The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 94, Issue 18

Scientific Articles | September 19, 2012

Preclinical Computational Models: Predictors of Tibial Insert Damage Patterns in Total Knee Arthroplasty: AAOS Exhibit Selection

Edward A. Morra, MSME¹; Christine S. Heim, BSc¹; A. Seth Greenwald, DPhil(Oxon)¹

¹ Orthopaedic Research Laboratories, 2310 Superior Avenue East, Cleveland, OH 44114. E-mail address for A.S. Greenwald: seth@orl-inc.com

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Investigation performed at Orthopaedic Research Laboratories, Cleveland, Ohio

J Bone Joint Surg Am, 2012 Sep 19;94(18):e137 1-5. doi: 10.2106/JBJS.K.00914

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Abstract

Abstract:

Computational models that predict clinical surface damage of the tibial insert during activities of daily living are emerging as powerful tools to assess the safety and efficacy of contemporary total knee arthroplasty designs. These models have the advantage of quickly determining the performance of new designs at low cost, and they allow direct comparison with the performance of classic, clinically successful designs. This study validated finite element and kinematic modeling predictions through comparison with preclinical physical testing results, damage patterns on retrieved tibial inserts, and clinically measured knee motion. There is a mounting body of evidence to support the role of computational modeling as a preclinical tool that enables the optimization of total knee arthroplasty designs and the auditing of component quality control before large-scale manufacturing is undertaken.

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Introduction

Introduction | Computational Modeling | Conclusions | Clinical and Manufacturing Relevance

Manufacturers and regulatory agencies share a common goal of having safe and effective total knee arthroplasty products available in the global marketplace. Several preclinical methods of testing total knee arthroplasty designs, including virtual computational modeling and physical laboratory wear simulation, are employed to predict damage patterns in polymer tibial inserts. However, the wear simulator method has been criticized for its poor correlation with clinical performance, long testing times, large expense, and difficulty in obtaining meaningful comparisons with other clinically successful designs.

This paper describes our computational modeling experience, dating back sixteen years and involving over forty-five total knee arthroplasty designs^1-3, in which the predicted damage patterns of the polymer inserts have correlated well with physical measurements of contact area and stress, laboratory wear simulations, and clinical retrievals, suggesting that computational modeling offers an alternative preclinical pathway for evaluating these systems.

Computational Modeling

Introduction | Computational Modeling | Conclusions | Clinical and Manufacturing Relevance

Two computational modeling algorithms are described. The finite element method (Fig. 1) provides snapshots of surface and subsurface stress states in the tibial insert at specific positions during activities of daily living. The kinematic method (Fig. 2) extends our understanding by providing a continuous simulation of total knee arthroplasty component motion and contact areas during an entire activity cycle.

Fig. 1

Finite element model. (Reproduced, with permission, from: Morra EA, Harman MK, Greenwald AS. Computational models can predict polymer insert damage in total knee replacements. In: Insall JN, Scott WN, editors. Surgery of the knee. Vol 1. 4th ed. p 271-83. Copyright Elsevier [2006].)

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Fig. 2

Kinematic model.

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Both methods use component geometries measured from the articular surfaces of actual components of implantable quality with use of a three-dimensional laser scanner, rather than relying on idealized computer-aided design models. The benefit of this reverse-engineering procedure is a determination of actual component fit, which relates directly to the accuracy of the manufacturing process.

The Finite Element Method

The finite element method allows a bridge to be constructed between the known in vivo loading of the normal knee and the locations and magnitudes of both surface and subsurface stress states associated with polymer insert damage for a given total knee arthroplasty design. A reproducible method of building finite element models^1-3 was developed with use of MARC software (MSC Software, Santa Ana, California), and validation checks involving multiple clinically utilized total knee arthroplasty designs were performed.

Direct measurement of physical contact area and stress with use of Fuji pressure-sensitive film is a methodology accepted by industry and regulatory agencies for predicting damage to the polymer tibial insert. A comparison between the contact stresses predicted by the finite element model and the pressure-sensitive film analysis for the Genesis Total Knee System (Smith & Nephew, Memphis, Tennessee) is shown in Figures 3-A and 3-B. The magnitude and shape of the contact areas and stress levels compared favorably.

