Fig. 16.1
Stress X-ray indicating degree of medial opening in a painful TKA
Classification System for Instability
Parrate and Pagnano defined three types of instability after TKA—extension instability, flexion instability, and genu recurvatum [13]. They were further subdivided into symmetric and asymmetric instabilities. The instabilities may be accentuated by component malposition, size, or overall limb alignment considerations. Extension instability is caused by relative over resection of the femur, tibia, or both. Correction can be managed by augmentation of the femur or increased tibial insert thickness. The joint line must be considered when addressing extension instability. Raising the joint line excessively can lead to limited knee flexion, altered patellofemoral mechanics, and midflexion instability.
Asymmetric extension instability occurs usually due to the lack of appropriate correction of varus and valgus deformities at the time of primary TKA. The most common occurrence is inadequate medial collateral ligament release at the correction of varus deformity. Inadequate correction of valgus alignment is more commonly the culprit when performing a primary TKA for valgus deformity and arthritis. Both inadequate ligament release and lack of correction of alignment deformities usually result in recurrence of the deformities and failure of the TKA due to instability or later asymmetric wear.
Flexion instability is most often seen in the patient with a well-aligned knee in the coronal plane and well-fixed components. It may occur in PCL-retaining knees when the PCL is disrupted or stretched. It occurs in PCL-substituting knees when the femur is undersized, displaced anteriorly, or with soft tissue stretching of the posterior capsule and secondary restraints to the flexion space. Dislocation of the knee may occur with either design and is a dramatic but fortunately rare event. Surgeons should become familiar with examining knees in flexion over the side of the examining table so they can become familiar with acceptable and unacceptable flexion space laxity. The presence of a “posterior sag sign” may be demonstrated in posterior cruciate-retaining knees. Recurrent effusion, repeated aseptic aspirations, and difficulty descending and ascending stairs are frequent clinical complaints in patients with flexion instability.
Recurvatum deformity is very difficult to treat and often associated with neurogenic causes such as polio or end-stage rheumatoid arthritis with recurrent synovitis and capsule laxity. Relatively minor deformities in patients without polio or RA can be managed with intentional under-resection of the distal femur (or augmentation with a standard resection) or possibly posterior transfer of the femoral origins of the MCL and LCL as described by Krackow and Weiss [14]. Practically, however, this deformity is most reliably treated with a hinged prosthesis with an extension stop. The hinge should be positioned such that the patella component is properly positioned relative to the trochlea as this condition is often accompanied by significant quadriceps weakness, and optimal patellofemoral mechanics should be an important secondary goal [13].
Gap Balancing in Revision Total Knee Arthroplasty
The earliest examples of flexion instability were recognized in early cruciate-sacrificing devices implanted in patients with patellectomies who ultimately sustained posterior tibial dislocations [15]. The kinematics of flexion initiation at the knee are complex and begin with a posterior pull from the hamstrings on the posterior proximal tibia. The first motion of the tibia is posterior translation relative to the femur. In a normal knee and cruciate-retaining TKA, the PCL then resists this translational movement, and the force is converted to a flexion motion at the knee. Cruciate-sacrificing knees are dependent on the “uphill principle” which causes the collateral ligaments to tighten as the femur translates anteriorly on the tibia resulting in flexion rather than dislocation [15]. Additionally, the patella helps prevent posterior dislocation by buttressing against the anterior femur and resists too much posterior translation of the tibia through the patellar tendon attachment.
Changes in the flexion/extension gap will alter the kinematics of this process and may result in a poorly functioning TKA. Consequently, understanding the basic kinematics of the knee is critical to the concept of balanced flexion and extension gaps . This principle is foundational to ensuring that the prosthetic knee flexes, extends, and remains stable in both primary and revision knee arthroplasty [16]. Failure to obtain proper balancing results in asymmetry in the collateral ligaments during either flexion or extension leading to increased polyethylene wear, decreased range of motion, and anterior knee pain [17, 18]. Rotational malalignment must also be considered in the unstable TKA, which can accentuate the ligamentous imbalance. When balancing a primary or revision TKA, the surgeon must understand the distinction between ligament tension and gap size. The tension is largely affected by the management of the soft tissue envelope around the knee, while gap size is modulated primarily by the implant size and position [19, 20]. A properly balanced knee will have correct axial alignment, symmetric tension on the collateral ligaments throughout both flexion and extension, and the flexion and extension gaps will be equal.
