Anterior cruciate ligament reconstruction: A literature review of the anatomy, biomechanics, surgical considerations, and clinical outcomes

Anterior Cruciate Ligament Reconstruction: A Literature Review of the Anatomy, Biomechanics, Surgical Considerations, and Clinical Outcomes Leslie S. Beasley, MD,* Daniel E. Weiland, MD,* Armando F. Vidal, MD,* Anikar Chhabra, MD, MS,* Andrea S. Herzka, MD,* Matthew T. Feng,† and Robin V. West, MD* Anterior cruciate ligament (ACL) ruptures are some of the most common knee injuries seen by sports medicine physicians. However, given the complex anatomy and function of the ACL, reconstruction of this ligament is anything but straightforward. The last decade has seen much advancement in ACL reconstruction, with an improved knowledge of the biology and biomechanics of graft incorporation, new choices for graft material and graft fixation devices, and more accelerated rehabilitation protocols. Although there are numerous studies in the literature on ACL reconstruction, there is yet to be a consensus among surgeons on the “best” graft choice and the “optimal” fixation device. This is generally attributed to the small sample size in most studies, which prohibits a definite conclusion of superiority of one technique over another. Additionally, it is difficult to directly compare the results from one study to another because there is tremendous heterogeneity between studies. This review is intended to examine the anatomy, biomechanics, surgical considerations, and clinical outcomes after ACL reconstruction that have been highlighted in the literature during the past 10 years. Oper Tech Orthop 15:5-19 © 2005 Elsevier Inc. All rights reserved. KEYWORDS ACL reconstruction, ACL anatomy, ACL biomechanics, ACL outcomes, knee ligament, review

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he integrity of the anterior cruciate ligament (ACL) is important to athletes who participate in running, cutting, and jumping sports. While some athletes can continue to participate in their sport after an ACL injury, many require reconstruction. Although an improvement in function can be achieved by current techniques of ACL reconstruction, the biologic and physiologic characteristics of the normal ACL are not fully restored. There are many surgical considerations when planning ACL reconstruction. Currently, most ACL reconstructions are performed with bone–patellar tendon– bone (BPTB) or hamstrings autograft. However, there has been recent interest in the use of allograft tissue given the early reports of good

*Department of Sports Medicine, University of Pittsburgh Medical Center, Pittsburgh, PA. †University of Pittsburgh School of Medicine, Pittsburgh, PA. Address reprint requests to Leslie S. Beasley, MD, University of Pittsburgh Medical Center, Department of Sports Medicine, 3200 South Water Street, Pittsburgh, PA 15203.

1048-6666/05/$-see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1053/j.oto.2004.11.003

outcomes and less donor site morbidity.1-4 In addition to graft choice, other areas under current investigation are graft fixation and graft tensioning. Although our understanding of the biology and biomechanics of graft incorporation have improved, the initial stability of the graft is dependent on the fixation method. Therefore, for the knee to withstand the increased activity associated with modern rehabilitative protocols, initial fixation strength is critical. In this work, we have reviewed the recent literature on graft incorporation, graft choice, and fixation as well as the clinical outcomes after ACL reconstruction that have been reported from 1994 through the present to help clarify the extensive and sometimes confusing data on this topic.

Anatomy and Biomechanics Optimal evaluation and management of the ACL hinges on a thorough understanding of ligament anatomy and function. The complexities of ACL anatomy and physiology have been debated in the literature, and various authors have offered a 5

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ovoid,13,15 or circular.16 The ligament’s posterior border is a convex arc complementing the curvature of the condyle’s articular surface, whereas the anterior border has a less predictable contour. The ligament fans out10,17,18 from its relatively narrow midsubstance to insert on the femur over an average area of 113 to 170 mm2.12,15,16 Multiple studies have identified separate bundles of the ACL.11,12,14,18-20 The more proximal anteromedial (AM) bundle comprises 49% of the femoral footprint, whereas the more distal posterolateral (PL) bundle accounts for the remaining 51%.16 The tibial attachment consists of a wide, oval fossa14 covering a mean area of 136 to 150 mm2.15,16 The fan-shaped tibial attachment has been found to be both larger and stronger than the femoral.12,14 The AM and PL bundles are named after their tibial attachments; the AM is anterior and medial to the PL. An intermediate bundle has also been described and is depicted as being anterolateral.21 Between the femur and tibia, the ACL averages a length of 31 to 38 mm14,15 with a midsubstance width of 10 to 12 mm.14,15,22 The middle third of the ligament is its most narrow portion, having an irregularly circular cross-sectional area of 35 mm2; this is unaffected by knee flexion angle.23 The fanning out of the ligament begins 10 to 12 mm from either insertion14 and results in a tripling of cross-sectional area at the ligament insertion sites.16 Over most of its length, its AM bundle courses anterior to the PL in all positions of flexion and extension, which results in tensioning of the AM with flexion, and the PL with extension.14,19,24 This unique loadsharing pattern helps distinguish the AM and PL bundles of the ACL.16

Microanatomy

Figure 1 (A) Schematic drawing of medial aspect of the lateral femoral condyle illustrating the femoral attachment of the anteromedial (AM) and posterolateral (PL) bundles of the anterior cruciate ligament (ACL) in the sagittal plane. (B) Schematic drawing of the tibial plateau illustrating the tibial insertion of the AM and PL bundles of the ACL in the axial plane.

diverse array of concepts, nomenclature, and reconstructive techniques. In 1938, Herzmark described the cruciate ligaments as vestigial structures,5 whereas Hey Groves and others advanced the cruciates as major knee stabilizers.6-9 Nearly a century later, we have come to agree on the basic anatomy and biomechanics of the ACL.

Gross Anatomy The ACL runs from the posteromedial aspect of the intercondylar notch on the lateral femoral condyle to a triangular space on the tibia between the medial intercondylar eminence and the anterior horns of the menisci (Fig. 1). Grossly, the femoral attachment has been described as planar10 or concave,11-13 and its projection as semicircular,12,14

The ACL is unusual, being intraarticular yet extra-synovial. Deep to the synovial covering lies the paratenon, the thickest and outermost of 3 connective tissue layers that surround the ligament. Beneath this layer lies the endotenon and epitenon, dividing the ligament into subfascicles and fascicles, respectively.25 Collagen fibers from the ligament predictably interweave through transitional zones of fibrocartilage and mineralized fibrocartilage to achieve ligament insertion into bone.26 As the fibrocartilage is avascular, the ACL must take most of its blood supply from surrounding synovial soft tissue and little from adjacent bone. The overlying synovial fold is rich in vessels, receiving branches from the middle geniculate artery and occasional contributions from the lateral inferior geniculate artery to form a vascular plexus. Similarly, branches of the tibial nerve innervate the ACL via the synovial fold.27 Nerve fibers accompany the vascular plexus and send axons to penetrate the ligament. Studies have found Golgilike tension receptors near the ligament ends28 and apparent Golgi tendon organs along the ligament surface.29 Within the deep substance of the ACL, there are a few mechanoreceptors accounting for the ACL’s afferent role in knee proprioception.17,29,30 Most mechanoreceptors are localized to the tibial half of the ligament and consist of slow-adapting Ruffini types, which perceive position within limits of motion, and rapidly-adapting Pacinian corpuscles, which sense motion in

Anterior cruciate ligament reconstruction any position.30 Free nerve-endings are present infrequently but are found exclusively within 5 mm of the femoral insertion.30 The near-absence of pain fibers is consistent with the clinical presentation of the ACL in isolated biomechanical failure before joint distension from blood.

