Cruciate Ligaments

The cruciate ligaments consist of a highly organized collagen matrix that accounts for approximately three-fourth of their dry weight. The majority of the collagen is of type I (90%), and the remainder is type III (10%). The cruciate ligaments are named for their attachments on the tibia and are essential for knee joint function. They act to stabilize the knee joint and prevent anteroposterior displacement of the tibia on the femur. The presence of numerous sensory endings also implies a proprioceptive function. They receive the majority of their blood supply from the middle geniculate artery.

The ACL originates from the medial surface of the posterior lateral femoral condyle and courses anteriorly, distally, and medially to the tibial attachment, which is a wide depressed area anteriolateral to the medial tibial spine in the intercondylar fossa. The average length of the ligament is 38 mm, and the average width is 11 mm. The ACL is the primary static stabilizer against anterior tibial translation. Biomechanical testing has shown that the ACL provides an average anterior translation restraint of 87.2% of the applied load at 30 degrees flexion and 85.1% at 90 degrees flexion. The ACL consists of two bundles, the anteromedial bundle which is tight during knee flexion, and the posterolateral bundle which is tight during knee extension. The ACL also plays a lesser role in resisting internal rotation. The maximum tensile strength of the ACL is approximately 1725+/− 270N.

The posterior cruciate ligament (PCL) originates from the lateral surface of the posterior medial femoral condyle in the intercondylar notch, courses distally, and attaches to the tibia in a depression, posterior to the intraarticular surface of the tibia and extends distally 1 cm. The PCL has an average length of 38 mm and an average width of 13 mm. The PCL is considered the primary stabilizer of the knee because it is located close to the central axis of the rotation of the joint and is almost twice as strong as the ACL. The PCL has been shown to provide approximately 95% of the total restraint to posterior translation of the tibia on the femur. It consists of an anterolateral and posteromedial bundle. The anterolateral bundle forms the majority of the PCL and is taut during knee flexion, whereas the posteromedial bundle is smaller and is taut during knee extension. Injuries to the PCL are less common than injuries to the ACL and usually result from hyperextension or an anterior blow to a flexed knee. Of significant degenerative changes that involve the medial compartment, 90% of cases have been associated with chronic PCL injuries.

Collateral Ligaments

The medial collateral ligament (MCL or tibial collateral ligament) connects the medial epicondyle of the femur to the medial tibia and serves to stabilize the knee specifically during valgus knee stress. The MCL consists of superficial and deep portions. The superficial MCL, as described by Brantigan and Voshell, consists of parallel and oblique portions. The anterior parallel fibers arise from the sulcus of the medial epicondyle of the femur and consist of heavy, vertically orientated fibers coursing distally to insert on the medial surface of the tibia. This insertion is on average 4.6 cm inferior to the tibial articular surface and is just posterior to the insertion of the pes anserine. The posterior oblique ligament and deep fibers of the MCL run from the medial epicondyle and blend to form the posteromedial knee joint capsule. The superficial MCL functions as the primary restraint against valgus stress, a restraint to external rotation of the tibia, and a weak restraint to anterior tibial translation in ACL-deficient knees. The parallel fibers of the superficial MCL are under tension from full extension to 90 degrees of flexion but become maximally taut at 45–90 degrees of flexion. The deep MCL extends from the femur to the midportion of the peripheral margin of the meniscus and tibia. Anteriorly, the deep MCL is clearly separated from the superficial MCL by a bursa, but posteriorly the layers blend together. The deep MCL also functions as a weak secondary stabilizer against valgus stress.

The lateral collateral ligament (LCL or fibular collateral ligament) serves as restraint during varus stress of the knee. The LCL originates on the lateral epicondyle of the femur just anterior and distal to the origin of the gastrocnemius. It runs beneath the lateral retinaculum to insert on the head of the fibula, where it blends with the insertion of the biceps femoris. In biomechanical studies the LCL provides 55% varus restraint at 5 degrees of knee flexion and 69% restraint at 25 degrees of knee flexion. There is decreased varus resistance from the posterolateral capsule as the knee is flexed, leading to an increase in total varus restraint by the LCL.

