Chapter 2: Background
2.2. Spinal pathology and treatments
2.2.3. Decompression
In patients in whom severe symptoms persist and functional impairment develops, surgery is the recommended option. Decompression surgery used in LSS aim to decompress the neural elements, without occur instability of the segment. Such decompression surgery usually leads to relief of pain in the legs and low back pain [41]. Decompressive surgical procedures include laminectomy and hemilaminectomy, hemilaminotomy, fenestration, and foraminotomy [40]. The complication rates for decompression surgery range from 14 % to 35
% or more [42]-[45]. Typical complications of decompression surgery include inadequate decompression with significant residual stenosis, instability of segment, renewed nerve compression, and reossification. All of these complications result in renewed nerve compression [44]-[47].
Decompression surgery may cause as mentioned above if weight bearing structures are compromised. Therefore, instrumented is necessary when preexisting or surgically induced instability is present. Pedicle screw instrumentation is a popular method of strong fixation to achieve stabilization rate (Figure 2.12). For stabilization of one spinal functional unit, four pedicle screws are usually used.
risks ems utilize rtheless all vertebral fle
The basic r bar spinous
the use of p
ss the interv a non-rigi low for dis exion throu rationale of processes
pedicle screw divided in s: postopera e are insert empt to red vertebral dis
id means o traction acr gh band tec f interspinou
as a treatm
ws is techni nto three ca
ative neurap kage or loss
Pedicle scre
ies have em process dev ted between duce neural
sc, or promo of connecti
ross the int chnology.
us process ment for st
ically dema ategories: 1 praxias or p of correctio
ew instrume
merged for vice [51] an n the spinou compressio oting lumba
ng otherwi tervertebral
device for tenosis is th
anding and a 1. Infection permanent n on [49].
entation [50
managing nd dynamic us processe
n by preven ar flexion. D ise standard l disc or re
inserting an hat the sym
associated w ns: deep in
neurologica
0].
lumbar ste c pedicle scr es of the lum
nting lumba Dynamic pe
d pedicle s egulate the
n implant b mptoms of
with certain fections. 2.
al disorders.
enosis in a rew system.
20
claudication are often relieved with the lumbar spine in the flexed position, and worsened in extension. Conceptually, a device that induced some flexion of the motion segment would increase the caliber of the spinal canal and the intervertebral foramen. Furthermore, if such a device merely prevented extension, it could minimize the narrowing of the spinal canal and foramen observed with extension. Additionally, and interspinous process device might also provide for some degree of distraction which could unload both the facet joints and intervertebral discs, potentially reducing back pain. Finally, on a practical note, the ability to access the interspinous area with a small incision and minimal paraspinal muscle stripping implies that the implantation of such a device could be performed in much less traumatic way than current decompression techniques. The interspinous process device can be called the non-fusion surgery is to restore normal physiological motions, or to allow restrained motions within a certain range, through various mobile non-fusion devices that aim to avoid or alleviate adjacent segment disease.
Four such interspinous process devices have been designed and are currently available:
the Coflex(Paradigm Spine, Wurmlingen, Germany), the Wallis (Abbott spine, Bordeaux, France), the Diam (Medtronic, Tolochenaz, Switzerland), and X-Stop (Medtronic, Tolochenaz, Switzerland). As a general note, at the time of this writing, the four devices described here are in various stages of clinical development.
(1) X-STOP
The X-STOPconsists of a titanium oval spacer with two lateral wings to prevent lateral migration (Figure 2.13). It is inserted into the interspinous space without disruption of the interspinous ligament. A biomechanical study demonstrated that the force required to insert the device in the appropriate position is 4.5 times less than the force required to break off the spinous process with the device placed too caudally or cranialy, suggesting that the device insertion is relatively safe. The body of biomechanical and clinical literature for the X-STOP far exceeds that of the other devices and as such it is described in the most detail here. It has
been formally evaluated in patients with computed tomography or MRI confirmed lumbar stenosis who complained of leg, buttock, or groin pain relieved by flexion, with or without back pain. In one article, it is speculated that the implant may confer some benefit to patients with pressure-related discogenic back pain, under the hypothesis that the implant provides some distraction and thus decreases pressure within the intervertebral disc.
(2) Wallis
The Wallis system consists of an interspinous blocker made from PEEK (Polyetheretherketone) with two woven Dacron ligaments which wrap around the caudal and cranial spinous processes (Figure 2.14). The interspinous ligament is removed and the Dacron ligaments are inserted around the caudal and cranial spinous processes. At the end of the procedure, the interspinous ligament is repaired. The designer of this device advocates the following indications for its implantation: recurrent disc herniation after primary discectomy, primary discectomy for voluminous herniated disc, discectomy for herniation of a transitional disc segment, disc degeneration adjacent to a previous fusion, and isolated Modic I lesion leading to chronic low back pain.
