Abstract:
An external fixator system comprises a clamp adapted to couple a fixator pin to a connecting rod. The clamp includes a slot for transversely receiving the connecting rod. A bolt is inserted through a bore passing transversely to the slot to engage a pin connector holding a fixator pin. The pin connector has a rod-engaging surface that wedges the connecting rod into the slot thus increasing the clamp&#39;s rigidity by preventing rotation of the clamp around the rod and rotation of the pin connector in the clamp body. An aiming device is also disclosed, which attaches to at least two connecting rods to guide the insertion of fixator pins between the rods. The aiming device is adjustable to accommodate various distances between the rods and has different modes of attachment to the rods to enable the clamp installation either above or below the rods.

Description:
RELATED APPLICATIONS 
     This application is a Divisional application of U.S. application Ser. No. 08/841,529, filed Apr. 23, 1997 U.S. Pat. No. 5,976,133 the entire teachings of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     External skeletal fixator systems are used on fractured bones or joints during osteosynthesis typically for more serious injuries involving multiple or compound fractures. Pins are drilled through a patient&#39;s skin and into a fractured bone. Clamps secure the pins to a common connecting rod, creating a rigid frame that immobilizes the bone. 
     The Kirschner-Ehmer external fixator system is commonly used in veterinary orthopedic surgery. The system includes: a standard connecting rod; clamps adapted to slide over the ends of the connecting rod; fixator pins; and bolts for tightening the clamps around the connecting bars, which bolts have through-holes near the head for receiving the fixator pins&#39; shafts. 
     The Kirschner-Ebmer system precludes installation of an additional clamp between two installed clamps on the connecting bar or removal of an intermediate clamp without disassembly of the entire connecting rod. This is because the Kirschner-Ehmer clamp includes a narrow slot leading to a wider channel, into which channel a connecting rod is inserted axially. The connecting rod will not fit into the slot to allow for transverse mounting. Consequently, the surgeon must anticipate the number of clamps required and slide them onto the connecting rod before insertion of the end pins into the bone. This is especially limiting if an additional pin or clamp is required at the completion of surgery or at subsequent patient visits. In addition, the Kirschner-Ehmer clamp undergoes plastic deformation when tightened, permanently deforming and fatiguing the material. Consequently, reuse of the clamp is discouraged. Furthermore, the Kirschner-Ehmer clamp is inapplicable in ring fixators such as the Ilizarov external ring fixator. 
     A fixator clamp produced by Synthes® permits transverse installation of a clamp on a connecting rod between two installed clamps without rod disassembly. It includes a slot for receiving a connecting rod, a hole for receiving a pin in an orthogonal direction relative to the connecting rod, a clevis-shaped clamp, and a bolt and nut which simultaneously secure the pin in the hole and the clamp on the connecting rod. This design is limited to orthogonal pin placements relative to the connecting rod and is mechanically complex. 
     The Synthes® and Kirschner-Ehmer designs are particularly susceptible to loosening under repeated cyclic loading. The Kirschner-Ehmer device relies entirely on the axial tension in the bolt to preclude pin rotation. A slight amount of loosening due to cyclic loading reduces axial tension in the bolt, allowing the pin to rotate relative to the connecting rod. The Synthes™ design relies on a clamp that touches the connecting rod at only two points, rendering this device susceptible to loosening. 
     Improved external fixator clamp designs have been described in U.S. patent application Ser. No. 08/643,512, filed May 6, 1996, entitled “External Fixator System”, filed by Kraus, et al., the teachings of which are incorporated herein in their entirely by this reference. There, the clamp body had a slot that was wide enough to transversely receive and snap-fit over the connecting rod. A hook-shaped bolt, rather than the eye-bolt of the Kirschner-Ehmer designs, was used to hold the fixator pin while securing the clamp body to the rod. This configuration enabled the clamp body to be conveniently added in the middle of a rod. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an external fixator clamp that incorporates a number of advantages achieved by the designs described in 08/643,512 application. The present clamp body has a slot that is wide enough to also transversely receive the rod. Innovations, however, are introduced that relieve a number of tolerances in the manufacture of the clamp, thus substantially reducing the per unit cost, while simultaneously increasing the clamp&#39;s rigidity. 
