Successful outcomes with total. Constraint in Primary Total Knee Arthroplasty

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1 Constraint in Primary Total Knee Arthroplasty 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 , Seattle, WA J Am Acad Orthop Surg 2005;13: Copyright 2005 by the American Academy of Orthopaedic Surgeons. 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. Rotatinghinge knee implants usually are recommended for patients with severe deformity or instability that cannot be managed with a varus-valgus implant. Successful 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 defined 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 examined, instability was the cause of nearly 25% of all the total knee revisions performed. 1,2 Instability occurs when the available ligaments and soft-tissue structures, in combination with the prothesis 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. Instability may be the result of generalized soft-tissue laxity, inadequate flexion/extension gap balancing, improper component position or alignment, 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 Volume 13, Number 8, December

2 Constraint in Primary Total Knee Arthroplasty range from flat-on-flat, posterior cruciate ligament (PCL) retaining, unconstrained articulations to fully linked, maximally constrained, simple hinge designs. However, the added degrees of implant stability carry potential, and sometimes actual, disadvantages. As the amount of constraint is increased, stress transmitted to the modular implant-host or prosthesis-host interface also increases. The heightened stress may result in increased backside polyethylene wear in modular tibial components or in early implant loosening, and ultimately to failure. 3 Most authors therefore recommend using the least amount of implant constraint necessary to achieve a satisfactory result. 4 Constraint Terminology and General Principles Little standardization exists in the terminology used by implant manufacturers and surgeon-investigators to describe the degree of constraint within a particular arthroplasty design. Furthermore, many studies substitute brand-specific names for descriptive generic terminology, adding to the difficulty of comparing designs. The major implant categories in present use, from the least to the most constrained, are as follows: (1) PCL-retaining (often called cruciate-retaining, or CR); (2) PCLsubstituting (often called posteriorstabilized, or PS); (3) unlinked constrained (sometimes called varusvalgus constrained, or VVC); and (4) rotating-hinge knee implants. Common brand-specific terms for the VVC design include the NexGen Legacy Constrained-Condylar Knee (Zimmer, 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 stability is desired because of soft-tissue deficiencies. Cruciate-Retaining Implants CR (PCL-retaining) implants are minimally constrained prostheses that depend on an intact PCL to limit posterior translation of the tibia on the femur. Potential benefits of CR implants (over either PCLsacrificing or PCL-substituting designs) include the following: fewer patellar complications, increased quadriceps muscle strength, improved stair-climbing ability, preserved 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 design features (eg, a tibial post and femoral cam, deeply dished articular surfaces, and a third condyle ) that limit excessive tibial translation of the knee arthroplasty after resection of the PCL (Figure 1). By allowing rollback, increasing the amount of distraction tolerated before subluxation occurs, and increasing varus-valgus constraint, the cam-post mechanism improves both anterior-posterior and translational stability. Recently, interest has developed in using highly conforming tibial inserts to increase stability. 8 Some designs may eliminate the need for resection of intercondylar notch bone stock and the use of a tibial post, which has the potential to wear. Various methods of achieving posterior stability are used by each implant design, with theoretic benefits to each design. However, no comparative clinical studies confirm the superiority of one design over another. Regardless of the method used to achieve posterior stability, there are reported intraoperative and postoperative benefits of a PS prosthesis over a CR design. These benefits include relative ease of ligament balancing, greater versatility in the presence of different types of knee deformity, easier correction of severe deformity by eliminating a tight PCL, increased predictability in restoration of knee kinematics, improved range of motion, and potentially minimized polyethylene wear because of the option to use more congruent articular surfaces Furthermore, the PCL can rupture postoperatively when it is overzealously recessed intraoperatively, is tight postoperatively because of an altered joint line, or is damaged by synovitis from inflammatory arthropathy, resulting in failure. 9 The use of PS implants avoids these problems. A potential problem with PS implants, however, is tibial post polyethylene wear from the cam-post mechanism. Excessive wear particulate 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. 516 Journal of the American Academy of Orthopaedic Surgeons

