How Internal Rotation of the Knee Affects Joint Health

Internal rotation of the knee is the inward twisting of the shinbone (tibia) relative to the thighbone (femur), and it happens every time you bend your knee. The movement is small compared to the knee’s main bending-and-straightening action, but it plays a surprisingly large role in how the joint distributes load, how ligaments get injured, and how surgeons plan everything from ACL reconstruction to total knee replacement. Understanding this subtle rotation helps explain a range of knee problems that seem unrelated on the surface.

How the Knee Rotates During Normal Movement

Most people think of the knee as a simple hinge, but it actually combines bending with a small amount of twist. As the knee bends, the tibia rotates inward; as the knee straightens, the tibia rotates outward. This coupling between flexion and rotation is sometimes called the “screw-home mechanism” because the tibia essentially screws itself into a locked position at full extension.¹ The lock provides the stability you rely on when standing still without much muscular effort.

During walking, this rotation follows a predictable pattern, but with a wrinkle. Three-dimensional gait analysis has shown that the screw-home movement occurs as expected during the late phases of the stride, producing roughly 17° of rotation. However, early in the stance phase, the tibia briefly rotates externally even though the knee is starting to flex, a reversal known as the “paradoxical screw-home movement,” measuring about 6°.² This paradoxical twist is a normal part of weight acceptance and does not indicate anything wrong with the knee. It just shows that the joint’s rotational behavior under real-world loading is more complex than what you see in textbook diagrams of a non-weight-bearing knee.

This coupling between flexion and internal rotation is not unique to humans. Comparative anatomy research has found the same asymmetry between the medial and lateral compartments of the knee in many mammalian species, suggesting the pattern is deeply conserved across evolution.³

The Structures That Control It

Several tissues work together to keep internal rotation within a safe range. The anterior cruciate ligament (ACL) is the most important single restraint. The ACL stretches as the tibia rotates inward: one biomechanical study found that ACL length increased in a straight-line fashion, about 0.7 mm for every 5° of internal rotation, up to 30°.⁴ That progressive tensioning is what makes the ACL the primary check on inward twist.

When researchers cut the ACL in cadaver knees, internal rotation increased by roughly 3° to 4°. Cutting additional structures on the outside of the knee, specifically the anterolateral ligament (ALL) and the iliotibial band (ITB), added more rotational looseness. The combined effect of losing the ACL plus those lateral structures was about 6° to 7° of additional internal rotation.⁵ A separate cadaver study confirmed that sectioning both the ALL and ITB in an already ACL-deficient knee produced increases in internal rotation ranging from 1° to 12° depending on the flexion angle, with about a fifth of the knees showing the largest increases of 8° to 12°.⁶ So while the ACL does the heavy lifting, the lateral structures act as important secondary restraints, particularly at deeper flexion angles.

On the muscular side, the popliteus is a small muscle at the back of the knee that actively produces internal rotation. Ultrasound-based stiffness measurements have confirmed that the popliteus stiffens during isometric internal rotation and during weight bearing, and its force decreases as the knee flexes during single-leg standing.⁷ The hamstrings, particularly the semitendinosus and semimembranosus on the inner side, also contribute to internal rotation when the knee is bent. The quadriceps, by contrast, generate a slight external rotation force because of how the patellar tendon attaches to the tibia.

Sex Differences in Rotational Laxity

Women’s knees tend to allow more internal rotation than men’s. A cadaveric comparison found that female knees had greater internal laxity from full extension through 50° of flexion, with the largest difference being about 8° at 50° of flexion. Male knees had roughly 42% greater internal rotational stiffness between 0° and 30° of flexion.⁸ In living subjects, women showed about 20% more internal-external rotation laxity and about 24% less rotational stiffness compared with men across the menstrual cycle.⁹

These differences in baseline laxity are one piece of the puzzle behind women’s higher rates of ACL injuries, though they are far from the full explanation. Hormonal fluctuations, neuromuscular control patterns, limb alignment, and differences in landing mechanics all play roles. The rotational laxity difference does mean, though, that what counts as “normal” internal rotation varies by sex, and clinicians need to consider this when interpreting examination findings.

