Abstract:
A method of replacing an ACL with a graft. The method provides for the drilling bone tunnels in a femur and a tibia. A replacement graft is provided having first and second ends. A biodegradable composite screw is provided. The screw is made from a biodegradable polymer and a bioceramic or a bioglass. At least one end of the graft is secured in a bone tunnel using the biodegradable composite screw.

Description:
TECHNICAL FIELD  
       [0001]     The field of art to which this invention relates is surgical procedures for the repair of an anterior cruciate ligament, more specifically, a surgical procedure for affixing an anterior cruciate ligament graft into a bone using a biodegradable interference screw.  
       BACKGROUND OF THE INVENTION  
       [0002]     The knee joint is one of the strongest joints in the body because of the powerful ligaments that bind the femur and tibia together. Although the structure of the knee provides one of the strongest joints of the body, the knee may be one of the most frequently injured joints, e.g., athletes frequently stress and tear knee ligaments. The large number of ligament injuries has given rise to considerable innovative surgical procedures and devices for replacing and reconstructing torn or dislocated ligaments, typically involving grafting autografts, allografts, or a synthetic construct, to the site of a torn or dislocated ligament. For example, the replacement of an anterior cruciate ligament (ACL) may involve transplanting a portion of the patellar tendon, looped together portions of semitendinosus-gracilis (hamstring) tendons, or donor Achilles tendons, to attachment sites in the region of the knee joint.  
         [0003]     Tears or ruptures of an anterior cruciate ligament of a knee (ACL) typically require a major surgical intervention wherein a replacement graft is mounted to the ends of the bones surrounding the knee in order to reconstruct the knee. A ruptured or damaged ACL typically results in serious symptoms such as knee instability resulting in diminished ability to perform high level or recreational sports, or in some cases daily activities relating to motility. Although the use of knee braces may alleviate some of these symptoms, the potential long term effects of a damaged ACL include meniscal damage and articular cartilage damage.  
         [0004]     The basic steps in a conventional ACL reconstruction procedure include: harvesting a graft made from a portion of the patellar tendon with attached bone blocks; preparing the graft attachment site (e.g., drilling holes in opposing bones of the joint in which the graft will be placed); placing the graft in the graft attachment site; and rigidly fixing the bone blocks in place within the graft site, i.e., the holes or “bone tunnels”. The screws used to fix the graft in place are called “interference screws” because they are wedged between the bone block and the wall of the bone tunnel into which the bone block fits. Typically, there is very little space between the bone block and the inner wall of the bone tunnel in the bone at the fixation site.  
         [0005]     Several types of surgical procedures have been developed to replace the ACL. Although repair would be a preferred procedure, it is not typically possible since the end of the torn ACL is typically not of sufficient length to reattach successfully. However, reconstructions can be made to a damaged ACL.  
         [0006]     There are several types of conventional replacement grafts that may be used in these replacement procedures. In all procedures tibial and femoral tunnels are drilled by the surgeon using conventional techniques. Known, conventional drill guides and drills are used. In one type of procedure known as a bone-tendon-bone procedure, an autograft tendon is harvested from the patellar tendon along with an attached bone block on one end harvested from the patella and a harvested bone block on the other end harvested from the tibia. In order to secure the graft in the knee, one end is mounted into the tibial tunnel and other end is mounted into the femoral tunnel. This is done by mounting the opposed bone blocks in the tibial and femoral tunnels, respectively, in the following manner. A guide pin is passed through the tibial tunnel, into the fermoral tunnel and out through the lateral femoral cortex. Suture is used to attach the graft to the proximal end of the guide pin. The distal end of the guide pin is then pulled out of the lateral cortex of the femur and the graft is pulled into the knee (femoral and tibial tunnels). Once the bone blocks are emplaced in the respective tibial and femoral tunnels, the blocks are secured in place in the following manner. One method of securing or fixing the ends of the graft in the tunnels is to use a conventional metallic interference screw. The screw is inserted into the opening of a tunnel and placed in between the graft and the interior surface of the bone tunnel. It is then turned and screwed into the tunnels, thereby forcing the end of the graft against an interior surface of the bone tunnel. The ends of graft are secured and maintained in place in the tunnel by means of a force fit provided by the interference screw.  
