Shawn M. Brubaker, William Mihalko, Thomas E. Brown, and Khaled J. Saleh
DEFINITION
Bone loss and indications for the use of metallic augments in revision total knee arthroplasty (TKA) usually are guided by classification of the bony defect.
Several classification systems have been developed and introduced that describe and quantify proximal tibial bone loss with the intent of guiding preoperative planning and intraoperative treatment and aid in forming a prognosis postoperatively.
Classification systems assess the bony defect in terms of size, location, presence or loss of cortical containment, and symmetry or disparity across the tibial plateau.20
One of the most widely used systems is the Anderson Orthopaedic Research Institute (AORI) bone defect classification system.
The AORI bone defect classification system divides bone loss of the distal femur or the proximal tibia into three types, based on the radiographic status of the metaphyseal bone.5–7
Distal femoral metaphyseal defects are graded as FI, FII, or FIII.
Proximal tibial metaphyseal defects are graded as TI, TII, or TIII.
The metaphyseal region of the proximal tibia is defined as the bone cephalad to the tibial tubercle.
Type I defects of the proximal tibia have intact metaphyseal bone with no component subsidence or loss of the primarily reconstructed joint line.
Minor defects may be present that will not compromise the stability of the tibial component at the time of revision surgery; in such cases, primary type reconstruction components may potentially be used.
Type II defects of the proximal tibia have damaged metaphyseal bone with component subsidence or joint line alteration due to loss of metaphyseal bone.
Bone loss in type II defects can involve either the lateral or, more commonly, the medial tibial plateau as well as the entire proximal tibia.
Defect reconstruction with cement, augments, or, possibly, bone grafting is required to reestablish the joint line at an appropriate level, and revision stemmed components usually are required.
Collateral ligament origins and insertions are preserved in TII type defects.
Type III defects of the proximal tibia have deficiency of the proximal metaphyseal bone that involves a major segment of the proximal tibia.
This type of defect may involve the tibial tubercle, with resulting patellar tendon detachment and loss of extensor mechanism function.
The medial collateral ligament also may be detached or functionally incompetent (ie, pseudo-laxity) as a result of bony deficiency.
The broad insertion of the medial collateral ligament on the proximal medial metaphysis of the tibia renders incompetence or frank loss of attachment due to tibial bone loss less likely as compared to femoral condylar bone loss, where the origin of the medial collateral has a much smaller footprint or area of attachment to the medial epicondyle.
Defect reconstruction with augments and structural bone graft is required to fill the bony deficiency. Longstemmed revision implants are required, including hinged modular revision components.
A custom component rarely is required because of the availability of modular oncology or limb-preserving systems.
In summary, proximal tibial bone defects are classified as intact, or type I (TI); damaged, or type II (TII); and deficient, or type III (TIII).
By definition, the use of metallic augments is restricted to type TII or TIII defects. TIII defects with frank bony deficiency often require bulk structural allograft, either alone or in combination with metallic augmentation.
The major indication for isolated metallic augmentation is a type II tibial bone defect.6
ANATOMY
The native tibial plateau has a 3-degree varus slope in the coronal plane relative to the mechanical and anatomic axes of the tibia, to match the 3-degree valgus slope of the femoral condyles.
The distal femur has a 3-degree valgus slope relative to its mechanical axis and a 9-degree valgus slope relative to its anatomic axis in the coronal plane.
In the sagittal plane, the proximal tibia has a posterior slope of 9 to 10 degrees on average, with a range of 4 to 12 degrees.5 The medial tibial plateau is mildly concave, and the lateral tibial plateau is mildly convex.
About 60% of ground reaction forces are transmitted through the medial tibial plateau; the remaining 40% are transmitted through the lateral plateau.
The trabecular bone of the proximal tibia is densest in its most proximal 1 cm and is responsible for load transmission.
The strength and stiffness of the proximal tibial bone are dictated by trabecular architecture and bone density, with the most dense area being central in each plateau.2
The bone also is more dense between the plateaus under the tibial spine and decreases in density toward the periphery of the plateau.
The subchondral, epiphyseal, and metaphyseal bone is relatively denser in the medial tibial plateau due to higher load transmission compared to the lateral tibial plateau.19,20
The tip of the fibular head is approximately 1 cm below the surface of the lateral tibial plateau.20
FIG 1 • A. Vascular anatomy of the knee. AP (B) and lateral (C) views of post–primary total joint arthroplasty anatomy of the knee.
The tibial tubercle is 25 to 40 mm below the joint surface, and the average insertion point of the patellar tendon is 29 mm distal to the tibial plateau.
The patella tendon averages 44 mm in length and ranges from 35 to 55 mm in length.14
Therefore, the distal pole of the patella averages 15 mm above the joint surface and ranges from 12 to 16 mm.
The fibular head is the most commonly used bony reference for joint line restoration in the revision TKA setting.