3 Hannah Morgan, MD, et al Figure 1 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.) 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.) 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 syndrome, in which a soft-tissue nodule forms that can wedge into the intracondylar notch during knee flexion, causing an audible and painful clunk. Other disadvantages include potential raising of the joint line, the need for additional bone resection to accommodate the femoral box and keel, and the risk of dislocation or instability in flexion. 6,12 Despite the dissimilarities between CR and PS implants, most studies have found no significant differences in function, patient satisfaction, or survivorship of the two designs in unselected patient cohorts However, CR and PS implants may not function similarly in particular subgroups of patients (eg, patients with patellectomy, rheumatoid arthritis, or large varus or varusflexion deformity). Varus-Valgus Constrained Implants VVC implants have a tall (often reinforced) tibial post and a deep femoral box, which provide more inherent coronal plane stability than do PS prostheses (Figure 2). Because there is no axle connecting the tibial and femoral components, these implants are sometimes referred to as unlinked constrained implants. To a variable extent, depending on design, VVC implants limit varus-valgus tilt as well as rotation (Figure 3). The stem extension 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 (cemented stems). VVC knee implants may be used for both primary and revision arthroplasty. They are often helpful in treating patients with severe valgus deformities, collateral ligament deficiency, bone defects, and residual instability or irreconcilable flexionextension imbalances after PS implants. These implants have an acceptable survival rate at intermediate follow-up, but little is known Volume 13, Number 8, December

4 Constraint in Primary Total Knee Arthroplasty Figure 3 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.) 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.) about their performance beyond 10 years Drawbacks of VVC implants include the need to remove femoral intercondylar 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 potential drawbacks are failure or fracture of the tibial intercondylar eminence 20 and recurrent instability despite an intact intercondylar eminence. 4 However, regardless of the potential problems, VVC implants remain an important tool in the armamentarium of the surgeon who performs knee arthroplasties. Rotating-Hinge Knee Implants Rotating-hinge knee implants are highly constrained devices most often used for complex revision arthroplasty performed for severe bone loss and/or complex instability and for oncologic surgery. The tibial and femoral components are linked with an axle that restricts varus-valgus and translational stresses (Figure 4). To decrease the overall amount of constraint, these components permit rotation of the tibial bearing around a yoke on the tibial platform. This configuration provides a great degree of inherent stability; for that reason, these implants are very useful in salvage situations (eg, significant bone loss, severe deformity, unreconstructable 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 potential drawback still exists of forces applied across the knee being transmitted to the constraining portions of the implant or to implant-bone interfaces, leading to aseptic loosening or to unusual mechanisms of prosthesis failure. 4,21 However, the long-term durability seen in younger oncology patients suggests that the rotating hinge can dissipate some of the forces. Another drawback to rotatinghinge knee implants is that a larger 518 Journal of the American Academy of Orthopaedic Surgeons

5 Hannah Morgan, MD, et al 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 prosthesis and the potential difficulties that could be encountered with future revision procedures. As a result of these issues, many knee surgeons recommend a hinged knee prosthesis only for patients with severe collateral ligament insufficiency, for those with marked bone loss, in complicated salvage cases, or in elderly patients 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 fixation interfaces, typically to diaphyseal bone. Figure 5 Management The surgeon must decide, based on patient factors and the type of knee deformity, how much constraint is necessary for the primary TKA patient. The degree of actual or potential instability should be assessed, and the least-constrained implant that will correct that instability should be chosen (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) Uncomplicated Primary Total Knee Arthroplasty The typical patient undergoing an index knee replacement has mild or moderate coronal plane deformity, intact posterior cruciate and collateral 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 augment cemented fixation), the possibility of more nearly normal knee kinematics, and some native varusvalgus stability. 24,25 The disadvantages 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 balanced, as well as the potential for in- Volume 13, Number 8, December

