Sports Medicine over the past two decades.14 Intrinsic risk factors are further divided into
anatomic and neuromuscular risk factors. Anatomic risk factors include anthropometric differences, increased Q-angle/pelvis width, ACL femoral notch size and properties, foot pronation and navicular drop, body mass index (BMI), age, gender, biomechanical and neuromuscular changes during pubertal maturation, increased joint laxity, and increased hamstring flexibility. Intrinsic neuromuscular risk factors are movement patterns, ground reaction forces, muscle-activation patterns, timing and magnitude of muscle activation, imbalance muscle firing pattern, and increased fatigue. Possible extrinsic factors contributing to ACL injury are climate conditions, role of playing surface, and shoe-to-playing surface interaction.
Intriguing data have been published on potential intrinsic anatomic risk factors involving ACL injuries. Growth and development associated with puberty and decreased control of hamstrings in young females may increase the possibility of an ACL injury. However, there is still much disagreement in the current findings and further research is required to clarify these potential risk factors. Researchers agree that the findings on anatomic risk factors improve our understanding of the mechanism(s) involved in ACL injury. Nevertheless, as Griffin and colleagues stated: “one must appreciate that if anatomical factors are found to be definitely associated with an increased risk for injury, they may be more difficult to modify than are environmental, hormonal, and/or neuromuscular factors.”2
These controlled laboratory studies provide strong theoretical support to clinical observations. However, as initially stated by Markolf et al.15 and Pope et al.,16
and recently affirmed by several others17,18
a shallow medial tibial plateau lead to a greater risk for injury, while a deeper medial tibial plateau concavity provides a more stable seating of the medial femoral condyle which also provides a protective ACL role (restraint against anterior tibial translation), even if muscular protection is insufficient.19
According to Hasheim et al.23 “the depth of the medial
tibial concavity may be a more critical risk factor in anterior cruciate ligament injury than the slope;” for a 1mm decrease in tibial plateau depth concavity, there is a three-times greater risk for a non-contact ACL. Third, during a jump–landing event, co-flexion of the hip and knee joints must occur to protect the ACL from non-contact injury.19 Furthermore, the rate of hip flexion (not the hip flexion angle) at ground contact is paramount, as a slower rate of hip flexion, compared with knee flexion, will allow the tibia to undergo anterior translation, resisted solely by the ACL. In addition, Hashemi et al.,19
stated: “relative trunk
position at the ground contact has the potential to either reduce (trunk flexion) or facilitate (trunk extension) a hip and knee flexion dysynchrony and influence risk for ACL failure.” The co-flexion of hip flexion and knee joints is of interest as Branch and Hunter24
had reported that
ACL-deficient subjects (compared with healthy subjects) had a greater anterior shift of their pelvis (hip flexion), kept their hips less abducted and more externally rotated during the stance phase compared with non-injured control subjects. At the same time, the ACL-deficient subjects’ planted side knee was in a greater varus and an externally rotated position and the ankle was more externally rotated than the subjects in the control group. Branch and Hunter24
concluded that
the cumulative kinematic changes of the hip, knee, and ankle in the ACL-deficient subjects translated to a compensatory early turning of the body towards the cut.
the human
neuromuscular system is not able to respond in a timely fashion to protect the knee joint from injury. Thus, the question remains in terms of how much potential ‘active’ neuromuscular control one may have over the knee joint in preventing an ACL injury during cutting, pivoting, and landing (identified as high-risk ACL injury causing) movements, especially during a fatigued state. However, Hashemi et al.,19
suggested
that both male and female athletes are susceptible to this delay. As such, why is there a large gender disparity in ACL injury rates, with women having two to seven times higher risk for injury compared with men in high-participation sports such as soccer and basketball?1,2 We will address this matter below.
It is of interest that most ACL injury prevention research has focused on neuromuscular risk factors, which Renstrom et al.1
categorized as
“modifiable factors”. Although research on neuromuscular/modifiable risk factors has demonstrated possible temporary benefits, non-contact ACL injury rates have not yet decreased.19–21
Recently, Hashemi et al.19 and Stijak et al.22 suggested several anatomic
mechanisms that may increase/protect the ACL from non-contact injuries. First, a steeper posterior tibial slope has a potentially higher anterior tibial translation that lead to an increased risk for non-contact ACL injury. A mild tibial slope, with increased knee joint flexion, “creates a posteriorly directed shear force, [which] will resist anterior tibial translation.”19
According to Hashemi et al.,19 this anatomical feature plays an equal, if not, greater ACL-protective role than the active hamstrings. Second, 62
have indeed provided thought-provoking non-contact ACL mechanisms of injury or mechanisms protecting that ACL injury from injury. However, several factors must be considered in terms of the proposed injury mechanisms.
Hashemi et al.19 Another paper published in 2010,25 compared magnetic resonance
imaging (MRI) scans of 20 males and 20 females with non-contact ACL tears with 20 males and 20 females who only had a meniscal pathology on MRI scan. When non-injured ACL men and non-injured women were compared, the latter group had significantly deeper medial tibial plateau. However, no statistical differences were noted in the tibial plateau between healthy women and women who had sustained a non-contact ACL injury. Men with injured ACLs had significantly deeper medial and lateral tibial plateaus and a significantly steeper posterior slope than healthy men.25
According to data from Bisson and Gurske-DePerio,25 men should have a
similar rate of non-contact ACL injuries rather than the often suggested large gender disparity in non-contact ACL injury rates. Indeed, the available injury data on non-contact knee ligament injuries for male athletes support this premise. This is confirmed from the available data on both genders in different sports. In basketball (60.3% for males versus 63.8% for females),26,27
missing >10 days of activity (233 for males versus 195 for females),28,29
in lacrosse, based on injury severity, classified as and
soccer, again based on injury severity, (215 for males and 245 for females).30,31
game and practice, respectively),32
In football (North American), a high number (29.1 and 36.1%, of non-contact ACL injuries also occur.
US MUSCULOSKELETAL REVIEW
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