[this video was sponsored by Brilliant]
This is the ACL, or anterior cruciate ligament. Here in the United States,
around a quarter of a million people injure their ACLs each year, and all things being equal,
female athletes injure their ACLs 4 to 6 times more frequently than their male counterparts.
Biomechanics labs have been studying this phenomenon for decades now,
looking for anything that might explain the disparity. Maybe it has something
about women’s skeletal anatomy, or their landing mechanics, or maybe hormones?
I read about this subject a lot for my undergrad degree,
which was over a decade ago now. And back then, nobody really knew why this was
happening. They had some good ideas, but like with everything in science, it’s complicated.
I figured that with 10 years of additional research, we must be
getting a little closer to understanding the root of the problem. And it turns out,
we still don’t have a clear answer. But we do have some good ideas, and in this video,
we’re gonna take a look at some of the possible explanations.
So it’s 2022, what does the research say about the ACL gender gap?
Hello and welcome, if you’re new here, my name is Patrick and this channel is
all about anatomy and physiology. Let’s start off with a little crash course on knee anatomy.
The knee itself is made of the tibia, femur, and patella, or kneecap. But for this video,
we’ll spend most of our time looking at the joint between the tibia and femur,
what’s called the tibiofemoral joint.
As far as muscles, the knee can move into extension thanks to the quadriceps,
and move into flexion thanks to the hamstrings.
Now, there is a lot of connective tissue between those two bones. And the first thing we see are
two little concave cups of dense connective tissue and fibrocartilage called menisci,
or a meniscus in the singular. Without any soft tissue, the tibia itself has a relatively
flat top, so the menisci help stabilize the femur in the joint and act like a cushion
for the femur on top of the tibia. The rest of these rope looking things are ligaments,
which is connective tissue that helps keep the knee together. And that big one right in
the middle is the ACL, or anterior cruciate ligament. For the completionists out there,
there’s also the PCL, or posterior cruciate ligament. Cruciate just means they cross
each other in the knee. And on the sides are the medial collateral ligament and
lateral collateral ligaments, or the MCL and LCL respectively. Collateral just means “on the side”,
while medial and lateral just tell you which side of the knee it's on.
Now while your knee shouldn’t move side to side, it’s still subject to stresses
that could deform it under side to side stress, like if you got tackled by a football player.
Biomechanics experts call that kind of stress either valgus stress or varus
stress — valgus comes from lateral to medial, while varus comes from medial to lateral. So,
we can think of the MCL as the ligament that would get hurt from a purely valgus force,
like in this illustration, while the LCL would be injured during a purely varus force.
Now anatomy diagrams make it seem like these connective tissue structures are all separate
and distinct, but in reality, there’s a lot of overlap between the menisci, ligaments,
and the connective tissue capsule around the whole knee. In fact, a single injury
on the soccer pitch can damage the MCL, medial meniscus, and ACL all in one fell swoop. This
type of injury is actually so common that it gets its own name — the unhappy triad.
ACL tears themselves can happen in a bunch of different ways, but there are two main
mechanisms of injury: they’re either contact or non-contact injuries. 30% of ACL tears are
from contact injuries — the injury comes from an external force like getting tackled. The other
70% of ACL injuries are non-contact injuries — the injury comes from landing or cutting wrong.
A study from 1990 found that the most common method of non-contact ACL tears
was through multiplanar movement — things like combining both a rotation and a valgus force,
or hyperextending the knee while internally rotating the tibia.
Other studies have shown that the ACL could also be torn through a forward
sliding of the tibia on the femur — something called shear force. And the
quad muscles can actually contribute quite a bit to the shear stress on the tibia.
But people have also torn ACLs by decelerating quickly,
changing direction, or landing and twisting their knee.
All that’s to say there’s not just one mechanism of injury for non-contact tears.
That’s why a lot of the research I’ll be talking about in this video seems overly
specific — researchers are focusing on one factor within a complicated physics problem.
Speaking of, we should define the problem first.
