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Extensor Mechanism Failure Associated With Total Knee Arthroplasty: Prevention and Management

Parker, David A. MBBS; Dunbar, Michael J. MD; Rorabeck, Cecil H. MD

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Journal of the American Academy of Orthopaedic Surgeons: July 2003 - Volume 11 - Issue 4 - p 238-247
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As many as 50% of revision total knee arthroplasty (TKA) procedures have been attributed to patellofemoral complications.1 The incidence of these complications has decreased markedly as surgical techniques and component design have been refined. Earlier studies2-4 reported incidences of 10% to 35%; more recent studies5,6 cite rates of 1% to 12%. However, patellofemoral complications continue to be a significant source of postoperative morbidity and revision surgery. Possible complications include patellofemoral instability, extensor mechanism disruption, soft-tissue impingement, prosthetic wear or loosening, and osteonecrosis.

Prosthetic Design

Early prosthetic designs did not allow the option of patellar resurfacing. Initial experiences with unresurfaced patellae revealed a high rate of persistent patellofemoral discomfort among patients with rheumatoid arthritis or osteoarthritis, which stimulated the development of a patellar implant. Based on an anatomic study of 80 arthritic patellas, Aglietti et al7 initially described a dome-shaped patellar prosthesis for use with TKA. Subsequently, both symmetric domeshaped and asymmetric, conforming, anatomic-shaped designs have become widely used. Hsu and Walker8 reported that increased conformity of the patellar component decreased the predicted amount of deformation and wear, whereas Matsuda et al9 found that conforming patellas had higher contact stresses than did domeshaped implants, which reduced the load sharing between the patellar component and the quadriceps tendon. Both types of component demonstrated contact stresses higher than the yield strength of polyethylene. Nevertheless, satisfactory long-term results have been reported with both conforming and dome-shaped components. Whichever style is used, it is important that the patellar component be an appropriate match to the femoral component throughout the full range of motion.5,6

The high failure rate of metalbacked prostheses during the 1980s led to the development of all-polyethylene patellar components (Fig. 1). Failure of the metal-backed implants was predominantly caused by wear of the thin polyethylene with resultant metallosis and, less commonly, by loosening of the uncemented implants. Also, patellar implants should have multiple small pegs because a single large peg produces a greater increase in patellar strain and may cause fracture.2

Figure 1
Figure 1:
Three types of patellar implant components. Left, All-polyethylene component with single central peg. Center, All-polyethylene component with three pegs. Right, Metal-backed component.

Femoral component design is also an important factor in patellofemoral function. Kirk et al10 compared two different prosthetic designs and concluded that the higher incidence of patellofemoral complications with a specific implant was caused by the less anatomic design of the femoral component. Mont et al6 attributed the elimination of patellofemoral complications primarily to improvements in femoral component design, especially in relation to patellofemoral contact. Early improvements in the femoral component included the addition of a trochlear flange and a more anatomic trochlear groove that improves patellar tracking. A broad, deep trochlear groove is important to accommodate the patellar component congruently throughout the full range of motion. Eckhoff et al11 showed that the trochlear groove lies lateral to the midplane between the femoral condyles along a line between the anatomic and mechanical axes of the femur. Therefore, the component groove should extend far enough distally to contain the patella in deep flexion. Such prosthetic design improvements have positively affected patellofemoral function after TKA. Improved surgical technique also has helped decrease the incidence of complications, as has greater appreciation of the importance of femoral and tibial component rotation, joint line alteration, and soft-tissue balancing.

Although both anatomic and clinical studies support the use of an asymmetric femoral component, Harwin5 reported only a 0.6% rate of subluxation using a symmetric femoral component design. No advantage has been shown to using an asymmetric trochlear groove and/or asymmetric component; in fact, surgical technique may be the most critical factor.12 In a patient whose patella is not resurfaced, the femoral component should be compatible with the native patella, including a long, deep trochlear groove. However, no designs have been able to exactly reproduce normal kinematics, and no femoral component has been designed specifically for the native patella. Thus, the longevity of the articulation between the native patella and the prosthetic trochlea requires further study.