How Bone Shape Influences Rotation

Not all knees are built the same, and the slope of the tibial plateau, the flat-ish top of the shinbone, has a measurable effect on rotational behavior. The lateral (outer) and medial (inner) sides of the plateau can have different backward slopes. When the difference between those two slopes is large, the knee tends to rotate more internally. One cadaveric study found that increasing this slope difference by 10° produced about 7.6° more internal rotation at full extension.¹⁰

This bone geometry also matters clinically. In patients with ACL injuries, a steeper lateral posterior tibial slope correlated with a more dramatic pivot-shift test result, the clinical sign of rotational instability.¹¹ In healthy female athletes, those with a lower medial-to-lateral slope ratio and a more steeply inclined overall tibial slope went through greater internal rotation excursions during landing tasks. For every 0.1 decrease in the medial-to-lateral slope ratio and each additional degree of overall tibial slope, internal rotation excursion increased by about 2.2°.¹²

The practical takeaway is that two people can have the same ligaments, same muscle strength, and same movement patterns, but different amounts of internal rotation simply because their bones are shaped differently. Surgeons increasingly factor tibial slope into their planning, particularly when deciding whether a patient needs additional procedures beyond a standard ACL reconstruction.

Internal Rotation and Knee Injuries

Excessive or poorly controlled internal rotation is a central player in ACL tears. The pivot-shift mechanism that ruptures the ACL involves a combination of internal rotation and valgus (knock-knee) force. Laboratory recreation of the pivot shift has shown that coupling internal rotation torque with valgus torque best reproduces the characteristic anterolateral subluxation of the lateral tibial plateau.¹³ This is why sports that demand sudden cutting and pivoting, like soccer, handball, and basketball, carry high ACL injury rates.

A large prospective study of over 750 female elite handball and soccer players examined knee biomechanics during cutting maneuvers. Players who had previously torn their ACL showed lower peak internal rotation moments on the previously injured leg compared with injury-free players, suggesting a protective movement adaptation after injury. Interestingly, none of the knee-specific biomechanical variables measured during cutting predicted which players would go on to suffer a second ACL injury.¹⁴ That finding highlights how difficult it remains to identify who will reinjure their knee based on movement analysis alone.

Internal rotation also matters for the kneecap. When the tibia rotates inward excessively, it can shift the line of pull on the patellar tendon and increase stress on the undersurface of the patella. Taping techniques that limit tibial internal rotation have been shown to reduce pain during lunges and single-leg squats in people with patellofemoral pain syndrome, even though the taping did not measurably change lower-limb movement patterns on motion capture.¹⁵ The pain relief without kinematic change suggests the tape may work partly through sensory feedback rather than purely mechanical correction.

Internal Rotation and Long-Term Joint Health

After an ACL tear, rotational mechanics tend to stay abnormal even if the knee feels stable in day-to-day life. Research has found that in ACL-deficient knees, the tibia remains internally rotated throughout the stance phase and late swing phase compared with the uninjured side. This persistent offset loads cartilage regions that were not typically loaded before the injury, potentially accelerating the development of osteoarthritis.¹⁶ In knees that already have osteoarthritis, a related but distinct problem appears: greater femoral internal rotation and dysfunction with the screw-home mechanism during extension.¹⁷ In other words, altered rotation can both result from and contribute to joint degeneration, creating a feedback loop.

Measuring Rotational Laxity in the Clinic

Assessing how much internal rotation a knee allows is trickier than measuring how far it bends or straightens. Surgeons have traditionally relied on manual tests, twisting the tibia inward by hand with the knee at 30° and 90° of flexion and comparing the feel to the other side. But the subjective nature of these tests has driven development of specialized devices.

