Allograft Reconstruction of ACL-Deficient Knee Treatment & Management

Updated: Dec 24, 2015
  • Author: Bart Eastwood, DO; Chief Editor: Thomas M DeBerardino, MD  more...
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Treatment

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-patella 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. [17]

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 in which bone may be deficient.

Shino [18] and Noyes independently reported good results using allografts in the 1980s, as did Yoldas [19] and Lawhorn in 2003. [20] So, why is the allograft 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, 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 [21, 22, 23] :

  • 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.

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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. [24] Potential 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. This decreases the risk of viral transmission. [25, 26]

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. [27, 28, 29]

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. [30] Later, this was lowered to approximately 1 in 10-20 million based on unpublished calculations by tissue bank workers. Moreover, no cases of transmitted viral disorders have been documented since the advent of the laboratory standards established more than a decade ago.

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Surgical Therapy

A major advantage of allografts is that there are a greater variety of tissues available for reconstruction. Bone-patellar tendon-bone has been used most commonly, and although some advocate its use in primary cases, most are used in revisions. Its popularity 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 due to its size, length, relative ease of insertion, and accommodation to being split into 2 bundles as part of an increasing trend for PCL reconstruction. Hamstring, tensor fascia lata, and other tissues, such as anterior and posterior tibial tendons, have also been used with varying success. [31] Rene Verdonk of Belgium has reported good success in revisions with these tibial tendons with up to an 8-year follow-up.

After proper thawing or rehydration and implantation, the incorporation of both autograft and allograft follows a similar sequence. The original structure acts as a scaffold for revascularization, cell repopulation, and remodeling. [32] However, 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. [33]

Once remodeling is complete, implanted allografts appear histologically similar to native ACL. However, this does not necessarily translate into strength or stability. Shino [18] showed histologic maturity at 18 months, whereas Arnozky [34] showed dog allograft histologically resembling normal ACLs at 1 year. Using a goat model, Drez [35] and Jackson [23, 27] independently showed similarities with native ACL at 26 weeks. Although it is now understood that the goat model is not applicable to humans regarding time of incorporation, Drez showed the maximum load-to-failure of allografts to be 43% of the native ACL, and Jackson showed this failure to be 27% of native ACL versus 62% for autografts.

Future and controversies

The risks of disease transmission would seem to have become infinitely small, but, as evidenced by fatal infections noted already, this risk has not been reduced to zero. It is imperative that the surgeon constantly monitors the source of his or her grafts and has a very specific protocol of response in the face of an adverse surgical outcome when infection is a possible diagnosis. [36]

With a supply of safe graft materials, other than a national graft shortage or insurers or the hospital denying coverage for the additional costs, strength and long-term results become the main concern.

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.

Admittedly far from ideal, allografts offer an off-the-shelf material with a relatively good record. Although prospective long-term results are unknown, 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 autograft materials without the problems associated with patella tendon grafts. For revisions and situations in which no autograft material is available, it offers hope where none might otherwise exist.

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Complications

Infection following any surgical procedure is certainly one of the accepted, yet feared, complications. However, significant publicity has surrounded three infections and subsequent deaths following orthopedic allograft transplants. [37]

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

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