Bone loss and subsequent defects are often encountered in revision total knee arthroplasty and occasionally in primary total knee arthroplasty. The variability in size and location of these defects has led to the development of a multitude of techniques aimed at restoring the physical integrity of the knee and supporting prosthetic replace- ment. Techniques frequently reviewed in the literature include filling minor defects with cement; augmentation of cement with screws, wires, or mesh; bone grafting;
metal augmentation with blocks or wedges; and custom components.
Modularity in total knee systems has earned its acceptance by providing utility in the management of this wide spectrum of bony defects. Consequently, as the array of modular options including offset stems, stem extensions, variable femoral and tibial prosthetic body options, and modular augmentations have evolved, custom implants are now rarely needed. The clinical acceptance of modular metal wedges and blocks is due in large part to their effectiveness in managing the variety of clinical situations that face the knee arthroplasty surgeon.
Bone defects that remain contained by the cortical rim, both in the tibia and in the femur, are generally best managed with bone grafting techniques. A number of authors have reported success using structural as well as morsellized allograft in these contained defects.1For very large contained defects, a combination of bulk and mor- cellized graft may be most appropriate, usually offloaded with extended prosthetic stems.
When the cortical rim of either the distal femur or proximal tibia is breached, the reconstructive options are challenging. In younger patients, structural allograft may be an option for consideration, yet this is tempered by reported problems including host-graft nonunion,
disease transmission, and possible late collapse or resorp- tion of the allograft. Indeed, there is a trend in revision centers away from bulk, structural allograft when other options are readily available.
Surgical techniques other than the use of modular or custom implants include shifting of the prosthesis to a region of more supportive host bone stock and/or pos- sibly downsizing the prosthesis. These intraoperative choices represent compromises that may be accompanied by potentially undesirable consequences. On the tibial side, downsizing the tray and shifting away from a com- promised cortical rim results in increased unit force transmission across the component to the underlying bone. Reduction of cortical rim contact coupled with an increased reliance on cancellous bone, tray subsidence may result. One clinical study suggests that translation of the tibial tray greater than 4mm may lead to higher component loosening and failure.2 Downsizing of a femoral component to accommodate anterior or poste- rior bone loss may inadvertently lead to flexion space instability.
Recognition of the limitations associated with the techniques mentioned previously led to the development of modular metal wedges and block augmentations.
The first wedged augmentation of a tibial component was reported by Jeffery et al.3 The first clinical series reporting use of modular metal wedges for the man- agement of bone deficiency was by Brand et al. in 1989.4 Modular metal augmentations are now readily incor- porated in modern knee reconstruction systems. In this chapter we discuss the relative indications for femoral or tibial augmentations with modular augments, the justification for their use in modern reconstructive surgery, limitations with this approach, and techniques employed.
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Modular Augments in Revision Total Knee Arthroplasty
J. Bohannon Mason
BONE LOSS: GENERAL CONSIDERATIONS
Bone deficiencies and bone loss are encountered in both primary and revision settings. In a primary knee extreme varus, valgus, or flexion deformities may preoperatively herald the presence of bone defects, which, if ignored, may threaten the component reconstruction. Varus or valgus angulation, in the extreme, can lead to significant bone loss on either the tibial or femoral side of the joint.
Although such extreme defects are less commonly encountered in primary knee arthroplasty in clinics today, progressive or rapid bone loss associated with avascular necrosis, neglect, or trauma may result in bone defects that require augmentation. Inflammatory arthropathy, such as rheumatoid arthritis, may result in severe cyst formation and bone loss.
The bone defects seen in revision knee arthroplasty generally occur with component loosening, component removal, or from osteolysis. Several authors have described classification schemes for bone loss about the knee.5Deficiencies on the tibial side are typically central cavitary, peripheral, or a combination. On the femoral side, the loss of structural host bone that requires aug- mentation is usually distal or posterior (Figure 9-1).
