Failure mechanisms in joint replacement

12

Failure mechanisms in joint replacement

M B U R K E and S G O O D M A N , Stanford University Medical Center, USA

12.1

Introduction

Joint replacement has revolutionized the treatment of arthritic disorders of the hip, knee, shoulder, and other articulations in the body. According to the American Academy of Orthopaedic Surgeons, there were 220 000 primary total hip replacement, 108 000 partial hip replacements, and 418 000 primary total knee replacements performed in the United States in 2003.1 However, during this same time period, there were also 36 000 revision total hip replacements and 33 000 revision total knee replacements. These latter procedures cost $1.66 billion and $1.47 billion in hospital costs respectively. As the general population continues to age, the number of joint replacements will continue to increase. Although the longevity for joint replacements has continued to improve, revision surgeries are increasing in number, and the accompanying burden on the patient, their family, and society is substantial. The most common reasons for revision surgery include implant loosening, wear and periprosthetic osteolysis, infection, recurrent dislocation, malalignment, stiffness, periprosthetic fracture, and implant failure or fracture. In addition, when an implant is placed in bone, there is a redistribution of stresses and subsequent remodeling in the bony bed according to Wolff's law. The rearrangement of the bony architecture in the presence of an implant can have adverse consequences. In this chapter, we will explore some of the mechanisms of failure of joint replacements in current and past usage. We will concentrate on specific etiologies of failure, including implant loosening, wear and the generation of wear debris, dislocation, bony remodeling of the implant bed, and the phenomenon of stress shielding, and failure due to surgical technique. Some of these concepts will also be developed in other related chapters.

12.2

Wear and debris

When materials are in contact with each other and undergo relative motion, wear of the materials occurs. As the reason for performing a joint replacement is to

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obtain pain-free motion and improved function of an articulation, it is not surprising that all implants for total joint replacement undergo wear. The concepts of friction, wear, and lubrication are critical to understanding one of the most important challenges in joint replacement surgery today: the construction of a joint replacement that will last a lifetime while the patient partakes in normal daily activities. The subject of tribology is discussed in Chapter 2, and in other chapters on the different materials for total joint replacement. In this section, the clinical aspects of wear and particle generation will be discussed. Wear often occurs at multiple interfaces of a joint replacement. McKellop has classified wear of joint replacements into four types or modes.2,3 Mode 1 occurs at interfaces that are normally supposed to articulate and undergo wear, for example, the metal ball and polyethylene insert of a hip replacement, or the metal-on-polyethylene articulation of a knee replacement. Mode 2 wear occurs between one normal side of an articulation and another side that should not normally articulate. Mode 2 wear occurs, for example, when a metal ball of a hip replacement burrows through the polyethylene insert to articulate with the metal backing that surrounds the polyethylene. Mode 3 wear occurs when a `third body' particle or other structure, not normally present at that location, interposes itself in a joint articulation. Examples of type 3 wear include wear caused by retained cement, metallic or bone particles, or broken wires that have migrated into a total hip articulation. Mode 4 wear occurs between two surfaces that are not normally meant to undergo wear due to relative motion, for example, socalled `backside wear' of the acetabular polyethylene insert against the metal backing of a modular cementless cup. Another example of mode 4 wear is impingement of the prosthetic femoral neck on the side of the acetabular component. The clinical consequences of wear of joint replacements are threefold.4 First, as wear proceeds, the tolerances between the bearing surfaces become altered. This may lead to changes in the biomechanics, function, and range of motion of the joint (which may be increased or decreased), impingement, subluxation, or dislocation. Second, wear may subsequently alter the physicochemical properties of the bearings, surface coatings, and other treatments. Third, wear of the materials generates particulate debris which may lead to a chronic synovitis, foreign body, and chronic inflammatory reaction, periprosthetic osteolysis, loosening, or pathologic fracture. Prosthetic by-products due to wear may have both local and systemic consequences. With a metal-on-plastic articulation such as a hip joint, progressive wear may compromise the biomechanics of the joint such that sliding occurs in addition to rolling. Patients may complain of the hip suddenly giving way or feeling unstable. Continued wear may lead to impingement of the prosthetic neck on the polyethylene liner, disruption of the locking mechanism of a cementless metalbacked cup and dislodgement of the liner. As the femoral head bores into the cup, the range of motion may become restricted; impingement of the prosthetic

