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ABSTRACT: The rotational constraint characteristics of 4 commercially available posterior-stabilized im-plants (Triathlon, NexGen, PFC, and Genesis II) were determined. The torques required to achieve up to 620° of internal and external rotation were quantified at hyperextension, 0°, 15°, 60°, and 90° of flexion. The effect of lubrication was tested by comparing the torque values generated in dry and serum environments. Re-sults indicated that substantially higher torques were generated in the dry environment, compared with the serum environment. In extension, the principal design features influencing the torques were interaction of the box and post, as well as the insert anterior lip. In high-er flexion angles, the amount of rollback, coupled with the posterior lip geometry, most heavily influenced the generation of torque. Comparison of the 4 designs showed that the Triathlon generated the least torques to 610° of rotation, whereas the PFC and Genesis II were the most constrained.

[J Knee Surg. 2008;21:315-319.]

IntroductIon

Several studies have documented evidence of rotary motion between the insert and baseplate of a total knee arthroplasty (TKA).6,8,13-15,18 This motion, which can cause backside wear, osteolysis, and loosening,15 results from rotational stresses generated at the articulating sur-

face. The rotational constraint of a knee design thus plays an important role. The more constrained the design, the greater the potential for torques generated at the articulat-ing surface to be transferred to the interface between the polyethylene tibial insert and the metallic baseplate and to the bone-implant interface. For example, Bradley et al4 found evidence of rotational backside wear in 71% of the more constrained PFC posterior-stabilized trays; however, only 31% of the PFC cruciate-retaining components dis-played the same.

Articular surface constraint can be quantified experi-mentally using a multi-axis test frame. Testing can be performed dry or with a lubricant. The American Society for Testing and Materials standard on constraint testing specifies the use of a lubricant.2 However, results can vary depending on which method is used.

The purpose of this study was to quantify the effects of measuring constraint in a dry versus a lubricated envi-ronment and to quantify rotational constraint of 4 TKA designs in a lubricated environment. The Triathlon PS (Stryker Orthopaedics, Mahwah, NJ), PFC Sigma Stabi-lized (DePuy-Mitek, Warsaw, Ind), NexGen LPS (Zimmer Inc, Warsaw, Ind), and Genesis II PS (Smith & Nephew, Andover, Mass) designs were studied.

MaterIals and Methods

Test SamplesThis study evaluated 3 samples of each knee design.

All samples represented medium-sized components with the thinnest modular polyethylene insert offered by the manufacturer.

Test ApparatusAll testing was performed on a multi-axis servo-

hydraulic test frame (MTS Bionix 858; MTS Corp, Eden

Rotational Constraint of Posterior-Stabilized Total Knee Prostheses

Safia Bhimji, MS Mark Kester, PhD

Thomas Schmalzried, MD

Ms Bhimji and Dr Kester are from Stryker Orthopaedics, Mahwah, NJ; and Dr Schmalzried is from Orthopaedic Hospital, Los Angeles, Calif.

Ms Bhimji and Dr Kester are employees of and Dr Schmalzried is a consultant for Stryker Orthopaedics. However, no financial support was received for this article.

Correspondence: Mark Kester, PhD, Stryker Orthopaedics, 325 Corporate Drive, Mahwah, NJ 07430.

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Prairie, Minn). The test frame consisted of 2 actuators: a coupled axial-torsional actuator oriented vertically to ap-ply compressive loads and internal-external torques to the components, and a side actuator, which is an axial actua-tor oriented parallel to the floor to apply shear loads.

Test StructureThe femoral components and tibial baseplates were

rigidly mounted to stainless steel holding fixtures. The in-serts were assembled to the baseplates according to surgi-cal protocol. The insert-baseplate assembly was mounted at a 0° slope to a bath, and the bath was then mounted to a bearing table connected to the side actuator of the test frame. The assembly was mounted so that the side actua-tor applied a load in the anteroposterior (AP) direction of the component assembly, and the bearing table allowed unrestricted mediolateral motion. The femoral component was mounted to the axial-torsional actuator via fixturing that allowed flexion angle adjustment and unrestricted varus-valgus rotation. Figure 1 presents a photograph of the test setup. To initially align the femoral component to the insert at each flexion angle, a 200-N compressive

force was applied while the femoral component was ro-tated through 610°. The insert was allowed to float in the AP and mediolateral directions to find its natural resting position.