Patellofemoral Instability

Patellofemoral instability has been one of the most common reasons for revision surgery after TKA (Fig. 2), with a reported incidence of subluxation and/or dislocation as high as 27% in earlier series.3,4 Recent studies5,6 have shown the benefit of improved surgical technique and prosthetic design, with reported incidences of subluxation and/or dislocation ≤1%. Instability may be related to several factors, including femoral and patellar component design, as well as malalignment of any of the three components, malrotation of the femoral or tibial component, or soft-tissue imbalance. Instability also may be related to overstuffing of the patellofemoral joint or to asymmetric resection of the patella. Preoperative patellar subluxation or dislocation should be assessed because it may indicate an anatomic imbalance that would predispose to postoperative instability, which should be addressed intraoperatively.

Figure 2
Figure 2:
Patellar instability 3 years after initial TKA. A, Sunrise radiograph of a lateral subluxation of an unresurfaced patella. B, Sunrise view of a lateral dislocation of a resurfaced patella.

Patellar component positioning has an important effect on patellar tracking. In a cadaveric model, central placement achieved optimal patellofemoral mechanics, and medial placement produced increased patellar tilt.13 However, most authors5,6 still advocate some medialization to avoid a laterally placed component, which would increase the risk of subluxation. Reproducing the normal patellar thickness is important because excessive resection can predispose to fracture, and inadequate resection will result in limited flexion and patellar maltracking. An oversized or anteriorly positioned femoral component also can lead to increased patellofemoral pressure. Asymmetric patellar resection frequently leads to patellar tilt and instability.

Femoral and tibial component alignment also is critical. Correct axial alignment is important for all aspects of TKA function; in an ideal alignment, the weight-bearing line is placed at the center of the knee joint. Knees with fixed preoperative valgus or varus alignment can develop postoperative patellofemoral instability if the deformity remains uncorrected. Lynch et al2 found an increased incidence of subluxation in patients with preoperative valgus alignment that was not corrected intraoperatively. Kirk et al14 reported on a series of 15 knees with patellar dislocation after TKA, of which 13 had preoperative varus alignment. Preservation of the level of the joint line also has been shown to be important in decreasing patellofemoral complications by avoiding elevation of the joint line and patella baja.15

Femoral and tibial component rotation is critical to patellar stability. Internal rotation of the femoral component will medialize the trochlear groove relative to the patella, resulting in lateral subluxation. Similarly, excessive internal rotation of the tibial component will cause lateralization of the tibial tubercle, also resulting in lateral patellar subluxation. Berger et al16 found a direct correlation between combined internal rotation of the two components and the severity of patellofemoral complications. Akagi et al17 found improved patellar tracking and less frequent lateral retinacular releases done in knees with femoral components externally rotated to the posterior condylar axis compared with those parallel to the axis.

Femoral component rotation can be assessed in several ways. Techniques for cutting the femur parallel to the transepicondylar axis, perpendicular to the anteroposterior axis, or externally rotated 3° to the posterior condylar axis have been described,18 and each of these axes should be assessed (Fig. 3). The posterior condyles also can be cut parallel to the tibial cut with the flexion gap tensioned and the knee at 90° of flexion. When using the posterior condylar axis, it is important to assess for lateral condylar wear and hypoplasia, particularly in the valgus knee, because these can lead to internal rotation of the femoral component. Poilvache et al18 found the transepicondylar axis to be the most reliable guide; this axis also has been shown to most closely approximate the flexion axis of the knee. In practice, it is prudent to observe all of the available anatomic landmarks; the transepicondylar axis is probably the best guide if significant disparity exists.

Figure 3
Figure 3:
Axes of femoral component rotation. APA = anteroposterior axis, TEA = transepicondylar axis, TFG = tensioned flexion gap, PCA = posterior condylar axis. (Reprinted with permission from Lonner JH, Lotke PA: Aseptic complications after total knee arthroplasty. J Am Acad Orthop Surg 1999;7:311-324.)

