Spinal fixation plate

A bone fixation plate assembly (10, 210, 210') for fixation of the spine includes a mechanism (26) for allowing movement between adjacent vertebrae (18) while maintaining engagement of the bone fixation plate (12, 212, 212') with the vertebrae (18).

TECHNICAL FIELD 
The present invention relates to a bone fixation plate for anterior 
fixation of the spine. More particularly, the present invention prevents 
axial displacement of adjacent vertebrae while allowing the adjacent 
vertebrae to move relative to each other. 
BACKGROUND OF THE INVENTION 
Spinal fixation has become a common approach in treating spinal disorders, 
fractures, and for fusion of vertebrae. A common device used for spinal 
fixation is a bone fixation plate. A typical bone fixation plate includes 
a relatively flat, rectangular plate having a plurality of apertures 
therethrough. A corresponding plurality of fasteners, i.e., bone screws, 
are provided to secure the bone fixation plate to a bone, such as two 
adjacent spinal vertebrae. The screws are firmly tightened to secure the 
bone fixation plate to the bone or bones to be fixed. Typical examples of 
such bone fixation plates are illustrated in U.S. Pat. No. 5,364,399, 
issued Nov. 15, 1994, to Lowery et al. and U.S. Pat. No. 5,601,533, issued 
Feb. 11, 1997, to Trebing et al. These patents are cited as exemplary of 
the general state of the art with regard to bone fixation plate 
technology. In general, these types of devices can be utilized for 
anterior fixation of the spine for cervical, lumbar, and/or thoracic 
fixation. 
The basis of anterior fixation or plating is to approach the spine from 
anterior or anterio-lateral approach and use the screws to solidly mount 
the bone fixation plate to the affected vertebrae. This approach is 
commonly used in cases of trauma, tumors, and degenerative conditions. 
Often, in addition to the application of a bone fixation plate, graft 
material may be combined in attempt to permanently fuse together adjacent 
vertebrae. The graft material can consist of bone grafts obtained from 
other bones in the patients body or from cadaver bones. 
A common problem associated with the use of such bone fixation plates is 
the tendency of the bone screws to "back out" or pull away from the bone 
into which they were fixed. This problem occurs, primarily, due to the 
normal motion of the body and spine. The spine is a very dynamic entity 
and is constantly moving. This problem is especially prevalent in areas of 
high stress such as the spine. This is a particularly important problem 
because as the screw becomes loose and pulls away from the bone, the head 
of the screw can rise above the surface of the bone fixation plate and, 
possibly, even work its way completely out of the bone. This condition can 
create a number of potentially serious problems given the number and 
proximity of blood vessels and other critical structures near the 
locations of anterior spinal plate fixation. 
A number of various designs have been brought forth in attempts to prevent 
screws from pulling away from the bone and/or to prevent the screws from 
backing out or pulling away from the surface of the bone fixation plate. 
For example, the Lowery et al. patent, discussed above, discloses an 
anterior cervical plating system incorporating a locking screw which 
engages the heads of the bone screws used to secure the cervical plate to 
the vertebrae. The locking screw is positioned above the bone screws and 
is driven against the heads of the bone screws to and rigidly fix the bone 
screws to the plate. However, for this locking mechanism to work, the 
distance between the heads of the bone screws must be kept to a minimum, 
thereby limiting the potential applications of the bone fixation plate. 
Additionally, while the Lowery et al. patent allows for the bone screws to 
be angled, if the screws are not angled exactly the same amount, which is 
very difficult to achieve, the locking screw cannot adequately contact 
both bone screw heads. 
Another example of a mechanism for preventing bone fixation screws from 
backing out or becoming dislodged from the bone is set forth in the 
Trebing et al. patent discussed above. The Trebing et al. patent discloses 
a mechanism whereby the bone fixation plate is threaded and is used in 
combination with a bone screw having both bone engaging threads and a 
threaded portion near the head of the bone screw which is complimentary to 
the threaded hole in the bone fixation plate. In this mechanism, the screw 
is rigidly fixed to the bone fixation plate. However, it is possible to 
lock the bone screw to the bone fixation plate while leaving a gap between 
the bone fixation plate and the bone. This problem can cause inferior 
fixation of the bone or even total failure of the fixation. 