Other Ligaments

Another important ligament of the knee is the medial patellofemoral ligament (MPFL), which serves to stabilize the patella. The MPFL connects the medial border of the patella to the femur and prevents lateral translation of the patella. Anteriorly, the transverse ligament (or anterior intermeniscal ligament) of the knee connects the anterior edges of the medial and lateral menisci. The transverse ligament prevents anterior translation of both menisci during knee extension. Collectively, the posterolateral corner (PLC) also plays a role in knee stability. The static stabilizers of the PLC include the LCL, the popliteus tendon, and the popliteofibular ligament. The PLC functions primarily to resist varus force, as well as posterolateral rotation of the tibia, especially when the cruciate ligaments are deficient. Cruciate tears often accompany PLC injuries, as both are strong stabilizers of the knee.

Hamstring Autograft

Hamstring autografts are frequently used by harvesting the patient’s ipsilateral semitendinosus and/or gracilis tendons from their sight of insertion on the tibia. Hamstring tendon grafts are often harvested through the same incision through which the surgeon drills the tibial tunnel for the ACL reconstruction, which decreases donor-site morbidity and improves cosmesis. In a study by Gupta et al., patients treated with a hamstring autograft experienced less postoperative pain for up to 6 hours than those treated with bone patellar tendon bone (BPTB) autografts. However, 6–48 h after operation, patients treated with either hamstring or BPTB autografts showed similar pain on a visual analog scale. Hamstring tendons tend to have less long-term postoperative pain associated with them than other autografts such as BPTB autografts, which have been correlated with kneeling pain. A meta-analysis revealed 17.4% kneeling pain in the patellar tendon group compared with 11.5% kneeling pain in the hamstring group. Numerous studies have compared the strength of various graft types. Hamner et al. demonstrated quadruple hamstring tendon grafts to have greater strength and stiffness than patellar tendon grafts during biomechanical analyses (quadruple hamstring tensile load = 4090 N; stiffness, 776 N/m).

Downsides to the hamstring autograft include graft-harvesting morbidity related to sore hamstrings and subsequent weakened knee flexion. The deficit in knee flexion is thought to be more significant at greater flexion angles. Aune et al. found this knee flexion deficit to be statistically significant at the 6-, 12-, and 24-month follow-up when compared to patellar tendon grafts ( P < .01; mean flexion at 240 degrees/s relative to unaffected side in hamstring group = 80%–90% compared with near 100% in patellar tendon group). Patients who undergo hamstring autograft must often adhere to a longer, more cautious recovery. The reason for this is because the graft lacks the bony component similar to that of the BPTB graft, requiring more time for integration into the native femur and tibia. Rodeo et al. reported that soft tissue–bone grafts such as the hamstring graft incorporate into the subject’s bone within 8–12 weeks, which is about 2–6 weeks longer than grafts containing bone. In addition, Brophy et al. analyzed the Multicenter Orthopaedic Outcome Network (MOON) data from 2002 to 2005 and demonstrated an increased risk of infection postoperatively in hamstring autografts when compared with that in BPTB autografts (odds ratio [OR] = 4.6 [95% confidence interval (CI) = 1.2 to 17.9; P = .026).

Bone Patella Tendon Bone Autograft

BPTB autografts consist of harvesting the patient’s ipsilateral middle third of the patellar tendon with adjacent bone from the patella and tibia. The inclusion of bone plugs in the autograft provides bone-to-bone healing and is thought to have faster incorporation than other soft tissue graft types. For this reason, some argue that the use of BPTB autografts for ACL reconstruction is better suited for young, highly active patients desiring a quick recover to high-intensity sports.

