The weak link in anterior cruciate ligament reconstruction: what is the evidence for graft fixation devices?
The ACL is a ligamentous structure composed of mainly type I collagen. It originates from the posteromedial aspect of the lateral femoral condyle and inserts onto the anterior and lateral aspect of the medial tibial spine. The ACL has two bundles (anteromedial and posterolateral) that are named for their tibial insertion. The cross-sectional area at the midsubstance is between 26 mm and 44 [mm.sup.2], and the length varies between 22 mm and 41 mm. (3, 4, 6) The middle geniculate artery is the primary blood supply; however, there are also minor contributions from the inferomedial and inferolateral geniculate arteries. The ACL also has a proprioceptive function, and it has been shown to contain several mechanoreceptors, including Pacinian and Ruffini corpuscles. (3)
The forces transmitted through the ACL vary based on the integrity of the menisci, the other knee ligaments, and the position of the knee. In a series of elegant studies, Morrison (7-9) demonstrated that ACL loads of 169 N could be expected during normal level walking while ascending and descending stairs generated 67 N and 445 N of force, respectively (Table 1). The ultimate load to failure of the native femur-ACL-tibia complex in young cadaveric specimens is approximately 2,160 N with a mean stiffness of 242 N/mm. Older specimens have lower ultimate loads to failure (average of 496 N) and less mean stiffness (124 N/mm). (10, 11)
Based upon the breaking load of the native ACL, Noyes and coworkers (12) have estimated that the ACL strength needed for most activities was 454 N (100 pounds). Therefore, the initial fixation strength of an ACL graft required for these activities should be greater than 450 N. Clinically, excellent outcomes have been seen using graft fixation techniques that only had a load to failure strength of 248 N. (3, 4, 13, 14) It remains unclear if fixation strength of greater than 450 N is in fact needed.
The ultimate loads to failure and stiffness of commonly used grafts in ACL reconstruction have been shown to be significantly greater than the native ACL's strength and are presented in Table 2. Although it should be noted that these values are actually less after the grafts are harvested and artificially fixed during ACL reconstruction, they still far exceed the 450 N suggested by Noyes and coworkers. (4, 12, 14-17)
Structured rehabilitation or surgical reconstruction are the two primary treatment options for patients with ACL tears. Many young, active patients choose to undergo surgical reconstruction in order to resume sports and also to attempt to protect the menisci and articular cartilage from damage due to knee instability. Structured rehab is traditionally reserved for older, less active patients; however, older patients who maintain active lifestyles may also elect to undergo surgical reconstruction.
ACL reconstruction is one of the most widely performed orthopaedic procedures in the USA, and it is thought that more than 200,000 procedures are performed each year. (18, 19) The success rates for this procedure in younger patients range from 80% to 95% for experienced surgeons. Improper placement of the bone tunnels is the most common cause of failure. (20, 21) Patients are typically able to return to their pre-injury activities 6 months to 1 year after surgery. Despite this high success rate, there are still risks involved in ACL reconstruction, and these include infection, deep venous thrombosis, arthrofibrosis, bleeding, and graft failure. Improvements in operative techniques, technology, and rehabilitation have mitigated some of these issues. In seeking to minimize graft failure, however, surgeons continues to face difficult decisions with respect to graft choice and fixation method. We will explore some of the evidence supporting various types of grafts and their fixation.
There are a variety of autograft and allograft options available for ACL reconstruction, each with its own associated risks and benefits. Autograft options include a central third bone-patellar tendon-bone (BPTB), 4-strand hamstring tendon consisting of the gracilis and semitendinosus (HS), or the less commonly used quadriceps tendon-bone graft.
The BPTB and HS grafts are by far the most commonly used autografts, and both have a good clinical track record. Advantages of BPTB graft include relative ease of harvest, secure fixation, increased graft incorporation, and excellent graft strength. Some of the disadvantages include risk of patella fracture, patellar tendonitis, anterior knee pain, and loss of sensation. HS graft, on the other hand, has been associated with reduced donor site morbidity, including less anterior knee pain, better knee extension, and increased graft strength. Disadvantages include increased difficulty harvesting the graft and weakness of knee flexion. (3, 18, 22, 23)
In terms of clinical outcomes, Freedman and associates showed in a systematic review that the BPTB autograft had a lower failure rate (1.9%) than the HS autograft (4.9%) in ACL reconstructions. (24) A more recent Cochrane systematic review by Mohtadi and colleagues (2) that examined ACL outcomes after BPTB versus HS autograft was unable to find sufficient evidence to recommend one graft over the other for ACL reconstruction. Interestingly, they did find increased knee stability across all tests (pivot shift, Lachman) for BPTB graft but could not associate this with better functional outcomes. Additionally, they did not have long enough follow up to assess whether this increased stability reduced the development of osteoarthritis. As anticipated, patients experienced more anterior knee pain and loss of extension with this graft. Conversely, HS graft resulted in some loss of flexion. Similarly, Spindler and coworkers (25) performed a systematic review comparing BPTB and HS grafts and was unable to recommend one graft over the other.