Biomechanics Normal function of the knee relies on a complex interplay between motion and stability. An understanding of the biomechanics of the ACL can only occur in conjunction with that of the entire knee joint. The knee possesses 6 degrees of freedom, and the interaction of the bony femur, tibia, and patella, as well as the ligamentous structures and menisci in these planes allows the knee joint to withstand tremendous forces during athletic activity. Changes in any of these components can alter the biomechanics of the knee joint, greatly increasing the loads and functional demands placed on the remaining structures. The cruciate ligaments of the knee are essential in providing passive restraint to anterior–posterior knee motion. The primary function of the ACL is to prevent anterior translation of the tibia relative to the femur. Other functions of the ACL include resisting internal rotation of the tibia and varus or valgus stress of the tibia in the presence of collateral ligament injury. During the past 10 years, many studies have been performed that further define the biomechanical properties of the ACL. Early scientists searched for an isometric portion in the ACL that could be used to aid in graft placement. Recent studies have shown that fibers in the ACL are recruited depending on the 3-dimensional changes in the joint.31 This concept helps explain how the ACL can fail variably depending on the position of the knee and the direction of the load. Furia and coworkers32 showed that isometric measurements of the ACL varied depending on fiber origin from the femoral attachment. They suggest that clinically, intraoperatively measured elongation of up to 3 mm in reconstructed ACLs is acceptable as long as it recreates the pattern of the native ACL. During the past decade, many experiments have looked at the mechanical properties of the ACL during passive motion versus vigorous activity. Recent evidence has shown that during normal everyday function, the ACL receives small loads that are only 20% of its failure capacity.31,33 In a cadaveric model, strain levels for the AM bundle and PL bundle between 10° and 110° of passive flexion were equal to or less than zero.34 Significant differences in strain measurements were found only at flexion angles approaching 120°,34 which is consistent with the findings of Fleming and coworkers35 who showed that stationary bicycling is a rehabilitative exercise that permits increased muscle activity without subjecting the ACL to higher strain values. The ACL does, however, see significant force under weight-bearing conditions. Using a cadaveric model, Torzilli36 demonstrated an “anterior neutral shift” of the tibia with respect to the femur when a quadriceps force or a compressive load was applied to ACL-deficient knees. Using a porcine model, Li and coworkers37 showed increased anterior tibial

7 translation, internal tibial rotation, and in situ forces on the ACL under compressive axial forces. This concept has been demonstrated in vivo as well. Fleming and coworkers38 implanted transducers in the intact ACL and observed a significant increase in ACL strain as the knee was stressed in weight-bearing conditions. It is postulated that the posterior slope of the tibial plateau plays a significant role in causing the tibia to slide anteriorly under axial pressure. This is compounded by the posterior compression force vector that must be countered by the extensor mechanism of the knee.36,38 There has been increasing interest in reconstructing both the AM and PL bundles of the ACL in a “double-bundle reconstructive technique.” Both the AM bundle and the PL bundle likely have independent roles in providing stability to the knee. Using a robotic manipulator, Sakane and coworkers39 observed that at knee flexion angles of 0° and 45°, the in situ forces of the PL bundle were larger than the AM bundle, and the PL bundle was affected much more by knee flexion throughout the full range of motion. Bach and coworkers40 could only demonstrate a trend toward higher strain in the PL bundle between 15° and 30° of flexion. Both of these studies, however, were limited by their respective cadaveric models. In vivo, there is most likely a complex relationship of force distribution through the separate bundles of the ACL that has not yet been fully elucidated.

Surgical Considerations: Graft Incorporation Integration of the anterior cruciate ligament graft into the bone tunnel is critical for the success of ACL reconstruction. Although little is known about the interosseous healing of graft tissues in bone tunnels, it is generally believed that BPTB grafts integrate more rapidly and predictably than soft tissue tendon grafts. With a BPTB graft, it has been shown that bone remodeling occurs and that the ACL insertion site is reestablished by 6 months after reconstruction.41-43 However, no studies have been able to establish how the site of tendon insertion onto the bone plug integrates and remodels within the tunnel.44,45 In the case of soft tissue grafts, healing begins with predominantly cellular fibrous tissue interspersed along the load axis.41,42,45-49 Connective tissue has been shown to continue to surround the graft for up to 12 months after implantation.41-43 Because it is not possible to track graft incorporation into bone after ACL reconstruction in humans, animal studies have been used to define the histology, biochemistry, and biomechanics of this phenomenon. Recently, several augmentation techniques, including the addition of exogenous growth factors and gene therapy have shown promise in improving the integration of grafts within bone tunnels.

Histology and Biochemistry of Graft Incorporation Various recent histologic studies in animals have contributed to the understanding of the postoperative incorporation of

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8 the tendon graft after ACL reconstruction.45-54 The histologic considerations of postoperative tendon graft incorporation are discussed in detail by Zelle and coworkers (see Zelle et al: Biological considerations of tendon graft incorporation within the bone tunnel, pp 36-42, this issue).

Biomechanics of Graft Incorporation Several studies have compared the pull out strength of boneattached and bone-free grafts in tunnels over time. However, the results of these studies are inconsistent due to variability in graft placement, difficulty achieving proper tunnel orientation in animals, graft failure or stretch before remodeling, and the inability for animals to follow a uniform rehabilitation protocol. Using a rabbit extra-articular graft model, Kawakami and coworkers46 demonstrated no significant differences in pullout strength when comparing bone-attached and bone-free tendon grafts at 2 and 4 weeks. However, Park and coworkers55 found that BPTB grafts had a higher load to failure and increased stiffness when compared with soft tissue grafts at 4 and 8 weeks using an intraarticular graft model. No significant differences were present at 2 and 12 weeks. Tomita and coworkers50 demonstrated greater pullout strength in BPTB grafts at 3 weeks, but these differences normalized at 6 or 12 weeks in a canine model. In each of the above studies, before 6 to 8 weeks, most of the grafts failed at the graft-tunnel sites, but after 8 weeks they failed in the midsubstance of the tendon. This difference demonstrates the progression and stability of the healing grafts at the insertion sites over time. Given the differences between BPTB and soft tissue graft incorporation, alternative methods to increase the biomechanical strength of soft tissue graft incorporation have been described. Greis and coworkers56 have demonstrated recently that increasing both the tendon length within the tunnel and the tendon fill of the tunnel significantly increases force to failure.