         [0007]     Another surgical procedure for the replacement of an anterior cruciate ligament involves providing a graft ligament without attached bone blocks. The graft can be an autograft or an allograft. The autografts that are used may typically be harvested from the hamstring tendons or the quadriceps tendons. The allografts that are conventionally used are harvested from cadaveric sources, and may include the hamstring tendons, quadriceps tendons, Achilles tendon, and tibialus tendons. If desired, and if readily available, it may possible to use synthetic grafts or xenografts. Tibial and femoral tunnels are similarly drilled in the tibia and femur respectively using-conventional techniques, drill guides and drills. Once the tunnels have been drilled, the surgeon then pulls the graft through the tibial and femoral tunnels using conventional techniques such that one end of the graft resides in the tibial tunnel and the other end of the graft resides in the femoral tunnel. For example, one conventional technique for pulling a graft through the tunnels is to attaché the graft to the proximal end of a guide pin using conventional surgical suture. The guide pin is then passed through the tibial tunnel, into the femoral tunnel, and out though the femoral cortex. The distal end of the guide pin is then pulled out of the lateral cortex of the femur and the graft is pulled into the knee (femoral and tibial tunnels). After the surgeon has emplaced and positioned the ends of the graft in the respective tunnels, the graft ends need to be secured and fixed in place to complete the replacement procedure. One method of securing or fixing the ends of the graft in the tunnels is to use a conventional metallic interference screw. The screw is inserted into the opening of a tunnel and placed in between the graft and the interior surface of the bone tunnel. It is then turned and screwed into the tunnels, thereby forcing the end of the graft against an interior surface of the bone tunnel. The ends of the graft are secured and maintained in place in the tunnel by means of a force fit provided by the bone screw.  
         [0008]     Interference screws for anchoring ligaments to bone are typically fabricated from medically approved metallic materials that are not naturally degraded by the body. One potential disadvantage of such screws is that once healing is complete, the screw remains in the bone. An additional disadvantage of a metal screw is that in the event of a subsequent rupture or tear of the graft, it may be necessary to remove the metal screw from the bone site. Metallic screws may include a threaded shank joined to an enlarged head having a transverse slot or hexagonal socket formed therein to engage, respectively, a similarly configured, single blade or hexagonal rotatable driver for turning the screw into the bone. The enlarged heads on such screws can protrude from the bone tunnel and can cause chronic irritation and inflammation of surrounding body tissue.  
         [0009]     Permanent metallic medical screws in movable joints can, in certain instances, cause abrading of ligaments during normal motion of the joint. Screws occasionally back out after insertion, protruding into surrounding tissue and causing discomfort. Furthermore, permanent metallic screws and fixation devices may shield the bone from beneficial stresses after healing. It has been shown that moderate periodic stress on bone tissue, such as the stress produced by exercise, helps to prevent decalcification of the bone. Under some conditions, the stress shielding which results from the long term use of metal bone fixation devices can lead to osteoporosis.  
         [0010]     Biodegradable interference screws have been proposed to avoid the necessity of surgical removal after healing. Because the degradation of a biodegradable screw occurs over a period of time, support load is transferred gradually to the bone as it heals. This reduces potential stress shielding effects.  
         [0011]     In order to overcome the disadvantages that may be associated with metal interference screws, interference screws made from biodegradable polymers are known in this art. For example, it is known to use an interference screw made from polylactic acid. Ideally, the biodegradable interference screw will rapidly absorb or break down and be replaced by bone. However, it is known that screws made from polylactic acid tend to maintain their structural integrity for very long periods of time thereby preventing the desired bone in growth. Attempts have been made to improve the bone regeneration process by using other biodegradable polymers and copolymers of lactic acid that resorb or absorb more quickly. The problem often associated with these quicker absorbing polymers or copolymers is that the bone regeneration may proceed at a much slower rate than the rate of resorption, resulting in premature mechanical failure of the screw and a resulting pull out of the graft end from the femoral tunnel. Some of the absorbable interference screws of the prior art may take several years to absorb, and may result in a fibrous tissue mass or cyst being left behind, not bone. This lack of bone in-growth may create fixation problems if the ACL is torn again, necessitating a new graft replacement. In addition, if the screw absorbs too slowly, the screw will need to be removed in the event of a subsequent failure of the graft.  
