Fig. 9.1
Intraoperative image of 4.5 screws being placed in medial tibial bone defect as “rebar” for cement
Fig. 9.2
Intraoperative image of screws being placed level with tray in medial tibial bone defect
Fig. 9.3
Intraoperative image of cemented tray over screws to fill medial tibial bone defect
Modularity in total knee systems has earned its acceptance by providing utility in the management of a wide spectrum of bony defects. Consequently, custom implants are now rarely needed as the array of modular options have evolved to include offset stems, stem extensions, variable femoral and tibial prosthetic body options, and modular augmentations. The clinical acceptance of modular metal augments is due in large part to their off the shelf availability and flexibility in effectively managing the variety of clinical situations that face the knee arthroplasty surgeon.
Bone defects that remain contained by the cortical rim can often be successfully managed with morcellized bone grafting techniques [1]. For very large contained defects, a combination of bulk and morcellized graft may be most appropriate, usually offloaded with extended prosthetic stems. However, in many surgeons’ hands, newer sleeve and cone options are replacing these grafting techniques.
When the cortical rim of either the distal femur or proximal tibia is breached, the reconstructive options are more 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 resorption of the allograft [2]. Indeed, most revision centers rarely use bulk, structural allograft in revision arthroplasty.
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 possibly downsizing the prosthesis. These intraoperative choices represent compromises that may be accompanied by potentially undesirable consequences including component subsidence, loosening, and failure [3]..Downsizing of a femoral component to accommodate bone loss may inadvertently lead to flexion space instability.
Recognition of the above limitations led to the development of modular metal augments. Brand et al. reported the first clinical series of modular metal wedges for the management of bone deficiency in 1989 [3]. Modular metal augmentations are now readily incorporated in modern knee reconstruction systems and include augments of the individual condyles, cones, sleeves, and wedges [4]. In this chapter, we discuss the indications, limitations, and techniques for the use of femoral or tibial modular augmentations in total knee arthroplasty.
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. Extreme defects related to severe disease, progressive or rapid bone loss associated with avascular necrosis, neglect, or trauma may result in bone defects that require augmentation even in primary knee arthroplasty. Inflammatory arthropathies , 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 [4]. Engh’s classification syste m is similar for both femoral (F) and tibial (T) sides; Grade 1 is a minimal metaphyseal defect, Grade 2A is a loss of medial or lateral condyle, Grade 2B is the loss of both condyles, and Grade 3 is a severe condylar bone loss with absent MCL/LCL. An algorithmic approach to preoperative grading of deficiencies can help in strategically planning for the appropriate combination of components, stems, and augments. The most common patterns of bone loss that require modular augmentation include the medial or lateral tibia in association with varus and valgus collapse, respectively, and a combination of distal and posterior femoral augmentation with femoral component failure.
Preoperative radiographs can help identify patients who may require tibial or femoral augmentation. Brand et al. have proposed a method for estimating tibial defect size based off of preoperative anterior-posterior radiographs [3]. A line is drawn down the central axis of the tibia. A perpendicular line is then drawn at the top of the intact tibial plateau. A tibial defect exceeding 15 mm from the horizontal line may require augmentation and should be considered in preoperative planning of the reconstruction.
Estimation of the need for augmentation on the femoral side can be more difficult (Figs. 9.1, 9.2 and 9.3). The metallic bulk of the femoral implant makes visualization of the distal femur difficult even with multiple oblique views. Knowledge of the prosthetic design and history may be of benefit in preoperatively determining the need for femoral augmentation if defects are not obviously apparent. Preoperative estimation of bone stock after component removal can be helpful in planning augmentation options that should be available.
Modular Metal Augmentation
Modular augmentation represents an attractive option in reconstructive surgery, allowing a surgeon to provide an implant construct customized to the defects encountered, reestablish correct component levels with respect to the joint line, maintain or reestablish limb alignment , and adjust soft tissue balance (Figs. 9.4 and 9.5).
Fig. 9.4
Modern revision knee systems allow for the use of augments of varying thickness, as here on the posterior and distal femur
Fig. 9.5
Radiograph of a posterior augment the femoral component
The mechanical strength of augmentation wedges and blocks has been investigated. In vitro studies have focused on two areas of interest [5]. The 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. Older designs relied exclusively on cement fixation of the augment to the prosthesis. All mechanisms of augment fixation have been used successfully in the short term with clinical experience up to 5 years reported. The long-term concerns include loosening, dissociation of the augments, and possible fretting leading to third-body wear.
Brand et al. reported a revision of a non-modular tibial tray for polyethylene failure in which they had previously applied a 5 mm wedge with cement for a medial tibial defect [3]. 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. Fehring et al. found that tensile strain within the cement-bone interface was less with block augments compared with wedges [5]. However, the maximal strain differential between blocks and wedges was only slight, arguing that the augment that best fills the defect should be used.
Patel et al. reported on 102 primary knees revised with type-2 defects treated with modular augments and stems with mean follow-up 7 ± 2 years (5–11). There were 18 tibial augments and 176 femoral augments implanted, all of which were fixed to the component by cement. There was a 92% survivorship at 11 years and a 14% rate of nonprogressive lucent lines that was not associated with outcomes or survivorship or implant utilized [6].
The first clinical series reporting the use of metal wedges for tibial bone deficiencies was reported by Brand et al. [3]. 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, cemented tibial 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.
Rand reported a series of 28 primary knees with defects up to 18 mm, majority medial, at a mean follow-up of 27 months. Clinical scores for all patients were rated as good to excellent despite nonprogressive radiolucent 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 [7].
Tibial Component Augmentation
Modular augments used beneath the tibial tray are typically either wedge-shaped, which fit above an oblique bone resection, or are more commonly blocks. Hemi-wedges can be used to fill small peripheral defects, whereas full-wedge augments can be used to correct axial alignment beneath the tibial 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 a unicondylar defect (medial or lateral) and supporting anterior or posterior cortical bone at the level of the tray-bone resection.
Indications
Tibial augmentation with modular metal wedges or blocks is usually applied to defects of 5–20 mm in depth, particularly when these defects fail to support more that 25% of the tibial base plate (Fig. 9.6). Several factors guide the decision to use modular augments. Since the 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 likewise 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 additionally limited by the extensor mechanism. Resection levels greater than 20 mm below the native joint line place the tibial tubercle and extensor mechanism in jeopardy, particularly if on the lateral side.
Fig. 9.6
Contained tibial defects are easily managed with a tibial augment, allowing cortical rim contact with the prosthesis
Extensive proximal tibial bone loss over both medial and lateral surfaces of the proximal tibia may be handled with thicker polyethylene inserts. Tibial bone loss may exceed the height of the modular polyethylene inserts available for a given knee system. Additionally, as the polyethylene insert’s thickness increases, the stresses at the insert locking mechanism increase, potentially leading to increased micromotion. Elevating the tibial base plate and reducing the thickness of the polyethylene insert required can offset this negative biomechanical consequence. 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 (Fig. 9.7). As the tibial base plate is elevated with augments from below, the stem is effectively shortened, suggesting consideration of a longer stem (Figs. 9.8 and 9.9).
Fig. 9.7
A full tibial tray augment can raise the joint line of the tibial tray and decrease tibial polyethylene thickness
Fig. 9.8
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 significantly compromised to require a tibial augment, a longer stem extension should be considered
