Allograft Reconstruction of ACL-Deficient Knee

Background

Multiple techniques are available for reconstruction of the anterior cruciate ligament (ACL). Controversy certainly exists as to which autograft is best and which methods of placement and fixation should be used. An obvious issue is, What is the better choice when both autografts and allografts are available to the surgeon and patient? [12(See Treatment for a discussion of the advantages and disadvantages of autografts and allografts.)

The supremacy of free bone-patellar tendon-bone autograft was briefly challenged in the 1970s and 1980 by proponents of artificial ligaments in the form of Gore-Tex and Proplast. Poor experiences with these nontissue substitutes led surgeons to choose other graft materials, including allografts. This trend was accelerated after Jackson and others developed the technique of arthroscopically assisted ACL reconstructions during the mid-1980s. [3]

Instances exist in which autograft is not available because of multiple reconstructions or combined ligament injuries. In addition, after discussing the advantages and drawbacks of the various graft materials available, patients may choose not to use autograft material. In these situations, other graft sources must be considered.

This article deals with the grafts available and is aimed at providing the reader with an increased confidence in choosing from various materials. It does not cover surgical indications or techniques, which are addressed in other articles in Medscape Drugs & Diseases.

Anatomy

The anatomy of the knee is reflective of its function in ambulation. Knee stability and pain-free range of motion are important in maintaining daily function. Most commonly, overuse, age, and traumatic injuries cause structural damage to the knee that may limit its function. Therefore, a thorough understanding of the anatomy of the knee is essential to properly diagnosing and treating knee pathology.

The ligaments of the knee joint can be divided into the extracapsular ligaments and the intra-articular ligaments. The extracapsular (external) ligaments are as follows:

  • Patellar ligament
  • Medial collateral ligament (MCL)
  • Lateral collateral ligament (LCL)
  • Oblique popliteal ligament
  • Arcuate popliteal ligament

The intra-articular ligaments are as follows:

  • ACL
  • Posterior cruciate ligament (PCL)
  • Posterior meniscofemoral ligament

The ACL attaches posterior to the attachment of the medical meniscus on the anterior intercondylar area of the tibia and passes superior, posterior, and lateral, where it attaches to the posterior part of the medial side of the lateral condyle of the femur.

The PCL arises from the posterior intercondylar area and passes on the medial side of the ACL to attach to the anterior part of the lateral surface of the medial condyle of the femur.

The menisci are wedge-shaped and attach at their ends to the intercondylar area of the tibia. The medial meniscus is C-shaped and firmly adheres to the deep surface of the MCL medially, the ACL anteriorly, and the PCL posteriorly. Because of these attachments, the medial meniscus is less mobile than the lateral meniscus.

For more information about the relevant anatomy, see Knee Joint Anatomy.

Pathophysiology

An image depicting ACL injury can be seen below.

Anterior cruciate ligament reconstruction aims to Anterior cruciate ligament reconstruction aims to reduce instability episodes in an attempt to preserve the meniscus. When meniscal injury has occurred, the knee becomes degenerate with time.

Prognosis

Long-term published clinical studies comparing allografts with autografts are few. Indelicato et al [4and Shelton et al [5showed generally good results in comparison of the tissues. One study showed improved long-term outcomes with autograft over allograft as well as with not smoking and with normal body mass index. [6In another study, an overall trend of fewer patellofemoral symptoms and better range of motion with allografts was noted. Shelton described a trend of increased pivot glide with allograft, which was not statistically significant.

Although happy with their allograft results, all of these authors remained cautious with their outlook, echoing the sentiments of Beynnon that it may take years to see a pattern for overall failure for any graft type. [7]

Beynnon theorized that the initial and 2- to 3-year outcome studies may not accurately assess longer-term results. [8He showed that reestablishing anteroposterior (AP) stability does not predict future graft behavior. Using strain gauges in autograft reconstructions, he showed that strain characteristics established at the time of surgery were a more powerful predictor of long-term results. Grafts that varied most from normal strain patterns in the early postoperative period showed long-term failure. This is disturbing when bench studies have shown that tensioning allografts in the human cadaver knee to fully achieve AP joint stability increased forces in the graft at all angles of flexion.

Authors have long proclaimed dangerous strain and shearing in terminal extension. Of particular note, the good results that Indelicato [4and Shelton [5achieved all predated the era of accelerated rehabilitation protocols popularized by Shelbourne. [9In fact, the allograft protocols included limited arcs and crutch weightbearing for up to 12 weeks.

With all of this in mind and with the knowledge that allografts take longer to remodel and mature, the following question remains: Should there be concern with allografts in general, and specifically in relation to ongoing trends in accelerated rehabilitation? Although Shelbourne has not suggested this, should his autograft axiom be applied? It allows activity based on the status of rehabilitation and not on graft biology.