Obviously, multiple permutations of any bone loss classi- fication schemes are seen clinically, depending in large part on the mode of failure, the failed component type, and preexisting host bone stock. The most common pat- terns of bone loss that require modular augmentation include medial tibia in association with varus angulation, lateral tibial augmentation seen with valgus failure, and a
combination of distal and posterior femoral augmenta- tion with component failure.
Preoperative radiographs can help identify patients who may require tibial or femoral augmentation. Brand et al.4have proposed a method for estimating tibial defect size based off of preoperative anterior-posterior radi- ographs. This technique is illustrated in Figure 9-2. A line is drawn down the central axis of the tibia. A perpendi- cular line is then drawn at the top of the intact tibial plateau. A second perpendicular line is extended to the base of the tibial defect. A differential measurement, cor- rected for magnification, exceeding 15mm may require augmentation and should be considered in preoperative planning of the reconstruction.
Estimation of the need for augmentation on the femoral side is slightly more difficult. The 3-dimensional shape of the distal femur captured on 2-dimensional film, along with the metallic bulk of the femoral implant, make visualization of the distal femur difficult. Additionally, the bicondylar overlay on lateral films may lead to underesti- mation of unicondylar defects. Although oblique x-rays may be of benefit, evolving computed tomography tech- niques with subtraction algorithms hold great promise for accurate preoperative prediction of bone loss. Addi- FIGURE 9-1. Modern revision knee systems allow for the use of
augments of varying thickness, as here on the posterior and distal femur.
20mm
FIGURE 9-2. The size of a peripheral tibial defect can be meas- ured off the reconstructed joint line based on the uninvolved side.
(Adapted from Mason and Scott5 by permission of Lippincott Williams & Wilkins.)
tray or to substitute for more extensive proximal cortical bone loss. Block augments, sometimes referred to as step wedges, are employed when bone loss at the cortical rim includes segmented medial (or lateral) bone and sup- porting anterior or posterior cortical bone at the level of the tray-bone resection.
Fehring et al.7 found that tensile strain within the cement-bone interface was less with block augments compared with wedges. However, the maximal strain differential between blocks and wedges was only slight, arguing that the augment that best fills the defect should be used.
The long-term results of revision knee arthroplasty with modular augments have not been reported. The first clinical series reporting the use of metal wedges for tibial bone deficiencies was reported by Brand et al.4 In this series, 22 knees in 20 patients were included. Modular metal wedges used to customize the tibial implant. Three of the 22 knees were revision cases. In each case a small tibial cemented stem extension was employed. Six knees, at average 37 months’ follow-up, revealed radiolucent lines beneath the tibial wedge; however, no tibial tray was judged to be loose. Rand8reported a series of 28 primary knees at a mean follow-up of 27 months in which defects up to 18mm were treated. The majority of these were medial bone defects. Clinical scores for all patients were rated as good to excellent despite nonprogressive radi- olucent lines beneath 13 of the 28 tibial wedges. In a follow-on study of the same patient cohort, no significant degradation in the radiographic follow-up of the wedges was noted.9One patient failed due to patella complica- tions. Despite the use of modular metal augmentations in revision knee reconstruction in multiple clinical series, no other clinical series have focused specifically on the role of modular metal augmentations in the success or failure of the reconstruction.
TIBIAL COMPONENT AUGMENTATION
Modular augmentation represents an attractive option in reconstructive surgery, allowing a surgeon to produce a custom implant, reestablish correct component levels with respect to the joint line, maintain or reestablish limb alignment, and adjust soft tissue balance.
Indications
Tibial augmentation with modular metal wedges or blocks is usually applied to defects of 5 to 20mm in depth, particularly when these defects fail to support more that 25% of the tibial base plate (Figure 9-4). Several factors guide the decision to use modular augments. Since the tionally, careful study of the prosthetic design and knowl-
edge of the history of the prosthesis may be of benefit in preoperatively determining the need for femoral aug- mentation if defects are not obviously apparent.