Various other mechanisms used to prevent bone screws from pulling out of 
bones include cams which engage and lock the screws and the use of 
expanding head screws which expand outwardly when adequate force is 
applied thereto to engage the holes in the bone fixation plate. All of 
these particular designs have drawbacks including potential for breakage 
or requiring particular precision and alignment in their application in 
order to correctly work. 
Yet another apparatus for preventing bone screw back-out from a bone 
fixation plate is shown in U.S. Pat. No. 5,578,034, issued Nov. 26, 1996, 
to Estes. The Estes patent discloses a system for internal fixation of 
bone which includes a bone fixation plate having a number of bores 
therethrough, a corresponding number of screws each having an enlarged 
head portion and an elongated shaft portion defining bone engaging threads 
thereabout and a non-threaded portion between the head and the threaded 
portions, and a corresponding number of screw anti-backout members each 
having a bore therethrough. The screw anti-backout members are inserts 
positioned within the bores of the fixation plate and are initially sized 
to slidingly receive an elongated screw shaft therethrough. During 
application of the fixation plate, the bone screws are advanced through 
the bone fixation plate bores and the screw anti-backout members which are 
positioned within the plate bores to screw the bone fixation plate to the 
underlying bone. Thereafter, the apparatus is sufficiently heated to 
shrink the bores of the screw anti-backout members, thereby trapping the 
non-threaded portion of the screw shafts located between the fixation 
plate and the threaded portions. The anti-backout collars are immobilized 
within the bore of the fixation plate. Thus, the collar and fixation plate 
remain in fixed relationship to each other after fixation to the 
underlying bone. 
Other types of inserts or collars have been used with bone fixation plates 
for a variety of reasons such as those shown in U.S. Pat. No. 4,388,921, 
issued Jun. 21, 1983, to Sutter et al. and U.S. Pat. No. 5,607,428, issued 
Mar. 4, 1997, to Lin. Sutter et al. discloses a bone fixation plate in 
which sleeves are placed in openings provided in a bone fixation plate. A 
screw is placed through the sleeve and into the underlying bone. By 
tightening the screw, the sleeve is clamped in place with relation to the 
bone fixation plate thus assuring that the fixation plate will stay 
rigidly connected with the screws. 
The Lin patent discloses a bone fixation plate having a direction adjusting 
ring disposed in at least one hole in the fixation plate. Upon insertion 
and tightening of the threaded bone screw, arresting edges of the 
direction adjusting ring are urged into engagement within the hole to 
securely fix and retain the direction adjusting ring therein. 
Both the Sutter et al. and Lin patents disclose, similar to those patents 
described above, an insert that is rigidly disposed or fixed in a hole or 
aperture in the fixation plate. They do not allow for movement of the 
insert with relation to the bone fixation plate. 
Therefore, it would be desirable to provide a bone fixation assembly which 
would allow positive, rigid fixation of a bone fixation plate to a bone, 
such as adjacent vertebrae, while allowing movement, stress, or dynamic 
load sharing of the adjacent vertebrae thereby enhancing the bone 
rebuilding process and enhancing the success of a bone graft. 
Additionally, it would be desirable to have a bone fixation plate assembly 
which would prevent the application of high stress on the screws and plate 
which would lead to failure of the assembly, including the actual breakage 
of the screws and closure of the previously formed gap between the 
adjacent vertebrae. Additionally, it would also be desirable to provide a 
bone fixation plate assembly that allows the fixation screws to be locked 
to the plate to prevent the screws from backing out of the plate while 
allowing the plate to be firmly seated against the underlying bone. 
Further, it would desirable to provide a bone fixation plate assembly 
which requires no small parts nor requires no additional steps to lock the 
screws to the bone fixation plate. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a bone fixation plate assembly 
for fixation of the spine includes a mechanism for allowing movement 
between adjacent vertebrae while maintaining engagement of the bone 
fixation plate with the vertebrae.

DETAILED DESCRIPTION OF THE INVENTION 
A bone fixation plate assembly constructed in accordance with the present 
invention is generally shown at 10 in FIG. 1 and in an exploded view in 
FIG. 2. 
More generally, referring additionally to FIG. 4, the bone fixation plate 
assembly 10 includes a substantially flat elongated bone fixation plate 12 
having a longitudinal axis (A) defined by a first end 14 and a second end 
16. The length of the elongated bone fixation plate along the axis (A) 
should be sufficient enough to span between adjacent vertebrae 18. The 
elongated bone fixation plate 12 also includes an upper surface 20 and a 
lower surface 22. the lower surface 22 is adapted to engage the vertebrae 
18. 
