introduction Designs include unconstrained posterior-cruciate retaining (CR) posterior-cruciate substituting (PS) constrained nonhinged hinged fixed versus mobile bearing History 19th century interposition of soft tissues for reconstruction of articular surfaces 1950s Walldius designs first hinged knee replacement 1958 MacIntosh and McKeever introduce acrylic tibial plateau prosthesis to correct deformity 1960s Gunston introduces first cemented surface arthroplasty of knee joint 1970 Guepar develops a new hinged prosthesis based on design by Walldius that increases motion and decreases bone loss ~1973 "total condylar prosthesis" is introduced which is first to resurface all three compartments (PCL sacrificing) Concepts in Prosthetic Design Femoral rollback definition the posterior translation the femur with progressive flexion importance improves quadriceps function and range of knee flexion by preventing posterior impingement during deep flexion biomechanics rollback in the native knee is controlled by the ACL and PCL design implications both PCL retaining and PCL substituting designs allow for femoral rollback PCL retaining native PCL promotes posterior displacement of femoral condyles similar to a native knee exhibits paradoxical anterior translation in the first 40 degrees of flexion PCL substituting tibial post contacts the femoral cam causing posterior displacement of the femur Constraint definition the ability of a prosthesis to provide varus-valgus and flexion-extension stability in the face of ligamentous laxity or bone loss importance in the setting of ligamentous laxity or severe bone loss, standard cruciate-retaining or posterior-stabilized implants may not provide stability design implications in order of least constrained to most constrained cruciate-retaining posterior-stabilized (cruciate-substituting) varus-valgus constrained (non-hinged) rotating-hinge Modularity definition the ability to augment a standard prosthesis to balance soft tissues and/or restore bone loss options include metal tibial baseplate with modular polyethylene insert more expensive than all-polyethylene tibial component has an equivalent rate of aseptic loosening compared with all-polyethylene tibia component metal augmentation for bone loss modular femoral and tibial stems advantages ability to customize implant intraoperatively disadvantages increased rates of osteolysis in modular components backside polyethylene wear micromotion between tibial baseplate and undersurface of polyethylene insert that occurs during loading Fixation options include cemented proven survivorship and function high viscosity cement has longer working time cementless trabecular surface allows for long term biologic fixation Cruciate-Retaining (CR) Design Design minimally constrained prosthesis that depends on an intact PCL to provide stability in flexion Indications arthritis with minimal bone loss, minimal soft tissue laxity, and an intact PCL varus deformity < 10 degrees valgus deformity < 15 degrees Radiographs radiographs won't show box in the central portion of the femoral component as PS knees have (see PS knee radiographs) Advantages avoids tibial post-cam impingement/dislocation that may occur in PS knees more closely resembles normal knee kinematics (controversial) less distal femur needs to be cut than in a PS knee improved proprioception with preservation of native PCL newer poly-options can allow for PCL substitution via anterior-stabilized or ultra-congruent shapes in cases of PCL insufficiency without loss of functional results Disadvantages tight PCL may cause accelerated polyethylene wear loose or ruptured PCL may lead to flexion instability and subluxation Posterior Stabilized (PS) Design Design slightly more constrained prosthesis that requires sacrifice of PCL resection of PCL increases the flexion gap in relationship to extension gap so posterior must be matched to avoid flexion-extension mismatch femoral component contains a cam that engages the tibial polyethylene post during flexion polyethylene inserts are more congruent, or deeply "dished" Indications previous patellectomy reduces risk of potential anteroposterior instability in setting of a weak extensor mechanism inflammatory arthritis inflammatory arthritis may lead to late PCL rupture deficient or absent PCL Radiographs lateral radiograph will show the outline of the cam, or box, in the femoral component Advantages easier to balance a knee with absent PCL arguably more range of motion easier surgical exposure Disadvantages cam jump mechanism with loose flexion gap, or in hyperextension, the cam can rotate over the post and dislocate treatment initial closed reduction by performing an anterior drawer maneuver final revision to address loose flexion gap tibial post polyethylene wear patellar "clunk" syndrome mechanism scar tissue gets caught in box as knee moves into extension treatment arthroscopic versus open resection of scar tissue additional bone is cut from distal femur to balance extension gap Constrained Nonhinged Design Design constrained prosthesis without axle connecting tibial and femoral components (nonhinged) large tibial post and deep femoral box provide varus/valgus stability rotational stability Indications LCL attenuation or deficiency MCL attenuation or deficiency flexion gap laxity moderate bone loss in the setting of neuropathic arthropathy Radiographs Advantages prosthesis allows stability in the face of soft tissue (ligamentous) or bony deficiency Disadvantages more femoral bone resection necessary to accommodate large box aseptic loosening as a result of increased constraint Constrained Hinged Design Design most constrained prosthesis with linked femoral and tibial components (hinged) tibial bearing rotates around a yoke on the tibial platform (rotating hinge) decreases overall level of constraint Indications global ligamentous deficiency hyperextension instability seen in polio or tumor resections resection for tumor massive bone loss in the setting of a neuropathic joint gross flexion/extension mismatch despite appropriate soft tissue releases Radiographs Advantages prosthesis allows stability in the face of soft tissue (ligamentous) or bony deficiency Disadvantages aseptic loosening as a result of increased constraint large amount of bone resection required Mobile Bearing Design Design minimally constrained prosthesis where the polyethylene can rotate on the tibial baseplate PCL is removed at time of surgery Indications young, active patients (relative indication) Advantages theoretically reduces polyethylene wear increased contact area reduces pressures placed on polyethylene (pressure=force/area) Disadvantages bearing spin-out mechanism occurs as a result of a loose flexion gap tibia rotates behind femur treatment initial closed reduction final revision to address loose flexion gap All-polyethylene base plates Design tibial plate is a solid block of polyethylene as opposed to a metal tray with a poly insert. Indications no clear indications Advantages less expensive decreased rates of osteolysis Disadvantages lose modular flexibility Outcomes studies show equivalent functional outcomes with decreased cost Highly Congruent Liners Design medial compartment concavity allows lateral compartment to translate between flexion and extension this creates a medial pivot Indications no clear indications Advantages may better create native knee kinematics Outcomes studies show equivalent outcomes and survivorship in short and mid-term studies Patient Specific Instrumentation Design femoral and tibial cutting block instrumentation based on imaging specific to patient's anatomy Indications no clear indications at this time Advantages less instrumentation to process peri-operatively Outcomes no obvious cost benefit from PSI no obvious benefit in postoperative TKA alignment no obvious benefit in outcomes or patient satisfaction