BIOMECHANICS
448 www.spinejournal.com March 2011
SPINE Volume 36, Number 6, pp 448–453
©2011, Lippincott Williams & Wilkins
Copyright © 2011 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Two-Level Noncontiguous Versus Three-Level
Anterior Cervical Discectomy and Fusion
A Biomechanical Comparison
Michael A. Finn, MD,* Mical M. Samuelson, MS,* Frank Bishop, MD,* Kent N. Bachus, PhD,† and Darrel S.
Brodke, MD†
Study Design. Biomechanical study.
Objective. To determine biomechanical forces exerted on interme-
diate and adjacent segments after two- or three-level fusion for treat-
ment of noncontiguous levels.
Summary of Background Data. Increased motion adjacent to fused
spinal segments is postulated to be a driving force in adjacent segment
degeneration. Occasionally, a patient requires treatment of noncontigu-
ous levels on either side of a normal level. The biomechanical forces
exerted on the intermediate and adjacent levels are unknown.
Methods. Seven intact human cadaveric cervical spines (C3–T1)
were mounted in a custom seven-axis spine simulator equipped with
a follower load apparatus and OptoTRAK three-dimensional tracking
system. Each intact specimen underwent five cycles each of flexion
extension, lateral bending, and axial rotation under a 1.5 Nm mo-
ment and a 100-Nm axial follower load. Applied torque and motion
data in each axis of motion and level were recorded. Testing was
repeated under the same parameters after C4–C5 and C6–C7 dis-
kectomies were performed and fused with rigid cervical plates and
interbody spacers and again after a three-level fusion from C4 to C7.
Results. Range of motion was modestly increased (35%) in the
intermediate and adjacent levels in the skip fusion construct. A
significant or nearly significant difference was reached in seven
of nine moments. With the three-level fusion construct, motion
at the infra- and supra-adjacent levels was significantly or nearly
significantly increased in all applied moments over the intact and
the two-level noncontiguous construct. The magnitude of this
change was substantial (72%).
Conclusion. Infra- and supra-adjacent levels experienced a marked
increase in strain in all moments with a three-level fusion, whereas
the intermediate, supra-, and infra-adjacent segments of a two-level
fusion experienced modest strain moments relative to intact. It would
be appropriate to consider noncontiguous fusions instead of a three-
level fusion when confronted with nonadjacent disease.
Key words: skip fusion, adjacent level disease, intermediate segment,
multilevel fusion, cervical biomechanics. Spine 2011;36:448–453
C
ervical arthrodesis remains a common and effective
treatment of degenerative, traumatic, neoplastic, and
infectious pathologic conditions. Increased scrutiny of
long-term results has, however, led to recognition of the de-
velopment of “adjacent-level disease” in up to 25% of treated
patients at 10 years.
1
Although there remains some debate
over whether adjacent-level degeneration is a consequence
of the natural progression of the disease process or whether
it is hastened by the fusion itself, a significant amount of in
vitro,
2–5
in vivo,
6
and finite element
7,8
biomechanical data sug-
gests that the latter is more likely. In these studies, the adjacent
levels have been demonstrated to be subjected to increased
stress, strain, and intradiscal pressure after fusion, factors
that are believed to promote the degenerative cascade through
mechanisms including nutrient deprivation, buildup of waste
products, increases in the relative amount of type 1 collagen,
and decreases in proteoglycans, chondroitin sulfate, and type
2 collagen.
9–11
The results of several studies have demonstrated that the
biomechanical effects on the adjacent segments are greater
after the fusion of multiple segments. Weinhoffer et al
12
dem-
onstrated a progressive increase in adjacent-level intradiscal
pressure after the fusion of one or two segments in the lum-
bar spine. Dmitriev et al
5
examined the immediate stability
endowed by three different fusion constructs in the cervical
spine over two and three levels and reported a trend toward
increased motion in flexion/extension at the supra- and in-
fra-adjacent levels with three-level constructs. Lopez-Espina
et al
8
performed a finite element analysis of the effect of single-
and double-level fusion on the adjacent levels and found that
both contributed to increased stress in the adjacent levels, the
magnitude of which was greater in the case of two-level
From the Departments of *Neurosurgery and †Orthopedic Surgery, University
of Utah School of Medicine, Salt Lake City, UT.
Acknowledgment date: February 26, 2009. First revision date: September 9,
2009. Second revision date: December 17, 2009. Acceptance date: Decem-
ber 17, 2009.
The device(s)/drug(s) is/are FDA-approved or approved by corresponding na-
tional agency for this indication.
No funds were received in support of this work.
One or more of the author(s) has/have received or will receive benefits for
personal or professional use from a commercial party related directly or in-
directly to the subject of this manuscript: e.g., honoraria, gifts, consultancies,
royalties, stocks, stock options, decision making position.
Address correspondence and reprint requests to Darrel S. Brodke, MD, Uni-
versity Orthopaedic Center, 590 Wakara Way, Salt Lake City, UT 84108;
E-mail: Darrel.Brodke@hsc.utah.edu
DOI: 10.1097/BRS.0b013e3181fd5d7c