Practical implications of study findings
Screening of athletes to identify potential risk factors predisposing to ACL injury has obvious advantages.
Our study identifies diameter (size) of ACL as one such important potential risk factor.
Subjects with thinner ACL could negate this risk by undertaking special neuromuscular exercises targeted to decrease the knee loading [2,
10,
11].
Improving the strength of the adjacent muscles and making them function as ACL agonists can significantly decrease the biomechanical loading of ACL,
thus decreasing the injury risk [4].
Specific exercise routines that induce ACL hypertrophy may be developed for subjects at risk of ACL injury,
especially adolescents.
Few molecules inducing proliferation of ACL cells have been identified [4].
More such highly effective molecules may be identified.
Why ultrasound of ACL?
MRI is more accurate in assessing ACL size than ultrasound; however,
using MRI of knee to screen a large number of aspiring athletes can be extremely expensive and time consuming and can significantly overburden the already strained healthcare resources.
It is thus debatable whether performing an MRI of the knee in an aspiring athlete without symptoms is justifiable [5].
Ultrasound is a promising modality that can be used as an excellent screening test to detect subjects especially aspiring athletes prone to ACL injury.
Ultrasound can be an ideal screening examination to assess the size of a normal ACL due to its wide availability and short completion time and also due to its economical and nonionizing radiation nature [6-9].
Limitations of study
Major limitations of our study include ACL diameter as the measured criteria to determine ACL size and evaluation of only tibial aspect of the ACL by ultrasonography.
Since stress in a ligamentous tissue is a product of force divided by cross-sectional area (CSA),
minimum CSA would be an excellent measure compared to diameter in assessing ACL size.
Measuring cross-sectional area of ACL by ultrasound would require further studies and may be difficult due to inability to clearly separate ACL from adjacent soft tissue.
Volume would also be a very good criterion to assess ACL size; however,
it is not possible to measure ACL volume by ultrasound.
If the ACL is assumed to have a fairly regular and characteristic shape and aspect ratio (CSA/length),
the diameter should be an appropriate surrogate measure for CSA [4].
It is possible to examine only the tibial part of ACL by ultrasound and not the whole ACL.
Sometimes it may be difficult to distinguish ACL from surrounding tissue on ultrasound.
This can be minimized by using dynamic ultrasound.
Utilizing the unaffected ACL from opposite extremity to represent the diameter of the torn ACL,
which cannot be measured after injury,
is also a limitation of the study.
However,
Jamison et al.
in 2009 compared the bilateral knee ACL sizes using MRI in healthy subjects and concluded that no remarkable disparity is present between normal unaffected ACLs in the same individual [12],
thus demonstrating that the contralateral ACL is a suitable substitute for the torn ACL for diameter or size measurements.
The ACL diameters computed by us are within the range of previously published data (0.83 ± 0.146 cm) [6].
The ACL injury mechanisms that our study depended on were self-reported. Our study did not take into account the levels of physical activity of the subjects,
which may alter the tensile and other mechanical properties of the ACL [4,
13].
It is possible that professional sportspersons may have stronger and firmer ligaments than recreational sportspersons or nonathletes because of recurrent loading of the ligaments [4].
However,
to our knowledge,
no published data is available to support this hypothesis [4].
Our study assumes matching past physical activity level between control and ACL-injured subjects [10].
Our study also assumes matching ACL injury risk due to extrinsic and intrinsic factors other than ACL diameter between control and ACL-injured subjects [4].
In the future,
it is possible that the subjects from the control group may also suffer an injury of the ACL [4].
We tried to control this variable to a significant extent by matching the ages of these two groups; however,
disparity in the levels of physical activity affecting both past and future likelihood of an ACL injury remains a shortcoming [4].
Additional research studies are needed to validate the differences that we observed amongst control and ACL-injured subjects.
Acknowledgements:
Major parts of this poster presentation are derived from an original research article already accepted for publication and having the same title and the same authors [14].
Financial support came from Medical Research Center,
Hamad Medical Corporation,
Doha,
Qatar.