should be used cautiously since any long term series of injections can have adverse side effects.

Indications for surgery

• Degeneration of the disk causing collapse of the disk and pinching of the nerve root
• Prior disk surgery with disk space collapse and persistent pain
• In conjunction with surgery for removal of a ruptured disk where the rupture has occurred on both sides of the disk. Or when a massive rupture exists at the center of the disk. In this situation, the facets may be partially or completely removed on both sides thus requiring a fusion
• Instability associated with a long history of back pain
• Spondylolesthesis : When the joints degenerate such that one vertebra slips over the one below
• Congenital abnormalities or spondylolysis of the vertebra (fracture at the base of a facet)
• Surgery for spinal stenosis
• Removal of a facet during surgery
• Repeat ruptured disk surgery at the same level
• Long standing back pain in which medication and physical therapy do not help. In this case, back or leg pain is severe enough to limit an individual’s everyday activities or quality of life.

Bone grafts

Bone grafts: In order to fuse two segments of bones together, small pieces of bone are placed between the segments to promote growth. These pieces are called bone grafts. Bone grafts may be taken (harvested) from the patient (autograft) or obtained from a bone bank, from bone taken from a cadaver which is specially treated and sterilized (allogrqaft). In some cases, other materials may also be used to enhance fusion.

• Autograft bone: is usually harvested from the iliac crest, the top portion of the pelvic bone. The bone is removed in two forms- short sticks of bone from the surface of the crest (cortical bone) and small fragments of bone from the bone marrow at the center of the crest (cancellous bone). When the fusion is performed posteriorly (from behind), bone from the back of the iliac crest is harvested. When performed anteriorly (from in front), bone from the front of the iliac crest is used.
• Allograft bone: comes in many forms depending on the bone from which the graft was harvested. The type of bone used depends on the specific procedure and will be discussed with that procedure.
• Demineralized Bone Matrix (DBM): is produced from banked human bone taken from cadavers. Considerable care is taken to obtain the bone using a sterile technique and is tested for sterility throughout its production. The bone is then demineralized (calcium removed). This special allograft bone may be processed in the form of gel, putty and sheets that can be added to a graft site to enhance fusion. The gel can be processed so that it may or may not become firm after being applied.
• Bone Morphogenic Protein (BMP): is a new material that whenadded to a fusion site has the ability to greatly enhance bone growth. Several companies are currently developing BMP for clinical use.
• A bone stimulator: is a special battery with wires that creates a low steady current that stimulates bone growth. The wires from the stimulator are wrapped around pieces of the graft bone and the battery is placed beneath the skin.

Warning to Smokers: Smoking significantly increases the chance that a lumbar fusion will fail. Without smoking, a good fusion occurs in approximately 90% of the time. With smoking the incidence drops to only 70%.

Procedures for Lumbar Spine Fusion:

Posterior Fusion:

1. An incision is made in the middle of the back overlying the vertebrae to be fused. The muscles are elevated off of the bone to expose the spinous processes, laminae, facets and transverse processes on both sides.
2. The posterior fusion is usually carried out along with a decompressive laminectomy in which the spinous processes, laminae, and a portion of the facets are removed to take pressure off the dura and nerve roots.
3. The surface of the remaining exposed bone is partially removed to expose the inside of the bone for better fusion.
4. Wedges of cortical bone(the hard surface bone) and cancellous bone (the bone marrow) from the pelvic bone are laid down between the vertebrae to be fused.
5. Sometimes bits of autograft bone may be added to the packing to supplement the bone taken from the patient. Likewise strips of DBM may be added to enhance fusion
6. The implantation of a bone stimulator increases the chance of a satisfactory fusion, especially if the patient is a cigarette smoker. Posterior fusion with pedicle screws

One of the factors that decreases the chance of fusion is motion at the fusion site. If there is instability present such as seen in cases of spondylolesthesis, eliminating motion by binding the two vertebrae together increases the chance of a good fusion--such is the function of a pedicle screw.

