9Failures in Patellar Replacement in Total Knee Arthroplasty 9 9

Chapter 9 · Failures in Patellar Replacement in Total Knee Arthroplasty – J.A. Rand

9 Failures in Patellar Replacement in Total Knee Arthroplasty

J. A. Rand

Summary

Patellar failure after TKA is often multifactorial. A care- ful assessment of patient factors, implant design and surgical technique must be performed. If there are major problems with implant design or component position- ing, revision of the entire arthroplasty may be necessary to correct the patellar failure and ensure a durable result.

Isolated revision of the patellofemoral joint for any reason must be approached cautiously, as a high failure rate is often encountered.

Introduction

In a recent series of revision total knee arthroplasties (TKA), extensor mechanism problems comprised al- most 12% of the reasons for reoperation [1]. Reasons for failure in the patellofemoral joint are multifactorial and may be related to patient selection, implant design, sur- gical technique, or combinations of these factors. There- fore, any discussion of patellar component failures must consider multiple potential reasons for failure. Unfortu- nately, most studies of patellofemoral complications have not considered the importance of the tibiofemoral joint for the complication. Anterior knee pain, patellar instability, fracture, loosening, wear, extensor mecha- nism rupture, and a variety of miscellaneous problems affecting the patella can adversely affect the results of a TKA.

The anatomy and kinematics of the patellofemoral joint are complex. There is variability in the orientation of the patellar groove in both the coronal and transverse planes.The patellar groove is oriented approximately per- pendicular to the epicondylar axis. Since there are sub- stantial individual variations in alignment and patellar tracking, the design of the femoral component needs to accommodate these variations. The patella undergoes a medial shift in early flexion followed by a lateral shift in deep knee flexion beyond 90°.In deep flexion,the contact area on the patella moves distally on the lateral facet of the patella,resulting in a decrease in contact area.The contact areas and kinematics of the patella are altered in TKA and are affected by implant orientation and design of the femoral and patellar components.

Etiology

Patient selection is an important variable influencing extensor mechanism complications. Patellar complica- tions are increased in patients with a diagnosis of patellofemoral arthritis, obesity, osteoporosis, valgus deformity, post-traumatic arthritis, and prior proximal tibial osteotomy (

).

A diagnosis of osteoarthritis and obesity has been as- sociated with an increased risk of patellar complications [2]. In the presence of valgus deformity, varying degrees of lateral femoral condyle hypoplasia make rotational po- sitioning of the femoral implant difficult. In knees with

⊡ Fig. 9-1. Merchant X-ray demonstrating patellar subluxation bilaterally with fracture on the left knee in a patient with preoperative patellofemoral arthritis

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preoperative valgus, a lateral retinacular release was nec- essary in 102 of 134 knees to treat intraoperative patellar subluxation [3]. The presence of patella infera following proximal tibial osteotomy or post-traumatic arthritis can result in impingement between the patella and the tibial component, resulting in pain or patellar instability. The patient with severe osteoporosis is at risk of patellar frac- ture following patellar resurfacing.

Surgical technique is an important variable influenc- ing patellar complications.A midvastus or subvastus sur- gical approach results in improved patellar tracking and less frequent need for a lateral retinacular release than does an anteromedial arthrotomy. Using meta analysis, a lateral retinacular release was required in ten of 164 (6%) subvastus approaches compared with 31 of 172 (18%) medial parapatellar approaches [4–6]. Femoral and tib- ial component position affect patellar alignment and complications. Patellar complications are diminished by maintenance of the joint line and patellar height, lateral placement of the femoral component on the femur, medial placement of the patellar component on the patella, and posterior placement of the tibial component on the tibia.The femoral component should not be flexed on the femur, and the trochlear flange should be aligned with the anterior femoral cortex. If the trochelar portion of the femoral component is prominent, the extensor mechanism will be displaced in an anterior direction, re- sulting in increased lateral retinacular tension.The result is a potential decrease in knee motion and possible patellar maltracking. In severe cases, the patella may mechanically catch on the trochlear flange of the femoral component. Any deviation of the femoral, tibial, or patellar components from these ideal locations can ad- versely affect patellar alignment, leading to patellar failure. Internal malrotation of either the tibial or the

femoral component will adversely affect patellar track- ing (

).