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femoral neck on the side of the cup may cause subluxation or dislocation. With further erosion of material, the head may come to articulate with the metal backing of a cementless shell (mode 2 wear) or pierce the polyethylene completely into the cement mantle of a cemented cup. Although wear may have mechanical consequences, in a metal-onpolyethylene articulation, hundreds of thousands of polyethylene particles around 0.5±5 m in size are generated with every step.3,4 These particles undergo phagocytosis and invoke an adverse foreign body and chronic inflammatory reaction that can have serious local consequences. When the number of wearassociated particles overload local homeostatic mechanisms, a state of disequilibrium occurs.5,6 This leads to upregulation of pro-inflammatory cytokines, chemokines, eicosanoids, the nitric oxide and other metabolic pathways, that stimulate the degradative pathways and inhibit the formative pathways of bone.4,7±13 This tilts the balance in favor of bone destruction, called periprosthetic osteolysis. The cellular processes involved in this reaction will be described in further detail in Chapter 15. Interestingly, in some patients, progressive wear may evoke little or no osteolysis, whereas in others, seemingly minor wear is associated with large osteolytic lesions. There may be a genetic basis for some of these idiosyncratic reactions. In most cases, wear and progressive osteolysis are silent, that is, are asymptomatic until significant wear, synovitis, and loss of bone stock occur.14±16 The eventual symptoms may include those from a chronic synovitis, or due to microfractures or frank breakage of the bone with displacement. Clinically, a chronic synovitis leads to swelling, pain, and warmth of the joint, simulating a joint infection. However, aspiration, microscopic analysis, and culture of the synovial fluid will yield a sterile synovitis containing mostly macrophages, lymphocytes, and wear debris, rather than bacteria and polymorphonuclear leukocytes classically seen in infection. Chronic synovitis may lead to expansion of the joint space, capsular, and ligamentous laxity and complaints of joint instability. This may lead to subluxation or even recurrent dislocation of the joint. Progressive wear and periprosthetic osteolysis undermine the bone stock that forms the foundation of the cementless implant or the surrounding cement mantle. With continued loading, micromotion of the implant within bone results from the lack of support for the prosthesis. This micromotion further compromises the underlying bone, resulting in macromotion and eventually, frank loosening or failure of the implant (Fig. 12.1). Pathologic fractures through areas of particle-induced osteolysis are usually acute painful events, often without prior symptoms. In addition, avulsion of a tendinous insertion may occur through osteolytic bone, such as avulsion of the greater trochanter. The mainstay of treatment of osteolysis is prevention.4,14±16 Careful patient selection, choosing the optimal bearing couple, detailed pre-operative planning

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12.1 These radiographs demonstrate extensive osteolysis and `cement disease' around both the femoral and acetabular components: (a) anteroposterior view; (b) frog lateral view; (c) cross-table lateral view. Both components were revised utilizing impaction grafting of a new press-fit acetabular component and an extensively porous coated long-stemmed revision femoral component. An allograft femoral strut graft was utilized: (d) anteroposterior view and (e) cross-table lateral view.

and meticulous surgical technique are important principles to follow. Periodic clinical and radiographic surveillance is also critical so that early progressive osteolysis can be identified and the patient informed of the different treatment options.4,14±16 The basic principles of treatment include debridement of the debris and synovium, revising the worn articulation and any malaligned components, reconstructing lost bone stock, and stabilizing any fractures as necessary.

12.3

Implant or bone fracture

The incidence of fracture around total joint replacements is increasing. This is due to the increasing prevalence of patients with arthroplasties, expanding indications (to include younger and more active patients), and an increase in revision surgeries. While trauma affects a cross-section of society, arthroplasty patients who sustain fractures have additional confounding variables including osteolysis, diminished bone stock and the presence of surgical implants. Furthermore, medical co-morbidities often influence the decision-making process in this relatively elderly population.

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Patients with an increased risk for periprosthetic fractures include those who have osteoporosis (low bone mass), osteomalacia (pathologic poor bone quality), and those patients who are prone to injury. Therefore, this group includes patients who are elderly, have a history of chronic steroid use, neurologic deficit due to stroke or neuropathy, alcoholism or metabolic bone disease. Fortunately, many fractures can be avoided by meticulous surgical technique and by diligent post-operative care. All total joint patients should be followed closely to evaluate for the presence and progression of periprosthetic osteolytic defects, as these represent a frequent site of fracture. Revision of worn components can minimize these defects and bone stock can be restored. Fractures around total joints can be divided into those that occur intraoperatively vs. postoperatively. Intra-operative fractures are not uncommon and can be influenced by implant design and surgical technique. The incidence is much greater during revision procedures than primary operations, and when using press-fit cementless components rather than cemented ones.

12.3.1 The hip Fractures occur at several key steps during a total hip arthroplasty (THA) including dislocation, broaching, reaming, and impacting press-fit acetabular and femoral components. The anterior bow of the femur or anatomic variability can lead to fracture when the stem and femur do not match well. Fractures can occur around press-fit acetabular shells that are eccentrically or under-reamed. Most surgeons under-ream cementless acetabular components by one or two millimeters based on perceived bone quality. Under-reaming is used to improve the press-fit of a component. This is thought to increase initial stability and bone ingrowth potential. It may be, intuitively, an attractive option for osteoporotic patients; however, unfortunately, this population is prone to fracture. Reaming line-to-line decreases the fracture risk at the potential expense of implant stability. Clearly, under-reaming increases the force necessary to impact an acetabular cup (approx 2000 N for 2 mm under-reaming and 3000 N for 4 mm of under-reaming).17 In a study of cadaveric specimens, Kim et al.17 fractured 18 sockets, and had a clear predominance of fractures with 4 mm of under-reaming. This laboratory data is consistent with Sharkey et al.'s (1999) operative experience in which 13 fractures occurred during seating of the acetabular component.18 In their study, 8 of 13 fractures occurred in hips under-reamed by 2 mm; 3 of which were under-reamed by 3 mm and only 1 of which was underreamed by 1 mm. This effect can be exacerbated during the insertion of elliptical acetabular shells. The monoblock elliptical design has been shown to be an independent risk factor for fracture during impaction.19 Fractures of the femur after total hip replacement are often detected intraoperatively by direct observation, but visualization of acetabular fractures can be obscured by the implant. For similar reasons radiographs clearly show femur

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fractures, while acetabular fractures can be more subtle. Therefore, when intraoperative acetabular fracture is suspected, direct observation should be made by removing the component if necessary, especially if the component is not fully seated or loose. In the post-operative setting, oblique radiographs or a computed tomography (CT) scan with metal suppression can be helpful. Prior healed fractures can leave weakened bone that is prone to re-fracture. Screw holes concentrate stress, thereby predisposing to fractures. For this reason, many surgeons bypass such stress-risers by at least two cortical diameters.

