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. S uccessful outcomes with total knee 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 c ru- ciate ligament (PCL)–retaining, un- constrained articulations to fully linked, maximally constrained, sim- ple hinge designs. However, the add- ed degrees of implant stability carr y 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. Battist a. 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 This problem especially occurs in implant designs with fixed femoral components and a posterior tibial slope. Another disadvantage of PS implants is soft-tissue impingement, including the patellar “clunk” syn- drome, in which a soft-tissue nodule forms that can wedge into the intra- condylar notch during knee flexion, causing an audible and painful “clunk.” Other disadvantages in- clude potential raising of the joint line, the need for additional bone re- section to accommodate the femoral box and keel, and the risk of disloca- tion or instability in flexion. 6,12 Despite the dissimilarities be- tween CR and PS implants, most studies have found no significant differences in function, patient sat- isfaction, or survivorship of the two designs in unselected patient cohorts. 13-15 However, CR and PS im- plants may not function similarly in particular subgroups of patients (eg, patients with patellectomy, r heuma- toid arthritis, or large varus or varus- flexion deformity). Varus-Valgus Constrained Implants VVC implants have a tall (often re- inforced) tibial post and a deep fem- oral box, which provide more inher- ent coronal plane stability than do PS prostheses (Figure 2). Because there is no axle connecting the tibial and fem- oral components, these implants are sometimes referred to as unlinked constrained implants. To a variable extent, depending on design, VVC im- plants limit varus-valgus tilt as well as rotation (Figure 3). The stem exten- sion is important in transmitting stresses generated by the constrained articulation away from the fixation interfaces at the joint line to more normal diaphyseal bone (cementless stems) or along a broader surface area of implant-cement-bone contact (ce- mented stems). VVC knee implants may be used for both primary and revision arthro- plasty. They are often helpful in treating patients with severe valgus deformities, collateral ligament defi- ciency, bone defects, and residual in- stability or irreconcilable flexion- extension imbalances after PS implants. These implants have an acceptable survival rate at interme- diate follow-up, but little is known Figure 1 A through D, Sagittal plane kinematics of a posterior-stabilized total knee arthroplasty. The tibial post engages a femoral box during knee flexion, substituting for the resected posterior cruciate ligament and providing posterior stability during gait. However, the polyethylene on the tibial post can be a source of wear or impingement, and dislocation over the post (when the flexion-extension gaps are not balanced) can cause failure of the TKA. (Courtesy of Zimmer, Warsaw, IN.) Figure 2 Varus-valgus constrained implant (Maximum Constrained Knee). These implants feature a tibial post that engages in a deep femoral box to provide stability. (Courtesy of Biomet, Warsaw, IN.) Hannah Morgan, MD, et al Volume 13, Number 8, December 2005 517 about their performance beyond 10 years. 16-19 Drawbacks of VVC implants in- clude the need to remove femoral in- tercondylar bone to accommodate the femoral box, which decreases the remaining bone stock available for revisions, and potentially higher rates of aseptic loosening as a result of increased constraint. 16 Other po- tential drawbacks are failure or frac- ture of the tibial intercondylar emi- nence 20 and recurrent instability despite an intact intercondylar emi- nence. 4 However, regardless of the potential problems, VVC implants remain an important tool in the ar- mamentarium of the surgeon who performs knee arthroplasties. Rotating-Hinge Knee Implants Rotating-hinge knee implants are highly constrained devices most of- ten used for complex revision arthro- plasty performed for severe bone loss and/or complex instability and for on- cologic surgery. The tibial and fem- oral components are linked with an axle that restricts varus-valgus and translational stresses (Figure 4). To de- crease the overall amount of con- straint, these components permit ro- tation of the tibial bearing around a yoke on the tibial platform. This con- figuration provides a great degree of inherent stability; for that reason, these implants are very useful in sal- vage situations (eg, significant bone loss, severe deformity, unreconstruc- table ligamentous deficiency, flexion/ extension gap imbalances). Historically, aseptic loosening was seen more commonly in uniplanar hinged knee devices because of the tremendous degree of constraint, which prohibited rotational motion. These are no longer in use. The poten- tial drawback still exists of forces ap- plied across the knee being transmit- ted to the constraining portions of the implant or to implant-bone interfaces, leading to aseptic loosening or to