CLINICAL JUSTIFICATION AND RESULTS USING MODULAR METAL AUGMENTATION
The mechanical strength of augmentation wedges and blocks has been investigated. In vitro studies have focused on two areas of interest.6The first is the fixation of the augment to the prosthesis. Most modern designs rely on a screw or snap-lock mechanism, occasionally augmented with cement (Figure 9-3). Older designs relied exclusively on cement fixation of the augment to the prosthesis. All mechanisms of augment fixation have been used success- fully in the short term with clinical experience up to 5 years reported. The long-term concerns include loosen- ing, dissociation of the augments, and possible fretting leading to third body wear. Brand et al.4reported a revi- sion of a nonmodular tibial tray for polyethylene failure in which they had previously applied a 5mm wedge with cement for a medial tibial defect. After 5 years in vivo, the medial wedge maintained 77% of the sheer strength of control and showed no evidence of corrosion, fretting, or impending failure.
Modular augments used beneath the tibial tray are typically either wedge shaped, which fit above an oblique bone resection, or are blocks. Hemiwedges can be used to fill small peripheral defects, whereas full wedge augments can be used to correct axial alignment beneath the tibial
FIGURE 9-3. Screw-on or snap-fit mechanisms are used for attachment of the augment to the prosthesis in most modern systems.
tibial diaphysis tapers distal to the joint line, resection to the supportive tibial host bone requires the use of a smaller base plate or risks overhanging metal, which can be particularly problematic to the patient postoperatively.
Tibial defects rectified by downsizing the tibial base plate, with greater resection of bone to the depth of the defect, may limit the opposing femoral component sizing choices. The depth of modular augmentation, too, is limited by several practical considerations. First, most commercially available augments do not taper as the host bone metaphysis does. Larger tibial augments may like- wise expose a sharp prosthetic edge at the base of the augment. This modular overhang may cause pain and should be avoided if other options for reconstruction are suitable. The depth of a modular augmentation is addi- tionally limited by the extensor mechanism. Resection levels greater than 20mm below the native joint line place the tibial tubercle and extensor mechanism in jeopardy, particularly if on the lateral side.
Extensive proximal tibial bone loss over both medial and lateral surfaces of the proximal tibia may be handled with thicker polyethylene inserts. However, as the poly- ethylene insert’s thickness increases, the stresses at the insert locking mechanism increase, potentially leading to increased micromotion. This negative biomechanical consequence can be offset by elevating the tibial base plate, and reducing the thickness of the polyethylene insert required. Full tibial base plate augments or bilateral matched medial and lateral augments can be used to raise the tibial tray closer to the native joint line (Figure 9-5).
As the tibial base plate is elevated with augments, the stem is effectively shortened, suggesting consideration of a longer stem (Figure 9-6).
FIGURE 9-4. Uncontained tibial defects such as this medial defect are easily managed with a tibial wedge augment, allowing cortical rim contact with the prosthesis.
FIGURE 9-5. Tibial bone loss may exceed the height of the modular polyethylene inserts available for a given knee system. In this instance medial and lateral augments are paired to elevate the tibial joint line.
FIGURE 9-6. A full modular wedge augment was used in this patient who had experienced valgus failure of his prior implant. A short stem extension was selected. Despite initial stability, implant loosening occurred at 3-year follow-up. When host bone is sig- nificantly compromised to require a tibial augment, a longer stem extension should be considered.
The size of the wedge or block is then determined by measuring the distance between the undersurface of the tibial tray and the depth of the cortical defect. Most revi- sion systems provide resection guides for the various modular components (Figure 9-8). However, in obese patients who require deep resection levels or have lateral defects, these resection guides may be difficult to use and the resection may require free-hand adjustments. A narrow oscillating saw or high-speed bur can be particu- larly useful in these situations. The selection of a modular augment typically mandates the use of a stem. Con- sequently, intramedullary alignment systems are most helpful and can prevent errors including medial or lateral displacement of the augment, excessive or reversed slope of the tibial tray, and large errors in axial alignment in the AP plane. Offset stems can be useful in avoiding compo- nent overhang.
Estimating the height of the joint line can be difficult in cases with extensive bone loss associated with ligament laxity. Although the kinematic relationship between the femoral and tibial components is most important, the surgeon should strive for accurate joint line restoration.
