Where Are We Now?
In the current study, Hollnagel and colleagues  performed a retrospective study correlating hamstring cross-section on pre-operative MRI and intra-operative autograft size. The authors used 8 mm as the standard acceptable thickness required for ACL reconstruction, positing that graft thickness < 8 mm is at an increased risk to rupture . They found that pre-operative MRI measured hamstring and graft thickness (four-strand graft as measured in the operating room) had a good correlation using their MRI protocol (1.5 T and 3 T).
Numerous options are available for grafting a torn ACL, including using patellar tendon bone, quadriceps tendon, allografts of various kinds, or hamstring tendon. Hamstring autografts are a popular graft choice among orthopaedic surgeons, and in certain areas of the world, it is the dominant graft choice for ACL reconstruction . But it can be difficult to predict graft diameter prior to surgery, and several studies have implied comparable laxity values with other graft choices, albeit with a higher incidence of failure which one study attributes to diameter size .
Four strand semi-tendinosus and gracilis grafts have shown comparable biomechanical properties and clinical results to bone-tendon-bone, and may decrease anterior knee pain and donor site morbidity of bone-tendon-bone grafts . One multi-center study has shown a direct correlation between graft size and Knee Injury and Osteoarthritis Outcome Scores (KOOS) and IKDC with increasing graft size . Grafts below 8 mm in diameter had a much higher failure rate, particularly in patients younger than 18-years-old. In another systematic study, a graft diameter below 8 mm had a 6.8 times greater relative risk of failure . Other potential sources of concern include technical errors such as graft fixation or tunnel malposition.
Hamstring tendons vary in size and because its cross-sectional area is associated with strength of collagenous tissues, there is a concern that diminutive grafts are at a greater risk of failure or might require augmentation or other biomechanical constructs to be successful. Because of this, there is a push to determine hamstring graft size pre-operatively . The most common methods are MRI, CT, ultrasound, or anthropometrics (patient height, weight, age, sex).
Where Do We Need To Go?
Multiple large registry studies have shown higher revision rates among hamstring autografts versus bone-tendon-bone. These studies, while registering large cohorts, offer little information regarding graft size or diameter [2, 4]. Our ultimate goal should be to determine what is a reliable construct of a hamstring graft that will avoid failure. Because typical preparations of hamstring grafts do not always achieve a graft of 8 mm in diameter, different constructs or grafts with augmentation might be more appropriate graft options . Some surgeons believe that patients with higher BMI or activity level might be better served with a different graft choice or construct. When determined pre-operatively, appropriate counseling and determination of the best option can be discussed with the patient. It is also possible that there are circumstances where a smaller graft might be acceptable such as a lower level athlete, smaller stature or an older less active patient.
We also need to determine the mechanism of graft failure in patients with grafts less than 8 mm. It is possible that failure actually occurs because of inadequate ligamentization of the smaller graft in its bony tunnel and not because of the graft disruption itself . Finally, we still need to determine what causes disruption.
How Do We Get There?
For years we have been seeking the answer to the perfect graft choice in ACL reconstruction without attaining an answer. Randomized controlled studies comparing various graft choices might lend insight into this perplexing problem, but these trials are expensive and difficult to control. Multi-center registries frequently seen today lack data control, evaluation of laxity, and outcome data, short of failure. An ACL graft may technically fail but still lead to excellent clinical outcomes if laxity is not evaluated. Future prospective studies, looking specifically at technical aspects of graft placement should evaluate the cause(s) of failure for these grafts and determine whether such problems are related to patient-related issues or technical aspects. Incorporating patient-specific data may allow us to determine best graft choice for the reconstruction along with technical aspects of care that lead to a successful reconstruction. It is possible we may never determine a single best ACL reconstructive technique.
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2. Gifstad T, Foss OA, Engebretsen L, Lind M, Forssblad M, Albrektsen G, Drogset JO. Lower risk of revision with patellar tendon autografts coimpared with hamstring autografts, a registry study based on 45,998 primary ACL reconstructions in Scandinavia. Am J Sports Med. 2014;42:2319-2328.
3. Hollnagel K, Johnson BM, Whitmer KK, Hanna A, Miller TK. Prediction of autograft hamstring size for anterior cruciate ligament reconstruction using MRI. Clin Orthop Relat Res. [Published online ahead of print]. DOI: 10.1097/CORR.0000000000000952
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