The ultimate goal of gap balancing is to obtain equal gap size in flexion and extension with appropriate alignment and stability. Ries described three general principles that should be considered when balancing the knee: (1) tibial insert thickness or tibial augments affect both the flexion and extension gap equally, (2) extension gap problems can be corrected by adjusting the distal femoral cut with either augmentation (loose extension gap) or further resection of the distal femur (tight extension gap), and (3) solitary flexion gap problems are handled on the femoral side with anterior/posterior shifting of the femoral component or changing the femoral component size (effectively increasing the AP diameter of the implant) with the use of metal augments as necessary [21, 22]. The process can be simplified by obtaining the correct flexion gap first and matching the extension gap to the flexion gap. This decreases the number of possible combinations from nine to just three: equal flexion/tight extension, equal flexion/equal extension, and equal flexion/loose extension [19]. In reality, however, the surgeon must be simultaneously considering the flexion and extension gaps as decisions are made as to proper augmentation, implant size, and tibial insert thickness to avoid altering the joint line and patellofemoral mechanics.
The flexion gap stability is often a bigger challenge than the extension gap as it is affected by more variables: posterior tibial slope, size of the tibial/femoral components, polyethylene thickness, posterior femoral bone loss, and revision stem offset. In contrast, the extension gap is decided primarily by the proximal/distal femoral position. Because of this, many authors advocate correcting the flexion gap as the first step in the algorithm. The size of the flexion gap is best modulated by the size of the femoral implant which increases primarily in AP diameter as the component sizes increase. The surgeon can utilize posterior femoral augments to decrease the flexion space or to allow component upsizing in the presence of posterior condyle bone loss. Offsetting of the femoral component is possible with most modern revision systems and is used to assure the anterior flange of the femoral component is positioned against the anterior femoral bony cortex. This step assures the entire femoral thickness is used to “fill” the flexion space. There are several ways to fix components with stems and include rigid press-fit stems without cement fixation, “hybrid” cement fixation with “snug” stems and implant and metaphyseal cement fixation, or totally cemented stems (with or without cement plugs). All have their proponents and have been used successfully. The current trend is to avoid rigid cementless stems and offsets as there appears to be higher rates of (stem tip) pain in these patients. Fixation methods will be discussed in further detail later in this chapter.
Management of Bone Loss/Joint Line Restoration
Complex revision TKA cases can present with significant bone loss, which can make the subsequent revision reconstruction challenging. The etiology of bone defects is multifactorial and can include subsidence of loose implants, prosthetic wear resulting in osteolysis, infection, periprosthetic fractures, osteonecrosis, or stress shielding [23]. Since the ultimate goals in revision TKA are to create a pain-free knee with a functional range of motion, this is accomplished by correction of coronal and sagittal alignment, balanced flexion/extension gaps with an appropriate-sized implant and optimized ligamentous stability, stable and durable fixation of the implants, and preservation of as much host bone as possible [24]. There are a variety of reconstructive options available including cement and screws, metal augments, impaction and bulk allografts, metaphyseal cones and sleeves, or mega-prostheses [25–28]. The selection of these is dependent on the location and quantity of osseous defects present at the time of reconstruction as well as which soft tissues are intact.