Helpful techniques available to the surgeon include com- paring the patella ligament height to the contralateral knee or to the knee prior to reconstruction, as well as radiographically examining the contralateral, uninvolved joint line, and extrapolating the height of the proximal fibula to the native joint line.
Femoral Component Augmentation
The use of modular metal augmentations on the femoral side has received less attention in the literature. Current knee systems include augments of variable thicknesses for the medial and lateral condyles both distally and posteri- orly, or in combination. A few systems provide anterior femoral augments. As surgeons become more conscious of soft tissue balance, the role of femoral joint line restoration and correct axial rotation is prioritized.
Failure to restore the joint line or properly rotate the com- ponents relative to each other can compromise knee kinematics. Knee flexion and patella tracking may be adversely affected.
Rotational alignment is discussed elsewhere in this text. However, modular femoral augments may help facil- itate accurate restoration of component rotation. Lateral femoral condylar hypoplasia is often associated with valgus axial alignment. When recognized, lateral condylar hypoplasia is easily managed with posterolateral modular augmentation on the femoral component. Inattention to the relative hypoplasia in this situation may lead to inter- nal rotation of the femoral component, particularly if a posterior condylar referencing system is used. Likewise, a frequently encountered situation in revision arthroplasty Surgical Technique
In reconstructing the deficient proximal tibia with modular augments, the objectives remain restoration of alignment, soft tissue balancing, and a near-anatomic replication of the joint line to restore knee kinematics. In primary and revision knee arthroplasty the initial resec- tion level is selected with optimal preservation of host bone stock. The residual peripheral defects are then assessed. It is important to determine the flexion- extension gap relationship between the femoral and tibial trial components. This is particularly true when trial distal femoral augments are considered, as the tibial resec- tion level equally affects the flexion and extension space.
With the trial femoral component in position the knee is brought into full extension and the rotational alignment of the tibial tray relative to the tibial host bone is deter- mined and marked on the proximal tibia. This step is important before preparing the proximal tibia for an augment. The axial rotation of the tibial tray relative to the tibia determines the anterior to posterior (sagittal) orientation of the wedge or block resection. Failure to note this rotational alignment may result in difficulty matching the modular augment to the prepared resection, or inadvertent internal or external rotation of the tibial tray (Figure 9-7).
FIGURE 9-7. It is important to determine the tibial tray rota- tional alignment prior to resection of the defect. This ensures proper seating of the tibial tray with the augmentation and also ensures the correct rotational relationship to the femoral compo- nent. (Adapted from Mason and Scott5by permission of Lippincott Williams & Wilkins.)
is the revision of an improperly rotated femoral com- ponent. The common error is internal rotation of the femoral component relative to the epicondylar axis.
Restoration of proper rotational alignment at revision surgery may require external rotation of the femoral com- ponent. The availability of posterior modular augments can be of particular benefit (Figure 9-9). When femoral component failure requires removal of the implant, there is often loss of distal femoral bone.
Additionally, distal resection of bone to achieve a stable bone surface elevates the prosthetic-bone interface.
Modular distal femoral augmentation can help reduce this artificial elevation of the joint line. References for femoral joint line mirror the discussions above on tibial joint line restoration. The epicondyle can be used as a relative bony reference point, however, the distance from the epicondyle to the joint line varies from patient to patient.10Anterior femoral augments, although less com- monly employed, may be of benefit if the prosthetic stem forces the femoral component anteriorly. Anterior-poste- rior femoral stem translation is available now with most systems. Combined with the flexibility of cementing a smaller diameter femoral stem, it is uncommon that the femoral component cannot be placed flush to the ante- rior cortex of the femur, obviating the need for space- occupying anterior augments (Figure 9-10).
The modular femoral augments are particularly useful in restoring proper anterior-posterior dimension to the femoral component. As is frequently the case in revision surgery, the flexion space is capacious compared with the extension space. Posterior augmentation of the femoral component allows proper sizing of the prosthesis, maximizing medial-lateral bone coverage and
addressing the extension-flexion mismatch (Figure 9-11).
