Biologics in Orthopedic Surgery: Ligament Reconstruction in the Knee

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.