Augmentation Techniques for Graft Incorporation Improvements in soft tissue graft healing to bone are critical to allow more aggressive and earlier rehabilitation protocols and an earlier return to sports and activities. Recent studies in animals describing various augmentation techniques show promise. One technique that has been described involves enveloping a soft tissue graft with periosteum. This has been shown in a rabbit model to lead to a significantly improved histologic healing processes and improved biomechanical pullout strength at multiple time points before 12 weeks. Additionally, earlier bony ingrowth was evidenced, whereas the insertion site remained unaffected.57-60 Recently, there has been interest in augmenting tendon to bone healing within the bone tunnel using osteoinductive growth factors. Rodeo and coworkers61 first demonstrated that the addition of recombinant human BMP-2 led to histologic evidence of earlier bone ingrowth as well as a greater attachment strength at earlier time points in soft tissue grafts when compared with controls. In addition, Bone Protein

(Sulzer Biologics, Wheat Ridge, CO), a combination of multiple growth factors purified from noncollagenous protein extract of bovine femurs, increased new bone formation and tensile strength, most notably at 8 weeks.62 The addition of abundant amounts of bone marrow stromal cells to the bone tunnel also has been shown to improve the tendon– bone insertion healing by forming fibrocartilaginous attachments at early time points.63 Gene therapy and tissue engineering may also play a role in soft tissue graft augmentation in the future, as demonstrated by Martinek and coworkers.64 In this study, BMP-2 gene-transferred ACL grafts demonstrated a broad matrix of chondro-osteoid at the tendon bone interface 4 weeks postoperatively, which then progressed to a normalappearing ACL insertion. In addition, the stiffness and ultimate load to failure were enhanced with this technique. It is unclear at this point what role the addition of growth factors, Bone Protein, stromal cells or gene therapy will play in the future of ACL reconstructions; however, these areas currently are under active investigation.

Conclusions As demonstrated by the paucity of studies, all of which are in animal models, very little is really known about soft tissue graft incorporation into bone tunnels in the clinical setting. Varied results are attributed to different animal models, different types of grafts and graft fixation, and lack of uniform postoperative protocols. Understanding the exact mechanism of soft tissue graft healing to bone tunnels will help define what augmentation techniques will be most beneficial, leading to more rapid graft incorporation, accelerated rehabilitation, and to an athlete’s earlier return to sports.

Surgical Considerations: Graft Selection The ideal graft material should reproduce the complex anatomy of the native ACL, provide the same biomechanical properties of the native ACL, permit secure fixation, promote rapid biologic incorporation to allow for accelerated rehabilitation, and minimize donor site morbidity. Although numerous graft alternatives both autogenous and allogenic have allowed athletes to return to their sport, no single graft source meets all of the above criteria.3,65-67 Commonly used autogenous grafts include BPTB, quadrupled hamstring tendon and quadriceps tendon with or without bone. Allograft options include BPTB, hamstring tendon, Achilles tendon, and anterior or posterior tibialis tendon. The orthopaedic surgeon must choose a graft source and fixation technique in an effort to restore knee stability; however, this choice has become complicated by the plethora of literature that supports the use of one graft choice or one fixation device over another. The purpose of this section is to review the scientific and clinical data pertaining to graft selection options for ACL reconstruction.

Patellar Tendon Jones described the use of BPTB graft in ACL reconstruction in 196368; it has since become the “gold standard” for pri-

Anterior cruciate ligament reconstruction mary ACL reconstruction. The BPTB graft has many theoretical advantages, including graft strength, stiffness, and potential for bone-to-bone healing. Noyes and colleagues69 were the first to investigate the structural and mechanical properties of the native ACL in comparison to graft alternatives, including BPTB, single tendon gracilis and semitendinosus, quadriceps, fascia lata, and iliotibial tract. They reported the mean ultimate tensile strength and stiffness of the normal ACL to be 1725 N and 182 mol/L/mm, respectively. The only graft alternative to exceed these normal ACL values was the central third BPTB specimens. The BPTB graft (13.8-mm wide) demonstrated 168% of the ultimate tensile strength and nearly 4 times the stiffness of the native ACL. A similar investigation demonstrated the ultimate tensile strength of a 10-mm central third BPTB graft to be 2977 N, slightly higher than the value of 2900 N, which Noyes had reported for the 13.8-mm graft. This discrepancy was attributed to the clamping design and technical differences between the studies. Woo and coworkers70 identified the significance of specimen age and orientation when trying to determine the tensile properties of graft materials. Younger specimens tested in the anatomical orientation exhibited greater linear stiffness (242 ⫾ 28 N/mm) and ultimate load (2160 ⫾ 157 N). These numbers may more accurately reflect the properties of a normal ACL. Although caution must be used when interpreting the results of cadaveric biomechanical studies, the data to date suggest that the central third BPTB graft’s initial tensile strength and stiffness are comparable to if not greater than the native ACL. The graft strength in vivo following implantation is dependent on multiple factors such as fixation, extent of necrosis, and remodeling. The relative clinical significance of initial graft strength is unclear. Despite high subjective and objective success rates (see Clinical Outcomes section), the use of autogenous BPTB graft is most commonly criticized for its donor site morbidity. Postoperative complications include patellar fracture,71 quadriceps weakness,72,73 and patellar tendonitis.74 Anterior knee pain is the most common postoperative complication, ranging from 5% to 55%.34,75-78 The cause of postoperative anterior knee pain in patients with ACL reconstruction remains unclear. Patellofemoral pain has been documented in 28% of ACL-deficient patients treated nonoperatively79 and as many as 28% of patients treated with hamstring reconstructions.74 Sachs and coworkers73 were among the first to note the association between anterior knee pain and postoperative flexion contracture and quadriceps weakness after ACL reconstruction. In a retrospective analysis, Aglietti and coworkers78 demonstrated that the rate of postoperative patellofemoral pain decreased from 40% to 21% when comparing open repair with postoperative casting to modern arthroscopic technique with early postoperative range of motion. The concept that early range of motion can prevent the postoperative complication of patellofemoral pain is further supported by Shelbourne’s work.80 He compared the results of 602 patients who had undergone BPTB ACL reconstruction regardless of preexisting patellofemoral pain or chondromalacia to the results of 122 younger athletes without

9 prior injury or symptoms. There was no statistically significant difference in the anterior knee pain scores between these 2 groups. Both the operative and control groups reported little or no anterior knee pain during sporting activities (94% and 92%, respectively). The authors conclude that immediate restoration of knee hyperextension after ACL reconstruction prevents scar tissue formation and eliminates anterior knee pain. It remains unclear whether patellar tendon graft harvest can be associated with increased postoperative anterior knee pain. Although some studies comparing hamstring autograft to BPTB autograft techniques have noted, no difference in anterior knee pain, difficulty kneeling, or patellofemoral crepitus between groups,81,82 others noted an increased rate of anterior knee pain, difficulty kneeling or patellofemoral crepitus in the BPTB group.83