Improved outcomes are noted if the joint line is elevated less than 8 to 10 mm.10,13
Pre- and postoperative radiographs of the primary procedure allow the most accurate determination of the postarthroplasty joint line.
Blood supply to the proximal tibia is both endosteal and periosteal (FIG 1A).22
The endosteal blood is supplied via a nutrient artery branching from the posterior tibial artery that enters the tibia posteriorly and distal to the soleal line.
The proximal tibial periosteal blood is supplied by the medial and lateral inferior genicular arteries as well as the anterior tibial recurrent artery.
The medial and lateral inferior genicular arteries arise from the popliteal artery and pass deep to the collateral ligaments to supply the medial and posterolateral periosteum of the proximal tibia.
The anterior recurrent tibial artery is an ascending branch that arises from the anterior tibial artery just after it passes through the proximal tibiofibular interosseous membrane and supplies the anterolateral periosteum of the proximal tibia.
All of these vessels also contribute to the anterior anastomotic peripatellar ring.
Neurovascular injury during primary and revision TKA is rare.
The popliteal neurovascular bundle is at greatest risk during proximal tibial resection.
These vessels are 3 to 12 mm posterior to the articular surface of the tibia when the leg is extended and 6 to 15 mm posterior when the knee is flexed to 90 degrees.24
At the level of the tibial resection the distance is approximately 2 cm posterior to the cut surface.
The popliteal artery and vein are anterior to the tibial nerve at this level.15
Most revisions do not put the tibial artery trifurcation at risk unless more than 30 mm of proximal tibia is resected, which occurs when a tumor prosthesis is utilized for proximal tibia replacement.
Most neurovascular injuries during primary and revision TKA result from tourniquet use in the patient with peripheral vascular disease.23
The proximal tibial anatomy of the TKA requiring revision total knee arthroplasty is highly variable (FIG 1B,C).
Component subsidence, osteolysis, fracture, and infection all can alter the usual post-arthroplasty anatomy of the proximal tibia.
Pre- and postoperative radiographs of the primary procedure can be invaluable during preoperative planning of the revision surgery to determine actual bone loss and true change in component position.
The type of component noted on the postoperative primary radiographs can be helpful in determining the amount of bone that was originally resected from the proximal tibial plateau.
Cruciate-retaining implants typically are cut with 3 to 7 degrees of posterior slope in the proximal tibial resection, whereas cruciate-sacrificing and cruciate-substituting or posterior stabilized implants typically have 0 degrees of posterior slope.
Consequently, in the revision setting where posterior stabilized or “super stabilized” or “total stabilized” type components are used, additional anterior resection of the tibial plateau may be required to restore neutral slope.
Some revision systems call for a slight amount of slope to match the design of the stemmed tray, and the proximal tibial should be resected accordingly.
Bone defect classification schemes as outlined earlier for damaged or deficient metaphyseal tibial bone can greatly aid in understanding the pre-revision anatomy.
PATHOGENESIS
Multiple causes may lead to failure of the primary TKA, including aseptic loosening, deep infection, flexion instability, complex regional pain syndrome, malalignment, postoperative stiffness, and extensor mechanism complications.21
Proximal tibial bone loss in primary total knee failure can be attributed to the following factors: implant malalignment primarily or due to bony collapse; aseptic loosening with implant migration; or intraoperative bone loss during implant removal.9
Bone insufficiency or osteopenia from stress shielding, osteolysis, or osteonecrosis can be underlying factors contributing to preor intraoperative bone loss.
The quantity of osteolysis is affected by implant design and the quality of polyethylene, as well as host response to particulate debris.
Disuse osteopenia may contribute to massive proximal tibial bone loss in the presence of periprosthetic fracture.
Component loosening and resultant implant failure occur primarily via two common routes: aseptic, due to osteolysis; and septic, due to bacterial infection.
Osteolysis of the proximal tibial bone is a result of polyethylene particulate debris from wear at the bearing interface as well as “backside” wear.3
Backside wear results from micromotion between the polyethylene insert and the modular tibial tray.
Somewhere on the order of billions of submicron polyethylene particles per year can be generated at the bearing surface.
These particles generate histiocytic and macrophage response, where intercellular signaling pathways are activated that promote osteoclast activity and bone resorption.
Osteolytic lesions are either focal or expansile, depending on the submicron particle burden as well as the host response to the particles.6
The process begins in areas of soft or exposed bone that are subject to intra-articular synovial fluid pressure.
Debris-filled synovial fluid and resultant areas of osteolysis follow the paths of least resistance around tibial component of the implant.
Susceptible areas include uncovered bone of the tibial plateau, ie, areas covered by neither the component nor cement.
Screw holes and areas on the undersurface of the tibial tray that lack the bony ongrowth surface of a press-fit tibial component also offer pathways to the metaphyseal bone.
As mentioned previously, the metaphyseal bone is stronger centrally and more resistant to significant osteolysis; however, the periphery is weaker and more prone to osteolysis.
