Osseous Deficiencies in Total Knee Replacement

Robert A. Sershon, MD

Kevin B. Fricka, MD

Nancy L. Parks, MS

C. Anderson Engh Jr, MD

BACKGROUND

Osseous deficiencies in complex primary and revision total knee arthroplasty (TKA) settings can pose significant surgical challenges. Causes of bone loss are broad and often multifactorial, including periprosthetic osteolysis, subsidence of loose implants, fracture, stress shielding, osteonecrosis, or infection.

Bone loss can involve the femur and/or the tibia. The extent of bone loss ranges from minimal to the loss of the entire metaphysis. Reconstruction is dictated primarily by the amount of bone loss and secondarily by ligamentous integrity. Tools available to reconstruct bone loss include particulate or bulk allograft, solid metal augments, porous metal augments, and segmental solid implants. For increasing defect size, intramedullary stems become necessary to offload the metaphyseal construct. Interface management is critical to successful bone reconstruction.

When a bone defect is repaired, the once “single” host-implant interface turns into a “double” interface: host-augment-implant. The interface between the host bone and the graft or porous augment must be supportive and healthy enough for graft healing or ingrowth fixation to a porous surface. The counter-face must allow cement fixation to the implant. The final goal is to achieve a well-functioning final construct that possesses structural stability, appropriate balancing, and proper alignment.

When bone loss is anticipated or encountered intraoperatively, a classification system allows for a methodological approach to managing the deficiency. A classification system should allow preoperative prediction and preparation for surgical reconstruction, realizing that radiographs likely underestimate the amount of bone loss. The classification also guides intraoperative decision-making. It is imperative to prepare for severe deficiencies in order to have the necessary equipment and components available for any scenario.

In this chapter, we present a systematic approach for management of osseous deficiencies in TKA, encompassing preoperative assessment, implant removal, description of existing classification systems, and reconstruction options based upon the Anderson Orthopaedic Research Institute (AORI) classification for bone loss in the setting of revision knee arthroplasty.

PREOPERATIVE ASSESSMENT

Preoperatively, the surgeon should attempt to establish the mechanism of implant failure, gauge ligamentous stability, estimate bone loss, and plan for the upcoming reconstruction. The most common causes of failure for modern TKA implants include aseptic loosening, infection, instability, fracture, and stiffness. The frequency of wear, osteolysis, and patellofemoral complications has decreased substantially since the 1990s and early 2000s due to changes in surgical technique, polyethylene processing, sterilization techniques, and implant design.¹,² Irrespective of failure mode, revision arthroplasty must correct the factors that led to failure of the primary arthroplasty.

Essential elements of the physical exam include gait assessment, skin integrity and evidence of prior incisions, range of motion with the presence or absence of contracture, extensor mechanism competency, ligament competency, neurovascular exam, and spine and hip examination.³ Laboratory tests include a screening erythrocyte sedimentation rate and C-reactive protein level, with possible fluid aspiration including cell count, culture, D-dimer, and alpha-defensin assay to differentiate septic from aseptic loosening.⁴

In most instances, radiographs are the major source of preoperative information regarding the mechanism of failure and degree of bone loss. Comparisons should be made with prior imaging studies when available. Recommended views include standing anteroposterior, lateral, Merchant, and standing full-length views of the bilateral lower extremities. These views allow for evaluation of femoral and tibial implant position and size, assessment of current bone stock, interface fixation, critique of patellar height and coronal position, and coronal alignment. Rarely, fluoroscopically guided spot views can also be obtained to better evaluate the status of the bone-cement or bone-implant interface for tibial and femoral components.⁵ Routine use of MRI or CT is not recommended due to increased cost and failure to significantly alter management.⁶ X-rays of the contralateral knee can identify the joint line relationship to bony landmarks, such as the femoral epicondyles for the femur and the fibular head for the tibia.

Bone loss encountered at the time of revision surgery generally exceeds the preoperative radiographic estimation.⁷ This is especially true in the case of osteolysis, in which the remaining bone is often of such poor quality as to be structurally incompetent; when it is adequately debrided, a much larger defect than predicted must be reconstructed.