(3) DIAM
The DIAM for intervertebral assisted motion, is an interspinous implant that consists of a silicone core surrounded by a polyester outer mesh which is secured to the cephalad and caudal spinous processes by two polyester tethers (Figure 2.15). These tethers are inserted through the interspinous processes using attached steel needles and then are secured to the device by means of two titanium crimps.
(4) Coflex and Coflex-F
The Coflex device is made of titanium. It was originally developed as an interspinous U-shaped and is placed between two adjacent spinous processes (Figure 2.16) [52][53][54].
After implantation, the lateral wings are crimped towards the spinous processes to improve fixation. The U-shaped structure is designed to allow the lumbar spine to have controlled
move versi Cofle
ement in fo ion called th
ex [56].
Fi
orward and b he Coflex-F
igure 2.13: X
backward b F (Coflex ri
X-STOP [5
Fig
Figure 2.16
22
bending. To ivet) has als
5]
gure 2.15: D
6: Coflex an
o improve st so been dev
Figu
DIAM [55]
nd Coflex-F
tability in a veloped, wh
ure 2.14: Wa
F [56].
all motions, hich adds a
allis [55]
a modified rivet to the d e
Recently, many studies have evaluated the biomechanical behaviors of the Coflex and Coflex-F devices. Tsai [57] used cadaveric lumbar L4 and L5 segments with implanted Coflex device to examine their biomechanical behavior, and the results showed that the implanted Coflex device can provide stability for the lumbar spine in flexion-extension and axial rotation, except in lateral bending. Kong [58] reported 1-year follow-up outcomes after Coflex device implantation and traditional fusion for degenerative spinal stenosis. The results indicated that both the Coflex device and traditional fusion reduced the range of motion (ROM) at the surgical segment, but fewer effects were found at the adjacent segments with the Coflex device as compared with the increasing ROM with traditional fusion. Kettler [56]
compared the Coflex and Coflex-F devices using biomechanical experiments and found that both implants had strong stability in extension. However, the Coflex implant could not compensate the instability in flexion, lateral bending, and axial rotation as well as the Coflex-F did. Wilke [59] examined the biomechanical effects of different interspinous process devices for flexibility. The Coflex device had the best stabilizing effect in extension but poor stability in flexion. In lateral bending and axial rotation, the Coflex device had neither a stabilizing nor a destabilizing effect. Inconsistent results regarding the biomechanical effects of the Coflex device have been shown in previous studies. In addition, these studies are mostly a short-segment analysis focused on the surgical segment. The effect of the Coflex device and the Coflex-F device on adjacent segments is still not clear.
Therefore, the first subject was to investigate the biomechanical differences between the Coflex device and the Coflex-F at surgical and adjacent segments by using finite element (FE) analyses on a five-segment spinal model. In addition, the study also compared these two interspinous process implantations with pedicle screw fixation.
2.2 ery in cases ors such a morbidities [ Spinal fusi ipulation [6 ure 2.17). It goal of the anterior spi ebral bodies ebrae. These pted from fib
Figure 2.17:
surgery gery is nee y of the ver rtebra relativ
ility can m s of LSS ran as type of
[60]-[67].
on is define 68], and ai t is an effec e procedure
inal column s to mainta e bone graft bulae, illia,
This radiog
ded in case rtebral body ve to one b make nerve
nge from 40 f decompre
ed as a bony ims to com ctive techni
is to restor n. In gener ain disc he fts may be a the iliac cre
graph demo
24
es of severe y >3 mm), s below) or sc root comp 0-90 % in th ession, dur
y union betw mpletely eli
ique for tre re disc heigh ral, bone gr eight and to autografts, a
est, or ribs.
onstrates a s
e degenerati spondylolist coliosis (late
pression. S he literature ation of f
ween two ve iminate mo
ating degen ht, enlarge rafts are pl o accelerat allografts or
solid bony u
ion disc, in thesis (>5 m
eral curvatu Success rate e and depen follow-up,
ertebrae spa ovement by nerative spin
the stenotic laced into e bone gro r synthetic m
union betwe y the motio nal instabili c foramen, a the interfac owth into n materials wh e variety of atients and
ing surgical on segment ity, and the and support ce between neighboring hich can be
L4 [69].