As osteolysis progresses, the tibial component loses its bony support, and a radiolucent line forms between the component itself or the component cement mantle and the underlying metaphyseal bone.
The tibial component is aseptically loose when the radiolucent line is circumferential and no direct bony support remains.
Both expansile osteolysis and component loosening can lead to component subsidence, usually into a varus position.
The medial tibial plateau is dimensionally larger than the lateral tibial plateau and potentially has more exposed cancellous bone when symmetrical tibial components are utilized.
Septic loosening is the result of bacterial infection of the knee.
Classically, the organism most commonly isolated from the knee was Staphylococcus aureus ; however, more recent studies suggest that Staphylococcus epidermidis may be equally common, although less virulent.4,27 Bacterial infection occurs through one of several routes.
The knee may be contaminated at the time of implantation, although this happens in less than 0.5% of cases.
Hematogenous inoculation of the joint with bacteria also may occur in the face of a previously well-functioning prosthesis.
Dental work causes bacteremia 100% of the time, and other procedures such as gastroesophageal endoscopy and colonoscopy all introduce potential for bacteremia and late hematogenous spread to the implanted knee.
All surgical procedures put a prosthesis at increased risk for infection.
Less virulent organisms may not lead to significant bone loss intrinsically, but more virulent organisms often result in a loose component.
A third route of infection is direct contamination of the knee by trauma, ie, traumatic arthrotomy.
Most prosthetic infections are treated with two-stage revision arthroplasty of the knee, in which either a static antibiotic spacer or a dynamic articulated type of spacer is used for the first stage of the revision.
Preformed antibiotic-loaded polymethylmethacrylate spacers are commercially available, but the concentration of antibiotic in the cement is predetermined by the manufacturer.
In some instances, the concentration in a preformed spacer may not be sufficient to overcome a virulent organism or to meet the preference of the surgeon.
Articulated spacer molds allow the surgeon to select the quantity and type of antibiotic to be loaded into the cement spacer.
The second stage of the revision takes place once serum inflammatory markers have returned to within normal limits and the patient has responded clinically to both the antibiotics in the spacer and those given intravenously.
The time between the first and second stage of the revision TKA for infection is typically 8 to 12 weeks.
Explantation of a well-fixed primary component leads to potentially significant bone loss when it is removed from an osteopenic tibial metaphysis.
Occasionally a static antibiotic spacer may subside or displace in the joint between the revision stages, further contributing to bone loss.
A comparison study of static and articulated spacers showed greater bone loss with static spacers than with articulated spacers.8
Maximizing bone coverage with either type of spacer appears to preserve bone stock.
The AORI classification also applies to bone loss, either present or anticipated, at the second stage of the staged revision knee arthroplasty.
NATURAL HISTORY
The osteolytic process is clinically silent until the prosthesis loosens.
A loose prosthesis causes startup pain as well as cyclic pain with loading, provided that fracture or gross diplacement of the component has not occurred.
The pain may decrease with weight bearing as the component reseats itself and motion at the bone implant interface decreases.
Expansile osteolysis and component subsidence or proximal metaphyseal fracture my result in gross lower extremity deformity, usually varus, and occasionally may also present with hyperextension if the component subsides into an anteriorly sloped position.
Radiographic examinations of the implanted knee should be performed at regular intervals to detect the presence or progression of osteolysis.
The presence of osteolysis and its rate of progression depend on the polyethylene type and processing of the implant as well as the host response to particulate debris.
Many modular systems that use polyethylene inserts gammairradiated in air demonstrate accelerated wear patterns with high particle production.
Many of these implants demonstrated osteolysis within 36 months of implantation.17
Specific timeframes for aseptic component loosening are difficult to predict globally, because there are many contributing host and implant variables.
Septic failure of TKA is noted in the perioperative period in 0.5% of patients.
Delayed hematogenous infection occurs in 1% to 2% of TKAs over the lifetime of the implant.20
Infection must be ruled out in all painful TKAs.
Differentiation between septic and aseptic loosening of the tibial component is of paramount importance, because the management of each type is different, and a misdiagnosis can result in loss of limb.
PATIENT HISTORY AND PHYSICAL FINDINGS
The patient history and physical findings are centered around differentiating between septic and aseptic loosening, the amount of bony destruction and the ligamentous insufficiency present, and the resultant instability of the knee.
Patients with symptomatic osteolysis or loosening of the tibial component usually describe gradual onset of pain that is worse with startup type of activities such as arising from the bed or getting out of a chair or a vehicle.
They also commonly describe increased pain with the first steps of ambulation, which may decrease as the component settles into the remaining bony mantle of the proximal tibia.
There is less or no pain with rest.
The infected TKA will cause pain at rest, and patients who have deep infection may describe a temporal pain pattern with increasing or steady pain from the day of surgery to the day of clinical evaluation with no remittance.
