Sometimes patients get mixed up on what a spinal fusion is and what the purpose of internal spinal instrumentation (rods, hooks, and screws often known as 'hardware') is.
In a spinal fusion, the surgeon takes the spine with its many moveable vertebra and transforms it into one long solid column with no movement. This is done by stripping the muscles away from the spine, causing bleeding, repositioning the individual vertebrae (if needed), and then packing new bone around the segment so that it can heal in one solid mass. Decades ago, the patient was placed in bed in large body casts for 6 to 12 months as the bone healed, but today support for the fusion comes from the spinal instrumentation system.
This allows the healing patient to be more mobile and no longer restricted to bed. The spinal hardware thus has only one purpose: to hold the spine in the proper alignment while the bone fuses it into one solid column. Once the spine has fused, there is no longer any need for the hardware. Yet it is left in the body unless it protrudes or causes pain or other problems on its own.
The ultimate success of a spinal fusion rests on whether the fusion heals solidly in the desired position along its entire length. Thus, the spinal fusion succeeds if it remains in place with no pseudoarthroses, or areas of unhealed bone. It is very difficult to assess whether a patient has a pseudoarthrosis today even with the most highly detailed MRI because the nonhealed area may be in the form of a miniscule crack that cannot be seen on film. Patients can often function relatively normally with a pseudoarthrosis or two for years, so ordinarily there is no medical intervention unless a definite problem arises.
If the Scoliosis patient is having further spinal surgery, though, the doctor will usually expose the entire old fusion with a posterior incision and check it visually. If necessary, bone can be added to shore up the fusion and prevent future problems. Note that the only sure proof way to assess a fused spine for possible pseudoarthroses with current technology is through exploratory surgery.
Eventually, if the spinal bone fails to fuse properly or if it cracks decades later, the spine bends back and forth at that spot and may cause increasing levels of pain. As a patient with a pseudoarthrosis bends, the hardware rods and screws are also stressed back and forth, and they may eventually snap. This occurrence can turn into an emergency if the spinal hardware protrudes from the body or if it is in a position that is damaging muscles or nerves. Sometimes patients do not know that a piece of spinal hardware has broken, however, or that a screw or a hook has become loose and is moving in their backs. However, usually x-rays (or MRIs, if fine detail is needed) can pick up these problems.
Virtually every spinal fusion utilizes autologous bone, self-donated by the patient from the hip or ribs during surgery. The mixing of the patient's own new and old bone results in a more dependably solid fusion. If the patient is having a spinal fusion primarily in the thoracic (or midback area), the surgeon may choose to use rib bone in a procedure called a thoracoplasty, since this will also cosmetically reduce the rib hump that results from a curved spine rotating the ribs more prominently on one side. However, this procedure results in more pain after surgery, reduces lung capacity for about 3 months afterward, and may require a chest tube during and after surgery for a few days to prevent a hemothorax (blood in the chest cavity). Adults can even develop permanent slight lung impairment as a result of a thoracoplasty.
In addition, the use of autologous bone, whether from ribs or hips, increases the length of time of surgery and causes more blood loss, so it may not be desirable in certain populations of surgery patients, such as the older people. For various reasons, then, the surgeon may elect to use bank (or cadaver) bone, especially if the patient already has one or more bone donation sites from previous surgeries or the patient's bone is considered undesirable for the procedure.
The surgeon may also choose to use bank bone if the patient has developed pain, unrelated to the spinal correction, in a previous bone donation site. There is reportedly no danger in contracting HIV, the virus that causes AIDs, or other diseases from bank bone.