Hamstring Tendons Given the associated morbidity associated with BPTB graft ACL reconstruction, many surgeons have turned to autogenous semitendinosus and gracilis tendons as an alternative. The graft as well as the fixation for soft tissue grafts has evolved allowing for improved outcomes and increased popularity. Theoretical advantages to this graft choice include increased graft strength, stiffness, and cross-sectional area, as well as decreased donor site morbidity, and preservation of the extensor mechanism. Theoretical disadvantages of this graft include increased tendon-tunnel healing time and hamstring weakness. The 4-stranded hamstring graft has a superior biomechanical profile. The hamstring graft has evolved from a single strand of either gracilis or semitendinosus to a quadruple loop of both tendons. Single-strand hamstring graft had a documented ultimate tensile load of 838 to 1216 N,69 whereas quadrupled hamstring graft has an ultimate tensile strength of 4108 N,84 nearly 3 times the ultimate strength of a normal ACL. In addition, the stiffness of both quadrupled hamstring and BPTB secured with interference fit screw fixation is comparable.85 Quadrupled hamstring graft has a cross sectional-area of 55 mm2, which exceeds that of the 10 mm BPTB graft, which averages 32.3 mm2.84 Several studies have been designed to compare clinical outcomes with hamstring tendon ACL reconstruction to the “gold standard” BPTB ACL reconstruction.74,81-83,86-88 One meta-analysis comparing these 2 graft choices89 integrated results from 4 controlled trials to determine whether there were differences in outcomes between these 2 groups.74,83,86,87 The authors report that BPTB graft patients, when compared with hamstring autograft patients, have an increased chance of attaining a statistically stable knee, and nearly a 20% increased chance of returning to preinjury level of sports. By aggregating the data from these 4 different studies, statistically significant outcomes between graft groups were noted. Caution must be used when interpreting the results of this meta-analysis, however, as it is important to analyze the technical details of the included studies as these predictably influence outcomes. The results of chronic ACL reconstruction are likely different than treatment of the acute rupture. Similarly, the results of double-stranded ham-

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10 string graft with suspensory type fixation should not be combined with quadruple strand hamstring with interference fixation because the biomechanical differences between these constructs are confounding variables. Corry and coworkers83 were the first group to describe increased laxity with arthrometric examination in female patients who had undergone quadruple hamstring graft ACL reconstruction. This same trend has been demonstrated in several independent investigations.81 In a 2- to 5-year follow-up of quadruple hamstring versus. BPTB graft ACL reconstruction with interference screw fixation, the female hamstring patients had statistically significant increases in side to side KT-1000 testing as well as Lachman and pivot shift examination.81 These differences disappeared between 2 to 5 years. The authors suggest that the between-sex discrepancy noted at 2 years in the hamstring group could be secondary to decreased bone density in female patients. They hypothesize that the myofibroblasts implicated in ligamentization after ACL surgery could contribute to the relative tightening of the hamstring graft noted from 2 to 5 years after surgery. Several authors have since noted this trend toward increased laxity in female patients treated with hamstring autograft.88,90 However, this gender difference is not noted by all authors.82,91 Several studies comparing hamstring autograft to BPTB autograft have demonstrated a trend toward increased laxity or decreased rates of return to sport in the hamstring group.74,88 Most of these studies used suspensory-type fixation for the hamstring autograft, which has inferior biomechanical properties and likely contributes to the laxity seen in the hamstring groups. Two studies using interference screw fixation to secure the hamstring graft demonstrated no difference in subjective outcomes or return to sport.81,82 Decreased isokinetic muscle torque in the hamstrings following hamstring autograft is common at 1 year follow-up, but this weakness seen at 1 year follow-up subsequently diminished with time.82,83,86 A recent prospective randomized clinical trial compared the results of 60 BPTB autograft to 60 quadruple strand hamstring allograft ACL reconstructions.92 At 2-year follow-up no difference was found in visual analog scale, IKDC score, KT1000 side-to-side laxity measurements, the functional knee score for anterior knee pain, muscle strength recovery, or return to sports activity. A statistically significant higher prevalence of kneeling pain, and a larger cross-sectional area of decreased skin sensitivity existed in the BPTB group, whereas a statistically significant increase in prevalence of femoral tunnel widening was found in the hamstrings group. Tunnel widening did not correlate with any other outcome measures. The authors conclude that, overall, the 2 techniques yielded equal results. This study is comprehensive with an excellent scientific study design. The authors report a zero percent complication rate, no graft failures, and 100% patient satisfaction. These results exceed those previously published.

Quadriceps Tendon The quadriceps tendon graft for ACL reconstruction was first described by Marshall in 1979.93 He described the use of

partial thickness quadriceps-prepatellar retinaculum-patellar tendon graft for ACL reconstruction. The use of this graft choice did not gain popularity after its introduction because biomechanical testing revealed that its maximal load to failure was only 14% to 21% that of the normal ACL,69 and clinical outcomes using this technique were marginal.94,95 Subsequently, the use of full-thickness central third quadriceps tendon-bone graft was popularized by Blauth96 in 1984 and Fulkerson97 in 1995. Conflicting data regarding the biomechanical properties of the central-third quadriceps tendon-bone construct exist. One group98 found the quadriceps tendon construct had an impressive cross sectional are of 64.4 mm2 compared with 36.8 mm2 for the BPTB; however, the quadriceps ultimate load to failure was significantly lower than the BPTB (33.5 vs. 53.4 N/mm2). A second group99 reported that the ultimate load to failure of the quadriceps graft to be 1.36 times that of a comparable width patellar tendon graft. There is no clear explanation for the disparity of these 2 group’s results. Few long-term outcome studies have been undertaken to evaluate the central third quadriceps graft. One study demonstrated no difference in outcome between quadriceps tendon and BPTB graft ACL reconstruction at 1 year.100 There are several theoretical advantages, however, to the use of quadriceps graft choice. When harvesting the quadriceps graft the patellar tendon and fat pad are left unviolated. This prevents infrapatellar scarring and the risk of patella baja. In addition, the location of the incision proximal to the superior pole of the patella avoids an injury to the infrapatellar branch of the saphenous nerve, a common nuisance with BPTB graft harvest. Because long-term data for quadriceps tendon graft are scant, most surgeons do not use this graft source for primary ACL reconstruction. It is useful as an alternative autogenous graft when the surgeon does not wish to use allogenic tissue.