Late hematogenous infection presents with sudden onset of pain in a previously well-functioning prosthesis.
An acute knee effusion and loss of range of motion and stiffness also may present in this setting.
Complete neurovascular examination of the affected extremity should be carried out, along with focal examination of the knee.
Visual inspection for edema and erythema is pertinent in the face of infection.
Range of motion and patellar tracking should be evaluated along with ligamentous stability and overall limb alignment. A difference between active and passive extension demonstrates extension lag versus flexion–contracture.
Specific attention should be directed toward evaluating the functional status of the medial collateral and posterior cruciate ligaments in a knee with cruciate-retaining implants.
A quadriceps active test and posterior sag signs are useful to evaluate the functional status of the posterior cruciate ligament (PCL).
Varus and valgus stress tests in extension, in midflexion, and at 90 degrees of flexion are useful to evaluate the collateral ligaments for instability. If present, instability may be graded according to laxity.
Anterior and posterior drawer tests evaluate status of the PCL, conformity of the bearing surface, and proper function of cam–post mechanism as well as flexion gap.
If significant posterior subluxation is seen on posterior sag testing, it suggests a nonfunctioning PCL in a cruciate-retaining TKA.
The quadriceps active test also demonstrates failure of the PCL or of the cam–post mechanism in a posterior stabilized implant.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Plain film radiographic views of the affected knee are required (FIG 2).
Multiple orthogonal views, including oblique views, may illustrate areas of bone loss more fully.
The skyline view, while part of a complete radiographic analysis, does not allow visualization of the tibial component.
Weight-bearing anteroposterior and lateral views can be helpful to load the knee and show the component position of maximum subsidence as well as resultant limb.
It is important to obtain tangential views to properly align the bone implant interface with the beam of the radiograph.
If plain radiographs are thought to underrepresent the true volume of osteolysis, consideration may be given to CT with metal suppression.25
Conversely, if there is doubt regarding actual loosening of the component, technetium-99m bone scanning can illustrate areas of increased bone turnover that may indicate possible looseness of the tibial component.
In cases in which infection is suspected, the bone scan can be compared to an indium-111–tagged white blood cell scan.
Aspiration of the knee joint with culture of a definitive organism is the ideal confirmation of an infected TKA.
FIG 2 • Radiographs of the knee. A. AP view. B. Bent knee lateral view. C. Skyline view.
DIFFERENTIAL DIAGNOSIS
Osteolysis
Infection
Tibial component loosening
Tibial component subsidence
Periprosthetic proximal tibial fracture
NONOPERATIVE MANAGEMENT
Nonoperative management of a loose tibial component due to osteolysis with or without subsidence and fracture should be considered only if the revision procedure will place the patient's survival in jeopardy or make their quality of life worse than it was at presentation.
The same caution applies to patients with indolent infections. Virulent organisms can cause bacteremia and sepsis. Emergent surgical explantation of the components may be required to prevent septic shock and death.
SURGICAL MANAGEMENT
Ideally, the patient is medically optimized for revision knee surgery and all medical comorbidities have been addressed before the surgery.
Risk stratification should be performed by medical subspecialists based on the patient's medical history and on the recommendations of a general medical doctor.
The patient should be fully informed of the risks compared to the benefits of the revision total knee surgery and of the bone loss management plan.
Intraoperatively, the need for metallic augmentation is confirmed by segmental defects, defects that involve more than a quarter of the cortical rim, or defects that will result in less than 60% direct bony support of the revision component confirm.17
Preoperative Planning
Evaluating the component position and classifying the present bone loss on plain films is the cornerstone of preoperative planning.
Metaphyseal bone quality should be evaluated and intraoperative bone loss anticipated.
Templating should be carried out with the focus on restoring the joint line and filling the bony defect. The broad insertion of the medial collateral ligament presents an issue only for severe bone defect unless it is found to be incompetent on the preoperative physical examination. Close attention should be given to the planned level of resection and its relation to the insertion of the extensor mechanism. Wedge augments should be avoided if possible, but are indicated if needed to spare bone around the tibial tubercle.
The condition of the patient's skin should be carefully evaluated in the physical examination.
All previous skin incision and their age should be noted.
An anterior midline incision is preferred, but it may be necessary to use the most lateral incision to protect the patient from skin necrosis in areas between incisions. A 7-cm skin bridge should be maintained between all incisions if at all possible.
Patients with significant potential for necrosis may need to undergo a “sham” procedure to ensure the skin is viable.28 This involves an incision just through the skin without involving the extensor mechanism to verify healing potential.
The existing implant in the knee should be identified, and the operative reports of the previous surgery should be obtained prior to surgery.
Positioning
The patient is positioned supine on the operating room table.
A well-padded, non-sterile tourniquet is placed as proximally as possible on the operative lower extremity. All bony prominences are well padded.
A bump may be placed under the hip and pelvis on the operative extremity side, depending on the surgeon's preference.