un- usual mechanisms of prosthesis fail- ure. 4,21 However, the long-term dura- bility seen in younger oncology patients suggests that the rotating hinge can dissipate some of the forces. Another drawback to rotating- hinge knee implants is that a larger Figure 3 Anteroposterior (A) and lateral (B) views demonstrating stability of a varus-valgus constrained total knee arthroplasty. This drawing depicts the degree of coronal plane and rotational constraint provided by the tall, wide tibial spine in the deep femoral box of a VVC device. Most VVC implants are similar and permit 2° to 3° of varus-valgus stability and approximately 2° of internal-external rotation. (Courtesy of Zimmer, Warsaw, IN.) Figure 4 Rotating-hinge implant. In these prostheses, sometimes called “linked constrained” devices, femoral and tibial components are united by an axle that defines the flexion-extension arc in the sagittal plane. (Courtesy of Biomet, Warsaw, IN.) Constraint in Primary Total Knee Arthroplasty 518 Journal of the American Academy of Orthopaedic Surgeons bone resection is necessary to accept the housing of the implant. This is a concern because of both the reduced amount of bone supporting the pros- thesis and the potential difficulties that could be encountered with future revision procedures. As a result of these issues, many knee surgeons rec- ommend a hinged knee prosthesis only for patients with severe collat- eral ligament insufficiency, for those with marked bone loss, in compli- cated salvage cases, or in elderly pa- tients with comminuted fractures around the joint. 22,23 Long stems are required on the femoral side in these implants to limit the stress placed on the interfaces at the joint line and to transmit the stresses away from fix- ation interfaces, typically to diaphy- seal bone. Management The surgeon must decide, based on patient factors and the type of knee deformity, how much constraint is necessary for the primary TKA pa- tient. The degree of actual or poten- tial instability should be assessed, and the least-constrained implant that will correct that instability should be chosen (Figure 5). Uncomplicated Primary Total Knee Arthroplasty The typical patient undergoing a n index knee replacement has mild or moderate coronal plane deformity, intact posterior cruciate and collat- eral ligaments, adequate bone stock, and a normal extensor mechanism. In these patients, either a CR or PS implant may be chosen; both exhibit apparently similar survivorship into the second decade. As previously noted, the potential benefits of a CR implant include femoral bone preservation, tibial bone preservation (when pegs are used instead of a keel or stem to aug- ment cemented fixation), the possi- bility of more nearly normal knee kinematics, and some native varus- valgus stability. 24,25 The disadvan- tages of a CR prosthesis include the potential for greater difficulty achieving flexion and extension gap symmetry because of the additional ligament (PCL) that must be bal- anced, as well as the potential for in- Figure 5 Algorithm for selecting degree of constraint in primary total knee arthroplasty. CR = posterior cruciate–retaining, MCL = medial collateral ligament, PCL = posterior cruciate ligament, PS = posterior stabilized (PCL-substituting), TKA = total knee arthroplasty, VVC = varus-valgus constrained (unlinked constrained) Hannah Morgan, MD, et al Volume 13, Number 8, December 2005 519 creased polyethylene wear when the PCL is too tight postoperatively. When the PCL is sectioned and a PS component is used, ligament balanc- ing is less difficult and may be more reliable. PS devices are versatile and, therefore, may be used in patients with more complex deformities. For these reasons, familiarity with PS designs may be useful for surgeons who perform joint replacements only occasionally and require a sin- gle, versatile surgical technique. Severe Varus Deformity With Collateral Ligaments Intact Although a severe varus defor- mity may occur in isolation, more commonly it occurs in combination with a flexion contracture. 10 In the presence of such a deformity, deter- mining the degree of prosthesis con- straint is important. Equally impor- tant, however, is addressing the deformity and ligament contractures themselves. One of the risks in patients with severe preoperative var us deformity is that the knee will retain some re- sidual varus postoperatively. 