Better patient outcomes found with flexible rather than rigid IM rods for TKA

Researchers noted that use of the flexible rods reduced the chance of medial-lateral overhang.

Issue: November 2012

Flexible intramedullary rods placed into the femur during total knee arthroplasty reduced medial-lateral overhang and achieved better functional outcomes and patient satisfaction scores than rigid rods, according to recently presented research.

Perspective from Bryan D. Springer, MD

“The femoral components implanted with a flexible rod trended to a smaller size, which have the advantage of reduced chance of overstuffing the patellar region, smaller medial/lateral size, reduced chance of medial-lateral overhang and improved chance of getting to deep flexion,” study presenter Evan Leibowitz, MS, a research engineer at Stryker Orthopaedics, said. “With this, it seemed that the flexible rod might lead to a proper sizing and alignment of the femoral component.”

Leibowitz and colleagues conducted three studies that compared traditional rigid rods with flexible, intramedullary rods to assess the effect of the rods on implant size, to determine how implant alignment and anterior bow of the femur affect implant size and position, and whether that has an impact on patient satisfaction.

Rigid rods required a large reamer hole and migrated anteriorly in the sagittal plane as described in the literature, Leibowitz said. However, he noted that the flexible rods slid into the center canal of the femur because of its flexibility.

Virtual surgeries performed with rigid and flexible rods in small, average and large bows showed the flexible rods were more sensitive to the anterior bow of the femur, the study showed. Angular deviation was greatest in the largest bows at 7.5° compared to the average bows at 4.5°, with small bows showing no significant change.

The virtual surgeries also showed flexible rods made it possible to use smaller implant sizes “while maintaining the same distal and posterior tangencies,” Leibowitz said. The use of smaller sized implants can result in a reduction in medial-lateral overhang.

Kirby Hitt, MD, director of the Division of Adult Reconstructive Surgery at Scott & White Healthcare conducted a retrospective study of 600 patients undergoing total knee arthroplasty (TKA) and showed a trend toward smaller implant sizes when the flexible rods were used. In cases with flexible rods, Leibowitz said mid-size components were more frequently used and larger sized components were rarely used.

A prospective and randomized study of 100 patients who underwent TKA with either flexible or rigid rods showed that the flexible rod group had a 6% improvement in range of motion at 2-year follow-up.

“Preoperatively, they had a lower range of motion versus the rigid rod and 2 years later they beat out the rigid rod population,” Leibowitz said.

SF-36 and Knee Society Scores (KSS) trended higher for the flexible rod group compared to the rigid rod cohort. SF-36 physical scores improved 17.5% and mental scores improved 6% for the flexible rod group compared to rigid rod cohort. KSS scores improved 16.5% for pain and motion and 6.5% for function in patients outfitted with flexible vs. rigid rods. – by Renee Blisard Buddle

Reference:

Leibowitz E. #12-2732. Presented at: 13th EFORT Congress; May 23-25, 2012; Berlin.

For more information:

Evan Leibowitz, MS, can be reached at 325 Corporate Dr., Mahwah, NJ 07430; email: evan.leibowitz@stryker.com.

Disclosure: Leibowitz is an employee of Stryker.

Perspective

Bryan D. Springer, MD

Alignment in total knee arthroplasty (TKA) has mainly focused on component position in the coronal plane. Little attention has been given to the position of the femoral component in the sagittal plane. This prospective randomized study by Leibowitz and colleagues demonstrated superior clinical and functional outcomes when the femoral component was placed in line with the patients femoral bow in the sagittal plane using a flexible intramedullary (IM) femoral alignment rod. The authors also demonstrated that flexing the femoral component in line with the sagittal bow allows for more appropriate sizing of the femoral component, theoretically improving sagittal plane kinematics compared to a rigid IM rod.

One must be cautious about ascribing significant improvement in clinical and functional outcomes based on one parameter as there are multiple variables (surgical and patient related) that can influence patient outcomes following TKA. Caution also should be observed in association with excessive flexion of the femoral component, as this can lead to post impingement in extension with a posterior stabilized design and the potential for poly wear and post fracture. This study, however, should foster more discussion on the best placement of femoral component position in all planes as well as optimal femoral sizing.

Bryan D. Springer, MD

OrthoCarolina Hip and Knee Center

Charlotte, N.C.

Disclosures: Springer is a consultant for Stryker Orthopaedics and Convatec Surgical, receives educational honoraria from Depuy Orthopaedics and is on the editorial board of the Journal of Arthroplasty.