Each has different technical, performance considerations
In reconstructive surgery of the anterior cruciate ligament (ACL), debates over autograft versus allograft are ongoing. (See "Allograft versus Autograft in ACL Reconstruction," AAOS Now, October 2015.) But that's not the only decision that surgeons must make. They can also choose from three primary autograft tendons (patellar, hamstring, and quadriceps) and five primary allograft tendons (patellar, hamstring, tibialis, Achilles, and quadriceps).
During the 2015 annual meeting of the American Orthopaedic Society for Sports Medicine, Lars Engebretsen, MD, PhD, of Oslo University Hospital in Norway, focused on issues that should be taken into consideration when choosing the appropriate tendon for ACL reconstruction (Figs. 1-3).
Autograft: The big three
"We as surgeons should be able to use all three autograft choices: bone–patellar tendon–bone (BPTB)—the historic gold standard—hamstring, and quadriceps tendon," said Dr. Engebretsen. "The advantages of BPTB are fast bone-to-bone healing and ease of harvest. The graft is as strong as the original ACL, and the results are excellent in physically demanding patients."
However, BPTB has its disadvantages. These include elevated anterior knee pain, possible late chondromalacia, patella fracture, patella tendon rupture, and injury to the infra-patellar branch of the saphenous nerve. In addition, the following contraindications for using BPTB autograft have been identified:
- disorders of the patellofemoral joint
- history of patellar tendonitis
- skeletal immaturity
- active Osgood-Schlatter disease
- the prospect of reharvesting a BPTB autograft for revision surgery
According to the Scandinavian ACL registry, hamstring autografts have a higher rate of revision than patellar tendon autografts, noted Dr. Engebretsen. However, no differences have been found in anterior laxity, functional results, and International Knee Documentation Committee scores. In comparing the three types of grafts, Dr. Engebretsen noted the following:
- BPTB grafts are generally more statically stable but exhibit more extension lag and anterior knee pain than the other two types of autografts.
- Hamstring tendon grafts have a tendency toward more flexion weakness and greater laxity.
- Quadriceps tendon grafts are the least-studied and seem to result in less anterior knee pain, but otherwise yield similar outcomes.
According to Dr. Engebretsen, tunnel widening generally occurs in the first 3 to 4 months. The reason is likely multifactorial, including the bungee-cord effect (elastic longitudinal deformation), the windshield wiper effect (sagittal intratunnel graft motion), and stresses from accelerated rehabilitation. The incidence of tunnel widening is greater with hamstring grafts than it is with BPTB grafts; use of EndoButton fixation is also associated with a higher incidence of tunnel widening compared to use of Transfix and interference screw fixation.
Tunnel widening may also occur more frequently with bioabsorbable screws than with metal interference screws. Overall, however, no adverse effects on outcomes have been reported, although widening can complicate revision ACL replacement.
Allografts: Pluses and minuses
Dr. Engebretsen also summarized considerations for the use of ACL allografts. Advantages of allograft include the following:
- no donor site morbidity
- possibility of larger grafts
- decreased surgical time
- faster rehabilitation
Concerns or drawbacks to using allografts include cost, issues with graft incorporation, and at least some slight degree of risk of disease transmission; for hepatitis and HIV, for example, the risk is less than 1 in 1 million.
Allograft use may warrant consideration in older patients, but not necessarily in patients younger than 25, due to "unacceptably high" failure rates, said Dr. Engebretsen. Allografts may also be appropriate in revision cases and in those involving knee dislocations.
Whether to use fresh or irradiated tissue is also an issue. The U.S. Food and Drug Administration requires a sterility assurance of 10–3—meaning a 1 in 1,000 chance or lower that a living microbe exists. With fresh frozen grafts, screening and serological tests are used to ensure safety.
Irradiating a graft, however, reduces graft strength. A graft subjected to a dose of 1.5 to 2.0 Mrad has been associated with reduced knee stability and higher failure rate after implantation. This is problematic because a dose of 2 Mrad is needed to kill most bacteria and spores, and not even 5 Mrad exposure will kill HIV, Dr. Engebretsen said. Although irradiation may itself have undesirable effects, the use of antioxidants and greater collagen crosslinks has been shown to maintain tendon strength with exposure to radiation.
The following issues must also be taken into consideration when deciding to use an ACL allograft:
- slower incorporation
- anteroposterior laxity
- lesser load to failure at 1 year
In conclusion, Dr. Engebretsen noted that an athlete's sport may guide graft choice. "In Norway, we tend to use BPTB for pivoting sport, such as alpine skiing. Quadriceps grafts are on the rise and are used for the same sports as BPTB. Hamstrings are used often in nonpivoting sports."
Conflict of interest information for Dr. Engebretsen can be accessed at www.aaos.org/disclosure
Terry Stanton is a senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org
- Although BPTB autograft is the "gold standard" for ACL reconstruction, it should not be used in younger patients or those with a history of patellar tendonitis, Osgood-Schlatter disease, or other disorders of the patellofemoral joint.
- Tunnel widening may be an issue with autografts, in part tied to the type of fixation used.
- Although the risk of infection is low with allografts, the amount of irradiation required to ensure safety may result in reduced strength.