Management of the Patella in Revision Total Knee Arthroplasty

Kevin I. Perry, MD

Arlen D. Hanssen, MD

Historically there has been a relative rarity of published literature regarding the management of the patella during revision total knee arthroplasty (TKA). One of the primary reasons for this phenomenon is that during the majority of revision TKA, the existing patellar component can be retained if it is well-fixed, has only slight wear, and tracks well.¹,²,³,⁴,⁵,⁶,⁷ By definition, all patellar components should be removed in cases of revision TKA associated with deep periprosthetic infection. In aseptic revision TKA, patellar component retention occurs in approximately 60% to 70% of cases.⁷,⁸ Retention of a well-fixed patellar prosthesis that is of an unmatched design with the femoral trochlea has been shown to be clinically acceptable as long as the mismatch is not severe and the patella tracks well in the new femoral trochlea.³,⁸ The purpose of this chapter is to briefly review the surgical principles of management of the patella during revision TKA and in particular to emphasize the surgical techniques available for the bone-deficient patella.

It is critically important to recognize that a successful clinical outcome for patellar function and long-term durability of the patellar construct are highly dependent on proper axial and rotational positioning of the femoral and tibial components.⁹,¹⁰ This concept is most often overlooked when attempting an isolated revision of the patellar component and is one of the reasons that isolated patellar revision has been associated with an increased incidence of lateral retinacular release, suboptimal clinical outcomes, and an increased risk of postoperative complications.¹¹,¹²,¹³ In most cases, the reason that a patella has maltracking, aseptic loosening, prosthesis fracture, or excessive wear is because of associated femoral and/or tibial component malposition. Therefore, it is essential to ensure proper femoral and tibial component position before proceeding with patellar component revision and recognize that isolated patellar component revision is almost never warranted.

SURGICAL CONSIDERATIONS

During revision TKA, it is imperative that extensor mechanism disruption is avoided at all costs. Traditionally it has been common practice to evert the patella during surgical exposure; however, it is quite evident that lateral subluxation of the extensor mechanism with concomitant anterior subluxation on external rotation of the tibia from beneath the femur provides excellent exposure of the knee joint during revision TKA. It is the authors’ opinion that patellar eversion is never required during revision TKA except when the knee is fully extended. Release of all adhesions in the lateral gutter facilitates lateral subluxation of the extensor mechanism. Occasionally in particularly stiff knees, extensile exposures such as a rectus snip or tibial tubercle osteotomy may be required to safely expose the knee.

The primary indications for revision of a patellar component are listed in Table 71-1. If patellar component revision is deemed necessary, the quality and quantity of remaining patellar bone stock will determine the choice of the new implant or a variety of alternative surgical techniques. Reasons for patellar bone loss include overresection of the patella at the primary procedure, patellar osteolysis, infection, or iatrogenic bone loss associated with removal of the existing patellar implant, particularly with removal of well-fixed metal-backed patellar components. In patellar components with metal lugs, a diamond wheel cutting tool is used to side cut the lugs at junction of the baseplate.¹⁴ In the aseptic revision, it is reasonable to simply leave these lugs in place and cover them when implanting the new patellar prosthesis. Certain metal-backed patellar component designs are extremely difficult to remove without loss of significant patellar bone. In these circumstances and if the other criteria for patellar component retention are met, it seems reasonable to retain these metal-backed patellar components for the alternative of treating a patella with severe bone loss or fracture after their removal is much less desirable.

Once the prior patellar component has been removed, the patella should be prepared by removing fibrous tissue and any remaining cement deemed necessary for removal. If cement or metal lugs are well-fixed and will not interfere with fixation and positioning of another patellar implant, it is acceptable in the absence of infection to retain this material rather removing any additional patellar bone. Patellar bone stock should then be assessed with regard to quality, quantity, and location of remaining bone to determine whether there is enough bone stock to provide adequate fixation for the new component. Whenever possible, it is most desirable to implant another patellar prosthesis.