Allograft The use of allogenic tissue for primary ACL reconstruction is gaining popularity in the United States. Commonly used allograft sources include: BPTB, hamstrings tendons, anterior tibialis tendon, posterior tibialis tendon, and Achilles tendon with bone block. The use of allograft tissue has several theoretical advantages including reduced surgical time and availability of any graft size specification. Its greatest advantage, however, is the elimination of donor site morbidity. This should result in a less painful and easier recovery for the patient. In addition, because no incision is required for graft harvest, the surgery has an optimal cosmetic outcome. The disadvantages of allograft use include risk for disease transmission,101,102 immunologic reactions, effects of processing and sterilization, slower remodeling and incorporation time,103,104 and increased cost. Of these disadvantages, the risk of disease transmission is most worrisome to most orthopaedic surgeons. The rate of potential disease transmission from allograft tissue has decreased significantly during the past 2 decades. In the late 1980s the HIV transmission rate from allograft was estimated to be less than 1 in 1.7

Anterior cruciate ligament reconstruction million.101 This rate was based on screening with a questionnaire and HIV-1 and P24 antibody testing. Since then, tissue banks have employed polymerase chain reaction (PCR) testing to screen for both HIV and HCV which significantly decreases the window of occult viremia undetectable by assay.102 According to the last published American Association of Tissue Bank Statistics, during the past 5 years more than 2 million musculoskeletal allografts have been used with no documented incident of viral infectious disease transmission caused by an allograft.105 Gamma irradiation is often used as a secondary means for sterilization. Several studies have demonstrated that HIV expression is present after doses of 2.5 and even 5 Mrad of irradiation106,107, the upper limit of most tissue bank protocols. Gamma irradiation of allograft tissue with greater than 2 Mrad has a deleterious effect on its biomechanical properties and is not recommended.108,109 Because virucidal doses of gamma irradiation cannot be administered without deleterious biomechanical effects, its necessity is questionable. Although extremely rare, bacterial disease transmission from infected allograft tissue has been reported. In 2001, a 23 year old man died from overwhelming clostridium sordellii sepsis from an infected allograft.110 In response, the CDC has issued new guidelines for tissue banks. Methods to maintain sterility and screening for both viral and bacterial pathogens in allograft tissue continue to grow, and overall, the risk for disease transmission is extremely low. Because the risk for disease transmission with use of allograft tissue has diminished with modern screening technology, the use of allograft tissue in primary ACL reconstruction has gained popularity. There is an increasing body of literature that supports this practice. Shino and coworkers111 retrospectively reviewed quadriceps strength and anteroposterior laxity in a group of 92 ACL reconstruction patients with successful outcomes at 18 to 36 months following surgery regardless of graft selection. In this review there were 47 allograft and 45 autograft reconstructions. Patients chose which graft they preferred after the risks and benefits were discussed, and there was a tendency for older and less active patients to choose autograft. Regardless of the graft material, the reconstructed knee had increased anterior translation by instrumented drawer testing compared with the healthy knee, but there was no difference between the allograft and the autograft groups. Allograft recipients demonstrated statistically significant increased knee extension strength in comparison to the autograft group. Several authors have compared outcomes of allograft ACL reconstruction to those performed with autograft.112115 Harner and coworkers112 compared 64 patients with fresh-frozen nonirradiated allograft reconstructions to 26 patients with autograft reconstructions at 3- to 5-year follow-up. They found no statistically significant difference in IKDC ratings, laxity testing, or return to sports. The only statistically significant difference detected was in range of motion. Patients receiving an autograft had a higher incidence of limited knee extension (⬎6° side to

11 side difference). Caution must be used in generalizing these results to current practices with allograft ACL reconstruction because the postoperative rehabilitation protocol was less aggressive than what is commonly used today. In addition, the graft selection was determined by the patient’s preference and the enrollment in the study was based on the patient’s willingness to participate in a follow-up evaluation. Peterson and coworkers113 analyzed the long-term subjective and objective outcomes of patients receiving autograft versus allograft BPTB ACL reconstructions. Each group consisted of thirty patients. At 5-year follow-up there was no statistically significant difference in presence of pain, giving way, effusion, Lachman, pivot shift, and instrumented ligamentous testing. There was one graft failure secondary to rupture in each group. There was a trend that did not reach statistical significance toward an increased incidence of positive pivot glide in the allograft group and an increased extension loss in the autograft group. Kustos and coworkers115 performed a retrospective comparison of 53 autograft and 26 allograft ACL reconstructions performed between 1996 and 2002. They found no statistically significant difference in subjective assessment, Lachman, pivot shift, and range of motion. There was a trend toward increased anterior translation with Lachman testing in the autograft group, but this difference did not reach statistical significance. Indelli and coworkers1 recently reported their 3- to 5-year follow-up results on 50 patients who underwent ACL reconstruction with cryopreserved Achilles tendon allograft with bioabsorbable interference screw fixation. Their study group included 5 professional and 8 college-level athletes. The overall outcome was normal or nearly normal in 94% of their patients and 92% returned to their preinjury level of activity. The average KT-1000 side-to-side difference was 2.3 mm and average tibial tunnel widening measured on lateral radiograph was 2.7 mm. These results are comparable to Aglietti’s results in patients undergoing ACL reconstruction with BPTB or hamstring autograft.92 In summary, BPTB autograft is still the most commonly used graft source for primary ACL reconstruction, but hamstring autograft and allograft tissue grafts are becoming increasingly popular. Advances in fixation for soft tissue grafts have provided similar results in hamstring versus BPTB graft reconstructions. Recent data on the use of allograft tissue suggests that an equal result can be expected with this technique. Although animal studies suggest that a soft tissue graft has delayed biologic incorporation when compared with autogenous BPTB grafts, the literature to date does not support the need for a less aggressive rehabilitation protocol for patients who receive soft tissue graft ACL reconstructions. Good-to-excellent results can be expected with any of these graft selections provided the surgeon’s expertise with the selected technique, proper selection of fixation devices, and formal rehabilitation.

