Flexible shaft and method for manufacturing same

A shaft coupling for transmitting power including a flexible hollow rod extending in a longitudinal direction and having pairs of helical slots formed thereon. The pairs of slots are spaced from each other along the longitudinal direction of the rod. The shaft is manufactured by providing a hollow rod and advancing the rod with a combined rotational and translational motion. An opening is machined across a diameter of the rod during its advancement to form a pair of intertwined helical slots.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a flexible shaft and method for 
manufacturing same. More specifically, it relates to a flexible shaft that 
has particular application as a coupling for transmitting power, for 
example, in bone drills, medullary reamers, flexible bone plug 
introducers, and other types of flex drivers. 
2. Prior Art 
Various types of flexible shafts are known for a variety of applications. 
In the medical field of orthopedics, medullary reamers are used to enlarge 
the medullary canals of bone in preparation for the insertion of fixation 
devices, performing an intramedullary osteotomy, stimulating bone growth, 
the insertion of a plug to preclude bone cement from migrating while it is 
in its viscous state, and for other reasons. The medullary canals of bone 
are seldom straight. More typically, the canal will have some degree of 
curvature to it. Should a straight and rigid series of reamers be employed 
to enlarge the canal, there is considerable likelihood that the reamer, in 
not being capable of following the bone's curvature, will not remove the 
desired uniform amount of bone tissue. In such a situation, excession 
tissue removal occurring in at least one plane will be experienced as the 
reamer is advanced. For this reason, medullary canals are almost always 
prepared with reamers having a flexible shaft. 
These flexible medullary reamers consist of spiral or helically wound metal 
wire(s) or strip(s), which comprise the shaft of the reamer. A 
disadvantage of this design is that these reamers can be operated only in 
the forward or clockwise mode of rotation. If operated in the reverse or 
counter-clockwise mode, which occasionally is required to free a lodged 
reamer and to facilitate normal removal, the shaft unwinds, thus rendering 
the reamer permanently deformed, unusable, and unrepairable. This adds 
considerably to the cost of maintaining a serviceable set of medullary 
reamers. Further, a lodged cutting head may subsequently be extremely 
difficult, if not impossible to remove without further violation of the 
involved bone and surrounding tissues. 
Another disadvantage of said design is the extreme difficulty in their 
proper and thorough cleaning after use. The spiral or helically wound 
metal shafts contain many voids of various sizes. Blood and tissue readily 
infiltrate such voids and become trapped within the confines of the shaft. 
When the reamer is in use, the voids are considerably distorted and 
enlarged as the reamer is advance towards and within the medullary canal, 
thus providing ready access for the particles of tissue. Prior to use, all 
medullary reamers are sterilized and hopefully, the blood and tissue 
particles not evacuated during the cleaning process and remaining within 
the interstices of the reamers, are at least rendered harmless. However, 
depending upon the amount and composition of the extraneous particles and 
their degree of isolation from the sterilizing process, said particles may 
not be rendered sterile. Even in a sterile condition, these foreign 
particles may still cause problems of infection should they become 
dislodged from the confines of the reamer and come into contact with the 
patient's internal tissues. Medical professionals recognize this problem 
but acquiesce to using these reamers for lack of an acceptable 
alternative. 
A further disadvantage of this medullary reamer is that the torsional load 
it is subject to when in use results in poor power transfer and varying 
degrees of distortion of said shaft. If the power source providing the 
rotational energy to the reamer is great enough, said coils may tighten 
sufficiently to adversely affect the intended flexibility of the shaft. 
Another disadvantage associated with a spiral or helically wound reamer is 
the trauma it imposes to surrounding tissues. This results when the shaft 
of the reamer is not completely within the medullary canal as would occur 
during the initial reaming process. As the shaft rotates, that portion 
remaining outside of the medullary canal can become excessively flexed and 
distorted, thus enlarging the voids between the coils of the shaft. As the 
flexed shaft rotates, tissue lying outside of the canal and unintended for 
removal, becomes trapped within the voids and are torn from their 
underlying structures. 
SUMMARY OF THE INVENTION 
In is therefore an object of the present invention to overcome the 
drawbacks of the prior art and to provide a flexible shaft which operates 
as an efficient coupling for transmitting power and which can be rotated 
in both directions. 
It is a further object of the present invention to provide a medullary 
reamer shaft which can be easily and effectively cleaned between cases. 
It is a further object of the present invention to provide a method for 
manufacturing a flexible shaft from a variety of materials, whereby the 
strength and flexibility of the shaft can be,, controlled during the 
manufacturing process. 
These and other related objects are achieved according to the invention by 
a shaft coupling for transmitting power, including a flexible hollow rod 
extending in a longitudinal direction and having pairs of helical slots 
formed thereon. The pairs of slots are spaced from each other along the 
longitudinal direction of the rod. Each pair of slots consists of a first 
helical slot intertwined with a second helical slot. Each slot has the 
same length extending along the longitudinal direction of the rod. Both 
slots extend along the longitudinal direction to the same extent, i.e., 
they begin and end at the same point along the longitudinal direction. In 
other words, both slots are completely disposed within a single region. 
