Download The X-Stop Interspinous Process Distraction for Treatment of

The X-Stop Interspinous Process Distraction for Treatment of Lumbar Spinal
Stenosis under Local Anesthesia
Ken Hsu, MD, James Zucherman, MD
The X-Stop is a titanium spacer inserted between the spinous processes under local anesthesia for minimally
invasive surgical treatment of spinal stenosis.
Spinal stenosis is narrowing of the spinal canal, lateral recess or neural foramen resulting in neural
compression. The clinical symptoms of spinal stenosis include extremity pain, radiculopathy, sensory or
motor deficit, gait abnormalities, neurogenic claudication, bladder or bowel dysfunction. Spinal stenosis is
classified as congenital, acquired or combined. There are many causes for this condition; however, the most
common cause is degenerative in nature.
Although -spinal stenosis had been observed in animals and found in Egyptian mummies, our understanding
of this condition was not gained until about fifty years ago. Spinal stenosis was probably first described in
humans in 1803, by Portal of France, who observed that narrowed spinal canals were associated with leg
pain and atrophy.
However, it was not until 1949 that Verbiest described the anatomic changes of hypertrophic articular
processes causing spinal canal stenosis. Subsequently, Kirkaldy-Willis wrote about the "three joint complex"
and the pathological changes found in spinal stenosis. Degenerative processes may start in one or two of
the three joint complexes, including the disc anteriorly and the two facet joints posteriorly. With time, all
three joints are involved. The degeneration of the joints also cause abnormal motion which produces
osteophyte formation. Ultimately, disc protrusion or osteophyte formation, hypertrophy of the facet joints
and ligamentum flavum result in spinal stenosis. Medical literature regarding this condition became more
available after the mid 1970's. The significant increase in the reported cases of spinal stenosis is related to
the introduction of axial imaging provided by CT scan and MRI.
Spinal stenosis did not have the socioeconomic significance that we see today until the 1970's. The aging of
our population is resulting in a higher incidence of degenerative stenosis. The average life expectancy during
the Roman Empire 2000 years ago was 30 years. In 1900, it was 45. In 1996, it was 76 with 12% of the
U.S. population over 65 years. The estimated life expectancy will be 82 in the year 2026 with 20% of the
population over the age of 65.
Until the recent past, many of the patients with spinal stenosis were told that nothing could be done. They
were advised to simply decrease their activities. Many of those patients lived out their lives in rocking chairs
as idle spectators to daily activities.
Today, there is greater emphasis on the quality of life and physical fitness. There is better care for the aged.
Patients are now demanding more aggressive treatment.
In meta-analysis of 74 journal articles related to the surgical treatment of lumbar spinal stenosis, it was
found that the mean time from the onset of symptoms to surgical decompression was 46.5 months, or 3.87
years. Although some patients developed a rapid progression of symptoms and disability, many patients
experienced a slow decline in physical function and gradual worsening of symptoms.
For the majority of patients, conservative treatments are tried first. Non-surgical management includes rest,
controlled physical activity, back brace, physical therapy, non-steroidal anti-inflammatory medications and
epidural injections. Surgical decompression is performed for severe disabling pain not responding to
conservative -treatment, progressive neurological deficit, bladder or bowel dysfunction due. to cauda equina
syndrome. Decompressive surgery performed under general anesthesia is not without risks, especially in the
degenerative spinal stenosis patients who are usually advanced in age with multiple medical problems.
Commonly, the mean anterior-posterior diameter of the central spinal canal is 12 mm, with a mean cross
sectional area less than 100 mm square. The minimal cross section area is 77 mm plus/minus 13 mm
square available for the neural elements. The shape of the spinal canal is also important. For example, a
"trefoil" configuration to the central canal is predisposed to early spinal stenosis. The intervertebral foramen
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has an inverted teardrop shape with a cross sectional area ranging from 40 to 160 mm square. The
intervertebral foramen changes significantly on flexion and extension as well as on lateral bending and axial
rotation. Flexion increases central canal and foraminal dimensions, while extension decreases them.
Patients seek relief for "neurogenic claudication" or leg pain, which is aggravated by walking and relieved by
sitting or flexing the spine forward. Patients with lumbar spinal stenosis assume a more flexed or rounded
posture as they walk. They can walk longer periods by leaning forward supported by a cane, walker or
shopping cart. Frequently, the stenotic spinal segment is limited to one or two levels and it is not necessary
to flex the entire spine forward or round the whole back. If the one or two involved levels of stenosis are
flexed forward, the symptoms can be relieved. In our experience, if only the stenotic spinal segment (or
segments) is flexed forward, then an elderly patient who walks with a kyphotic spine or rounded back can
stand up straight and walk without difficulty.
The new treatment method we developed involves placing a spacer called X-Stop between the spinous
processes. This implant is a bumper or spacer that keeps the stenotic segment in the sitting or flexed
position when the patient stands up. It is indicated for patients with lumbar spinal stenosis who can sit
comfortably, but have difficulty standing and walking.
Until the time the X-Stop was conceived, the spinal process was thought as an essentially useless and weak
anatomical structure. We found that by the time symptoms of spinal stenosis develop the spinous processes
become hypertrophied. Sometimes it is as though the spinous processes become a weightbearing bone. We
have radiological, CT scan and MRI evidence of spinous process enlargement in thickness, height, and
density. Sagittal reconstruction of CT images frequently demonstrate abutment of one spinous process to
the next at the stenotic level.