Significant kneeling pain and decreased knee extension due to quadriceps weakness postoperatively are two downsides to the BPTB autograft. Corry et al. showed that at 1-year postoperatively, 55% of patients who received a BPTB autograft experienced kneeling pain compared with 6% of patients who received a hamstring autograft. At 2 years, the number for BPTB autograft patients experiencing kneeling pain dropped to 31% and remained at 6% in hamstring autograft patients. A 21-study meta-analysis revealed 17.4% kneeling pain in the patellar tendon group compared with 11.5% kneeling pain in the hamstring group. Some argue that kneeling pain can be decreased by implementing a more rigorous rehabilitation program. Researchers found less patellofemoral pain with an accelerated rehab, attributing the improvement to early range of motion and quadriceps strengthening. Corry et al. also demonstrated a significant amount of thigh atrophy compared to the contralateral leg with BPTB grafts, indicating a decreased extensor mechanism at 1 year after surgery. This difference, however, was no longer significant at 2-year follow-up. BPTB graft harvesting also introduces the risk of fracture in the proximal tibia and patella. Other complications include damage to the patellar articular cartilage, increased risk of patella tendon rupture, and risk of damage to the saphenous nerve below the patella. Though the BPTB autograft may offer a quicker return to high-intensity activity, pain with kneeling, quadriceps weakness, and harvest morbidity must be considered when using a BPTB autograft.

The choice between the two most popular autografts for ACL reconstruction, BPTB versus hamstrings, remains a debate today. Poehling-Monaghan et al. performed a systematic review of 12 publications investigating ACL graft choice and found no statistically significant differences between the BPTB and hamstring autografts in regard to manual laxity or graft failure rates. Manual laxity was measured via Lachman and pivot-shift tests, and instrumental laxity was measured using a KT-1000 arthrometer. They did find that BPTB grafts were associated with increased kneeling pain as well as an increased frequency of osteoarthritis after 5 years. In terms of clinical outcomes measured by International Knee Documentation Committee (IKDC), Lysolm, and Tegner scores, no significant difference was noted between hamstring and BPTB autografts. In another meta-analysis, Samuelsen et al. found a small increased risk in graft rupture requiring revision in hamstring grafts compared to BPTB grafts (2.84% compared to 2.80% in the BPTB grafts), with a number needed to treat of 235.

Quadriceps Autograft

Although less common, quadriceps autograft has shown to be another viable option for ACL reconstruction, and in recent times, it has been gaining popularity. Slone et al. performed a systematic literature review of 14 studies and found the quadriceps tendon autograft to be a safe, reproducible, and versatile graft in regard to ACL reconstruction. The quadriceps tendon is harvested from the patient’s ipsilateral knee using an anterior approach proximal to the patella. In a study by Lee et al. with patients treated with hamstring autografts versus quadriceps autografts, they were found to have similar knee joint stability and functional outcomes postoperatively. The quadriceps tendon autograft had a better outcome in terms of knee flexion than the hamstring graft. Chen et al. found only mild harvest-site tenderness in a group of 12 patients who received a quadriceps autograft at 18-month follow-up, and Fulkerson et al. reported no quadriceps morbidity in their 28-patient study.

The quadriceps graft can be difficult to harvest as the bone is denser, curved, and in close proximity to the suprapatellar pouch. The lack of long-term quadriceps tendon studies has made it a less popular choice than other autograft options, though it is gaining popularity in both research and usage. It remains clear that more research including direct comparison studies between BPTB, hamstring, and quadriceps autograft tendons for ACL reconstruction is needed to provide a better understanding of outcomes.

Allograft

An alternative to using a patient’s own tissue is allograft from cadaveric tissue. Typical allografts include patellar, hamstring, quadriceps, achilles, or tibialis anterior/posterior tendons from a cadaveric donor. The benefits of allograft for ACL reconstruction include decreased morbidity and postoperative pain because the tissue is not harvested from the patient. The lack of donor-site morbidity may also increase the esthetic outcome for the patient, as there are no additional incisions made for graft harvesting. Allografts are associated with shorter surgery time, smaller incisions, less postoperative pain, and hence a quicker return to activities of daily living. Shino et al. reported good to excellent results in 94% of patients receiving an Achilles or peroneal tendon allograft at a 3-year follow-up interval. Indelicato et al. reported 93% of patients with a Lachman score of grade I or less and 78% having a negative pivot shift receiving a BPTB allograft at 27 months of follow-up. At a 3- to 5-year follow-up, Harner et al. found similar outcomes comparing allograft and BPTB autograft ACL reconstructions. Allografts tend to be used more often in ACL revisions and have provided a useful alternative to autograft options. However, longer follow-up studies have shown some downsides to using allograft for ACL reconstruction. For example, the MOON data analyses by Kaeding et al. indicated a 5.2 times greater risk of retear in allograft reconstructions than that in BPTB autografts (OR = 5.20; 95% CI: 2.60–10.44; P < .01). In addition, other MOON data analyses indicated the use of allograft in young patients as the only predictor for retear in a 2497-patient cohort.

