The Effect of Femoral Cutting Jig Design on Restoration of Femoral Offset in Posterior-Referenced Total Knee Arthroplasty

Coyle, Ryan M. MD
Bas, Marcel A. MD
Rodriguez, Jose A. MD
Hepinstall, Matthew S. MD

The authors are from the Center for Joint Preservation and Reconstruction, Department of Orthopaedic Surgery, Lenox Hill Hospital North-well Health, New York, New York.

Drs Coyle and Bas have no relevant financial relationships to disclose. Dr Rodriguez is a paid consultant and designer for Smith & Nephew, Medacta, ConforMIS, and Exactech. Dr Hepinstall is a paid consultant for Stryker, Corin, Exactech, and KCl Acelity and has received institutional research support from Stryker, Cymedica, KCl Acelity, Flexion, and Kolon TissueGene.

Correspondence should be addressed to: Ryan M. Coyle, MD, Center for Joint Preservation and Reconstruction, Department of Orthopaedic Surgery, Lenox Hill Hospital Northwell Health, 130 E 77th St, Black Hall, 11th Fl, New York, NY 10075 ( [email protected]).

Received September 26, 2018

Accepted January 14, 2019

Orthopedics 42(6):p 316-322, November 01, 2019. | DOI: 10.3928/01477447-20190906-05

Femoral component sizing and rotation in total knee arthroplasty (TKA) affects patellofemoral tracking and tibiofemoral mechanics. Posterior referencing is said to optimize restoration of posterior condylar offset. However, it typically allows the surgeon to select 3° to 5° of external rotation to the posterior condylar axis, inevitably changing the offset of one or both condyles. The axis about which external rotation occurs varies between the jigs of various TKA systems. The location of this axis can result in a medial, central, or lateral reference point for posterior offset restoration. Variations in jig design will result in varying posterior offset changes at the same jig setting, with differential effects on balance between the flexion and extension gaps. Using identical Sawbones in a controlled laboratory setting, 9 TKA instrumentation systems were examined. Two systems referenced medially, 1 referenced laterally, and 6 referenced centrally. The authors measured distal and posterior resections in both 3° and 5° of external rotation to the posterior condylar axis. They calculated changes in both distal and posterior joint lines using resection measurements and implant specifications. Posterior resection thicknesses were greatest with instruments that referenced laterally and least with instruments that referenced medially. With increasing external rotation, instruments that referenced off the lateral femoral condyle introduced the greatest mismatch between the distal and posterior joint lines, as compared with instruments that referenced centrally or medially. Surgeons should be aware that laterally referencing systems can differentially restore distal and posterior joint lines at higher settings of femoral external rotation, potentially introducing incongruity between flexion and extension gaps if the posterior slope of the tibia is maintained. This may be particularly problematic if flexion laxity is increased by posterior cruciate ligament sacrifice. [Orthopedics. 2019; 42(6):316–322.]