Loading ParametersA 3800-N compressive load was applied between the

femoral component and insert while the femoral compo-nent was rotated between 65° of internal-external rota-tion at 5° per second for 5 cycles. The rotation was then incrementally increased to 610°, 615°, and 620° for 5 cycles each. The insert was allowed to float in the AP and mediolateral directions throughout testing. Testing was performed in hyperextension and at 0°, 15°, 60°, and 90° of flexion. The hyperextension angle was design de-pendent and represented the angle at which the femoral component first contacted the anterior side of the insert post.

Test ProcedureEach sample of the Triathlon PS design was first test-

ed in air at all flexion angles. Axial load and displacement, side load and displacement, and torsional torque and angle were recorded at 12.5 Hz throughout the loading process.

After testing had been completed in air, Alpha Calf Fraction Serum (Hyclone Labs, Logan, Utah) diluted with water (1 part serum to 6 parts water) was added to the bath, with the fluid level completely covering the level of tibiofemoral articulation. The temperature was heated to 37°C using a heater (approximately 30 min to heat), and circulation was maintained using a pump. The data collec-tion process was then repeated at each flexion angle for all samples of all 4 knee designs.

AnalysisArticular surface constraint was measured at each

flexion angle as the amount of torque required to rotate the femoral component on the insert from 220° to 120°. Comparisons between designs and between test environ-

Figure 1. Test setup.

1Figure 2. Rotational constraint at 15° flexion. Both air and serum are depicted.

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Figure 3. Rotational constraint at 90° flexion. Both air and serum are depicted.

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ments were made by overlaying plots of rotational torque data as a function of femoral rotation. The peak-to-peak torque values between 610° of rotation were also com-puted and compared across designs.

results

Effects of Test Environment on ConstraintPlots of rotational constraint for one sample of the Tri-

athlon knee design at 15° and 90° are displayed in Figures 2 and 3, respectively. The plots show results of testing in both a dry and a lubricated environment. A positive angle denotes internal rotation of the femoral component on the insert, and a negative angle denotes external rotation. The results demonstrate a substantial increase in the torque required for rotation in the dry environment, compared with the lubricated environment. This trend remained consistent across the 620° of femoral rotation and across flexion angles. On average, torques generated in the dry

environment were 45% larger than those generated in the lubricated environment.

Effects of Knee Design on ConstraintPlots comparing rotational constraint across all 4 knee

designs in the lubricated environment can be found in Fig-ure 4. Between 610° of rotation, Triathlon was the least constrained followed by either NexGen or Genesis II, de-pending on flexion angle. The PFC system demonstrated the most constraint across all angles. Constraint charac-teristics within 610° of rotation have clinical effects, as this range represents rotations likely to occur in vivo at the flexion angles tested.1,3,7,11,12

Beyond 610° of rotation, all designs demonstrated higher torques. In internal rotation, NexGen was the least constrained at hyperextension, 60°, and 90°; and Triathlon was the least constrained at 0° and 15°. PFC and Gen-esis II were the most constrained across all flexion angles. In external rotation, NexGen was the least constrained at hyperextension and at 15°; Triathlon was the least con-strained at 0°; and Genesis II was the least constrained at 60° and 90°. The PFC components did not reach 620° of rotation in hyperextension due to impingement of the femoral component on the anterior side of the tibial post, which causes the rotational torque levels to exceed a 35-Nm limit set to prevent damage to the inserts. The NexGen components were not tested to 620° of rotation at 90° of flexion. At this angle, the femoral component articulated close to the posterior edge of the insert due

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Figure 4. Rotational constraint across designs in a lubricat-ed environment.

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to the very posterior location of the femoral condyles on the insert. At 20° of rotation, the femoral condyles tended to sublux off the insert and, in some cases, deformed the posterior lip of the insert.