The quality of allograft tissue is variable due to different company and processing standards. Allografts are harvested from cadaveric donors and treated using a variety of techniques including cryopreservation and gamma irradiation for sterilization. The sterilization process may affect the structural stability of the tissue. For example, >3 millirad (mrad) can result in weakening of the graft tissue, which limits sterilization techniques to using <3 mrad. Another concern in allograft tissue is the risk of infection. There have been three reported cases of disease transmission from ACL BPTB allografts, including HIV and Hepatitis C Virus (HCV). Allograft tissue is also slower to heal than autograft tissue and can be associated with an immune response because it is foreign to the patient’s body. Kaeding et al. demonstrated that allografts were associated with a four times higher failure rate than autografts for ACL reconstruction in the pediatric population (age range of 10–19), putting their efficacy into question for younger patients. Though these disadvantages exist, allograft remains a viable option for select ACL reconstructions in modern orthopedic practice, especially in patients older than 40 years, with multiple ligament reconstructions, and revision procedures. Kuechle et al. reported 96% of 28 patients over the age of 40 years who received an ACL allograft (either freeze-dried fascia lata or Achilles tendon) to have a 0–1 Lachman test, KT-1000 arthrometer readings within 5.5 mm of the contralateral knee, and 55% achieved the same or better preoperative activity level measured by IKDC classification.

There have been extensive studies evaluating the best graft choice for ACL reconstruction. In a meta-analysis of 21 studies including 1348 patients, Freedman et al. reported an overall failure rate of 1.9% in BPTB autografts compared with 4.9% in the hamstring autograft group. BPTB autografts remain the most widely used graft type to date. Patellar tendon autografts are recommended especially for younger, competitive athletes looking for a quicker return to sports. The bone-to-bone healing in BPTB may offer an advantage over other graft types in the young, active patient. Soft tissue grafts including the hamstring are also successful and have continued to grow in popularity as they offer high tensile strength, less donor-site morbidity, and consistent patient satisfaction. Allografts are used less frequently yet have proven to be successful in select patients. One study examined patients receiving soft tissue ACL allografts and reported that 79 of 84 patients returned to a desired activity level postoperatively. Allografts are successful in older, less active patients due to their lack of harvest morbidity and easier rehabilitation, with a longer healing time being more accepted by this demographic.

Large multicenter data sets such as the MOON cohort have provided valuable insight into the performance and outcomes of various ACL graft choices in the recent years. The MOON cohort analyses suggest a decreased risk of infection with BPTB autografts (0.3%) compared with hamstring autografts (1.3%) and all types of allografts (1.0%). It also concluded that there is a significantly increased risk of retear with allografts when compared with BPTB autografts. A MOON study of 2683 patients found that 3.2% of BPTB autografts, 4.6% of hamstring autografts, and 6.9% of allografts tore over the course of a 2-year follow-up. Though a higher percentage of hamstring grafts suffered retear than BPTB autografts, this did not reach statistical significance ( P = .12). Ultimately large database studies, such as those using the MOON cohort, provide objective data that surgeons can use when deciding what is the best ACL graft choice for the patient.

Synthetics and Xenograft

To date, synthetic graft and xenograft reconstructions have not had much success; however, they are still an area for further research in ACL graft technology. Legnani et al. provided a review of literature of artificial grafts in 2010, in which they recognized carbon grafts as the first synthetic graft form. The carbon grafts were eventually abandoned due to serious sequelae including early rupture, inflammatory synovitis, and deposition in the liver. Polytetrafluorethylene (PTFE) grafts followed and were implemented in clinical trials. After initial positive results, Paulos et al. demonstrated a 76% complication rate in PTFE grafts. Eventually in 1993 the PTFE graft was removed from the market after reports of complications, including inguinal lymph node involvement. The Dacron polyester graft, introduced in 1989, also reached clinical trials but was removed from the market due to various studies demonstrating rupture, increased knee osteoarthritis, and synovitis. Other synthetics such as the Kennedy ligament augmentation device have shown complication rates up to 63%, including increased infection risk.