In a specific application, the objects of the invention are achieved by a 
medullary rotational reamer for clearing, enlarging or otherwise modifying 
the medullary space of bones. The reamer includes a flexible shaft with a 
cutting head at one end and an adapter piece at its opposite end for 
connecting the shaft to a rotational drive element thereby causing 
rotation of the shaft. The flexible shaft consists of a hollow rod 
extending in a longitudinal direction and having a plurality of pairs of 
helical slots formed thereon. The pairs of slots are spaced from each 
other along the longitudinal direction of the rod. A plurality of 
continuous surfaces are also formed on the rod. Each surface extends 
circumferentially around the rod and is located between adjacent spaced 
pairs of slots to form a helical slot interruption. The shaft, the cutting 
head and the adapter piece are integrally formed and made from a material 
selected from the group consisting of stainless steel, titanium, chrome 
cobalt molybdenum alloy, a carbon fiber composite, or any suitable 
material. 
Each pair of slots includes a first slot having a first series of points 
and a second slot having a second corresponding series of points. The 
corresponding point from the first and second slots are disposed 
diametrically opposite each other on the hollow rod. 
The method of manufacturing a flexible shaft according to the invention 
includes the following steps. Initially, a hollow rod is provided that 
extends in a longitudinal direction. The hollow rod is rotated around its 
longitudinal central axis and moved along in the longitudinal direction. 
An opening is machined across a diameter of the rods during the step of 
rotating and moving to form a pair of intertwined helical slots. The rate 
of rotation of the hollow rod is adjustable with respect to movement along 
the longitudinal direction to control the pitch of the helical slots. The 
hollow rod is made from a material selected from the group consisting of 
stainless steel, titanium, chrome cobalt molybdenum alloy, a carbon fiber 
composite, or any suitable material. 
The step of machining includes one of wire electrical discharge machining, 
water jet machining, laser machining, spark erosion machining, and rotary 
cutting machining. The step of machining further includes machining for a 
predetermined period of time to form a pair of slots having end points. 
The rod is then advanced to form a continuous surface devoid of slots 
adjacent one of the end points. The steps of machining and advancing are 
repeated to form a plurality of slots separated by continuous surfaces. 
Each of the continuous surfaces extends circumferentially around the rod to 
form a helical slot interruption. A cutting head and adapter piece are 
machined onto opposite ends of the rod, or connected on by welding or 
other suitable means. The shaft, the cutting head and the adapter piece 
are electro-polished for medical applications. The shaft is cleaned by 
inserting a brush or cannulated tube into the hollow rod. Pressurized 
water is forced through the cannulated tube, against an inner surface of 
the hollow rod, and through the slots to clean the shaft. Pressurized 
water may also be sprayed on the external surface of the rod to clean it.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now in detail to the drawings, and in particular FIG. 1, there is 
shown a prior art reamer 10 having a connection member 11 provided for 
attachment to a drive means such as an electric motor. At the other end of 
reamer 10, a drill bit 15 is provided so as to enable drilling of an 
intramedullary canal. Connecting bit 15 and connection means 11 is an 
elongated shaft 17 which is made of a long strip of metal wound in a 
spiral-like fashion to form coils 19 which extend throughout the 
longitudinal extent of shaft 17. Shaft 17 has not been found to be an 
effective means of transmitting torque from connection means 11 to drill 
bit 15, particularly when drill bit 15 encounters an obstruction tending 
to stop its motion. Shaft 17 by virtue of its coil-like construction, 
allows connection means 11 to be rotated by the drive motor with respect 
to bit 15, thereby enabling energy to be stored up therein. When the 
energy stored up in shaft 17 exceeds the forces which are retaining bit 15 
in a stationary position, bit 15 will then jump forward. In some cases, 
this may cause damage to the structure of the bone or surrounding tissue. 
A further disadvantage of the prior art reamer is evident when bit 15 
encounters an obstruction which is not easily removed. The surgeon 
operating reamer 10 may be tempted to reverse the direction of operation 
thereof, to loosen bit 15 from the obstruction. If this is done, and bit 
15 is not removed from the obstruction thereby, the reversal of the motion 
of the drive motor will cause shaft 17 to uncoil. This not only 
irreparably damages shaft 17 but also increases the potential for the 
damage to the surrounding bone tissue. In a further aspect, if sufficient 
reverse motion of connection means 11 is made with respect to bit 15, 
shaft 17 may become sufficiently widened so as to prevent its extraction 
from the opening formed by bit 15. Accordingly, several disadvantages in 
the prior art reamer 10 are self-evident. 
A further disadvantage of the prior art reamer stems from the fact that a 
second inner coil 12 is disposed within shaft 17, as can be seen in FIG. 
