Extrusion Resistance of an Injectable Nucleus Replacement in the Human Cadaver Spine
Jared Walkenhorst, M.S.1, Dennis Lee, M.S.1, David Spenciner, Sc.M.2
1
Spine Wave, Inc. Shelton, CT, 2RIH Orthopaedic Foundation, Providence, RI
INTRODUCTION: The NuCore Injectable Disc Nucleus (IDN) is an in-situ curing
hydrogel intended as an adjunct to discectomy. The IDN replaces excised nuclear tissue
and is injected through an annular deficit created during a nucleotomy. Because it
interdigitates with the surrounding tissue, it is important to understand how well it is
constrained by the surrounding tissue and whether it will remain in place even under
high physiologic loading.
5,000
OBJECTIVE: The purpose of this study was to evaluate how well the IDN would
resist extrusion from the disc space by testing human cadaveric anterior column units
(ACU: a functional spinal unit with posterior elements removed) in compression after
injection with IDN. ACUs were tested in the neutral posture. In addition, because some
literature indicates that disc herniation is more likely in extreme bending, ACUs were
also tested in flexion.
4,500
4,000
Failure Load (N)
3,500
Figure 1: Hyperflexed Extrusion Set-Up
3,000
2,500
2,000
1,500
Compression Flexion Test of 706-T11/T12 with posterior-lateral partial
discectomy (8/12/03). Stiffness = 735.0 N/mm. Yielding = 2411.8 N at 5.96
mm. Ultimate Load = 2891.6 N at 7.78 mm. No observed extrusion.
1,000
500
3,500
0
#1
3,000
Results of the hyperflexion testing are shown in Table 2 and Figure 5. In the flexed
compression testing of lumbar ACUs, the average load to failure was 2,637 N. The load
to failure was lower than neutral testing because the flexion caused narrowing of the
anterior disc space resulting in an earlier collapse of the anterior rim. In all eleven cases,
failure of the ACU was by endplate fracture, failure of the anterior rim, or cancellous
bone failure. No samples failed by extrusion of IDN. In three cases, small volumes
(<0.1 mL) of IDN was expulsed through the annulotomy at or after failure of the bone.
It is believed the IDN extrusion was caused by disc space collapse brought on by bony
failure. In addition, in one case, natural nucleus was expelled through the annulotomy
during the test, while IDN remained in place.
REFERENCES: 1. Nachemson et al., Acta Orthop Scand, 1963. 2. Nachemson et al., J
Bone Joint Surg 1964. 3. Wilke et al., Spine 1999.
#4
#5
#6
#7
#8
*#9
#10
Figure 4: Extrusion in Neutral Posture
Stiffness Calculated from
Linear Regression
Initial Yielding
1,500
Toe Region Due
to Fixture
Rotating into 12
deg Flexion
1,000
5,000
4,500
4,000
500
3,500
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
3,000
Figure 3: Sectioned Test Specimen.
IDN is intact within the disc space
Load (N)
0.0
Figure 2: Typical Force-Displacement Curve
In the neutral compression testing of lumbar ACUs, the average load to failure was
3,555 N. Results are presented in Table 1 and Figure 4. In all ten cases, failure was by
endplate fracture or cancellous bone failure. No samples failed by extrusion of IDN.
#3
2,000
Displacement (mm)
RESULTS: Each specimen was sectioned after testing to verify presence of the IDN
and observe its state. Each section was photographed. As shown in Figure 3, the IDN
remained intact and in place up to and after bony failure. In some cases of endplate
failure, IDN migrated into the cancellous bed of the vertebral body, but did not extrude
through the annulotomy.
#2
* IDN extrusion observed well after endplate failure.
2,500
Load (N)
METHODS: Each ACU was given a partial nucleotomy (≈1.5mL) through a standard
posterior 5 x 5 mm cruciate annulotomy. Each specimen received an injection of IDN
such that the material filled the cavity to the inside of the annulotomy. After curing of
the IDN, each ACU was subjected to compression testing in a servohydraulic test frame
to failure. Ten ACUs were tested in neutral posture, eleven were tested in flexion with
the angle oriented in the plane of the annulotomy as shown in Figure 1. The maximum
load and failure mode were observed and recorded from Force Displacement Curves as
shown in Figure 2. After testing, each specimen was sectioned and examined.
Ultimate Load
Linear Regression
2,500
2,000
1,500
1,000
500
Table 1: Extrusion Test Results for Neutral Set-Up
Sample
Extrusion
Load (N)
Failure
Load (N)
634 L2/L3
NONE
3608.8
634 L5/S1
NONE
4701
Failure Type
Cancellous
Failure
Cancellous
Failure
635 L1/L2
NONE
2897.6
Endplate Failure
637 L1/L2
NONE
3279.5
637 L3/L4
NONE
4129.1
636 L1/L2
NONE
634 L3/L4
75 L3/L4
Table 2: Extrusion Test Results for Hyperflexion Set-Up
Sample
Failure Load
(N)
673 L3/L4
NONE
2093
Failure Type
Central Endplate
Failure
688 L1/L2
2751 a
NA
Anterior Rim Failure
2040
NP Extrusion
688 T11/T12
(2040)
Endplate Failure
673 L1/L2
NONE
3093
Cancellous Failure
Endplate Failure
709 T12/L1
NONE
2769
Anterior Rim Failure
3717.2
Endplate Failure
706 T11/T12
NONE
2892
Cancellous Failure
NONE
3976.5
NONE
2826
Cancellous Failure
3096.4
Endplate Failure
Cancellous
Failure
706 L2/L3
NONE
706 L3/L4
2611
2651
3154
Endplate Failure
707 L2/L3
NONE
1544
Cancellous Failure
Central Endplate
Failure
2992.2
Endplate Failure
707 L4/L5
NONE
2139
Cancellous Failure
a
75 L1/L2
NONE
532 L1/L2
NONE
Average
N/A
3555.2
S.D.
N/A
581.7
Note a:
IDN extrusion through annulotomy
observed after 8.5-mm
of displacement @ 3100 N.
N/A
N/A
708 L2/L3
Average
S.D.
Note a:
Note b:
NONE
4320
3227
2637
N/A
764
Extrusion of IDN after yielding
Extrusion of Natural Nucleus
#1
*
#2
#3
#4
#5
#6
#7
*
#8
#9
#10
#11
* Extrusion of IDN after yielding of ACU
** Extrusion one IDN particle coincident with ultimate failure
Extrusion
Load (N)
b
0
Anterior Rim Failure
N/A
N/A
Figure 5: Extrusion in Hyperflexion Posture
DISCUSSION: Literature has predicted the loads through the disc during
daily living to be approximately 1200 N.[1][2][3] All tests exceeded this
load prior to any failure. The three cases of small IDN extrusion
occurred well after 1200 N was reached. To understand what happens
after bony failure, some tests were continued after failure was observed.
In those tests, the IDN migrated through the fractured endplate and into
the cancellous bed. When the disc was sectioned, much of the IDN was
no longer in the disc space. It is believed the IDN migration into the
vertebra was caused by disc space collapse brought on by bony failure.
CONCLUSIONS: The results of this study indicate the IDN was
successfully secured by the surrounding tissue within the disc space.
Extreme compressive loads did not cause extrusion of the IDN prior to
bony failure of the segment. These results indicate the IDN performs well
under high physiologic loads.