Overall, male athletes are more likely than female athletes to tear their ACLs,
mostly because they have the opportunity to play contact sports. But when we look at
comparable sports though, like boys soccer and girls soccer,
we see that girls injure their ACLs way more often, usually though non-contact injuries .
By the early 90s, researchers saw that this was a thing, and they had a good idea of how
ACL injuries happened. They also had some ideas about anatomical differences between
men and women that might predispose women to non-contact knee injuries.
So scientists decided to ask: how can we tell if
women’s skeletons predispose them to the kinds of forces that tear an ACL?
Unfortunately for the researchers, there is no standardized male or
female skeleton. When anatomists talk about sex differences, they’re talking
about averages — women tend to have wider hips, men tend to have bigger shoulders,
but there’s still a ton of variation from person to person within sex. Plus, factors like age,
activity, and hormones can literally change our bones. So scientists would have to dig way
deeper than assumptions about “the female skeleton”. And they realized that right away
with one of the biggest assumptions about ACL injury rates — it’s gotta be the hips, right?
When it comes to measuring this problem, the actual width from hip to hip isn’t that
interesting for this discussion because we want to see how the hips influence the knee.
Luckily, if you measure the angle between the anterior hip, patella, and tibial tuberosity,
you get something called the Q angle, or quadriceps angle. The idea was that a wider
Q angle leads to more valgus stress at the knee, which might predispose the athlete to
non-contact ACL tears. Men’s average Q angle is 5 to 14 degrees while women’s average is 10 to
18 degrees. So women do have bigger Q angles on average, but there’s some overlap there.
The Q angle hypothesis was probably the leading explanation behind
the ACL gender gap when I was in undergrad, but the last decade of experiments
haven’t been able to show that a wider Q angle contributes to ACL injury. A 2012 study took 12 female soccer players that had experienced ACL injuries, and 12 that hadn’t
and measured their Q angles plus a few other anatomical factors. Both groups had Q angles
ranging from 14 to 18 degrees and the researchers found no association between Q angle and injury.
Then in 2015, a handful of surgeons published a paper where they looked
at at 53 male athletes with non-contact ACL tears. They took a bunch of measurements on
them and found that Q angle couldn’t predict tear rates in this experiment either.
Then in 2017, a bunch of radiologists re-evaluated 86 MRIs from past patients who’d
ruptured their ACLs and compared them to over a hundred MRIs of patients with
intact ACLs who’d gotten knee MRIs for other reasons. Like the other studies,
they measured a bunch of angles, including Q angle and found that it couldn’t predict ACL injury.
Basically, every time we look at the anatomical factors shared by athletes that tear their ACLs,
Q angle doesn’t matter. I know it seems like it should, but it doesn’t work out.
Some of the other hip angles might matter though — here, follow along if you can.
If you scoop your pelvis forward like you’re trying to tuck your tail between your legs, you’re
doing a motion called a posterior pelvic tilt. If you do the opposite, popping your booty out
and arching your back, you’re doing an anterior pelvic tilt. This position gets the hip joint,
the femur in its socket, to twist inward. Now for you anatomy nerds out there, this is called
femoral anteversion. Sure enough, women tend to have a more anteriorly tilted pelvis than men
We’re still not totally sure if an anterior pelvic tilt is a risk factor by itself,
or if it just creates other risk factors by affecting the biomechanics down the leg.
Like more anterior pelvic tilt changes the mechanical advantage of the gluteus medius,
a muscle that inserts on the lateral femur, and keeps our legs from collapsing inward
into valgus stress. And more femoral anteversion stretches the hamstrings,
which puts them in a worse position for a strong contraction.
Now experts have wondered if strong hamstrings might prevent the tibia
from sliding too far forward, counteracting that shear stress
I mentioned earlier. And finding a hip position that weakened the hamstrings
was an interesting hypothesis for the ACL gender gap, but we’ll come back to that.
Because anterior tilt also predisposes the person to a foot positioning kind of
like flat footedness called subtalar joint pronation. The idea is that
a flatter foot gets the tibia to rotate internally when the foot hits the ground,
and that stresses the ACL. And back in 1992, research in The Journal of Athletic Training
showed that subjects with ACL injuries tended to pronate more than their non-injured counterparts.
