555

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Total

Knee

Findings Backed

David Piraino1 Richmond1 Harris Freed1 George Belhobek1 Jean Schils1 Bernard Stulberg2

Bradford

One

Replacement:

in Failure Patellar

technique

for

Our

AJR

7, 1990;

accepted

after

re-

This work was supported in part by a grant from Johnson and Johnson Corporation, Braintree, MA. 1 Department of Radiology, A-21 , Cleveland Clinic Foundation,

1 Clinic Center,

9500 Euclid Ave.,

Cleveland, OH 441 95-5021 . Address reprint requests to 0. Piraino. 2 Department of Orthopaedic Surgery, Cleveland Clinic

Foundation,

Cleveland,

OH 44195-5021.

0361 -803X/90/1 553-0555 C American Roentgen Ray Society

of Porous-Coated

Metal-

Component

replacing

the

articular

surface

of the

patella

in total

knee

replace-

ment is the use of a cementless porous-coated metal-backed polyethylene component. Anteropostenor, lateral, and Merchant or skyline radiographs in 10 cases of porouscoated metal-backed components that failed were evaluated for component alignment, component displacement, radiolucencies, loose beads, and integrity of the patellar component. Component failure was documented by surgery in all 10 cases. Seven cases of failure involved the patellar pegs or junction of the pegs with the metal backing and three cases involved the polyethylene portion. In the seven cases of peg-metal backing failure, three cases showed loose beads before failure and seven cases showed displacement of the metal backing at time of failure. All three cases of polyethylene failure showed narrowing or displacement of the polyethylene portion. experience

shows

that

polyethylene, and disruption backed patellar components. failure.

Received February vision April 20, 1990.

Radiologic

155:555-558,

displacement

of the

metal

backing,

displacement

of patellar pegs are seen in failed porous-coated Loose beads may be seen before peg-metal

September

of the

metalbacking

1990

A total knee replacement replaces the articular surface of the knee with metal and polyethylene components. Most knee prosthetic designs have tibial, femoral, and patellar components. Patellar articular surface replacement in total knee replacements is recommended to decrease patellofemoral joint pain and complications [1 ]. Early patellar components were all polyethylene. Metal backing was added to these components to transmit load more uniformly to the bone, to decrease the tendency of the polyethylene to deform under load, and to add metal porous coating to achieve bone ingrowth for biological fixation [2]. Figure i shows the type of porous-coated metal-backed patellar component evaluated in this study (Microloc prosthesis by Johnson and Johnson, Braintree, MA). The Microloc prosthesis is one of several porous-coated metal-backed patellar components available. The polyethylene portion is dome-shaped and articulates with the anterior groove in the femoral component of the total knee prosthesis. The metal backing or metal plate is a titanium alloy that does not extend to the outer margin of the polyethylene. Metal pegs (two or three) are attached to the metal backing. The porous coating is made of small metal beads sintered to the pegs and metal backing. The porous coating allows biological fixation of the patellar component by bone ingrowth between the metal beads.

Retrospectively,

radiographs

of i 0 known

evaluated to determine if any specific failure or at the time of failure. Materials

and

radiologic

failed findings

patellar

components

are seen before

were

component

Methods

Ten cases of surgically

confirmed

These cases were not consecutive.

patellar component failure were evaluated retrospectively. Radiographs of the patella before and after failure were

PIRAINO

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556

available for all patients. The radiographs were evaluated for component alignment, displacement of portions of the patellar component, developing component radiolucencies, loose metallic beads, and integrity of patellar components. Radiographic projections included anteroposterior films, lateralfilms, and Merchant [3] or skyline patellar views. In several patients, tangential radiographs also were obtained after positioning the bone-metal interface parallel to the radiographic

beam under fluoroscopy. A standard format was used for evaluating radiographs. Alignment of the femoral and tibial components was evaluated on the anteroposterior and lateral films. The varus or valgus alignment at the knee was measured along with the alignment of the femoral and tibial components with respect to the femur and tibia. Patellar alignment and positioning were evaluated on the lateral and Merchant views.

The patellar tilt, lateral vs medial placement,

and size of the patellar

component vs patellar length were measured [4]. The radiographs were evaluated by at least two radiologists. The second interpreter knew the results interpretation. Differences in opinion were resolved by The reviewers had no clinical information except that the had surgery for revision of the patellar component.

experienced of the first consensus. patients had

Results The results are summarized in Tables 1 and 2. Table i shows each of the cases evaluated, along with the findings on radiographs obtained before and at the time of failure.