Alignment of the femoral component with the epi- condylar axis or the AP axis appears to be best. Femoral component rotation parallel to the epicondylar axis re- sulted in the most normal patellar tracking and decreased shear forces early in flexion [7].Rotating the femoral com- ponent either internal or external to the epicondylar axis adversely affected patellar tracking. There is a close re- lationship between the femoral epicondylar axis and the patellar axis. Placing the tibial component perpendicular to the epicondylar axis resulted in correct rotation in 73%

of cases [8]. In a study of 102 TKAs, there was a mean of 6.2° of internal rotation in the knees with anterior knee pain compared with 0.4° of external rotation in the con- trol knees [9]. In a comparison of 30 TKAs with patellar complications and 20 controls without, combined inter- nal rotation of 1°–4° resulted in lateral patellar tracking and tilt,3°–8° patellar subluxation,and 7°–17° patellar dis- location or patellar failure [10].

Reproduction of patellar thickness, correct size and position of the patellar component,and balance of the ex- tensor mechanism are necessary for a satisfactory result.

A lateral retinacular release was required for 17% with medial compared with 46% with a central placement of the patellar implant [11]. The amount of bone resected from the patella will affect patellar tracking and patellar strain. Resection of excessive patellar bone can result in weakening of the patella, leading to fracture or implant fixation in poor-quality bone predisposing to loosening.

Thickening the patella at the time of resurfacing will tighten the lateral retinaculum,resulting in patellar tilt or subluxation. If the patella is resurfaced, the original patellar thickness should be reproduced. Asymmetric resurfacing of the patella should be avoided. In a series of

⊡ Fig. 9-2a, b.CAT scans of (a) femoral and (b) tibial component with internal malrotation

a b

Chapter 9 · Failures in Patellar Replacement in Total Knee Arthroplasty – J.A. Rand

21 TKAs with asymmetric resurfacing of the patella, 11 knees were revised, recommended for revision, or had anterior knee pain [12]. A lateral retinacular release for patellar maltracking is not innocuous. Patellar complica- tions occurred in14% of 540 knees with in comparison to 7% of 510 knees without a lateral retinacular release [13].

Complications in the lateral release group consisted of patellar radiolucency in 11, patellar fracture in nine, hematoma in seven, extensor lag in seven, patellar insta- bility in five, and patellar implant loosening in three [13].

Implant design affects patellar alignment and patel- lar tracking. A trochlear groove that is asymmetric or deep produces a decrease in shear and compressive force on the patella compared with a symmetric or shallow trochlear groove design [14]. Patellar complications oc- curred in 15 of 148 TKAs with a shallow trochlear groove compared with one of 153 TKAs with a deep trochlear groove [15] (

).

In a comparison of 150 TKAs performed with a stan- dard design with 150 TKAs with a 3° external rotation built into the femoral component (resulting in a lateral- ized trochlear groove), the prevalence of lateral retina- cular release was decreased from 14% to 5% and patellar maltracking from 12% to 5% [16]. Therefore, selection of a femur with a deep, offset trochlear design is preferable to minimize patellar complications. The patellar implant may be a central dome, offset dome, or anatomical in shape. Failure of the patella due to wear and deformation of patellar components are observed with all polyethylene and metal-backed patellar component designs. All poly- ethylene dome designs are susceptible to deformation, while metal-backed designs are susceptible to deforma- tion of the polyethylene over the metal or to dissociation of the metal backing. In a study of six different TKA designs, contact pressures in the patellofemoral joint at

knee flexion angles greater than 60° exceeded the yield point of the polyethylene for all designs [17].Anatomical- ly shaped patellar implants provide increased contact area over dome-shaped implants when aligned correctly but will have a decreased contact area with slight tilt or malrotation. Eighteen of 75 anatomically shaped patellar implants had a complication [18].For this reason,a dome- shaped patella is preferred to the anatomical shape, as it is more forgiving of minor malalignment. A study of a two-peg anatomical, three-peg dome, and three-peg anatomical patellar component design found improved results with the three-peg designs [19].

Either an inset or onlay patellar design may be used.