Posterior fusion with pedicle screws (Figure 5)

One of the factors that decrease the chance of fusion is motion at the fusion site. If there is instability present such as seen in cases of spondylolesthesis, eliminating motion by binding the two vertebrae together increases the chance of a good fusion--such is the function of a pedicle screw.

1. The exposure of the spine is the same as for an ordinary posterior fusion. Next, a specially designed screw is inserted through the center of the pedicle and anchored into the body of the vertebra
2. This is done on both sides and in all the vertebrae to be fused
3. The screws are then bound together by attaching a rod to each screw head. The construct is usually made of titanium
4. Strips of iliac crest bone are then laid down across the transverse processes and adjacent bone

Click an image below for a descriptive animation sequence

Lumbar Interbody Fusion:

Bones fuse together more readily when the bones are under compression. The center of rotation for forward and backward bending between two vertebrae is just in front of the spinal canal. Therefore a fusion between the two bodies is more likely to fuse than a fusion posteriorly.

The height of the collapsed disk space can be restored by first separating the vertebral bodies before inserting the fusion bone thereby decompressing a compressed nerve root

Since the disk is removed prior to inserting the bone, this eliminates the possibility or future disk rupture.

Anterior Interbody Fusion:

Using only bone:

The procedure is carried out from in front. A general surgeon exposes the front of the lumbar spine for the neurologic or orthopedic surgeon, usually by an incision just below the umbilicus. This is probably the most dangerous part of the procedure since the large blood vessels that go to and from the legs are directly in front of the spine. Bleeding from these vessels may be hard to control and injury to the large vein may result in formation of blood clots.

1. Once the spine is exposed, the anterior annulus of the disk is opened. The disk and cartilage are removed. The upper and lower surfaces of the interspace are scored with a burr, curettes or small chisels.
2. Wedges of iliac crest bone are then packed into the interspace supplemented with smaller pieces of cancellous bone.
3. All bleeding is controlled and the incision closed with sutures. This is a relatively demanding surgical procedure even after the general surgeon exposes the spine

Using a cage:

As with the posterior approach, anterior lumbar interbody fusion has been considerably simplified through the use of special instrumentation for inserting a cage. Once the spine is exposed, the procedure for inserting the cage is almost exactly the same as with a posterior fusion except that the cages are inserted from in front of the body of the vertebra and not the back.

1. The disk space is widened using a bullet shaped distractor that is hammered into the disk space
2. Additional disk is removed using a reamer. The reamer also breaks through the cortical bone above and below to expose the cancellous bone within the body of the vertebrae
3. The disk space and adjacent bone are then tapped
4. The cage is prepared. The center of the cage is filled with bits of cancellous bone taken from the iliac crest. The cage is then screwed into the tapped hole in the interspace and adjacent vertebral bodies

Click an image below for a descriptive animation sequence

Using a bone dowel:

This is similar to using a cage except that a threaded bone dowel is implanted

Relative contraindications to ALIF (Anterior lumbar interbody fusion).

An ALIF should not be carried out

1. If a nerve root is compressed then a posterior approach is desirable
2. Three or more levels of vertebrae are involved, particularly in a patient over 60 years of age or in the presence of osteoporosis (bone thinning)

Because an ALIF performed with a threaded cage, threaded bone dowel or bone ring is much easier to do, there has been a marked increase in the use of this procedure for back pain. It may be wise to get a second opinion before having this procedure.

Posterior Interbody Fusion:

Using only bone:

The procedure begins with a decompressive laminectomy along with removal of a portion of the facets. Using specially designed instruments, the disk is completely removed from the interspace (space between two vertebral bodies) as well as the cartilage lining the upper and lower surface of the interspace. An instrument is then inserted deep into the interspace and the vertebral bodies are pried apart. Bone grafts taken from the posterior iliac crest are then placed into the emptied interspace. The graft is inserted on one side while the bodies are held apart by the instrument inserted on the other side after which, the bone on the other side is inserted. After bleeding is controlled, the incision is closed. The patient is then placed in a lumbar spinal brace until there is X-ray evidence of fusion.

