I undertake more Anterior Cruciate Ligament (ACL) reconstruction operations in males than females, but this disguises the fact that for the equivalent exposure to twisting and turning sports, females are known to have a higher risk (estimated 4-6 x) of ACL injury. My operation numbers are skewed because in the UK more men continue with higher risk sports after school years, hence they dominate the numbers in clinic.
But if you are a teenage or adult female playing competitive football, netball, basketball or other similar sports (most ACL injuries occur during landing from a jump or abrupt changes of direction while running), then ACL tears are common and you are highly likely to have team mates who have suffered this career-threatening injury.
This blog explores why the risk is higher in females (for the same exposure time to the same movements). There are lots of factors thought to contribute, many of which are interrelated, some with more evidence that others.
Anatomical Differences
Smaller size ligament – on average women have smaller size knees than men, and this includes a smaller anterior cruciate ligament which will rupture at a lower force of injury. Women also have a narrower (tighter) intercondylar notch – which may put more pressure on the ACL in certain loading positions.
Model of cruciate ligaments crossing centre of knee joint within the intercondylar notch.
Lower muscle mass – men tend to have a higher percent muscle mass. As better muscular control of joint movement is known to reduce strain on ligaments, lower muscle mass in females will increase risk.
Biomechanical Differences
Wider pelvis in women – this leads to an increased angle between the femur (thigh bone) and tibia (shin bone) – known as the Q angle – which puts higher stresses on the knee ligaments during rotational movements. It also causes more issues with kneecap tracking in women than men.
Increased Q angle at knee caused by wider female pelvis.
– Dynamic knee valgus describes how when some individuals land from a jump, the knee can drop inwards, putting the ACL (and medial knee ligament) under increased strain. For more details see my recent blog on “Avoiding Knee Injuries in Sport”. It is more common in women, and can be easily identified by an experienced observer. This brief video shows a high risk individual: active knee valgus during drop jump test – YouTube . Dynamic knee valgus is a modifiable risk factor, so teenage girls and women who have this landing pattern should be encouraged to use a training programme such as the PEP Program (aclstudygroup.com).
Hormones and menstrual influences
The ACL has hormone receptors for both oestrogen and progesterone, leading to subtle differences in ligament laxity during the ovulatory cycle. Some studies have shown higher rates of ACL injury during the ovulatory phase (compared to the follicular phase), and possibly a protective effect of contraceptive pill use on the risk of ACL rupture.
Generalised joint laxity
This is a known risk factor for ligament injuries, and is more common in women. In particular, if your knee hyperextends (goes beyond straight), you are putting your ACL under tension in a position where you have reduced muscular control, so any additional force is taken directly by the ligament and may lead to rupture.
So what can you do? – Modifiable risks …
You can’t change the shape of your bones, or your hormones, or the natural laxity in your ligaments, but you can identify those athletes at higher risk, and you can do something to correct neuromuscular imbalances with the right training to improve your jump landing biomechanics. It is surprising how much difference these programmes can make – See my “avoiding knee injuries” blog for more details.