     Rather than a relatively long bolt to secure the pin to the clamp, a pin connector is used that is inserted into a connector bore in the clamp body. This connector has a pin bore for receiving the fixator pin and a rod-engaging surface. The connector is urged into the connector bore until the fixator pin is braced against the clamp body and the rod-engaging surface wedges the connecting rod into the slot to ensure that the rod is seated in the back of the slot. The interference between the connector and rod provides increased resistance to the rotation of the pin relative to the clamp body and rotation of the clamp body relative to the rod. Moreover, since the rod will always be fully seated in the slot by the connector, the clamp body need not snap-fit onto the rod. The snap-fitting functionality required precise manufacturing tolerances in the machining of the slot since the clamp was generally rigid, allowing for only slight amounts of flexing under reasonable forces. In contrast, in the present invention, the slot is preferably machined to the width of the rod. 
     In the preferred embodiment, a bolt is inserted into a distal end of the connector bore to mate with threads of the pin connector to thereby draw the pin connector into the connector bore. Alternatively, the connector could be externally threaded, but in either case, the connector should have a short overall length to reduce the amount of clearance necessary to couple the connector to the clamp. That is, when the clamp is constructed, the connector extends only partially across the width of the clamp body, the remainder of the distance being covered by the distally inserted bolt or similar fixture. 
     In general, according to another aspect, the invention also features an aiming device and method to assist in the installation of fixator pins. The device has a frame that is adapted to be attached to first and second connecting rods, between which the pin is to be connected. A guide is provided on the frame to facilitate the pin installation process. 
     In specific embodiments, the guide may be a bore in the frame or alternatively a sleeve, which is made part of the frame. This guide may provide for pin installation at various angles. The frame is preferably adjustable so that clamps for the pins can be installed either above or below the rods. The frame is also preferably expandable to accommodate various distances between the connecting rods. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1 is a perspective view of an external fixator clamp system including multiple fixator clamps that couple fixator pins to a common connecting rod to immobilize the fractured bone; 
     FIG. 2 is a perspective, exploded view of the fixator clamp of the present invention; 
     FIG. 3 is a perspective, partially cut-away view of the inventive fixator clamp, when constructed; 
     FIG. 4 is a cross-sectional view of the constructed fixator clamp identifying areas of contact between the pin connector, clamp body, and connecting rod; and 
     FIGS. 5A and 5B are plots of applied load as a function of displacement or position comparing the performance of the inventive clamps relative to commercially-available Kirschner-Ehmer clamps. 
     FIGS. 6A,  6 B, and  6 C are top, side, and bottom plan views of a connector showing the relevant dimensions in one implementation; 
     FIG. 7 is a side view showing the dimensions of the modified connector having a pin bore wide enough to accommodate a cannula; 
     FIGS. 8A,  8 B, and  8 C are right side, top, and left side views of the clamp body showing the dimensions used in the construction of one implementation that is compatible with the connector shown in FIGS. 6A-6C; 
     FIG. 9 is a perspective view of an aiming device of the present invention; and 
     FIG. 10 is a side plan view of the inventive aiming device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of an external fixator system  10  incorporating clamps  100 , which have been constructed according to the principles of the present invention. As is common in these systems  10 , a number of the clamps  100  are used to connect fixator pins  12  to a connecting rod  14 . The pins  12  are typically constructed from stainless steel and preferably have positive-profile threads  125 . The fixator rod  14  may also be constructed from stainless steel, but alternatives such as carbon fiber or acrylic may be substituted. According to the invention, the clamps  100  may be constructed from 316 or 304 stainless steel, carbon fiber, or fiber-glass. 
     The inventive system has applications in a variety of environments. Low cost coupled with the fact that patient mobility is regained immediately after installation makes the system applicable in the veterinary setting. These qualities, however, make it also useful in certain human applications, for example, treatment on the battlefield or in third world countries. 
     The most extreme stresses occur at the pin-bone interface, which can cause premature loosening. It is therefore important to select pins of maximum stiffness. As pin stiffness is proportional to the fourth power of pin radius, positive-profile threaded pins are preferred. A positive-profile threaded pin has threads of greater diameter than the pin shaft, resulting in a stiffer pin, and increased pin-bone adhesion. These are preferred over negative-profile threaded pins which have threads cut into the shaft, and therefore have drastically reduced pin stiffness. 