A synthetic graft that has shown some promise is the Ligament Advanced Reinforcement System (LARS) ligament. The LARS ligament is composed of polyethylene terephthalate and contains sutures intended to allow growth inside the joint space. Lavoie et al. followed up 47 patients who underwent ACL reconstruction with the LARS ligament and found favorable postoperative results. Patients reported satisfaction with activities of daily living, recreational ability, and postoperative pain levels; however, patients on average did not reach preinjury activity levels. Jia et al. performed a 7-year follow-up in patients with a LARS ligament ACL graft and found an overall complication rate of 2.2%. They found decreased knee laxity measured by KT-1000 arthrometer, improved Lysholm scores, and an 87% return to sport. Given these outcomes, they deemed the LARS ligament ACL reconstruction graft to have a good prognosis. In another study comparing outcomes of artificial ligaments such as the LARS ligament, Jia et al. found the clinical outcomes and complication rates for synthetic grafts to be similar to those of BPTB and hamstring autograft ACL reconstructions.

Xenografts, or grafts from another species used for human implantation, are currently being investigated. A study by Zaffagnini et al. attempted to find a method to reduce the immunological reaction associated with xenograft placement in a different species. They have demonstrated that treatment with alpha-galactosidase can decrease the immune reaction associated with non–self-antigen alpha-Gal found in the porcine ligament. After a 2-year follow-up of three patients receiving the “Z-Lig device” which uses this concept, they observed a subjective improvement in Tegner scores, and one-legged hop tests postoperatively indicated functional return to a high level versus the unoperated knee at 12 and 24 months. Most importantly, blood tests of all three patients showed the absence of major anti-Gal and anti–non-Gal antibodies, demonstrating successful attenuation of the biological immune reaction to the foreign ligament. Further research into producing and testing a viable synthetic or xenograft ligament for ACL reconstruction is necessary before they may be considered a reliable graft option.

Tendon-to-Bone Healing

Tendon-bone healing is paramount to the success of a reconstruction when an autograft or allograft is entirely of soft tissue. For example, the popular hamstring graft undergoes tendon-to-bone healing, which requires the healing of a newly introduced tendinous structure to the femoral and tibial bone tunnels. Cole et al. characterize the tendon-to-bone healing as either a compression or suspension fixation. Compression fixation secures the tendon at its bony insertion point, and suspension fixation uses fixation distant from the insertion site, such as the cortical periosteal or endosteal surface, cancellous, or cortical bone. Many researchers have shown that the native ACL insertion into bone can be broken down into four zones: (1) ligament, (2) unmineralized fibrocartilage, (3) mineralized fibrocartilage, and (4) bone. Genin et al. describe this transition, characterized by the increasing stiffness in collagen-fiber alignment and increased mineralization. In ACL reconstruction, the tendon graft is directly fixed into a bone tunnel creating a direct graft tendon-to-bone interface or bone-to-bone interface for BPTB and quadriceps grafts.

There are three phases of ACL graft soft tissue integration: (1) early, (2) proliferative, and (3) ligamentization. Cuti et al. describe an initial “early” phase, characterized by graft necrosis, hypocellularity, and blood vessel formation in an animal model. This phase typically lasts from weeks 1–4 postoperatively. The graft necrosis stimulates a cytokinetic and cellular response, drawing in neutrophils and macrophages via the release of tumor necrosis factor-a, interleukin 1-b, and interleukin 6. These cells work to eliminate waste and continue to release cytokines to facilitate the inflammatory response. The formation of granulation tissue fills the gap between the bone and the graft and is rich in vascular endothelial growth factor (VEGF) and fibroblast growth factor, promoting revascularization and recruitment of fibroblasts, respectively.