Higher level evidence does exist comparing BPTB and HS autografts. Taylor and colleagues (26) performed a randomized controlled trial of BPTB and HS autografts fixed using similar femoral and tibial fixation methods and found that both grafts provided similar objective, subjective, and functional outcomes at an average of 36 months post-reconstruction. Nationwide database studies from Denmark and Norway published in 2014 showed that BPTB grafts may be slightly better than HS grafts in ACL reconstructions. (27, 28) Rahr-Wagner and associates (28) evaluated 13,647 ACL reconstructions and found that BPTB ACL reconstructions had a 0.16% 1-year and 3.03% 5-year revision rate, while the HS grafts had a 0.65% 1-year and 4.45% 5-year revision rate. Persson and coworkers (27) in a study of 12,643 patients found that patients with HS grafts had twice the risk of revision compared with patients with BPTB grafts. Overall, the clinical significance of these findings remains unclear and more high quality level 1 studies are needed to determine whether BPTB or HS autografts are better for ACL reconstruction.
Central third quadriceps tendon autograft is less commonly used and have been associated with less anterior knee pain and graft site morbidity compared to BPTB grafts. (3-4, 29) This graft is typically used in revision surgery because it is more difficult to harvest, and most surgeons are unfamiliar with the technique.
Commonly used allograft tissues include BPTB, Achilles tendon, tibialis anterior, or tibialis posterior tendon. Advantages of using allograft include the availability of larger grafts, less postoperative pain, shorter operative time, the possibility of multiple ligament reconstruction, faster immediate postoperative, and avoidance of donor site morbidity. (3, 20) Disadvantages include delayed graft incorporation, disease transmission, potential immune reaction, increased cost, and decreased mechanical properties due to processing. A meta-analysis of 76 studies looking at the outcomes of autograft BPTB versus allograft BPTB showed that reconstruction with BPTB autograft resulted in lower graft rupture rate, improved knee stability, and single leg hop test. (30) Furthermore, the BPTB autograft patients were more satisfied postoperatively compared to the allograft patients.
The Biology of Graft Healing
A brief review of the biology of ligament-to-bone healing will help to facilitate an understanding of the importance of initial graft fixation. All ACL grafts undergo the same sequential process of incorporation into the host knee after surgery. (22, 31, 32) This process starts out with an inflammatory period, during which the graft undergoes some degeneration. This is followed by a revascularization period in which the host cells begin to migrate into the graft tissue. These early phases of graft healing are marked by decreased graft strength, which makes it critically important that the graft fixation device is able to withstand the forces seen during early rehabilitation. The graft-healing phase then follows, marked by an increase in graft strength, though the graft does not achieve the strength it had at initial fixation.
The bone-to-bone healing afforded by BPTB autograft gives these grafts an advantage over the soft tissue HS autografts, because the bone-to-bone healing is similar to fracture healing. With the bone-to-bone healing of BPTB autografts, the graft is usually fully healed to host bone by 6 weeks. Conversely, soft tissue grafts usually incorporate into host bone by 8 to 12 weeks after surgery. (22, 31) Although both autograft and allograft tissue undergo the same sequence of steps during the graft incorporation process, allograft tissue have a slower rate of biological incorporation and can take up to 6 months to incorporate. (22, 31, 33) Therefore, consideration should be given to protecting patients after allograft reconstruction for a longer time period than patients who have had autograft ACL reconstruction. The need for a longer period of protection was highlighted in an animal study by Jackson and colleagues (34) in which they compared BPTB autograft and fresh frozen BPTB allograft ACL reconstructions and found that after 6 weeks, the autografts had a greater cross-sectional area compared to the allograft. (22) Furthermore, at 6 months, the autograft demonstrated a better biologic response with respect to collagen composition and had significantly higher loads to failure values (1337 N versus 578 N) and improved anterior-posterior stability.
Graft fixation is arguably the most important part of ACL reconstruction as it is the weakest link in the early postoperative period. The ideal graft fixation has to be strong enough to avoid failure, stiff enough to restore knee stability, and secure enough to avoid slippage. During the first 6 to 12 weeks after surgery, as the graft incorporates into the host bone, the fixation must be able to withstand the demands of an accelerated rehabilitation program that is focused on early weightbearing, early return of full range of motion, and neuromuscular coordination and strengthening. There are a variety of methods by which the bone and soft tissue grafts can be fixed, and this can be done either in the bone tunnel or through a cortical based fixation away from the joint (Fig. 1). The choice of graft fixation device is a critical decision in ACL reconstruction, because poor graft fixation can lead to early clinical failure through micromotion at the graft-bone interface and graft slippage leading to a loss of graft tension.