Classification of Bone Defects
There are multiple different classification systems that have been developed to categorize bone loss and to guide treatment preoperatively, though ultimate determination of the bone defect is made in the operating room once the previous implants are removed [23]. The Anderson Orthopaedic Research Institute (AORI) is simple and is useful in describing bone defects and is now probably the most widely accepted classification for bone loss during revision TKA [29]. Bone defects in the distal femur and proximal tibia are divided into three types: type 1 has minor osseous defects not compromising component stability with intact cortical rim and relatively preserved joint line. Type II has more extensive bone loss with damaged metaphyseal bone and is further classified into subtypes (A and B) depending on if one or both condyles/plateaus are involved. Type III defects constitute cases with extensive metaphyseal bone loss compromising a major portion of either condyle or plateau and may involve compromise of the patellar tendon or collateral ligaments (Table 16.1) [29]. The validity of this classification system was analyzed by Mulhall who compared preoperative films to intraoperative observations and found that the AORI system predicted the bone loss correctly 67% of the time in the femur and 82% of the time in the tibia on the preoperative films [30].
Table 16.1
AORI bone defect types
Type 1 defect | Intact metaphyseal bone |
Good cancellous bone at or near a normal joint line level | |
Type 2 defect | Damaged metaphyseal bone |
Loss of cancellous bone that requires cement fill, augments, or small bone grafts to restore a reasonable joint line level | |
2A, one femoral or tibial condyle | |
2B, both femoral and tibial condyles | |
Type 3 defect | Deficient metaphyseal bone |
Deficient bone compromises a major portion of either condyle or plateau; these defects usually require a large structural allograft, a rotating hinged component, or custom component |
Treatment Options for Managing Bone Loss
The guiding principles in managing bone loss in revision TKA are to preserve as much native bone stock as possible and to rebuild bone stock where defects exist. This ideally creates a foundation to obtain initial and long-term fixation of the components. The surgeon must be aware of ligamentous integrity and the integrity of the extensor mechanism preoperatively as well. Instability may be both secondary to bone loss and ligamentous failure or imbalance.
Tibial Bone Loss
Increased Resection
Small, contained bone defects measuring less than 1 cm in depth (AORI type I) can usually be managed with cement, metal augments, or increased bony resection. Increasing the bone resection from the proximal tibia and distal femur seems to be a simple “fix” to minor bone defects and can be useful if minimal resection remedies the problem. However, the surgeon must realize that there is bone loss associated with the index arthroplasty, additional bone loss associated with the TKA failure, and further bone loss from component removal at the time of revision arthroplasty. Further resection could result in decreased metaphyseal bone strength and a decrease in the tibial component size which can increase contact forces on the tibia. A study by Harada demonstrated an abrupt decrease in tibial bone strength after 5 mm of resection from the joint line [31]. Consequently, removal of substantial bone should be limited, and no more than 1–2 mm should be resected at the time of revision [23]. Cortical implant contact and stem augmentation can help remedy the weakened bone encountered with tibial bone resection at the time of revision surgery. Metaphyseal cones are creatively a relatively new way to augment metaphyseal bone loss as well.
Component Shift
The surgeon can consider shifting the tibial component to areas with greater bone stock to avoid small osseous defects (<5 mm). This is of limited value in larger defects because significant shifts in the component placement can have deleterious effects on ligament kinetics [23]. Additionally, a study by Lee suggested that the tibial tray not be shifted more than 3 mm medially, and Daines and Dennis recommend no more than 2 mm lateral shifting of the component [23, 32]. In addition, shifting the component more than 3 mm which results in implant-bone overhang can result in pain. Additionally, the use of offsets in the tibia can create complex problems in implant removal should the knee need revision in the future.
Cement Reconstruction
Polymethyl methacrylate (PMMA) cement , with or without the use of screws, is another option for smaller bony defects. This is often indicated in peripheral defects of ~10% or less of the total condylar surface [33]. This is a simple and economical fix for smaller osseous defects, but is not a good option in larger defects (>10 mm) [23]. There are several pitfalls related to using PMMA when managing bony defects. First, large masses of cement create a significant exothermic reaction that can lead to osseous necrosis. Secondly, during the curing process, PMMA can lose 2% of its volume resulting in some collapse and loss of support [34]. This loss of volume will be accentuated with larger masses of cement. Lastly, PMMA has demonstrated inferior load transfer properties than metal augments and should be avoided in larger defects [34]. If used in the small defects, PMMA has demonstrated good results in primary TKA. Ritter demonstrated 13 of 47 primary TKAs with bone defects treated using PMMA with an average of 6.1 years of follow-up had evidence of radiolucent lines on radiographs but no progression to component loosening [35].