12

Surgical Considerations: ACL Graft Fixation and Tensioning Graft Fixation The introduction of accelerated rehabilitation protocols after ACL reconstruction ]has also placed new emphasis on the importance of graft fixation. The increasing activity with modern rehabilitation protocols places stresses on the graft before incorporation is complete. Therefore, initial stability is dependent on the strength of the fixation device rather than the strength of the graft. It has been estimated that a reconstructed ACL needs to withstand up to 454 N of tensile strength for activities of daily living.69,116 There are 2 main concerns with regard to graft fixation in ACL reconstructive surgery. The first issue centers on graft choice. Bone plugs and soft tissue grafts differ in regards to their viscoelastic properties and their fixation options. This translates into very different biomechanical properties for each graft/implant combination. The second issue that needs to be considered is the difference between femoral sided and tibial-sided fixation. The bone density of tibial metaphyseal bone is less than that of the femoral metaphyseal bone within the same individual.117 The significance of this is debated, but it has been postulated that decreased bone density negatively affects the biomechanical properties of interference screw fixation for soft tissue grafts.117,118 Additionally, the orientation of the femoral and tibial tunnels is very different. During weight-bearing work, the tibial tunnel and graft are colinear, potentially transmitting greater direct forces to the tibial bone-graft interface. These issues have led some to refer to tibial fixation as the “weak link” in ACL reconstruction.117 BPTB grafts allow for stable early fixation and early osseous integration via interference screw fixation. Kurosaka and coworkers119 were the first to demonstrate the excellent biomechanical properties of interference screws for BPTB grafts. Subsequently, various biomechanical reports have supported their use for BPTB fixation.83,117,119 Recently, there has been interest in bioabsorbable interference screw fixation and transcondylar fixation devices for BPTB grafts. Bioabsorbable interference screws have been advocated by some because of a reduced risk of graft laceration or bone plug fracture, improved compatibility with magnetic resonance imaging and because they do not compromise future revision surgery to the extent of metal screws.117 Additionally, they have been shown to have comparable biomechanical characteristics when compared with their metal counterparts.120-123 However, screw breakage and drive failure has been reported with their use.124 Recent biomechanical studies of femoral transcondylar fixation devices for BPTB grafts also appear to demonstrate similar biomechanical properties when compared with interference screw fixation.125 Interference screw fixation of BPTB grafts can be affected by screw divergence and gaps between the bone plug and tunnel. In a porcine model, screw divergence greater than 15° has been shown to decrease ultimate tensile loads by as much as 50%.126 Additionally, gap sizes greater than 3 to 4 mm have been shown to negatively affect pullout strength.127 But-

L.S. Beasley et al ler and coworkers127 recommended using a 9-mm interference screw for gaps of 3 to 4 mm and a 7-mm interference screw for gaps of 1 to 2 mm. They found no difference in pullout strength between these 2 screws if the gap was 1 to 2 mm. The clinical significance of this, however, is not clear.117 Reliable fixation of quadrupled hamstring grafts has historically been problematic. Emphasis on aggressive, early rehabilitation has stimulated considerable interest in developing reliable and strong soft tissue fixation devices. Numerous implants for both femoral and tibial soft tissue fixation have appeared in recent years. These devices can generally be broken down into extraarticular (suspensory type) fixation methods or apertural (anatomic) fixation methods. Options for fixation on the femoral side include interference screws (metal and bioabsorbable), transfixation devices such as the Trans-Fix (Arthrex, Naples, FL), Rigid-Fix (Mitek Products, Norwood, MA) or Bone Mulch Screw (Arthrotek, Inc., Warsaw, IN), and suspensory devices such as a suture post or the EndoButton device (Acufex Microsurgical, Inc., Mansfield, MA). Similarly, tibial-sided fixation options include various metal and bioabsorbable interference screws, staples, screw and washer constructs such as the WasherLoc (Arthrotek, Inc., Warsaw, IN), and screws used as a suture post. Additionally, novel implants such as the Intrafix device (Innovasive Devices, Mitek, Westwood, MA) have appeared in recent years. This device involves positioning of a polyethylene sheath concentrically within a quadrupled hamstring graft, which is then expanded by inserting a complimentary polyethylene interference screw. The reported biomechanical characteristics of this type of fixation have been favorable.128 Kousa and colleagues128 recently reviewed the fixation characteristics of 6 femoral and 6 tibial fixation devices in porcine bone. They tested specimens with a single-cycle load-to-failure test as well as a cyclic loading test followed by an additional single-cycle load-to-failure. On the femoral side, they found that the Bone Mulch Screw was superior to other techniques in single-cycle load to failure with a yield load of 1112 N.128 The other tested devices included the EndoButton CL (1086 N), RididFix (868 N), SmartScrew ACL (794 N), BioScrew (589 N), and the RCI screw (546 N). They found that the Bone Mulch Screw had the least residual displacement on cyclic loading and the greatest stiffness (115 N/mm) when compared with the other fixation options. Similarly, other recent studies have demonstrated the biomechanical superiority of transfixation devices and the EndoButton over interference screw fixation for soft tissue grafts on the femur.129 However, it should be noted that both the extensive bone formation that occurs around the Bone Mulch Screw and the implant’s large size raise concern for potential future revisions. On the tibial side, Kousa and coworkers128 found that the Intrafix implant (Innovasive Devices, Inc.) was the strongest in the single-cycle load-to-failure (1332 N) of the devices they tested, followed by the WasherLoc (975 N), tandemspiked washer (769 N), Smartscrew ACL (665 N), Bioscrew (612 N), and Softsilk screw (471 N). Furthermore, the Intrafix device demonstrated the least residual displacement on

Anterior cruciate ligament reconstruction cyclic loading and the greatest overall stiffness (223 N/mm). However, another recent study examining the biomechanical characteristics of the Intrafix device compared with a 35-mm bioabsorbable interference screw in paired human tibia revealed different results. Caborn and coworkers130 found that in human tibia of known bone mineral density, there was no statistically significant difference between these 2 devices. The load to failure of the Intrafix device averaged 796 N, which was significantly lower than previously published results in more dense porcine bone.128 Additionally, the displacement at failure was significantly greater in the Intrafix group when compared with the bioabsorbable screw: 17.3 mm and 10.9 mm respectively. They speculated that the increased slippage might occur as a result of subtle alignment differences between the sheath channels and the hamstring graft strands. Overall, there are numerous devices available commercially for femoral and tibial sided fixation. Biomechanical studies on this topic need to be examined cautiously. Study methods vary considerably, which makes comparing implants difficult. Additionally, animal bone which is commonly used for these studies, typically has a higher and more consistent bone mineral density than human bone.117 As noted, these bone mineral differences may influence fixation strength, especially for interference screw and Intrafix-type devices.117,118 Biomechanical studies also reflect the status of the fixation at the time of implantation and do not address biological issues or practical issues such as ease of implantation, postoperative morbidity, implant cost, or ease of revision surgery. More importantly, convincing evidence of a link between fixation type and clinical outcome has not been established.117 Choices regarding fixation options should be based on sound biomechanical and biologic principles as well as the surgeon’s comfort and experience using the implant.

