Anterior Cruciate Ligament Reconstruction: General Considerations

Natalie A. Squires MD

Robin V. West MD

Christopher D. Harner MD

Introduction

The anterior cruciate ligament (ACL) is the primary restraint to anterior translation of the tibia and is the secondary restraint to tibial rotation, varus, and valgus stress.¹^,² ACL injury alters the kinematics and the stability of the knee. Acute tears may be associated with meniscal tears in 42% to 77% of cases,³^,⁴^,⁵^,⁶^,⁷^,⁸ and chondral injuries in 20% to 23% of the cases,³^,⁵ resulting in the “syndrome of the ACL deficient knee.” Even an isolated ACL tear can result in subsequent meniscal or cartilage injury, leading to early degenerative changes. Age and activity level play an important role in the natural history of the ACL deficient knee. Older patients tend to be more sedentary, and a lower activity level places less stress on the knee; while younger, active patients place more stress on the knee. Chronic instability is common in ACL deficient knees. Marzo et al. found that many patients who were treated nonsurgically with an ACL injury had recurrent instability. Seventy-six percent of patients with an “unacceptable” result reported instability, and 45% of patients with an “acceptable” result had instability.⁹

Altered mechanics puts the knee at risk for degenerative changes. The focus of this chapter is on the general considerations of anterior cruciate ligament reconstruction surgery. ACL reconstruction is 90% successful in restoring knee stability and patient satisfaction.¹⁰ Reconstruction, however, is not necessary in all patients who sustain an injury. The indications for surgery are not absolute. The most commonly accepted indications include a patient’s inability to participate or function in his or her chosen athletic field, instability that affects activities of daily living, and an associated repairable meniscal injury or a multiple ligament knee injury with instability.

Although the relative indications may be widely agreed upon, there is little consensus on which reconstruction technique provides the most stable and functional outcome. Ideally the reconstruction should closely reproduce the ACL anatomy and biomechanical properties, and the graft should have structural properties similar to the ACL.

Anatomy

The ligament’s cross sectional area is 44 mm², with an ultimate tensile load of 2,160 N, a stiffness of 242 N/mm, and a strain of 20% before failure.¹¹^,¹² The forces in the intact ACL range from 100 N during passive knee extension to about 400 N with walking, and up to 1,700 N with cutting and acceleration-deceleration activities.¹⁵^,¹⁶ The ACL experiences loads that exceed its failure capacity only with unusual loading patterns on the knee. The goal of surgery is to restore stability and normal range motion. The success of anterior cruciate reconstruction is influenced by graft type, tunnel placement, graft tension, and fixation. Functional and clinical outcomes are also influenced by the rehabilitation regimen employed.

Graft Selection

Many factors, including the age of the patient, the activity level, the preoperative exam, and coexisting morbidities, should be considered when selecting a graft for an ACL reconstruction. Graft sources can be compared by many different criteria. The criteria that are most commonly used for graft comparison are the biomechanical properties, the biology of the healing, the ease of harvesting the graft, the fixation strength, the morbidity associated with the graft, and the “return to play” guidelines.

Table 34-1 Biomechanical Properties of Various ACL Graft Tissues¹³^,¹⁴

Tissue	Ultimate tensile load (N)	Stiffness (N/mm)	Cross sect area (mm²)	Reference no.
Intact ACL	2,160	242	44	11
BPTB (10 mm)	2,977	620	35	12
Quadruple HS	4,090	776	53	13
Quad tendon (10 mm)	2,352	463	62	14

Many graft options exist, including synthetic ligaments, autograft, and allograft tissue. Autograft options include the
patellar tendon, hamstrings, and quadriceps tendon. Allograft choices consist of the quadriceps tendon, patellar tendon, Achilles tendon, hamstrings, fascia lata, and anterior and posterior tibialis tendons. Patellar tendon autografts are historically the most popular graft choice because of their strength characteristics, ease of harvest, rigid fixation, bone-to-bone healing, and good clinical outcomes. However, donor site morbidity of patellar tendon autografts has led to the investigation and use of alternative graft sources.