12.3.2 The knee Fractures around a total knee arthroplasty (TKA) can occur in the patella, femur or tibia. Intra-operative fractures are rare, but result from high loads transmitted to the patella, overzealous impaction of the components or from imprecise bone cuts. Fractures occur around stems (as they are points of stress concentration) and are more common in patients who have ipsilateral hip and knee arthroplasties (due to stress concentration). Constrained implants also predispose to fractures via rigid transmission of torsional stresses. Removal of part of the anterior femoral cortex (notching) when performing a TKA may predispose the femur to supracondylar fracture, although this is controversial. This fracture is due to removal of the cortical origin of the trabecular bone in the distal femoral condyles. Therefore, it is important to properly size the femoral component and determine its position in the sagittal plane. The relative risk of fracture, however, has been a matter of debate. In biomechanical studies, the strength of notched femurs decreased in both bending and torsion by 18 and 39% respectively, and when loaded to failure, they resulted in a different fracture pattern from non-notched femurs.20 This effect was exacerbated by osteoporosis (as a function of the polar moment of inertia).21 In several series of patients who sustained supracondylar fractures, a disproportionate number of patients had `notched' femurs (rates of 10±46%).22±26 The relative risk of fracture after femoral notching has recently been evaluated by Ritter et al.27 In their review of 1089 total knee arthroplasties, they found 328 `notched' distal femurs. After an average five-year clinical follow-up, they experienced no fractures within this group. They did, however, have two fractures above non-notched femurs. They did not measure any excess risk of femur fracture after notching.27 Given these studies, we can conclude that the risk of fracture is small after anterior femoral notching, but it remains inadvisable given the ample evidence linking notching with decreased strength, and a preponderance of notched femurs in cohorts of patients who experienced supracondylar fractures. A precise technique is also advisable when cutting the intercondylar notch for a posterior stabilized implant. An imprecise, trapezoidal, or shallow resection

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can cause the square box of the femoral component to act as a wedge when impacted. Excessive force used to seat an ill-fitting component therefore can lead to fracture. Although this fracture is rare, it deserves attention from surgeons and design engineers. Long-stemmed knee components are particularly challenging to implant for several reasons. They are frequently used in revision procedures with patients who have poor bone quality or quantity. In accordance with their design, the stem±cortex interface is a site of stress concentration. Preservation of cortical bone strength is important, hence reaming must be done carefully. Offset stems can improve intramedullary fit in bones with abnormal anatomy, decreasing the potential for fracture. Patellar fractures are the most common periprosthetic fracture around a TKA. These fractures occur due to direct impaction from patient falls or from large tensile forces through porotic bone with prosthesis anchoring holes. Predisposition to fractures has been reported with increased patellar resection, insufficient patellar resection (overstuffing the joint), asymmetric patellar resection or malpositon of the femoral or tibial components Preservation of residual patellar thickness of at least 10 mm as well as protection of its blood supply can help minimize the risk of fracture. Flexion of the femoral component effectively over-stuffs the patello-femoral joint. Internal rotation of either the femoral or tibial component leads to increased risk of pain, mal-tracking, dislocation, and patellar fracture.28 Furthermore, patellar fractures can result from increased joint reactive forces that occur with high knee flexion, or with changes in joint-line position. Patellar fractures have also been attributed to large central pegs rather than three peripheral smaller pegs.28 Compromise of the patellar blood supply can also lead to weakening of the patella. The blood vascular supply to the patella is provided by a peri-patellar anastamosis from the geniculate vessels and the anterior tibial recurrent vessels traversing retrograde through the infrapatellar fat pad. During surgery, the medial contribution is compromised by the medial parapatellar arthrotomy. The anterior±inferior contribution is compromised by resection of the fat pad. Care, therefore must be taken to preserve the remaining lateral blood supply. Commonly a lateral retinacular release is necessary to centralize patellar tracking, which can further compromises the patellar blood supply precipitating the cascade of osteonecrosis, fracture, implant loosening, and failure. Therefore effort should be directed towards optimizing patellar tracking prior to performing a lateral retinacular release. This includes maintaining proper femoral and tibial component external rotation. Some surgeons release the tourniquet prior to performing a lateral release to ensure the extensor mechanism is not entrapped/constrained under the tourniquet. When a lateral release is necessary, the surgeon should attempt to visualize and protect the superior lateral geniculate vessels at the inferior margin of the vastus lateralis.