Tibial component rotation also is important, particularly in more conforming designs. The center of the component should be in line with the medial third of the tibial tubercle or internally rotated 18° compared with the plane of the tubercle.16 Sufficient surgical exposure of the entire tibial plateau is necessary to visualize landmarks for accurate assessment of component alignment. Excessive internal rotation must be avoided because it will lateralize the tubercle. Placement of femoral and tibial components in the coronal plane also is important. Medial placement of the femoral component medializes the trochlear groove, and medial placement of the tibial component lateralizes the tubercle; both can cause lateral subluxation of the patella. Although these components generally should be centered over the condyles, lateralization of both components is indicated if the patient's anatomy cannot be perfectly matched.

Assessment of patellar tracking should be done intraoperatively with trial implants and after implantation of definitive implants. Use of a thigh tourniquet can alter patellofemoral tracking; if maltracking exists, the tourniquet should be deflated and tracking reassessed before lateral retinacular release. The patella should independently track centrally within the trochlear groove without any tilting or subluxation. This overestimates the need for lateral release before closure of the knee joint because of the detachment of the vastus medialis muscle and medial retinaculum during exposure. Therefore, it is appropriate to place a single suture or towel clip in the extensor mechanism when testing tracking intraoperatively to better mimic the postoperative setting. If tilting or subluxation remains, all facets of component alignment should be checked and, if necessary, corrected (Table 1).

Table 1
Table 1:
Potential Problems in the Presence of Patellar Subluxation Before Considering Lateral Retinacular Release

If there is no identifiable component malalignment or malrotation, then the etiology most likely relates to soft-tissue imbalance. A tight lateral retinaculum can lead to subluxation and should be addressed with a lateral release. Lynch et al2 advised doing retinacular release well lateral to the patella to avoid the circumpatellar anastomosis. To avoid increasing the risk of quadriceps tendon rupture, the release should not extend medially at its proximal extent, and division of the vastus lateralis insertion to the superolateral patella should be avoided. If this does not correct the problem, the proximal realignment can be completed by advancing the vastus medialis muscle as part of the closure. Lateral release and proximal realignment usually correct most degrees of patellar subluxation. If frank dislocation or persistent subluxation is not corrected by proximal realignment, tibial tubercle malposition may be the cause, and a tubercle transfer may be required. Earlier studies indicated a high complication rate with distal realignment and recommended proximal realignment only, cautioning against the use of tubercle transfer.19 However, in later studies, satisfactory correction of patellar maltracking was achieved with this technique with minimal complications.14,20 Kirk et al14 reported15 cases of patellar dislocation after TKA that were all successfully treated using a modification of the Trillat procedure. The osteotomy should consist of a long fragment and should be tapered distally to avoid a stress riser. Fixation can be done with either screws or wires (Fig. 4). Transfer of the tibial tubercle is rarely necessary, and correct component alignment and soft-tissue balance should ensure good patellar tracking in most cases.

Figure 4
Figure 4:
Tibial tubercle osteotomy used for exposure during revision of failed TKA caused by metal-backed patellar implant wear with extensive metallosis. A, Long tapered fragment with intact lateral soft tissues. B, Wires through the medial tibial cortex, with proximal wire through the tubercle and the distal two wires around the tubercle. C, Wires tightened and tubercle secured.

Extensor Mechanism Disruption

Patellar Fracture

Patellar fracture is an uncommon complication of TKA (Fig. 5). Although the reported incidence ranges from 0%4 to 6%,21 most series2,5,6,22 report an incidence of 1% to 2%, with occurrence more common in men than in women. Such fractures usually are described on the basis of location, integrity of the extensor mechanism, and stability of the implant. Goldberg et al23 described a classification for patellar fractures complicating TKA based on the integrity of the extensor mechanism and fixation of the patellar implant. They found that fractures not associated with componentloosening, extensormechanism disruption, or major malalignment generally had good results with nonsurgical management. Other fractures required surgery, with a high proportion of unsatisfactory results. Most fractures usually are vertical (similar to stress fractures), without disruption of the extensor mechanism, and often are incidental findings that require no specific management. In most series3,5,6,24 of postoperative patellar fractures, more than half were managed nonsurgically with good or excellent results.