In an in vitro comparison of an inset to an onlay design, patellar strain was increased by 28% for the inset and 22%

for the onlay design over the unresurfaced value [20]. In a retrospective comparison of 135 resurfacing patellae with 116 inset patellae, patellar tilt, subluxation, and later- al retinacular release were less frequent with the inset than with the resurfacing design [21].

Are extensor mechanism complications different in resurfaced and unresurfaced patellae of TKA? Contro- versy has surrounded the need for patellar resurfacing at the time of TKA. This controversy arises from differing results which are clearly influenced by studies using TKA designs that do not allow congruent tracking of the native patella and surgical techniques that led to patellar malalignment. These results are further complicated by differing rates for reoperation that are influenced by the ease of resurfacing of the painful unresurfaced patella but not treatment of anterior knee pain in the resurfaced patella. Selective resurfacing attempts to identify those individuals who will have an improved clinical result by resurfacing while avoiding the complications of unneces- sary resurfacing. The best data regarding the results of patellar resurfacing derive from randomized,prospective studies of patellar resurfacing. Using meta analysis of nine randomized,prospective series,there were 518 resur- faced and 542 unresurfaced patellae followed for 2–10 years [22–30].Anterior knee pain was present in 38 (7.3%) of the resurfaced and 118 (21.8%) of the unresurfaced patellae. Knee scores were similar in both groups.

Patellar complications occurred in 14 (2.7%), leading to reoperation in ten (1.9%) of the resurfaced patellae.

This is in contrast to patellar complications in 37 (6.8%) of unresurfaced patellae, leading to reoperation in 36 (6.6%) [22–30]. If anterior knee pain persists in the un- resurfaced patella, will pain be relieved by resurfacing?

Using meta analysis of 60 knees with secondary patellar resurfacing, 36 (60%) were improved, 12 (20%) un- changed, and 12 (20%) worse after resurfacing [25–29, 31, 32]. Therefore, selective patellar resurfacing may be the best approach considering patient demands, implant design, patellar articular cartilage, and intraoperative patellar alignment.

⊡ Fig. 9-3. Merchant X-ray demonstrating lateral patellar tilt and sub- luxation in a TKA design with a shallow trochlear groove. There is also some internal rotation of the femoral component

9

Patellar instability may manifest as pain and weakness, intermittent giving way, or episodes of patellar disloca- tion. Patellar tilt and subluxation are frequently encoun- tered on routine radiographs of asymptomatic patients.

Why some patients with patellar malalignment are asymptomatic while others are symptomatic remains an enigma. The etiology of patellar instability can be traced to patient selection,implant design,surgical technique,or trauma [33]. Preoperative patellar subluxation, preopera- tive valgus, or patellofemoral arthritis have been corre- lated with an increased prevalence of lateral retinacular release and increased postoperative patellar malalign- ment [3]. Implant designs that have a shallow trochlear groove, fixed axis of rotation, or unrestricted rotation have had a high prevalence of patellar subluxation and dislocation. In a study of 289 TKAs using a design with a shallow trochlear groove and followed up, 14 knees re- quired revision for patellar maltracking [32]. Problems with surgical technique are a frequent reason for patellar instability. Internal malrotation of the femoral or tibial component, excessive valgus knee alignment, lateral placement of a patellar component on the patella, medial translation of the femoral component on the femur,thick- ening the patella with resurfacing, or oversizing the femoral component onto the anterior surface of the femur may contribute to patellar instability.

Treatment of patellar instability must be directed at the etiology. Implant malposition is best treated by com- ponent revision. Soft-tissue imbalance should be treated by a proximal realignment consisting of a lateral retinac- ular release and vastus medialis advancement. Although distal realignment of the tibial tuberosity can correct patellar instability, it creates a compensatory deformity, alters patellar kinematics, and may predispose to patellar tendon rupture. Most published studies of treatment of patellar instability did not assess implant rotation, mak- ing interpretation of the results difficult. In our own se- ries, the etiology of patellar instability was failure to bal- ance the extensor mechanism in nine knees , tibial com- ponent malrotation in four,atraumatic medial retinacular tears in four, quadriceps weakness in four, activity-relat- ed tears of the medial retinaculum in two, and a traumat- ic tear of the medial retinaculum in one knee [34].

Recurrent subluxation occurred in four of 14 knees following proximal realignment, while two of nine knees sustained a rupture of the patellar tendon following com- bined proximal and distal realignment. One of two knees treated by revision had a recurrent patellar subluxation.

