We re-examined experimental model results using half-body specimens with intact extensor mechanisms and navigation to evaluate cruciate-retaining (CR) and posterior stabilized (PS) total knee arthroplasty (TKA) component gaps through an entire range of motion. Six sequential testing regimens were conducted with the knee intact, with a CR TKA in place, and with a PS TKA in place, with and without 22 N traction in place at each stage. Each of 10 knees was taken through six full ranges of motion from 0˚to 120˚at every stage using a navigated knee system to record component gapping. No significant difference was found between loaded and unloaded component gaps, and no significant differences were found in component gapping between CR and PS TKAs throughout a full range of motion. Flexion-extension gap measurements were significantly different from previously published data (at 90˚flexion). No difference was found in kinematics when comparing CR and PS TKA component designs. Our results suggest that intact extensor mechanisms may be required to perform proper kinematic studies of TKA. Our findings provide evidence that the extensor mechanism may play a major role in the flexion-extension gaps in cadaveric knees. ß 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:627-632, 2014.Keywords: flexion-extension gap; cruciate retaining; posterior stabilized; total knee arthroplasty Total knee arthroplasty (TKA) is performed to relieve pain and restore function in patients with knee arthritis. Good clinical outcomes depend on achieving proper alignment of the implants and stability of the soft tissue envelope. The most common prostheses for primary TKA are cruciate retaining (CR) or posterior stabilized (PS) designs. To achieve stability, the surgeon must know what effect posterior cruciate excision will have on the biomechanics of the implanted prosthesis.In the literature on kinematics of TKA, discrepancy exists between biomechanical and clinical outcome studies when comparing CR versus PS designs. [1][2][3][4][5][6][7][8] Mihalko and Krackow 3 performed a biomechanical study investigating knee flexion-extension gaps in TKA before and after posterior cruciate ligament (PCL) resection and found a significant increase in flexion gap after PCL resection. Their study was conducted on specimens without an intact extensor mechanism using static measurements at 0˚and 90˚. They concluded that either additional distal femur could be resected or the posterior tibial slope could be decreased to balance flexion-extension gaps when performing PS TKA.To evaluate CR and PS TKA constructs, it is critical to understand the flexion-extension gap between the femoral and tibial articulating surfaces during knee motion. We re-evaluated the biomechanical study results using half-body specimens with intact extensor mechanisms and navigation to evaluate PS and CR component gaps through an entire range of motion. MATERIALS AND METHODSThe lower halves of five fresh-frozen non-preserved (one female, fou...

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