Function of the Spine

The spine is the structure that supports and stabilizes the body and enables motion. It gives humans the ability to perform activities such as walking, bending and sitting. It also provides protection for the spinal cord and nerve roots.

Spine Anatomy

The spine is composed of bones called vertebrae that are stacked on top of each other and separated by softer, pliable intervertebral discs. The discs are composed of two parts: the annulus fibrosus and the nucleus pulposus. The annulus is a tough, flexible outer ring that encompasses a jelly-like nucleus. Together, the annulus and nucleus work as a "shock absorber" to distribute loads.

Regions of the Spine

The spine is divided into four regions. The cervical region contains the seven upper-most vertebrae in the neck.  The thoracic region contains 12 vertebrae in the mid-back. The lumbar region contains five vertebrae in the lower back. At the base of the spine is the sacrum and coccyx bone, commonly known as the tailbone

About Total Disc Replacement

The Treatment

The ProDisc^TM-L total disc replacement has been determined to be safe and effective in the treatment of degenerative disc disease (DDD) at one level from L3-S1.  During the total disc replacement procedure, Dr. Tooke removes the diseased intervertebral disc and replaces it with a motion preserving ProDisc^TM-L implant.  The ProDisc^TM-L total disc replacement surgery is intended to:

·          Remove the diseased disc

·          Restore normal disc height

·          Significantly reduce discogenic back pain

·          Preserve motion in the affected vertebral segment

·          Improve patient function

The Design

The ProDisc^TM-L implant design is based on a ball and socket principle.  The ProDisc^TM-L implant is composed of three implant components – two metal endplates and a plastic inlay. 

The endplates have a patented central keel and small spikes for initial fixation to the vertebrae, and a plasma sprayed titanium coating on all bone contacting surfaces to promote bony integration. The ProDisc^TM-L implant materials have historically been used in hip and knee replacements and have been used in spinal arthroplasty procedures for two decades.

The ProDisc^TM-L implant is available in a variety of sizes to allow Dr. Tooke to accurately match the patient’s anatomy.

Question:  How many types of disc replacements are available?

Answer:  There are currently two types of disc replacements available.  This means that the U.S. Food and Drug Administration has approved them for use in the lumbar spine. In the future there will be cervical disc replacements, some of which are currently being studied in FDA trials under what is called an Investigational Device Exemption.

Question:  What are the two types of replacements now available for the lumbar spine?

Answer:  The first one which received approval is the Charite^®, which is from DePuy Spine_TM, a Johnson and Johnson company.  It was developed in Germany and implantation begun in the early 1990’s.  The second is the ProDisc^TM, which I will be implanting.  It is from Synthes^®, a large orthopaedic implant company.  French orthopaedic surgeon, Dr. Marnay, developed it and began implantation of the ProDisc^TM in 1990.

Question:  Is one any different from the other?

Answer:  They have similarities and differences.  Both have metal  (Cobalt – Chrome alloy) on high-density polyethylene bearing surfaces.  This is the material combination with which there is the most experience in the orthopaedic joint replacement field. 

The Charite® has three moving parts, a polyethylene core which slides between two metal plates with concave surfaces which are attached to the adjacent vertebra.  The stability of this implant is highly dependent on the stability of the native spinal structures including the facet joints and the part of the disc (annulus) which remains after much of the disc has been removed during the preparation for the disc implant insertion.

The ProDisc^TM has only two moving parts because the polyethylene part of the implant is locked to one of the metal plates and moves only in relation to the other.  This is the same type of configuration that has been used in total hip and knee replacement for approximately the last twenty-five years.  The actual bearing surfaces are a ball and socket configuration which is extremely stable and does not involve any sliding, or back and forth movement, and based on my training and clinical experiences, is less dependent on native spinal structures for stability.

Question:  How is a disc replacement implant inserted?

Answer:  The disc replacement is inserted from the front through an abdominal incision.  This is the same surgical approach as for an anterior inter-body fusion, with which we have a great deal of experience.  The anatomical structures of the abdomen are moved to the side, revealing the front of the vertebral column and the discs.  Most of the disc is removed, leaving some intact tissue on right and left sides.  The appropriate sized ProDisc^TM is determined, inserted, and locked into place via special cuts made in the adjacent vertebra.  The implant is then inserted and its position confirmed with X-ray.  The wound is then closed.  An animated video has been created to assist in understanding the procedure.

Question:  Is disc replacement an alternative to other procedures?   Is it done instead of something else, or is it for a problem that did not previously have a treatment?

Answer:  Disc replacement is used to treat some of the problems we now treat with fusion surgery.  It is useful in the treatment of conditions in which a lumbar disc is the primary source of pain and where back pain, rather than leg pain, is the primary symptom.   There remain many problems for which fusion is the only available treatment.

Question:  Is it better than fusion surgery?

Answer: The FDA currently approves disc replacement surgery only for problems at one spinal level.  Therefore, for the time being, a spinal fusion is considered for multiple level problems. Although, in Europe, many cases involving more than one spinal level have been performed successfully. 

Disc replacement has certain advantages over fusion especially in terms of postoperative recovery.  The recovery period is about 6 to 12 weeks postoperatively, whereas the postoperative recovery for fusion is 6 to 8 months.  In most of the studies comparing the outcomes of disc replacement and fusion two years postoperatively, the results are very similar.  A recent study has reported outcomes after ProDisc^TM to be a little better than fusion two years postoperatively.

 There is a theoretical advantage, which will take a long time to prove.  In fusion, one or more segments are permanently stiffened, while motion is retained with disc replacement.  With a spinal fusion, the adjacent mobile segment may degenerate and develop arthritic changes, which cause recurrent pain years later.  These changes are probably accelerated by the fact that there are increased stresses at the junction between the mobile and stiffened parts of the spine.  Therefore disc replacement has the potential to decrease so-called “adjacent segment disease.”

Question:  Can a disc replacement wear out?

Answer:  Based on my experience of performing many hundreds of other joint replacements in the knee and hip, it is my perspective that there is not a definitive answer available in the disc replacement literature.   The French experience has not revealed major wear problems at 16 years post-operatively.  However it is unlikely that many of the patients in Dr. Marnay’s original series were overweight (see below). A joint replacement implant is a mechanical device, and we all know that all machines eventually wear out.  The issue is one of how long that will take and will it take more years than the patient has left to live.  We know from hip and knee replacement experience that longevity of an artificial joint is related to size of load (weight) and activity level.  Therefore the risk of a hip replacement failing in a heavy 75 year old before that person dies is very remote.  In contrast, a heavy 35 year old who will become very active as soon as the pain resolves and has a life expectancy of many more years presents a much greater problem.  The 35 year old would always be counseled that one or more revision surgeries would probably be necessary over the course of his/her lifetime.   I would predict that young heavy patients will be identified as poor candidates and that middle-aged people of normal weight will emerge as being the best candidates.  Unlike hip and knee replacement patients, elderly spinal patients are not good disc replacement candidates due to other health issues.

Question:  Can a disc replacement be revised if it wears out?