The nonoperative extremity is secured to the table.
A bean bag, horizontal post, or leg-holding device such as an Alvarado leg positioner may be used to hold the leg in a flexed position during the procedure.
TECHNIQUES
APPROACH
A standard midline incision with a medial parapatellar arthrotomy is ideal. The skin is incised from 5 cm (or more, if necessary) proximal to the patella. The incision is centered over the patella and patellar tendon to the tibial tubercle (TECH FIG 1A).
Some surgeons prefer to bring the distal half of the incision slightly medially so that it terminates at the medial edge of the tibial tubercle. Preference is always given to the previous skin incision.
The anterior midline incision is carried down to the extensor mechanism, and full-thickness flaps are elevated both medially and laterally (TECH FIG 1B).
The medial flap usually must only be carried medially to the distal aspect of the vastus medialus obliquus muscle.
Laterally, the flap is raised sufficiently to allow for possible eversion of the patella after the arthrotomy is performed.
An anterior medial parapatellar arthrotomy is then performed (TECH FIG 1C). The previous arthrotomy location is again preferred.
Care should be taken to preserve a cuff of tissue on the medial aspect of the patella to facilitate closure of the arthrotomy.
TECH FIG 1 • Skin incision followed by extensor mechanism exposure and medial parapatellar arthrotomy. A. An anterior midline incision is centered over the patella, using the previous incision. B.Elevation of a full-thickness flap both medially and laterally, with medial parapatellar arthrotomy marked out. C. Medial parapatellar arthrotomy complete with V-Y turndown.
COMPONENT EXPLANTATION
Once the joint is opened, scar tissue is resected as necessary for adequate exposure. Specifically, the medial and lateral gutters should be restored with care to avoid injuring the collateral ligament origins.20,28 The suprapatellar pouch also should be restored via resection of scar tissue under the quadriceps tendon and from the anterior aspect of the distal femur just proximal to the femoral component.
The subperiosteal flap is elevated off the proximal and anteromedial aspect of the tibia to allow exposure to the entire tibial component and proximal tibial bone.
The tibial insert is removed from the tray to allow adequate visualization.
The tibial tray is then explanted. In cases of a loose tibial component, intraoperative bone loss usually is minimal, and special techniques for tibial component removal are not required.
In the septic knee with a well-fixed tibial component, bone-sparing techniques are used to remove the component.
Stemmed tibial revision components are recommended when using augments to provide additional bony stabilization of the implant, thereby shifting some of the load from the damaged or deficient metaphyseal tibia to the diaphysis.
MEDIAL OR LATERAL HEMIWEDGE AUGMENT
The explanted proximal tibia is evaluated intraoperatively, and bone loss is compared to the preoperative radiographs. The appropriate preoperative plan is followed with anticipated changes verified by intraoperative findings.
Intramedullary reaming is followed by placement of an intramedullary alignment guide, and a minimal transverse proximal tibial “skim” resection is taken after the tibial resection block is pinned in place (TECH FIG 2A,B).
The appropriate wedge augment cutting guide is then selected and positioned over the previously placed cutting guide pins, and medial or lateral wedge cuts are performed (TECH FIG 2C).
Once the proximal tibial surface is adequately prepared, a stemmed trial component that reflects the intramedullary stem to be used with attached augments is put in place (TECH FIG 2D).
When adequate bony support is achieved, the joint surface is restored, and flexion and extension gaps are balanced, then the component to be implanted is constructed to match the trial and appropriately cemented into place (TECH FIG 2E).
The goal is to place the tibial component directly onto a viable cortical rim of bone by converting noncontained defects into contained defects and to have a rigid press-fit intramedullary stem to support the tibial tray.
This process applies to all of the wedge and block techniques.
TECH FIG 2 • Medial or lateral hemiwedge augment. A. Intramedullary reaming should be carried to the depth of the stem available in the revision system in use. B. The block is pinned into place after the guide is placed over the intramedullary reamer or trial stem extension, and a “clean up” or skim cut is made. C. The pins from the previously used cutting block are maintained, the hemiwedge block is slid over the pins, and the hemiwedge cut is performed. D. The trial tibial component is assembled and placed on the tibia, and if appropriate fit and stability are obtained, the final components are assembled. E.The final component is cemented into place after tibial preparation is complete, and excess cement is removed after the assembled tibial component is impacted into place. (Courtesy of DePuy Orthopaedics, Inc., Warsaw, IN.)
FULL-WIDTH WEDGE AUGMENT
Intramedullary reaming is followed by placement of an intramedullary alignment guide, and a minimal tranverse proximal tibial “skim” resection is taken after the tibial resection guide is pinned in place.
An appropriately angled full-width wedge augment cutting guide is then selected and positioned over the previously placed cutting guide pins, and the oblique full-width wedge cut is performed (TECH FIG 3).
