Why women footballers have an increased risk of ACL injuries

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The high number of ACL injuries sustained in women’s football is something that affects players around the globe. 

Nearly every club in each country from Europe to South America has someone out of action with an ACL tear at the moment and some clubs have several players unavailable. 

Numerous studies have attempted to identify the reasons behind this and probably more research has been conducted into ACL injury management than for any other aspect of rehabilitation. 

A quick scan of the usual internet search engines returns countless articles on ACL injuries spanning the last thirty years and more; but despite our increased knowledge, these injuries remain a problem within the game (De Ste Croix et al, 2015; Dugan SA, 2005; Gans et al, 2018; Hewett et al, 2006).

Burnham and Wright (2017) highlighted the occurrence of ACL injury in sports with periods of rapid deceleration, such as football, where the rates of ACL injury in women’s football are twice as high as men. 

Arendt (2007) reported that ACL tear rates in female athletes are estimated at 2 to 8 times those experienced by male athletes. We also know that the risk of repeat or recurrent injury is high (Paterno et al, 2012; Wright et al, 2007). 

ACL tears continue to dominate the injury lists at clubs up and down the country despite the skills and demands required for the women’s game being no different from those of the men’s. 

Stopping, starting, changing direction, jumping and landing, etc. are all essential skills for a footballer to possess, whether male or female. 

However, the incidence of ACL injuries in the women’s game is far greater than in men’s football (Prodromos et al, 2007; Walden et al, 2011) and this applies to players at all levels. You don’t have to be at the top end of the footballing tree to suffer a cruciate ligament tear. 

First things first, though, and before we can discuss preventative strategies it’s important to understand the injury mechanics and recognise that injuries to the ACL usually occur in one of two different ways. 

These are sustained either through making direct contact with an opponent or by non-contact mechanisms such as jumping and landing awkwardly. 

Hewett et al (2006) recorded that 70% of ACL injuries in female athletes were sustained through non-contact mechanisms instead of collisions or in the tackle. 

This is supported by extensive research that crosses into other sports where the incidence of female ACL is high.

Stuelcken et al (2016) noted that in terms of injury mechanics, a similarity exists with netball where non-contact ACL injuries occur frequently, and with basketball as observed by Leppanen et al (2016) and Renstrom et al (2008).

So in sports that involve a considerable amount of twisting and turning at high speed together with jumping and landing on a fixed foot such as in netball, for example, the known risk factors for ACL injury are easily identifiable.

Junge and Dvorak (2004) described the most common injury mechanics of ACL rupture as a sudden deceleration with a change of direction when the foot is fixed, so clearly, that information has been available for a while. 

But before attempting to go into the reasons why ACL injuries feature so prominently in the women’s game it will be helpful to take a look at some basic anatomy of the knee joint first. 

Basic Anatomy 

The knee is a synovial hinge joint comprising of the articulation of the bones of the tibia and femur, lined with articular cartilage and principally stabilised externally by strong collateral ligaments reinforced with a capsule and internally by the anterior and posterior cruciate ligaments. 

The anterior cruciate ligament (ACL) lies deep within the knee and connects the femur to the tibia. Its major mechanical function is to prevent excessive forward movement of the shin with the knee in various degrees of flexion (Liu-Ambrose, 2003), whilst the posterior cruciate ligament (PCL) is an important restraint of posterior tibial translation.  

Anatomically, the ACL runs within the knee from the medial surface of the tibial plateau and extends upwards, backward and laterally to insert on the lower aspect of the femur. 

Both cruciate ligaments are static stabilisers that play an intricate part in adding stability to the knee throughout its full range of movement (Logterman et al, 2018) in conjunction with the medial and lateral collateral ligaments of the knee which provide stability in a side-to-side direction. 

However, although anatomical features play a prominent part in injury pathology applicable to the female athlete, several other factors are also thought to affect the predisposition of the female footballer to ACL tears. 

A study specifically designed for female athletes was commissioned by the International Olympic Committee in 2008; and together with recommendations from previous studies, three major underlying sources of ACL injuries in women’s football were agreed upon (Renstrom et al, 2008). 

In addition to anatomically-related gender differences, Renstrom et al, (2008) cited altered biomechanics and hormonal influences as significant contributors either collectively or on an individual basis to the female athlete's predisposition to an ACL injury. 

Anatomical differences 

Renstrom et al (2008) suggested that the anatomical differences between the male and the female knee can influence the potential for ACL injury in the female athlete. 

This coincides with earlier work by Arendt and Dick (1995) who identified gender-specific differences including a narrow intercondylar notch, a weak or smaller ACL than males, and increased anterior-posterior laxity of the knee joint. 

Anatomically, the area within the knee where the ACL inserts on the femur is narrower in women than in men, leaving less space for the ACL to move freely (Yu et al; 2002).
 