26 Be- cause of this tendency, various tech- niques to correct varus deformity and to balance ligaments intraoper- atively have been described. Laskin and Schob 27 reported on medial cap- sular recession, a procedure in which the medial capsular flap is elevated distal to the pes anserinus and al- lowed to slide as the knee is stressed into valgus. This aggressive medial soft-tissue release allows correction of the deformity and ligament bal- ancing without leading to postoper- ative instability. Laskin and Schob 27 also emphasized that the PCL is of- ten contracted in patients with se- vere varus, further contributing to the deformity. A medial release alone may not correct the flexion and varus contractures; therefore, a PCL release and subsequent use of a PS implant may be necessary to avoid the higher incidence of postop- erative pain, radiolucencies, reduced flexion, and increased need for revi- sion that accompany a r etained, con- tracted PCL. 10 A medial epicondylar osteotomy is another method of achieving ade- quate soft-tissue release in a severe varus knee. In contrast to other tech- niques that involve extensive sub- periosteal stripping, this procedure avoids ligament damage. Engh and Ammeen 28 reported excellent pa- tient satisfaction, stability, motion, and deformity correction. Regardless of the technique used, the varus de- formity must be corrected, and bal- ance of the coronal plane ligament must be reestablished to minimize the likelihood of premature failure of the TKA. Most unselected cohort series (in which most patients have varus de- formities) have shown excellent re- sults into the second decade of im- plant survivorship, with either CR or PS implants. However, only one study 10 that compares CR and PS implants has been performed in the context of severe varus or varus- flexion deformities. In this series, Kaplan-Meier survivorship, range of motion, and pain-related outcomes were worse in patients with fixed varus (or varus-flexion) deformities >15° who were treated with CR de- vices, compared with patients treat- ed with PS implants or with those who did not have such varus defor- mities and were treated with CR de- vices. 10 Severe Valgus Deformity With Collateral Ligaments Intact Patients may present with a val- gus deformity that resulted from lateral compartment bone loss and soft-tissue contracture, medial col- lateral ligament (MCL) attenuation, or overcorrected proximal tibial os- teotomy. The major concern, and the focus of the preoperative assess- ment of the valgus knee, should be the status of the MCL. It may be n or- mal, attenuated but present, or ab- sent. The surgical treatment of these knees depends on the type and de- gree of deformity and the condition of the MCL. When the MCL is present and functional, either a CR or PS implant may be used. Howev- er, similar to the tight varus knee, before the level of constraint is se- lected, it is essential to balance the knee in the coronal plane. Regard- less of which implant is chosen, when the MCL is intact, then resto- ration of the mechanical axis to neu- tral, releases of lateral-sided struc- tures (as appropriate), and placement of a suitably sized polyethylene in- sert usually suffice to correct the de- formity and balance the knee. Numerous descriptions of step- wise techniques for performing lat- eral releases and ligament balancing have been published. 29,30 Most rec- ommend evaluating the knee both in flexion and extension and sequen- tially approaching the tight struc- tures in each position. A selective lateral release of the lateral retinac- ulum and iliotibial band and of the posterior capsule may be performed as necessary. Release of the popli- teus tendon and release or advance- ment of the lateral collateral liga- ment also may be performed in severe cases. In addition, externally rotating the tibial baseplate to inter- nally rotate the tibial tubercle may help patellar tracking in the patient with a valgus deformity. A VVC implant may be chosen for patients who present with severe deformity, especially when the me- dial structures are attenuated or when the patient is elderly. 17 Regard- less of the technique used, it is es- sential that the mechanical axis be restored to normal to avoid the poor clinical outcomes that may result from patellar maltracking or coronal plane instability, seen in patients with residual excess valgus postop- eratively. Although uncommon, patients with severe deformity and/or severe ligament insufficiency may have complex instability present in both Constraint in Primary Total Knee Arthroplasty 520 Journal of the American Academy of Orthopaedic Surgeons flexion and extension. This instabil- ity exceeds the typical coronal plane laxity observed in the MCL-deficient knee; a far more constrained implant is required to gain satisfactory stabil- ity at the time of arthroplasty. This complex instability is usually a re- sult of sequelae from severe trauma or the multiply operated knee. Be- cause the stems of VVC implants may not withstand forces generated by knees with severe varus-valgus laxity, a rotating-hinge knee implant may be the best option in these pa- tients. However, only limited pub- lished follow-up is available regard- ing contemporary rotating-hinge designs at intermediate follow-up or longer for this clinical setting; 31 therefore, the decision to use a hinged implant should be carefully considered. Patients With Rheumatoid Arthritis Patients with rheumatoid arthri- tis present special concerns for the surgeon, not only because of medi- cal, anesthetic, and associated mus- culoskeletal problems, but also be- cause of the tendency for generalized ligamentous laxity or attenuation and joint deformity. These patients may present with severe o r fixed val- gus deformities. Most patients with rheumatoid arthritis present with minimal coro- nal plane