Autografts and allografts undergo a similar process of incorporation including graft necrosis, cellular repopulation, revascularization, and collagen remodeling. However, allografts have been shown to have a slower biologic incorporation.¹⁷ Healing of the site of the graft attachment may be responsible for most of the graft strength observed after transplantation. From a biologic standpoint, patellar tendon grafts, compared with soft-tissue grafts, have the advantage of bone-to-bone healing. Bone-to-bone healing is similar to fracture healing and is stronger and faster than soft tissue healing. The graft is usually incorporated into the host bone by 6 weeks during bone-to-bone healing. Soft tissue grafts usually take 8 to 12 weeks to incorporate into the host bone.¹⁸ The biomechanical properties of ACL grafts (Table 34-1) can vary significantly depending on the donor’s age, the size of the graft, and the testing methods.

Ease of Harvest

The goals of graft harvest should be to obtain an adequate graft specimen and to minimize donor site morbidity. The ease of graft harvest is surgeon dependent. Each graft represents a unique set of technical challenges and potential pitfalls that should be thoroughly understood prior to harvest.

Patellar tendon autografts require harvesting a tibial tubercle and a patellar bone plug. Large, deep cuts can increase the risk for a stress fracture in the proximal tibia or patella. Also, the patellar articular cartilage is at risk if the patellar cut is too deep. Trapezoidal bone cuts, instead of triangular ones, reduce the risk of articular cartilage penetration.

The quadriceps tendon is more difficult to harvest than the patellar tendon. The differences in the quadriceps tendon harvesting technique occur because the proximal patella has denser cortical bone than the distal pole, the surface is curved instead of flat, and the proximal patella has close adherence to the suprapatellar pouch. Fulkerson has described a technique to harvest the quadriceps tendon safely and efficiently.¹⁹

Hamstring harvest requires a thorough understanding of the anatomy of the gracilis and semitendinosus insertions. The sartorius is split to expose the underlying tendons. The hamstring tendons can either be left on their insertion or detached from the tibia during the harvest, and a closed or open tendon stripper is then used to retrieve the tendons. Care should be taken to harvest the entire tendon and not to amputate it prematurely.

Morbidity of the Graft

Anterior Knee Pain

Anterior knee pain is a common problem encountered after ACL reconstruction. Symptoms can occur anywhere along the extensor mechanism and typically involve the patellar or quadriceps tendons, patellofemoral joint, or the peripatellar soft tissues. It has been suggested that anterior knee pain may be related to the choice of graft material. The literature remains mixed on this subject, but most studies show a strong tendency for a decrease in anterior knee pain with the use of hamstring autografts compared with patellar tendon autografts.²⁰^,²¹^,²²^,²³^,²⁴^,²⁵ However, no difference regarding anterior knee pain has been shown in the comparison of patellar tendon autografts and allografts.²⁶ It has been suggested that anterior knee pain is related to the loss of motion and poor rehabilitation techniques rather than to graft choice. Sachs et al.²⁷ demonstrated a correlation between the development of patellofemoral symptoms and the presence of a flexion contracture and quadriceps weakness. Shelbourne and Nitz²⁸ noted a decrease in patellofemoral symptoms following an accelerated rehabilitation protocol, which they attributed to early range of motion and restored quadriceps strength. In an ACL reconstruction meta-analysis of 21 studies of patellar tendon autografts and 13 studies of hamstring grafts, there was a 17.4% incidence of anterior knee pain in the patellar group and a 12% incidence in the hamstring group.¹⁰

Quadriceps and Hamstring Strength

Quadriceps and hamstring weakness is another concerning issue following graft harvest. Most studies have shown no significant difference between the various grafts with regards to quadriceps strength. However, Rosenberg et al.²⁹
found a significant quadriceps deficit with isokinetic testing and a significant decrease in the quadriceps cross-sectional area on computed tomography scan when they compared 10 randomly selected patients who had undergone an ACL reconstruction with patellar autograft 12 to 24 months before testing.

When they compared athletes who had undergone either a patellar tendon allograft or autograft ACL reconstruction, Lephart et al.³⁰ found no significant difference between the two groups with regards to thigh circumference, quadriceps strength and power, and functional performance tests. The quadriceps index (involved leg/uninvolved leg at 60 degrees per second) had similar results between the allografts and autografts, with the indexes averaging 90% to 95%. The findings in this study indicate that harvesting the central third of the patellar tendon does not diminish quadriceps strength or functional capacity in highly active patients who have intense rehabilitation.