Fixation strength is especially important on the tibial side, which is usually the site of fixation failure, because the metaphyseal region of the tibia has less bone density than the femur and the graft experiences forces that are more collinear within the tibial tunnel. (21, 22, 35-37) Both location of the fixation (femur or tibia) and type of graft (bone or soft tissue) should, therefore, inform choice of fixation.
ACL reconstruction with a BPTB autograft and interference screw fixation for both the tibia and femur is considered by many to be the gold standard graft and fixation technique. However, the fact that other graft materials, such as the HS autograft, have enjoyed increased popularity has resulted in the creation of new fixation devices to improve the outcomes of ACL reconstruction. Similarly, the desire to improve on the interference screw fixation of BPTB grafts has also led to breakthroughs in the development of new fixation devices. Currently, available fixation options include interference screws (metal and bioabsorbable), staples, suture and post, cross pins, expansion bolts, suspension devices (cortical, cancellous or cortical-cancellous), or even an implant-free press-fit fixation technique. We will explore some of the evidence for these fixation devices.
Femoral Fixation: Bone-Patellar-Tendon-Bone Graft
Most surgeons who use BPTB grafts prefer to use interference screw fixation, which fixes the graft close to the joint line and compresses the bone block against the host bone in the tunnel. Some of the advantages of interference screw fixation include ease of insertion, decreased slippage during cyclic loading, and aperture fixation (near the joint). These result in the creation of a stiffer construct. (14, 21, 22) Early interference screws were metal; however, now there are a variety of bioabsorbable screws. Bioabsorbable screws have comparable initial strength and ease of insertion as their metal counterparts. (21) The bioabsorbable interference screws have several advantages, including MRI compatibility, decreased risk of graft laceration during insertion, and facilitation of revision surgery by avoiding the need for implant removal. However, there are also some disadvantages to their use, including the increased cost, screw breakage, failure during insertion, and potential foreign body reaction. (20-22) The introduction of cannulated screws has reduced some of the concerns over screw divergence from the femoral bone plug. Screw divergence angles greater than 20[degrees] are associated with decreased load to failure. (38) In terms of screw length and size, longer screws provide better fixation, and in BPTB grafts, the screw should only engage the bone plug. (39) The diameter of the screw should be 1 mm larger than the tunnel diameter for soft tissue grafts and the same size as the tunnel diameter for bone plug grafts. (37) Additionally, if there are concerns about bone quality, one should consider hybrid fixation. Overall, biomechanical studies have shown that both the metal and bioabsorbable screws are able to withstand the typical forces experienced by the graft during early rehab.
Cross pin fixation can be used on both the femoral and tibial side; however, it is more commonly used on the femoral side. Cross pins are usually 2 mm to 3 mm in diameter and are available in either metal or bioabsorbable forms. These pins pass across the bone and through the bone plug and from a biomechanical standpoint have been found to perform similar to interference screws when used in bone plugs measuring at least 9 mm to 10 mm. (21, 40, 41) This device provides fixation closer to the joint line and produces less tunnel widening. The principle mode of failure of this fixation is bone plug fracture. This can be prevented by using larger (at least 9 mm) bone plug diameters.
Suspensory fixation is another option for femoral fixation, and the EndoButton[R] (Smith & Nephew Arthroscopy, Inc.) is a commonly used cortical fixation device. The EndoButton[R] anchors the graft away from the joint, and this has led to concerns about graft micromotion and tunnel widening, which can lead to increased anterior laxity. (41) The EndoButton[R] is a good salvage option in BPTB cases when there is femoral tunnel posterior wall blow-out. (14, 41)
In an effort to minimize the potential complications associated with the placement of hardware, a hardware-free press-fit technique for fixing the femoral bone plug was developed. (42) Advantages of the technique include the ability to obtain MRI without concern for artifact and no hardware to be removed at revision surgery.