TABLE 71-1 Indications for Patellar Component Revision

Aseptic Loosening

Severe patellar osteolysis

Patellar component fracture

Moderate to severe patellar component damage

Malposition or mismatch affecting patellar tracking

Removal required due to presence of active infection

In general, remaining patellar bone stock of >8 to 10 mm occurs in approximately 85% to 90% of revision patellar procedures, and implantation of standard pegged or biconvex polyethylene patellar components can be successfully performed.⁶,⁷,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰,²¹ However, if the remaining patellar bone stock measures <8 to 10 mm, the bone-deficient patella, alternative implants, or reconstructive techniques are usually required (Table 71-2). The location and quality of remaining patellar bone, particularly the absence of peripheral rim support, dictate the choice of any given surgical technique used to address the bonedeficient patella. It is the authors’ opinion that patellectomy is never indicated during revision TKA with the sole exception being resection of a grossly necrotic and osteomyelitic patella.

TREATMENT OPTIONS AFTER PATELLAR COMPONENT REMOVAL

Adequate Remaining Patellar Bone Stock

Standard three-Pegged Polyethylene Patellar Components

With 10 to 15 mm of remaining bone, it is usually possible to prepare the patella by creating new lug holes in areas of retained cancellous bone and by using any small areas of cavitary bone loss as additional areas of cement fixation. It is preferable that there are only minimal areas of peripheral segmental bone loss, and in the remaining areas of sclerotic bone, small drill holes or ridges created with a saw or burr can be also created for additional fixation.

TABLE 71-2 Treatment Options for the Bone-Deficient Patella

Techniques Without Patellar Component Implantation
	Patellar resection arthroplasty Gull-wing osteotomy Cancellous impaction bone grafting
Techniques With Patellar Component Implantation
	Screw or pinning with cement augmentation Transcortical wiring Structural bone grafting Three-pegged porous metal monoblock patella Porous metal baseplate

FIGURE 71-1 Schematic of patella prepared for implantation of an inset biconvex patellar component.

Biconvex Polyethylene Patellar Components

If the patella has enough generalized cavitary bone loss to preclude support for a traditional pegged patellar button, the patella can be prepared to accept a polyethylene biconvex patellar component (Fig. 71-1). It has been shown that the use of a biconvex patellar component is possible for patellae with as little as 5 mm of central bone provided that there is good peripheral support of the patellar implant.²⁰ The primary disadvantage of this technique is that the overall patellar composite height is less than the normal patellar height or other techniques that restore the anatomic thickness of the patella (Fig. 71-2).²⁰ In a clinical series of 89 biconvex patellar components implanted during revision TKA, 10- and 14-year survivorship using aseptic loosening as an end point were 98% and 86%, respectively.¹⁶ In this revision series and an adjacent report using this implant during primary TKA, patellar fracture was associated with a radiographical measurement of central patellar thickness of <6 mm.¹⁶,¹⁷ Aseptic failure was associated with patellar osteonecrosis and absence of a superior rim of supporting bone.¹⁶,¹⁷

FIGURE 71-2 Merchant view radiograph of a biconvex patellar component with resultant decrease in anteroposterior patellar height.

The Bone-Deficient Patella

In approximately 10% of cases, when patellar component revision has been deemed necessary, patellar bone stock is insufficient enough that alternative treatment strategies are required.¹⁹ Severe patellar bone deficiency, i.e., <8 to 10 mm of remaining bone, has historically been a difficult problem that has adversely affected clinical outcomes of revision TKA. In the decades of the 1980s and 1990s, the most common approach was to simply remove the patellar component also known as patelloplasty or patellar resection arthroplasty.⁸,²² Over the past several decades, a variety of alternative treatment options have emerged to address patellar bone deficiency during revision TKA. These alternative reconstructive options can be classified as those that include implantation of a patellar button or those that do not include implantation of another patellar implant.

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