     FIG. 2 is an exploded view of one of the inventive clamps  100 . Generally, the clamp  100  comprises a clamp body  110 , a pin connector  112  that receives the fixator pin  12 , and some means for urging or drawing the pin connector  112  into a connector bore  116  so that the fixator pin  12  is braced against the clamp body  110 . In the illustrated embodiment, the pin connector is engaged by a ⅜″ long hex-head bolt  114  inserted from the distal side of the clamp body  110 . The external  10 - 32  threads  132  of the bolt  114  mate with the internal threads  118  of the pin connector  112 . 
     The clamp body  110  has a generally “U”-shaped cross-section defining a slot  120 . The height h of the slot  120  is chosen so that the clamp body  110  may be transversely fitted over the rod  14 , enabling the rod  14  to be inserted into the slot  120  in the direction of arrow a. The mouth of the slot  120  can have opposed chamfered surfaces  121  to facilitate the insertion. Since the clamp body  110  can transversely receive the rod, the clamp  100  is also applicable to other systems such as Ilizarov ring fixator rods along with type-two, -three fixator systems. 
     In the preferred embodiment, the height h of the slot  120  is machined to the outer diameter d of the connecting rod  14 . This dimensioning ensures that there is a slight interference fit between the rod and the slot, limiting play in the clamp body  110 -rod  14  interface even before the clamp is secured. Alternatively, the slot height h can be machined to be somewhat larger than the outer diameter d of the rod  14  or smaller, enabling the clamp to be snap fit over the rod  14 . In this later case, the clamp body  110  flexes somewhat enabling the slot height h to marginally increase and accept the rod  14 . The force necessary to install the clamp body, however, should not exceed the force that the typical surgeon can apply. 
     The pin connector  112  has a radially-extending pin bore  122  that receives the fixator pin  12 . Preferably the pin bore  122  should be dimensioned to smoothly receive the connector pin  12 , but without excessive play between the pin  12  and bore&#39;s inner walls. When the clamp  100  is fully constructed, the pin connector  112  is pulled into the connector bore  116  so that the shaft  124  of the pin  12  is braced against the proximal outer surface  126  of the clamp body  110 . 
     In the preferred embodiment, the pin connector  112  has a dome-shaped proximal end  128  and is internally threaded  118  at its distal end. The dome shape ensures that the outer surface will not catch or snag during patient movement. The internal thread arrangement yields a low overall length  1  of the connector  112  which is helpful when constructing the clamp. The pin connector  112  additionally has a chamfered distal end  130 , which engages the connector rod  114  when the clamp is fully constructed. 
     In alternative embodiments, the closed pin bore  122  is replaced by a slot-type bore yielding a hook-shaped member similar to that disclosed in the 08/643,512 application while retaining the rod-engaging surface  130  and short overall length disclosed here. The drawback associated with this embodiment, however, is the cost of machining the pin-slot, which is typically high due to its comparatively more complex shape relative to hole  122  and the concomitant reduction in rigidity of the connector due to the extended slot. 
     FIG. 3 is a perspective view of the constructed clamp  100  with a small cut-away to show the junction between the pin connector  112  and the connecting rod  14 . Tightening the bolt  114  draws the pin  12  against the distal outer surface  126  of the clamp body  110 . This action braces the pin against the clamp preventing the rotation of the pin connector  112  and thus the pin  12 . Moreover, the pin  12  is prevented from moving axially with respect to connector  112 . 
     Tightening the bolt  114  additionally draws the rod-engaging surface  130  of the connector  112  into engagement with the connecting rod  14 . This interference has a number of effects that further prohibit any movement of the tightened clamp  100 . First, friction between the rod-engaging surface  130  and the outer surface of the rod  14  further prevents any rotation of the pin connector  112 . Additionally, the force exerted by the pin connector at this junction pushes the rod  14  in the direction of arrow b. This ensures that the rod  114  is seated against the back wall  134  of the slot  120  providing a good rigid mechanical junction between the connecting rod  114  and the clamp body  110  preventing rotation of the clamp body around or sliding along the connecting rod  14 . 