Carter and Edinger³¹ compared the hamstring and quadriceps isokinetic results 6 months postoperatively in 106 randomly selected patients who had undergone ACL reconstruction using either patellar tendon or hamstring autografts. No statistically significant differences were found with regard to knee extension or flexion strength when evaluating the different graft sources. However, the majority of patients had not achieved adequate strength to safely return to unlimited activities at 6 months postoperatively.

Hamstring strength has been shown to be decreased at deep flexion angles following ACL reconstruction with hamstring autograft. Nakamura et al.³² evaluated hamstring strength at 2 years postoperatively in 74 consecutive patients who had undergone hamstring ACL reconstruction. Similar to other studies, recovery of peak flexion torque was over 90%. However, the recovery was less at 90 degrees knee flexion. These results suggest that the loss of hamstring strength after harvesting may be more prominent at deep flexion angles.

Yasuda et al.³³ isolated the effect of hamstring harvest from morbidity secondary to ACL reconstruction. They were able to demonstrate that tendon harvest did not affect quadriceps function, but hamstring function was affected. Soreness that affected activity level usually resolved at 3 months. Isometric hamstring strength returned by 3 months and isokinetic strength returned within 12 months.

Donor Site Complications

Reported complications with patellar tendon autograft are rare and include patella infera, patella fractures, patellar tendon ruptures, tendonitis, and numbness.³⁴ Numbness can also occur with hamstring autograft, due to injury of the superficial branch of the saphenous nerve.³³^,³⁵

Outcomes and Other Complications

Freedman et al.¹⁰ performed a meta-analysis of 21 studies (1,348 patients) from 1966 to 2000, with a minimum follow up of 24 months, comparing patellar tendon and hamstring tendon autografts. The rate of graft failure was significantly lower in the patellar tendon group (1.9% versus 4.9%). Laxity was measured clinically with the KT-1000 arthrometer and with pivot-shift testing. The results showed that a significantly higher proportion of the patellar tendon group (79%) had a side-to-side difference of less than 3 mm compared to the hamstring group (73.8%). The patellar tendon group had a significantly higher rate of subjects requiring lysis of adhesions (6.3% versus 3.3%), and the patellar group also demonstrated a higher rate of anterior knee pain (17.4% versus 11.5%). The hamstring group had a higher rate of hardware removal following the reconstruction (5.5% versus 3.1%). Infection rates between the two groups were not significantly different (0.5% in the patellar group versus 0.4% in the hamstring group).

Allografts

The fear of disease transmission with allografts has largely been eliminated with the development of modern donor screening and testing procedures. The American Association of Tissue Banks (AATB) first printed its guidelines in 1986 to ensure sterility and quality during allograft processing. Since then, the guidelines have been revised and updated six times, most recently in 1996. A potential cadaveric donor must first pass through a detailed medical, social, and sexual history questionnaire completed by the next of kin or life partner. A physical examination is also performed to detect for hepatosplenomegaly, lymphadenopathy, cutaneous lesions, and other signs of infectious diseases. Laboratory tests, required by the Food and Drug Administration (FDA) and the AATB, are performed. These tests include blood cultures, harvested tissue cultures, antibodies to HIV types 1 and 2, hepatitis B surface antigen, hepatitis C, syphilis, and human T-cell lymphotrophic virus.³⁶^,³⁷^,³⁸^,³⁹

Despite the extensive screening, there is still a window of vulnerability between the infection and the production of antibodies by the donor. To decrease this window, more than 50% of tissue banks use polymerase chain reaction (PCR) testing to directly detect the viral antigens. The use of PCR can dramatically increase the detection of HIV. PCR is very sensitive and can detect as few as 5 to 20 viral DNA copies per sample tested.³⁹

To date, there have been three reported cases of disease transmission from bone-patellar tendon-bone allografts used to reconstruct the ACL.³⁷ The first reported case was HIV in 1985, and there were two reported cases in 1991 of hepatitis C.³⁹

In the past, the high-dose radiation used for sterilization resulted in weakened structural properties of the graft tissue. The alternative use of ethylene oxide sterilization resulted in adverse surgical reactions, most commonly chronic effusions. Ethylene oxide does not alter the mechanical properties of the graft and can effectively remove micro-organisms. However, it leaves behind a chemical residue, which may result in chronic synovitis and subsequent graft failure.³⁸ The current sterilization techniques are gamma radiation
and cryopreservation. Cryopreservation has been shown to have no effect on the structural properties of ligament, tendon, or meniscal tissue.