Clinical Outcomes of Femoral Fixation Devices for BPTB grafts
In a cohort study examining bony incorporation of bioabsorbable interference screws, Cox and associates (43) found that at a mean of 3 years postoperatively, there was no evidence of tunnel narrowing. Calaxo[R] osteoconductive interference screw (Smith & Nephew Endoscopy, Andover, MA) and Milagro[R] bioabsorbable screw (DePuy Synthes, Warsaw, IN) were used for fixation of the bone plugs on both the femoral and tibial side of the BPTB grafts, and there were no significant differences found between the two screw types in terms of the Knee Injury and Osteoarthritis Outcome Score (KOOS) outcomes, tunnel widening, graft integrity, tibial or femoral bone reactions, or femoral screw degradation. The only difference was that the Calaxo[R] screws in the tibial tunnel were more likely to be graded as degraded or partially degraded compared to the Milagro[R] screws. It should be noted that the Calaxo[R] screw was subsequently removed from the market due to reports of postoperative complications related to the screw degradation properties. Similarly, in a randomized controlled study, Bourke and coworkers (44) showed that although both the Calaxo[R] and Milagro[R] screws demonstrated favorable subjective and objective outcomes at 2 years postoperatively, their use did not lead to bone formation. In fact, both screws were associated with tunnel widening (Fig. 2). In this study, the Calaxo[R] screw resorbed over the course of 6 months, which was faster than the Milagro[R] screw, which was still resorbing at 2 years. Additionally, Milagro[R] screws were associated with a high incidence of intratunnel cyst formation. Both of these high quality clinical studies bring into question some of the most highly touted benefits of bioabsorbable screws, namely the ability to degrade and incorporate into the bone tunnel. More long-term studies will be needed to further examine these early findings.
Maletis and colleagues (45) performed a randomized controlled trial comparing outcomes of ACL reconstruction with BPTB and quadruple HS grafts fixed with bioabsorbable interference screws on both the tibial and femoral sides. This small study showed that at 2 years postoperatively both groups had significant improvements in KT-1000 arthrometer and outcome scores. Although there were some small differences between the two groups, the clinical significance of those findings is not clear. Overall, both grafts performed well with the bioabsorbable interference screw fixation.
The clinical outcomes of interference screws versus other fixation devices was studied by Drogset and associates (46) in a prospective, randomized multicenter study comparing 2-year outcomes of BPTB autograft fixed with metal interference screws and double-looped HS autografts fixed with a bone mulch screw on the femur and WasherLoc (TM) on the tibia. Overall, both groups had similar clinical and subjective outcome scores. However, the fact that double looped-HS group underwent more meniscus surgery in the follow-up period raises some concerns about the fixation, and longer-term studies are needed to determine the cause of this discrepancy. Similarly, Gorschewsky and coworkers (47) showed that bioabsorbable cross-pin fixation was comparable to bioabsorbable interference screw fixation for the less commonly used bone-quadriceps tendon autograft. They showed a trend towards better subjective satisfaction and clinical outcomes, with the cross-pin group having a better IKDC score.
Widuchowski and colleagues (48) have shown that 15 years after femoral press-fit fixation of BPTB autograft, patients have good clinical and self-reported outcomes. In this study, 75% of patients had a normal or near normal IKDC score, and a majority had clinically stable knees on laxity testing. However, 67% of the patients were found to have some degenerative changes, but there was no correlation between the degenerative changes or knee stability and the subjective scores. Pavlik and associates (49) showed that 85% of their 285 patients who had press-fit femoral fixation had good outcomes according to their IKDC scores at a mean follow-up of 35.8 months. The return to sport rate was 69%, and KT-1000 arthrometer showed decent knee stability, with only a 1.91 mm side-to-side difference. Longer-term follow-up and higher quality studies are needed, before this fixation technique can be recommended for broader use.
The available evidence supports the use of interference screw for femoral fixation of the bone plug in BPTB graft ACL reconstruction. The decision on whether the metal or bioabsorbable interference screw is more superior will require further long-term studies. However, the bioabsorbable screws do have some interesting advantages that should be taken into consideration. There continues to be a great deal of emerging data on the other bone plug fixation options in the femur, and high quality long-term studies will help guide the choice of fixation.
Femoral Fixation: Hamstring and Other Soft Tissue Grafts
There are similar options available for soft tissue fixation on the femoral side, and these include interference screws, cross pin, and cortical-based suspensory fixation devices. Similar principles apply to fixation of these grafts with aperture fixation resulting in increased stiffness and reduced motion within the tunnel, which may affect graft healing. An additional consideration with soft tissue grafts is the reduction of graft stretching and slippage with aperture fixation. (14, 20, 21, 41) With this in mind, hybrid techniques have been developed combining interference screws with EndoButton[R] or EndoPearl[R]. (21)
EndoButton[R] cortical fixation has enjoyed great popularity as a fixation device for the femoral side of HS grafts. The EndoButton[R] suspends the graft from the anterolateral cortex of the distal femur and has been shown to have failure loads of up 644 N, while the newer version, the EndoButton[R] CL, was shown to have a failure load of 1345 N, which is higher than the failure load of the interference screw. (21, 50) The most common failure mode was found to be tearing at the suture tendon interface. (21) Despite the great biomechanical results and early clinical outcomes of this fixation, there are some concerns about this device. The fact that the graft is fixed so far away from the joint leads to micromotion and graft elongation within the tunnel during cyclic loading that is referred to as the "bungee effect." This has been implicated in tunnel widening as well as inhibition of tendon-to-bone healing. (41, 51, 52) It is also thought that tunnel widening allows for anteroposterior graft motion creating a "windshield wiper" effect. The causal nature of the "bungee effect" on radiographic bone tunnel enlargement remains theoretical and has been called into question by a cadaveric study performed by Brown and coworkers. (50) The study compared bone-graft motion of suspensory and aperture fixation and found no difference between the two methods of fixation. Additionally, the clinical effect of these observations remains unknown.
