Cervical Kinematics After Fusion and Bryan Disc
Rick C. Sasso, MD and Natalie M. Best, BS
Introduction: Disc arthroplasty has been shown to provide short-
term clinical results that are comparable with those attained
with traditional anterior cervical discectomy and fusion. One
proposed benefit of arthroplasty is the ability to prevent or delay
adjacent level operations by retaining motion at the target level
and eliminating abnormal adjacent activity. This paper com-
pares motion parameters for single-level anterior cervical
discectomy and fusion and disc replacement patients at the
index level and adjacent segments.
Methods: Radiographic data from patients enrolled in a
prospective, randomized clinical trial were selected for kinematic
assessment of cervical motion. All patients received either a
single-level fusion with allograft and anterior cervical plate
(Atlantis anterior cervical plate, n=13) or a single-level
artificial cervical disc (Bryan Cervical Disc prosthesis, n=9)
at either C5/C6 or C6/C7. Flexion, extension, and neutral lateral
radiographs were obtained preoperatively, immediately post-
operatively, and at regular intervals up to 24-month time points.
Cervical vertebral bodies were tracked on the digital radio-
graphs using quantitative motion analysis software (QMA,
Medical Metrics) to calculate the functional spinal unit motion
parameters including range of motion (ROM), translation, and
center of rotation. If visible, the functional spinal unit
parameters were obtained at the operative level, and also the
level above and the level below.
Results: As expected, significantly (P<0.006 at 3, 6, 12, and
24mo) more flexion/extension motion was retained in the disc
replacement group than the plated group at the index level. The
disc replacement group retained an average of 6.7 degrees at 24
months. In contrast, the average ROM in the fusion group was
2.0 degrees at the 3-month follow-up and gradually decreased to
0.6 degrees at 24 months. The flexion/extension ROM both
above and below the operative level was not statistically
different for the disc-replaced and fusion patients, however,
mobility increased for both groups over time. The anterior/
posterior translation that occurs with flexion/extension motion
remained unchanged for the disc replacement group at the level
above the target disc preoperatively and postoperatively. In
contrast, the translation increased for the level above the fusion.
At the 6-month follow-up, the increase in translation was
significantly greater for patients that were fused (P<0.02) than
for patients that received a disc replacement. This change was
not significant at 12 months.
Discussion: Previous studies have shown the Bryan disc to
maintain mobility at the level of the prosthesis. The long-term
clinical benefit of maintenance of motion is postulated to be the
ability to delay or avoid adjacent level operations. This study
reveals that there is no difference in flexion/extension ROM at
the level above and below either a fusion or Bryan arthroplasty.
There is, however, an increase in anterior/posterior translation
at the cephalad adjacent level in patients with arthrodesis while
the Bryan arthroplasty retains normal translation for the same
amount of flexion/extension at the adjacent level.
Conclusions: The Bryan disc may delay adjacent level degenera-
tion by preserving preoperative kinematics at adjacent levels.
Key Words: cervical spine, kinematics, arthroplasty, artificial
disc, Bryan artificial disc replacement, fusion
(J Spinal Disord Tech 2007;00:000–000)
and myelopathy.1Because of limitations specific to this
procedure, investigators have developed alternatives to
fusion that attempt to address the kinematic and
biomechanical issues inherent in fusing a cervical motion
Adjacent segment degeneration requiring reopera-
tion has been documented at a rate of 2.9% of patients
per annum by Hilibrand et al.2Other reports have helped
to shed light on the recurrence of neurologic symptoms
and degenerative changes adjacent to fused cervical
levels.2–5In patients undergoing cervical fusion, 25% will
have new onset of symptoms within 10 years of that
fusion.2Segments adjacent to a fusion are subjected to an
increased range of motion (ROM) and increased intra-
Intervertebral disc replacement is designed to
preserve motion, avoid the limitations of fusion, and
allow patients to quickly return to routine activities. It
avoids the morbidity of bone graft harvest,10,11pseudar-
throsis, complications of anterior cervical plating, and the
side effects of cervical immobilization.
nterior cervical discectomy and fusion (ACDF) is a
proven intervention for patients with radiculopathy
CE: jeyasriED: geethaOp: vpBSD:200297
Copyrightr2007 by Lippincott Williams & Wilkins
Received for publication June 16, 2006; accepted February 12, 2007.
From the Indiana Spine Group and Indiana University School of
Medicine, Indianapolis, IN.