When evaluating the femur radiographically, careful scrutiny of the distal femoral interface and the posterior condylar interface, which are the most common sites for interface loosening and bone loss, is warranted. Bone deficits involving the medial or lateral femoral metaphysis may indicate ligamentous deficiency. Femoral bone defects tend to be larger in posterior cruciate-substituting designs, hinged implants, and stemmed implants. When evaluating the tibia, a careful assessment of the integrity of the tibial tuberosity, the remaining bone stock with reference to the fibular head, quality of the metaphyseal bone, and the tibial slope is required (Fig. 42-1A to C). Tibiofemoral, distal femoral, and proximal tibial anatomic angles should be measured and compared to prior radiographs if available.

IMPLANT REMOVAL

It is important to identify the manufacturer and design of the prosthesis preoperatively in the event that design-specific disassembly or removal tools are required. Following adequate exposure, the foremost goal of implant removal is to retain bone stock. The sequence of component removal depends on exposure, implant stability, and surgeon preference. The polyethylene insert is removed first to decrease tension and enhance exposure. The tibial tray is often easier to remove next because the tibial interface consists of a two-dimensional interface, compared to the multiple interface planes of the femur.

FIGURE 42-1 Standard radiographs (AP (A), lateral (B), and sunrise (C)) identify a cruciate retaining cementless femur with cemented tibia. The patella is stable with probable osteolysis. The tibia has varus alignment suggestive of loosening with extensive metaphyseal osteolysis and cement-implant radiolucencies.

Circumferential exposure of the tibia prior to attempted component extraction prevents excessive bone loss, fracture, ligamentous, or neurovascular injury. Extraction of the tibial component should begin at the medial implant – cement interface, with the use of microsaws, hi-speed pencil-tipped burrs, and osteotomes. The central metaphyseal cancellous bone often cannot be accessed and will be sacrificed. Peripheral bone support is typically maintained, resulting in a contained central defect that is easier to reconstruct compared to an uncontained or segmental defect. Rarely, a long, well-fixed stemmed tibial component is present that requires a tibial tubercle osteotomy for removal. Following extraction of the tibial implant and stem, any remaining cement is removed using reverse curette and cement-splitting osteotomes. The trial tibial component aligned by the intramedullary canal is inserted to protect the remaining bone stock and becomes the initial reference for flexion and extension gaps.

Removing the femoral component also requires adequate exposure and disruption of the implant-bone (cementless fixation) or implant-cement (cemented fixation) interface. The anterior, medial, lateral, and posterior implant interfaces are freed with osteotomes, microsaws, or a pencil-tip burr. In some cementless femurs, a Gigli saw is used. The femoral notch interfaces are then released with curved and straight osteotomes. The femoral implant is extracted with a universal extraction device, using force judiciously to avoid damaging the remaining bone stock.

BONE DEFECT CLASSIFICATION

Several bone defect classifications have been described. The most helpful classifications are easy to use, reproducible, and guide reconstruction.⁸

Massachusetts General Hospital Classification

The Massachusetts General Hospital femoral-only defect classification system separates osseous defects into major and minor categories and then further into contained or uncontained defects.⁹ It classifies femoral defects into major and minor types according to the epicondylar level (above versus below), volume (>1 or ≤1 cm³), and containment (contained vs uncontained). Contained defects have cancellous bone loss only, with no significant cortical loss. Uncontained defects have cortical bone loss that results in lack of support for a portion of the implant. Condylar dissociations are categorized as uncontained.

University of Pennsylvania Classification

The University of Pennsylvania (UPENN) classification quantifies bone loss on a continuous numerical scale from 1 to 100 points.¹⁰ The system is set up as a finite element grid based on standard preoperative (AP) and lateral knee radiographs. The classification is intended to anticipate and accurately quantify bone loss preoperatively and has been found to be valid and reliable.⁷ While the UPENN system is precise, quantitative, and readily allows for comparisons of bone loss severity, it lacks the therapeutic recommendations present in other classification systems.

Huff and Sculco Classification

The Huff and Sculco classification is based on the Anderson Orthopedic Research Institute (AORI) bone defect classification.¹¹ The basic patterns of bone loss are cystic, epiphyseal, cavitary, and segmental. Cystic defects are small defects in the trabecular bone that do not affect implant stability. Epiphyseal defects involve cortical bone loss in the epiphyseal/metaphyseal regions, often compromising implant stability and requiring augmentation with stems. Cavitary defects consist of massive, intracortical, metaphyseal defects that require bulk allograft reconstruction or metaphyseal filling cones or sleeves with stems. Segmental defects are a combination of epiphyseal and cavitary defects, with a large extent of bone loss that may involve collateral ligament attachments. Reconstruction options again include bulk allograft or prosthetic reconstruction with stems and often require hinge-type revision prostheses.

