Knee biomechanics

The Knee Joint Biomechanics

The knee joint is a complex synovial joint with tibiofemoral and patellofemoral articulations. It comes under a lot of stress as it is required to have a large ROM coupled with the ability to withstand high forces. It also joins the two longest bones in the body together, these lone levers can therefore put a substantial amount of stress on stabilizing ligaments.

The surfaces of the tibia and femur are not as congruent as other joints such as the hip, despite this the knee is said to be able to move in 6 degrees of freedom. This means the knee joint can move in three dimensions (abduction/adduction. flexion/extension, internal/external rotation, anterior/posterior draw, medial/lateral shift and compression/distraction) these degrees of freedom can be tested as accessory movements. SO although the knee as a joint is able to flex and extend and also give some rotation, it needs these 6 degrees of freedom to do this.

Active knee flexion is typically around 130 degrees with 160 usually attainable passively.  In order for the knee to be picked up appropriately to swing past the supporting leg in a gait cycle then approximately 67 degrees of flexion is required. If the knee was not able to reach this angle then a circular motion centered at the standing ankle would occur- this causes the center of gravity to move upwards and then downwards which requires a greatly increased energy expenditure. This gait is frequently observed in new amputees with prosthetic and also post TKR.
The knee is also required to flex when it is the opposite leg in the swing phase (to approx 15 degrees) this is to dissipate impact force when the opposite leg hits the ground allowing a constant height when walking as the impact energy is absorbed by stretching the quads.
 The internal and external tibial rotation that occurs at the knee is often forgotten but is a key component in gait due to the "screw home" phenomenon. This phenomenon is difficult to imagine and even more difficult to explain. Its main purpose is to disperse energy when walking. When the foot swings for heel strike the pelvis rotates so that the hip moves forward. This movement requires external rotation of the hip. The stance occurs and the femur is internally rotated against the locked knee. Tibial external rotation causes inversion of the foot at the sub-talar joint, this raises the arch of the foot. As flexion then occurs the tibia rotated internally everting the foot and absorbing the energy.
That broadly covers the general knee kinematics mainly when looking at the gait cycle.


This leads us to the smaller scale articular mechanics of the knee

These small movements are very important in how the knee moves. If you look at the arcs of the femoral condyles they are much longer A-P than the tibial plateau, if they purely rolled they would roll off the back of the tibia at full flexion. Therefore as the knee flexes an anterior slide occurs allowing correct articulation. If it was a complete roller like in the elbow then only sliding would occur but due to the need for tibial rotation this is not the case.
When the knee locks the front of the femoral condyles presses into the anterior horns of the menisci, this causes the ACL to be tense and the PCL slack.

Articular Mechanics
The force both external and internal that occurs at the knee imposes considerable forces at the articular cartilage. This can occur in two planes horizontal (compressive) or vertical (shear or friction)
Compression-  The amount of compressive force is relative to the amount of load and the area of contact. If the knee was fully conforming then all pressure would be distributed throughout the whole knee decreasing the amount of pressure.This is not the case; the medial compartment is quite conforming but the lateral is less so with a flat tibial plateau. this is due to the screw home effect as most tibial rotation is needed in the lateral compartment. The body has adapted for this creating the menisci to be wedge shaped so that the force is as distributed as possible. When the weight isnt distributed correctly higher contact stress occurs which explains the prevalence of OA following meniscectomy.
The load carrying mode of the minisci is performed by squeezing the meniscus out of the joint. This causes the menisci circumference to increase. As it expands hoop tension occurs due to the strong fibres that are around the menisci periphery which transmits to tension on the tibial plateau due to strong inserting ligaments. The radial strength of the meniscus is considerably less than the hoop strenght which is why bucket handle tears occur.

Friction- there is remarkably low friction in synovial joints. If high amounts of friction occurred then cartilage would wear away extremely quickly due to the friction forces that would be applied. The reason for such low friction between cartilage and surrounding structures is the presence of synovial fluid and something called fluid film lubrication. when compressive forces occur at a microscopic level synovial fluid is squeezed out, this stops surfaces actually technically coming together as they are seperated by a thin layer of synovial fluid, then when movement occurs the fluid becomes trapped between the two surfaces in something termed the hydrodynamic effect. This cycl protects cartilage much like a thin layer of oil on a chain.


Soft Tissue Mechanics
The knee joint needs both the active stability coming from the muscles and the passive stability from the ligaments to keep the joint in its correct and most efficient form. The quads, hamstring and gastrocnemius control flexion/extension and internal/external rotation of the knee however in doing this they cause anterior/posterior shear forces that are restricted by the cruciate ligaments.  The ligaments around the knee develop tensile strength to resist displacement.
In passive structures there can be primary and secondary restraints. The primary restraint is a structure that is well aligned to deal with a force- for instance the ACL is well aligned to resist the anterior draw. However the MCL and menisci act as secondary restraints resisting this movement.
The patellafemoral joint is heavily loaded during weight bearing with an estimated  joint force of 5.5x body weight at 90 degrees flexion. In the frontal plane a force termed the Q force is generated which is the force put on the quadriceps tendon- usually parallel to the femoral shaft. This angle can be changed due to tibial rotation, hip rotation and quad tension. in men the clinical Q angle is 12-15 degrees and 15-18 degrees in women. Contracting the quads therefore displaces the patella laterally, this is resisted by the VMO.