The anatomy and geometry of the knee is uniquely suited for its motion and offers both static and dynamic stability, allowing it to withstand many multiples of our body weight during daily activities.

The femur, tibia, and patella make up the tibiofemoral and patellofemoral joints of the knee. The femoral condyles are rounded, especially posteriorly, and have a posterior offset relative to the femoral shaft, allowing for deep flexion.¹,² The medial condyle is larger and more circular in comparison to the lateral condyle and has a more uniform radius of curvature, allowing it to remain mostly stationary during knee flexion while the lateral condyle translates posteriorly, creating the posterior femoral rollback. This crucially allows the distal femur to externally rotate and promotes patellar engagement with the trochlear groove during knee flexion. Anteriorly, the condyles flatten and merge, forming the trochlear groove and centrally to form the intercondylar notch.¹,³

The articular surface of the tibia, also known as the tibial plateau, is asymmetric like the distal femur, conferring stability to the knee. In the coronal plane, the tibial plateau has a slight inward tilt that matches the medial to lateral asymmetry of the femoral condyles. In the sagittal plane, the plateaus are slightly posterior in relation to the tibial shaft axis and usually have a posterior slope. One study analyzing MRIs found that for males the medial tibial slope ranged from −3° to 10° (average 3.7) and the lateral tibial slope ranged from 0° to 9° (average 5.4). In females the medial tibial slope ranged from 0° to 10° (average 5.9) and the lateral tibial slope ranged from 1° to 14° (average 7.0).⁴ The tibial slopes are variable between different sexes and population and depend on the imaging and reference axis used. The more increased lateral tibial plateau slope enhances the posterior rollback.

Additional congruence between the femoral condyles and the medial and lateral tibial surfaces is granted by the menisci, which also behave as loading and stabilizing gaskets.¹,⁴,⁵,⁶ The medial and lateral menisci increase the effective joint surface, reducing contact forces by dissipating axial load into hoop stress. The medial tibial plateau is larger and more concave and is effectively deepened by its meniscus anchored along the tibial margins.⁶ In contrast, the lateral tibial plateau is smaller and more convex, with a centrally fixed lateral meniscus that remains more mobile on the periphery to accommodate the posterior rollback of the lateral condyle.⁷

The ligaments of the knee also play an important role in motion and stability. The medial and lateral collateral ligaments offer stability to varus and valgus stresses in the coronal plane. The superficial MCL is the major medial stabilizer, while the LCL provides lateral stability. The anterior and posterior cruciate ligaments provide stability to anteroposterior stresses in the sagittal plane. In ACL-deficient knees, the tibia subluxates anteriorly leading to cartilage wear in the posterior-medial aspect of the knee compared to the anterior-medial aspect. In PCL-deficient knees, the tibia subluxates posteriorly interfering with the proper posterior rollback and thus terminal flexion.

During normal gait, 60% to 70% of weightbearing forces in the stance phase pass through the medial compartment of the knee. Small changes in alignment lead to significant changes in load distribution in each compartment, which may predispose to or accelerate arthritis.⁸,⁹,¹⁰

Restoration of lower extremity alignment with proper TKA component alignment normalizes the distribution forces across the implant. Malpositioned components change the load distribution in the compartments and predispose to early failures.¹¹,¹²

Motion patterns of the knee are complex. The knee primarily flexes and extends but also rotates in the axial plane (internal-external rotation) and the coronal plane (varus-valgus rotations).¹³ Although the knee is believed to have two rotation axes in the sagittal plane,¹⁴ several studies recommend assessing knee motion in the sagittal plane by a line connecting the medial and lateral epicondyles—the transepicondylar line (TEL).³,⁴,¹⁴,¹⁵ When viewed parallel to the TEL, the posterior projections of the femoral condyle are two concentric circular outlines with the larger medial outline reflecting the larger radius of curvature of the medial femoral condyle (Fig. 6-1). The flexion-extension gap, the area between the TEL and tibial joint surface, remains constant throughout flexion and allows for constant tension on the collateral ligaments (Fig. 6-2).¹⁶