Apertures 24 can be disposed at each end 14,16 of the bone fixation plate 
12. A mechanism 26 for allowing movement between adjacent vertebrae 18 
while maintaining engagement of the bone fixation plate 12 with the 
vertebrae 18 is disposed in the bone fixation plate assembly 10. 
More specifically, referring to FIGS. 2 and 4, the mechanism 26 for 
allowing movement between the adjacent vertebrae 18 includes an insert 28 
disposed in at least one of the apertures 24 for receiving an retaining a 
screw 40 therein. The insert 28 is rotatably fixed within the aperture 24 
and can freely rotate therein. When the screw is inserted and tightened 
within the insert 28, both the screw 40 and the insert 28 can rotate 
axially about the longitudinal axis of the screw 40 while simultaneously 
preventing axial movement of the screw 40 with relation to the bone 
fixation plate 12. 
Additionally, the insert 28 can be fixed within the aperture 24 with a 
degree of side to side play between the insert 28 and the bone fixation 
plate 12 such that not only can the insert 28 rotate freely within the 
aperture 28, the insert 28 can move from side to side or wobble within the 
aperture 24. By wobbling or rocking within the aperture 24, the insert 28 
allows for longitudinal movement of the underlying bones with respect to 
the screws 40 to be tolerated and absorbed by the bone fixation assembly 
10. That is great movement and shifting of the underlying bones to which 
the assembly 10 is affixed to can be acceptably tolerated without failure 
of the assembly 10 or damage to surrounding tissues or structures. 
The ability of the insert 28 to rotate and/or wobble within the bone 
fixation plate is important as it is well understood according to Wolff s 
Law that bones grow along lines of stress. Therefore, in order for fusion 
to occur between two or more vertebral bodies, a graft placed between the 
vertebral bodies must be loaded or stressed for solid bone fusion to 
occur. The biology of bones makes the fusion process very dynamic. Graft 
material slowly resorbs as osteoclasts remove bone and osteoblasts replace 
bone with new living bone. With a rigid plate construct, where the screws 
hold the vertebral bodies at a fixed apart distance, any resorption of the 
bone graft reduces or eliminates the compression or stress forces on the 
graft. Accordingly, these rigid plate constructs can induce or cause a 
condition known as psuedoarthrosis. Therefore, in accordance with the 
present invention, it is important to maintain the graft under load and/or 
stress while allowing the plate to compensate for the bone dynamics. This 
can be accomplished utilizing the mechanism 26 as described herein. 
The insert 28 is mounted within the aperture 24. Preferably, the aperture 
24 is a circular bore. The bore can include at least one internal annular 
recess 13 disposed in either the upper surface 20, the lower surface 22, 
or both surfaces of the bone fixation plate 12 and a passageway 15 
therebetween defined by the bore of the aperture 24. The, insert 28 is in 
mating engagement with the internal annular recess or recesses 13 to lock 
the insert 28 within the bone fixation plate 12. 
The insert 28 includes a locking mechanism 42 for locking the screw 40 
within the bone fixation plate 12. Preferably, the locking mechanism 42 
includes threads disposed within an axial bore 33 which are adapted engage 
and retain the screw 40 therein. Referring to FIG. 2, the insert includes 
an annular recess 30 defined at one end by a circumferential threaded 
screw receiving flange 32 and at the other end by a frustoconical or 
wedge-shaped insert flange 34. The axial bore 33 extends from the 
circumferential threaded screw receiving flange 32 to the end of the 
frustoconical insert flange 34. The annular recess 30 and the 
frustoconical insert portion 34 can be biased outwardly and can include a 
transverse slot 36 disposed therein which bisects both the annular recess 
30 and the insert flange 34. 