One such device, a rotational measurement device (RMD), achieved readings that were on average within 2° of an invasive navigation system used in surgery. In healthy knees, 95% showed side-to-side differences of less than 5° on this device, compared with a threshold of 13° or more that clinicians use to diagnose pathological rotational looseness. Of note, simply measuring rotation at the foot with a boot-mounted tool overestimated rotation by an average of 34° and showed poor correlation with the actual tibial rotation.¹⁸ A separate rotameter device tested in both live subjects and cadavers also demonstrated high reliability and strong agreement with navigation-system readings.¹⁹

These tools remain largely research instruments for now, but they underscore an important clinical point: rotational laxity is real and measurable, and the side-to-side comparison is what matters most. A knee that has 25° of total internal rotation is not necessarily abnormal if the other knee measures the same.

Surgical Options for Restoring Rotational Control

Standard ACL reconstruction does a good job restoring forward-backward stability, but surgeons have increasingly recognized that it does not always fully correct the rotational component. This realization has driven interest in lateral extra-articular tenodesis (LET), a procedure that tightens the outer side of the knee to limit internal rotation. A systematic review of biomechanical studies found that seven out of eight papers analyzing tibial rotation reported that LET reduced internal rotation in the ACL-deficient knee compared with the native state.²⁰

More recent biomechanical work has confirmed that LET increases the restraint to internal rotation compared with the native anterolateral tissue, particularly at 30°, 60°, and 90° of flexion.²¹ Clinical trial evidence supports the addition of LET for high-risk patients: it reduces graft rupture rates and improves control of the pivot-shift type of instability. That said, no single LET technique has emerged as clearly superior, and the procedure is generally recommended selectively rather than for everyone getting an ACL reconstruction.²²

Training, Taping, and Footwear

For people who are not heading into surgery, there are simpler ways to influence internal rotation. A six-week neuromuscular training program in young female netball athletes significantly reduced knee internal rotation angle during single-leg landing.²³ Programs like this typically emphasize landing technique, hip and core strengthening, and balance work. The goal is not to eliminate internal rotation, which is a normal and necessary component of movement, but to ensure muscles control it rather than allowing it to spike during high-force activities like cutting and landing.

Footwear can also make a difference. A pilot study comparing shoe conditions during running found that stability shoes produced less knee internal rotation than neutral shoes or shoes with foot orthoses, with mean differences ranging from about 1.7° to 6.1° depending on the phase of the stride.²⁴ Whether those relatively small angular changes translate into meaningful injury prevention is still debated, but for runners dealing with knee pain linked to excessive rotation, a stability shoe is a low-risk option worth trying.

Internal Rotation After Knee Replacement

Recreating normal internal rotation is one of the hardest challenges in total knee arthroplasty. The artificial surfaces do not perfectly replicate the asymmetric geometry of the natural tibial plateau, and decisions about implant positioning have direct consequences for rotational kinematics.

Placing the tibial component in slight internal rotation during surgery actually decreased tibial internal rotation during flexion in a computational modeling study, while moderate external rotation of the component produced kinematics closer to those of a natural knee.²⁵ This is one reason surgical guides typically aim for slight external rotation of the tibial tray.

Whether to keep the posterior cruciate ligament (PCL) during the procedure also affects rotation. In one design that retains the PCL, knees averaged about 15° of passive internal rotation from full extension to 90° of flexion, compared with only 7° when the PCL was removed.²⁶ Even more striking, a study of bicruciate-retaining designs, which keep both the ACL and PCL, found that native rotational kinematics survived meniscus removal and femoral component placement but were lost once the tibial surface was replaced.²⁷ The tibial plateau’s natural geometry appears to be the master controller of the rotation pattern, and current implant surfaces do not fully replicate it.

In-Toeing in Children

Parents frequently worry when a child walks with the feet pointing inward, and this concern often gets framed as a knee rotation problem. In most cases, pediatric in-toeing is caused not by the knee itself but by one of three sources higher or lower in the leg: increased femoral anteversion (the thighbone is twisted more forward than average), internal tibial torsion (the shinbone itself is twisted inward), or metatarsus adductus (the front of the foot curves inward). All three are common developmental variants that typically resolve on their own as the child grows. Surgical correction is rarely needed and is generally reserved for cases that persist past skeletal maturity and cause functional problems. The key clinical step is distinguishing which level the twist is coming from, since each has a different natural history and a different threshold for intervention.