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The Journal of Bone and Joint Surgery (American) 86:1031-1037 (2004)
© 2004 The Journal of Bone and Joint Surgery, Inc.

Backside Wear of Modular Ultra-High Molecular Weight Polyethylene Tibial Inserts

Michael A. Conditt, PhD1, Sabir K. Ismaily, BS1, Jerry W. Alexander, BS1 and Philip C. Noble, PhD2

1 Institute of Orthopedic Research and Education, 6550 Fannin Street, Suite 2512, Houston, TX 77030. E-mail address for M.A. Conditt: mconditt{at}bcm.tmc.edu
2 Barnhart Department of Orthopedic Surgery, Baylor College of Medicine, 6550 Fannin Street, Suite 2625, Houston, TX 77030

Investigation performed at the Institute of Orthopedic Research and Education, Houston, Texas

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: The capture mechanisms of modular tibial total knee components may allow relative micromotion between the insert and the base-plate, leading to wear at the nonarticulating (backside) surface. Although retrieved components often display laxity in the capture mechanism in the unloaded condition, the magnitude of the relative motion that actually occurs under physiologic conditions has not been determined. This study was performed to assess the impact of different modes of knee-loading on the relative micromotion between the insert and the base-plate and the relationship between the duration that the implant had been in situ and the severity of backside wear.

Methods: Twenty-one posterior-stabilized total knee replacements of one common design (Insall-Burstein II) were retrieved at one to 100 months after implantation. The extent and severity of backside wear was graded with use of stereomicroscopy. All components were soaked in a bath (of physiologic saline solution at 37°C for four days prior to reassembly. The relative micromotion between the insert and the base-plate of each specimen was measured in vitro in two different conditions: with no axial load and with a combination of loads and torques simulating the stance phase of gait.

Results: The capture mechanism laxity between the insert and the tibial base-plate in the unloaded condition was approximately eight times larger than the micromotion measured during simulated gait. The capture mechanism laxity allowed a mean (and standard deviation) of 618 ± 226 µm of total relative micromotion compared with 103 ± 54 µm of relative micromotion during the gait cycle. Under both loading conditions, the predominant direction of interface motion was medial-lateral. No correlation was found between the magnitude of capture mechanism laxity and the relative micromotion measured during simulated gait (p = 0.11). Larger polyethylene protrusions on the backside surface did not correlate with less micromotion (p = 0.48) or with capture mechanism laxity (p = 0.06).

Conclusions: For the implant design that was studied, capture mechanism laxity between the modular insert and the base-plate in the unloaded condition was an order of magnitude larger than and not indicative of the micromotion that occurred during simulated physiologic loading. In addition, polyethylene protrusions into the screw-holes of tibial base-plates did not seat or lock the insert in place and reduce relative motion.

Clinical Relevance: While some clearance between the insert and the base-plate is required to allow assembly of modular tibial components at the time of surgery, the amount of relative interface motion during a functional activity such as normal gait, which can produce potentially damaging wear debris, is unknown. However, the compressive forces applied to the articular surface during a functional activity may substantially reduce micromotion between the insert and the base-plate relative to the unloaded condition.


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