Once the proximal tibial surface is adequately prepared, a trial stemmed component that reflects the intramedullary stem to be utilized with the attached augment is trialed.
When adequate bony support is achieved, the joint surface restored, and flexion and extension gaps balanced then the component to be implanted is constructed to match the trial and appropriately cemented into place.
TECH FIG 3 • Full-width wedge augment. Reaming is carried out as illustrated for the hemiwedge technique, and a skim cut is performed if necessary. Then the full-width wedge block is pinned into place according to the technique for the system being used. The system in this illustration allows the block to be rotated, and an oblique skim cut can be made. Trialing and component assembly and insertion are carried out as in Tech Fig 2. (Courtesy of DePuy Orthopaedics, Inc., Warsaw, IN.)
MEDIAL OR LATERAL BLOCK OR STEP AUGMENT
The explanted proximal tibia is evaluated intraoperatively, and bone loss is compared to the preoperative radiographs. The appropriate preoperative plan is followed, with anticipated changes verified by intraoperative findings.
Intramedullary reaming is followed by placement of an intramedullary alignment guide, and a minimal transverse proximal tibial “skim” resection is taken after the tibial resection guide is pinned in place (TECH FIG 4A).
The appropriate block augment cutting guide is then selected and positioned over the previously placed cutting guide pins, and medial or lateral step cuts are performed.
Once the proximal tibial surface is adequately prepared, a trial stemmed component that reflects the intramedullary stem to be used with attached augments is trialed (TECH FIG 4B).
When adequate bony support is achieved, the joint surface is restored and flexion and extension gaps balanced. The component to be implanted then is constructed to match the trial and appropriately cemented into place (TECH FIG 4C).
TECH FIG 4 • Medial or lateral block augmentation. A. Reaming and skim cutting are carried out as previously illustrated. Then the step cut is performed with the cutting block attached to either the intramedullary guide or a trial. B. The trial is assembled, and fit is evaluated. Additional freehand cleanup may be carried out to improve bone-to-component apposition in all of the illustrated techniques. C.The assembled revision component with attached block augment is cemented into the now-prepared tibia. (Courtesy of DePuy Orthopaedics, Inc., Warsaw, IN.)
METAPHYSEAL CONE AUGMENTATION
The explanted proximal tibia is evaluated intraoperatively and bone loss is compared to the preoperative radiographs. The appropriate preoperative plan is followed, with anticipated changes verified by intraoperative findings.
Intramedullary reaming should be carried to the depth of the stem available in the revision system in use (see Tech Fig 2A). Careful attention to tibial alignment while reaming is important. It may be necessary to ream slightly out of line with the tibial shaft to allow adequate space for the metaphyseal cone.
Intramedullary reaming is followed by placement of an intramedullary alignment guide to evaluate for stem-totray mismatch. Some metaphyseal cone augmentation systems do not allow for offset stems.
Once the appropriate-sized diaphyseal engaging stem is selected, the proximal tibia is sequentially broached with the appropriate-sized trial stem attached to the broach to provide proper alignment (TECH FIG 5A).
The option for rotational disparity between the tibial tray and the cone is available in some systems. This allows the surgeon to rotate the broach if necessary to improve filling of a proximal metaphyseal defect.
Broaching is carried out until rotational stability is obtained.
Some systems then use the proximal surface of the broach as the cutting guide. This necessitates placing the broach at the level determined by preoperative planning and intraoperative assessment. Ideally, this position resects 2 mm or less of the proximal tibial metaphysis (TECH FIG 5B).
Once the proximal tibia is resected and broached, a trial cone is placed and rotation is marked on the anterior tibia, a properly sized trial tibial tray and stem are assembled and placed through the cone, lack of excessive rotational disparity between the two is verified, and tibial tray rotation is marked (TECH FIG 5C).
Both the tray with stem and the cone are removed, and the trial cone is assembled to the stemmed trial tibial tray. The assembled trial is placed back onto the proximal tibia, and trialing is completed as previously described.
The selected trial is removed from the tibia and left assembled as a model for assembly of the final components.
Final assembly of the tibial components is done to match the trial, and the tibial cone is impacted onto the Morse taper of the revision tibial base plate, with care taken to match the trial model cone rotation on the trial stem.
TECH FIG 5 • Metaphyseal cone augmentation. A. After intramedullary reaming, a trial stem of the appropriate length is attached to the metaphyseal broach. Sequential broaching is carried out until good metaphyseal fill is obtained and the top of the broach is at the level of the planned skim cut. The handle is removed, and the broach is left in place. B. A skim cut is taken off the top of the broach. Careful attention should have been taken during broaching to ensure that the proximal surface of the broach rests at the planned level of the “clean up” cut. C. The trial tibial component is assembled and placed on the tibia. Careful attention should be given to tibial tray versus metaphyseal cone rotation. They may not be aligned with each other, depending on how the cone broach was rotated during broaching to gain maximal metaphyseal fill. The trial component assembly should be referenced during final component assembly. D. The final component is cemented into place after tibial preparation is complete. Care should be taken to keep the metaphyseal cone bone ingrowth surfaces free of cement during component insertion and impaction.