The TightRope[R] (Arthrex, Naples, FL) and ToggleLoc [TM] (Biomet, Warsaw, IN) are two other femoral cortical suspensory devices. However, a recent biomechanical study in which they were cyclically loaded showed that they both lengthened greater than the 3-mm threshold for clinical failure that was established. (33) This finding is concerning, and more studies will be needed to determine the clinical relevance of this finding. Additionally, there are currently no clinical studies that evaluate the use of these implants; therefore, the surgeon should carefully evaluate the available data prior to using these devices.
In terms of cross-pin fixation, the TransFix[R] (Arthrex Inc., Naples, FL) and RigidFix[R] (Depuy Mitek, Inc., Raynham, MA) are two examples of this type of fixation used to secure soft tissue grafts. Biomechanical studies have shown that these devices are able to withstand the forces seen during early rehab, and that there was no significant difference in the loads to failure of these devices compared to the EndoButton[R]. (14) Cross-pin fixation results in the graft being fixed closer to the joint line, which again is thought to reduce the graft motion within the tunnel. The Bone Mulch[TM] Screw (Biomet Inc., Warsaw, IN), which is also a suspensory fixation device that provides more of a proximal tunnel toggle fit, was shown to have the best biomechanical fixation characteristics. (53) Kousa and associates (53) tested the fixation of quadruple HS graft using several suspensory femoral fixation devices and found that the Bone Mulch[TM] Screw provided the best biomechanical fixation in terms of load to failure, stiffness, and amount of displacement.
Clinical Outcomes of Femoral Fixation Devices for HS and other Soft Tissue Grafts
In a meta-analysis of level 1 and 2 clinical studies on femoral fixation of soft tissue HS autograft, Colvin and colleagues (54) found a trend towards fewer surgical failures with interference screw fixation. However, no differences were found in the postoperative IKDC outcome scores. In 2012, Han and coworkers (55) also performed a systematic review of the outcomes of intratunnel versus extratunnel fixation of HS autograft. The intratunnel fixation group consisted of the grafts that had been fixed with an interference screw while the extratunnel fixation group consisted of fixation by button, staple, or post fixation. Both groups of patients had comparable knee stability measurements and objective outcome scores at minimum of 2-year follow-up. However, the intratunnel fixation group was able to start full weightbearing and engage in running and jogging earlier than the extratunnel group. Despite this early progression in rehab, the return to sports timing was still similar for the two groups.
Recently, Gifstad and colleagues (56) performed a prospective randomized study looking at the 2-year outcome of femoral fixation of HS grafts with the Bone Mulch[TM] Screw or the EZLoc[TM] (Arthrotec Inc., Warsaw, IN) femoral fixation, which is a cortical based fixation device. No significant differences were found in the clinical findings, knee outcome scores, or muscle strength between the two groups.
Fauno and associates (57) performed a prospective randomized study comparing femoral fixation of HS autografts with the TransFix[R] (Arthrex Inc., Naples, FL) or EndoButton[R] and found that at 1 year postoperatively, there was more tunnel widening in the EndoButton[R] group. Despite the increased tunnel widening there were no significant differences in clinical outcomes. Similarly, in a prospective randomized trial, Rose and coworkers (58) showed that at 1 year postoperatively, there were no differences in knee range of motion, knee stability testing, or outcome scores between a bioabsorbable interference screw (Arthrex, Inc.) and a bioabsorbable transfixation device (TransFix[R], Arthrex, Inc., Naples, FL).
Ma and colleagues (59) compared the outcomes at a mean of 35 months of HS fixation using a bioabsorbable interference screw (ConMed Linvatec, Largo, FL) and EndoButton[R] fixation. They did not find any differences in clinical outcomes when comparing the two fixation devices; however, they did note that there was significant tunnel enlargement in both groups. Additionally, the bioabsorbable screws were still present at 2 to 4 years after surgery.
In an interesting clinical trial, Arneja and associates (60) randomized patients undergoing HS autograft ACL reconstruction with bioabsorbable interference screws fixation of the femoral graft to either EndoPearl[R] (ConMed Linvatec, Largo, FL) augmentation or no augmentation. At up to 18 months postoperatively, they found that the augmentation with the EndoPearl[R] resulted in significantly decreased knee laxity as measured by KT-1000 knee arthrometer.