Anderson Orthopaedic Research Institute Classification

We prefer the AORI classification system, which is the reconstructive template for this chapter. Bone deficiency is assessed intraoperatively after the prosthesis is removed (Fig. 42-2). Three aspects define the classification:

femoral (“F”) or tibial (“T”) location
severity of bone loss—minimal, moderate, severe, defined as type 1, 2, or 3, respectively
single or both condyle/plateaus, defined as A or B, respectively.¹²

SURGICAL RECONSTRUCTION

Reconstruction options for bony defects can be guided according to the AORI classification system. In order to achieve a stable knee that possesses structural stability, appropriate balancing, and proper alignment, the axial and rotational alignment as well as the joint line must be restored. Cement augmentation, modular metal augmentation, and bone grafting replace lost metaphyseal bone. Intramedullary stems determine axial alignment and offload the metaphyseal reconstruction. After removal of components, the tibial reconstruction is addressed first. The joint line is recreated with trial components. Component rotation, flexion-extension gaps, and the need for constraint are then assessed. As an overview, type 1 defects possess intact metaphyseal bone with a relatively normal joint line. These can typically be treated with a primary implant, with or without modular augmentation. Type 1 defects may require a longer stem. Type 2 defects demonstrate damaged metaphyseal cancellous bone that requires revision components with augmentation (bone graft, modular augments, metaphyseal sleeves, or trabecular metal cones) and stem fixation to offload stress and support the metaphyseal reconstruction. Type 3 defects universally require structural allografts or large porous metal augments, often necessitating segmental replacements secondary to the ligamentous insufficiency. Type 3 defects by definition often have ligament deficiency and require higher levels of constraint or hinged implants (Table 42-1).

AORI Type 1 Defect (“Intact” Metaphysis)

A type 1 defect indicates that there is adequate metaphyseal cancellous bone to support the implant and the joint line has not been substantially altered.¹² Revision style components are not typically needed, and stems are optional. These defects may only require a particulate allograft, cement with screws, or small solid augments attached to the component.

Although bony defects may be minimal, close attention must be paid to restoring the joint line and correcting
any previous malrotation of components. Joint line aberrations following knee arthroplasty are associated with worse clinical outcomes, mid-flexion instability, and altered patellofemoral mechanics.¹³,¹⁴ The native joint line is located 10 to 15 mm proximal to the fibular head and 25 to 30 mm distal to the medial epicondyle.¹⁵,¹⁶ These landmarks can be used as rough predictors of joint line restoration in all revisions.

FIGURE 42-2 Medical illustrations of the Anderson Orthopaedic Research Institute (AORI) Bone Loss Classification System.

Cement and Screws

Small defects involving one condyle or plateau can be managed with bone cement in isolation or with axially aligned screws for additional support and cement.⁶,¹⁷,¹⁸,¹⁹ Bone cement with screws is easy to employ, cost-effective, and appropriate in patients with small osseous defects. Supportive screws should be introduced perpendicular to the implant, serving as support pillars. The surgeon must ensure that the screw heads are not sitting proud to the intact cut surface in order to avoid unintentional implant misalignment (Fig. 42-3).

TABLE 42-1 Overview of the Anderson Orthopaedic Research Institute (AORI) Bone Defect Classification System

Bone	Tibia	Femur	Joint Line	Reconstruction	Stemmed Component	Ligaments	Constraint
Intact	T1	F1	Normal	Cement	Optional	Intact	Unconstrained
Damaged	T2	F2	Uneven	Bone graft, solid augments, occasional porous augments	Yes	Variable damage	Variable constraint
Deficient	T3	F3	Altered	Allograft, segmental replacement, sleeve/cone	Yes	Often compromised	Required, often hinged

Particulate Graft

A particulate autograft or allograft is most often utilized for contained AORI type 1 or 2 defects, where the bone graft can be captured.¹⁹ When impacted, particulate graft, peripheral host bone support, and a stem create a
stable reconstruction. Although cost-effective when compared to metal augments, widespread use of particulate bone grafting for defects larger than type 1 bone loss has diminished in North America.²⁰,²¹,²²,²³ Our current indication for the use of the particulate allograft is in small to moderate sized, contained defects in younger patients who may require future revisions.

