Low backache, a major health problem in the aging population, can be caused by a variety of pathologies, one of which is degenerative disc disease (DDD).
A lumbar motion segment consists of all structures holding two adjacent lumbar vertebrae together to allow for movement in that segment. Each of these structures is capable of contributing to segmental motion and include from front to back: anterior longitudinal ligament, intervertebral disc, intervertebral foramen, ligamentum flavum, facet joints, interspinous ligaments and supraspinous ligaments.
A degenerated lumbar disc looses height due to desiccation of the nucleus pulposus with increasing fibrotic changes within its substance leading to increased compressibility as opposed to a normal disc. The intradiscal pressure falls and the stiffened annulus fibrosus bulges outwards radially like a ‘flat tyre.’
A reduced disc height in itself significantly impacts related structures in the involved spinal segment over time particularly the facet joints, subjecting them to abnormal loads resulting in osteoarthritic changes. The reduced stretching of the yellow ligament (ligamentum flavum) leads them to undergo remodelling with thickening. The consequent loss of elasticity and increased mass result in its bulging into the spinal canal, leading to secondary spinal canal stenosis. The shape of the dural tube in the axial plane which is oval changes with the morphometric changes in the facet joints and ligamentum flavum to triangular and eventually trefoil in appearance with crowding of cauda euina roots. These changes result in ‘limping’ termed neurogenic intermittent claudication. This phenomenon of ‘claudicare’ or limping as a result of canal stenosis was first described in race horses.
Bambakidis defined radiographic instability of a motion segment as translational motion of >3 mm in levels above L5-S1 or 5 mm at L5-S1, or motion segment angulation of >10° on lateral flexion/extension radiographs.
Kirkaldy-Willis studied the pathology and pathogenesis of lumbar spondylosis and stenosis and described the three-joint complex composed of the zygapophyseal joints and the intervertebral disc.
They postulated that rotation and compression injuries led to degenerative changes of the three-joint complex. The results of these changes to the three joint complex create disc-related back pain, segmental instability, unstable spinal stenosis, degenerative spondylolisthesis, retrolisthesis, degenerative scoliosis, and rotational deformities.
Lumbar fusion is used to treat pain attributed to abnormal motion or mechanical insufficiency resulting in segmental instability produced by the degenerative change described above. Often the compressed nerves are released simultaneously to reduce back and leg pain. In the last 40 years, indications for fusion have remained largely constant.
Replacing a degenerated joint instead of fusing it was considered for the spine due to the success of total knee and hip arthroplasty. The first described total disc replacement was the Fernstorm steelball endprosthesis in the late 1950s.
The ideal TDR patient is likely earlier in the Kirkaldy-Willis degenerative cascade than a fusion patient. The ideal TDR candidate may be an individual between 35 and 45 years old, with back pain severe enough to impact activities of daily living and/or work. This usually coincides with patient’s self-assessment on the Visual Analog Scale of a Pain score of 4/10 or more. Symptomatic DDD with or without radicular pain is the primary indication for TDR.
Indications for TDR are based on patients’ clinical problems, on several image findings, and other information.
- Low back pain (DDD) caused by osteochondrosis
- Sciatica associated with degenerative spondylolisthesis <3 mm
- Sciatica after nucleotomy
In summary, TDR candidates should have failed sufficient nonoperative treatment for a minimum of 6 months for symptomatic DDD, have no structural abnormalities like circumferential lumbar spinal stenosis or facet arthritis, have BMD T score >-1.0 on DEXA scan for osteopenia, no significant psychological issues, and diagnostic studies including discography confirming the disc as the pain generator. All major contraindications should be absent.
Patients with more advanced DDD with no nerve compression and reduced disc height (>4 mm) had superior satisfaction rates. Patient satisfaction rates are 85% for mono- and 65% for bi-segmental TDR. More favorable results in patients with isolated disc disease compared with multi-level disease.
Contraindications to TDR fall under two broad categories:
1) Painful conditions not caused by the disc, and
2) Conditions that may compromise long-term device functionality:
End stage disc resorption, disc height <3 mm
Pars defect: Spondylolysis, Spondylolisthesis
Vertebral body compromise like fractures L4, L5, severe osteoporosis
Evidence of segmental instability
Pseudoarthrosis following earlier spinal surgery
TDR is for pure discogenic pain with physiologic failure of the involved disc to take loads consistent with activities of daily living. The traditional patient would be one who wakes up in the morning feeling on top of the world with no back ache. The pain comes during routine days at a specific time consistent with a certain amount of physical activities which loads the spine resulting in failure of the specific segment to bear loads. The low back pain responds to lying down more than analgesics. For technical reasons this surgery is restricted to last 2 levels of Lumbar spine.
It is important to redefine pathologies not treatable by TDR which include:
Nerve root compression with radicular symptoms
Positive SLRT with pain below knee
Multiple level beyond 2 levels recommended: L4-5 and L5-S1
Lumbar spine stenosis
Arachnoiditis and Epidural Scarring