Overall, these high quality studies have been unable to identify which device provides the best femoral fixation for the soft tissue graft. Hopefully, as longer outcome studies become available, a clearer picture will emerge as to which device is superior. In the meantime, from the available evidence, it appears that the interference screw may provide a small advantage for soft tissue fixation.
Tibial Fixation: Bone-Patella-Tendon-Bone Graft
The tibial fixation is considered to be the weakest link in ACL fixation due to the differences in bone density and the forces experienced by the graft. Similar to the femoral fixation of BPTB grafts, interference screws are also more commonly used for tibial fixation. The procedure can be performed with metal or bioabsorbable screws and both screw types have been shown to have similar biomechanical properties that are able to withstand the forces on the graft during early rehabilitation. Currently, a screw of at least 8 mm to 9 mm in diameter and 20 mm in length is advocated for tibial fixation.
Staple fixation of the bone plug with either a single or double staple has been advocated in the past for tibial fixation, especially in cases of graft tunnel-length mismatch. However, in a biomechanical study comparing double staple fixation versus interference screws, it was found that a 9 mm x 30 mm interference screw had a higher load to failure (758 N versus 588 N) and less bone plug breakage (1% versus 27%) than the double staple fixation. (21, 61)
A screw used as a post linked with a suture can be used a back up to tibial interference screw fixation in cases where there is poor bone quality or if the bone plug is fractured. In some cases, a washer may also be used with the screw. Steiner and coworkers (62) in a cadaveric study using older specimens (age range 48 to 79 years old) found that suture with a post combined with interference screw fixation resulted in a load to failure of 674 N, which was similar to the intact ACL (560 N) in their study. This type of fixation fixes the graft away from the joint and does not provide the same type of compression afforded by the interference screw. (41)
Clinical Outcomes of Tibial Fixation Devices for BPTB Grafts
In a prospective, randomized multicenter study, Kaeding and associates (63) compared metal (titanium cannulated interference screw) and bioabsorbable (Phantom [TM] bioabsorbable polymer interference screw; DePuy, Warsaw, IN) interference screws used for fixation of both the femoral and tibial bone plugs in BPTB autografts and found no clinical differences. At 1-year follow-up, both groups had similar knee range of motion and KT-1000 side-side differences. However, at 2-year follow-up, more patients in the bioabsorbable screw group reported activity levels in the strenuous category compared to those in the metal group. The significance of this is unclear in light of the fact that all other outcomes were comparable.
Similarly, at 7 years postoperatively, Drogset and colleagues (64) found only a minor difference in the outcome of metal versus bioabsorbable interference screw fixation of both the femoral and tibial sides of BPTB grafts. With the exception of a better pivot shift result in the bioabsorbable interference screw group, all other parameters were similar. Given these findings, the investigators recommended against the use of bioabsorbable screws because of the limited advantages over the metal screws. The meta-analysis by Shen and coworkers (65) also failed to show any significant differences between the outcome for knee joint stability and function between the metal interference versus bioabsorbable screw fixation groups. However, they did find that the bioabsorbable screws were associated with significantly more knee joint effusions.
Similar to the findings of bone plug fixation on the femoral side, the available evidence supports the use of interference screw for tibial fixation of the bone plug in BPTB graft ACL reconstruction. The use a metal or bioabsorbable interference screw continues to have no clear evidence supporting the use of one over the other; however, the bioabsorbable screws do have some interesting advantages that should be taken into consideration. More high quality studies will be needed to identify the optimal fixation device for the tibial bone plug in ACL fixation.
Tibial Fixation: Hamstring and Other Soft Tissue Grafts
Similar to bone plug fixation, soft tissue fixation on the tibial side can be achieved with the use of interference screws, staples, a screw and washer, or a screw post and suture.
Interference screws are arguably the most commonly used fixation device for soft tissue grafts. Longer screws are required to adequately fix these grafts, and the concerns about graft laceration remain, especially with metal screws. The Intrafix[R] (DePuy Mitek, Inc., Raynham, MA) device has circumvented this concern by having an outer sheath that protects the graft. This particular device has been shown to have the highest load to failure (1,332 N) and stiffness (223 N/mm) of the tibial soft tissue fixation devices (Table 3). In a study involving six different tibial soft tissue fixation devices, the Intrafix[R] device showed superior strength and resistance to graft slippage. (66) Occasionally, a suture post is used as a second level of fixation. Once the interference screw is placed, the free ends of the sutures from the tibial end of graft are tied around a screw with or without a washer used as post.
