Constraint in Primary Total Knee Arthroplasty Abstract Instability is an important cause of failure following total knee arthroplasty. Increasing component constraint may reduce instability, but doing so also can cause increased forces to be transmitted to fixation and implant interfaces, which may lead to premature aseptic loosening. Constraint is defined as the effect of the elements of knee implant design that provides the stability needed to counteract forces about the knee after arthroplasty in the presence of a deficient soft-tissue envelope. Determining the amount of constraint necessary can be challenging. Most primary total knee arthroplasties are performed for knees without substantial deformity or the need for difficult ligament balancing in these cases, either a posterior-stabilized or a posterior cruciate��� retaining design is appropriate. In certain situations, such as patients with prior patellectomies, rheumatoid arthritis, or substantial preoperative deformities, a posterior-stabilized knee may be favored. With their large posts, varus-valgus constrained implants typically are reserved for patients with substantial coronal plane instability, which is difficult to balance with a posterior-stabilized or cruciate-retaining implant alone. Rotating- hinge knee implants usually are recommended for patients with severe deformity or instability that cannot be managed with a varus-valgus implant. Sknee uccessful outcomes with total arthroplasty (TKA) depend on many factors, one of which is the degree of constraint inherent in the prosthesis design. Constraint is de- fined as the effect of the elements of knee implant design that provide the stability needed in the presence of a deficient soft-tissue envelope. In two recent reports, in which a total of nearly 500 failed TKAs were exam- ined, instability was the cause of nearly 25% of all the total knee revi- sions performed.1,2 Instability occurs when the avail- able ligaments and soft-tissue struc- tures, in combination with the pro- thesis articular design and limb alignment, are unable to provide the stability necessary for adequate function in the presence of stresses transmitted across the knee joint. In- stability may be the result of gener- alized soft-tissue laxity, inadequate flexion/extension gap balancing, im- proper component position or align- ment, or ligamentous insufficiency. Such instability may occur in any plane. To address instability in primary TKA, implants with varying degrees of constraint are available. These Hannah Morgan, MD, Vincent Battista, MD, and Seth S. Leopold, MD Dr. Morgan is Acting Instructor, Department of Orthopaedics and Sports Medicine, University of Washington Medical Center, Seattle, WA. Dr. Battista is Assistant Program Director, Orthopaedic Surgery Residency Program, William Beaumont Army Medical Center, El Paso, TX. Dr. Leopold is Associate Professor, Department of Orthopaedics and Sports Medicine, University of Washington Medical Center. The views expressed in this manuscript are those of the authors and do not reflect the official policy of the Department of Defense or the United States Government. Reprint requests: Dr. Seth S. Leopold, University of Washington Medical Center, 1959 NE Pacific Street, Box 356500, Seattle, WA 98195. J Am Acad Orthop Surg 2005 13:515- 524 Copyright 2005 by the American Academy of Orthopaedic Surgeons. Volume 13, Number 8, December 2005 515

range from flat-on-flat, posterior cru- ciate ligament (PCL)���retaining, un- constrained articulations to fully linked, maximally constrained, sim- ple hinge designs. However, the add- ed degrees of implant stability carry potential, and sometimes actual, dis- advantages. As the amount of con- straint is increased, stress transmit- ted to the modular implant-host or prosthesis-host interface also in- creases. The heightened stress may result in increased backside polyeth- ylene wear in modular tibial compo- nents or in early implant loosening, and ultimately to failure.3 Most au- thors therefore recommend using the least amount of implant con- straint necessary to achieve a satis- factory result.4 Constraint Terminology and General Principles Little standardization exists in the terminology used by implant manu- facturers and surgeon-investigators to describe the degree of constraint within a particular arthroplasty de- sign. Furthermore, many studies substitute brand-specific names for descriptive generic terminology, adding to the difficulty of comparing designs. The major implant categor- ies in present use, from the least to the most constrained, are as follows: (1) PCL-retaining (often called cruciate-retaining, or CR) (2) PCL- substituting (often called posterior- stabilized, or PS) (3) unlinked con- strained (sometimes called varus- valgus constrained, or VVC) and (4) rotating-hinge knee implants. Com- mon brand-specific terms for the VVC design include the NexGen Legacy Constrained-Condylar Knee (Zim- mer, Warsaw, IN) and the Total Condylar III (Johnson & Johnson, Braintree, MA). Both are unlinked, constrained prosthetic alternatives to rigid or rotating-hinge prostheses for complex knee reconstructions in which additional coronal-plane stabil- ity is desired because of soft-tissue de- ficiencies. Cruciate-Retaining Implants CR (PCL-retaining) implants are minimally constrained prostheses that depend on an intact PCL to limit posterior translation of the tib- ia on the femur. Potential benefits of CR implants (over either PCL- sacrificing or PCL-substituting de- signs) include the following: fewer patellar complications, increased quadriceps muscle strength, im- proved stair-climbing ability, pre- served proprioceptive fibers, lowered shear forces at the tibial component��� host interface, improved bone-stock preservation on the femoral side, and retention of more nearly normal knee kinematics. In addition, CR implants avoid the tibial post���cam impingement or dislocation over the tibial post that can occur in PS implants.5-7 Posterior-Stabilized (Cruciate-Substituting) Implants In contrast with CR implants, PS (PCL-substituting) implants have de- sign features (eg, a tibial post and femoral cam, deeply ���dished��� artic- ular surfaces, and a ���third condyle���) that limit excessive tibial transla- tion of the knee arthroplasty after re- section of the PCL (Figure 1). By al- lowing rollback, increasing the amount of distraction tolerated be- fore subluxation occurs, and increas- ing varus-valgus constraint, the cam-post mechanism improves both anterior-posterior and translational stability. Recently, interest has developed in using highly conforming tibial in- serts to increase stability.8 Some de- signs may eliminate the need for re- section of intercondylar notch bone stock and the use of a tibial post, which has the potential to wear. Various methods of achieving poste- rior stability are used by each im- plant design, with theoretic benefits to each design. However, no com- parative clinical studies confirm the superiority of one design over an- other. Regardless of the method used to achieve posterior stability, there are reported intraoperative and postop- erative benefits of a PS prosthesis over a CR design. These benefits in- clude relative ease of ligament bal- ancing, greater versatility in the presence of different types of knee deformity, easier correction of severe deformity by eliminating a tight PCL, increased predictability in res- toration of knee kinematics, im- proved range of motion, and poten- tially minimized polyethylene wear because of the option to use more congruent articular surfaces.8-11 Fur- thermore, the PCL can rupture post- operatively when it is overzealously recessed intraoperatively, is tight postoperatively because of an altered joint line, or is damaged by synovitis from inflammatory arthropathy, re- sulting in failure.9 The use of PS im- plants avoids these problems. A potential problem with PS im- plants, however, is tibial post poly- ethylene wear from the cam-post mechanism. Excessive wear particu- late debris can lead to osteolysis. None of the following authors or the departments with which they are affiliated has received anything of value from or owns stock in a commercial company or institution related directly or indirectly to the subject of this article: Dr. Morgan and Dr. Battista. Dr. Leopold or the department with which he is affiliated has received research or institutional support from Zimmer, Inc. Dr. Leopold or the department with which he is affiliated has received nonincome support (such as equipment or services), commercially derived honoraria, or other non-research���related funding (such as paid travel) from Zimmer, Inc. Dr. Leopold or the department with which he is affiliated serves as a consultant to or is an employee of Zimmer, Inc. Constraint in Primary Total Knee Arthroplasty 516 Journal of the American Academy of Orthopaedic Surgeons