         [0012]     Accordingly, what is needed in this art is a novel method of performing an ACL replacement graft procedure using a novel interference screw made from a biodegradable material which rapidly absorbs or degrades and promotes bone in-growth.  
       SUMMARY OF THE INVENTION  
       [0013]     Therefore, it is an object of the present invention to provide a novel method of replacing a ruptured or injured anterior cruciate ligament with a graft using a novel biodegradable interference screw consisting of a composite of a biodegradable polymer and a biodegradable ceramic or bioglass.  
         [0014]     Accordingly, a novel method of repairing an anterior cruciate ligament in the knee is disclosed. A replacement graft is provided having a first end and a second end. A bone tunnel is drilled in the tibia. A bone tunnel is also drilled in the tibia. The first end of the graft is mounted in the femoral bone tunnel. The second end of the graft is mounted in the tibial bone tunnel. A biodegradable, composite interference screw is provided. The interference screw is made from a copolymer of poly (lactic acid) and poly(glycolic acid) and a bioceramic. The biodegradable screw is inserted into the femoral bone tunnel between an interior surface of the femoral bone tunnel and the first end of the graft. The interference screw is rotated such that the screw is substantially contained within the femoral bone tunnel, and the first end of the graft is fixed in place between the interference screw and a section of the interior surface of the femoral bone tunnel.  
         [0015]     These and other features, aspects and advantages of the present invention will become more apparent from the following description and accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1A  is a side view of a biodegradable interference bone screw useful in the method of the present invention.  
         [0017]      FIG. 1B  is an end view of the interference bone screw of  FIG. 1A .  
         [0018]      FIG. 1C  is a cross-sectional view of the inference bone screw of  FIG. 1B  taken along view line A-A.  
         [0019]      FIG. 2  is a side view of a driver device useful for emplacing the bone screw of  FIG. 1  in a bone tunnel.  
         [0020]      FIG. 3  illustrates a bone-tendon-bone graft prior to emplacement in a knee for an ACL reconstruction.  
         [0021]      FIG. 4  shows a guide wire placed into the femoral tunnel between the tunnel wall and the bone block.  
         [0022]      FIG. 5  illustrates a conventional tap being used to tap a hole between the wall and the bone block.  
         [0023]      FIG. 6  shows a biodegradable interference screw being inserted into the femoral tunnel between the tunnel wall and the bone block.  
         [0024]      FIG. 7  illustrates a guide wire placed into the tibial tunnel between the tunnel wall and the bone block.  
         [0025]      FIG. 8  illustrates a conventional tap device being used to tap a hole between the tunnel wall and the bone block.  
         [0026]      FIG. 9  illustrates the screw being inserted into the tibial tunnel between the tunnel wall and the bone block.  
         [0027]      FIG. 10  is a side view of the knee after the ACL replacement procedure has been completed.  
         [0028]      FIG. 11A  is a histological section of a PLA/PGA bone pin containing β-tricalcium phosphate and surrounding tissue.  
         [0029]      FIG. 11B  is a histological section of a PLA bone pin and surrounding tissue.  
         [0030]      FIG. 11C  is a histological section of a PLA bone pin and surrounding tissue.  
         [0031]      FIG. 11D  is a histological section of a PLA bone pin containing β-tricalcium phosphate and surrounding tissue.  
         [0032]      FIG. 11E  is a histological section of a PLA/PGA bone pin containing β-tricalcium phosphate and surrounding tissue.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The novel interference screws of the present invention are a composite of a biodegradable polymer or copolymer and a bioceramic. The term biodegradable as used herein is defined to mean materials that degrade in the body and then are either absorbed into or excreted from the body. The term bioceramic as defined herein is defined to mean ceramic and glass materials that are compatible with body tissue. The bioceramics are preferably biodegradable.  