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12.3.3 Classification of periprosthetic fractures There are multiple classification systems for each of the locations where periprosthetic fractures occur. Key principles are reflected in each of the classification systems. They include the position of the fracture relative to the implant's fixation, the remaining bone stock, and the stability of the implant. This has important repercussions on implant retention and fixation options in each of these locations. The Vancouver classification for proximal periprosthetic femur fractures is the most widely utilized system. It classifies the fracture based on location relative to the stem: Above the level of the prosthesis (A), at or just below the tip of the prosthesis (B), or well below the level of the prosthesis (C). Treatment is guided by an evaluation of the remaining bone stock and implant stability. In type A fractures, those of the greater tuberosity are distinguished from those involving the lesser tuberosity. Both tuberosities are sites of insertion of major muscles around the hip. Type B fractures (at the level of the stem) are subdivided based upon implant stability, and bone stock. Subtype B1 is a stable implant with a fracture at or below the level of the stem. Subtype B2 fractures occur with loose stems and adequate bone stock. Subtype B3 fractures occur in association with severe loss of bone stock (either due to osteolysis or comminution). In type C fractures, the fracture occurs well below the level of the prosthesis, rendering the implant unaffected by the fracture. Acetabular fractures around a prosthesis are classified by Peterson and Lewallen based upon implant stability.29 Stable implants within a fractured acetabulum are Type I while Type II fractures render the acetabular shell grossly loose. Fractures about the femoral component of a TKA (supracondylar fractures) have been classified by Lewis and Rorabeck with regard to the degree of fracture displacement and the stability of the femoral component.30 Type I fractures have a stable component with a non-displaced fracture. Type II fractures have a displaced fracture with a stable femoral component. A type III fracture is any pattern that results in an unstable prosthetic component. Tibial fractures are similarly classified based on location (tibial plateau, adjacent to the stem, distal to the prosthesis or at the tibial tubercle), the implant (stable vs. unstable), and timing (intra-operative vs. post-operative). Clearly, each factor has treatment implications. Patellar fractures are integrally related to extensor mechanism function and the stability of the patellar component (in the case of resurfaced patellae). Hozack et al. classified patella fractures based on displacement, extensor mechanism function, distal pole displacement, and failure of prior (non-operative treatment).31

12.3.4 Treatment options Periprosthetic fractures are occasionally treated with activity modification, restricted weight-bearing, immobilization, and close radiographic and clinical

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follow-up. More often, however, they are treated with osteosynthesis and bone grafting to optimize anatomic alignment, and provide sufficient bony stability to allow joint motion and to help restore lost bone stock. Periacetabular fractures often go unnoticed intra-operatively. The fracture site is obscured from the surgeon's vision by the implanted cup and surrounding soft tissues. The only indication that a fracture has occurred may be sudden seating of a tight-fitting component, or the subtle change in pitch heard during impaction. If an intra-operative fracture is suspected, the cup should be removed to inspect the underlying bone. Sharkey et al. demonstrated that initial stability of the cup is critical to overall outcome. Unstable shells fail to gain stability leading eventually to revision surgery.18 Treatment is therefore directed toward obtaining a stable construct in the face of fracture. In their series, three of the four fractures diagnosed on a delayed basis migrated and/or failed clinically. Of the fractures that were initially diagnosed intra-operatively, cup fixation and the fractures were reenforced with acetabular screws. These patients had 6±8 weeks of restricted weight-bearing. The combination of activity restriction and improved cup stability (by placing screws) resulted in improved outcomes (though compromised compared with patients who do not experience fractures). Initial stability must be achieved to optimize patient outcome. Haidukewych et al. reiterated this point with a larger cohort of patients.19 Their incidence of acetabular fracture was 0.4% (21 of 7121 hip arthroplasties). In their cohort of patients, they obtained immediate fixation with 17 components by rim fixation, despite the fracture, but had to exchange four components for multi-holed shells with screws to gain stability. In each of their patients, the fractures healed and the acetabular components performed well. Cup designs were evaluated, and elliptical designs were associated with increased fracture risk. Intra-operative fractures are usually non-displaced, but traumatic postoperative fractures can result from high-energy trauma and may be associated with areas of osteolysis and bearing wear. In this situation stability should be achieved through supplemental acetabular shell screws and, where necessary, plate and screw fixation if major portions of the supporting walls and columns are involved. The application of plates often requires increased soft tissue dissection, blood loss, and operative risk to neurovascular structures. Bony defects can be grafted with structural or morselized bone graft and fragments stabilized with well-described techniques of acetabular column and wall buttress plating. Small wall fractures can be ignored if they do not impact the stability of the component. Large wall fractures should be stabilized with internal fixation with or without bone graft. Column fractures should be plated. Medial wall fractures should be bone grafted and large defects may necessitate the use of an anti-protrusion cage or ring. Periprosthetic fractures around the femoral component are usually treated operatively, because they compromise the stability of the implant. Nevertheless,

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non-operative treatment with activity and weight-bearing modification or with traction and close follow-up for signs of progression or loosening is occasionally appropriate. Operative treatment of a periprosthetic fracture is usually the use of cerclage cables or wires, plates or allograft struts. Severe cases with compromised bone can be treated with proximal femoral replacement or with tumor or customized prostheses. Fractures of the greater trochanter influence hip abductor strength, which is a critical component of gait, function, and hip joint stability. Therefore, fractures that remain non-displaced can be treated with a period of toe-touch weightbearing with close radiographic follow-up. However, if the fracture occurs intraoperatively or if the fracture displaces, rendering the abductor mechanism compromised, the trochanter should be stabilized with wires or a trochanteric plate. If structurally significant osteolysis is present, it should be grafted and the bearing surfaces exchanged. Treatment of femoral shaft fractures around a femoral hip stem depends upon component stability, fracture comminution and remaining bone stock. Vancouver type B1 fractures can be reliably treated with osteosynthesis plates stabilized by screws, cerclage wires/cables. Cortical strut allograft bone can be used to augment bone stock on the anterior and/or lateral surfaces. These fractures reliably heal, but can increase the risk of infection and/or hip instability. Vancouver B2 fractures (fractures around a loose femoral stem), have adequate bone stock for revision surgery. The loose stem must be removed from the proximal fracture fragment (and may require a proximal trochanteric osteotomy). The proximal femur is reconstructed around a new femoral stem that must achieve 5 cm of distal fixation (in the intact distal femoral shaft). The proximal fracture fragments can be stabilized with plates and screws and/or allograft augmentation. These fractures are at increased risk for non-union, malunion and infection (Fig. 12.2). Vancouver B3 fractures occur in femurs with severe osteolysis and/or comminution. The reconstruction requires revision of the femoral stem with allograft struts or a proximal femoral structural allograft. This can jeopardize the attachment of the hip abductors, and hence the stability of the hip. Another alternative is a tumor prosthesis. Owing to the high risk of instability with these constructs, securing the component with a constrained acetabular liner may be advisable. Similar concepts govern the treatment of fractures around a TKA. Function relies upon the restoration of a stable, properly aligned implant. The collateral ligaments, like the hip-abductor mechanism, must be structurally competent. If the fracture is non-displaced and stable, with adequate bone stock, it is reasonable to attempt to treat the fracture in a long-leg cast or functional brace with a 6±12 weeks of protected weight-bearing. If a fracture above a TKA is displaced, rigid internal fixation, enough to allow postoperative knee motion must be obtained with either a blade-plate, condylar screw with side-plate, peri-articular locking plate, or intramedullary