     In the preferred embodiment, the clamp body  110  does not substantially squeeze-down on or close over the connecting rod when the bolt  114  is tighten. It is only the interference between the rod-engaging surface  130  and the rod  14  that prevents movement between the rod  14  and clamp  100 . 
     FIG. 4 is a cross-sectional view illustrating the major points of contact that increase the rigidity of the constructed clamp  100  relative to those in the prior art. The first area of contact  410  is between the rod-engaging surface  130  and the outer surface of the connecting rod  14 . This prevents rotation of the rod  14  with respect to the clamp body  110  and ensures the rod is fully seated in the slot  120 . The force exerted by the rod  14  against the pin connector  112  additionally pushes the pin connector into engagement with the internal wall of the connector bore  116  at the second contact area  412 . This further prevents the rotation of the pin connector  112  when loads are placed on the pin  112 . 
     In other implementations of the pin connector  112 , the pin bore  122  has a diameter which is large enough to accommodate a cannula, e.g., {fraction (3/16)} inch. This modified connector is useful when using the clamp  100  as a guide for drilling and inserting the pins  12 . The modified connector is installed in the clamp body  110 , and the clamp body  110  is installed on the connecting rod  14 . The cannula is inserted into the enlarged pin bore  122  and used to predrill through the soft tissue of the patient with a trocar to the bone. The positive profile pin is then inserted down through the cannula and installed in the bone. Once completed, the cannula is removed along with the modified connector replaced with one appropriate for the narrower diameter pins  12 . The pin connector is then inserted into the clamp body  110  and then tightened to secure the pin  12  to the rod  14 . 
     FIGS. 5A and 5B are graphs of load as a function of position comparing the performance of commercially-available Kirschner-Ehmer external fixator clamps relative to the fixator clamps of the present invention. The data was obtained using a specially designed jig that allowed application of clamps on a vertically-oriented {fraction (3/16)} inch connecting rod supported with a triangular brace. A 3.2 millimeter pin was placed perpendicular, horizontal in the experimental set up, to the connecting rod. A device for applying a load to the 3.2 millimeter pin was constructed so that a concave 12 mm diameter bearing would apply a load on the 3.2 mm pin directly over a load cell. A servo-hydraulic mechanical testing machine was used for application of the force as well as collecting load and actuator pin data. The distance from the center of the clamp bolt to the applied force was 25 mm. A preload of 10 N force was applied on the 3.2 mm pin in a position ramp at a rate of 0.01 mm/sec. This continued for 4 mm. 
     FIG. 5A is data collected when the clamps were tightened to 40 inch-pounds. The inventive clamp (dark line)  510  showed a greater resistance to deformation than the commercially-available Kirschner-Ehmer clamp (grey line)  512 . As illustrated in FIG. 5B, the invention&#39;s greater resistance to deformation also occurred when the clamps were tightened to 70 inch-pounds. Again, the inventive clamp (dark line)  510  showed a greater resistance to deformation than the commercially-available clamp (grey line)  512 . 
     FIGS. 6A,  6 B, and  6 C show the dimensions for the connector  112 . FIG. 6A is a top view showing the outer diameter of the connector. FIG. 6B is the side view of the connector  112  showing the ⅛ inch diameter pin bore  122 . The distance c between the pin bore center and the bottom extent of the connector  112  is relevant to ensure that the rod-engaging surface  130  engages the connecting rod  14  while the pin  12 , held in the bore  122 , is simultaneously braced against the clamp body  110 . The rod-engaging surface  130  is constructed by a chamfer on the end of the connector formed at an angle of 29.5. Finally, FIG. 6C shows the connector  112  from the bottom. This view shows the end diameter, the internal 10-32 internal threading, and its depth. 
     FIG. 7 is a side view of the modified connector  112  showing the {fraction (3/16)} inch diameter pin bore  122 . This version is appropriate for use as a drill guide, the pin bore  122  being wide enough to receive the cannula. 
     FIGS. 8A,  8 B, and  8 C show the dimensions for the clamp body  110 . FIG. 8A is a side view showing the dimensions of the slot. FIG. 8B is the top view. Finally, FIG. 8C is another side view showing the overall dimensions of the clamp body  110 . 
     FIG. 9 is a perspective view illustrating the operation of an inventive aiming device that is compatible with the clamps  100  but may also be used with other clamp systems. The aiming device is most useful when installing pins  12  in type-two fixator systems in which the pins are secured to two connecting rods  12 , one on either side of the limb  20 . 