We have studies on the spinous process strength. In the study on "Failure Load of the Lumbar Spinous
Process During Flexion and Extension," Yerby, Lindsey and Kreshak measured the in-situ loads of the
interspinous spacer and related these implant loads to the failure loads of the spinous processes using a
cadaver model. The mean in situ load of the spacer was 100.5 plus/minus 65.3 Newtons. The mean failure
strength of the L3 spinous process was 1033 plus/minus 505 Newtons and was significantly greater than the
mean failure strength of the L4 spinous process (765 plus/minus 374 Newtons). The difference between the
mean failure strength of the L3 and L4 spinous processes is likely due to the different loading directions. The
results suggest a failure was to occur as a result of extreme extension, failure would likely originate within
the most caudal spinous process. There was a significant positive relationship between the spinous process
failure load and bone mineral density. This suggests that patients with low bone mineral density should be
examined thoroughly before being considered for interspinous process spacer surgery.
In the study on "Loading of an Interspinous Process Spacer During Axial Rotation," Yerby, Lindsey and
Kreshak measured the lateral contact load between the spinous processes and the implant stops and
compare these loads to the endurance limit of the implant. The combined results from the biomechanical
and fatigue testing demonstrated that there is relatively little load on the implant during axial rotation. The
expected maximum load on the implant during axial rotation (16.4 Newtons) was less than 15% of the
endurance limit (111 Newtons) and less than 6% of the failure load (276 ' Newtons). Any load between the
endurance limit would theoretically result in infinite life for the implant. The results reflected a safety factor
of over 6 times the expected daily load on the implant.
The spacer we are currently using is made of titanium in the shape of an oval cylinder. It has two parts, the
spacer itself and the wing. It is placed between the spinous processes anterior to the supraspinous ligament
as close to the lamina as possible; however, it is outside the spinal canal and away from the nerves. The
procedure requires no general anesthesia. Only local anesthesia is used. It can be performed in less than
one hour. There is no soft tissue or bone resection. For the surgical procedure, the patient is placed in the
right lateral decubitus position with the lumbar spine flexed as much as possible. A skin incision of only 5 cm
is made. Paraspinal musculature is elevated from both sides of the spinous processes. The supraspinous
ligament must be preserved. A curved probe is used to locate the space between the spinous processes. The
space is gently widened with a dilator. The oval cylinder spacer is inserted from the right side bottom up.
The wing is attached and locked in position by tightening the nut.
Since February of 2000, the safety and efficacy of the X-Stop for the treatment of symptomatic lumbar
spinal stenosis has been evaluated in a prospective, randomized multi-center study involving 200 patients.
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For 100 patients, X-Stop is being used. The other 100 patients are receiving conservative treatment with
epidural blocks. The inclusion criteria for the study include the following: patients must be 50 years or older,
they have leg/buttock/groin pain relieved when flexed such as when sitting, and the stenosis in the spinal
canal or neuroforamen is 50% or more when compared to the segments above and below. Exclusion criteria
include a fixed motor deficit, Cauda Equina Syndrome, severe stenosis at more than two levels, significant
peripheral neuropathy, significant peripheral vascular disease, scoliosis of more than 25°, spondylolisthesis
greater than grade I, pathological fracture, severe osteoporosis, severe obesity with BMI greater than 40
kg/m2, and active infection or systemic disease. We have six month follow up results. Eighty-two patient
suffering from neurogenic "claudication at the L3-4 and/or L4-5 levels have been enrolled in the randomized
prospective study. Forty-seven of these patients have received the X-Stop implants and 35 have been
treated conservatively, including epidural injections. Prior to surgery, all patients completed an initial Zurich
Claudication Questionnaire. This questionnaire is a self-administered survey developed to be a spinal
stenosis specific outcome measure consisting of symptom severity, physical function and satisfaction scales.
The Zurich Claudication Questionnaire has been previously validated for spinal stenosis patients treated with
laminectomy. In the current study, the questionnaire scores are compared between the X-Stop and the
epidural groups at six weeks, six months, one year and two years following surgery. The two groups are
compared statistically using a Significance Test for Proportions with a level of significance of 0.05. After six
months, the Zurich Claudication Questionnaire scores reveal 73% of the X-Stop patients were significantly
improved compared to 14% of the epidural patients had improved. This difference was significantly different
(P less than 0.001). 95% of the X-Stop patients reporting some improvement in symptom severity, and
91% reported an increase in physical function. The primary reason for the failure in the X-Stop group was
misdiagnosed co-morbidities. It was also noted that the X-Stop patient population tends to dramatically
increase their activity level almost immediately. In many ways, this short-term preliminary follow up
compares favorably to traditional surgical treatment, as well as non-surgical treatment in selected cases.
The surgical procedure is minimally invasive, inexpensive and safe. There were no intraoperative
complications.
Although this procedure was initially performed to provide relief of extremity or radicular pain, there was a
significant number of patients who were also able to obtain relief of their low back or axial pain. Recently, a
study was completed by Swanson and Yerby showing that with the X-Stop in place there was load reduction
in the posterior aspect of the disc of 40% to 60%. This is important since it is widely accepted that most
disc herniations originate from annular tears in the posterolateral region. During extension, the X-Stop
appears to redirect a large portion of the load away from the disc and transfer that load to spinous process.
This is a significant finding in terms of clinical applications. The decrease in posterior annular pressure could
improve back pain by relieving pressure on the nerve endings that are present in the outer one-third of the
annulus. We will be performing a controlled clinical study to evaluate the effectiveness of the X-Stop in the
treatment of axial back pain.
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