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12.2 These radiographs are of a patient who mis-stepped 4 weeks after right THA for osteoarthritis of the hip. The patient sustained a Vancouver B2 periprosthetic femur fracture around the proximal end, and the prosthesis subsided within the bone (a). The old femoral component was excised, the fracture reduced and stabilized with cerclage wires and an extensively porous coated stem was placed (b).

nail. Intramedullary fixation is attractive because it can be performed with minimal dissection and can be used reliably for posterior cruciate retaining prostheses (as a nail will easily fit through the area between the femoral condyles). If, on the other hand, a posterior-stabilized implant is present, the surgeon must be aware of the variable presence of a pre-drilled hole in the central box created for this contingency. If a hole is not present, one can be created with a carbide drill, but other methods of fixation may be simpler. Fractures that compromise knee stability via avulsion of the collateral ligaments require fixation or revision to stemmed constrained implants. If the origin of the collateral ligaments is disrupted, the fracture requires bone grafting, or if there is soft-tissue interposition, the fracture site should be visualized directly, and rigid internal fixation should be obtained. A loose prosthesis should be removed, and replaced with stemmed components over a fracture that is stabilized and bone grafted as necessary. In the rare case where there is significant comminution, with minimal remaining bone stock, distal femoral allograft around a stemmed distal femoral component or a custom implant can be considered. Occasionally, factors such as the patient's medical condition, the presence of chronic infection or multiple injuries necessitate long-term traction treatment or amputation. Treatment of periprosthetic tibial fractures is dependent upon implant stability and the location of the fracture. Fractures of the tibial plateau with a stable component (type I) are treated with screw(s) and buttress plate as necessary. If these fractures occur intra-operatively, the fracture site and tibial plateau should be offloaded and bypassed by use of a stemmed component. If the fracture results in a loose component or a type II fracture (one that occurs around the stem of the tibial component) the implant should be revised utilizing bone grafting and stemmed components. Type III fractures (those that occur distal to the tibial component) usually do not affect the stability or function of the implant and should be treated non-operatively with casting and/or bracing. Type IV fractures (avulsions of the tibial tubercle) affect the extensor mechanism.

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Small non-displaced avulsions can be treated with extension bracing and activity modification. Fractures that displace or render the extensor mechanism incompetent require internal fixation to re-establish extensor continuity. The treatment of patellar fractures depends upon the stability of the polyethylene component, the function of the extensor mechanism and the remaining bone stock. A stable component may be retained. An extensor mechanism that is non-functional due to a displaced patellar fracture requires internal fixation with a tension band and a new component if it is loose. However, these operations are frequently unsuccessful due to non-union. Partial or complete patellectomy remains a viable option when insufficient bone stock or comminution precludes fixation and re-implantation. In patients who demonstrate failure of the above treatments, the final option is transplantation of an allograft comprising the quadriceps tendon, patella, patellar tendon and tibial tuberosity. Non-operative treatment consisting of six weeks in an extension brace with minimal weightbearing is appropriate only for non-displaced transverse or vertical patella fractures in which the patellar component remains well fixed. Some of these factors cannot be influenced by the patient and/or physician, however, early treatment of osteolysis and proper implant selection can minimize the risk.

12.3.5 Implant fracture Because of the use of modern super-alloys and better prosthesis designs, implant fracture is a problem that has largely been solved. Occasionally a fracture is seen in a prosthesis made with a suboptimal design or poor manufacturing methods. Cast implants with large grain size, and numerous impurities and asperities are predisposed to fracture. Although uncommon with modern implants, the majority of remaining fractures occur in the region of the femoral head and neck and are related to implant selection. The calcar femorale is normally composed of dense, strong bone that supports and protects the hip prosthesis. In the rare case when an arthroplasty is performed in a patient with a deficient calcar (due to tumor, unstable intertrochanteric or subtrochanteric fracture or during revision surgery) a calcar replacing prosthesis, designed to withstand the high stresses of this region should be selected to prevent fatigue fracture. There is also a higher incidence of prosthetic fracture with some modular implants but this has largely been resolved due to better engineering design. The remainder of prosthetic fractures usually occur in ceramic total hip components. This was an unacceptably common mode of failure with first generation ceramic implants. Up to 13.4% of ceramic femoral heads failed due to fracture, hence many companies stopped their sales. However, the newer (third generation) manufacturing techniques of hot isostatic pressing, laser marking and proof testing have lowered the fracture rate to approximately 0.004%. Isostatic pressing helps increase the density of the ceramic to the ideal