Contact stress as measured with pressure-sensitive Fuji film (Fig. 3-A) and as predicted by finite element modeling (Fig. 3-B).

Fig. 3-A

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Fig. 3-B

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An example of the consequence of incongruous fit between the femoral component and the proximal surface of a mobile-bearing tibial insert, as predicted by its finite element model, is indicated in Figure 4-A. The proximal contact stress distribution propagates to the distal interface, creating contact patterns along the edge (Fig. 4-B). Similar contact patterns can be appreciated on the distal surface of a specimen of the same design after physical laboratory wear simulation⁴ (Fig. 4-C). A comparison of Figures 4-B and 4-C indicates that the finite element model was capable of accurately predicting the locations of abrasion even in an extreme case of incongruent component fit.

The finite element model was predictive of unexpected independent wear test results.

Fig. 4-A

Proximal contact stress as predicted by finite element visualization of incongruent proximal component fit.

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Fig. 4-B

Distal contact stress as predicted by finite element visualization of unusual perimeter contact. Blue and red arrows mark points of predicted contact on the lateral and medial portions, respectively, of the distal tibial insert surface.

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Fig. 4-C

Blue and red arrows mark corresponding points of distal abrasive wear produced by laboratory wear simulation; unusual perimeter abrasion is evident. (Reproduced, with permission, from: Morra EA, Harman MK, Greenwald AS. Computational models can predict polymer insert damage in total knee replacements. In: Insall JN, Scott WN, editors. Surgery of the knee. Vol 1. 4th ed. p 271-83. Copyright Elsevier [2006].)

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The similarity between the result of finite element modeling and analyses of clinical implant retrievals for the Duracon Total Knee System (Howmedica, Rutherford, New Jersey), a long-enduring and clinically successful design, further increases confidence in the validity of the computational methodology. Figure 5-A is a proximal view of the contact areas predicted by the finite element model for the heel-strike portion of walking gait. The image in Figure 5-B is an overlay plot that visualizes all damage pattern measurements from a series of seventeen retrieved tibial inserts of this design⁵. The progressive darkening of areas suggests a commonality of damage locations among the retrieved implants. It is apparent that a facsimile of the wear pattern visualized in Figure 5-B would emerge if the measured contact areas in Figure 5-A were swept by typical patient kinematics. A comparison of these images indicates that the finite element model again reasonably predicted distinctive damage patterns of the polymer tibial insert.

Proximal view of the contact areas as predicted by finite element modeling (Fig. 5-A) and a depiction of the actual abrasive wear scars from seventeen clinical retrievals (Fig. 5-B). (Reproduced, with permission, from: Morra EA, Harman MK, Greenwald AS. Computational models can predict polymer insert damage in total knee replacements. In: Insall JN, Scott WN, editors. Surgery of the knee. Vol 1. 4th ed. p 271-83. Copyright Elsevier [2006].)

Fig. 5-A

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Fig. 5-B

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The Kinematic Method

A computational kinematic model for the Duracon Total Knee System was created with use of KneeSIM software (LifeModeler, San Clemente, California), which offers a constraining soft-tissue envelope with active flexor and extensor muscle groups driving activity. Two activities of daily living, walking gait and deep knee bending, were simulated and animations of the resulting component motion and contact areas sweeping across the tibial insert were recorded.

Figure 6 visualizes cumulative contact areas during the activity cycles of walking gait and deep knee bending. The data are overlaid on Figure 5-B for comparison, clearly demonstrating that the patient kinematics predicted by the dynamic KneeSIM model closely matched the abrasive wear scars seen in the clinical retrieval data.

Fig. 6

Cumulative contact areas swept out by KneeSIM modeling of activities of daily living (green) overlaid on the clinical wear scar data from Figure 5-B.

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Several contemporary total knee designs were evaluated with use of KneeSIM to determine the expected weight-bearing flexion motion of each design¹. Simulation of a deep flexion activity was conducted to determine the maximum flexion angle achievable with each design. Impingement of the posterior cut surface of the femur (Fig. 7-A) against the tibial insert (Fig. 7-B) defined the maximum flexion angle achieved. Figure 8 compares the predicted results with other published research that utilized fluoroscopy or clinical examinations to determine average weight-bearing flexion motion^6-9. The KneeSIM results compared closely with the clinical results at lower maximum flexion angles. However, KneeSIM overpredicted the flexion angle results for designs that achieve high flexion, as the computational model used in this study did not account for calf-thigh contact, which often limits flexion in patient studies. The KneeSIM method appears to be a reasonable predictor of clinical knee motion outcomes.

