Patellar Bone Loss

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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.

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

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

• 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.