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3.98 g/cm, and optimizes the ceramic microstructure to increase its overall strength. Laser marking serial numbers decreases the notch effect. Finally, the components are proof tested past the limits of physiologic load (at least eight times body weight). These manufacturing methods have increased the strength of ceramic implants and decreased their failure rate.32

12.4

Dislocation

Dislocation after THA is the second most common reason for revision hip surgery. Dislocation is associated with increased costs when revision arthroplasty is required.33 The incidence of dislocation after primary arthroplasty has been reported between 0.6% and 7%, but most published reports for primary arthroplasty report 2±4%.34 Revision arthroplasty (compared to primary) is associated with a much higher dislocation rate; 7.4% in a review of the Mayo Clinic experience by Alberton et al.35 Several factors have been associated with increased rates of dislocation, including female sex, patient age over 70 years, or surgery performed for avascular necrosis, fractures, non-union, inflammatory arthropathy, neurologic disorders and alcohol and drug dependence. The risk of dislocation extends over the lifetime of the prosthesis. Berry et al. found the risk of dislocation is 1% at 1 month, 1.9% at 1 year, and continues to increases at a rate of 1% every 5 years to a maximum of 7%.36 Three-quarters of dislocations occur within the first year after surgery, but this long-term follow-up study calls into question the low dislocation rates found in short-term follow-up studies. The most common direction of dislocation is posterior. This occurs when the lower extremity the hip joint in flexion, adduction, and internal rotation. Anterior dislocation occurs much less often, and is due to hip extension with external rotation and abduction. These extreme positions should be avoided post-operatively. Abduction wedges, braces, and knee immobilizers can be used to help reinforce patient compliance. The factors that affect hip stability include the surgical approach, component position, prosthesis head±neck ratios, and abductor muscle function. The compressive and stabilizing force for the hip joint is provided by the abductor muscles. Function is optimized by re-creating the mechanical hip center-of-rotation, with anatomic trochanteric offset and appropriate leg lengths. A greater trochanter that is positioned either too superiorly or medially renders the abductors weak and the hip prone to dislocation. Similarly, fractures of the greater trochanter, avulsion of the gluteus medius tendon, or dysfunction of the central nervous system (such as post-cerebrovascular accident, Parkinson's disease, etc.), or the peripheral nerves (superior gluteal nerve paralysis) can lead to hip instability. As most dislocations occur posteriorly, a posterior approach that compromises the posterior capsule is associated with a higher dislocation rate than an anterior or lateral approach. In a meta-analysis by Kwon et al.37 the shortterm dislocation rates for an anterolateral approach was 0.7%, for lateral 0.43%,

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and 1.01% for the posterior approach. The risk of posterior dislocation can be minimized by performing a meticulous multilayered capsular repair (capsulorrhaphy). This fact was supported by Goldstein et al.,38 and Kwon et al.'s37 meta-analysis showed a relative risk reduction of 8.21 by performing a soft-tissue repair after a posterior approach to the hip. Surgical factors that lead to increased hip stability include increasing femoral head size, restoration of soft-tissue tension, and proper component positioning. Increasing the head±neck ratio is advantageous because it allows a greater degree of motion before primary impingement. In a cadaveric study by Bartz et al.,39 smaller head sizes were shown to decrease the effective hip range of motion prior to impingement and dislocation. Increasing the femoral head size from 22 to 28 mm increased the hip range of motion by 7.6ë prior to dislocation. Hedlundh et al.40 reported a 2.3 times greater recurrent dislocation rate for hips with a 22 mm vs. a 32 mm head. Increasing the head size also improves stability by increasing the distance the femoral component must travel to disengage the acetabular component ± the drop-distance. Unfortunately, increasing the femoral head size also increases the volumetric wear rate because for a given range of motion, increasing the femoral head diameter increases the contact surface. The elevated particulate load can lead to accelerated osteolysis. The ramifications of this, however can be offset to some degree by using ultra-highly crosslinked polyethylene, metal-on-metal or ceramic bearing surfaces. Impingement between the femur and pelvis (external impingement) can also lead to dislocation. Prevention is accomplished through proper placement of the acetabular shell, debriding major osteophytes, and restoring the natural anatomic offset (lateralization of the femur). Restoring offset also increases abductor tension, the crucial factor in creating a stable joint. Increasing abductor tension can be achieved by lateralizing the acetabular component, using a high-offset acetabular liner, increasing the femoral neck length, a high-offset femoral stem or by advancing the abductor mechanism via trochanteric osteotomy. Lateralizing the acetabular component can increase the joint reaction forces by increasing the body-weight moment-arm and hence the work generated by the abductor muscle complex. Lateralization of the hip center increases the torsional stresses on the femoral stem thereby potentially affecting stem stability and longevity. Using a high-offset acetabular liner has the same biomechanical effect, but it also increases the leg-length. Use of an increased neck-length also increases leglengths, offset and abductor tension. High offset femoral components have a lower neck-shaft angle, or a lower neck take-off point, and therefore generate offset without increasing leg-length. The soft-tissue sleeve can be tightened without lengthening the extremity by advancing the greater trochanter, either through an osteotomy, or by advancing the repair in a transtrochanteric approach. Non-union of the trochanter in this approach compromises the abductor integrity and has been shown to lead to a six-fold increased risk of dislocation.41

