Stability of the Medial Pivot Total Knee Prosthesis
Condition: Osteoarthritis of Knee
MeSH terms: Osteoarthritis, Knee
Study type: Interventional

Summary: 
      The purpose of this study is to compare objective and subjective measures of knee stability
      following total knee arthroplasty with a medial pivot design vs. a posterior stabilized
      design.
    

Detailed description: 
      Several different prostheses are available for use in total knee arthroplasty (TKA). These
      designs aim to replicate the normal kinematics of the knee joint while maintaining stability
      throughout a full range of motion. The posterior cruciate ligament (PCL) contributes to these
      functions in the native knee, and if preserved it can function similarly in the prosthetic
      knee. Prostheses in which the PCL is sacrificed, and its function not replaced by other
      means, have poorer outcomes. However, various problems with preserving the PCL, including PCL
      deterioration in arthritic knees and difficulties with proper tensioning, have led to the
      development of alternative ways to emulate normal femoral rollback and sagittal plane
      stability. These posterior-substituted (PS) designs substitute for the PCL with either a cam
      and post mechanism or a symmetrical ultra-congruent tibial insert.

      The cruciate-retaining (CR) and PS designs, together with advances in surgical techniques and
      component materials, have achieved excellent implant survivorship with rates of > 90% at 20
      years. Reports of functional outcomes, however, have been variable. One potential area for
      improvement in TKA is optimization of implant design to better approximate native knee
      kinematics. In the normal knee, the medial condyle remains stable in the sagittal plane,
      functioning like a ball-and-socket, whereas the lateral condyle translates anterior to
      posterior during flexion. The designs of the CR and PS knees do not allow for this
      medial-centered rotation. Analysis has revealed paradoxical anterior sliding of the femur
      during flexion, abnormal axial rotation, and condylar lift-off. Edge loading and increased
      sagittal plane motion may predispose to accelerated polyethylene wear. The posterior
      stabilized design uses a cam-and-post mechanism in which one piece of the prosthesis has a
      plastic post that fits into a slot in the other piece of the prosthesis.

      A newer design that attempts to address these issues is the medial pivot knee (MP). This
      design is characterized by an asymmetrical tibial insert in which the medial compartment is
      ultra-congruent, providing antero-posterior stability and the lateral compartment allows for
      rollback around a medial axis of rotation i.e uses a ball-and-socket mechanism. This design
      more accurately recreates normal knee kinematics, reduces anteroposterior instability, and
      avoids condylar lift-off. Early studies indicate improved polyethylene wear characteristics.
      Midterm studies report excellent implant survivorship and clinical outcomes.

      Several randomized trials have compared the MP knee favorably with other designs. Patients
      with bilateral TKAs with a different prosthesis on each side preferred their medial pivot
      knee to a PS, CR, or mobile-bearing (MB) design. A trial comparing the MP and PS designs
      found greater range of motion (ROM) and better patient-reported outcomes in the MP group at 2
      years. There also exists literature reporting poorer outcomes with the MP design. A trial
      involving 96 patients who had received both an MP knee and an MB knee on contralateral sides
      found lower ROM, higher complication rates, and worse patient reported outcomes in the MP
      knee.

      Given the lack of consensus in the literature, further investigation is warranted to
      determine the impact of the MP design on outcomes following TKA.
    

Intervention name(s): Medacta GMK posterior stabilized prosthesis; Medacta GMK Sphere prosthesis
Intervention type(s): Device; Device
Allocation: Randomized
Intervention model: Single Group Assignment
Masking: Single (Participant)
Primary purpose: Treatment