         [0034]     The biodegradable polymers that can be used to manufacture the composite screws used in the novel process of the present invention include biodegradable polymers selected from the group consisting of aliphatic polyesters, polyorthoesters, polyanhydrides, polycarbonates, polyurethanes, polyamides and polyalkylene oxides. Preferably, the biodegradable polymers are aliphatic polyester polymers and copolymers, and blends thereof. The aliphatic polyesters are typically synthesized in a ring opening polymerization. Suitable monomers include but are not limited to lactic acid, lactide (including L-, D-, meso and D,L mixtures), glycolic acid, glycolide, ε-caprolactone, p-dioxanone (1,4-dioxan-2-one), trimethylene carbonate (1,3-dioxan-2-one), δ-valerolactone, and combinations thereof. These monomers generally are polymerized in the presence of an organometallic catalyst and an initiator at elevated temperatures. The organometallic catalyst is preferably tin based, e.g., stannous octoate, and is present in the monomer mixture at a molar ratio of monomer to catalyst ranging from about 10,000/1 to about 100,000/1. The initiator is typically an alkanol (including diols and polyols), a glycol, a hydroxyacid, or an amine, and is present in the monomer mixture at a molar ratio of monomer to initiator ranging from about 100/1 to about 5000/1. The polymerization typically is carried out at a temperature range from about 80° C. to about 240° C., preferably from about 100° C. to about 220° C., until the desired molecular weight and viscosity are achieved. It is particularly preferred to use a copolymer of poly(lactic acid) and poly(glycolic acid). In particular, a copolymer of about 85 mole percent poly(lactic acid) and about 15 mole percent poly(glycolic acid).  
         [0035]     The bioceramics that can be used in the composite screws used in the novel process of the present invention include ceramics comprising mono-, di-, tri-, α-tri-, β-tri-, and tetra-calcium phosphate, hydroxyapatite, calcium sulfates, calcium oxides, calcium carbonates, magnesium calcium phosphates. It is particularly preferred to use a β-tritricalcium phosphate.  
         [0036]     In addition to bioceramics, bioglasses may also be used in the composite screws. The bioglasses may include phosphate glasses and bioglasses.  
         [0037]     The amount of the bioceramic or bioglass in the composite interference screw will be sufficient to effectively promote bone in-growth. Typically the amount will be about 2.0 Vol. % to about 25.0 Vol. %, and preferably about 15.0 Vol. %.  
         [0038]     The composite, biodegradable interference screws useful in the present invention are manufactured in conventional extrusion and molding processes using conventional extruding and molding equipment. In a typical process, dry biodegradable polymer pellets and dry bioceramic or bioglass are metered into a conventional heated screw extruder. The materials are heated and blended in the extruder for a sufficiently effective residence time to provide a viscous composite having a uniform distribution of the particles of bioglass or bioceramic. Then the viscous composite is cooled and chopped to form pellets of the homogenous composite. The interference screws may be molded in a conventional injection molder. In a typical injection molder, pellets of composite are fed into a barrel, passed through a heating zone to melt the polymer, then pushed forward through a nozzle and into the cavity of a chilled mold. After cooling, the mold is opened, and the part is ejected.  
         [0039]     A biodegradable interference screw  5  of the present invention is seen in FIGS.  1 A-C. The screw  5  is seen to have an elongate body  10  having a cannulated passage  20  therethrough, with proximal socket opening.  22  and distal opening  26 . The body  10  is seen to have a plurality of thread flights  30  extending from the outer surface  12 . The body  10  is seen to have distal end  14  and proximal end  16 . A driver  50  for inserting or emplacing the crew  5  in a bone tunnel is seen in  FIG. 2 . The driver  50  has an elongated rod member  60  having distal end  62  and proximal end  64 . Distal end  62  is seen to have a driver  63  extending therefrom having a hexagonal configuration for mating with socket  22 . The screw  5  is mounted to driver  50  by inserting the driver  63  of distal end  62  into the mating proximal socket end  22  of the passage  20 .  