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Implant position affects stability. Optimal positioning has been proposed by Barrack et al.42 from computer modelling to be 45ë of cup abduction with 20ë of anteversion. The femoral stem should be placed in 15ë of anteversion.42 Similar studies by D'Lima et al.43 demonstrated that closure of the actabular component to 35ë resulted in markedly decreased motion before impingement or dislocation. Stability could be achieved only with increased acetabular and femoral component anteversion. By contrast, opening the cup to 45ë resulted in markedly increased stability at all positions of femoral and acetabular version. Excessively abducting the acetabular component results in edge loading of the polyethylene and leads to early wear and excessive osteolysis and failure as demonstrated by Schmalzried et al.44 When a hip arthroplasty is performed through a posterior approach, the anteriorly displaced femur places the cup into relative retroversion. The opposite is true for an anterior approach. Aberrant anatomy (i.e. developmental dysplasia of the hip, retroverted acetabulum) can also lead to component malposition. Placement of the components outside of these limits, which can be exacerbated by excessive anteversion of the femoral and acetabular components, leads to impingement and higher dislocation rates. Lipped or oblique liners re-orient the acetabular face to optimize stability. They do not change the offset or leg-length, but highly elevated lips (20ë) can influence the effective version and/or abduction angle, thereby augmenting stability. The trade-off to these components is they can decrease range of motion and increase the chance of neck-liner impingement. Dislocation is best treated by prevention. This includes meticulous patient and implant selection, re-creation of appropriate soft tissue tensions, intra-operative trial reduction with documentation of stability and range of motion, and meticulous surgical repair. Nevertheless, joint instability often may be persistent. The surgeon should determine the direction of dislocation, individual, and relative positioning of the components, and evaluate for neurovascular injury, fracture, or component failure. The surgeon should also evaluate for the presence of infection. The hip can usually be reduced in the emergency room with muscular relaxation and conscious sedation. Occasionally, fluoroscopic guidance can assist with reduction. Should an open reduction be required, the patient and surgeon should be prepared to perform a complete revision, as indicated. After successful closed reduction, an abduction brace should generally be applied for a minimum of six weeks to allow capsular healing. Non-compliant patients can be placed in a hip spica cast. Immobilization (or limitation of motion) with limited weight-bearing for six weeks and abductor strengthening effectively treats two-thirds of dislocations, but is more effective in patients who dislocate within a year of the initial procedure.41 Patients who dislocate more than one year from the initial procedure are more likely to develop chronic instability and require surgical correction. This may reflect the role of bearing surface wear in generating an unstable joint.

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Surgical treatment for chronic instability is highly individualized and should be directed toward the cause of instability. Infections clearly should be treated with debridement, antibiotics and revision of the components (as discussed in Chapter 15, Section 15.2). Malpositioned components should be revised or occasionally treated with either lipped or oblique liners. Any loose or broken components should also be revised. Instability due to polyethylene wear is treated with femoral head and acetabular liner exchange (preferably to a more durable combination). Impingement should be treated by resection of osteophytes, increasing femoral offset and/or exchange of components as the operative findings dictate. Inadequate soft tissue tensions can be improved by increasing head/neck lengths and offset with stems or offset acetabular liners. If the modular components have been optimized and instability persists, a trochanteric advancement can be performed. Woo and Morrey (1982) demonstrated that revision surgery is effective approximately two-thirds of the time.41 In situations of persistent instability, when operative treatment fails or when patient health and physical demands limit surgical options, salvage procedures are indicated. These include placement of a constrained acetabular component, bipolar arthroplasty or girdlestone resection arthroplasty. Constrained devices capture the femoral head to improve stability to the detriment of range of motion and increased shear forces at the acetabular shell±bone interface (Fig. 12.3).

12.3 This patient had a primary THA with decreased femoral offset and increased acetabular anteversion (a and b). The patient sustained multiple hip dislocations despite brace treatment. Placement of a constrained acetabular liner (c) failed after four years due to accelerated wear with failure of the locking ring mechanism. Placement of a lateralizing polyethylene liner and large femoral head with increased neck length rendered the hip stable (d).

280

12.5

Joint replacement technology

Stress shielding

Bone is in a constant state of flux. This remodeling process allows bone to react to its environment and stressors. According to Wolff's law, bone is formed and strengthens along lines of mechanical stress. The corollary is that bone devoid of stress, atrophies (like most tissues in the body). Wolff's law is clinically apparent by the formation of osteophytes around an arthritic joint (increased stress causes hypertrophy of the bone) and under rigid internal fixation plates (bone atrophy). It also manifests in osteoporosis of bedridden, non-functional or neurologically impaired patients with atrophic bone. In the setting of THA, stress in the proximal femur is shared by both the host bone and the metal implants. Cemented or rigidly fixed femoral components that have distal fixation and are constructed of stiff material (relative to cortical bone) can be expected to support the majority of the patient's weight, thereby relieving stress on the proximal femur. This causes resorption of proximal femoral bone. By contrast, a less-rigid femoral implant (such as titanium alloy versus cobalt chrome alloy) that gains proximal fixation (proximally porous coated stems) shares more stress with a greater length of the femur. Stress in the proximal femur helps prevent proximal resorption. This helps prevent insufficiency fractures, improves implant stability and facilitates revision surgery by improved bone stock. The process of stress shielding is seen around acetabular, femoral, and total knee implants. Each of these supports bone and shields the bone from stress. In the proximal tibia and distal femur, stress shielding is seen under the articular surfaces, especially in metal-backed components. In the proximal femur, it is seen in the greater and lesser trochanters and along cementless stems that are distally well fixed. It is seen variably around acetabular components that distribute weight-bearing stress. Stress shielding is present to a lesser degree around cemented components, because of the presence of a lower-modulus intermediate material, poly(methylmethacrylate). Prevention or stress shielding is accomplished by selecting implants that allow native bone to support as much of the patient's weight as possible (loadsharing implants). Distal fixation of long stem femoral components should be used only if necessary for implant stability. The use of a collar on the femoral component to distribute load to the proximal femur is theoretically beneficial in mitigating adverse bone remodeling and stress shielding. However, the surgical precision necessary for this to occur is often difficult to achieve. Medical treatment with bisphosphonate medications has been used to preserve bone mass in osteoporotic patients. Bisphosphonates have been shown to preserve bone mass around total joint implants, but they inhibit normal osteoclast function and bone remodelling. This may lead to detrimental effects on the stability of cementless stems and overall durability of the arthroplasty. Furthermore, these medications are not without potential adverse systemic

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effects. Currently, the use of bisphosphonates to prevent stress-shielding after joint replacement is controversial.