Two knees developed deep infections. Distal extensor mechanism realignment has been recommended using a tibial tubercle transfer as treatment for patellar insta- bility. However, as distal extensor mechanism realign-

ment alters knee kinematics and is associated with rup- ture of the patellar tendon in some patients, proximal re- alignment is the preferred technique in the absence of implant malposition. Revision of the femoral and tibial components should be considered for those knees with component malposition.

Patellar fractures may present as anything from asymptomatic findings on routine radiographs to acute episodes with disruption of the extensor mechanism (

).

In a series of 12 464 TKA from the Mayo Clinic there were 85 patellar fractures, for a prevalence of (0.68%) [35].The prevalence of fracture is greater in men than in women, in resurfaced than in unresurfaced patellae, and in revision than in primary TKA. The etiology of patellar fracture includes trauma, avascularity, implant malalign- ment, obesity, excessive patellar bone resection, high activity level, large range of motion, inset patellar design, large central fixation lug,revision TKA,and osteoporosis.

The position of the tibial and femoral components affects the severity and results of patellar fractures. In a series of 36 patellar fractures, a good or excellent result was achieved in 15 of 16 with minor malalignment as com- pared with three of 20 with major malalignment [36].

Avascularity of the patella following a medial arthrotomy combined with a lateral retinacular release has been suggested as an etiology of fracture.Another explanation for the association of patellar fracture with lateral reti- nacular release is abnormal forces on a maltracking patella that necessitated the lateral release. In a series of 1146 TKAs,a patellar fracture occurred in 22 of 406 (5.4%) knees with as compared with 18 of 740 (2.4%) without a lateral retinacular release [37]. There was no significant difference in the prevalence of patellar fracture following lateral retinacular release in which the superior lateral genicular artery was spared or sacrificed [38].

A variety of classification systems have been used for patellar fractures after TKA. Differences in classification make comparison of results from various series difficult.

⊡ Fig. 9-4.Lateral X-ray demonstrating an asymptomatic patellar frac- ture with non-union

Chapter 9 · Failures in Patellar Replacement in Total Knee Arthroplasty – J.A. Rand

The easiest classification system is the one from the Mayo Clinic [35].Fractures are divided into three types based on the stability of the patellar prosthesis and on whether or not the extensor mechanism is intact. A type-I fracture has a stable implant and an intact extensor mechanism.A type-II fracture has a disruption of the extensor mecha- nism.A type-III fracture has a loose patellar implant and an intact extensor mechanism. The type-III fractures are divided into subtypes A and B, based on the quality of re- maining bone stock (A = good and B = poor with <10 mm or marked comminution). For the 38 type-I fractures, non-operative treatment consisted of observation in 27, immobilization in ten, and operative treatment including patellar component removal and patelloplasty in one knee.A fibrous union occurred in 16,non-union in 15,and union in six fractures.Of the 12 type-II fractures,only one was treated without an operation (resulting in a satisfac- tory outcome). Operative management with open reduc- tion and internal fixation resulted in union of one of six fractures. Partial patellectomy and tendon repair in five knees resulted in complications in three. Of the 12 type-II fractures, six had complications, five a reoperation, and seven either pain or weakness. A type-III fracture oc- curred in 38 knees. Of the 12 type-IIIA fractures, eight were treated with and four without an operation. Only two of the 12 fractures united. Two of four knees treated without an operation had pain. Complications occurred in five of five fractures treated by resection of the patellar components and internal fixation. Of the 12 type-IIIA fractures, seven knees had complications, three a reoper- ation, and seven pain or weakness. Of the 16 type-IIIB fractures, a satisfactory result occurred in three of four knees following non-operative treatment. Pain or weak- ness occurred in seven of 12 knees following operative management. Of the 28 type-III fractures, eight had a complication, three underwent reoperation, and 15 re- mained symptomatic. Considering the entire group of fractures, there were improved results and fewer compli- cations following non-operative than following operative management. Non-operative treatment of 46 fractures resulted in complications in four knees and in reopera- tion of one knee. Operative treatment of 32 fractures re- sulted in complications in 18 and reoperation of 16 knees.