During insertion of the insert 28 within the aperture 24, both the 
frustoconical insert portion 34 and the annular recess 30 are compressed 
as the frustoconical insert portion 34 engages the walls of the annular 
internal recess 13 of the bone fixation plate 12. That is, the 
frustoconical shape of the insert portion 34 acts as a ramping surface 
which, when in contact with the annular internal recess 13, gradually 
compresses the insert portion 34 and the annular recess 30 by compressing 
or closing the transverse slot 36. When the insert portion 34 and annular 
recess 30 are compressed, they are able to pass through the channel 15 of 
the aperture 24 until they reach the internal recess disposed in the lower 
surface 22 of the bone fixation plate or side opposite which the insert 28 
was inserted. Once the frustoconical insert portion 34 exits from the 
channel 15 of the aperture 24, the outward biasing force causes the 
frustoconical insert portion 34 to expand and engage the walls of the 
internal recess 13 to lock the insert 28 within the bone fixation plate 
12. Since the diameter of the uncompressed frustoconical insert portion 34 
is larger than the diameter of the bore of the aperture 24, the 
frustoconical or wedge shaped portion of the insert portion 34 prevents 
the withdrawal of the insert 28 from the aperture 24. Thus, the insert 28 
is locked into the bone fixation plate 12. 
The diameter of the annular recess 30 can also be smaller than the diameter 
of the bore or aperture 24 so that the insert 28 is free to rotate within 
the aperture 24. 
Alternatively, an insert 28' can be constructed in multiple parts as shown 
in FIG. 3. The insert 28' can include an axial bore 33' having threads 42' 
disposed therein. The insert 28' also can include a circumferential 
threaded screw receiving portion 32' and an annular recess 30'. A 
retaining member 29 which includes structure for engaging a portion of the 
annular recess 30' can be provided to retain the insert 28' within the 
recess 13 of the bone fixation plate 12. The retaining member 29 can be 
affixed to the annular recess 30' by means such as compression fitting, 
threaded engagement, or by other means well known to those skilled in the 
art. 
Referring to FIG. 4, a bone fixation plate assembly 10 is shown affixed to 
two adjacent vertebrae 18. The bone fixation plate 12 is mounted to the 
adjacent vertebrae 18 by way of a number of bone engaging screws 40. The 
screw 40 is in threaded engagement with the insert 28 as is shown in 
phantom at 42. 
Numerous embodiments of the invention are contemplated. For example, 
referring to FIGS. 5 and 6, a bone fixation assembly designated as 10 is 
shown. In this embodiment, the bone fixation plate 12 is substantially 
flat and includes two apertures 24 disposed at the end 14,16 of the bone 
fixation plate 12 and also includes at least one additional aperture to 
24' disposed between the ends 14,16 of the bone fixation plate 12. In this 
embodiment, the aperture 24' disposed between the ends 14,16 of the bone 
fixation plate 12 is equidistantly disposed between the ends 14,16 of the 
bone fixation plate 12 along the longitudinal axis (A). However, the 
aperture 24' disposed between the ends 14,16 of the bone fixation plate 12 
can be disposed at any point in between the ends 14,16 of the bone 
fixation plate 12. Additionally, the additional aperture 24' can be 
disposed adjacent to the longitudinal axis (A). The placement of the 
additional apertures 24' depends upon the application to which the 
assembly 10 is to be applied. 
Referring to FIGS. 7 and 8, an additional embodiment of a substantially 
flat bone fixation plate assembly 10 is provided wherein five apertures 
24, 24' are utilized. In this embodiment, two apertures 24 are disposed at 
each end 14,16 of the bone fixation plate 12. An additional aperture 24' 
is equidistantly disposed between the ends 14,16 and along the 
longitudinal access (A) of the bone fixation plate 12. 
As shown in FIGS. 9 and 10, a further embodiment of the assembly is shown. 
This embodiment is similar to the embodiment shown in FIGS. 7 and 8, 
however, the bone fixation plate assembly is arcuately shaped or curved 
transverse to the longitudinal access (A) to conform the bone fixation 
plate 212' to the curvature of the vertebrae. Utilization of a curved or 
formed fixation plate 12', 212' can be incorporated into any embodiment of 
the bone fixation assembly in accordance with the present invention and is 
not limited by the specific embodiments disclosed herein which are 
provided as examples of the possible designs which can be utilized with 
the present invention. 
Referring to FIGS. 11 and 12, a further embodiment of the bone fixation 
plate assembly of the present invention is shown at 210'. This embodiment 
includes a curved, elongated, a substantially bone fixation plate 212' 
including rectangular slot 44 having a recess 45 disposed therein. The 
elongated slot 44 is disposed substantially parallel to the longitudinal 
axis (A) of the bone fixation plate 212". 
