Stability of the Medial Pivot Total Knee Prosthesis

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.

Inclusion Criteria:

- Age 18-85, regardless of gender, ethnicity, or pathology

- Must require a total knee arthroplasty

- All subjects must have given signed, informed consent prior to registration in study.

Exclusion Criteria:

- Minors

- Any patients that are unable to consent

- Patients with active infection or osseous tumor of the operative extremity

- Patients undergoing revision surgery