The authors concluded that non-operative treatment should be selected for type-I and some type-III fractures.

Operative treatment should be used for type-II fractures.

In summary,treatment of patellar fractures must con- sider the etiology and carefully evaluate the position of the femoral and tibial components. Treatment of most patellar fractures should be non-operative,with an initial period of immobilization. Operative treatment should be reserved for those fractures with an extensor mechanism rupture. Revision of the femoral or tibial components may be required to address implant malrotation or mal- position.

Ruptures are the most difficult extensor mechanism problems to manage, with the highest frequency of fail- ure and complications. The location of rupture may be in the patellar tendon, in the quadriceps tendon, or in asso- ciation with a patellar fracture. At the Mayo Clinic, the prevalence of quadriceps tendon rupture was 24 of 23 800 (0.1%) and patellar tendon rupture was 18 of 8288 (0.17%) TKA [38, 39]. Quadriceps tendon tears are often associat- ed with systemic diseases such as diabetes mellitus or use of systemic corticosteroids [38]. Lateral retinacular re- lease, prior operation, and extensile surgical exposures may result in quadriceps rupture [38]. Finally, trauma from a fall or vigorous activity may rupture the tendon damaged by systemic disease or surgery [38]. The results of treatment of quadriceps ruptures after TKA have been variable. Excision of devitalized tissue and direct suture to the freshened patellar bone bed is recommended [38].

Reinforcement of the repair site with an autograft, allo- graft, or synthetic material may help to protect the repair.

Healing takes precedence over motion. Prolonged immo- bilization in extension followed by gradual range of motion in a brace is the preferred postoperative manage- ment. Partial ruptures of the quadriceps tendon have a better prognosis than complete ruptures. They present with quadriceps weakness but still have active extension.

Treatment consists of operative repair and protected re- habilitation.

Patellar tendon rupture is a more difficult problem than a quadriceps rupture as the patellar tendon is poor- ly vascularized. In a series of 18 patellar tendon ruptures, the etiology was difficult exposure in the stiff knee, extensive release of the patellar tendon at the time of sur- gical exposure,manipulation for limited motion,revision TKA, and distal extensor mechanism realignment for treatment of patellar maltracking [39]. Direct suture repair alone was unsuccessful in all nine attempts, and staple repair was successful in two of four attempts [40].

For acute ruptures direct repair must be augmented with an autogenous or allogeneic tendon graft or a synthetic ligament. Late reconstruction of a chronic rupture re- quires an Achilles tendon or whole extensor mechanism allograft. In the presence of a soft-tissue defect over the anterior aspect of the knee,an autogenous gastrocnemius flap may be used. In a series of seven repairs using semi- tendinosis augmentation, only one knee had a residual extensor lag of 10° [40]. Of nine knees treated with the Achilles allograft, three had a residual extensor lag [41].

Two grafts failed, requiring repeat repair, and there was

one infection. The use of a whole extensor mechanism

allograft requires careful patient selection, meticulous

surgical technique, and carefully supervised rehabilita-

tion. There must be adequate soft-tissue coverage, good

motion, a well-aligned or revisable TKA, and adequate

tibial bone at the level of the tibial tuberosity. The low-

demand patient who is able and willing to cooperate in

a prolonged rehabilitation program is the best candidate.

The results of allograft extensor mechanism reconstruc- tion have been variable. In one report, six revisions were performed in 18 of 27 allografts that were followed [42].

In a report of 40 allograft extensor mechanism recon- structions, 15 had a residual extensor lag and there were eight graft failures [43]. In summary, suture repair with augmentation should be used for acute ruptures while allograft reconstruction is reserved for chronic ruptures.

A high prevalence of residual extensor lag and a high complication rate should be anticipated.

Patellar implant loosening and wear are less frequent with modern than with older implant designs. Modern surgical technique with alignment of the implants on the epicondylar axis of the femur has improved patellar alignment and decreased the prevalence of these prob- lems. However, the problem of wear of patellar implants has not been resolved,as the contact stresses in most TKA designs exceed the yield point of the polyethylene of the patellar implant. Patellar failure may present as an inci- dental finding on radiographs or as symptomatic anteri- or knee pain and crepitation. The etiology of patellar implant loosening includes fixation in deficient bone, component malposition, patellar maltracking, patellar osteonecrosis, asymmetric bone resection, altered joint line, osteolysis, failure of bone ingrowth in cementless designs, and obesity [44].