Table 2 shows

the number

of cases with specific

radiologic

findings of polyethylene failure and peg metal failure. Of the 1 0 knee replacements evaluated, seven had component failure of the patellar pegs or junction of the patellar pegs with the metal backing (Fig. 2). Seven cases showed displacement of the metal backing relative to the patella, and seven cases showed loose beads at the patellar pegs at the time of failure diagnosis (Fig. 3). Three cases showed evidence of loose beads at the patellar pegs before actual displacement of the metal backing away from the patella occurred (Fig. 4). No cases showed metal-bone radiolucencies greater than 2 mm before component failure. Four cases of the peg-plate failures showed no loose beads and no evidence

ET AL.

AJR:155, September

1990

of metal bone radiolucency on the most recent radiograph before demonstration of component failure. Three of i 0 components had failure of the component with separation of the polyethylene from the metal backing (Fig. 5). Two of these had narrowing of the polyethylene component, and two had displacement of the polyethylene component away from the metal backing. All three polyethylene failures had narrowing and/or displacement of the plastic component at the time the failure was diagnosed. The postsurgical alignment of the patellar components was similar to that in a previous study by Gomes et al. [4], who evaluated patellar positioning in total knee replacements. Radiographs of the tibial and femoral components in these 1 0 cases did not show evidence of failure.

Discussion

The primary sites of patellar component failure in our study were at the peg-metal backing interface and at the polyethylene-metal interface. Previous studies on metal-backed components have also shown most failures to be related to the polyethylene-metal interface and peg-metal interface [5-8]. Shear forces on the patella cause separation of the polyethylene from the metal backing [6]. Once there has been partial separation of the polyethylene from the metal backing, part or all of the polyethylene may displace from the metal backing. This type of failure was shown on radiographs as narrowing or displacement of the polyethylene. Shear forces also cause fatigue fracture of the peg-metal backing interface [6]. Fatigue fracture at the peg-metal interface was shown on radiographs as displacement of the metal backing or fracture at the peg-metal backing junction. Fixation of porous-coated noncemented prostheses requires bone ingrowth into the small spaces between the metal beads. Rigid fixation occurs only in the areas of bone ingrowth. Animal studies have shown consistent bone ingrowth into pegs and failure of ingrowth into the metal backing of porous-coated knee replacements [6]. Patellar shear forces

Fig. 1.-A, Radiograph shows normal appearance of porous-coated cementiess metal-back patellar component (arrow shows position of polyethylene portion). B, Photograph of metal-back patellar component shows porous coating, porous-coated pegs (curved arrow), and polyethylene portion (straight arrow).

AJR:155,

September

TABLE

1: Findings

KNEE

1990

on Radiographs

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Case No.

of Patellar

REPLACEMENT

Component

COMPONENT

Failure

Findings Before Failure

1

Loose

2

Polyethylene

3

Loose

557

FAILURE

Findings After Failure

beads

narrowing

beads

1

ype

of Fail re u

Loose beads; metal plate displacement

Fracture

Polyethylene narrowing; polyethylene dis-

Separation of polyethylene from metal backing

placement Loose beads;

Fracture

at peg-plate

junction

Fracture

at peg-plate

junction

metal

at peg-plate

junction

plate displacement 4

None

Loose

beads;

metal

plate displacement 5

None

Loose

beads;

metal

Separation

plate displacement 6

None

7

None

8

Polyethylene

Polyethylene displacement Loose beads; metal plate displacement Polyethylene narrowing

narrowing

of beads

Separation of polyethylene from metal backing Fracture at peg-plate junction Polyethylene

narrowing

separation 9

Loose

beads

Loose

beads;

from

inner patellar peg

from metal backing Fracture at peg-plate

metal

and

of polyethylene junction

plate displacement i0

None

Loose beads; metal plate displacement

Fracture at peg-plate junction; separation of beads from inner

TABLE Type

2:

Summary

of Findings

on Radiographs

by Failure

No. of Patients With

Finding

Polyethylene Failure Polyethylene Polyethylene

narrowing displacement

Loose beads before failure Loose beads after failure Metal

backing failure Total

can then

displacement

lead to fatigue

With

Peg-Metal

Backing

Failure

2 2

0 0

0 0

3 7

0 3

7 7

after

fracture

at the peg-metal

backed

patellar components

backing

Radiologically, several patellar components were unremarkable before failure. This finding correlates with previous studies of failed porous-coated metal-backed patellar components in which the diagnosis offailure was unknown before surgery in 28% [7]. In our study, however, all i 0 cases demonstrated at least one abnormal radiologic finding of the patellar component at the time of failure diagnosis. Loose beads and polyethylene narrowing were seen before diagnosis of patellar component failure. Although loose beads have been shown in successful porous-coated metal-backed patellar components, their occurrence is rare. A previous study on porous-coated metal-backed patellar components of similar design showed only one loose bead associated with one patellar component in 40 knee replacements with an average follow-up of 1 2.9 months [9]. Wear of polyethylene components has been shown in most prosthesis designs [1 0]. Narrowing of the polyethylene of porous-coated metal-