12.6

Comment on surgical failure

Clearly, surgical technique is a crucial factor in successful arthroplasty surgery. As stated above, poor surgical technique can lead to fracture (either intraoperative or post-operative), accelerated implant wear, instability, impingement or stiffness, or limb-length discrepancy. Poor technique, in TKA, can result in soft tissue disruption (i.e., patellar tendon avulsion) or laceration of critical structures (medial collateral ligaments, posterior cruciate ligament) or neurovascular structures. It can result in persistent knee pain, patellar maltracking and early failure due to abnormal mechanics. Proper alignment and rotation of the TKA is critical to surgical success. Over 30 years ago, Lotke and Ecker45 recognized the importance of coronal alignment in implant survival. They noted five failures, all via fracture of the medial tibial plateau. Four of the five failures were in knees placed in excessive varus. Ritter et al.46 evaluated 421 knee arthroplasty patients with up to 13-year follow-up. They found eight failures, five in patients placed in varus (less than 4ë anatomic valgus), and three in patients placed in 5±8ë of anatomic valgus. No failures were reported in the patients placed in valgus. They concluded that malalignment is a significant contributor to mechanical failure, but varus malalignment is better tolerated than valgus.46 Similarly, Jeffery et al.47 reported an increased rate of failure in knees in which the axis of alignment (Maquet's line ± from the center of the femoral head to the center of the talus) failed to pass within the middle third of the knee. One-third of their knees were malaligned. The failure rate for the malaligned knees was 24% (compared with 3%) at 8 years.47 When the weight-bearing line of the lower extremity passes through the center of the knee, the prosthesis is evenly loaded. If, on the other hand, stresses are not shared evenly over the prosthesis, one side experiences lift-off, while the other experiences excessive compression. This leads to excessive wear, particulate debris, osteolysis, cement de-bonding and component failure. Component rotation is also a critical component to success. In a cadaveric study, femoral rotation was found to correlate best with the transepicondylar axis of the femur. Internal or external rotation of 5ë resulted in increased femorotibial joint forces as well as abnormal shear at the patellofemoral articulation. This leads to post-operative pain, patellofemoral subluxation, and early failure.48

12.7

Summary

Despite the fact that total joint replacement is an effective operation for relieving pain and improving function, there are still issues related to implant wear and the adverse effects of particulate debris, including periprosthetic osteolysis and

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implant loosening. Better bearing surfaces will undoubtedly improve implant longevity; however, a bearing surface that allows a lifetime of normal activity, including impact-loading sports, has not yet been achieved. Recurrent dislocation is the second most common problem that has recently been addressed by careful pre-operative planning, meticulous surgical technique, and the use larger diameter alternative bearing surfaces. The cause of instability of a joint replacement should be investigated and corrected. Implant failure due to fracture is uncommon due to better designs and manufacturing techniques; however newer modular prostheses and those made with inferior designs or materials, and poor supporting bone stock are risk factors. Stress shielding is a manifestation of Wolff's law and although not a major clinical problem, can lead to compromised bone stock for revision surgery. Periprosthetic fractures are extremely challenging for the reconstructive surgeon. These should be classified as to the location, the type of prosthesis in situ, the functionality and stability of the implant, and the quality of the surrounding bone. It is preferable, if possible, to fix fractures associated with stable, well-functioning implants rather than to deal with the fracture and perform an implant revision simultaneously.

12.8

Future trends

Total joint replacement is changing. No longer is a `one size fits all' mentality pervasive. The options for joint replacement are individualized for each patient. This has lead to concepts such as choosing the most functional, least expensive prosthesis for the level of activity of the patient (so-called `implant matching') to minimally invasive surgery (MIS) in which incisions and surgical dissections are kept to those that are necessary to accomplish the procedure only. Hopefully the concept of MIS will facilitate a quicker rehabilitation, early discharge, and return to normal activities, but this has not yet been conclusively shown. Modularity of implants will provide the surgeon with a host of options to reconstruct normal biomechanics `on the spot'. Newer bearing surfaces employing highly crosslinked polyethylene, metal-on-metal and ceramic-on-ceramic articulations may allow a wider range of activities. However, enthusiasm should be tempered by the unavailability of long-term follow-up. Computer-assisted surgery has the potential to improve implant alignment with `on-line' feedback in the operating room, but this concept is still in its infancy and multi-plane alignment cannot easily be assessed. One of the most exciting trends in modern arthroplasty surgery is multi-modal pain management, that is, the use of numerous adjunctive agents and modes of anesthesia and analgesia intra- and peri-operatively. Hopefully, this will allow the operative experience to be better tolerated by the patient and facilitate earlier more effective rehabilitation and return to function.

Failure mechanisms in joint replacement

12.9

283

References

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