Additionally, the size of the intercondylar notch of the female knee together with the ACL itself being slightly smaller and also of a different constitution in women could potentially be considered as precursors to injury. 

Hewett et al (2004), noted that after puberty females exhibit measurable quadriceps dominance, ligamentous dominance, leg dominance (lower extremity asymmetry) and trunk dominance compared to males.
 

This links the non-modifiable anatomical factors described by Yu et al (2002) as reinforced by Burnham and Wright (2017) with alterations in biomechanics that can have a cumulative effect over time.
 

Biomechanical influences: The ’Q’ angle

Renstrom et al (2011) noted that an increased ‘Q’ angle is often thought to be a standard risk factor for ACL injury in women footballers. 

The ‘Q’ angle is formed by the intersection of two imaginary lines drawn through the centre of the femur and tibia; with the intersection of these at the patella forming what we call the ‘Q’ angle. 

The first line starts at the lateral edge of the pelvis and extends down the femur through the centre of the patella. Where this line crosses a similar imaginary line drawn upwards through the centre of the tibia, forms the resultant angle we refer to as the ‘Q’ angle. 

Typical Q angles in men are often measured at 12 - 16 degrees while in women the angle is far greater; with an average of 16 -19 degrees as a comparison. So, biomechanically speaking, the wider the hips and pelvis, the greater the ‘Q’ angle. 

An increased ‘Q’ angle is thought to lead to an increased risk of the knee giving way on landing due to the differential stresses being placed on the knee; leading to uneven forces resulting in injury. 

It’s important to note however that ‘Q’ angles can vary from person to person. 

Measuring the ‘Q’ angle as part of a thorough biomechanical assessment can help to identify those players most likely to be considered at risk of ACL tears or other associated knee injuries. 

Hormonal factors 

The literature suggests an association between hormonal fluctuations and ACL injury (Herzberg et al, 2017). 

 
Studies to date suggest that knee ligament laxity and risk of an ACL injury may be increased during the ovulatory phase of the menstrual cycle and that suppression of follicular development and ovulation with hormonal contraceptives may reduce this risk (Herzberg et al, 2017).

The associations between laxity and hormones, coupled with increased generalised joint laxity may well contribute to the increased risk of ACL injury in female athletes after puberty (Hewett et al, 2006, Myer et al, 2008). 

We now accept that changes in female hormones during the menstrual cycle may well influence the potential for ACL injury and this is something that clubs are now addressing through individualised training plans. 

Prior et al (2019) however, cautioned against the implication that we wholly understand the ovarian hormonal relationships with connective tissues while stressing that in their opinion this remains unclear. 

Yet the emphasis placed on hormonal factors in predicting the potential for ACL injury is now greater than ever before. In conjunction with anatomical differences and biomechanical factors, we are far more knowledgeable about ACL injury risk factors than we were ten years ago.

Discussion

Despite our current knowledge, previous ACL injury has been shown to significantly increase the odds of sustaining another ACL tear; since it has long been recognised that previous injury significantly increases the risk of repeat injury to the same structure (Hagglund et al, 2006).

In certain cases of ACL tears, sometimes the injuries even occur to the contralateral knee (Wright et al, 2007; Paterno et al, 2012); and this too, is currently recognised as a common issue in women’s football.
 

Anatomical and biomechanical factors contribute to the risk of ACL injury (Walden et al., 2011) and there appears to be little doubt that when hormonal influences are considered as well, all three identified risk factors are important in the aetiology of ACL injuries in female athletes. 

Differences in ‘Q’ angles between men and women arising from the greater width of the female hips and pelvis translates to an increased potential for ACL injury in the female athlete through affecting the loads taken through the knee. 

Working distally from the hip and knee down the lower limb identifies further risk factors such as over-pronation of the foot which can lead to altered biomechanics. 

Chappell et al (2002), Myer et al (2011), Leppanen et al (2016) gave additional biomechanical risk factors for women's ACL injuries as follows:

• · Upright trunk position on landing 
• · Increased valgus stress
• · Increased internal hip rotation 
• · Strong quadriceps contraction at the time of ground impact 
• · Almost full knee extension on landing

Making allowance for hormonal factors is a topical subject for discussion in today’s sport in terms of injury prevention techniques; but although this information has been around for years the incidence rate of ACL injuries in the women’s game continues to rise. 

That there is a known higher incidence of ACL injuries in female athletes than in males (Prodromos et al, 2007; Walden et al, 2011) is currently not contested and remains a matter of concern for everyone involved. 

As highlighted by Nilstad et al, (2014) we need to utilise our increased knowledge of risk factors for lower extremity injuries to enable more targeted prevention strategies to be applied to reduce injury rates in female footballers.