peg

is not a specific

sign of compo-

nent failure as it may only indicate wear of the polyethylene. Displacement of the polyethylene from the metal backing, displacement of the metal backing from the patella, and fracture of the pegs are the radiologic findings we consider important in diagnosing failure of porous-coated metal-backed patellar components. Other studies with different types of successful porous-coated metal-backed patellar components have shown stable patellar fixation with follow-ups for as long as 3-4 years [i 1 -i 3]. These studies have not shown displacement of the metal backing or polyethylene in successful

patellar components

junction.

patellar

[i 1 -i 3]. Although

a controlled

study of

the prevalence of these findings in successful porous-coated metal-backed patellar components has not been done, previous studies indicate that these findings are rare in successful porous-coated metal-backed patellar components. In our study, the most common reason for revision of the

failed patellar components was pain. In one patient, revision was not needed until i year after radiologic diagnosis of patellar

component

failure.

The recent reports of failure with porous-coated metalbacked patellar components have made the use of these components have many

more controversial, potential advantages

but these components over non-metal-backed

still ce-

mented components. Also, complications of non-metalbacked patellar components have been relatively common and include patellar dislocation, patellar fracture, loosening, and wear of the polyethylene portion and avulsion of the ligamentum

patellae

[1].

ACKNOWLEDGMENT We thank

Joan

Hodge

for manuscript

preparation.

558

PIRAINO

ET AL.

AJR:155,

September

1990

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Fig. 2.-Merchant-view radiograph shows lateral displacement of metal backing and separation of porous coating (curved arrow) from inner metal core (straight arrow) of patellar pegs.

Fig. inferior (arrow)

3.-Lateral radiograph shows displacement of metal backing with fracture of metal peg-plate junction. Loose metallic beads are seen in joint space.

Fig. 4.-Lateral

radiograph

shows a

few loose beads (arrow) in component that later showed fracture at peg-plate

junction.

Fig. 5.-Radiograph shows portion of polyethylene (arrows) that has separated from metal backing and Is displaced inferiorly.

REFERENCES 1 . Ranawat

CS. The patellofemoral

joint in total condylar knee arthroplasty.

Clin Orthop 1986;205:93-99 2. Stulberg SD, Stulberg BN. Hamati Y, Tsao A. Failure mechanisms backed patellar components. C/in Orthop 1988;236:88-105

3. Merchant analysis

AC, Mercer of patellofemoral

AL, Jacobsen congruence.

of metal-

RH, Ccol CR. Roentgenographic J Bone

Joint

Surg

A: 1391 -1396 4. Gomes LS, Bechtold JE, Gustilo RB. Patellar prosthesis

(Am]

1974;56-

positioning in total

knee arthroplasty. C!in Orthop 1988:236:72-81 5. Lombardi AV, Engh GA, VoIz RG, Albrigo JL, Brainard BJ. dissociation of polyethylene in metal-backed patellar components knee arthroplasty. J Bone Joint Surg [Am) 1988;70-A(5):675-679

Fracture/ in total

6. Rosenberg AG, Andriacchi TP, Baren A, Galante JO. Patellar component failure in cementless 106-114

total

knee

arthroplasty.

Clin

Orthop

1988:236:

7. Bayley JC, Scott RD. Further observations on metal-backed patellar component failure. Clin Orthop 1988:236:82-87 8. Bayley JC, Scott RD, Ewald FC, Holmes GB Jr. Failure of the metalbacked patellar component after total knee replacement. J Bone Joint Surg (Am] 1988;70-A(5):668-674 9. Cheng CL, Gross AE. Lcosening of the porous coating in total knee replacements. J Bone Joint Surg (Br] 1988;70-B:377-381 1 0. Brick GW, Scott RD. The patellofemoral component of total knee arthroplasty. C/in Orthop 1988:231 : 163-1 78 11 . Landon GC, Galante JO, Maley MM. Noncemented total knee arthroplasty. C/in Orthop 1986:205:49-57 1 2. Rosenberg AG, Borden A, Galante JO. A comparison of cemented and cementless fixation with the Miller-Galante total knee arthroplasty. Orthop C!in North Am 1989;20(1):97-111 1 3. Hungerford DS, Kenna AV. Preliminary experience with a total knee prosthesis with porous coating used without cement. C!in Orthop

1983;176:95-107