The peak-to-peak torque values generated between 610° of femoral rotation were computed for each flex-ion angle. Results at 0° and 15° of flexion are shown in Figures 5 and 6, respectively. The graphs display the av-erage and standard deviation over the 3 samples tested. Comparisons across designs were made using a 1-factor analysis of variance (alpha = 0.05). Between 610° of ro-tation, Triathlon demonstrated the least amount of torque (differences were statistically significant with at least 1 design at each flexion angle). Consistent with their high-er conformity, PFC and Genesis II generated the largest amount of torque.

dIscussIon

The effects of friction between components contrib-ute substantially to constraint characteristics. Our results demonstrated that testing in a dry environment increased the torques generated at the articulating surface by 45% over those obtained in the lubricated environment. A con-sistent test environment is necessary to compare results across various loading conditions and implant designs. It is recommended that a lubricant of dilute calf serum be used to emulate the synovial environment of the knee joint.

When comparing across the 4 designs tested in the lu-bricated environment, Triathlon was the least constrained between 610° of rotation, whereas PFC and Genesis II were the most constrained. Resistance to rotation in the PFC design was caused mainly by box-post impingement, as previously reported.10 Impingement was greatest in hy-perextension and full extension, where the box and the post usually play a larger role in constraint. Anterior tibial lips can also add to the resistance, as was demonstrated

by the Genesis II design. Rotational constraint can be re-duced by increasing clearance around the tibial post, by rounding the post with respect to the coronal dimension of the intercondylar box, and by lowering anterior lip height. Mobile tibial bearings are another design-dependent way to reduce rotational constraint.

Whereas the tibial post and intercondylar box influ-enced rotational constraint in extension and hyperexten-sion, the tibiofemoral condylar geometry influenced ro-tational torques in deeper flexion. The inability to test the NexGen at 90° to 620° without damaging the insert bears discussion. As the anatomic knee flexes, the femur rolls posteriorly on the tibia while externally rotating on the tibia. If this motion is to be permitted after arthroplasty, the articular surfaces must accommodate rotation as the cam and post drive rollback. The NexGen was not able to accommodate as much rotation as the other designs tested due to the posterior position of the femoral condyles at 90° of flexion and the posterior insert geometry. When the femur was rotated to 620°, the NexGen tibial articulation would not permit the rotation without subluxation and damage to the posterior lip of the insert. To allow more rotation, the NexGen would have to decrease the rollback, extend the articular track, or remove material from the posterior intercondylar aspect of the femoral component. Design features such as rotary arcs in the articular surface of the tibial insert can enhance the ability to accommodate rotation.9 The arcs should extend far enough anteriorly and posteriorly to allow rotation at the extremes of flexion and extension.

There are shortcomings with this investigation. The test rig used a fixed center of rotation for internal and external rotations, which may not fully represent the in situ variability. The maximum flexion angle tested was 90°, which is less than most clinical series indicate as the maximum flexion angle. However, 90° covers joint loading under the majority of ambulatory conditions, in-

Figure 6. Peak-to-peak torque, at 5° flexion for 610° rota-tion. Differences were significant between Triathlon (Stryker Orthopaedics) and all other designs and between PFC (Depuy-Mitek) and NexGen (Zimmer Inc) and NexGen and Genesis II (Smith & Nephew).

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Figure 5. Peak-to-peak torque, at 0° flexion for 610° rota-tion. Differences were significant between Triathlon (Stryker Orthopaedics) and Genesis II (Smith & Nephew), and Tri-athlon and PFC (Depuy-Mitek).