Alternatively, should these patients be restricted as is commonly done in grafts without bone plugs due to fixation concern? This question is especially important with the potential earlier aggressive rehabilitation and return to activity that allografts allow by virtue of their decreased morbidity as compared with autografts. [101112]

The type of allograft, sterilization process, and patient characteristics also may have some effect on outcomes. Irradiated grafts in some studies show higher failure rates. [1314Allograft use in younger and more athletic populations have also shown higher failure rates in studies. [15]

In a study comparing autografts with allografts in 99 young patients with a symptomatic ACL-deficient knee (100 knees) who were followed up for a minimum of 10 years, more than 80% of all of the grafts were intact and had maintained stability at follow-up; however, the failure rate was three times higher in the allograft group than in the autograft group. [16]

In a meta-analysis and systematic review of prospective studies evaluating irradiated allograft against autograft for ACL reconstruction, Wang et al found that the former was inferior to the latter with respect to knee stability and subjective evaluation but that the two types of graft did not differ significantly with repsect to function and complications. [17 However, only a limited number of randomized controlled trials were included in the analysis, and more such studies, with longer follow-up periods, would be required to define failure rates more accurately.


Approach Considerations

Certainly, the use of autografts for reconstruction of an anterior cruciate ligament (ACL)-deficient knee presents some disadvantages. One is the need to add an incision to sacrifice important tissue. The other is the imposition of iatrogenic hardships, including patellofemoral symptoms, especially with bone–patellar tendon–bone grafts that can hinder rehabilitation and can contribute to range-of-motion loss, arthrofibrosis, and patella baja.

Reported cases of patellar tendon rupture and patellar fracture also cause concern. For hamstring constructs, two-strand grafts are neither as strong nor as stiff as desired, and four-strand grafts can affect knee-flexion torque when both semitendinosus and gracilis are harvested. [18]

Overall, concern with soft-tissue fixation continues to be a challenge, though advances are being made in this regard. Allografts would appear to be a rational choice. Their benefits include the sparing of autogenous tissue and the morbidity associated with their harvest, small incisions, shorter surgical times, and a larger choice of tissue types and sizes. This is especially important in revision cases where bone may be deficient.

Shino et al [19and Noyes independently reported good results using allografts in the 1980s, as did Yoldas et al [20and Lawhorn et al [21in 2003. Given these results, it is reasonable to ask why the allograft is not the universal choice.

Offsetting the list of allograft benefits is a litany of potential disadvantages (eg, potential for disease transmission, delayed incorporation, and decreased ultimate strength relative to autograft counterparts). Allografts also add another $1000 or so to the cost of a reconstruction. The actual surgical technique, however, including tunnel placement, tensioning, and fixation methods, should be similar for autografts and allografts.

Therefore, the three major clinical factors to consider in assessing allograft use are as follows [222324:

  • Potential disease transmission
  • Ultimate graft strength
  • Additional cost of the allografts

Whereas no true contraindications seem to apply to the use of allografts, some reports indicate that chronic instabilities tend to do better with autografts.

The 2014 guideline on the management of ACL injuries formulated by the American Association of Orthopaedic Surgeons (AAOS) recommends that practitioners use either autograft or appropriately processed allograft tissue in patients undergoing ACL reconstructions, on the grounds that the measured outcomes are similar, though with the caveat that these results may not be generalizable to all allografts or all patients (eg, young or highly active patients). [25]

Hybrid grafts (a combination of autograft and allograft) may be considered a viable alternative in selected patients. [26]

Procurement and Processing of Allografts

Since 1984, the American Association of Tissue Banks has set the minimum standards for procurement and processing of allogenic tissue. [27Potential donors undergo a series of examinations in addition to the physical examination, which include medical, social, and sexual histories. Any history of unprotected sex or exposure to a communicable disease results in an automatic rejection, as does other diseases.

On physical examination, abnormalities are sought, including signs of infectious disease. Routine blood and tissue cultures are obtained and examined for antibodies associated with HIV-1 and HIV-2, hepatitis, syphilis, and lymphoma. A major concern is the timing of HIV antibody production in an infected individual. This window averages 25 days but can be as long as 6 months.

To decrease the risk of missing unconverted donors, many tissue banks do a polymerized chain reaction (PCR) assay to detect viral antigens. This decreases the window to about 19 days with a confidence level of 95%. It adds approximately $120 to the overall cost of the graft. The assay decreases the risk of viral transmission. [2829]

Grafts can be harvested with aseptic or clean techniques. If a clean technique is used, a secondary sterilization process is needed. Heat or high-dose radiation can be used to kill virus particles, but this alters and weakens the collagen structure. Ethylene oxide, though excellent in removing microorganisms, was the culprit in earlier reports of poor allograft outcomes due to chemical residue resulting in synovitis and graft failures.