The advantage of modular metal augmentations for the distal femur over solid, nonmodular components is the ability to independently fit defects of each condyle and conserve host bone. The surgical technique for femoral preparation using modular augments is quite simple and familiar to most surgeons. An intramedullary guide is suggested. A stem is recommended when modular aug- ments are employed. As the height of the distal femoral augment increases, the rotational constraint implied by host bone contact within the intracondylar notch region of the component is decreased (Figure 9-12). Many systems allow the use of a constrained condylar designed
A B
FIGURE 9-8. (A) Asymmetric tibial bone loss. Note the use of an intramedullary guide and a graduated cutting jig. (B) Resection for modular medial tibial augment.
FIGURE 9-9. Bone loss is often seen in association with femoral component failure. The posterior femoral condylar bone is partic- ularly susceptible to osteolysis.
FIGURE 9-10. Modular metal augments can be very helpful in
reconstruction of deficient posterior bone loss. FIGURE 9-11. Posterior modular augments are used to up-size the femoral implant, assisting with flexion space management without affecting the extension space. A cemented stem is inten- tionally displaced posteriorly, allowing an anterior reference for femoral reconstruction.
FIGURE 9-12. Chamfer resections should be assessed and made with the appropriate sized distal femoral augment trial in place. In many revision cases in which distal augments are required, the chamfer resection is minimized. Implanting a condylar constrained femoral housing can increase the rotational stability of the recon- struction.
knee with cruciate substituting polyethylene inserts. If augments are employed, the extra depth of the box resec- tion of a constrained condylar designed knee provides additional rotational stability to the femoral implant.
Additionally, if late ligament instability occurs, the femoral component need not be exchanged to allow use of the condylar constrained tibial insert.
DISCUSSION
Although modular metal augmentation blocks and wedges do not restore host bone stock, properly applied, these augments allow immediate weight bearing and range of motion, transferring loads to intact host bone, while providing durable long-term implant stability.11 Additionally, the multiple sizes available with modular revision knee systems allow expedient reconstruction at a cost savings compared with custom implants. Recently, modular trabecular metal augments and semicustom tra- becular metal augments have become available. These augments offer the same modular benefits of solid metal augments, with the added potential for osteointegration and soft tissue interdigitation.
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4. Brand MG, Daley RJ, Ewald FC, et al. Tibial tray augmen- tation with modular metal wedges for tibial bone stock deficiency. Clin Orthop. 1989;248:71.
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prosthetic modularity and custom implants. In: Lotke PA, Garino JP, eds. Revision Total Knee Arthroplasty. Philadel- phia: Lippincott-Raven; 1999:207.
6. Brooks JP, Walker PS, Scott RD. Tibial component fixation in deficient tibial bone stock. Clin Orthop. 1984;184:302.
7. Fehring TK, Peindl RD, Humble RS, et al. Modular tibial augmentations in total knee arthroplasty. Clin Orthop.
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8. Rand JA. Bone deficiency in total knee arthroplasty: use of metal wedge augmentation. Clin Orthop. 1991;271:63.
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a follow-up study. Clin Orthop. 1995;321:151.
10. Rand JA. Modular augments in revision total knee arthro- plasty. Orthop Clin North Am. 1998;29:347.
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Modular metal wedges allow tremendous intraopera- tive flexibility in the management of tibial and femoral deficiencies. Load transfer to bone is more evenly dis- tributed by metal augmentation than by other reported techniques of reconstruction of bone defects. Modular augments do circumvent the potential complications associated with bone graft harvest, donor site morbidity, or allograft incorporation. Long-term data regarding the prosthesis–metal augmentation interface with newer snap-fit and screw-fit fixation methods remains to be proven. Current clinical data support the continued application of modular augmentations in revision knee arthroplasty. Modular augments are particularly applica- ble in revision cases with peripheral cortical defects. Large bone defects that occur in younger patients may still best be managed with bone grafting techniques, which attempt to restore bone stock for potential future surgery.
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