         [0040]     The biodegradable composite interference screws described herein are used in the novel ACL reconstruction procedure of the present invention in the following manner as illustrated if  FIGS. 3-10 . Prior to reconstructing the ACL using a bone-tendon-bone graft, a patient is prepared for surgery in a conventional manner. The patient&#39;s knee  100  is prepared for surgery in a conventional manner including swabbing the skin around the knee with a conventional antiseptic solution, and draping the knee. The knee  100  is then angulated by the surgeon in a conventional manner to facilitate the surgical procedure. The patient is then anesthetized in a conventional manner using conventional anesthetics, either general or local at the discretion of the surgeon. As seen in  FIG. 1 , the knee  100  is seen to have a femur  150  having a distal end  160  and a tibia  130  having a proximal end  140 . Proximal end  140  is seen to have a tibial plateau  141 . Extending from the distal end  160  of femur  150  are the femoral condyles  170  separated by notch  175 . For the sake of illustration, the tendons, cartilage, fascia, soft tissue and skin are not shown. The knee  100  is accessed by the surgeon using a conventional arthroscope that is inserted though a conventional cannula, that has been previously emplaced in the knee  100  in a conventional manner through an incision in the skin covering the knee  100 . A flow of sterile saline is initiated through channels in the arthroscope into the knee  100 . The stumps of the ACL are removed from the surfaces of the tibial plateau  141  and the chondryl notch  175  using conventional shavers that are inserted through the cannula. A bone-tendon-bone graft  200  is harvested and prepared by the surgeon in a conventional manner. The graft  200  is harvested by making an incision in the skin over the knee  100  down the anterior patella to the tibial. A conventional sagittal saw is then used to harvest the opposed bone plugs  220  that are connected by harvested patellar tendon segment  210 . The tendon segment  210  is cut from the patellar tendon in a conventional manner using a scalpel. If desired, a graft without bone blocks attached may also be used in the method of the present invention.  
         [0041]     The procedure continues by mounting a conventional tibial drill guide (not shown) to the proximal end of the tibia  130 . A conventional guide pin  250  is inserted into the drill guide and mounted to a conventional surgical drill. The guide pin  250  is seen to have elongated body  252  having distal cutting end  254  and proximal end  255  with suture mounting opening  257 . The guide pin  250  is drilled into the front of the tibia  130  in a conventional manner until the distal end  254  exits out from the tibial plateau  141 . The drill guide is then removed from the tibia  130  and a conventional surgical reamer is placed over the guide pin  250  and turned to ream out a tibial tunnel  280  having a passage  282 , an inner tunnel wall  283 , a top opening  284  out of the tibial plateau  141  and a bottom opening  286  out through the tibia  130 . Then the reamer and the guide pin  250  are removed from the tibial tunnel  280  and a conventional femoral aimer device (not shown) is inserted into tibial tunnel  280  and manipulated until the distal end of the femoral aimer engages the appropriate location on the femoral notch  175 . Then the guide pin  250  is inserted through a passage in the femoral aimer, and the guide pin  250  is mounted to a conventional surgical drill and drilled into the femoral notch such that the distal end exits out through the lateral side of the femur  150  and through the skin overlying that section of the femur  150 . Next, the femoral aimer is removed from the knee  100  and a conventional surgical bone reamer is placed over the guide pin  250  and moved through the tibial tunnel  280 , and a femoral tunnel  290  is drilled though the femur having a passage  292 , an inner tunnel wall  293 , an upper opening  294  out through the lateral side of the femur  130  and a bottom opening  296  out of the femoral notch  175 . The reamer is then removed from the bone tunnel  290 .  