     The aiming device  900  comprises a frame constructed from a proximal transverse member  916 , a connecting member  912  extending orthogonally away from the proximal transverse member  916 , and a distal transverse member  914  that extends parallel to the proximal transverse member on the distant side of the limb  20  into which a pin  12  is to be installed. 
     The proximal transverse member  916  has a clamp slot  918  near the top end, in the orientation of FIG.  9 . The clamp slot  918  houses a modified clamp body  910 , which enables the proximal transverse member  916  to be installed onto the connecting rod  14 . The modified clamp body  910  is retained in the clamp slot  918  by an extended thumb bolt  915  that has external threads  940  to engage a threaded connector bore  941  of the clamp body  910 . A cannula bore  920  is formed in the proximal transverse member  916 , above the clamp slot  918 . 
     The orientation of the cannula bore  920  with respect to a connecting rod  14  held in the modified clamp  910  is such that an installed pin may be attached to the connecting rod via the clamp  100  described with reference to FIG.  2 . That is, the distance between the center of the cannula bore  920  and the center of the slot in the modified clamp  910  is equal to the distance between the center of the pin bore  122  in the pin connector  112  and the center of the slot  120  in the clamp body  110  in the constructed clamp of FIG.  2 . 
     In alternative embodiments, the cannula bore  920  is replaced with a sleeve arrangement. Such a sleeve can be ridgedly attached to the member or pivotable, continuously or only at discrete angles, to enable pin placement at various inclinations with respect to the connecting rods  14 . 
     The connecting member  912  is received into a rectangular bore  922  in the bottom end of the proximal transverse member  916  in a mortise and tenon arrangement. The connecting member  912  is retained in the bore  922  with set screws  924 . 
     The distal transverse member  914  extends perpendicularly away from the connecting member  912 . A rectangular bore  926  is formed in the distal transverse member in which the connecting member  912  may slide to accommodate various tissue thicknesses of the patient&#39;s limb. The distal transverse member  914  has first and second slots  936 ,  938  for connecting it to the distant-side connecting rod in the type-two fixator system. 
     A clevis clamping member  928  fits over the bottom end of the distal transverse member  914 , the connecting member  912  extending through transverse passages  930  through the clevis member&#39;s legs. A thumb screw passing through the clevis member  928  and engaging a bottom end of the distal transverse member  914  is tighten to push the clevis member over the distal member  914  and secure the connecting member  912  in the rectangular bore  926  of the distal member  914  and thereby arrest the sliding of the distal member on the connecting member. 
     FIG. 10 is a plan view of the aiming device  900  better illustrating the location of the slots  936 ,  938  relative to the cannula bore  920  and modified clamp body  910 . An axis  942  of the cannula bore  920  intersects the distal transverse member  914  at point  943 , which is equidistant from the centers of the two slots  936 ,  938 . The distance between the point  943  and the centers of either of the slots  936 ,  938  is the same as the distance between the axis of the cannula bore  920  and the center of the slot in the modified clamp body  910 . This relationship is illustrated by line  944  which bisects the slot  936  and the clamp body  910 . As a result of the relationship, one of the inventive clamps  100  described with reference to FIG. 2 may be installed on the distant-side connecting rod  14  and will be in an ideal position to attach a pin inserted through the cannula  946  and installed in the bone  21  of the limb  20 . The two slots  936 ,  938  are provided to enable the clamp  100  to be installed above or below the distant-side connecting rod  14 . 
     The inventive aiming device  900  holds the cannula  946  as it is inserted through the patient&#39;s soft tissue to the surface of the bone  21 . A very narrow orthopedic pin may first be inserted, however, to properly locate the bone surface where the pin is to be set. Once the cannula has been inserted down to the bone, a pin  12  is inserted into the cannula and drilled into and through the bone  21  to the distant-side connecting rod  14 . Once the pin is properly set, the aiming device and cannula are removed and the pin  14  is attached to both connecting rods using clamps  100 . 
     In other applications, the proximal member  916  may be disconnected from the connecting member  912  by removing set screws  924 . In this configuration, it is useful as an aiming device for type-one fixator systems. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.