The substantially rectangular slot 44 allows the insert 28 to be adjustably 
positioned at various locations within the slot 44. By being adjustable, 
the assembly 210' is able to be utilized in a greater number of locations 
and applications. Additionally, the insert 28 disposed in the slot 44 can 
be allowed to slide freely within the slot 44 to accommodate movement of 
the underlying bone or bones. 
Referring to FIGS. 13 and 14, a further embodiment 210 is shown having as 
an elongated slot 44'. The elongated slot 44' is disposed at a 
substantially acute angle with respect to the longitudinal axis (A) of the 
substantially flat bone fixation plate 212. 
In a further embodiment shown in FIG. 15, an elongated slot 44" is shown 
disposed at one end 16 of the bone fixation plate 212 and parallel to the 
longitudinal axis (A). 
In all of the embodiments shown in FIGS. 11 through 15, the elongated slot 
44, 44', 44" accepts at least one insert 28, 28' therein. The inserts 28, 
28' can be affixed within the recess 45 in a manner similar to that 
discussed above. The elongated slot 44, 44', 44" allows for a substantial 
margin for adjustment and proper location of the screw 40 and the bone 
fixation plate 12, 212, 212' and, as discussed above, the inserts 28, 28' 
can be made to slide freely within the slot 44, 44', 44". 
The spacing or orientation of the apertures 24, 24', the slots 44, 44', 
44", and the inserts 28, 28' within the bone fixation plate assembly 10, 
210, 210' can be designed and selected so as to achieve a desired dynamic 
load sharing arrangement between the screws 40 disposed in the various 
combinations of apertures 24, 24' and slots 44, 44', 44" described above. 
That is, the bone fixation plate assembly 10, 210, 210' can be tailored to 
a specific application such that the load carried by each screw 40 can be 
distributed in a desired manner, including load shifting after the 
assembly 10, 210, 210' has been affixed to the underlying bone. Thus, the 
assembly 10, 210, 210' can accommodate the dynamic environment into which 
it is utilized without incurring the drawbacks of the prior art devices 
described above. 
Referring to FIG. 6, the bone fixation assembly 10 is shown including a 
bone engagement mechanism 46 disposed on the lower surface 22 of the bone 
fixation plate 12 for enhancing the engagement of the bone fixation plate 
12 to the vertebrae. Preferably, the bone engagement mechanism 46 includes 
a texturized surface disposed on substantially all of the lower surface 22 
of the bone fixation plate 12. In other embodiments, the texturized 
surface can be disposed on only a portion of the lower surface 22 as is 
shown in FIG. 6. 
The texturized surface can include etched, roughened, grooved, knurled, or 
like contouring of the lower surface 22 of the bone fixation plate 12 as 
is well known in the art. 
Referring to FIG. 16, the screw receiving portion 32 of the insert 28 is 
shown in greater detail. At least two rectangular recesses 38 are disposed 
therein. The recesses 38 are adapted to receive a device, such as a 
screwdriver or similar tool, to prevent rotation of the insert 28 within 
the aperture 24 during the insertion and tightening of the screw 40 
disposed therein to the underlying bone. The location and configuration of 
the recesses 38 within the screw receiving portion 32 of the insert 28 can 
be varied according to a desired or specific tool to be utilized for the 
purpose of preventing rotation of the insert means 28. In operation, the 
tool or device would be placed in mating engagement with the recesses 38 
thereby providing leverage to withstand the rotational or torque forces 
applied to the insert 28 via the threads 42 disposed in the bore 33 of the 
insert 28 as transmitted thereto by the screw 40. 
The screws 40 utilized for securing the bone fixation plate assembly 10, 
210, 210' to underlying bone are generally bone screws of the type well 
known in the art. Preferably, the screws 40 utilized in the present 
invention are of the type illustrated in FIG. 17. The screw 40 typically 
comprises a screw with a shaft having a head 50 and a tip 52. The shaft 48 
is threaded and includes two differently threaded portions 54, 56. Portion 
54 is threaded to threadingly engage with the threads 42 of the insert 28. 
The threaded portion 56 is threaded to specifically engage bone. An 
example of a screw which can be used with the bone fixation plate assembly 
10, 210, 210' of the present invention is shown in U.S. Pat. No. 5,601,553 
to Trebing et al., and is incorporated herein by, reference. 