Prosthetic Augments
Metal prosthetic augments are useful in uncontained type IIA defects and type IIB (bicondylar) defects of moderate size >10 mm [36]. Tibial augments come in a variety of shapes including both full and hemiplateau blocks and angular wedges with sizes that typically range from 5 to 15 mm. These augments allow alteration to both the flexion and extension gaps by permitting the filling of bone defects and placement of the implant closer to the joint line.
Femoral Bone Loss
The AORI classification system applies to femoral bone loss as well. Minor defects in the condyle can be filled with morcellized graft or femoral augments [36]. Femoral augments are generally block shaped and can increase by increments of 5–20 mm in size depending upon the implant system used [36]. When comparing prosthetic augment wedges to blocks in a biomechanical study, blocks were more stable and have been shown to have superior strain distribution from the implant to the underlying supporting bone [37, 38]. These augments can be applied quickly, allow for intraoperative custom modifications, offer excellent biomechanical properties, require very little initial bone resection, and are effective at restoring the joint line in revision settings [36]. They have been used with generally good success rates with 84–98% good to excellent results in several series [24, 33, 36, 39].
Localized Impaction Grafting
Morcellized autograft or allograft can be used to fill small contained lesions (type I or II) and is a good option in younger patients where restoration or preservation of host bone stock is preferable [40]. This has shown good results at midterm follow-up with several studies demonstrating new osseous trabeculation at the graft site and stable fixation of the components [27, 40–42]. More recently, some centers have advocated the use of impaction grafting for large uncontained defects. These centers use a wire mesh to transform the uncontained lesion into a contained defect for the impaction grafting. The early results with this technique are promising; Lonner used this technique in 14 revision TKAs with large uncontained defects and demonstrated significant improvement in knee society scores with an average increase in 47 points and no revisions at 2 years of follow-up [27, 43].
Structural Allografts
Structural allografts are a common option for large uncontained type II defects that are too large for augmentation alone and some type III defects. Dorr recommended using structural allografts in tibial defects affecting greater than 50% of the tibial plateau [40]. Femoral head, distal femoral, or proximal tibial allografts are the most commonly used allografts for large bone defects in revision TKA (Fig. 16.2). They provide the advantage of potentially restoring bone stock and are customizable at the time of surgery. Unfortunately, they are technically challenging to use, and bony incorporation takes a substantial amount of time and may never completely incorporate. Additionally, there is a very small theoretical risk of disease transmission [44]. The technical keys to large allograft reconstructions include developing a healthy, bleeding host bone interface to accept the graft, maximizing the host bone-allograft bone contact, and achieving stable fixation, which is often difficult to obtain [23]. Despite having a high complication rate for these cases reported in the literature, there are several studies that have demonstrated a high rate osseous integration when stable fixation is obtained at the time of reconstruction with implant survival rates ranging from 75 to 93% with medium- to long-term follow-up [25, 26, 45, 46]. Many of these defects will be managed by metaphyseal cones in the future.
