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TKR: Biomechanics & Prosthetic Design Considerations

- See:
- Total Knee Replacement Menu
- Axial Alignment
- Kinematics of the Knee Joint
- Metallurgy: TKR
- Rotation of TKR
- Unconstrained Prosthesis


- Discussion:
- inability to achieve precise alignment will ultimately lead to component loosening, the leading cause of TKA failure;
- designs with greater conformity between the femoral & tibial components tend to provide greater inherent stability;
- requirements for TKR:
- axis of rotation: 15 - 20 deg
- medial-lateral motion of 5 -10 degrees;
- flexion requirements:
- 65 degrees of flexion is required to walk at a normal pace
- 95 degrees of flexion to walk up and down stairs;
- 110 degrees is required to arise from chair;
- ref: Can a High-flexion Total Knee Arthroplasty Relieve Pain and Restore Function Without Premature Failure?

- Posterior Condylar Offset:
          - references:
- Does mismatch of the femoral component aspect ratio influence the range of knee flexion after posterior-stabilized total knee arthroplasty?
- Posterior condylar offset and flexion in posterior cruciate-retaining and posterior stabilized TKA.

- femoral roll back:
- as knee flexes, femur rolls back on tibia, increasing potential for further flexion by preventing posterior structures from impinging;
- rollback increases the length of quadriceps moment-arm;
- if femoral roll back is not designed in prosthetic knee, effective strength of quadriceps is reduced by about 30%;
- w/ posterior cruciate sacrificing TKR, the posterior projection of  femoral condyle should, ideally, be as large as possible;
- effects of increased conformity:
- increases the contact area (and reduces contact stress within the polyethylene)
- less risk of polyethylene wear, fracture, and delamination;
- limits the ability of the femur to roll back during flexion (increased shear stress to the fixation surfaces of the implants);
- ref:  Multicenter determination of in vivo kinematics after total knee arthroplasty.

- PCL retaining vs posterior stabilized prosthesis:
- Posterior stabilized prosthesis
- PCL retaining prosthesis

- effect of constraint on TKR:
- unconstrained prosthesis
- semiconstrained prosthesis:
- may be used for knees w/  flexion contractures of up to 45 deg and angular deformities of 20 to 25 deg;
- these types of components have a more conforming articulation and the contact area remains the same throughout knee motion;
- theoretically there is minimal roll back with knee flexion;
- in many semiconstrained components, saggital radius of component is constant, and therefore knee flexion would be expected to be limited to 120 deg;
- IB2
- basic Insall Burstein Knee is modification of original modification of original total condylar design;
- problems w/ IB1 include poor stair climbing ability (only 22%);
- total constrained prosthesis:
- these implants all restrict normal knee motion in one or more planes;
- stresses are high on implant & at implant-cement & bone-cement interfaces;
- higher incidence of loosening, breakage, and excessive wear;
- aseptic loosening of the components is most common complication;
- are more prone to wear from cement & polyethylene polyethylene particles

Evaluation of the Noiles hinged knee prosthesis. A five-year study of seventeen knees.

Adventures in mobile bearing knee design: a mid-life crisis.

Effects of total knee replacement design on femoral-tibial contact conditions.

Laxity and function in knee replacements. A comparative study of three prosthetic designs.

Optimizing Flexion After Total Knee Arthroplasty: Advances in Prosthetic Design.

Design may be Counterproductive for Optimizing Flexion After TKR.

Total knee replacement with natural rollback.

Tibiofemoral kinematics and condylar motion during the stance phase of gait

Physiologically shaped knee arthroplasty induces natural roll-back.

Patellar tracking in primary total knee arthroplasty.