as th he anterior l F) approach F approach nucleus befo PLIF approa udes the rem eus. In addi e space (Fig ified from t e is approac ure 2.18; blu
igure 2.18: C icates the A
The spinal vertebral b ures of this
common sur lumbar inte h, and transf
includes the ore implant ach, a partia moval of the
ition, a certa gure 2.18; re the PLIF me ched, an inf
ue arrow) [7
Common su ALIF approa
interbody ody, thus r device (Fi
rgical techn erbody fusio
foraminal lu e removal o ting an inter al laminecto e ISL, SSL, ain portion ed arrow) [7 ethod to pro ferior
niques for th on (ALIF) a umbar inter of the ALL, rbody fusio omy, discec LF, posteri of the facet 71]. Recentl ovide a min -laminectom
hniques for i arrow indic ates the TLI
e can repla hysiologica . First, the
he insertion approach, p rbody fusion the anterior on cage (Fig ctomy and n ior portions t joint can b ly, the TLIF nimally inva my and a un
insertion of cates the PL IF approach
ace the dege al disc heig
spinal fusi
of a spinal posterior lum
n (TLIF) ap r portions o gure 2.18; b nucleotomy
of the disc be removed F approach h asive surgic nilateral fac
a spinal cag LIF approach
h.
enerative d ght. In gene
on cage is
l cage can b mbar interb pproach. In g of the disc a
black arrow y are perform
c annulus, a to give the has been pr cal techniqu cectomy are
ge. The blac ch, and the b
disc and dis eral, there made of a and the total nerve roots roposed and e. After the e performed
ck arrow blue arrow
stension the are several a variety of d
26
biocompatible materials, including stainless steel, titanium alloy, carbon fiber-reinforced polymer (CFRP), and polyetheretherketone (PEEK) [73]. Due to the high mechanical strength of these materials, a spinal interbody fusion cage can provide better longitudinal support than a traditional bone graft, without causing collapse. Second, rough or specific designs can be found on the contact surfaces of spinal cages. In order to prevent cage slippage, rough contact surfaces, saw teeth, spikes or threads have been designed to increase stability between fusion devices and endplates. Third, these implants are usually designed to be hollow, with small pore or openings on the wall. These hollow cages can be filled with bone grafts to promote bone growth. Furthermore, only small amounts of cancellous bone are required, because there is no longer need for the cubic graft to be a spacer. The small pores and openings on the wall allow the growth of bone through the cage, resulting in bony fusion. Therefore, spinal fusion cages can avoid donor site morbidity and increase fusion rates.
Currently, many kinds of spinal cage designs are available on the market, which can be classified by the various surgical approaches used in their implantation. Large single lumbar cage designs are used for the ALIF procedure (Figure 2.19 A). Some paired cage designs are used strictly for PLIF procedures (Figure 2.19 B). In addition, some specific shapes of cages are designed for minimally invasive surgical techniques such as the TLIF procedure (Figure 2.19 C).
All types of interbody fusion approaches are recommended for combination with traditional posterior pedicle screw fixation to increase stabilization and fusion rates (Figure 2.20). A pedicle screw is a device composed of rods and screws contoured to restore lumbar lordosis and disc height, and can be used for unilateral or bilateral pedicle screw fixation.
Figu ditions of th ments, maint
nically dem ws surgery i Recently, th posterior spi minimally in s modified ce and the s
(A) arious lumba
l Ltd., Bettl USA); (C) A
20: Interbod
n is very su e lumbar sp tenance of manding and
dy fusion co
uccessful in
y fusion cag erland); (B) ryker Spine
ombined wit
n the treatm n provides s
compressio d with cert rological ris .21), which pedicle scr Coflex-F sp
th posterior
ment of de stabilization n. Howeve tain risks. T sk and impl
has been c rew fixation pacer is an . The rivets attachment t
nCage-Open ryker Spine,
New Jersey
pedicle scre
eformity as n of the spin er, the use
Typical com lant failures claimed to p n, was adopt interspinou s joining the
to the poste
n (Synthes S , Mahwah, N ted to interb us process d e wings of on of neural e screws is of pedicle
bilization of body fusion device with the Coflex nt. It retains y,
the a ing tissue,
ions, and op
Therefore, ex-F device ery. In addi na bone an ant in the in All types tional pedic be used for
used with t mmetric con ness. There ease stability wever, the Tr crossing scr e and pedic ition, Beca nd spinous nterspinous
of fusion cle screw fi
unilateral o the TLIF su nstruct will efore, supp
y of TLIF c ranslaminar ews before 9].
inous proce pedicle an me, thus spe
Figure 2
d subject w cle screw f
use of the process to process.
surgery a fixation to i
or bilateral p urgery to p
result in s lementation combined w r facet screw
they can tr
28
ess implant natomy, mi eding patien
2.21: Coflex
was to inve fixation, in PLIF surgi w fixation r raverse the
ts and mini inimizing m nt recovery.
x-F device [7
estigate the combinatio ical process ach which
are recom abilization a
ew fixation.
bility in min ment destabi
nslaminar f ral pedicle requires lon
facet joint,
imally inva muscle trau
.