Figure 5
Figure 5:
Patellar fractures. A, Sunrise radiograph of a vertical fracture with stable implant. B, Sunrise view of a vertical fracture with unstable implant. C, Lateral view of a transverse fracture with unstable implant.

Intraoperative fractures are more common in revision surgery but can occur in primary cases, especially if the patella is particularly thin. It is important to avoid overreaming or eccentric reaming, overcompression of the patellar clamps during reaming, and slippage of the reamer. Resurfacing patellas that are < 10 mm thick requires extra care to avoid fracture; implants with increased thickness can help minimize resection and restore patellar height in such cases.25

Postoperative fractures may be nontraumatic or traumatic. Nontraumatic fractures may be associated with any of several possible risk factors. Resurfacing of the patella has been shown to be associated with a higher rate of fracture.3,26 Other risk factors include those related to the patient (eg, osteoporosis1) and implant (eg, central peg design,25 cementless implants23). Technical factors include excessive or inadequate resection, devascularization of the patella, and patellar subluxation.26 Malalignment of the limb and implant malrotation also have been associated with increased risk of patellar fracture.16,27 Nontraumatic fractures are usually associated with one or several of these risk factors, especially osteonecrosis. The role of lateral retinacular release in nontraumatic fractures is unclear. Healy et al22 found increased incidence of fracture after lateral release, whereas Ritter and Campbell,24 in a large series, did not. The technical procedures that add to the risk of osteonecrosis also increase the risk of fracture (eg, quadriceps turndown). Traumatic postoperative fractures may result from a direct trauma or an indirect cause, such as eccentric quadriceps muscle contraction resulting in a proximal avulsion.

Management depends on the fracture pattern, stability of the implant, and integrity of the extensor mechanism (Fig. 6). Many fractures are asymptomatic and occur with stable implants and an intact extensor mechanism. Such fractures can be managed nonsurgically, usually with good or excellent results. Fractures with extensor mechanism disruption or dislocation and/or unstable implants require surgical treatment and have poor results in more than half of cases.23,25 Alignment of the implant also is important because knees with major malalignment have more severe fractures and the poorest outcomes.23,27

Figure 6
Figure 6:
London Health Sciences Centre algorithm for the management of patellar fractures.

Quadriceps Tendon Rupture

Quadriceps tendon rupture is an extremely rare complication.5,6,22 Lynch et al2 reported an incidence of 1.1% in a series of 281 TKAs; other authors26 have presented single case reports. With such small numbers, the etiology can only be speculated on and could include factors such as overresection of the patella with damage to the quadriceps tendon as well as vascular injury and incomplete healing after extended approaches such as V-Y turndown, manipulation, or trauma, particularly if there is preexisting tendon degeneration. In the study by Lynch et al,2 all three patients with quadriceps tendon rupture had a lateral release, possibly indicating reduced vascularity as an etiologic factor. Anterior extension of lateral release also may contribute to rupture and should be avoided.

Treatment of the rupture requires direct repair of the tendon. Rupture usually occurs near the distal insertion; thus, the tendon can be repaired directly to bone via drill holes or suture anchors using nonabsorbable sutures. Suture anchors are preferable if patellar bone stock is limited because drill holes may compromise the implant. Supplementation with an allograft may be required if there is any deficiency in the extensor mechanism that would prevent satisfactory repair. The knee is held in full extension for 6 weeks postoperatively before beginning gradual restoration of motion, with the priority being durable healing of the repair rather than rapid restoration of full preoperative movement. Unfortunately, surgical repair has had largely unsatisfactory results in the small series reported, with persistent extensor lag and limited range of motion.2

