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Biomechanics of the Patello-Femoral Joint

- See: Role of Knee Joint During Gait:

- Discussion:
    - patella increases the mechanical advantage of extensor muscles by transmitting forces across knee at greater distance (moment) from axis 
            of rotation;
    - increases functional lever arm of quads as well as changing direction of pull of quad mechanism;
    - approx 7 cm of translation from full flexion to extension;
    - at > 90 deg of flex, quad tendon starts to contact trochlea;
    - in full flexion when patella is entirely in intercondylar notch, it incr lever arm of quad by only 10%;
    - as knee starts to come into extension patella's contribution increases until 45 deg of flexion at which patella lengthens lever arm by 30%
            - it then decreases w/ more extension;

- Contact area: maximum contact area at 45 deg.
    - in this position: both the central ridge and the medial and lateral facets area in contact with the sulcus;
    - in full extension: the lower most portion of the patella is in contact, and it progresses proximally as the knee is flexed;
    - odd facet: contacts the femur only at maximum flexion;
    - relative lack of conformity that contributes to many of the problems noted in patellar tracking;

- Two important biomechanical functions:
    - transmission of tensile forces:
           - it is the principal site of insertion of quadriceps, it transmits tensile forces generated by quadriceps to patellar ligament;
           - despite low friction in the patello femoral joint, tension in the tendon above patella differs from that in the patellar ligament below it;
           - tension in extensor mechanism can be considerable in the normal patient, even under static conditions, rising to 6 times half the body 
                   wt in each knee during crouching;
           - at 25 deg, the force at the patello femoral joint is equal to that passing thru the tibio femoral joints;
           - at nearly full bend (before pts buttocks touch his heels) this value has risen to nearly 150% of force passing through tibiofemoral 
                  - this explains need for large area of thick articular cartilage on normal patella and on the femoral condyles;
           - max patellofemoral joint reaction loads occur at 35 deg of flexion & may approach a max of 3.5 to 4 times body wt w/ stair climbing
                   or descent;
           - beyond 90 deg of flexion, posterior surface of quadriceps tendon also comes into contact with the trochlea;
                  - when this occurs, compressive forces on the patellofemoral articulation are diminished owing to division of the load bearing 
                          between patellofemoral joint and the tendon of quadriceps mechanism;
    - increasing extension lever arm:
           - patella effectively increases lever arm of knee extension mechanism from axis of knee flexion-extension;
           - this incr knee extensor moment generated by contraction of quads;
           - Kaufer documented this mechanical enhancing function of the patella;
                  - w/ cadaveric knees & balanced quad force w/ restraining force at distal aspect of tibia at 0-120 deg deg of knee flexion;
                  - by calculating moment about knee axis (knee moment equals tibial force multiplied by tibial moment arm), he was able to 
                          determine effective quad moment arm (quadriceps moment arm equals knee moment divided by the quadriceps force).
                  - he found that patella serves to incr magnitude of quad moment arm & that contribution made by patella increases w/
                           progressing extension of knee, being almost 30 % at full extension;
            - Patellar biomechanics.

- Patellofemoral Joint Reactive Force:
    - patella is subjected to complex loading
    - w/ knee extension, it transmits almost all of force of quadriceps contraction and thus is loaded primarily in tension;
    - w/ knee flexion, its post surface contacts distal aspect of femur & is subjected to compressive force = patellofemoral joint rxn force;
    - loading on the surface creates 3-point bending configuration in patella.
    - this bending load results in tension at ant surface of patella, which is additive to that naturally generated by distraction from contraction of 
           the quadriceps.
    - relative contribution of these modes of loading of patella depends primarily on position of knee joint;
           - as knee moves into flexion, bending forces become increasingly important.
           - magnitude of tensile forces in anterior surface of patella reaches maximum near 45 degrees of knee flexion;
           - loads across patella have not been precisely measured, but they probably are on order of 3000 newtons of tensile load and may rise 
                   to 6000 newtons in young, trained men.
           - during normal activities such as stair-climbing, joint reactive forces may equal 3 time body wt, and doing deep knee bends can 
                   increase JRF to 7-8 times body wt