2. As coil 19 and inner coil 12 flex during use, the gaps within the coil 
are alternately opened and closed which allow foreign particles to 
penetrate into both coils. Since there is no way to clean the surfaces 
between inner coil 12 and coil 19, foreign particles may remain lodged 
within shaft 17. 
As can be seen in FIGS. 3, 4 and 5, reamer 20, according to the invention, 
consists of a single coil 22. Coil 22 is made from pairs of helical slots 
24 which are machined into a shaft 23, in a manner that will be described 
more fully below. Along the length of shaft 20, helical slot interruptions 
30 are formed which consist of a continuous surface extending 
circumferentially around shaft 23. Helical slot interruptions 30 provide 
added strength to shaft 23 and as a result reamer 20 does not require an 
inner coil. 
In addition, slots 24 may be machined wider than with the prior art devices 
such that, as can be seen in FIG. 5, regions 25 exist where it is possible 
to look completely through shaft 23 by aligning slots on the front side of 
the shaft with slots on the rear side of the shaft. The strength and flex 
characteristics of shaft 23 may be adjusted by controlling the pitch and 
width of slots 24. 
In addition, the position and number of helical slot interruptions 30 can 
affect the strength characteristics of shaft 23. Typically, along a 20" 
shaft, three helical slot interruptions 30 are spaced evenly along the 
length of shaft 23. This divides slots 24 into four sets of slots of equal 
length. The hollow interior of reamer 20 may accommodate an insert to 
stiffen the shaft, a guide pin which was previously positioned within the 
bones medullary space to pre-flex the shaft, or cleaning equipment. 
FIG. 6 shows a method for manufacturing shaft 23. Initially, a hollow rod 
40 is provided which can be made, for example, from stainless steel, 
titanium, chrome cobalt molybdenum, a carbon fiber composite, or any other 
suitable spring material. Rod 40 is advanced along a helical path by a 
combination of rotational motion 42 and translational motion 44. A machine 
head 46 is oriented so as to machine an opening across a diameter of rod 
40. When machine head 46 begins machining, a first helical slot begins at 
point 50 and a second corresponding helical slot begins at point 50', 
diametrically opposed to point 50. As rod 40 advances, a pair of 
intertwined helical slots are formed thereon. At point 51 and 
diametrically opposite point 51', machining is terminated. Rod 40 
continues to advance, for example, 1/4, 1/2 or 3/4 of a turn At points 52 
and 52' machining resumes and a second pair of intertwined helical slots 
are formed along rod 40. 
As can be appreciated, the pitch of helical slots 48 can be adjusted by 
changing the rate of rotational motion 42 with respect to the rate of 
translational motion 44. The speed at which slots 48 are machined is 
determined by the capacity of machine head 46. Shafts of a commercially 
acceptable quality have been machined by wire electrical discharge 
machining. However, helical slots 48 may also be machined by electrical 
discharge machining, water jet machining, laser machining or spark erosion 
machining. 
In the manufacture of medullary reamers, the cutting head and adapter piece 
may be machined onto the ends of rod 40 to form a reamer of unitary 
construction. A distinct advantage with this method of manufacturing is 
that additional time, which would otherwise be required to weld or 
otherwise connect the adapter and cutting head to the rod is avoided. In 
addition, a reamer of unitary construction may undergo electro-polishing 
which is a chemical treatment to polish the material and give it a smooth 
polished finish. The polished surface resists contamination from foreign 
particles and is easier to clean. Electro-polishing cannot easily be 
carried out on a structure consisting of several overlapping connected 
parts. 
In addition to the wide slots and hollow interior shown in FIG. 5, and in 
addition to the electro-polishing described above, reamer 20 may be 
further effectively cleaned by a cannulated tube 60 shown in FIG. 7. This 
tube is configured and dimensioned to slide within shaft 20. Pressurized 
water 61 or other cleaning fluid is forced through apertures 62 to clean 
the inside of shaft 23 as well as the opposed surfaces forming the slots. 
Because slots 24, as can be seen in FIG. 5, are generally wider than the 
prior art slots, they can be effectively cleaned by a pressurized fluid. 
As was mentioned earlier, rods of a variety of material may be machined 
according to the disclosed process to provide a variety of operating 
characteristics. The configuration and length of the helical slot may be 
combined with helical slot interruptions of varying configurations to 
provide a wide range of flexible shafts which are extremely strong and 
easily cleaned. The primary advantage of having the helical slot 
interruptions is that the shaft can be rotated in both directions. In 
other words, if the shaft is configured as a medullary reamer, the surgeon 
has all of the advantages attendant with the prior art reamers, but 
additionally has the ability to operate the reamer shaft according to the 
invention in a reverse or counterclockwise direction. The helical slot 
interruptions effectively prevent the coil from unravelling which is a 
serious problem where the helical slot extends uninterrupted along the 
entire length of the shaft. 
While only a single embodiment of the present invention has been shown and 
described, it is to be understood that many changes and modifications may 
be made thereunto without departing from the spirit and scope of the 
invention as defined in the appended claims.