A double staple is another fixation option, and the use of the "belt buckle" technique in which the graft strands are folded back on itself over the first staple has been shown to provide a biomechanically stronger construct than a single staple. However, the use of staples has been shown to have a greater displacement, lower failure load, and lower stiffness than other fixation devices such as the interference screw and suture post. (21)
The WasherLoc[TM] (Arthrotek, Biomet Inc., Warsaw, IN), which is a multiple pronged washer and screw, can be used to fix the tibia end of the HS graft and provides fixation through compression of the graft against the cortical bone. This has performed well biomechanically and was shown to have an ultimate load to failure of 905 N and stiffness similar to that achieved by interference screw fixation. (14, 21, 41)
Clinical Outcomes of Tibial Fixation Devices for HS and Other Soft Tissue Grafts
Volpi and associates (67) showed that at 5-year follow-up, bioabsorbable cross-pin fixation of the tibial side of the ACL reconstruction with the RigidFix[R] (DePuy Mitek Inc., Raynham, MA) resulted in similar clinical outcomes when compared to bioabsorbable interference screw fixation of both BPTB and HS autografts. Although each group had only 30 patients, these findings are promising because they show that cross-pin fixation of the tibia can provide a safe and reliable fixation option.
In a randomized controlled study, De Wall and colleagues (68) compared Intrafix[R] and hybrid fixation of HS autografts. They found at a minimum 2-year follow-up, centrally placed Intrafix[R] had the same clinical outcomes as hybrid fixation with a standard eccentrically placed metal interference screw and supplemental staple fixation. More specifically, the groups showed no differences in KT-1000 arthrometer testing, IKDC, Lysholm, or Mohtadi knee outcome scores. Although there were only 113 patients in the study, these results add some clinical validity to the excellent biomechanical testing results of the Intrafix[R] device.
Harilainen and coworkers (69) performed a prospective, randomized study looking at interference screw fixation for BPTB grafts compared to a femoral titanium suspension plate and tibial post fixation of double looped HS graft. At 5-year follow-up, there were no statistically significant differences between the two groups with respect to functional outcomes or clinical laxity testing. Given these similar results, they were unable to determine whether BPTB or HS grafts are superior in ACL reconstruction. However, they were able to show that the tibial post fixation of the HS graft performed similar to the interference screw fixation of the BPTB graft in terms of clinical outcomes.
In a later randomized controlled trial, Harilainen and associates (70) compared four different fixation techniques for HS grafts. This small study had four groups each containing 30 patients and examined the following femoral and tibial fixation combinations: femoral Rigidfix[R] cross pin and Intrafix tibia fixation; femoral Rigidfix[R] and BioScrew[R] (ConMed Linvatec Inc., Largo, FL) tibia; femoral Bioscrew[R] and Intrafix[R] tibia; or femoral and tibial Bioscrew[R]. They were able to show that there were no statistical or clinically relevant differences in outcome at 2-year follow-up for any of the fixation techniques for HS grafts. Additionally, they demonstrated that all four fixation techniques led to improved patient outcomes.
The available evidence suggests that both the interference screw and the Intrafix[R] device provide the best tibial fixation of soft tissue grafts in ACL reconstruction. Currently, fixation with the standard interference screw may hold a slight edge due to its longer track record, but emerging high quality evidence should be able to provide more answers with regard to the best soft tissue fixation device for the tibia. As mentioned earlier, the choice of metal versus bioabsorbable screw remains unclear, and more long-term studies are needed to guide this decision.
Despite all of the devices available for femoral and tibial fixation of bone plugs, interference screws remain the gold standard by which all of the other fixation devices are judged (Table 4). The surgeon should weigh the risks and benefits of using a metal versus a bioabsorbable screw in light of emerging evidence, because the early results have been inconclusive about which device is superior.
There is currently no recognized gold standard for soft tissue fixation, and the surgeon is encouraged to weigh the risks and benefits of using different devices. If aperture fixation is deemed to be important, then one should consider a device such as an interference screw or the Intrafix[R] device.
The future of ACL fixation devices is extremely bright, and new devices are constantly being introduced that promise to enhance bony incorporation, provide excellent initial fixation strength, and degrade within a reasonable amount of time to obviate the need for screw removal in a potential revision surgery.
Kirk A. Campbell, M.D., Christopher Looze, M.D., Joseph A. Bosco, M.D., and Eric J. Strauss, M.D.
Kirk A. Campbell, M.D., Christopher Looze, M.D., Joseph A. Bosco, M.D., and Eric J. Strauss, M.D., Division of Sports Medicine, Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, New York, New York.
Correspondence: Kirk A. Campbell, M.D., Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, 301 East 17th Street, Suite 1400, New York, New York 10003; email@example.com.
None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony.