Other research has contradicted that finding, but biomechanics experts get like,
inappropriately excited about pronation, so more research is coming, I’m sure. Alright,
well let’s keep going down the lower limb and maybe we can find some other sex differences.
If you measure the angle between the patella and the longitudinal axis of the tibia,
you get something called the patellar tendon tibial shaft angle. It’s greater
when the knee is extended and gets closer to zero as the knee bends. Regardless of sex,
a bigger angle predisposes people to ACL injury, because when that angle is bigger,
like when the quads are contracting, the shear stress on the knee increases.
A study from 2003 showed that women tend to have greater patellar tibial shaft angles than men by
a few degrees, and a bigger study from 2019 found the same thing, but it wasn’t statistically significant.
Now that angle changes depending on how straight or bent the knee is,
which lends itself to another question:
Strong quadriceps contractions put more anterior shear stress on the knee,
which can contribute to ACL tears, so would equally strong hamstring
contractions balance it out with posterior shear stress?
To find that answer, we’d have to measure the torque generated by someone’s quads,
then the torque generated by their hamstrings, and come up with a ratio. When we do, we see
that women have quads that are much stronger than their hamstrings, while men have a more
balanced ratio. This gave researchers the idea that women’s quad to ham ratios keep
their hamstrings from countering the anterior shear stress, predisposing them to ACL injuries.
Unfortunately for that hypothesis, a systematic review published in 2022 analyzed years of
research on this topic and found that quad hamstring ratio couldn’t predict ACL tears.
Strong hamstrings are great for athletic performance, but don’t explain the gap,
so we’ll have to keep looking for anatomy that explains the ACL gap.
Anyone getting tired by the lack of conclusiveness in this video yet?!
If you look at a knee from behind, you’ll find two big bumps, what are called the
femoral condyles. Between those condyles is a little canyon called the intercondylar notch,
and in that notch are the ACL and PCL ligaments. Now, usually, as knees get bigger and wider,
so does the notch, so researchers use a ratio of notch width to condyle width,
what’s called the notch width index, or NWI.
Back in the 80s, a team of researchers suggested that a smaller NWI predisposed people to ACL
injuries. They proposed that narrow notches might rub up on the ACL during hyperextension,
cutting or pivoting motions. And over time, this would fray the ACL like a rope until it snapped.
Women tend to have narrower NWIs than men, so this might help explain the difference in injury rates.
But everything I’ve presented in this video has been frustratingly indecisive up to this point,
so I’m sure we’ll be disappointed by this one too!
Well actually, hold up, this is a confident sounding title: narrow notch width is a risk
factor for anterior cruciate ligament injury in the pediatric population. Okay!
These researchers took MRIs from 68 patients aged 10-18 who had sustained
ACL tears on either one knee, or both knees, and MRIs from a control group.
Then they measured the notch width indices on all of them,
and found a statistically significant difference between the bilateral rupture
group and the control, as well as between the single leg rupture group and control.
But there’s another sex difference we haven’t mentioned yet, the ACL itself.
Researchers in Boston published a paper in 2021 where they scanned 108 knees and found that women
tended to have smaller ACLs relative to their knee size than men. There was no difference
in the quality of the ligament itself, just cross section, or width across the ligament.
According to the researchers, some of those other anatomical differences probably contribute
to the size of the ACL, which altogether contribute to the increased risk of ACL tear.
So we’ve got the notch and we’ve got ACL itself,
but other researchers have wondered whether joint laxity might contribute.
This is when the majority of synovial joints — those big bony joints like
the knees or wrists — have a bigger range of motion than the average reference joint. And
women tend to have greater incidence of general joint laxity than men.
That doesn’t mean all women have looser joints than men, just that women fall
into the “greater range of motion than average” category more often than men.
Sometimes laxity comes from connective tissue disorders like Ehlers-Danlos syndrome,
but in 5 to 15 percent of the general population, it just happens. And bringing it back to ACLs,
the idea is that knee joint laxity exposes athletes to all of those forces that predispose
them to ACL tears through those dangerous multi-planar movements regardless of sex.