Using a cage:

The above procedure is a very demanding operation. In the last few years instrumentation and an implant called a threaded cage has been developed to simplify the procedure. The procedure is the same as above up to the time the disk is removed.

• At this point, part of the disk is removed on each side of the center of the disk. The disk space is widened using a bullet shaped distractor tool that is hammered into the disk space. A special retractor is then used to protect the dura and nerve roots. (Figure 6a)
• More disk is removed using a drill or reamer. The drill or reamer also breaks through the cortical bone above and below to expose the cancellous bone within the body of the vertebrae. (Figure 6b)

 

Figure 6a - Disk space spread by distractor tip. Courtesy of Surgical Dynamics	Figure 6b - Disk material and adjacent cortical bone removed by drill. Courtesy of Surgical Dynamics

• The disk space and adjacent bone is then tapped. (Figure 6c)
• The cage is then prepared. The cage looks like a small barrel with a screw thread on the surface and multiple holes in the side. The center of the cage is filled with bits of cancellous bone taken from the iliac crest or from the lamina and spinous process bone removed during the procedure. The cage is then screwed into the hole in the interspace and adjacent vertebral bodies. The thread also acts to hold the adjacent bodies in alignment while the fusion takes place. This may take as long as a year. (Figure 6d)

Figure 6c - Hole tapped. Courtesy of Surgical Dynamics	Figure 6d - Threaded cage inserted. Courtesy of Surgical Dynamics

• The patient uses a brace following surgery for four to six months

Using a bone dowel:

The procedure is the same as described above for using a cage, except that a dowel of bone is used. Like the cage the bone dowel is threaded on its surface and hollow in the center. The dowel is filled with bits of cancellous bone. The use of bone dowels is relatively new, but may have certain advantages.

1. Metal cages impair the ability to determine that fusion has taken place using X-rays, CT scans and MRI. Using bone instead of metal considerably reduces this problem
2. The cage is very hard compared to vertebral bone. The cage thus has the tendency to cut into the adjacent vertebrae with time. The result is a tendency for the vertebrae to collapse over the cage with narrowing of the disk interspace. The bone dowel being softer than a cage reduces this problem
3. The cage will never become a part of the bone fusion. With time even the hard bone dowel should become part of the fusion

360 fusion:

• A midline incision is made in the low back over the vertebra to be fused and the muscles lifted off of the bone to reveal the spinous processes and lamina of the two vertebra to be fused (Figure 8A)
• Bone id removed from the lamina and part of the facets to expose the disk and the nerve roots (Figure 8B)
• Special instruments are placed into the disk space to remove the disk material (Figure 8C)
• Specially formed pieces of graft bone (Figure 8D) are carefully placed on each side in the emptied disk space (Figures 8E, 8F). Cancellous bone (bone marrow bone) chips are packed between and about the grafts
• The back part of the fusion is then carried our by placing bone across the transverse processes and remaining lamina and facets. (Figure 8G) Frequently pedicle screws are placed to help support the fusion
• The wound is then closed with sutures
• Under certain circumstances it is advisable to fuse the spine both in front and in back. Various combinations of the above procedures can be used. The procedures may be carried out together in one sitting or carried out at separate times. The following is an example of a 360 fusion carried out from behind