Cement is applied to the assembled component selectively, as most cones allow for bony ingrowth with a porous surface coating (TECH FIG 5D). If a diaphyseal press-fit stem is selected, cement is applied only to the proximal tibia and tibial base plate.
Selection of a cemented-type metaphyseal stem may require step-cementing of the stem and tibial base plate, with no cement applied to the ongrowth surfaced metaphysical cone. This allows direct bony contact between the cone and the bone, allowing for ingrowth if cement is applied sparingly to the stem to prevent extrusion with tibial component impaction. If care is not taken, cement could potentially cover the cone and prevent bony ingrowth.
FREE TRABECULAR METAL AUGMENTATION
The explanted proximal tibia is evaluated intraoperatively, and bone loss is compared to the preoperative radiographs. The appropriate preoperative plan is followed, with anticipated changes verified by intraoperative findings.
Intramedullary reaming to obtain good diaphyseal fill is followed by placement of an intramedullary alignment guide to evaluate for stem-to-tray mismatch.
If coronal mismatch is found, consideration should be given to an offset stem.
Intramedullary reaming is followed by placement of an intramedullary alignment guide, and a minimal tranverse proximal tibial “skim” resection is taken after the tibial resection guide is pinned in place.
The cavitary defect of the proximal tibia is curetted clean, and all membrane is removed.
The trabecular metal augment that most closely fits the defect is selected, and a high-speed burr is used to remove minimal amounts of bone to allow for a tight press-fit of the augment (TECH FIG 6A,B).
The augment is impacted into place. In cases in which the augment does not fully contact the surrounding bone, crushed cancellous allograft croutons can be combined with demineralized bone matrix to fill the peripheral void (TECH FIG 6C–E).
If an offset stem is selected to allow for good tibial coverage, or if the augment is placed off-center of the diaphysis to allow for best void fill, then a high-speed metal cutting burr can be used to trim the augment centrally.
Once clearance is obtained for the augment, the trial tibial stem with baseplate is inserted into the tibial diaphysis through the trabecular metal augment to verify fit.
When adequate bony support is achieved, the joint surface restored, and flexion and extension gaps balanced, the component to be implanted is constructed to match the trial and appropriately cemented into place.
The proximal portion of the tibial stem is cemented to the trabecular metal augment, and the stem is either press-fit in the tibial diaphysis or cemented per the preoperative plan (TECH FIG 6F).
TECH FIG 6 • Free trabecular metal augmentation. A. Trabecular metal augment sizing. The tibia is reamed as previously described, and a skim cut is taken off of the proximal tibia. The proximal metaphyseal defect is then sized by placing various-sized trials over the reamer on the tibial bone in an inverted position. B. Metaphyseal bone preparation. A tibial trial is constructed to determine the quantity and direction of any offset if required. A highspeed burr is then used to remove small amounts of bone, to allow full seating of the selected trabecular metal augment trial. Sufficient bone should have been removed to allow non-forceful seating of the trial, but full “fill” of the metaphyseal defect is not required. C. The trabecular metal augment trial and the tibial component trial are placed in the tibia simultaneously to verify lack of impingement between the two. If impingement is present, the augment may be either repositioned or directly trimmed with a burr to allow clearance of the tibial stem. (continued)
TECH FIG 6 • (continued) D,E. Once the trials have been inserted and appropriate fit has been achieved, the final augment is gently impacted into place. Excessive force may fracture the proximal tibia. Any defect that remains between the metaphyseal bone and the augment is one grafted to fill the void. Impaction grafting techniques may be used. F. Once the tibial augment is inserted and grafted, re-trialing of the tibial component is carried out. Additional removal of a small amount of the augment may be required to prevent impingement. The final component is impacted into the tibia with an adequate amount of cement to fill the void.
POSTOPERATIVE CARE
Postoperative care is directed by the intraoperative findings and the stability of the newly implanted component.
If a proximally cemented stemmed component is seated on cortical bone with all defects contained after use of an augment, then immediate full weight bearing may be allowed.
Range-of-motion exercises also may begin immediately if the skin over the anterior knee is in good condition postoperatively and the incision has been closed with no tension.
In situations where the skin is under tension or the wound appears tenuous immediately postoperatively, then range-of-motion exercises are delayed, with the leg held in extension for the first 48 hours.
The incision and skin are then reevaluated, and if there is no evidence of erythema or wound drainage, gradual range-of-motion exercises are started. The incision is watched carefully for drainage after range-of-motion exercises are initiated.
When the tibial component is not fully supported directly by native metaphyseal bone, then toe-touch weight bearing should be initiated until incorporation of any allograft that was used in conjunction with augmentation.