The “proliferative” phase of healing is characterized by increased cellularity during weeks 4–12 and includes the presence of mesenchymal stem cells that help to facilitate graft remodeling. Scheffler et al. described an invasion of myofibroblasts that takes place toward the end of the proliferative phase. The fibroblasts replace granulation tissue and exert tension on the cellular and extracellular matrix, restoring the “ligament-like” tension during the healing process. As the healing process continues, macrophages release transforming growth factor-beta (TGF-β), facilitating the creation of a fibrovascular scar that connects the graft to the native bone. These perpendicular type III collagen fibers are called “Sharpey-like fibers.” These bundles function to resist shear stress placed on the graft during the early phase of rehabilitation. At week four of a rabbit study, Liu et al. noted a dense connective tissue matrix joining the bone to the soft tissue hallucis longus tendon graft, and at 6 weeks, there was a fibroblastic cellularity with longitudinal collagen organization. At 8 weeks postoperatively in rabbit models, Kanazawa et al. found that type III collagen is deposited around the fibroblast-like cells that move toward the center of the healing semitendinosus tendon. Throughout the proliferative phase, bone tunnel remodeling also takes place, characterized by osteoblast activity producing new bone to fill the void of the tunnels.

Many studies have examined the mechanical stability of ACL grafts postoperatively. At weeks 6–8 postoperatively, several studies show that the graft is at its weakest mechanical point. Some suggest that this decreased mechanical stability can be attributed to the disorganized collagen network that is laid down during the healing process. This collagen network starts to become organized late in the healing process, when the graft begins to resemble a native ACL. At this point in rehabilitation, the graft must be loaded carefully to avoid compromise, but appropriate loading is required to ensure the continuation of graft integrity.

The final phase of ACL graft healing involves “ligamentization” of the tendon. At approximately 12 weeks, the graft finally starts to resemble a native ACL in strength. It is important to note, however, that the new ACL graft never truly replicates a native ACL because it has a different histologic and morphologic appearance. Hypercellularity and vascularization during the proliferative phase decreases to levels similar to those of the native ACL by 6–12 months in animal models. In a biochemical study by Marumo et al., longitudinally oriented collagen fibers and spindle-shaped cells were observed at 4–6 months postoperatively. They believe that the ligamentization process can continue for up to 1 year. As Cole et al. noted, the entire healing process plateaus at approximately 6 months postoperatively. A small margin void of bridging collagen or mineralization between tendon and bone can even remain. Suspensory fixation methods can even exhibit granulation tissue and irregular bone without collagen ingrowth between the graft and native bone for up to 6–14 months.

Bone-to-Bone Healing

Compared with soft tissue grafts, ACL grafts containing bone heal by a different mechanism, similar to fracture healing. Grafts that undergo this type of healing include BPTB and quadriceps because they are harvested with a bone plug. Direct contact between the bone tunnels in the tibia/femur with the ACL graft bone plug provides a site for bone-to-bone healing. Once the bone plug and native bone are in contact, the formation of new bone begins. Inflammatory cells, mesenchymal stem cells (MSCs), and chemotactic agents are abundant in the bone-to-bone interface. Granulation tissue forms and macrophages remove cellular debris as the osteoclasts continue to resorb bone. Osteoblasts eventually lay down osteoid, which is then mineralized to form new bone. This process can take anywhere from 6 to 12 months after ACL reconstruction.

Tomita et al. performed a randomized trial observing the intraosseous graft healing between a doubled flexor digitorum superficialis and a BPTB graft in canine ACL reconstructions. At 3 weeks postoperatively from BPTB autografts, they observed the anterior portion of the bone plug and tunnel interface filled with granulation tissue, whereas the posterior bone plug was in direct contact with the bone tunnel wall. On high-power magnification, they noticed few collagen fibers surrounding the intraosseous tendon portion, osteocyte necrosis throughout the bone plug (excluding the superficial layer), and limited change in the tendon-to-bone junction of the graft itself. At 6 weeks, the granulation tissue in the anterior gap remained, the posterior gap demonstrated new bone formation, and Sharpey’s fibers began to bridge the space between the bony wall and the intraosseous tendon portion of the graft. Also, the bone plug of the BPTB graft showed new bone formation and anchoring to the bone tunnel wall. A decrease in cartilaginous cells in the bone-to-tendon interface was also noted. At 12 weeks, more bony integration was seen in the posterior gap, perpendicular collagen fibers were increased in number, and granulation tissue in the anterior gap remained at levels similar to soft tissue ACL grafts. At 3 and 6 months postoperatively, the BPTB graft demonstrated a higher ultimate failure load than the flexor tendon graft, demonstrating its potentially quicker healing potential and early strength.