Fig. 16.2
Allograft used to replace lateral femoral condyle
Metaphyseal Cones/Sleeves
Large central contained cavitary or combined cavitary-segmental defects in the femur or tibia can be treated using metaphyseal sleeves or porous cones. The ultimate advantage of these devices is the long-term biologic fixation and avoidance of nonunion, resorption, and collapse of the structural allografts used in revision TKA [47]. Porous metal cones achieve ingrowth, and any type of revision prosthesis can be cemented to the center surface. Porous cones are modular in nature and allow the surgeon to choose a size and position that will best fit the defect. Metaphyseal sleeves are implant specific and are not as customizable as the porous unlinked cones. The primary difference between titanium or trabecular metal cones and sleeves is at the implant interface. Metaphyseal sleeves utilize a morse taper junction with the prosthesis rather than cement as with the unlinked tantalum or titanium cones. Results have been good in regard to osseous integration and implant stability for both cones and metaphyseal sleeves. One group followed 16 patients for 31 months after using tantalum cones for reconstruction and had no failures due to aseptic loosening [48]. Meneghini also followed 15 patients that had cones placed at the time of reconstruction for an average of 34 months with no reported failures [49]. Midterm data for the use of metaphyseal sleeves has also demonstrated good osseous integration with few failures [50, 51].
The surgical technique for tantalum or titanium cone placement begins with good exposure to adequately assess the bony defect of the metaphysis of the tibia and/or femur and the need for metaphyseal cone fixation. The most common scenario is a severe contained or uncontained osseous defect of the medial plateau or condyle with some degree of lateral plateau or condyle bone loss but with enough lateral bone present to provide some support. In more severe cases, both plateaus and condyles have large defects and the tapered shape of the cone creates an interference fit with the remaining cortical bone to provide all the structural support [47]. A trial stem or a reamer may be used to help align the finishing cut and to ensure proper position of the cone in the metaphysis. Once the correct size and shape is selected from the trial cones, specialized reamers or a high-speed burr are utilized to contour the metaphyseal bone to match the cone. Newer stem and reamer-based instrumentation has been developed to machine for the cones. Stability of the cone is achieved in two different ways: creating a press-fit wedge or by resting on intact bone distally. The cone is then impacted into place using an impactor. Once the cone is impacted into position, the surgeon may now insert the tibial tray and stem or femoral trial and stem to allow assessment of joint alignment, joint line height, stability, and motion. The central portion of the cone creates an artificially reconstituted proximal tibial metaphysis or distal femoral metaphysis that is receptive to cementation at the time of final implant positioning. Any remaining voids/defects around the periphery of the cone can be filled with morcellized cancellous bone graft and/or augments and cement.
Condyle-Replacing Hinge Prosthesis
Massive type III defects with loss of collateral ligament integrity often require a hinged prosthesis and should be considered, particularly in older, low-demand patients. The most significant advantage to using this technique is reduced surgical time and early fixation with the ability to mobilize the patient immediately postoperatively. These implants can also address bone and soft tissue defects that other implant systems cannot treat. It is generally wise to have a hinge implant system available as backup whenever tackling more than the simplest revision case.
Trial Cutting Guides in the Performance of Revision Total Knee Replacement
A trial cutting guide is a cutting guide which is shaped like a trial femoral implant. The guide is sized and shaped like the final implant but has cutting slots or surfaces that permit the performance of augment and/or box cuts through the trial (Fig. 16.3). These devices permit a trial reduction to be performed before the resections and, more importantly, allow the resections to be performed while the guide remains in place. Such devices permit evaluation of the flexion and extension gaps before bone resection and allow more reliable determination of bone loss versus soft tissue integrity. This permits selection of the appropriate augments required to establish a stable implant construct at the time of revision arthroplasty.
Fig. 16.3
Stryker Triathlon trial cutting guide with offsetting capability, augment cut surfaces, and box cut surfaces. The cuts can be made 5 mm increments supporting augments up to 15 mm at both the distal femur and posterior condyle. This allows one guide to perform all resections leading to improved accuracy and reproducibility
Surgical Technique: Trial Cutting Guide
Prior to incision, it is important to establish the collateral ligamentous integrity with an exam under anesthesia. This information is helpful and may indicate the level of constraint needed for the revision implant. Loose implants can mimic ligament instability, and thus the exam should be performed both before and after arthrotomy. As in all revision TKAs, good exposure is critical to successful surgery and safe removal of the previous implants. The implants should be removed carefully to avoid damage to the collateral ligaments, excessive bone loss, or damage to the extensor mechanism.
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