Comparing the Science of High-Flex Knee Implants

Removing additional 2mm of bone from the posterior condyles is necessary to accommodate safe high flextion.

While bone conservation is always important, removing an additional 2mm of bone is of little significance compared to the need to optimize the tibiofemoral contact area throughout the entire range of motion. Research has shown, and clinical studies have confirmed, that an implant system that provides a greater contact area during high flexion can help minimize the possibility of edge loading, and thereby reduce the likelihood of "digging" of the metal condyle into the articular surface when the knee is flexed beyond 125.˚1 On the NexGen CR-Flex and LPS-Flex Implants, this was accomplished by extending the radius of the posterior femoral condyles so the contact area remains high in deep flexion.

To extend the radius, the posterior condyles are slightly thicker, which also increases the posterior condylar offset (the distance from the most posterior point of the posterior femoral condyle to the tangent of the extended posterior cortex of the femoral shaft2). When the posterior condylar offset is increased, the potential for impingement of the femoral shaft on the tibial articular surface is reduced.2 Several newer competitive designs, such as the Stryker Triathlon, claim to be "high-flex friendly." However, head-to-head testing at Zimmer showed that the Triathlon had significantly less contact area compared to the NexGen Flex Implants at high flexion angles between 125˚ and 155˚.1

Another potential benefit of this additional posterior bone removal is that better access to the joint space is provided during minimally invasive procedures, which helps facilitate insertion and extraction of instruments and implants.

View Zimmer NexGen CR-Flex vs. Stryker Triathlon Graph

References

  1. Data on file at Zimmer.
  2. Bellemans J, Banks S, Victor J, Vandenneucker H, Moemans A. Fluoroscopic analysis of the kinematics of deep flexion in total knee arthroplasty: influence of posterior condylar offset. J Bone Joint Surg (Br). 2002; 84-B(1):50-53.
  3. Hitt K, Shurman J II, Greene K, et al. Anthropometric measurements of the human knee: correlation to the sizing of current knee arthroplasty systems. J Bone Joint Surg. 2003;85:115-122.
  4. Scott WN. Pearls on avoidance and treatment of intraoperative and postoperative complications: exposure of the stiff knee. Presented at: American Association of Hip and Knee Surgeons, Knee Society Specialty Day; March 25, 2006.
  5. Nelson CL, Kim J, Lotke PA. Stiffness after total knee arthroplasty. J Bone Joint Surg. 2005;87:264-270.
  6. Mihalko W, Fishkin Z, Krakow K. Patellofemoral overstuff and its relationship to flexion after total knee arthroplasty. Clin Orthop. 2006;449:283-287.
  7. Ortiguera CJ, Berry DJ. Patellar fracture after total knee arthroplasty. J Bone Joint Surg (Br). 2002;84:532-540.
  8. Puloski SKT, McCalden RW, MacDonald SJ, Rorabeck CH, Bourne RB. Tibial post wear in posterior stabilized total knee arthroplasty: an unrecognized source of polyethylene debris. J Bone Joint Surg. 2001;83-A:390-397.
  9. Callaghan JJ, O'Rourke MR, Goetz DD, Schmalzried TP, Campbell PA, Johnston RC. Tibial post impingement in posterior-stabilized total knee arthroplasty. Clin Orthop. 2002;404:83-88.
  10. Chiu YS, Chen WM, Huang CK, Chiang CC, Chen TH. Fracture of the polyethylene tibial post in a NexGen posterior-stabilized knee prosthesis. J Arthroplasty. 2004;19(8):1045-1049.
  11. Zimmer NexGen Flex Knee Design Rationale. 97-5964-004-00. Zimmer, Inc. 2004:16.
  12. Argenson JN, Komistek RD, Mahfouz M, Walker SA, Aubaniac JM, Dennis DA. A high flexion total knee arthroplasty design replicates healthy knee motion. Clin Orthop. 2004;428:174-179.
  13. Argenson JN, Scuderi GR, Komistek RD, Scott WN, Kelly MA, Aubaniac JM. In vivo kinematic evaluation and design considerations related to high flexion in total knee arthroplasty. Journal of Biomechanics. February 2005; 38(2):277-284.
  14. Li G, Most E, Sultan PG, et al. Knee kinematics with a high-flexion posterior stabilized total knee prosthesis: an in vitro robotic experimental investigation. J Bone Joint Surg. August 2004;86:1721-1729.
  15. Huang HT, Su JY, Wang GJ. The early results of high-flex total knee arthroplasty: a minimum of 2 years of follow-up. J Arthroplasty. 2005;20:674.

* Vanguard Complete Knee System Cruciate Retaining brochure, 2004. Form No. Y-BMT-884/071504/M.

†Vanguard Complete Knee System Design Rationale, 2006. Form No. Y-BMT-906R/031506/M.