Can We Improve Graft-to-Bone Healing?

An important aspect in the success of ACL reconstruction is the incorporation of the graft into the native knee during healing. The graft is placed through femoral and tibial bone tunnels during surgery. Poor graft-bone healing can cause early postoperative failure of the reconstruction.

Can We Improve the Biology of the Graft?

Improving the biology of an ACL graft is a topic of interest, particularly in graft remodeling and maturation. Using biologic augmentation to expedite graft maturation would allow for quicker healing and potentially improve clinical outcomes while decreasing the risk of early failure. Orrego et al. not only studied bone tunnel widening as previously mentioned but also investigated graft maturation by assessing signal intensity within the graft, with a low-intensity signal indicating graft maturation. At 6 months of follow-up, there was a significant difference in graft maturation in the platelet concentrate group compared with the control group. Another study injected PRP after surgical portals were closed and compared the MRIs with those of patients who did not receive the injection. They found increased maturation on MRI signal intensity at 4 and 6 months, but no statistical significance at 12 months. Radice et al. applied PRP gel to BPTB autografts and measured maturation by MRI heterogeneity. This study reported a shortened time by 48% (179 days) to reach graft maturation compared with the control (369 days). One could infer that this means potential graft healing in almost half of the time with PRP as an adjunct. Although MRI is a practical method to obtain objective data on graft maturation, there may be some interobserver error. Furthermore, there are several studies that claim there is no difference in graft maturation when evaluated by MRI. Rather than relying on MRI studies to evaluate graft maturation, Sanchez et al. published a study of second-look arthroscopy at 6–24 months after surgery with direct evaluation and histologic analysis of previous ACL grafts with and without PRGFs. They compared graft thickness and apparent tension, which were reviewed by blinded observers, and found that the ACL grafts treated with biologic augmentation rated higher, but this did not reach statistical significance. The histology was evaluated using the Ligament Tissue Maturity Index of Murray et al. and found the grafts with PRGF to have more mature tissue. Interestingly, they noticed a new connective tissue formed around the graft, which occurred more frequently in grafts augmented by PRGF. This highly vascular envelope became denser over time until it was nearly indistinguishable from the original graft. All these patients required a second-look surgery for other intraarticular pathology, which is not part of standard ACL reconstruction postoperative care. Therefore this study had a limited number of patients, and they may not be generalizable to ACL patients without postoperative pathology. A second-look arthroscopic surgery does provide the most direct and objective analysis; therefore this information is important. However, graft maturation has not been directly related to clinical outcomes in any of these studies and therefore may be clinically insignificant.

Undoubtedly, there is still significant controversy on the topic of biologic agent augmentation with ACL reconstruction. The results of clinical outcomes, graft-to-bone healing, and ACL graft biology are inconclusive at this time. However, using these biologic agents has not shown to cause harm; therefore more research is warranted to better define the beneficial qualities of biologic augmentation in ACL reconstruction.

Can We Improve Healing of Partial Tears?

Partial ACL tears represent only about 10%–28% of ACL tear patients, yet there are ongoing discussions in the orthopedic community regarding the treatment of incomplete tears. When compared with other ligaments of the knee, the ACL has poor ability for primary healing, usually requiring intervention to facilitate a full recovery. As described by Dallo et al., the synovial fluid of the knee joint has inhibitory properties preventing fibrin-platelet clot formation and fibroblast recruitment, which results in poor primary healing potential. In an attempt to overcome the poor healing potential of partial ACL tears, the efficacy and viability of biologic augmentation is a topic of growing interest. Though not yet popular in 2018, biologics may offer a potential option for treatment of incomplete ACL tears in the future. Options for augmentation include PRP injection, biologic scaffolds, and stem cell therapy.