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  Patellar bone grafting in the setting of revision total knee arthroplasty has been designed to restore patellar bone stock and to potentially improve function in patients with severe patellar bone deficiency or prior patellectomy.

  
  
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  The patellar bone-grafting procedure is performed as an alternative to either patella resection arthroplasty or patellectomy.

  
  
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  The outcome of any patellar intervention is highly dependent on optimal rotational positioning of the femoral and tibial components for proper patellar kinematics.

  
  
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  Bone grafting to a thickness 5 to 7 mm greater than the final planned patella thickness allows for compaction of bone graft and appropriate long-term patella thickness.

Introduction

During revision total knee arthroplasty (TKA), loose, malpositioned, or damaged patellar components with good residual bone stock are found approximately 40% to 50% of the time. However, the magnitude of patella bone loss occasionally precludes adequate fixation of another patellar prosthesis. There may be substantial loss of patellar bone stock due to asymmetric or excessive bone resection during earlier arthroplasties, osteolysis resulting from wear debris, prior infection, or bone loss during removal of a well-fixed patellar prosthesis. Results obtained with retention of existing patellar component or revision with good bone stock are superior to other treatment options. Therefore, it is generally preferable to insert another patellar implant as a part of a patellar revision, which is possible 30% to 50% of the time. Traditional treatment options in this setting have included either patellectomy or retention of the remaining patellar osseous shell.

Although primary or revision TKA in patients with a prior patellectomy is an acceptable procedure, functional outcomes have been inferior to those of TKA performed in patients with a patella. Patellectomies performed as a part of a revision TKA have been associated, in mid-term to longer-term follow-up, with persistent anterior knee pain associated with lateral subluxation and fragmentation of the patellar shell, inferior clinical knee scores, and difficulties with weakness or delayed disruption of the extensor mechanism. As a consequence, most authors have not performed a patellectomy in conjunction with a revision knee replacement and have attempted to retain the patellar osseous shell. Unfortunately, retention of the osseous shell (patellar resection arthroplasty) has also been associated with inferior clinical results, as reflected by lower knee scores, persistent retropatellar pain, patellar maltracking, difficulty with stair climbing, and delayed patellar fragmentation. Options other than patellar bone grafting have been described. The gull-wing osteotomy has been reported as a reliable option in the setting of revision TKA where there is a patella that cannot be resurfaced. However, it does not add bone stock to the patella and may be used in situations of more limited bone loss.

The concept of patellar bone grafting was initially described for TKAs in patients who had undergone a prior patellectomy. A structural bone graft 2.5 cm wide and 1 cm thick was secured in a subsynovial pouch of the patellar tendon in the anatomic location of the previous patella. The purpose of this procedure was to increase the moment arm of the extensor mechanism and to improve the stabilizing characteristics of the knee joint in the sagittal plane. On the basis of the results in six patients (seven knees), it was concluded that patellar bone grafting improved quadriceps leverage and was useful in restoring extensor mechanism function. The concept of tissue being sewn into the peripheral patellar rim to contain bone graft within the patellar shell evolved from the description by Cave and Rowe. They described a surgical procedure in which the degenerated surface of the patella is covered with a portion of the infrapatellar fat pad, which is elevated and sewn peripherally into the patellar rim to be interposed between the patella and the femoral trochlea.

Patellar bone grafting in the setting of revision TKA has been designed to restore patellar bone stock and potentially to improve function in patients with severe patellar bone deficiency or prior patellectomy. The procedure of patellar bone grafting described in this chapter relies on the presence of an osseous patellar shell, impaction of cancellous bone graft into the defect, and containment of the bone graft with soft tissue secured into the peripheral patellar rim. The patellar bone-grafting procedure is performed as an alternative to either patellar resection arthroplasty or patellectomy.

Indications and Contraindications

The indications for patellar bone grafting are a prior patellectomy or determination at the time of revision surgery that the magnitude of patella bone loss precludes fixation of another patellar implant, provided that the patient has properly aligned components and an intact extensor mechanism. Contraindications include patella bone stock that would support a prosthesis without grafting, ongoing infection, severe extensor mechanism maltracking that cannot be corrected at the time of revision, and disruption of the extensor mechanism.

Preoperative Evaluation

Pertinent preoperative clinical examination of patellar function in the setting of revision TKA includes assessment of range of motion (ROM), quadriceps strength, extensor mechanism competency (and observation of extensor lag), instability, and patellar tracking. Skin changes and effusion may provide additional evidence of local infection.

Laboratory tests in patients presenting with a painful TKA must include a complete blood count with differential, erythrocyte sedimentation rate (ESR), and C-reactive protein level (CRP). Any elevation in laboratory values raises the suspicion for infection. Aspiration is advisable whenever joint fluid is present. The fluid is sent for cell count, Gram staining, and aerobic, anaerobic, and fungal culture. The aspirate is examined for signs of purulence, bleeding, metallic or polyethylene debris, or change in viscosity.

Clinical findings of extensor mechanism incompetency, in particular an extensor lag, should alert the clinician that additional procedures such as extensor mechanism augmentation may be required.

Radiographs should include anteroposterior standing radiographs, Merchant patellar radiographs, and lateral radiographs made with fluoroscopic positioning and use of magnification markers to allow accurate measurements of patella height. Any evidence of osteolysis, polyethylene wear, component failure, loosening, or migration should be noted. The patella bone loss in patients considered for patellar bone grafting is categorized as severe cavitary patella bone loss with only the anterior cortex and variable amounts of the peripheral patellar rim remaining (or prior patellectomy).