Patellar implant loosening was a problem with early implant designs which had a small central fixation keel that was prone to breakage. Loosening occurred in 180 of 4583 (4.2%) TKAs [45]. Loosening was associated with five radiographic findings: bone-cement radiolucency, increased bone density, trabecular bone collapse, frag- mentation and fracture of the patella, and lateral sublux- ation of the patella [45]. Lateral retinacular release was performed in 132 of 180 (73%) TKAs with patellar loosen-

ing as compared with 2320 of 4104 (59%) without. Patel- lar maltracking with avascular necrosis from the lateral retinacular release were felt to be the reasons for patellar loosening [45]. Patellar implant design was changed to three pegs to improve fixation. A change from a single central fixation peg to three peripheral pegs was adopted by many manufacturers in the late 1980s. The three-peg designs have continued to have problems with loosening, but at a lower rate than some of the single- or two-peg designs.Therefore,implant design and surgical technique can contribute to implant loosening (

).

Deformation and wear of patellar components due to high loads in the patellofemoral joint,combined with lack of congruity between the dome-shaped patella and the femoral component design, are commonly seen at the time of revision TKA. In a retrieval study, wear was iden- tified in 65% of metal-backed designs and in 78% of all polyethylene designs [46].Wear of metal-backed patellar components remains a problem. Revision of the entire arthroplasty may be required. Once the metal backing is exposed, damage to the femoral component occurs with the generation of metal debris. The prevalence of failure of metal-backed patellar components is dependent upon the design, implant alignment, and duration of use. Sur- vivorship of a metal-backed patella at 9 years was 73% for TKAs with a shallow trochlear groove compared with 93% for TKAs with a deep trochlear groove [47]. Inset patellae with metal backing have a low failure rate.In a se- ries of 331 mobile bearing patellae followed up for a mean of 73 months, there were three patellar fractures and one dislocation but no failures from wear [48]. Revision of metal-backed patellar components is recommended in the presence of patellar malalignment, symptomatic crepitation, or significant wear or deformation observed at the time of reoperation for any reason. If there is cor- rect patellar tracking,dome-shaped polyethylene patellar

9

⊡ Fig. 9-5. Merchant (left) and later- al (right) radiograph of loose patellar implant. There is lateral subluxation of the patella and a femoral implant de- sign with a shallow trochlear groove

implants may be retained in the presence of mild defor- mation or wear at the time of revision of a femoral im- plant. Implant design and surgical technique must be carefully evaluated in the knee with failure of a patellar component.

A variety of miscellaneous soft-tissue complications may affect the patellofemoral joint.A fibrous nodule may develop at the junction of the proximal pole of the patel- la and the quadriceps tendon. The nodule becomes en- trapped in the intercondylar notch of a posterior stabi- lized design during flexion,resulting in a painful clunk on knee extension [49]. The prevalence of the patellar clunk syndrome was 32 of 900 (3.5%) TKAs [50]. Arthroscopic or open resection of the fibrous nodule is usually suc- cessful. Peripatellar fibrosis is a universal finding at the time of revision of a TKA, yet crepitation in the patellofemoral joint of a TKA may or may not be painful.

The prevalence of this complication is related to femoral component design being more frequent in those designs with a shallow trochlear groove. Three types of peri- patellar fibrosis have been described: circumferential, fibrotic bands, and nodular [51]. Arthroscopic débride- ment of peripatellar fibrosis resulted in 59% fair or poor results, with five knees having a recurrence of symptoms [51]. Patella infera after TKA can be a source of pain. A change in joint line of less than 8 mm has been found to be one of the variables associated with good results with a posterior stabilized TKA [36]. Both joint line elevation and shortening of the patellar tendon can occur,resulting in patellar impingement against the polyethylene of the tibial component.Since patellar tracking is improved with medial placement of a dome-shaped patellar implant on the patella, a portion of the lateral facet is often left un- resurfaced. In the presence of patellar tilt or subluxation, impingement of the unresurfaced lateral facet of the patella against the trochlea of the femur may occur.

Resection of the unresurfaced bone is usually curative.

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