Figure 8a - The bone of the fourth and fifth lamina and spinous processes are exposed. Courtesy of Medtronic Sofamor Danek	Figure8b - Part of the lamina and facets are removed to expose the nerve roots and the underlying disk (arrows) . Courtesy of Medtronic Sofamor Danek
Figure 8c - The disk space is cleaned out of disk material. Courtesy of Medtronic Sofamor Danek	Figure 8d - Specially made bone graft on instrument holder. Courtesy of Medtronic Sofamor Danek
Figure 8e - The bone graft is placed in the emptied disk space using the special holder. Courtesy of Medtronic Sofamor Danek	Figure 8f - Two bone grafts are placed, one on each side of the disk space. Cancellous bone is then packed around the grafts. Courtesy of Medtronic Sofamor Danek
Figure 8g - The fusion is completed by placing bone posteriorly over the transverse processes and remaining portion of the lamina and facets. Courtesy of Medtronic Sofamor Danek

Minimally Invasive Spine Fusion:

Surgical techniques have recently been developed that allow the surgeon to perform a lumbar spinal fusion and instrumentation in a minimally invasive manner. This results in less post-operative pain and a shorter recovery period in most cases

• Several different techniques have been developed. What each has in common is that small incisions are used for removal of the disk, placement of a bone graft or cage within the disk space and placement of pedicle screws and rods with minimal injury to the back muscles
• An example of a minimally invasive lumbar spine fusion is as follows:
• The spine is approached with a muscle splitting technique using a series of muscle dilators
  1. A small incision is made over the disk and the bone over the disk is exposed by passing progressively larger muscle dilators. Finally a tubular retractor about 1-2 inches in diameter is placed over the final muscle dilator and the muscle dilators removed
  2. The procedure is then performed through the tubular retractor using an endoscope, microscope or loupe magnification. If there are pinched nerves, bone or ruptured disc can be removed to un-pinch the nerves
  3. Fusion of the spine is then performed through the tube and is achieved by placing bone into the disc space (called an interbody fusion) or on top of the spine (called a posterolateral fusion). At times both are used to make sure an adequate fusion occurs
  4. The tube is then removed allowing the muscles of the spine to return to there normal position. The spinal instrumentation is then placed percutaneously (by puncturing the skin) or through a small incision in the skin using specially designed instrumentation systems that preserve as much of the normal anatomy of the spine as possible (Figure 9A,B)
  5. The role of the instrumentation is to allow the bone to heal and fuse (much like the role of a cast for a broken arm or leg) forming a solid piece of bone and therefore preventing movement between two or more vertebrae

  Figure 9a - Pedicle screws are placed in the tow vertebra to be fused using extenders. Courtesy M. Perez-Cruet, MD	Figure 9b - The extenders are linked together and attached to a devise that aligns the holes in the screw heads and the rod. The rod can then be passed through a small skin incision into the holes with minimal muscle injury. Courtesy M. Perez-Cruet, MD

Complications

• Significant complications can accompany a lumbar fusion whether it is performed from in front or from in back.
• Untoward effects of anesthesia
• Bleeding or hemorrhage with the possible need for blood transfusions
• Nerve root injury that could result in paralysis, loss of feeling, or loss of bowel and bladder control
• Infection
• Blood clots in the veins of the legs or pelvis
• Pulmonary embolus
• Tear in the covering of the nerves with leaking of cerebrospinal fluid
• Injury to blood vessels
• Injury to bowel or ureters
• Pseudoarthrosis- failure of the fusion to take place. Successful fusion may not be able to be determined for over one year
• Impotence due to retrograde ejaculation in which sperm goes into the bladder (ALIF)
• Prolonged ileus, a condition in which the bowel stops functioning
• Pain from the bone graft donor site
• Dislodgment or backing out of the bone or metallic implants
  
• Compression of a nerve root or cauda equinae by bone or metallic implants
  
• The possibility of unforeseen complications

Postoperative Care

• The patient is encouraged to turn frequently and take deep breaths
• Elastic stockings should be worn for the first two to three weeks
• A support garment is usually prescribed for the first four to six months
• Walking usually starts on the day after surgery and progressively longer walks are encouraged
• Driving an automobile is delayed until determined by your surgeon
• X-rays will be obtained at regular intervals as determined by your surgeon