When bony ongrowth cones or free trabecular metal augmentation is used with less than full bony support, consideration should be given to delaying full weight bearing until bony ingrowth occurs.
In cases in which partial weight bearing is initiated postoperatively, progression to full weight bearing can take place at 6 weeks postoperatively.
As with primary TKA, deep vein thrombosis prophylaxis is mandatory and should follow usual protocols.
We use 6 weeks of coumadin prophylaxis in the patient with no history of thrombosis or pulmonary embolism.
We also use fractionated low-molecular-weight heparin starting 18 to 24 hours after the completion of surgery to protect the patient in the interval after surgery where the international normalized ratio has not yet reached our target of 1.8 to 2.2.
OUTCOMES
Modular tibial augmentation systems were developed in the midto late-1980s and became broadly available by the early to mid-1990s; consequently, no long-term studies are available on the survivorship of this type of tibial component augmentation. A handful of midterm studies are available.
In an early report on the use of tibial tray augmentation, Brand et al1 reported no failures in 22 knees with an average follow-up time of 37 months. The average age of the patients at the time of surgery was 70 years. There were no failures requiring revision and no loosening of the tibial components. The incidence of no progressive radiolucent lines was 27%. All but one patient was pain-free.
Rand19 also reported early outcomes shortly after wedge augments became available. In his study, 28 knees in 25 patients who underwent revision TKA with a metal wedge augmentation for tibial bone deficiency were reviewed at an average of 2.3 years (range, 2 to 3.5 years) after surgery. A medial side wedge was used in 24 knees, and a lateral side wedge was used in four. The average preoperative bone defect size was 12 mm on the medial side and 8 mm on the lateral side. The preoperative knee scores improved from 53 to 89 at the last follow-up. There were 79% excellent and 21% good results. There were no complications or reparations. Radiolucent lines beneath the metal wedge were present in 13 knees, but none were progressive.
Haas et al11 used tibial augments combined with press-fit diaphyseal engaging stems to manage bone loss in revision TKA for aseptic failure. The average duration of follow-up was 3 years and 6 months (range, 2 to 9 years). Only patients who had had revision of the femoral component or the tibial component, or both, because of aseptic failure were included. The average preoperative Hospital for Special Surgery knee score was 49. Postoperatively, the knee score improved to an average of 76 points (range, 0 to 97 points). Eighty-four percent of patients had an excellent or good result.
Hockman et al12 reported midterm results with one modular revision prosthesis. Fifty-four consecutive Coordinate (Deputy, Warsaw, IN) revision total knee arthroplasties were eligible for minimum 5-year follow-up. Nine knees failed and required either revision or component removal. Eight additional knees were considered clinical failures. Metallic augmentation was used in 89% of the knees, and large structural allografts were required in 48% of the knees. Revisions with bone loss that required bulk allograft failed less often (19.2%) than revisions managed without bulk allografts (42.9%). Modular augments did not effectively address the bone loss and instability encountered in many instances at revision surgery. Survivorship of the implant was 79% at 8 years in their study.
Pagnano et al16 reported on early and midterm results using tibial wedge augmentation. Their mid-term report was a follow-up of their short term study of 28 knees in 25 patients. The patients were originally reviewed 2.3 years after surgery, at which time 79% had excellent results and 21% had good results. Their midterm report was of 24 knees in 21 patients with metal wedge augmentation for tibial bone deficiency. The patients were reviewed 5.6 years clinically and 4.8 years radiographically after surgery. Clinical results were excellent in 67% and good in 29%. Radiolucent lines at the cement bone interface beneath the metal wedge were present in 13 knees. Eleven of the radiolucencies were <1 mm in width, and 2 were 1 to 3 mm in width. The authors stated that metal wedge augmentation for tibial bone deficiency is a useful option. No deterioration of the wedge-prosthesis or wedge-cement-bone interface was noted at midterm follow-up.
Radnay and Scuderi18 reported a novel approach to tibial augmentation. They describe the use of free trabecular metal augments (see Techniques). Ten tantalum tibial cones were press-fit into the prepared cavitary defect of a series of revision TKAs. The stemmed tibial component was cemented into the implanted tibial cone and stems were press-fit in four knees and cemented in six knees. Offset stems were used in three tibias. At follow-up (average 10 months), radiographic evaluation revealed no evidence of loosening or change in position. Strength, range of motion, and stability were comparable to previously reported series of revision arthroplasties. The authors state that trabecular metal cones may eliminate the need for extensive bone grafting or structural allograft in revision knee arthroplasty.
COMPLICATIONS
Complications of revision TKA with metallic tibial augmentation can be divided into two categories: early and delayed.
Perioperative or early complications can include intraoperative damage to neurovascular structures, extensor mechanism and collateral ligamentous disruption, and early postoperative infection.
Delayed complications most commonly include osteolysis, aseptic loosening, and late septic prosthetic arthropathy.
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