6 Constraint in Primary Total Knee Arthroplasty creased polyethylene wear when the PCL is too tight postoperatively. When the PCL is sectioned and a PS component is used, ligament balancing 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 single, versatile surgical technique. Severe Varus Deformity With Collateral Ligaments Intact Although a severe varus deformity may occur in isolation, more commonly it occurs in combination with a flexion contracture. 10 In the presence of such a deformity, determining the degree of prosthesis constraint is important. Equally important, however, is addressing the deformity and ligament contractures themselves. One of the risks in patients with severe preoperative varus deformity is that the knee will retain some residual varus postoperatively. 26 Because of this tendency, various techniques to correct varus deformity and to balance ligaments intraoperatively have been described. Laskin and Schob 27 reported on medial capsular recession, a procedure in which the medial capsular flap is elevated distal to the pes anserinus and allowed to slide as the knee is stressed into valgus. This aggressive medial soft-tissue release allows correction of the deformity and ligament balancing without leading to postoperative instability. Laskin and Schob 27 also emphasized that the PCL is often contracted in patients with severe 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 postoperative pain, radiolucencies, reduced flexion, and increased need for revision that accompany a retained, contracted PCL. 10 A medial epicondylar osteotomy is another method of achieving adequate soft-tissue release in a severe varus knee. In contrast to other techniques that involve extensive subperiosteal stripping, this procedure avoids ligament damage. Engh and Ammeen 28 reported excellent patient satisfaction, stability, motion, and deformity correction. Regardless of the technique used, the varus deformity must be corrected, and balance 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 deformities) have shown excellent results into the second decade of implant 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 varusflexion 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 devices, compared with patients treated with PS implants or with those who did not have such varus deformities and were treated with CR devices. 10 Severe Valgus Deformity With Collateral Ligaments Intact Patients may present with a valgus deformity that resulted from lateral compartment bone loss and soft-tissue contracture, medial collateral ligament (MCL) attenuation, or overcorrected proximal tibial osteotomy. The major concern, and the focus of the preoperative assessment of the valgus knee, should be the status of the MCL. It may be normal, attenuated but present, or absent. The surgical treatment of these knees depends on the type and degree of deformity and the condition of the MCL. When the MCL is present and functional, either a CR or PS implant may be used. However, similar to the tight varus knee, before the level of constraint is selected, it is essential to balance the knee in the coronal plane. Regardless of which implant is chosen, when the MCL is intact, then restoration of the mechanical axis to neutral, releases of lateral-sided structures (as appropriate), and placement of a suitably sized polyethylene insert usually suffice to correct the deformity and balance the knee. Numerous descriptions of stepwise techniques for performing lateral releases and ligament balancing have been published. 29,30 Most recommend evaluating the knee both in flexion and extension and sequentially approaching the tight structures in each position. A selective lateral release of the lateral retinaculum and iliotibial band and of the posterior capsule may be performed as necessary. Release of the popliteus tendon and release or advancement of the lateral collateral ligament also may be performed in severe cases. In addition, externally rotating the tibial baseplate to internally 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 medial structures are attenuated or when the patient is elderly. 17 Regardless of the technique used, it is essential 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 postoperatively. Although uncommon, patients with severe deformity and/or severe ligament insufficiency may have complex instability present in both 520 Journal of the American Academy of Orthopaedic Surgeons

7 Hannah Morgan, MD, et al flexion and extension. This instability exceeds the typical coronal plane laxity observed in the MCL-deficient knee; a far more constrained implant is required to gain satisfactory stability at the time of arthroplasty. This complex instability is usually a result of sequelae from severe trauma or the multiply operated knee. Because 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 patients. However, only limited published follow-up is available regarding 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 arthritis present special concerns for the surgeon, not only because of medical, anesthetic, and associated musculoskeletal problems, but also because of the tendency for generalized ligamentous laxity or attenuation and joint deformity. These patients may present with severe or fixed valgus deformities. Most patients with rheumatoid arthritis present with minimal coronal plane deformity. Whether a CR or PS implant is more prudent in these patients is controversial because of concerns about the typically poor quality of the soft tissues and the potential for synovitis to cause late attenuation and rupture of the PCL. Although some have reported excellent results with a CR prosthesis at intermediate follow-up, 32 concern exists that late instability may occur with long-term follow-up. Hanyu et al 33 assessed the PCL intraoperatively 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 performed for instability in the CR group, whereas 6.5% of TKAs in the PS group (2/31) developed late dislocations (at 8 and 10 years postoperatively) over the tibial post. 33 In another retrospective study, CR implants in patients with rheumatoid arthritis were associated with inferior results compared with PS implants, principally because of late instability and progressive recurvatum 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 comment 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 disruption of the normal four-bar linkage of the knee. In the context of knee replacement, it has been hypothesized that loads on the PCL in the years following surgery may be increased, potentially resulting in late attenuation and instability. 11,35,36 Patellectomy 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 implants. 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 important to note that TKA patients with prior patellectomies generally have poorer outcomes and higher complication rates than do nonpatellectomized patients, even when PS implants 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 arthroplasty. A wide spectrum of MCL attenuation and functional laxity exists, ranging from mild valgus deformities with no ligament attenuation to severe valgus deformity with ligament attenuation or rupture. Many factors, such as patient age, activity level, host tissue compromise (ie, rheumatoid arthritis), bone-stock deficiency, and multiple prior knee surgeries, influence the choice of surgical technique and implant in a patient with an MCLdeficient 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 instability. A high index of suspicion that the MCL may not be competent should be maintained in patients with marked valgus deformity or a predisposing history (eg, rheumatoid arthritis, 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 sacrificed to obtain appropriate softtissue 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. Whiteside, 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 demonstrates moderate attenuation or when substantial valgus deformity exists, either a PS or VVC implant may be used. A number of series 16,18,19 have demonstrated accept- Volume 13, Number 8, December

