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Posterior Stabilized Prosthesis

- See:
      - Biomechanics of TKR
      - Joint Line Position
      - PCL Retaining TKR

- Discussion:
    - cruciate ligaments are needed for running atheletes, but are not as important to resist forces in thesagittal plane in reconstructed arthritic knee;
    - theoretically, PCL-substituting TKR allow exposure & ligament correction of PCL-sacrificing prosthesis while reproducing kinematic effects of PCL;
          - there is predictable roll back, which allows greater ROM and due to the CAM  effect, increases the lever arm;
          - in the report by Udomkiat P, et al (2000), 38 matched pairs of DJD knees from patients who underwent TKR w/ minimum 2 yrs of followup were studied to
                  compare the functional outcome between a cruciate retention and PS design w/ essentially identical articulation surfaces;
                  - there was no statistically significant difference between the two groups in the clinical evaluations;
                  - fluoroscopic kinematics showed that PS knee experienced AP femorotibial translation more similar to the normal knee during normal gait and deep knee bend;
                  - ref: Functional comparison of posterior cruciate retention and substitution knee replacement.  
    - in considering a posterior stabilized, forces tending to produce AP glide are  much smaller than tibio-femoral compressive force (approx 1/5);
          - hence, prosthesis need not provide much resistance to AP subluxation;
    - effects of removal of PCL:
          - tends to reduce flexion moment on the knee, which is compensated for by leaning the body forward;
          - transfers shear forces that are normally absorbed by PCL to interface between the bone and cement;
          - greater stress is also transferred to patella, and frxs are more frequent;
          - level walking gait analysis does not show any significant difference between retaining or removing PCL;
          - stairs:
                 - w/ stairs, pts w/o PCL shift their wt farther forward w/ each step;
                 - leaning forward results from shortened quadriceps mechanism lever arm w/o the roll back provided by the PCL;
                 - increases shear stresses at bone-cement interface during stair limbing because of lack of femoral rollback;
    - retention of PCL results in more normal gait pattern during stair climbing, partly
           because of the ability to maintain normal flexion moment of knee during stair climbing;
    - flexion:
           - in posterior stabilized TKR, average flexion angle is 100-115 deg;
    - effects of PCL and MCL release:
           - in the report by Saeki, et al., 6 knees from cadavers were tested for change in stability after release of the MCL with  posterior cruciate-retaining and substituting TKR.
                  - posterior-stabilizing post added little to varus and valgus stability;
                  - posterior-stabilized total knee arthroplasty was even more rotationally constrained in full extension than knee with intact MCL and PCL;
                  - ref: Stability After Medial Collateral Ligament Release in Total Knee Arthroplasy.  

- Indications for Posterior Stabilized Prosthesis:
    - excessive posterior roll back:
            - if tightness remains on medial side despite release of medial collateral ligaments & posterior capsule;
            - knee may be tight in flexion, & has excessive posterior roll back indicating that the PCL is too tight;
            - tight PCL causes excessive rollback, which results in a stiff & painful knee;
            - excision of PCL affords easier correction of fixed deformity;
    - insufficient rollback:
            - if preserved PCL is lax, knee demonstrates a posterior sag and no evidence of roll back;

- Design of Stabilized Prosthesis:
    - TKA designs that resect the PCL must provide AP stability by having congruent geometry in flexion to avoid sagittal instability;
          - this geometry permits essentially uniaxial flexion with less flexion arc & produces a quadriceps force which is weaker;
    - intercondylar spine (post):
          - PCL function can be built into prosthesis by including appropriately shaped intercondylar tibial eminence;
          - however, it transfers shear forces normally absorbed by the PCL to the bone cement interface;
                  - despite this, aseptic loosening and osteolysis are rarely associated with cemented posterior stabilized implants;
          - intercondylar spine on tibial component aritculates w/ transverse cam on femoral component
                  which has the effect of substituting for PCL;
                  - this spine articulates w/ transverse bar on the inner surface of intercondylar portion of femoral component;
                  - this bar forces the femur posteriorly on the tibial plateaus, enabling knee to flex more than 110 deg;
                  - bar also acts as a stabilizer when the knee is in flexion, helping to prevent posterior subluxation when patient stands from chair or climbs stairs;
          - interaction between the tibial spine and the femoral cam inaddition to modified center of curvature of the prosthesis, allows femoral rollback during flexion;
          - modifications and difference between systems:
                  - increasing the height in the tibial spine (and corresponding box cut):
                        - may increase medial-lateral instability, especially in extension;
                        - may or may not lead to increased polyethylene wear;
                        - may make future revision surgery more difficult w/ increased risk of knocking out one or both femoral condyles;
                  - shifting of the femoral cam posteriorly;
                  - note that in some TKR systems, the intercondylar spine remains a constant size for each femoral component size;
                        - consequence of this is that there will be a relatively large notch cut for a small femoral component, and this risks femoral condyle fracture if the component had to be removed;
                        - in case of a small patient, it might be wise to select a TKR system that offered intercondylar spine sizes proportional to femoral component size;
          - effect of spine on component loosening:
                  - in the report by Mikulak SA, et al, the authors studied loosening with press fit condylar PCL substituting TKR;
                  - the authors concluded that rotational forces were generated by impingement of the side walls of the intercondylar box on the polyethylene post;
                               - they felt that rotational stresses are transmitted to the modular interfaces and to the metal-cement interfaces, resulting in loosening and osteolysis;
                               - reduction in rotational constraint would be desirable;
                               - patients with bilateral total knee replacement may be at increased risk for this type of loosening;
                  - ref: Loosening and Osteolysis with the Press-Fit Condylar Posterior-Cruciate-Substituting Total Knee Replacement.  

- Complications:
    - Patellar Clunk Syndrome and Synovial Entrapment

Cruciate retained and excised total knee arthroplasty. A comparative study in patients with bilateral total knee arthroplasty.

The posterior stabilized total knee prosthesis. Assessment of polyethylene damage and osteolysis after a ten-year-minimum follow-up.

Implant design influences tibial post wear damage in posterior-stabilized knees.

Increased Long-Term Survival of Posterior Cruciate-Retaining Versus Posterior Cruciate-Stabilizing Total Knee Replacements

Post impingement in posterior stabilised total knee arthroplasty