Posterior Slope of the Tibial Component : Wheeless' Textbook of Orthopaedics

- See:
       - tibial component
       - proximal tibial cut
       - extra-medullary cutting guide;

- Discussion:
- viewed in saggital plane, line drawn between anterior & posterior rims of the tibial plateaus is approx 10 deg posteriorly;
- this radiographic tilt does not take the menisci into consideration (which minimize the tilt to about 3 deg);
- this tilt produces a component of shear during simple stance;
- this posterior tilt actually minimizes the magnitude of shear stress when compressive stress increases during flexion;

- Operative Considerations for Posterior Tibial Slope:
- sagittal alignment of the tibial component is determined by amount of posterior slope of the proximal tibial cut;
- most current systems strive for 3-7 deg of posterior slope, because bone in the anterior portion of the cut surface is weaker;
- if proximal tibia is cut perpendicular to long axis of tibia in saggital plane, tibial component subsidence is more likely;
- implant considerations:
- if implant has a thickness = anteriorly and posteriorly, then tibia must be resected w/ a downward slope of approx 10 deg;
- if tibial implant has built in slope (that is the anterior portion is thicker than the posterior portion), then the tibia may be resected at 90 deg;
- excessive posterior slope:
- posterior sloping of prosthetic joint line should not be over done, since it may lead to posterior tibiofemoral subluxation (excessive flexion gap);
- excessive posterior slope combined w/ a deep proximal tibial cut may result in valgus instability (from loss of posterior oblique ligament) and may result in loss
of the PCL (in which case a posterior stabilized prosthesis will be required);
- inadequate posterior tibial slope:
- under no circumstances should tibia be cut with an upward slope since this diminishes size of flexion space posteriorly;
- a transverse tibial cut more bone is removed anteriorly than posteriorly, which may result in anterior subsidence;
- quality of anterior cancellous bone quality of anterior cancellous bone decreases rapidly as one progresses downward below joint surface;
- to prevent subsidence into soft bone, one needs to preserve as much anterior bone as possible and use a posteriorly sloped resection line