8 Constraint in Primary Total Knee Arthroplasty able results with a VVC implant, generally without specific repair or reconstruction of the MCL; excellent or good outcomes in 80% of patients at intermediate follow-up have been reported. In patients with severe MCL attenuation, especially those with complex instability in addition 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 allograft reconstruction of the MCL may be done to treat medial-sided laxity, often in conjunction with a VVC implant. Advantages of imbrication 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 ligament reconstruction. Disadvantages of this technique include the potential for late attenuation or rupture of the repair (especially with host tissue compromise) and the difficulty of getting satisfactory ligament balance in both flexion and extension in severe valgus knees after imbrication or reconstruction. This results from the fact that no true isometric point for the MCL exists throughout the range of motion. Advantages of ligament reconstruction (either autograft or allograft) include decreasing the amount of implant constraint necessary (through use of PS or CR devices and decreasing the stresses transmitted to fixation interfaces. Disadvantages include technical difficulty of flexionextension ligament balancing, increased surgical time, and, with autografts, donor site morbidity. Intraoperative Injury of the Medial Collateral Ligament MCL injury may occur intraoperatively in a patient with no predisposing 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 intraoperative complete MCL injury. 25 The injuries were either midsubstance 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 intraoperative MCL injury appeared far more frequently in morbidly obese patients in one report. 40 Historically, iatrogenic MCL injury has been treated using VVC implants, although evidence-based support 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 components, and the low likelihood that VVC components will be available in the operating room at the time of what is expected to be an uncomplicated TKA, 16,25 it seems potentially advantageous to consider alternatives to this approach, when possible. One alternative to increasing implant constraint during a TKA when a previously normal MCL is injured intraoperatively is to perform a primary MCL repair or reattachment and to protect the repair postoperatively 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 Surgery knee score at a mean follow-up of 4 years was 93 (excellent). No patient required bracing beyond the initial 6-week period, and no patient demonstrated coronal plane instability. 25 Complex Instability It is not possible to anticipate every pattern of deformity that may occur in the context of primary TKA. For example, in occasional cases of coronal plane deformity, particularly varus deformity, the surgeon must choose between failing to obtain ligament balance and completely releasing the tightened medial structures, thus creating coronal plane instability. In such cases, similar to those of severe valgus previously described, the surgeon may have to decide between ligament reconstruction, VVC implants, and, in the most severe cases of ligament instability, rotating-hinge prostheses. In addition to patients with severe varus-valgus instability, others who may benefit from a contemporary design of rotating-hinge knee are elderly patients with comminuted distal femur fractures or periprosthetic fracture nonunion, patients with extensor-mechanism disruptions and unstable knees, and those with marked bone loss that cannot be treated with augmentation or joint-line adjustments. 41,42 Rotatinghinge knees may have potential long-term risks, but they offer potential reconstructive options for patients with severe, complex instability. Summary Deciding the amount of constraint to use in a particular TKA is an important, yet challenging, element of preoperative planning. Using an implant with insufficient constraint risks failure from instability, whereas using a device that has more constraint than is necessary can predispose the patient to aseptic loosening and bone loss. Clinical factors, such as rheumatoid arthritis, prior patellectomy, severe coronal plane deformity, and collateral ligament deficiencies or complex instability, all may influence the decision regarding the degree of constraint implant to use. For most primary knees without substantial deformity or a need for difficult ligament balancing, either a posterior-stabilized or a cruciateretaining design is appropriate. In every case, the least constrained im- 522 Journal of the American Academy of Orthopaedic Surgeons