Graft Tensioning Graft tension is an important factor in achieving a successful outcome after ACL reconstruction. Initial graft tension can significantly affect joint kinematics, graft survivorship, and can possibly predispose a knee to early degeneration. However, the optimal initial graft tension and fixation angle is not known, and remains the subject of considerable debate. Initial animal studies examining the effect of graft tension on knee stability and cartilage degeneration implied that overtensioning ACL grafts may predispose the knee to early degeneration without offering a significant increase in anteroposterior stability.119 High initial graft tension may cause the tibia to sublux posteriorly. This subluxation places tension on the PCL, causing the knee to be “locked-in,”131 which can result in impaired kinematics and cartilage deterioration. Recent clinical studies have demonstrated conflicting data on this issue. Yasuda and coworkers132 performed a prospective, randomized clinical trial of a 20 N, 40 N, or 80 N initial graft tension applied to doubled autogenous hamstring tendon grafts at the time of ACL reconstruction. They discovered a significant decrease in postoperative translation as a func-

13 tion of initial graft tension at 2 years postreconstruction. Anterior–posterior translation was 2.1, 1.4, and 0.6 mm, respectively, for their groups. Conversely, van Kampen and coworkers133 and Yoshiya and coworkers134 performed similar randomized studies using BPTB grafts. They detected no significant differences in postoperative stability with differing amounts of tension. This issue is further complicated by the fact that the behavior of a graft in response to increased tension is very dependant on the mechanical properties of the graft material and the fixation device. Karchin and coworkers135 recently demonstrated that the stiffness within a graft construct greatly influences the amount of tension required to restore stability to an ACL-deficient knee. They tested constructs of varying stiffness and discovered that high-stiffness constructs required substantially less tension than low-stiffness constructs to restore normal AP stability. They felt the highstiffness constructs better matched the biomechanical properties of the intact ACL and were less likely to predispose the knee to the complications of overconstraint.135 There is no consensus regarding initial graft tension. Excessively low graft tension may result in residual laxity and, conversely, excessive tension may overconstrain the knee and lead to impaired motion and cartilage damage. Most likely, the issue of ideal tension is multifactorial. Load-deformation characteristics of graft constructs are very dependant on the mechanical properties of the grafts as well as the devices used for fixation.136 The optimal initial graft tension is therefore both graft and fixation method dependent. Consequently, consensus recommendations regarding optimal graft tension cannot be made at this time.

Clinical Outcomes In a 10-year review of the literature on ACL reconstructions there have been 37 publications in major sports medicine journals reporting on clinical outcomes. The difficulty in critically comparing these outcome studies lies in the lack of uniform methodology of examination and reporting. In addition to the significant variables that exist between studies, from patient demographics to acuity of reconstruction, these studies also differ in terms of graft choice and fixation technique. Outcome variables reported include a variety of both subjective and objective scores. However, given the numerous subjective knee scoring systems and the various objective measurements of knee laxity, few articles are directly comparable in this respect either. Despite the disparity in outcomes reporting, many studies include the IKDC and Lysholm scores, the Tegner activity score, and a percentage of how many patients return to their previous level of activity. Objectively, the most commonly reported measurements were the Lachman and pivot shift tests and the KT-1000 (Medmetric Corp., San Diego, CA). In this comprehensive review of the literature, we have focused on extracting and comparing these outcome measurements to draw some clinically applicable conclusions.

L.S. Beasley et al

14 Table 1 Average Subjective Scores Between Groups Subjective Scores

All groups (37) BPTB (31) Hamstrings (17) Allograft (4)

IKDC (A/B)

Lysholm (ave)

Tegner (ave)

Percent Return to Previous Level of Sport

86% (18) 86.2% (18) 85.6% (11) 94% (2)

91.5 (20) 91.4 (20) 91 (9) 94 (1)

5.8 (17) 5.9 (16) 5.9 (8) 5 (1)

70.7% (12) 70% (12) 69% (6) 96% (2)

Number in parentheses indicates the number of studies reporting this variable.

Overall Results Data were extracted separately from each of these 37 studies.1-4,34,40,62,66,80-83,87,88,137-158 Specifically noted were the number of patients in each study, time to follow-up, percent of patients available at follow-up, and the average age of the patient population. Subjective patient satisfaction data, including IKDC, Lysholm, and Tegner scores and return to previous level of play, were noted. Objective measures extracted were the results of the Lachman and pivot shift examinations, KT-1000 side-to-side differences in laxity and graft failures. Not all studies included each variable and some studies included incomplete data. For example, one study81 stated that “90% of patients had a normal Lachman examination,” in which case the 90% were included with the normals; however, the remaining 10% were not designated grades II or III specifically, so they were not included. Therefore, the percent total for each measure may equal more than 100%. Additionally, some studies reported a “normal” as the lowest Lachman grade and others considered grade I the lowest possible score followed by grades II and III. Similarly for pivot shift results, some authors used “no pivot” as the lowest score and others used 1⫹ as the lowest score, all followed by 2⫹ and 3⫹. To minimize these discrepancies, results reported here will include “normal” and grade I Lachman together and “normal” and 1⫹ pivot shift results together. The main variable between the studies was the choice of graft tissue used for reconstruction. Fifteen of the 37 studies report on outcomes after BPTB autograft reconstruction, 15 compare hamstrings autograft with BPTB autograft, 3 studies report outcomes after hamstring autograft reconstruction alone, 2 compare autograft versus allograft reconstructions, and 2 report on allograft reconstruction alone. The minimum time to follow-up in these studies was 21 months, most hav-

ing a minimum of 2 year follow-up, and the average age of patients in these 37 studies was 28.3 years old (range of averages 19.4-54.5 years old), as some studies specifically reported on outcomes in an older population. In terms of subjective outcomes (Table 1), 18 studies reported IKDC results. On average 86% of patients rated their overall subjective assessment of their knees as normal (A) or nearly normal (B). There were 20 studies that reported Lysholm scores, averaging 91.5; 17 studies included postoperative Tegner activity scores, averaging 5.8; and 70.7% of patients reportedly returned to their previous level of athletic activity, as indicated by the 12 studies that included this variable. Objective outcomes extracted from the literature were results of the Lachman, pivot shift, and KT-1000 examinations as well as graft failure rates (Table 2). Twenty-two studies reported Lachman grades, however, as indicated above the reporting of the grading was not uniform. Some studies considered grade I as the lowest possible score, and others used “normal” as a grade below grade I. Most authors consider grade I normal; therefore, the “normal” and grade I results were combined and reported here together. There were an average of 88.6% normal and grade I knees on Lachman examination, 11.8% grade II and 3.7% grade III. For the same reasons, normal and 1⫹ pivot shifts were combined and reported together, as many authors reported 1⫹ as the lowest score possible, whereas others reported no pivot shift as their lowest score. Overall, 21 studies included pivot shift testing and an average of 92.6% were reported as normal or 1⫹, 9.0% of knees were graded as 2⫹, and 3.7% were graded as 3⫹. KT-1000 results were given in 25 of the studies. An average of 69.7% of knees demonstrated ⬍3 mm side-to-side difference in laxity, 23.5% of knees demonstrated 3 to 5 mm difference and 8.5% had ⬎5 mm side-to-side difference. The

Table 2 Average Objective Scores Between Groups Objective Scores Lachman 0/I All groups (37) BPTB (31) Hamstrings (17) Allograft (4)

88.6% (22) 89.6% (22) 83% (11) 92.5% (4)

II

0/1ⴙ

2ⴙ

3ⴙ

<3 mm

3-5 mm

>5 mm

Percent Graft Failure

92.6% (21) 92% (21) 89% (9) 96.7% (3)

9.0% 7.5% 10% 3.3%

3.7% 0.5% 5% 0%

69.7% (25) 73% (22) 63% (12) 71.3% (3)

23.5% 20.7% 30% 24.7%

8.5% 8% 11.2% 3.7%

7.3% (13) 8% (10) 8% (7) 0% (1)

Pivot Shift III

11.8% 3.7% 10.6% 1% 15% 23%* 6.3% 0%

Number in parentheses indicates the number of studies reporting this variable.