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cluding stair climbing. The contact areas of the different designs were not determined, which could affect the ser-vice life of the implant.5 However, reduction of oxidation appears to be more important than contact area for low wear and implant survival.16 Essentially all manufacturers now sterilize and package tibial polyethylene inserts in an oxygen-free environment. Retrieval studies indicate a favorable performance, with a dramatic reduction in pit-ting and delamination.17 Some manufacturers offer tibial inserts with no or negligible free radicals, although clini-cal results are not yet available.

conclusIon

The rotational torques generated by 4 posterior-stabilized knee implant systems were compared at dif-ferent flexion angles. Substantially higher torques were generated in air, compared with those generated in serum. The posterior-stabilized box-post design and anterior tib-ial lip affected rotational constraint in extension, whereas the tibiofemoral articular geometry had a greater effect on the ability to accommodate rotation in flexion. In ex-tension, implants should design in clearance between the cam and the post and avoid high anterior lips that can re-sist rotation. In flexion, the tibial and femoral geometries should accommodate rotation without damaging the pos-terior poly or allowing femoral subluxation off the tibial posterior lip.

reFerences

1. Andriacchi TP, Dyrby CO, Johnson TS. The use of func-tional analysis in evaluating knee kinematics. Clin Or-thop. 2003;(410):44-53.

2. ASTM International. Standard Test Method for Determi-nation of Total Knee Replacement. West Conshohocken, PA: ASTM International; 2005.

3. Benson LC, DesJardins JD, Harman MK, LaBerge M. Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee sim-ulator. Proc Inst Mech Eng [H]. 2002;216:409-418.

4. Bradley MP, Mayor MB, Collier JP. Differences in articu-lar track area of posterior-stabilized and cruciate-retaining

retrieved total knee implants. Orthopedics. 2004;27:1273-1278.

5. Buechel FF Sr, Buechel FF Jr, Pappas MJ, Dalessio J. Twenty-year evaluation of the New Jersey LCS Rotating Platform Knee Replacement. J Knee Surg. 2002;15:84-89.

6. Conditt MA, Ismaily SK, Alexander JW, Noble PC. Back-side wear of modular ultra-high molecular weight polyeth-ylene tibial inserts. J Bone Joint Surg Am. 2004;86:1031-1037.

7. Costigan PA, Deluzio KJ, Wyss UP. Knee and hip kinetics during normal stair climbing. Gait Posture. 2002;16:31-37.

8. Engh GA, Lounici S, Rao AR, Collier MB. In vivo dete-rioration of tibial baseplate locking mechanisms in con-temporary modular total knee components. J Bone Joint Surg Am. 2001;83:1660-1665.

9. Harwin SF, Greene KA, Hitt K. Early experience with a new total knee implant: Maximizing range of motion and function with gender-specific sizing. Surg Technol Int. 2007;16:199-205.

10. Klein R, Serpe L, Kester MA, et al. Rotational constraint in posterior-stabilized total knee prostheses. Clin Orthop. 2003;(410):82-89.

11. Lafortune MA, Cavanagh PR, Sommer HJ III, Kalenak A. Three-dimensional kinematics of the human knee during walking. J Biomech. 1992;25:347-357.

12. Li G, Zayontz S, DeFrate LE, Most E, Suggs JF, Rubash HE. Kinematics of the knee at high flexion angles: An in vitro investigation. J Orthop Res. 2004;22:90-95.

13. Mikulak SA, Mahoney OM, dela Rosa MA, Schmalzried TP. Loosening and osteolysis with the press-fit condylar posterior-cruciate-substituting total knee replacement. J Bone Joint Surg Am. 2001;83:398-403.

14. Parks NL, Engh GA, Topoleski LD, Emperado J. The Coventry Award. Modular tibial insert micromotion. A concern with contemporary knee implants. Clin Orthop. 1998;(356):10-15.

15. Rao AR, Engh GA, Collier MB, Lounici S. Tibial inter-face wear in retrieved total knee components and correla-tions with modular insert motion. J Bone Joint Surg Am. 2002;84:1849-1855.

16. Ritter MA. Direct compression molded polyethyl-ene for total hip and knee replacements. Clin Orthop. 2001;(393):94-100.

17. Silva M, Kabbash CA, Tiberi JV III, et al. Surface dam-age on open box posterior-stabilized polyethylene tibial inserts. Clin Orthop. 2003;(416):135-144.

18. Wasielewski RC. The causes of insert backside wear in to-tal knee arthroplasty. Clin Orthop. 2002;(404):232-246.

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