9 Hannah Morgan, MD, et al plant that provides satisfactory joint stability should be chosen; soft-tissue repair or ligament reconstruction may help decrease the level of constraint implant needed. Acknowledgment The authors gratefully acknowledge the support of Debbie L. Ames, whose assistance was invaluable in the preparation of this work. References 1. Fehring TK, Odum S, Griffin WL, Mason JB, Nadaud M: Early failures in total knee arthroplasty. Clin Orthop 2001;392: Sharkey PF, Hozack WJ, Rothman RH, Shastri S, Jacoby SM: Insall Award paper: Why are total knee arthroplasties failing today? Clin Orthop 2002;404: Cameron H, Hunter G: Failure in total knee arthroplasty: Mechanisms, revisions, and results. Clin Orthop 1982; 170: McAuley JP, Engh GA: Constraint in total knee arthroplasty: When and what? J Arthroplasty 2003;18: Callaghan JJ, O Rourke MR, Goetz DD, Schmalzried TP, Campbell PA, Johnston RC: Tibial post impingement in posterior-stabilized total knee arthroplasty. Clin Orthop 2002; 404: Gidwani S, Langkamer VG: Recurrent dislocation of a posterior-stabilized prosthesis: A series of three cases. Knee 2001;8: Most E, Zayontz S, Li G, Otterberg E, Sabbag K, Rubash HE: Femoral rollback after cruciate-retaining and stabilizing total knee arthroplasty. Clin Orthop 2003;410: Hofmann AA, Tkach TK, Evanich CJ, Camargo MP: Posterior stabilization in total knee arthroplasty with use of an ultracongruent polyethylene insert. J Arthroplasty 2000;15: Waslewski GL, Marson BM, Benjamin JB: Early, incapacitating instability of posterior cruciate ligament-retaining total knee arthroplasty. J Arthroplasty 1998;13: Laskin RS: The Insall Award. Total knee replacement with posterior cruciate ligament retention in patients with a fixed varus deformity. Clin Orthop 1996;331: Paletta GA Jr, Laskin RS: Total knee arthroplasty after a previous patellectomy. J Bone Joint Surg Am 1995;77: Beight JL, Yao B, Hozack WJ, Hearn SL, Booth REJ: The patellar clunk syndrome after posterior stabilized total knee arthroplasty. Clin Orthop 1994;299: Clark CR, Rorabeck CH, MacDonald S, MacDonald D, Swafford J, Cleland D: Posterior-stabilized and cruciateretaining total knee replacement: A randomized study. Clin Orthop 2001;392: Forster MC: Survival analysis of primary cemented total knee arthroplasty: Which designs last? J Arthroplasty 2003;18: Becker MW, Insall JN, Faris PM: Bilateral total knee arthroplasty: One cruciate retaining and one cruciate substituting. Clin Orthop 1991;271: Donaldson WF III, Sculco TP, Insall JN, Ranawat CS: Total condylar III knee prosthesis: Long-term follow-up study. Clin Orthop 1988;226: Easley ME, Insall JN, Scuderi GR, Bullek DD: Primary constrained condylar knee arthroplasty for the arthritic valgus knee. Clin Orthop 2000;380: Hartford JM, Goodman SB, Schurman DJ, Knoblick G: Complex primary and revision total knee arthroplasty using the condylar constrained prosthesis: An average 5-year follow-up. J Arthroplasty 1998;13: Lachiewicz PF, Falatyn SP: Clinical and radiographic results of the Total Condylar III and Constrained Condylar total knee arthroplasty. J Arthroplasty 1996;11: McPherson EJ, Vince KG: Breakage of a total condylar III knee prosthesis: A case report. J Arthroplasty 1993;8: Wang C, Wang H: Early catastrophic failure of rotating hinge total knee prosthesis. J Arthroplasty 2000;15: Barrack RL, Lyons TR, Ingraham RQ, Johnson JC: The use of a modular rotating hinge component in salvage revision total knee arthroplasty. J Arthroplasty 2000;15: Rand JA, Chao EY, Stauffer RN: Kinematic rotating-hinge total knee arthroplasty. J Bone Joint Surg Am 1987;69: Berger RA, Rosenberg AG, Barden RM, Sheinkop MB, Jacobs JJ, Galante JO: Long-term followup of the Miller- Galante total knee replacement. Clin Orthop 2001;388: Leopold SS, McStay C, Klafeta K, Jacobs JJ, Berger RA, Rosenberg AG: Primary repair of intraoperative disruption of the medial collateral ligament during total knee arthroplasty. J Bone Joint Surg Am 2001;83: Teeny SM, Krackow KA, Hungerford DS, Jones M: Primary total knee arthroplasty in patients with severe varus deformity: A comparative study. Clin Orthop 1991;273: Laskin RS, Schob CJ: Medial capsular recession for severe varus deformities. J Arthroplasty 1987;2: Engh GA, Ammeen D: Results of total knee arthroplasty with medial epicondylar osteotomy to correct varus deformity. Clin Orthop 1999;367: Whiteside LA: Selective ligament release in total knee arthroplasty of the knee in valgus. 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