Reprints: Rick C. Sasso MD, Clinical Orthopaedic Surgery, Indiana
University School of Medicine, Indiana Spine Group, 8402 Harcourt
Rd., Suite 400, Indianapolis, IN 46260 (e-mail: rsasso@indianaspi-
J Spinal Disord Tech?Volume 00, Number 00, ’’ 2007
Artificial disc implantation was first accomplished
by Fernstrom12in 1966. In the late 1980s
al13developed ametal-on-metal ball and socket cervical
disc replacement comprised of 316L stainless steel. In
May 2002, the Bryan cervical disc was the first cervical
prosthesis to be implanted in the United States as part of
a Food and Drug Administration (FDA) Investigation
Device Exemption (IDE) study. The Bryan disc (Med-
tronic Sofamor Danek, Inc, Memphis, TN) is a one-piece,
biarticulating, metal-on-polymer, unconstrained device
with a fully variable instantaneous axis of rotation14,15
and is the prosthesis used in this study.
One of the primary goals of cervical disc replace-
ment is to reproduce normal kinematics after implanta-
tion. This includes the anterior-posterior translation
routinely exhibited during flexion and extension. Motion
studies have been conducted on human cadavers in the
laboratory16,17but are limited by the mandatory artificial
design of in vitro evaluation. The optimal method of
assessing cervical kinematics is in a living human in vivo
model. A recent study evaluated in vivo motion with the
Bryan disc postoperatively as compared with preopera-
tive movement.18The most valid study design, however,
examines a prospective, randomized cohort of arthro-
plasty versus a control fusion population. The current
study provides these data. This study aims to evaluate the
ability of the Bryan disc device to maintain motion at the
implanted level and also assess the motion at adjacent
segments and compare this juxtalevel movement of
arthroplasty to conventional arthrodesis.
, Cummins et
MATERIALS AND METHODS
The first 22 consecutive patients enrolled in the
Bryan cervical disc arthroplasty FDA IDE trial at one site
are the subjects of this study. This is a prospective,
randomized assessment of arthroplasty versus ACDF for
the treatment of one-level cervical radiculopathy or
myelopathy resistant to nonoperative treatment. All 22
patients were operated by one surgeon from May 2002 to
April 2003. They received either a single-level anterior
cervical allograft/plate (Atlantis anterior cervical plate,
n=13) or an artificial cervical disc (Bryan Cervical Disc
prosthesis, n=9) at either C5/C6 or C6/C7. Twelve
patients had their procedure at level C5/6 and 10 at C6/7.
Thirteen patients (59.1%) were male. At the time of
surgery, the patient population’s ages ranged from 32 to
53 years with an average age of 42.4±5.4 years.
Evaluation of Kinematics
Radiographic data from consecutive patients en-
rolled in a prospective, randomized clinical trial were
selected for kinematic assessment of cervical motion.
Flexion, extension, and neutral lateral radiographs were
obtained preoperatively, immediately postoperatively and
subsequent regular intervals up to 24 months. Cervical
vertebral bodies were tracked on the digital radiographs
using quantitative motion analysis (QMA) software
(Medical Metrics; Houston, TX) to calculate the func-
tional spinal unit motion parameters including ROM,
translation, and center of rotation. This feature-tracking
software has been used in a number of other studies.19–21
If visible, the functional spinal unit parameters were
obtained at the operative level, and also the level above
and the level below. Translation was measured as
displacement of the cephalad vertebral body along a line
parallel to the cephalad endplate of the caudal vertebral
body and measured as a percent of the length of that
Significantly (P<0.006 at 3, 6, 12, and 24mo) more
flexion/extension motion was retained in the disc replace-
ment group compared with the plated group at the index
level. The disc replacement group retained an average
ROM of 6.7 degrees at 24 months. In contrast, the
average ROM in the fusion group was 2.0 degrees at the
3-month follow-up and gradually decreased to 0.6 degrees
at 24 months (Fig. 1).