KT-1000

Anterior cruciate ligament reconstruction overall graft failure rate, including traumatic re-ruptures was extracted from 13 studies and averaged 7.3%.

BPTB Studies There were 31 studies 2,4,34,40,62,66,80-83,87,88,138-142,144-146,148-157 reporting on the clinical outcomes after BPTB autograft ACL reconstructions (this includes the 16 studies on outcomes of BPTB alone and the comparison studies including a BPTB arm). In this group, the average patient age was 28.3 years old. Subjectively, of the 18 studies that reported IKDC scoring, an average of 86.2% reported a normal (A) or nearly normal (B) knee; of the 20 studies who reported Lysholm results, the average score was 91.4; and of the 16 studies who reported postoperative Tegner activity scores, the average was 5.9. Twelve studies reported return that an average of 70% of athletes returned to their previous level of play. In terms of objective measurements, 22 studies reported results of the Lachman examination with 89.6% being normal or grade I, 10.6% being grade II and 1% being grade III. Twenty-one studies reported on pivot shift grading. 92% were reported as grade 0 or 1⫹, 7.5% as grade 2⫹ and 0.5% as 3⫹. KT-1000 results were reported in 22 of the studies with 73% of knees having less than 3 mm of side-to-side difference, 20.7% having 3 to 5 mm difference and 8% having greater than 5 mm difference. The overall graft failure rate was reported in 10 studies and averaged 8%.

Hamstrings Studies There were 17 studies62,81-83,87,88,137,140,141,143,145-148,152,157,158 that reported the outcomes after ACL reconstruction with hamstrings tendon autograft, 12 of which used quadrupled hamstrings and 5 which used doubled hamstrings tendons (this includes the 4 studies on hamstrings alone and the 13 studies comparing hamstrings autograft reconstruction to BPTB). Subjectively, 11 studied reported and average of 85.6% normal (A) and nearly normal (B) knee results on the IKDC score, 9 studies reported a Lysholm score with an average of 91 and 8 studies reported Tegener postoperative activity scores with an average of 5.9. An average of 69% of patients returned to their previous level of athletic activity according to 6 studies. Objectively, of the 7 studies which reported Lachman scores, an average of 83% of knees were reported normal or grade I, 15% were reported as grade II and one outlying study141 reported 23% of their patients had a grade III Lachman. There were 9 hamstrings studies that reported pivot shift test results. 89% were reported as grade 0 or 1⫹, 10% grade 2⫹ and 5% grade 3⫹. Twelve studies reported KT-1000 results. 63% demonstrated less than 3-mm side-to-side difference, 30% reported a 3- to 5-mm side-to-side difference, and 11.2% reported a greater than 5-mm difference. In the 7 studies that reported graft failure rates, the average was 8%.

Allograft Studies Four studies1-4 reported on the outcomes after allograft ACL reconstructions (2 were studies on allograft alone and 2 compared allograft to autograft reconstruction). The average age of

15 patients undergoing allograft reconstruction was 32.9 years old. Subjectively, 1 study reported and average Lysholm score of 94, 1 reported postoperative Tegner scores averaging 5, and 2 studies reported IKDC scores, with an average of 94% of patients reporting a normal (A) or nearly normal (B) knee. Two studies included information on return to play, and 96% of patients reportedly returned to the same level of preinjury activity. In terms of objective measurements in the allograft group, all 4 studies reported Lachman results with an average of 92.5% of patients being normal or grade I, 6.3% being grade II, and no patients with a grade III knee. Three studies reported pivot shift and KT-1000 results. 96.7% of patients had 0 or 1⫹ pivot shift scores, 3.3% had 2⫹ scores and no patients had 3⫹ knees. An average of 71.3% of patients had side-to-side laxity differences of less than 3 mm, 24.7% had differences of 3 to 5 mm and 3.7% had differences ⬎5 mm. Only one study reported on overall graft failure, and they reported 0%.

Conclusions Freedman and coworkers159 recently published a meta-analysis of arthroscopic ACL reconstructions using BPTB versus hamstrings autograft from 1966 to 2000. They found that significantly more patients reconstructed with BPTB had ⬍3 mm side-to-side difference on KT-1000 testing, and had a lower rate of graft failure, however they had a higher rate of anterior knee pain relative to the hamstrings group. They concluded that the BPTB group had an overall greater patient satisfaction and better static knee stability than the hamstrings group. Our results from a review of the literature from the past 10 years yielded similar results in terms of knee stability. Overall, patients in our review report similar subjective scores regardless of reconstructive technique, with an average of 85 to 95% of patients reporting they feel that they have a normal or nearly normal knee. However, ACL reconstructions with a BPTB technique show a trend toward an objectively more stable knee, while the hamstrings reconstructions demonstrating slightly lower Lachman, pivot shift and most notably, KT-1000 scores. The clinical significance of an objectively more stable knee is unknown given the fact that many patients with laxity still rate their overall satisfaction and function highly.

Conclusions The last decade has solidified our understanding of the anatomy and function of the ACL with respect to the kinematics of the knee. Additionally, we have continued to study and learn more about ACL graft incorporation and are seeking ways to augment this critical process. Finally, we are continuing to sort out the best graft choice. Given the many good choices, this judgment may be case dependent and patient factors like age, functional demands and preexisting anterior knee pain may play a key role in decision making. Similarly, the optimal fixation device has yet to be determined and may also depend on the graft type used, the patient’s demands, age, and bone quality. Current studies suggest that several suitable implant options are available and the choice should be based on sound biomechanical and bio-

16 logic principles as well as the surgeon’s comfort with use of the implant. Current techniques of ACL reconstruction have been relatively successful at restoring stability to the ACL- deficient knee; however, there appears to be room for improvement. The question begs, can we do better? Can we improve on the 10% of patients with a suboptimal outcome? And what new techniques will the next decade bring? We can anticipate seeing improvement in graft incorporation, possibly with growth factor or gene therapy augmentation, as well as stronger and more reliable graft fixation devices. Additionally, reconstructive techniques have continued to evolve, and there are some who advocate the reconstruction of both the anteromedial and posterolateral bundles of the ACL graft. Early reports suggest that this technique may lead to better rotational stability than traditional single bundle reconstructions. In conclusion, although we have come a long way in the past decade, I think that with the continued research in this area, the future may hold even better outcomes for patients after ACL reconstruction.

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