Patellar Tendon Rupture

Patellar tendon rupture is also uncommon, but it is more frequently reported than quadriceps tendon rupture. Although earlier published incidences ranged from 0.2% to 5%,2,28,29 reported incidences in later series were <1%,5,6 suggesting a decreasing frequency of rupture as surgical technique improves. As with quadriceps tendon rupture, the low frequency makes study of etiology difficult. However, rupture should be preventable. In general, the patients most at risk are those with multiply operated knees. Possible specific etiologies include a stiff knee that causes difficulty everting the patella during exposure; trauma with hyperflexion, including postoperative manipulation; multiple procedures with subsequent devascularized tissue; and patient factors (eg, chronic steroid use, systemic disease).2,30 Distal realignment procedures have been implicated,29 as has excessive patellar resection with damage to the extensor mechanism.31 Component malalignment27 and hinged implants4 also are thought to place increased stress on the extensor mechanism.

Rupture can occur intraoperatively, in the immediate postoperative period, or as a delayed complication. Intraoperative avulsion can occur during exposure of a stiff knee if excessive force is applied to the tendon attachment while the patella is everted and the knee flexed. Rupture can occur in the early postoperative period, such as during manipulation, at any stage as a result of trauma, or as a delayed complication because of chronic attrition, such as may occur with impingement against the tibial insert.31

Preventing this complication requires vigilance during exposure when everting the patella and flexing the knee. If this is difficult, ancillary methods should be used. Careful posteromedial dissection from the tibia is important to allow external rotation of the tibia, which markedly decreases tension at the tendon insertion. If excessive tension remains, a quadriceps snip can be done; in revision surgery, a lateral release in association with division of the lateral gutter and infrapatellar scar tissue also is useful. Occasionally, a tubercle osteotomy or a quadriceps turndown may be necessary.

Treatment of tendon rupture can be difficult and has generally had unsatisfactory results, with few patients regaining full active extension or a satisfactory degree of flexion.29,30,32 Repair can be primary, with or without autograft augmentation, or with allograft reconstruction. If there is partial avulsion or avulsion with an intact periosteal sleeve, the tendon can be reattached primarily to bone either through drill holes or with suture anchors33 or staples.29 This repair can be augmented using a semitendinosus or gracilis tendon autograft left attached distally, particularly if the quality of the primary repair is poor or if there is soft-tissue defect.30 Primary repair of late ruptures generally has had poor results.29 Abril et al34 reported two cases of tendon rupture 1 month after TKA, with successful primary repair through drill holes and support by a figure-of-8 wire for 3 months. These results are difficult to reproduce with a direct repair, and a more extensive reconstruction is usually indicated.

Cadambi and Engh30 described a technique in which a semitendinosus tendon autograft is left attached distally and used to augment the patellar tendon by passing it along the medial border of the tendon through a drill hole in the patella and suturing it to itself distally. Although the mean extensor lag was 10° and flexion only 79°, the authors concluded that this technique was superior to primary repair or allograft reconstruction.

Allograft reconstruction was first described by Emerson et al,31 who used an allograft of quadriceps tendon, a patella with a cemented prosthesis, the patellar tendon, and the tibial tubercle. The tibial tubercle was attached to the tibia with two screws and a tension band wire. The quadriceps tendon allograft was then placed on slight tension and attached to host tendon, with the patella maintained in the appropriate position on the femoral component. One third of patients had extensor lag ranging from 20° to 40°, and there was a high complication rate. However, the authors still considered this technique a satisfactory option for extensor mechanism deficiency. They suggested that resurfacing of the allograft patella was unnecessary.

In a larger series, Nazarian and Booth35 described a modification of this technique using fresh-frozen allograft with unresurfaced patellas and the allograft tensioned with the knee in full extension. The mean extensor lag was 13° in 15 of 36 patients and the mean flexion, 98° in all patients. Although the authors quoted a success rate of 34 of 36 patients, 8 required repeat allograft and 12, a walking aid. Leopold et al32 also reported a high rate of failure using the technique described by Emerson et al,31 with progressive extensor lag and dependence on walking aids. They suggested that improvements were required to tension the graft intraoperatively and that alternative techniques should be considered. Other techniques have been described, including the use of a medial gastrocnemius flap,36 use of synthetic ligament augmentation,37 and patellotibial fusion,38 all with small numbers, short follow-up, notable complications, and persistent extensor lag.