Alternatively, the screw threads 54, 56 can utilize the same thread for 
both portions. 
In an alternative mechanism for fixation of the bone fixation plate 
assembly 10 to the vertebrae 18 as shown in FIG. 18, a headless screw 40 
can first be driven into and affixed within the bone. The bone fixation 
plate 12, 212, 212' of the bone fixation assembly 10, 210, 210' having an 
insert 28, 28' disposed therein would then be disposed over the portion of 
the screw 40 protruding from the bone and would be fixed to the bone 
fixation plate 12, 212, 212' and insert 28 by threadingly engaging the 
threaded screw receiving portion 32 of the insert 28, 28' with the threads 
of the portion of the screw 40 protruding from the bone. 
Referring to FIGS. 19 and 20, an alternative embodiment for both an 
alternative insert 70 and aperture 60 is shown. The aperture 60 which can 
be disposed in any plate 12, 212, 212' includes, in cross-section, 
substantially arcuate ends 62 and substantially straight sides 64 which 
can be machined into the bone fixation plate 12, 212, 212'. The ends 62 
are substantially hemispherical and matingly engage within the aperture 
60. Into the aperture 60, an insert 70 including substantially 
hemispherical portions 72 disposed at the ends of the insert 70 and 
substantially straight side portions 75 can be disposed. An axial threaded 
bore 76 in the insert 70 is provided to accept bone screws 40 therein. A 
transverse slot 78 is provided in the insert 70 to allow the insert 70 to 
be contracted or to be compressed to facilitate its insertion within the 
aperture 60. 
Referring specifically to FIG. 20, the biasing mechanism of the insert 70 
is shown in greater detail. The transverse slot 78 extends through the 
insert 70 substantially bisecting the insert 70. A portion 80 of the 
insert 70 defines a terminal margin of the slot 78. This portion 80 of the 
insert 70 allows the insert 70 to be compressed for insertion into the 
aperture 60. That is, the portion 80 acts as a hinge or spring to allow 
the insert 70 to compress and to expand. The insert 70 can be biased 
outwardly as described above for the insert 28. After insert 70 is 
disposed within the aperture 60, the insert 70 freely rotates within the 
bore 76 and also, the insert 70 can be moved or positioned to any angle 
such that the bore 76 is angled with respect to the axis (A) of the plate 
12, 212, 212'. The ability of the insert 70 to be angled with respect to 
the longitudinal axis (A) of the plate 12, 212, 212' allows for the screws 
40 to be inserted at angles with respect to the longitudinal axis (A) of 
the plate 12, 212, 212'. 
The advantage of the insert 70 being able to rotate in more than one plane 
is that it allows a surgeon to place the screw at any angle in necessary 
order to adapt the bone fixation assembly 10, 210, 210' for varying 
anatomy. Therefore, a surgeon could place the screws such that they are 
convergent or divergent and facing in the caudal or cephalad directions. 
Thus, in cases of severe deformity, the multi-angle positionable insert 70 
would allow easier correction of the deformity while still allowing for 
the utilization of the low profile plate 12, 212, 212'. 
The bone fixation plate assembly 10, 210, 210' of the present invention can 
be constructed of any suitable material. Preferably, the assembly 10, 210, 
210' and other components, such as the inserts 28, 28', and 70, are 
constructed of suitable materials which are compatible with the uses and 
environments into which the assembly 10, 210, 210' will be utilized. 
Preferably, the assembly 10, 210, 210' is constructed of metallic 
materials such a titanium, stainless steel, or other metal alloys. The 
material which the assembly 10, 210, 210' is constructed of should have 
strength and be non-reactive and non-antigenic to biological systems. 
Throughout this application, various publication are referenced by citation 
and number. Full citations for the publication are listed below. the 
disclosure of these publications in their entireties are hereby 
incorporated by reference into this application in order to more fully 
describe the state of the art to which this invention pertains. 
The invention has been described in an illustrative manner, and it is to be 
understood the terminology used is intended to be in the nature of the 
description rather than of limitation. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. Therefore, it is to be 
understood that within the scope of the appended claims, reference 
numerals are numerals are merely for convenience and are not to be in any 
way limiting, the invention may be practiced otherwise than as 
specifically described.