Generic physical knee model illustrating the posterior cut surface of the femur (Fig. 7-A) used to define maximum flexion (Fig. 7-B) in the computational kinematic knee model.

Fig. 7-A

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Fig. 7-B

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Fig. 8

Maximum weight-bearing flexion angles as predicted by KneeSIM compared with the available clinical literature for MRK⁶, Duracon⁷, JOURNEY⁸, and NexGen LPS-Flex Fixed⁹ total knee arthroplasty designs.

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Conclusions

Introduction | Computational Modeling | Conclusions | Clinical and Manufacturing Relevance

The presented results suggest that computational finite element and kinematic modeling tools offer an effective alternative for predicting the in vivo performance of total knee arthroplasty designs. The computational results compared favorably with evidence accepted by industry and regulatory agencies involving contact areas and stress measurements, laboratory wear simulation, clinical retrievals, and clinical measures of flexion motion.

These contemporary modeling tools will prove most useful in the product development stage to evaluate design concepts computationally prior to investing the time and expense required for prototype production and subsequent physical laboratory testing.

Clinical and Manufacturing Relevance

Introduction | Computational Modeling | Conclusions | Clinical and Manufacturing Relevance

The utilization of contemporary tools to reduce the expense burden of bringing new products to market is important in today’s emerging health care environment, in which minimizing cost is a predominant theme. The resulting information assists both medical device manufacturers and regulatory agencies in providing safe and effective products for the global marketplace while meeting the above challenge.

Morra EA; Rosca M; Greenwald JF; Greenwald AS. The influence of contemporary knee design on high flexion: a kinematic comparison with the normal knee. J Bone Joint Surg Am. 2008 Nov;90 Suppl 4:195-201.[PubMed][CrossRef]

Morra EA; Greenwald AS. Polymer insert stress in total knee designs during high-flexion activities: a finite element study. J Bone Joint Surg Am. 2005;87 Suppl 2:120-4.

Morra EA; Postak PD; Plaxton NA; Greenwald AS. The effects of external torque on polyethylene tibial insert damage patterns. Clin Orthop Relat Res. 2003 May;( 410):90-100.

Bell CJ; Walker PS; Sathasivam S; Campbell PA; Blunn GW. Differences in wear between fixed bearing and mobile bearing knees. Trans Orthop Res Soc. 1999;24( 2):962.

Morra EA; Harman MK; Greenwald AS. Computational models can predict polymer insert damage in total knee replacements. In: Insall JN, Scott WN, editors. Surgery of the knee. Vol 1. 4th ed. Philadelphia: Elsevier; 2006. p 271-83.

Mannan K; Scott G. The Medial Rotation total knee replacement: A clinical and radiographic review at a mean follow-up of six years. J Bone Joint Surg Br. 2009 Jun;91( 6):750-6.

Pennington J; Quinlan J; Doyle T; Bayan A; Theis JC. Results of porous-coated anatomic and Duracon total knee arthroplasty. J Knee Surg. 2010 Dec;23( 4):181-6.

Catani F; Ensini A; Belvedere C; Feliciangeli A; Benedetti MG; Leardini A; Giannini S. In vivo kinematics and kinetics of a bi-cruciate substituting total knee arthroplasty: a combined fluoroscopic and gait analysis study. J Orthop Res. 2009 Dec;27( 12):1569-75.

Kim TH; Lee DH; Bin SI. The NexGen LPS-flex to the knee prosthesis at a minimum of three years. J Bone Joint Surg Br. 2008 Oct;90( 10):1304-10.

Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

Topics

knee replacement arthroplasty ; tibial insert

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Edward A. Morra, Christine S. Heim, A. Seth Greenwald; Preclinical Computational Models: Predictors of Tibial Insert Damage Patterns in Total Knee ArthroplastyAAOS Exhibit Selection. The Journal of Bone & Joint Surgery. 2012 Sep;94(18):e137 1-5.

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