Delayed rupture of the patellar tendon also can be repaired using an Achilles tendon allograft. The allograft, attached to a fragment of calcaneus, is inset into the tibia and fixed in that position. The Achilles tendon is split and wound in a figure-of-8 fashion, through either the extensor mechanism or the patella. The former approach minimizes the risk of further devitalizing the blood supply to the patella. The allograft is then sutured back onto itself with the knee in full extension. Our experience with this technique in seven patients has been positive in terms of restoring extensor function to the point that a brace is not required. Most patients are left with an extensor lag, but this has not proved to be problematic.

Patellar Clunk and Soft-Tissue Impingement

Patellar clunk is a well-recognized complication of posterior stabilized TKA, with a reported incidence of up to 3.5%.39 It is caused by a proliferation of synovial and fibrous tissue at the superior pole of the patella at the quadriceps tendon insertion. This proliferation of tissue is itself caused by articulation of the region with the sharp anterior flange of the intercondylar notch in flexion. Presumably that, in turn, causes an inflammatory reaction that subsequently leads to development of a fibrous nodule in the notch when the knee is flexed. When the nodule is of sufficient size, painful dislodgement occurs as the knee is actively extended from a flexed position. The dislodgement usually occurs at approximately 30° of flexion and causes the painful clunk for which the syndrome is named. Symptoms usually present a mean of 1 year after the procedure.39

Prosthetic design seems to be the main risk factor for patellar clunk. Most newer prostheses have a smaller box with a deeper patellar groove and a more posterior position of the femoral cam, thereby decreasing the chance of soft tissue articulating with this region. The incidence of patellar clunk with these newer prostheses seems to be greatly reduced, although longer follow-up is necessary. Prevention may be helped by excision of the synovium on the posterior aspect of the quadriceps tendon in this region.

Although a trial of nonsurgical management may be undertaken, most patients with an established clunk require surgery to resolve the symptoms. Excellent results with arthroscopic resection through a superolateral portal have been described,39 although care must be taken to avoid scratching the femoral component or damaging the patellar polyethylene. An arthrotomy to remove this tissue is a simple procedure with a relatively rapid recovery time, especially if significant adhesions are expected to make arthroscopic visualization difficult or if other problems with the prosthesis must be addressed. Recurrence after successful removal is rare.

Other complications related to abnormal soft-tissue formation have been published. Thorpe et al40 reported 11 patients (in a series of 635 arthroplasties) who had painful patellofemoral dysfunction caused by intra-articular peripatellar fibrous bands. Nine of 11 implants were stabilized posteriorly and, although the etiology of the pathology is unknown, all patients had resolution of symptoms after arthroscopic removal of the bands.

Patellar Component Wear and Loosening

Patellar component wear is usually secondary to either maltracking or implant design.4 Markedly higher failure rates exist with polyethylene implants with metal-backed components than with cemented all-polyethylene components.22 However, it is unusual for wear of an all-polyethylene component to be sufficient to require revision surgery. Although metal backing of the patella improves load distribution, such implants have had a high rate of failure,41 primarily because the thin polyethylene rapidly wears and delaminates (Fig. 7), leaving a metal-on-metal articulation. Often, revision of the entire TKA is required. Although some surgeons have had good results with metal-backed implants, most now advocate use of a cemented all-polyethylene component.

Figure 7
Figure 7:
Failed metal-backed patellar implant. Left, Burnished metal backing of patellar component from articulation with femoral component. Center, Worn delaminated polyethylene. Right, Marked burnishing (arrow) of femoral component.

Patellar component loosening is rare. Incidence of up to 2% has been noted,42 but no cases of loosening have been reported in most recent large series using cemented all-polyethylene implants.5,6 However, loosening has been a reported problem with cementless, metal-backed implants. Healy et al22 found an increased rate of loosening in cementless implants and also reported loosening secondary to osteonecrosis. Patients with high activity levels and good range of motion are thought to be at increased risk, as are patients with malpositioned components or those with small central fixation lugs. Loosening also can occur secondary to fracture, maltracking, and osteolysis; in such cases, the underlying problem requires management. Improvements in femoral component rotation and femoral trochlear design should help decrease the problem of patellar loosening.