deformity. Whether a CR or PS implant is more prudent in these patients is controversial be- cause of concerns about the typical- ly poor quality of the soft tissues and the potential for synovitis to cause late attenuation and rupture of the PCL. Although some have repor ted excellent results with a CR prosthe- sis at intermediate follow-up, 32 con- cern exists that late instability may occur with long-term follow-up. Hanyu et al 33 assessed the PCL intra- operatively and performed CR TKA in patients only when the PCL was present and functioning normally. In their series, 10-year survivorship of the entire TKA cohort (both CR and PS) was 93%. No revisions were per- formed for instability in the CR group, whereas 6.5% of TKAs in the PS group (2/31) developed late dislo- cations (at 8 and 10 years postopera- tively) over the tibial post. 33 In another retrospective study, CR implants in patients with rheu- matoid ar thritis were associated with inferior results compared with PS implants, principally because of late instability and progressive re- curvatum deformity. 34 Although the authors concluded that a PS implant is more appropriate in the setting of rheumatoid arthritis to avoid these complications, they did not com- ment on the extent of the synovitis or the integrity of the PCL at the time of the index arthroplasty. Patients With Patellectomy Patellectomy leads to the disrup- tion of the normal four-bar linkage of the knee. In the context of knee replacement, it has been hypothe- sized that loads on the PCL in the years following surgery may be in- creased, potentially resulting in late attenuation and instability. 11,35,36 Pa- tellectomy also can cause decreased extensor mechanism power because of the loss of the fulcrum provided by the intact patella. A retrospective study showed that patellectomized patients treated with PS implants had better functional and pain scores than did those treated with CR im- plants. 11 The observation that use of PS devices leads to better results when TKA is performed in patients with prior patellectomies has been supported. 35 However, it is impor- tant to note that TKA patients with prior patellectomies generally have poorer outcomes and higher compli- cation rates than do nonpatellecto- mized patients, even when PS im- plants are used. 36 Medial Collateral Ligament Deficiency and Total Knee Arthroplasty Deficiency and instability of the MCL can create a great challenge for the surgeon performing an arthro- plasty. A wide spectrum of MCL at- tenuation and functional laxity ex- ists, ranging from mild valgus deformities with no ligament atten- uation to severe valgus deformity with ligament attenuation or rup- ture. Many factors, such as patient age, activity level, host tissue com- promise (ie, rheumatoid arthritis), bone-stock deficiency, and multiple prior knee surgeries, influence the choice of surgical technique and im- plant in a patient with an MCL- deficient knee. The diagnosis of an MCL-deficient knee should be made preoperatively so that the range of necessary implants is available at the time of surgery. The amount of valgus deformity should be noted, as well as the degree of MCL instabili- ty. A high index o f suspicion that the MCL may not be competent should be maintained in patients with marked valgus deformity or a predis- posing history (eg, rheumatoid ar- thritis, prior osteotomy). Choice of Implant In a knee with only a mild valgus deformity and ligament attenuation, either a CR or PS implant design may be used. Most authors agree that PS TKA components should be used when the PCL needs to be sac- rificed to obtain appropriate soft- tissue balance. In cases of grade 2 or lower MCL laxity, the extremity alignment can be corrected so that the lax compartment is loaded and closed with weight bearing. In this way, the varus-valgus constraint mechanism is not overtaxed. White- side, 37 Healy et al, 38 and Krackow et al 39 reported success in treating mild to moderate MCL laxity with CR implants, ligament balancing, and proximal MCL advancement when needed. When the MCL ligament demon- strates moderate attenuation or when substantial valgus deformity exists, either a PS or VVC implant may be used. A number of se- ries 16,18,19 have demonstrated accept- Hannah Morgan, MD, et al Volume 13, Number 8, December 2005 521 able results with a VVC implant, generally without specific repair or reconstruction of the MCL; excel- lent or good outcomes in 80% of pa- tients at intermediate follow-up have been reported. In patients with severe MCL attenuation, especially those with complex instability in ad- dition to coronal plane laxity, either a VVC or a rotating-hinge implant usually is required, especially when the patient is not a candidate for MCL reconstruction. Medial Collateral Ligament Repair or Reconstruction Advancement, imbrication, or al- lograft reconstruction of the MCL may be done to treat medial-sided laxity, often in conjunction with a VVC implant. Advantages of imbri- cation or advancement include the potential for increased component survivorship because of decreased stress transmission to the fixation