The most common technique is that of sterile harvest, antibiotic soaks, low-dose radiation, and storage by either freeze drying, freezing, or the newest technique, cryopreservation, which may cause less damage to the tissue during processing. [303132]

What is the risk of viral transmission, and should it cause concern? In 1989, Buck et al calculated a 1 in 1.5 million chance of HIV transmission in screened donors. [33Later, this was lowered to approximately 1 in 10-20 million on the basis of unpublished calculations by tissue bank workers. Moreover, no cases of transmitted viral disorders have been documented since the advent of the established laboratory standards.

Roberson et al systematically reviewed proprietary processing techniques for allograft tissues in ACL reconstruction and demonstrated no significant differences in patient-reported outcomes or biomechanical properties between the various techniques, with the exception of the Tutoplast process, which had an unacceptably high failure rate (45% at 6 years). [34]

Surgical Therapy

A major advantage of allografts is that more different tissues are available for use in reconstruction. Bone–patellar tendon–bone has been employed most frequently, and although some advocate its use in primary cases, its most common application is in revisions. The popularity of this allograft stems from its two bony attachment sites, which ease fixation.

Achilles tendon is also available, but it is used more commonly in posterior cruciate ligament (PCL) reconstruction because of its size, length, relative ease of insertion, and accommodation to being split into two bundles as part of an increasing trend for PCL reconstruction. Hamstring, tensor fasciae latae, and other tissues (eg, anterior and posterior tibial tendons) have also been used, with varying degrees of success. [35In Belgium, Verdonk has reported good success in revisions with the tibial tendons at a follow-up of up to 8-years.

After proper thawing or rehydration and implantation, the incorporation of both autograft and allograft follows a similar sequence, with the original structure acting as a scaffold for revascularization, cell repopulation, and remodeling. [36However, the timing of events varies, in that the remodeling and maturation process is prolonged by as much as 50% for allografts.

Grafts are weakest during this vascularization and maturation period. This has implications for the stresses that these tissues can withstand in the postoperative period. One study reported a higher graft failure rate with younger patients and with allograft, with a multiplicative effect when combined. [37]

Once remodeling is complete, implanted allografts appear histologically similar to native ACL. However, this histologic similarity does not necessarily translate into comparable strength or stability.

Shino et al [19showed histologic maturity at 18 months, whereas Arnozky et al [38showed dog allograft histologically resembling normal ACLs at 1 year. Using a goat model, Drez et al [39and Jackson et al [2430independently showed similarities with native ACL at 26 weeks. Although it is understood that the goat model is not applicable to humans regarding time of incorporation, Drez et al showed the maximum load-to-failure of allografts to be 43% of native ACL, and Jackson et al showed this failure to be 27% of native ACL vs 62% for autografts.

The risks of disease transmission via allograft would seem to have become infinitely small, but, as evidenced by fatal infections previously noted, this risk has not been reduced to zero. It is imperative that the surgeon constantly monitor the source of his or her grafts and develop a very specific protocol for response in the face of an adverse surgical outcome when infection is a possible diagnosis. [40]

With a supply of safe graft materials, aside from the possibility of a national graft shortage or insurers or of the hospital denying coverage for the additional costs, strength and long-term results become the main concern with allograft use.

The information above indicates the need to protect these grafts from aggressive early rehabilitation. Protection may include limited weightbearing and stresses placed across the joint. However, no data are available to support this protocol, and prospective comparative studies are needed. For primary cases, weighing the risk of outright allograft failure due to tissue weakness against the morbidities of autograft harvest still leaves the surgeon with a difficult decision. No clear answer exists.

Although admittedly far from ideal, allografts offer an off-the-shelf material with a relatively good record. Prospective long-term results are unknown, but many patients have done well clinically with this procedure as a primary reconstruction. However, with improved soft-tissue fixation, tripled semitendinosus without gracilis and quadriceps tendon grafts are becoming more appealing, in that they offer strong graft materials without the problems associated with patella tendon grafts. For revisions and situations in which no autograft material is available, this offers hope where none might otherwise exist.

Complications

Infection following any surgical procedure is certainly one of the accepted, though nonetheless feared, complications. However, significant publicity surrounded three infections and subsequent deaths that occurred after orthopedic allograft transplants. [41]

The strain level that damages grafts and the strain level necessary for graft development are not known at present. Proper graft placement certainly plays a critical role. Specifically with regard to allograft, the hydration status or how well thawed the graft is must be considered. If the graft is not allowed to recover fully from its frozen or freeze-dried state, postoperative tensioning and strain characteristics may drastically change soon after surgery.