Gamma radiation is an effective method of sterilization, but doses above 3.0 Mrad are necessary to kill viruses. Unfortunately, this level of radiation has detrimental effects on the graft strength. The difficulties associated with the sterilization of a cleanly procured graft have led to the development of a technique of aseptic harvest and a special cleaning process. This process consists of antibiotic soaks, multiple cultures, and low-dose radiation (<3.0 Mrad). This sterilization technique is the most commonly used process for producing a sterile ACL graft.³⁸ New low-temperature chemical sterilization methods with good tissue penetration have been developed. These appear to be sporicidal and do not seem to adversely affect the biomechanical properties of tissue. Other sterilization techniques, such as supercritical CO₂ and the use of antioxidants in combination with gamma irradiation are being developed.³⁹

Synthetics and Prosthetics

Synthetic ligaments have been shown to have an increased complication rate secondary to synovitis and poor post-op tensile strength when compared to allograft and autograft reconstructions.⁴⁰^,⁴¹^,⁴² Prosthetic implants have also had very limited success. They have been associated with recurrent instability, chronic effusions, and synovitis.

Authors’ Preference

The patellar tendon autograft continues to have the largest reported outcomes in the literature and is the mostly widely used graft source.³⁷ The hamstring tendons are gaining increasing popularity, mainly due to the excellent stiffness and tensile load properties, improved fixation techniques, reduced harvest morbidity, and excellent outcome and patient satisfaction scores. However, there continues to be a reported higher degree of instrumented (KT-1000) tested laxity for hamstring reconstruction and a lower return to preinjury activity levels.²⁰^,²¹^,²²^,²³^,²⁴

Allografts have gained a recent resurgence in the literature. Improved sterilization techniques, along with a wide range of graft sources, have lead to increased safety and availability. The benefits of decreased surgical morbidity and easier rehabilitation must be weighed against the higher costs of the allografts and a slower incorporation period.¹⁶

In our practice, we tend to use patellar tendon autografts in high-demand individuals who participate in cutting, pivoting, or jumping sports. We also favor the patellar tendon autograft in athletes who desire a “quick return to play.” Pre-existing anterior knee pain and certain lifestyle activities (kneeling for work, religion) are relative contraindications to the patellar tendon autograft. Quadruple hamstring autograft is our preference in “lower” demand patients, recreational athletes, younger patients with open growth plates, and for cosmesis. Contraindications to the hamstring autograft include generalized increased ligamentous laxity, competitive sprinters (terminal flexion weakness), and a previous hamstring injury. Our preference is not to use hamstring autografts in “high-demand athletes.” We opt for the patellar tendon allograft in lower-demand patients, older patients who prefer an easier rehabilitation, and in the multiple-ligament injured knee. We prefer not to use allograft in younger patients and do not use synthetic grafts.

Tunnel Placement

Tunnel placement is based on the anatomy of the innate ACL. The ACL is a collection of fibers that attach to the femur and tibia over a broad area. The tibial attachment is adjacent to the anterior horn of the lateral meniscus, and the femoral attachment is in the posterior aspect of the intercondylar notch on the medial wall of the lateral femoral condyle. The fibers of the ACL have been divided into two distinct bundles. The anteromedial bundle (AMB) originates from the proximal aspect of the femoral attachment and inserts on the anteromedial aspect of the tibial attachment. The posterolateral bundle (PLB) inserts onto the posterolateral aspect of the tibial attachment. When the knee is extended, the PLB is tight, while the AMB is moderately lax. With knee flexion, the AMB tightens and the PLB becomes loose.⁴³

Good clinical results following ACL reconstruction have been associated with anatomic tibial and femoral tunnel placement on the lateral radiographs.⁴⁴^,⁴⁵^,⁴⁶^,⁴⁷ Good outcomes have been related to a femoral tunnel placed 60% or more posteriorly along the Blumensaat line and to a tibial tunnel placed 20% to 40% posteriorly along the plateau (behind the Blumensaat line on a lateral in full extension).

Only gold members can continue reading. Log In or Register to continue