Range of Motion For Disc-Replaced and Plated Patients
pre3 month 6 month12 month 24 month
FIGURE 1. Flexion/extension ROM for
disc replaced and fusion patients; at the
target level and also at the adjacent
Sasso and BestJ Spinal Disord Tech?Volume 00, Number 00, ’’ 2007
r 2007 Lippincott Williams & Wilkins
The flexion/extension ROM both above and below
the operative level was not statistically different for the
disc-replaced and fusion patients, however, mobility
increased for both groups over time (Fig. 1). The mean
anterior/posterior translation that occurs with flexion/
extension activities remained unchanged for the disc
replacement group (8.3% of endplate preoperatively,
8.4% at 6mo, and 8.2% at 12mo) for the level above
the disc (Fig. 2). In contrast, the translation increased for
the level above the fusion (8.5% of endplate preopera-
tively, 12.4% at 6mo, and 11.0% at 12mo). This increase
was dramatic for some fusion patients, but was not
uniformly noted (Fig. 3). At the 6-month follow-up, the
increase in translation was significantly greater for
patients that were fused (P<0.02) than for patients that
received a disc replacement. This change was not
significant at 12 months, perhaps owing to variability in
level of patient effort.
The long-term clinical benefit of maintaining mo-
tion is postulated to be the delay or avoidance of adjacent
level degeneration. Disc replacement aims to preserve the
normal kinematics in the spine. The Bryan disc is one of
several arthroplasty systems currently being investigated
for use in the cervical spine. While each of these devices
has early outcome data, long-term studies assessing
adjacent segment degeneration are still pending.
Wigfield et al22reported favorable results on a 2-
year pilot study of the Prestige I Disc designed to address
the safety of the technique and to assess the stability of,
and motion allowed by, the device. They tried to target
patients most at risk for adjacent segment disease. Fifteen
patients were enrolled in the study. Motion was success-
fully preserved as radiographically demonstrated within
an appropriate physiologic range. Device stability was
concluded as no devices dislocated.
In a separate prospective nonrandomized study,
Wigfield et al9compared the effects of the Prestige Disc
and one-level anterior fusion on adjacent segment
motion. No significant difference in adjacent segment
motion was measured between the 2 groups preopera-
tively. Postoperatively, there was a significant increase in
adjacent segment motion in the fusion group (mean 9
degree increase) compared with a slight reduction in
adjacent level motion noted in the disc group. In the
fusion group, adjacent segment motion increased 5% at 6
months and 15% at 1 year. Subgroup analysis showed
that increased motion occurs predominantly in normal
rather than degenerative adjacent discs.
In May 2002, the Bryan Cervical Disc was the first
cervical prosthesis to be implanted in the United States.
Early clinical studies have shown clinical success and
preserved motion at the implanted site
particular interest is whether the kinematics are preserved
at the operated segment and at adjacent segments. ROM
with the Bryan disc has been studied and has shown no
significant change in ROM at the operated level and at
adjacent levels postoperatively.19,20This study found
similar results. However, the strength of this study is
that it is a prospective, randomized comparison with the
conventional ACDF. One of the major criticisms of
ACDF is that it does not preserve the normal kinematics
of the spine and therefore may result in degeneration at
adjacent segments potentially requiring an additional
fusion.2–9This study did show preserved motion with the
Bryan disc as compared with ACDF. Flexion/extension
ROM was not determined to be significantly different
between populations at adjacent segments. However,
there was a significant difference in translation at the
level above the fusion after the surgery. To accomplish
similar flexion/extension ROM at the level above the
fusion, increased translation was found in the fusion
group. This increased translation at the adjacent level
may place excessive loads on the annulus and the facet
joints above a cervical fusion.
This study showed that the Bryan disc maintains
mobility at the level of the prosthesis, comparable with
the preoperative ranges. In addition, the study corrobo-
FIGURE 2. A and B, Extension (A) and flexion (B) x-rays of the
disc space above a Bryan disc. The translation of the cephalad
vertebrae along the cephalad endplate of the caudal vertebrae
is relatively small.
FIGURE 3. A and B, Extension (A) and flexion (B) x-rays of the
disc space above a fusion. The translation of the cephalad
vertebrae along the cephalad endplate of the caudal vertebrae
is relatively large.
J Spinal Disord Tech?Volume 00, Number 00, ’’ 2007Bryan Disc Arthroplasty
r2007 Lippincott Williams & Wilkins
rates the findings of others and demonstrates that an
increase in anterior/posterior translation at the adjacent
level occurs in patients with arthrodesis.2–9It is therefore
postulated that the Bryan disc decreases risk for
degenerative translational motion. Further evaluation
needs to be done once long-term results have been
Normal flexion/extension ROM continues out to 2
years after Bryan disc arthroplasty. The flexion/extension
ROM at adjacent levels to the Bryan disc and a fusion are
not different, but to allow this same motion above a
fusion-increased translation is demonstrated.
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r 2007 Lippincott Williams & Wilkins