The vascular supply to the patella has been well described in anatomic studies that have demonstrated extensive extraosseous and intraosseous systems with contributions from all genicular vessels. The extraosseous vessels form an anastomotic ring, which is damaged to some extent during arthroplasty. The standard medial parapatellar approach divides the three medial contributors; lateral meniscectomy and lateral release can divide the two lateral contributors; and excision of the infrapatellar fat pad can damage the inferior part of the ring. Patellar resurfacing can cause damage to the intraosseous supply, putting the patella at risk for osteonecrosis. Scuderi et al43 reported decreased patellar vascularity on bone scan after lateral release, but follow-up studies showed possible revascularization within 60 days. In a large series of TKAs, Ritter and Campbell24 reported no increase in osteonecrosis in patients who had a lateral release. Healy et al,22 in one of the few series about osteonecrosis in TKA, reported an incidence of 1.4% in 211 TKAs. Failure to recognize this complication is probably the cause of the low reported incidence. The highest incidence is in patients who required a quadriceps turndown procedure for exposure in revision surgery. In our experience, radiographic evidence of osteonecrosis with sclerosis and flattening or fragmentation developed in 8 of 29 TKAs in which quadriceps turndown procedures were done as part of the revision procedure (Fig. 8).

Figure 8
Figure 8:
Patellar osteonecrosis after quadriceps turndown. A, Lateral radiograph demonstrating sclerosis and flattening of the patella. B, Sunrise radiograph showing patellar flattening, fragmentation, and lateral subluxation.

The natural history of osteonecrosis is poorly defined, except in symptomatic cases in which the sclerotic appearance and secondary fracture or fragmentation can be associated with prosthetic loosening. Prevention should include avoiding the turndown approach and, theoretically, minimizing lateral releases and fat pad resection. Use of the subvastus approach also preserves most of the medial supply, although these measures remain to be proved as effective osteonecrosis prevention techniques. Management of established osteonecrosis involves treatment of secondary complications. If asymptomatic, nonsurgical management is indicated, whereas patellar fragmentation and prosthetic loosening necessitate removal of the implant and loose bony fragments. The remaining patella should be preserved as much as possible, although the prognosis is guarded.

Management of the Failed Patellar Component

Appropriate management of a failed patellar component depends primarily on the cause of failure. If a component requires revision because of wear or loosening, considerations should include techniques for prosthesis removal, evaluation of the predicted remaining bone stock, the condition of the remainder of the extensor mechanism, and the state of the femoral component.

If technically possible, revision of the patellar component is preferred because it restores the extensor mechanism and provides better pain relief than does patellectomy or patelloplasty. However, when there is insufficient bone stock to seat a component, the chance of failure is high and such revision should not be undertaken. As mentioned, at least 10 mm of residual bone is required for resurfacing; the patella should be left unresurfaced when <10 mm of bone remains. In such a situation, to avoid further loss of tension of the extensor mechanism, resection arthroplasty and patelloplasty with reshaping of the residual patella to match the femoral component is preferable to a patellectomy. Complete revision and synovectomy is necessary if a failed metal-backed component has caused damage to the femoral component with subsequent metallosis.41 However, Barrack et al44 found that retaining well-fixed, undamaged, wellaligned patellar components at the time of revision surgery for other components resulted in equivalent outcome to that achieved with successful reimplantation.


Extensor mechanism failure is the primary reason for revision TKA. Advances in prosthetic design and surgical technique have led to a marked decrease in the incidence of such complications, but they continue to be a notable source of morbidity and unsatisfactory results. Because many complications are difficult to manage and often have relatively poor results, prevention is the cornerstone of management. Most of these complications can be avoided with appropriate prosthetic selection and attention to detail in surgical technique. When management is contemplated, a systematic assessment of the specific causative factors of the complication should be done to determine and apply the appropriate treatment.


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