2. Knee injury biomechanics

In a frontal impact, the occupant's lower extremities are among the first structures to interact with the vehicle interior. When the pelvis is restrained by the seat belt and the torso pitches forward, the knees travel toward the instrument panel (IP) and typically make contact before the upper body reaches the airbag. The knee–thigh–hip (KTH) complex therefore acts as a load path that can absorb energy, but it can also transmit injurious forces into the distal femur, patella, tibial plateau, and the ligaments that stabilize the knee joint. Designing an IP that manages this contact requires a working knowledge of what is being loaded, how it fails, and what numerical thresholds define acceptable versus injurious loading.

Anatomy of the Load Path

The knee is a modified hinge joint formed by the distal femur, the proximal tibia, and the patella. In IP contact, the load is applied anteriorly, usually to the patella and the anterior tibial tuberosity, and is transmitted proximally through the femoral condyles into the femoral shaft and hip. Several structures govern how that load is distributed and where failure initiates:

• Patella and patellofemoral joint: The patella is a sesamoid bone embedded in the quadriceps tendon. Under direct anterior loading it acts as a load-spreading plate, but it is also the most anterior structure and therefore the first point of contact. Patellar fracture and chondral damage are common outcomes of concentrated IP loading.
• Distal femur and femoral condyles: The condyles transfer load into the shaft. With the knee flexed, the condyles are loaded in a combination of compression and bending, and the supracondylar region is a frequent fracture site.
• Tibial plateau and proximal tibia: The plateau transmits load into the tibial shaft. Its cancellous bone is relatively weak, so plateau depression fractures occur when the load is applied through the patella with the knee near extension.
• Ligaments and capsule: The ACL, PCL, MCL, and LCL, together with the joint capsule, resist shear and rotation. They are rarely the primary injury in pure IP contact, but they determine how much of the applied load is converted into bone stress versus joint displacement.
• Knee flexion angle: The posture at contact strongly influences the load path. A flexed knee (roughly 60°–90°) loads the patella and distal femur more directly; a near-extended knee shifts load toward the tibial plateau and can produce a different fracture pattern.

Injury Mechanisms Specific to IP Contact

Three mechanisms dominate in instrument panel loading:

1. Direct anterior compression of the patella. A stiff or concentrated IP surface applies a localized force to the patella, producing patellar fracture, patellofemoral cartilage damage, and, at higher magnitudes, distal femoral fracture. The severity depends on contact area, IP stiffness, and the rate of loading.

2. Axial loading through the extended or slightly flexed knee. When the knee is near extension, the applied force is transmitted largely along the tibial axis, loading the tibial plateau and producing depression or split fractures. This mechanism is sensitive to the knee flexion angle and to whether the foot is planted on the floor, which changes the boundary condition at the ankle.

3. Bending and shear of the distal femur. With the knee flexed and the femur angled relative to the load direction, the distal femur experiences bending moments that can exceed the strength of the supracondylar region, producing a transverse or comminuted fracture. This is often the mechanism that governs IP design because it occurs at moderate flexion angles typical of real occupants.

Tolerance Criteria and Their Interpretation

Knee tolerance in IP loading is usually expressed as a force or moment limit measured at the knee, or as a deflection limit. The most widely referenced values come from cadaver and PMHS testing and are codified in regulatory and consumer test procedures. Representative values used in practice include:

These values are engineering references, not absolute pass/fail lines. They were derived under specific test conditions (loading rate, flexion angle, boundary conditions) and should be applied with the same configuration in mind. In practice, an IP design is evaluated by measuring knee forces and moments in a sled or component test and comparing them against the applicable criterion for the occupant size and posture being assessed.

From Criteria to Design Decisions

The practical consequence of these tolerances is that the IP must be compliant enough to limit peak knee force and moment, yet stiff enough to absorb energy without bottoming out against hard structures. A common approach is to provide a deformable knee bolster with a controlled crush characteristic: an initial compliant phase that limits the peak force, followed by a stiffer phase that absorbs the remaining energy over a longer stroke. The target is to keep the knee force below the fracture threshold while limiting deflection so the knee does not contact rigid substructure.

Consider a representative design check. Suppose a component test applies a 4 kN target load to a knee bolster at 60° knee flexion, and the measured peak knee force is 9.2 kN with a knee moment of 240 N·m. The force is below the 10 kN femur-fracture reference, but the moment is close to the upper end of the 200–300 N·m band. The design action is not simply to soften the bolster further, because that may increase deflection and risk bottoming out; instead, the crush curve should be reshaped so the initial peak is reduced while the stroke is preserved. This is the kind of trade-off that knee biomechanics informs.

Applying the Criteria in Assessment

When assessing an IP design, the knee biomechanics translate into a small set of measurable quantities: peak knee force in each axis, peak knee bending moment, and knee deflection relative to the femur. These are typically obtained from a Hybrid III or THOR dummy in a sled test, or from a component-level impactor test that reproduces the knee–bolster interaction. The assessment should record the knee flexion angle and foot condition alongside the measured values, because the same force at 90° flexion and at 30° flexion does not imply the same injury risk.

A useful discipline is to treat the tolerance values as a band rather than a single number. If the measured knee moment sits in the upper part of the 200–300 N·m band while force is comfortably below 10 kN, the design is likely acceptable but has limited margin for occupant variation (larger occupants, different postures, or a stiffer IP in production). In that case, the next design iteration should focus on the crush characteristic rather than on the peak force alone. Conversely, a design that keeps moment low but allows large deflection may be trading fracture risk for bottoming-out risk against rigid substructure, which can produce a sharp force spike late in the event.

The key takeaway for IP design is that knee injury risk is governed by the interaction of three variables: the load applied by the panel, the posture of the knee at contact, and the load path through the femur and tibia. Tolerance criteria give the numerical targets, but the design decisions that meet them come from shaping the panel's crush behavior and contact geometry so that the load is distributed and the peak is limited without sacrificing stroke.