The Hip: Preservation, Replacement and Revision

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

Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Tuesday, August 28, 2012 11:32 am