A study published in 2005 looked at 169 knees that had undergone ACL reconstruction,
and found that 42.6% of them had generalized joint laxity compared to 21.5% in the control group,
and 78.7% of them had hyperextension of the knee compared to 37% in the control group.
They concluded that generalized joint laxity, and specifically, knee laxity contributed to
ACL injury, but didn’t distinguish by sex. Unfortunately, their results are complicated
since athletes can get generalized joint laxity with repetitive motions. It’s common in swimmers,
gymnasts, and baseball pitchers, and it’s why there’s been an effort to limit the number
of pitches a teenage baseball player can throw in a game — to preserve their joints.
Okay, so those are a few of the possible anatomical risk factors. But as many of you
were probably guessing, scientists have also looked at hormonal differences and the effect
of periods. Unfortunately, we’re left with conflicting evidence and no real consensus.
First, a little endocrinology refresher: a menstrual cycle starts when someone menstruates,
then around two weeks later, they ovulate, or release an egg from their ovaries. If that egg
isn’t fertilized, the person menstruates and the cycle starts all over again. Hormonally,
they’ll increase circulating estrogen until /just/ before ovulation, at which point
luteal hormone spikes and progesterone starts increasing. Estrogen will dip down and create
another little bump. That said, there’s a ton of variation — everyone’s cycle is different.
So a logical first question was whether athletes tended to injure their ACLs in any particular
phase of their menstrual cycle when different hormones are at different levels. Unfortunately,
there’s evidence that it might be all of them separately. No good answer there.
A related question was whether taking birth control might be a factor, since oral birth
control messes with hormones. The idea is that increased progesterone, the main hormone in birth
control, encourages cells called fibroblasts to make a connective fiber called collagen which is
a tough protein that makes ligaments strong. But once again, we have conflicting results, and most
of our science comes from animal models. Research groups have found that hormonal birth control
users have less joint laxity, increased hamstring to quad strength ratios, and increased single leg
stability compared to non users. But again, different researchers have found the opposite.
At this point in my research for this video, I was starting to lose hope that I’d ever read about a
concrete risk factor for the ACL gender gap. And then to confirm that I wouldn’t, the authors of a
2009 review said “​​nearly every risk factor has contradictory data reported at some point”. Cool.
But even if we had high quality evidence pointing to an anatomical factor that cause the gap,
that’s still not actionable — you can’t tell someone to workout their intercondylar notches.
So research has turned to modifiable risk factors? And the answer is a resounding yes,
at least for female athletes. I know! Finally, I’m saying something without hedging it!
Now, there is no single best program for ACL prevention, but in general,
these programs try to instill good movement habits through plyometrics, strength training, technique
and balance exercises. And the cool thing about these programs is that we have a large enough body
of research to do meta analyses, or studies of past studies. And these analyses show that
prevention programs do prevent ACL injuries in female athletes when they’re adhered to.
Like, by a lot.
A research group did a meta-analysis of those meta analyses and published
their results in the Journal of Orthopaedic Research in 2018.
They found that these injury prevention programs cut ACL injury rates in half
of all athletes studied, and reduced non-contact ACL tears in female athletes by
67%. So these things may take time, but they’re effective for athletes in non-contact injuries.
But as great as these programs can be, they still take time, money,
and training to implement at the school level.
And one thing I didn’t expect going into this video was the amount of
research that’s looked at how financial and public health factors contribute to
the ACL gender gap. Things like socioeconomic status, housing,
and education level can keep people from getting the best care, which bleeds into sports medicine.
A review published in February of 2022 described how male athletes in general are
more likely to undergo ACL reconstruction and more likely to return to sport after recovery.
Although as of 2017, teenage girls do have the highest rates of reconstruction surgery.
The same review found evidence that Black and Latino patients
were less likely to get reconstructive surgery, to delay surgery for longer,
have fewer physical therapy sessions, and regain less of their quadriceps function than white patients.
So there's still a ton of research to do in this area.
Now, I love this topic because it shows how you
need to understand other sciences in order to solve problems within anatomy.
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