         [0042]     Referring to  FIG. 3 , the graft  200  is illustrated proximal to the knee  100  having the tibial tunnel  280  and femoral tunnel  290  drilled and reamed in the tibia  130  and femur  150 , respectively. The guide pin  250  is seen to reside in the knee  100  with the elongated body  252  of guide pin  250  substantially contained within tibial tunnel  280  and femoral tunnel  290 , with distal end  254  exiting out through opening  294  and proximal end  255  exiting out from opening  286 . Next, the surgeon threads sutures  230  through the suture tunnels  222  in bone blocks  220 . The suture through the top bone block  220  is also threaded through opening  257  of guide pin  250 . The surgeon then pulls guide pin  250  distally such that the graft  200  is displaced into the knee  100  with upper bone graft  220  located in passage  292  of femoral tunnel  290  and lower bone block  220  located in passage  282  of tibial tunnel  280 . An optional step of tapping the bone block and boned tunnel is illustrated in  FIGS. 4 and 5 . A guide wire  300  is seen to be inserted into femoral bone tunnel  290  between bone block  220  and inner tunnel wall  293 . Then, a conventional cannulated bone tap  310  is inserted over guide wire  300 . The bone tap  310  has elongated cannulated member  310 , having a transverse handle  314  mounted to proximal end  312  and a tapping/cutting end  318  mounted to distal end  316 . The tapping cutting end  318  is rotated by rotating handle  314 , causing an opening to be cut and threads to be tapped between inner wall  293  and bone block  220  in the femoral tunnel  290 . Then, as seen in  FIG. 6 , a biodegradable interference screw  5  mounted to a driver  50  is mounted to the guide wire  300  and threaded into the femoral tunnel  290  between the bone block  220  and the inner wall  293 , thereby securing the upper bone block  220  in the passage  292  of femoral tunnel  290 . The guide wire is then removed from the femoral tunnel  290  and inserted into opening  286  of and into passage  280  of tibial tunnel  280  between the lower bone block  220  and the inner wall  183  as seen in  FIG. 7 . Then, the surgeon tensions the graft  200  by pulling proximally on sutures  230  connected to lower bone block  220 . Then, the bone tap  310  is inserted into tibial tunnel  280  over the guide wire  300  and an opening and threads are cut and tapped between inner wall  283 , and bone block  220 . Finally, the bone tap  310  is removed and as seen in  FIG. 9 , a biodegradable interference screw  5  is mounted over the guide wire  300  and threaded into the tibial tunnel  280  between inner wall  282  and lower bone block  220 , thereby securing the lower bone block  220  in tibial tunnel  280 . This completes the ACL reconstruction, and the graft  200  is now secured in the knee  100 . The complete reconstructed knee  100  is seen in  FIG. 10 . The surgeon then checks the knee for proper flexion and completes the procedure in a conventional manner by removing the scope and portal, and conventionally closing and/or suturing and bandaging all incisions.  
         [0043]     The following examples are illustrative of the principles and practice of the present invention although not limited thereto.  
       EXAMPLE 1  
       [0044]     Biodegradable composite bone pins  1  were prepared in a conventional manner and into the femurs of mammalian laboratory animals. The pins were of the following three compositions: A) composites of 15/85% by volume β-tricalcium phosphate and (85/15)poly (lactide co-glycolide); B) poly(lactide); and C) composite of 15%/85% by volume β-tricalcium phosphate and poly(lactide). About 24 months after implantation, the animals were euthanized and histological sections were obtained. As seen in  FIG. 11A , a bone pin  500  having a Composition (A) demonstrated a significant degree of absorption when compared with the original diameter indicated by arrows  505 , and significant tissue (bone) in-growth. In addition, minimal tissue reaction was observed. As seen if  FIGS. 11B and 11C , bone pins  510  and  520  having Composition (B) exhibited minimal absorption compared with the original diameters indicated by arrows  515  and  525 , respectively. As seen in  FIG. 11D , a bone pin  530  having Composition C showed minimal absorption compared with the original diameter indicated by arrows  535 . And, as seen in  FIG. 11E , a bone pin  540  having Composition A demonstrated a significant degree of absorption compared with the original diameter indicated by arrows  545 , and significant tissue (bone) in-growth. Minimal tissue reaction was observed.  
         [0045]     The novel ACL graft replacement method of the present invention using a composite interference screw made from a bioaborbable polymer and a bioceramic or bioglass has many advantages. The advantages include having improved bioabsorption and bone replacement, improved tissue in-growth, and minimizing tissue trauma. In addition, there is an optimal balance between stiffness and elasticity of the screws.  
         [0046]     Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.