As the knee flexes, the tibia shifts slightly from varus to valgus and also internally rotates (femur externally rotates).¹⁷ The internal-external rotation of the tibia with
respect to the femur occurs about the tibia’s long axis (Fig. 6-3A).¹⁴,¹⁵,¹⁸ The rotation of the tibia during flexion and extension is accommodated by the medial and lateral asymmetry in the tibiofemoral joint described previously.¹,⁶ The medial compartment has a concave tibial surface and a more immobile meniscus in comparison to the convex tibial surface, which has a more mobile meniscus in the lateral compartment. This results in less posterior translation of the medial tibiofemoral contact point in comparison with the lateral tibiofemoral contact point, resulting in tibial internal rotation (femoral external rotation) during knee flexion (Fig. 6-3B).¹,¹⁵,¹⁷,¹⁸ This rotation totals approximately 30° over the entire flexion-extension arc. When the knee reaches full flexion, the lateral tibiofemoral contact point has translated posteriorly to the edge of the tibial surface with 30° of external
rotation and 5° of valgus rotation.¹⁹ Variation in the anteroposterior translations of the tibiofemoral contact points in different studies can be explained by different anatomic references, activity, and foot positions.¹⁹

Patellofemoral kinematics are also affected with knee flexion and extension. In stance, while the knee is extended, the patella lies proximal and lateral to the trochlear groove. As the knee flexes, the femur begins to externally rotate and the patella enters the trochlear groove and tracks along the groove.²⁰ The patella itself also begins to flex as it moves distally with knee flexion, causing the patellofemoral contact point to move distally.²¹,²² At terminal flexion, the patella sinks between the two femoral condyles, making contact with each (Fig. 6-4).² The patella’s flexion-extension motion is about an axis located transversely to the femoral condyle, slightly anterior and distal to the TEL (Fig. 6-4).³,²⁰ Patellofemoral kinematics is altered by femoral and tibial tubercle variations as well as ligamentous laxity.²³,²⁴,²⁵,²⁶

There is significant variation in lower extremity alignment. Individual differences in height and bone morphology, including degenerative changes, affect knee alignment. The mechanical axis of the lower extremity is determined by drawing a line from the center of the femoral head to the center of the ankle.²⁷ In normal limbs, the mechanical axis usually passes through the medial tibial spine but is also dependent on height and pelvic width as noted above. The mechanical axis can be subdivided into the femoral mechanical axis and the tibial mechanical axis. The femoral mechanical axis is measured from the center of the femoral head to the center of the intercondylar notch of the distal femur. The tibial mechanical axis is measured from the center of the proximal tibia to the center of the ankle.

The anatomic axis of the lower extremity is based on the relationship of the intramedullary canals of the femur and tibia.²⁷ The anatomic axis of the femur is created by a line drawn proximal to distal in the intramedullary canal bisecting the femur in half. The angle between the anatomical and mechanical axis of the femur is usually 5° to 7°. The anatomic axis of tibia is created by a line drawn proximal to distal in the intramedullary canal bisecting the tibia in half. The anatomic axis and mechanical axis of the tibia are often the same, though the anatomic axis can vary if there are bony angular deformities.

Knee alignment is described as the orientation of the thigh (femur) with the leg (tibia, fibula, and ankle). It can be further described in the coronal and sagittal perspectives (Fig. 6-5).

In the coronal view, while standing, the ground reaction force passes from the hindfoot through the ankle joint to the center of the hip. When the knee is well aligned, it is centered on this load-bearing axis (LBA). Coronal malalignment occurs when the knee center deviates significantly from the LBA. The varus knee deviates laterally with the respect to the LBA, overloading the medial tibiofemoral compartment, while the valgus knee deviates medially, overloading the lateral compartment (Fig. 6-6).¹⁰,²⁸,²⁹,³⁰,³¹ This can bias locations of osteoarthritis based on the particular compartment of the joint that sees this increased load.

In the sagittal view, the knee center is slightly posterior to the LBA. Genu recurvatum results when the knee center is located significantly posterior to the LBA, creating a hyperextension deformity. By contrast, a flexion contracture results in a knee center which is anterior to the LBA (Fig. 6-7).

Radiographic assessment of knee alignment is best made with AP and lateral weightbearing views.¹⁰,²⁵,²⁸,²⁹ Complete alignment of the limb requires inclusion of the hip, knee, and ankle using a full-length radiograph (such
as a standard 36-inch cassette) or from digitally stitched images.²⁵,³¹,³² To minimize variability and rotation errors, the knee should be positioned with the flexion plane facing straight ahead rather than basing this off of the patella facing ahead due to the variability of the latter.²⁵,²⁹ To assess patellar orientation, axial (Merchant) views should be obtained, and the TEL be used as a reference (Fig. 6-8).⁴,²⁴,³³ A CT scan may also be used to assess rotational variation.³⁴