FIGURE 42-3 Postoperative AP radiograph showing a cruciate retaining (CR) total knee replacement with use of cement and screws to fill a bony defect.

AORI Type 2 Defect (“Damaged” Metaphysis)

Type 2 defects affect one condyle/plateau (type 2A) or both condyles/plateaus (type 2B) and are characterized by moderate areas of bone loss. Type 2 bone loss includes femoral metaphyseal loss distal to the femoral epicondyles and tibial loss proximal to the tibial tubercle. The metaphyseal bone stock is “damaged” but not completely deficient. Unlike type 1 defects, type 2 bone defects provide inadequate support for an implant without the use of stems. The tibia is prepared first, since its position will establish the flexion-extension gaps and assist in setting the rotation of the femoral component. For type T2A defects that have a minimally damaged contralateral plateau, the tibial cut is made perpendicular to the tibial axis at the level of the intact plateau (Fig. 42-4). For tibial defects in the range of 10 mm in depth, the gap can be filled with a solid augment on the undersurface of the implant. Porous cones and sleeves can be used for smaller defects but are more commonly employed in the setting of tibial plateau defects greater than 10 mm.⁶,¹⁸,²⁴,²⁵ Porous augments are not cemented to the host bone, allowing for eventual osseointegration. The tibial component is cemented to the porous augment.

Type T2B defects lack a bony reference for the joint line. Defects less than 10 mm in thickness can be restored with a thicker tibial tray or medial/lateral augments as previously described. Larger type 2B tibial defects will require porous cones or sleeves (Fig. 42-5). Both cones and sleeves provide a large surface area for biologic fixation and are accompanied by a tibial stem. Cones are not integral to the stem and thus allow greater freedom in placing cones in position for maximal host-cone contact. Further, the final orientation of the tibial stem is not dictated by the placement of the cone. However, smaller cones may limit the diameter of the stem and often require fully cemented stem reconstruction. In contrast, sleeves are directly attached and sized to the tibial stem, allowing metaphyseal cementless sleeve and press-fit diaphyseal stem fixation. The downside of sleeves is that the proximal position of the tibial tray is dictated by the tibial intramedullary canal. When the position of the tibial tray dictated by the canal is found to be suboptimal, smaller diameter, shorter stems can be cemented through the metaphyseal augment or offset stems can be utilized.⁶

Once the tibial joint line is reconstructed, femoral bone defects are addressed. Intramedullary femoral guides typically determine the distal femoral resection. Type F2A defects rely on the reconstructed tibia and contralateral intact femoral condyle to determine the level of distal resection and assist with femoral component rotation. When using a gap-balancing technique, if ligament damage prevents determination of femoral rotation, the epicondylar axis can be used as the main reference for rotation. The deficient femoral condyle is augmented with solid metal augments. Larger type F2A defects with scant metaphyseal bone can be treated with porous metal augments. Alternatively, deficient single femoral condyles can be reconstructed with the use of intramedullary aligned femoral sleeves combined with solid metal augments. The choice of porous augments or sleeves is based on surgeon preference. Porous augments and sleeves attached to stems have essentially replaced bulk allograft in all but the youngest patients with type 2 defects.

Type F2B femoral defects, by definition, require posterior and distal augmentation (Fig. 42-6). The effects of augments on flexion and extension spaces are similar to those encountered in primary arthroplasty.²⁶ Spacer blocks referencing the tibial reconstruction can identify differences in the flexion-extension gaps and relative defect sizes. Typically, the extension space is tighter than the flexion space. Increasing femoral component size can address the flexion space until the femoral component becomes excessively wide in the medial-lateral dimension. In this scenario, a small amount of flexion laxity can be addressed with increased constraint or slight elevation of the joint line. Type F2 defects do not typically necessitate
a hinge reconstruction. Small defects 1 cm³ or less are easily filled with solid metal augments. Larger defects require porous cones fitted to the defect or metaphyseal sleeves attached to an intramedullary stem.

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