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Caption: Figure 1 Examples of commonly used ACL fixation devices. (A) From left to right: EndoButton[R], Mulch[TM] Screw, Rigidfix[R], BioScrew[R], RCI screw and SmartScrew[R]. (B) From left to right: WasherLoc [TM], tandem spiked washer, Intrafix[R], BioScrew[R], SoftSilk [TM], and SmartScrew[R]. (Figure 1A reproduced from Kousa P, Jarvinen TLN, Vihavainen M, et al. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction-Part I: Femoral site. Am J Sports Med. 2003 Mar-Apr;31(2):174-81; and Figure 1B reproduced from Kousa P, Jarvinen TLN, Vihavainen M, et al. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction--Part II: Tibial site. Am J Sports Med. 2003 Mar-Apr;31(2):182-8; With permission.)
Caption: Figure 2 Do bioabsorbable screws actually absorb and promote tunnel narrowing? These sagittal CT scans of the same patient show that although the Calaxo[R] screw (A) resorbs by 6 months, there is still persistent tunnel widening up to 2 years later. Similarly, the use of the Milagro[R] screw (B) in another patient also resulted in persistent tunnel widening at 2 years after implantation, and this screw also had much slower reabsorption than the Calaxo[R] screw. (Reproduced from Bourke HE, Salmon LJ, Waller A, et al. Randomized controlled trial of osteoconductive fixation screws for anterior cruciate ligament reconstruction: a comparison of the Calaxo and Milagro screws. Arthroscopy. 2013 Jan;29(1):74-82. With permission.)
Table 1 Forces on the ACL and PCL during Regular Activities of Daily Living Activities ACL (N) PCL (N) Level Walking 169 352 Ascending stairs 67 641 Descending stairs 445 262 Descending ramp 93 449 Ascending ramp 27 1215 (Adapted from Brand J, Weiler A, Caborn DNM, Brown CH, Johnson DL. Graft fixation in cruciate ligament reconstruction. Am J Sports Med. 2000 Sep-Oct;28(5):761-74. With permission.) Table 2 Ultimate Load to Failure and Stiffness of Grafts Commonly Used in ACL Fixation Graft Selection Ultimate Stiffness Strength to (N/mm) Failure (N) Native ACL 2160 242 Native PCL 1867 Patellar tendon 2977 455 Quadrupled hamstring tendon 4140 807 Quadriceps tendon 2353 326 (Adapted from Brand J, Weiler A, Caborn DNM, Brown CH, Johnson DL. Graft fixation in cruciate ligament reconstruction. Am J Sport Med. 2000 Sep-Oct;28(5):761-74. With perission.) Table 3 Failure Strength and Stiffness of Various Fixation Devices Fixation Ultimate Stiffness Load to (N/m) Failure (N) Patellar tendon Metal interference screw 558 -- Bioabsorbable interference screw 552 -- Soft tissue (Femoral) Bone Mulch[TM] Screw (Biomet, Inc.) 1,112 115 EndoButton[R] (Smith & Nephew Endoscopy) 1,086 79 RigidFix[R] (DePuy Synthe) 868 77 SmartScrew[R] ACL (ConMed Linvatec) 794 96 BioScrew[R] (ConMed Linvatec) 589 66 RCITM Screw (Smith & Nephew Endoscopy) 546 68 Soft tissue (Tibial) Intrafix[R] (DePuy Synthes) 1,332 223 WasherLoc[TM] (Arthrotek) 975 87 Tandem spiked washer (Arthrotek) 769 69 SmartScrew[R] ACL 665 115 BioScrew[R] 612 91 SoftSilk[TM] (Acufex Microsurgical, 471 61 Mansfield, MA) Table 4 Comparison of the Biomechanical Strength, Time to Biological Incorporation, Commonly Used Fixation Device, Graft Site Morbidity and Return to Play of Commonly Used ACL Grafts Biomechanical Property Graft Tensile Load Stiffness (N) (N/mm) Biologic Incorporation Patellar 2,977 620 Bone-to-bone healing tendon (6 wks) autograft Quadruple 4,090 776 Soft-tissue healing hamstring (8-12 wks) Patellar Similar to Similar to Bone-to-bone tendon patellar patellar tendon healing, slow allograft tendon autograft incorporation autograft (> 6 mo) Quadriceps 2,352 463 Bone-to-bone healing tendon and soft-tissue (6-12 wks) Biomechanical Property Graft Method of Graft Site Outcomes/Return Fixation Morbidity to Play (months) Patellar Interference Anterior knee 4-6 tendon screw pain, larger autograft incision Quadruple Variable Hamstring Increased laxity, 6 hamstring weakness Patellar Interference None > 6 tendon screw allograft Quadriceps Variable Similar to Limited data tendon patellar tendon autograft (Adapted from: West RV, Harner CD. Graft selection cruciate ligament reconstruction. J AmAcad Surg. 2005 May-Jun; 13 (3):197-207.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Author:||Campbell, Kirk A.; Looze, Christopher; Bosco, Joseph A.; Strauss, Eric J.|
|Publication:||Bulletin of the NYU Hospital for Joint Diseases|
|Date:||Jan 1, 2016|
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