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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