Surgical Techniques

After standard medial parapatellar arthrotomy, thorough débridement of the joint, and opening of the medial and lateral gutters, the femoral and tibial components are addressed. It is important to achieve proper axial and rotational alignment. Proper external rotation of the femoral component relative to the epicondylar axis and appropriate rotation of the tibial component allow for central tracking of the patella. Lateral releases or medial capsular imbrications, or both, are occasionally indicated for enhanced tracking.

The patellar bone stock is evaluated intraoperatively. If patellar bone grafting is entertained based on preoperative radiographs, it is helpful to retain the pseudomeniscus of scar tissue and most of the peripatellar fibrotic tissue to facilitate suture fixation of the tissue flap to the patellar rim. The patellar shell is prepared by removing all fibrous membrane in the crevices of the remaining patella bone. The tissue flap is created from one of several sources. This may be a large flap of peripatellar fibrotic tissue or a free tissue flap obtained from either the suprapatellar pouch or the fascia lata in the lateral gutter of the knee joint. If no periarticular tissue of sufficient size or strength can be obtained, allograft fascial graft may be used, although this is not preferred.

The tissue flap is sewn to the peripheral patellar rim and peripatellar fibrotic tissue with multiple, nonabsorbable size-0 sutures to provide a watertight closure. A small purse-string opening is left in one portion of the tissue flap repair to facilitate delivery of bone graft into the patellar defect. Cancellous autograft is harvested from the metaphyseal portion of the central part of the femur during preparation of the femur for the revision implant. In the absence of locally available cancellous autograft, cancellous allograft bone has been used. The bone graft is prepared by morselizing the bone into small fragments approximately 5 to 8 mm in height and width; this size facilitates tight impaction of the bone graft into the patellar shell–tissue flap construct. The bone graft is tightly impacted through the opening of the fascial flap into the patellar bone defect, with enough volume so that the final patellar construct has a height approximately 5 to 7 mm thicker than the final desired patellar thickness.

The tissue flap is then closed completely to contain the bone graft within the patellar shell. The adequacy of the suture repair is examined to ensure that the tissue flap securely contains the impacted bone graft. The peripatellar arthrotomy site is provisionally repaired with several sutures or towel clips to mold the patellar construct in the femoral trochlea as the knee is placed through the full ROM. Postoperative rehabilitation is the same as in the usual protocol after revision TKA.

Complications

The basic principles of revision TKA technique minimize postoperative complications. Restoration of axial alignment and soft tissue balance is of primary importance. Additionally, rotational alignment of the tibial and femoral components allows for proper alignment and function of the extensor mechanism. Subluxation or dislocation of the patella can occur with improper alignment or soft tissue imbalance. Fragmentation of the bone-grafted patella can occur and may lead to suboptimal results (i.e., anterior knee pain and reduced extensor mechanism strength). Infection is always of concern in revision procedures, especially when allograft is used.

Outcomes

The senior author has reviewed his mid-term results of nine patellar bone-grafting procedures in the setting of revision TKA with an average follow-up of 37 months. Knee Society scores improved from 39 to 91 points for function and from 40 to 84 points for pain. The point of greatest patellar thickness measured intraoperatively before grafting ranged from 7 to 9 mm. Patellar thickness on immediate postoperative Merchant radiographs averaged 22 mm, whereas at the time of final follow-up, patellar thickness averaged 19.7 mm, demonstrating excellent retention of patellar thickness.

Two cases required revision in this series. The first was for arthrofibrosis and instability. At the time of revision, a small core biopsy of patellar bone obtained from the region of the medial patellar facet revealed empty lacunae and fragments of dead bone spicules without evidence of revascularization. However, the patellar tissue flap was intact and had the gross appearance of smooth fibrocartilage. The second case required revision at 55 months after patellar bone grafting because of aseptic loosening of the femoral component. At the time of revision, restoration of patellar bone stock was observed. Along with femoral revision, a new patellar component was placed in the new viable patellar bone.

Pearls and Pitfalls

Postoperatively, the retropatellar surface of the construct undergoes continued remodeling against the prosthetic femoral trochlea, with the tissue flap serving as an interpositional tissue arthroplasty. This retropatellar remodeling allows the patellar construct to assume the shape of the prosthetic trochlear groove during ROM movements of the knee. The design and shape of the femoral trochlea may be an important factor in proper remodeling.

The outcome of any patellar intervention is highly dependent on optimal rotational positioning of the femoral and tibial components for proper patellar kinematics. This fact should be taken into strong consideration during the decision-making process for tibial and femoral revision. Isolated patellar revision has been shown to have a high incidence of associated perioperative complications. It may be that, with isolated patellar bone grafting, the graft is vulnerable to femoral or tibial malrotation, which would predispose toward lateral patellar subluxation and abnormal remodeling of the patellar bone graft.

In a study comparing patellar replacement with retention of the osseous shell, the latter procedure yielded lower-quality results, although it should be noted that patients in whom the patella was not suitable for another patellar implant may have had other factors that predisposed toward a lower-quality result. In view of these findings, the results obtained with patellar bone grafting described here appear to be more comparable with the outcomes after implantation of another patellar implant rather than retention of a patellar osseous shell.

Postoperative Care

Rehabilitation in the setting of patellar bone grafting is dictated by the rehabilitation needs of the other components of the revision procedure. ROM exercises may begin immediately as tolerated.

Summary

Severe patellar bone deficiency that precludes fixation of another patellar implant has remained a rare yet important problem during revision TKA. In contrast with the treatment alternative of patellectomy or retention of the osseous shell, patellar bone grafting imparts the potential for restoration of patellar bone stock and may improve functional outcome by facilitating patella tracking and improving quadriceps leverage. The procedure is simple to perform and does not require sophisticated instrumentation or a long learning curve. On the basis of the current satisfactory short-term to mid-term clinical results, the authors believe that this surgical procedure is an important addition to the armamentarium of surgeons performing revision TKA.