75].
e biomecha on with AL s needed to
may make
mmended f and fusion r
Unilateral nimally inv lization and facet screw
screw fixati ng passage t and necess
asive surger uma, blood
anical behav LIF or TLIF o remove p e the Cofle
for combin rates. A ped
pedicle scr vasive surge
d a decrea w is recom ion (Figure through the sitating a lar
ry, such as d loss, skin
vior of the F in fusion parts of the ex-F cannot
nation with dicle screw rew fixation ery, but the se in spine mmended to 2.22) [76].
e lamina for rge surgical
surge The third s Coflex-F slaminar fac bar fusion.
igure 2.22:
x-F has be er, the effec subject was and suppl cet screw f
unilateral p
een adopted tiveness of to investiga lemented w fixation imp
pedicle screw
d to combi this proced ate the biom with one u
planted into
w fixation a
ne interbod dure is still u mechanical unilateral p o the L3-L4
and translam
dy fusion i unclear.
characterist pedicle scre
4 segment i
minar facet s
in minimal
tics of TLIF ew fixation
30
Chapter 3 Materials and Methods
3.1 Coflex and Coflex-F in non-fusion surgery
The first subject of following sections includes FE modeling and simulation technique of this study. Five FE models of the lumbar spine were constructed for this study. The first model was the intact lumbar spine. The other four models were the defect lumbar spine, the defect lumbar spine combined with Coflex, defect lumbar spine combined with Coflex-F, and defect lumbar spine combined with pedicle screw fixation.
3.1.1 FE model of intact lumbar spine (Intact model)
To create a three-dimensional FE model, computed tomography scan DICOM files of the L1 to L5 lumbar spine of a middle-aged male were obtained at 1-mm intervals. The commercially available visualization software Amira 3.1.1 (Mercury Computer Systems, Inc., Berlin, Germany) was used to describe cross-section contours of each spinal component in accordance with gray scale value (Figure 3.1). Then, the three-dimensional surface geometries were constructed through sequential processed cross-section contours as shown in Figure 3.2 A. Each spinal component was exported as a Drawing eXchange Format (DXF) file and converted to the Initial Graphics Exchange Specification (IGES) file as shown in Figure 3.2 B. The FE analysis software ANSYS 9.0 (ANSYS Inc., Canonsburg, PA) was used to reconstruct the FE model by converting the IGES file to ANSYS Parametric Design Language (APDL) code in Figure 3.2 C. The INT model was an osseo-ligamentous lumbar spine, which included the vertebrae, intervertebral discs, endplates, posterior bony elements, and all seven ligaments (Figure 3.3 A).
An eight-node solid element (SOLID185) was used for modeling the cortical bone, cancellous bone, posterior bony element, cartilage endplate, and annulus ground substance.
The cortical bone and cancellous bone were assumed to be homogeneous and transversely
isotropic [81]. The posterior bony element and cartilage endplate were assumed to be homogeneous and isotropic [82]. The intervertebral disc consisted of annulus ground substance, nucleus pulposus and collagen fibers embedded in the ground substance. The nonlinear annulus ground substance was simulated by using a hyper-elastic Mooney-Rivlin formulation [83][84]. The collagen fibers simply connected between nodes on adjacent endplates to create an irregular criss-cross configuration. These irregular angles of collagen fibers were oriented within the range of the Marchand’s [85] study. In the radial direction, twelve double cross-linked fiber layers were defined to decrease elastic strength proportionally from the outermost layer to the innermost. Therefore, the collagen fibers in different annulus layers were weighted (elastic modulus at the outermost layers 1-3: 1.0, layers 4-6: 0.9, layers 7-9: 0.75, and at the innermost layers 10-12: 0.65; cross sectional areas at the outermost layers 1-3: 1.0, layers 4-6: 0.78, layers 7-9: 0.62, and at the innermost layers 10-12: 0.47) based on previous studies [86][87]. The nucleus pulposus was modeled as an incompressible fluid with a bulk modulus of 1666.7 MPa by eight-node fluid elements (FLUID80) [81]. The 43 % of the cross-sectional area in the disc was defined as the nucleus, which was within the range of the study by Panagiotacopulos (30-50 %) [88] Therefore, approximately 47 % to 49 % disc volume was assigned to nucleus pulposus. All seven ligaments and collagen fibers were simulated by two-node bilinear link elements (LINK10) with uniaxial tension resistance only, which were arranged in an anatomically correct direction [89]. The cross-sectional area of each ligament was obtained from previous studies [82][87][90][91], and material properties of the spine are listed in Table 3.1. The facet joint was treated as having sliding contact behavior using three-dimensional eight-node surface-to-surface contact elements (CONTA174), which may slide between three-dimensional target elements (TARGE170). The coefficient of friction was set at 0.1[92].
The initial gap between a pair of facet surfaces was kept within 0.5 mm as shown in Figure 3.3 (b) [81]. The stiffness of the spinal structure changes depending on the contact status, so