Allograft Complications and Risk Factors

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Chapter 71 Allograft Complications and Risk Factors

Chadwick C. Prodromos, Brian T. Joyce

Introduction

Allograft use has been increasing because donor site morbidity is avoided. Although this benefit is well understood, the drawbacks are more complicated and less well understood. The purpose of this chapter is to collect information on potential allograft risk and allograft complications to help surgeons in their risk-benefit analyses. Some of the information contained in this chapter can be found elsewhere in the chapters on stability results (see Chapter 69) and infections (see Chapter 70). Potential allograft complications/risks can be divided into three categories: (1) graft failure or increased laxity and late graft laxity; (2) infection; and (3) disease transmission. The first two also occur with autografts; the third is allograft specific. Potential causes for increased allograft laxity are: (1) radiation sterilization; (2) nonradiation sterilization; (3) freezing; (4) increased donor age; and (5) increased allograft shelf life. Potential causes for increased allograft infection and disease transmission risk are: (1) failure to follow tissue-handling guidelines; (2) fraudulent procurement practices; (3) lack of sterilization; (4) foreign body effects; and (5) harvest/preparation contamination.These are summarized in Box 71-1. It is of interest to note that two of these factors involve human error by individuals not within the surgeon’s control. This highlights the inherently increased risk in allografts versus autografts attendant to the fact that so many delicate and exacting tasks must be properly performed before the graft gets to the operatingroom. Most tissue banks are run meticulously and with great care. However, human error, or intentional human misbehavior, can occur with the surgeon unaware.

Areas of Morbidity

Graft Failure and Laxity

The meta-analytic data presented in Chapter 69 showed allografts to have a failure rate two to three times that of autografts, even when radiated grafts were removed from consideration. It also showed the allograft normal stability rate to be significantly lower. This effect occurred in both bone–patellar tendon–bone (BPTB) and soft tissue grafts. This material is covered in more detail in that chapter. Despite these overall worse stability results, it should be pointed out that some reports show excellent allograft stability rates.¹

Delayed Graft Failure

There is some evidence that allografts have a tendency toward late failure,²^–⁴ whereas autografts have shown very little tendency to late failure.⁵ This perhaps mirrors the experience with bone allografts, which can fracture years after clinical implantation. This late failure has been accompanied by biopsy evidence of late or absent recellularization,^2,^3,⁶ which may be causally related. It may be that allograft-implanted patients should be counseled that late failure can occur. A longer period of follow-up for allograft patients may be indicated than is necessary with autografts.

Infection

It is not clear whether the overall infection rate is higher for allografts than for autografts, but there is evidence that it may be so. The Centers for Disease Control and Prevention (CDC), in a well-known analysis of one surgicenter’s experience, showed no infections in autografts and sterilized allografts but a 3% infection rate ⁷ in unsterilized allografts. In our clinical experience the only infected anterior cruciate ligament reconstruction (ACLR) failures we have seen have been in allografts. We have seen one patient with two infected allografts from two different tissue banks operated at two different hospitals by different surgeons for each procedure. This patient went on to autograft revision without incident.

Disease Transmission

The rate of disease transmission risk is difficult to evaluate because routine viral testing of post-allograft patients is not carried out, although it is certainly very low. A recent report, however, included a patient who had hepatitis B transmitted from a donor who initially tested negative for hepatitis B and whose allograft was radiated. After the recipient contracted hepatitis, repeated, more sophisticated testing showed that that the donor was indeed the source of the hepatitis virus.⁸ Studies would indicate that the current sub–3Mrad levels of radiation now most often employed would be expected to kill neither the hepatitis nor human immunodeficiency viruses.^9,¹⁰ Higher levels in the 5Mrad range are probably necessary. Newer protocols using such higher doses with radioprotectant may provide the answer.¹¹

Potential Causes of Increased Laxity

Radiation Sterilization

Radiation has been clearly shown to weaken grafts.¹²^–²⁰ Newer protocols are intended to mitigate or eliminate this effect, but 2-year follow-up data are currently lacking.

Chemical Treatments

Non-radiation cleaning (e.g., Allowash, Lifenet, Virginia Beach, VA), sterilization (e.g., Biocleanse, Lifenet),²¹ cryoprotectant,²² radioprotectant (e.g., Clearant, Los Angeles, CA), tutoplasty, ethylene oxide, and other techniques are used on allografts. The more aggressive these procedures, the more they affect the material properties of the graft. In one recent study the mechanical properties of the grafts were substantially altered by a cryoprotectant technique such that the surgeon was unable to use the graft.²² Questions about these grafts can only partially be answered by studies that analyze strength at time zero. It is the response of the graft to cellular and vascular ingrowth and ligamentization that determines ultimate strength. The effects of such treatments on this process have been little studied.

Freezing

Most grafts are frozen and rethawed at least twice. Grafts are initially frozen after procurement pending the results of cultures. Typically they are then thawed and prepared and then refrozen while a recipient is identified. Freezing has been shown to adversely affect tissue properties.²³ Cryoprotectants are used to protect grafts from the deleterious effects of freezing, but it is unknown how well they work. The temperature of freezing also plays a role. Thus it is reasonable to wonder how much a single cycle of freezing may alter the graft or the subsequent ligamentization remodeling process. Further complicating this issue is the fact that some grafts are only frozen, whereas some are freeze-dried or lyophilized. It is unknown whether the addition of drying to freezing is a benefit because it removes potentially damaging ice crystals, is of no consequence, or is deleterious because it further modifies the tissue.