ACL Graft: Allograft

/ OPA Orthopedics/

 

Allografts (aka “donor tissue”) initially seemed to be the holy grail of graft tissue for ACL reconstruction, but as it turns out, most things that seem too good to be true are to good to be true. 

There are many donated tissue allografts that are used for ligament reconstruction, including the Achilles tendon, Tibialis Anterior, Posterior Tibialis, or hamstring tendons.  The concept is that we are really using the ACL graft as a ‘scaffold’ that is incorporated into the bony sockets and to some extent this scaffold is populated by cells from the graft recipient.  Allograft tissue is harvested from organ donors and processed by tissue banks usually with any number of proprietary processes to sterilize the tissues as well as limit the number of living cells (irradiation, chemical sterilization, etc).  Because these tendon scaffolds have very few cells at baseline, and after treatment they have even fewer, they are relatively devoid of living proteins and cells, so they do not generate an immune response as would say a heart, liver, or kidney transplant.  Allografts don’t require any kind of tissue matching, and they can be frozen and stored for a significant amount of time, so they are generally readily available. 

Initially, allografts were seen as a huge benefit to surgeons, who saw the opportunity to get grafts of almost any size with no harvest-site discomfort or complications.  Indeed, I started using allografts early on in my career when they’d initially become available, and particularly in my high-level athlete patients, because this was the population who I thought would benefit the most by eliminating graft harvest site issues.  Unfortunately, I appreciated more failures in these patients than I had previously with the patient’s own grafts – the seemed a little looser, and I saw a few more re-ruptures with minimal trauma.  So did my colleauges in their young athletes...so did my colleagues across the country. 

Because ACL graft failure is relatively uncommon if you consider the number of ACL’s done in this country (see “How good is ACL reconstruction, really?”), it takes a lot of ACLs performed to see if there is a significant difference in the performance of different graft types.  Now there have been a number of large, multi-center studies that have demonstrated clearly that ACL reconstructions with allograft tissue have a statistically-significantly greater likelihood of failing when compared to autograft tissues such as bone-patellar tendon-bone (BTB) or hamstring autograft (HS).  In some populations, such as young athletes[i] or military cadets[ii] the failure rates of allograft can be as high as 30% to 44%, respectively! 

Some tissue processing techniques (e.g. Gamma irradiated grafts) have been associated with weakening the tissue, so some authors suggest that appropriately conditioned allografts may not be as bad as studies have shown.  Still, I have seen and it has been reported, that some allografts are simply ‘digested’ by the host recipient.  Occasionally, I’ll see a patient who tore their allograft ACL, and when we operate to revise their ACL graft tear we will find there is no torn graft in the knee, because the graft is simply absent, like no surgery had ever been done.  The crux remains how to adequately sterilize and process the donated tissue while maintaining its strength. 

ADVANTAGES:

  1. There is no graft harvest morbidity to the ACL patient.
  2. Very large grafts are available, so graft size is not limited by patient size.
  3. In patients who have torn their ACL and multiple other ligaments (PCL, MCL, LCL), it is nice to use allograft for some of the repairs rather than taking graft from the uninjured knee.
  4. If a person’s own graft tissue (i.e. HS) is small, then allograft can be used to increase the girth of the graft.

DISADVANTAGES:

  1. Re-rupture rates are high in some patient populations, up to 44%.
  2. There is an extremely slight, but not non-existent risk of disease transmission, as with any donated or transplanted tissue.
  3. Studies have shown that allografts reach their mature strength slower.  So while the surgery is easier on the patient and more efficient for the surgeon, the ultimate healing time to return to all athletics with low risk of re-tear may actually be longer.

The Take Home Message:  So in my practice, there is a role for allografts for relatively sedentary populations, as an augmentation or to support a patient’s own graft tissue, or in the case where there are multiple ligaments torn and I want to avoid a lot of harvests to an already very injured knee.  I rarely like the idea, however, of using a tissue with some controversy surrounding it when there are better options available.  The allograft is the “Honda scooter of grafts” – It is easy to drive, you can park it anywhere, it’s dirt cheap with respect to graft pain, but they don’t work well to haul things or race, and there might be some reliability and safety issues.  You might not want it as your only vehicle.

[i] Barrett, AM, et al. American Journal of Sports Medicine. 2011 Oct;39(10):2194-2198.
[ii] Pallis, M, et al. American Journal of Sports Medicine. 2012 June;40(6):1242-1246.