surfaces and avoidance of allograft tissue, as would be needed for liga- ment reconstruction. Disadvantages of this technique include the poten- tial for late attenuation or rupture of the repair (especially with host tis- sue compromise) and the difficulty of getting satisfactory ligament bal- ance in both flexion and extension in severe valgus knees after imbrica- tion or reconstruction. This results from the fact that no true isometric point for the MCL exists throughout the range of motion. Advantages of ligament recon- struction (either autograft or allo- graft) include decreasing the amount of implant constraint necessary (through use of PS or CR devices and decreasing the stresses transmitted to fixation interfaces. Disadvantages in- clude technical difficulty of flexion- extension ligament balancing, in- creased surgical time, and, with autografts, donor site morbidity. Intraoperative Injury of the Medial Collateral Ligament MCL injury may occur intraoper- atively in a patient with no predis- posing deformity. In one series of 600 consecutive knees with either varus or neutral alignment that were treated with primary TKA, 16 knees (2.7%) sustained an inadvertent in- traoperative complete MCL inju- ry. 25 The injuries were either mid- substance disruptions or complete avulsions of the ligament from bone during the procedures. Although this can occur in patients with normal body mass, 25 the incidence of intra- operative MCL injury appeared far more frequently in morbidly obese patients in one report. 40 Historically, i atrogenic MCL inju- ry has been treated using VVC im- plants, although evidence-based sup- port for this approach is lacking because of the relative infrequency of the complication. 3,16,19 Because of the expected higher rates of aseptic loosening, the increased resection of bone required to implant such com- ponents, and the low likelihood that VVC components will be available in the operating room at the time of what is expected to be an uncompli- cated TKA, 16,25 it seems potentially advantageous to consider alterna- tives to this approach, when possi- ble. One alternative to increasing im- plant constraint during a TKA when a previously normal MCL is injured intraoperatively is to perform a pri- mary MCL repair or reattachment and to protect the repair postopera- tively with a hinged brace for 6 weeks. In one series, 16 knees were successfully treated primarily with reattachment or repair and bracing. The average Hospital for Special Sur- gery knee score at a mean follow-up of 4 years was 93 (excellent). No pa- tient required bracing beyond the initial 6-week period, and no patient demonstrated coronal plane instabil- ity. 25 Complex Instability It is not possible to anticipate ev- ery pattern of deformity that may occur in the context of primary TKA. For example, in occasional cas- es of coronal plane deformity, partic- ularly varus deformity, the surgeon must choose between failing to ob- tain ligament balance and complete- ly releasing the tightened medial structures, thus creating coronal plane instability. In such cases, sim- ilar to those of severe valgus previ- ously described, the surgeon may have to decide between ligament re- construction, VVC implants, and, in the most severe cases of ligament in- stability, rotating-hinge prostheses. In addition to patients with se- vere varus-valgus instability, others who may benefit from a contempo- rary design of rotating-hinge knee are elderly patients with comminut- ed distal femur fractures or peripros- thetic fracture nonunion, patients with extensor-mechanism disrup- tions and unstable knees, and those with marked bone loss that cannot be treated with augmentation or joint-line adjustments. 41,42 Rotating- hinge knees may have potential long-term risks, but they offer poten- tial reconstructive options for pa- tients with severe, complex instabil- ity. Summary Deciding the amount of constraint to use in a particular TKA is an impor- tant, yet challenging, element of pre- operative planning. Using an implant with insufficient constraint risks fail- ure from instability, whereas using a device that has more constraint than is necessary can predispose the pa- tient to aseptic loosening and bone loss. Clinical factors, such as rheu- matoid arthritis, prior patellectomy, severe coronal plane deformity, and collateral ligament deficiencies or complex instability, all may influ- ence the decision regarding the de- gree of constraint implant to use. For most primary knees without sub- stantial deformity or a need for diffi- cult ligament balancing, either a posterior-stabilized or a cruciate- retaining design is appropriate. In ev- ery case, the least constrained im- Constraint in Primary Total Knee Arthroplasty 522 Journal of the American Academy of Orthopaedic Surgeons plant that provides satisfactory joint stability should be chosen; soft-tissue repair or ligament reconstruction may help decrease the level of con- straint implant needed. 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Constraint in Primary Total Knee Arthroplasty 524 Journal of the American Academy of Orthopaedic Surgeons