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SECTION I: SYMPOSIUM: Papers Presented at the 2006 Meeting of the Knee Society

Porous Tantalum Patellar Augmentation: The Importance of Residual Bone Stock

Ries, Michael, D; Cabalo, Adam; Bozic, Kevin, J; Anderson, Martin

Section Editor(s): Laskin, Richard S MD, Guest Editor

Author Information
Clinical Orthopaedics and Related Research: November 2006 - Volume 452 - Issue - p 166-170
doi: 10.1097/01.blo.0000229359.27491.9f

Abstract

Bone loss commonly occurs after failure of a primary total knee arthroplasty (TKA). Femoral and tibial defects encountered during revision TKA (revTKA) have been well described.8,24 However, patellar bone loss also can be present, which may compromise the integrity of the extensor mechanism.

When patellar bone loss is not severe, revision with a cemented patellar component can provide satisfactory results.22 However, trabecular bone is necessary for optimal cement fixation, and for management of more severe patellar bone loss when the patella is fragmented or a “bony shell,” cemented revision may result in early loosening.9,22 Several methods have been described to treat severe patellar bone loss during revTKA.12 Resection patellar arthroplasty or leaving a “bony shell” is a surgical option when the existing bone stock is not sufficient to support a revision patellar implant.1,2,19,20,23 Gull-wing osteotomy is a technique to improve patellar tracking without restoring the deficient bone stock.25 Cancellous bone allograft placed into a synovial pouch is effective in improving bone stock.11,12 However, the patellar bone graft is subject to patellar compressive forces, and full thickness of the patella may not be restored. The average thickness of the patella is 24 mm, which is important in maintaining effective strength of the extensor mechanism.13 Whole patellar allografting restores normal patellar height and has been described to treat extensor mechanism discontinuity and patellar bone loss associated with fracture after TKA.5,6,17 However, allograft complications of resorption or rejection can occur.

Alternatively, patellar height and thickness can be restored using a trabecular metal patellar component.3,16,18 Trabecular metal is made from porous tantalum, and provides a rough, highly porous surface, similar to cancellous bone, with favorable bone ingrowth properties.4 Early results with a trabecular metal patellar component for management of severe bone loss after TKA have been favorable.16,18 Animal studies indicate fibrous tissue also can attach directly to the metal surface, which could provide tendon or ligament fixation directly to the implant.10,15 However, clinical confirmation of ligament or tendon attachment to trabecular metal has not been established.

Since clinical studies demonstrate favorable results of trabecular metal augmentation of bone defects and animal studies have demonstrated direct fibrous tissue attachment to the trabecular metal surface, we presumed a trabecular metal implant could be used to replace absent bone stock in a patellectomized knee.

METHODS

This study is a retrospective double cohort case series. Patients were assigned to one of two groups based on the amount of patellar bone stock present at the time of surgery. The same patellar implant was used in both groups. The clinical results were compared between the two groups to determine the effect of residual patellar bone stock. During the study period we treated patients with severe patellar bone loss who were not candidates for conventional cemented patellar component revision with a porous tantalum augmentable patellar implant (Augmentable Patella™, Zimmer, Warsaw, IN). We retrospectively reviewed 16 consecutive patients (18 knees) who had a primary or revision TKA using a porous tantalum augmentable patellar implant. There were 10 men and six women. The average age at the time of surgery was 63 years (range, 46-88 years). The preoperative diagnosis was prior patellectomy in six knees (five of which had previous TKAs), isolated patellar component loosening after TKA in five knees, failed TKA in six knees, and osteoarthritis with severe patellar bone loss in one knee. One patient had bilateral simultaneous patellar component reconstructions for patellar osteonecrosis with patellar component loosening. One patient who underwent trabecular metal patellar component reconstruction after TKA with prior patellectomy had loosening of the patellar component and was treated with revision to another trabecular metal implant.

Patients were divided into two groups based on the amount of patellar bone loss present at the time of reconstruction. Group 1 (seven knees) had no residual patellar bone stock in contact with the trabecular metal patellar component from prior patellectomy or severe osteolysis. Group 2 (11 knees) had a minimum of 50% of the implant covered by bone for implant fixation. Bone fragments that were present but either excised during the surgical procedure or not in contact with the patellar component were not considered to provide any portion of the implant bone coverage.

The trabecular metal patellar reconstruction was performed by cementing the polyethylene patellar component into the trabecular metal surface. After removal of the failed patellar component, remaining patellar bone was prepared with a reamer for the trabecular metal implant. The implant was sutured to the undersurface of the extensor mechanism using nonresorbable Number 2 TICRON® sutures (Tyco, Gosport, England) through the peripheral holes in the implant. The sutures were placed through drill holes in the remaining patellar bone if there was bone covering the implant, and otherwise were placed directly through soft tissue and tied over the dorsal surface of the extensor mechanism. The trabecular metal implant was positioned slightly proximal to the level of the joint line and oriented in the center of the femoral component trochlear grove.

After surgery, patients were treated with a routine total knee replacement protocol that included unrestricted passive and active assisted range of motion, isometric quadriceps strengthening, and ambulation using crutches or a walker weight bearing as tolerated.

Clinical and radiographic assessment was performed at 6 weeks, 3 months, 6 months, and each year after surgery. Patients were examined at each time interval by their treating surgeon (MR, KB, MA). Knee Society questionnaires were completed by the patients preoperatively and at each out patient follow up appointment. Radiographs were evaluated by each treating surgeon and an independent observer (AC) who was unaware of the patient's clinical outcome. Loosening was defined by: (1) radiographic displacement of the patellar component of more than 2 cm on serial patellar or lateral radiographs; (2) palpation of a freely mobile patellar component within the knee on physical exam; and (3) erosion of the patellar component through the extensor mechanism with associated extensor mechanism discontinuity. Preoperative and postoperative Knee Society scores, extensor lag, range of motion, and complications were compared between the two groups.

RESULTS

All seven tantalum patellar implants in Group 1 migrated and loosened within 1 year. In two of these seven knees soft tissue necrosis overlying the patella developed, leading to extensor mechanism disruption. One of these patients (Fig 1A-E) was treated with removal of the trabecular metal implant and allograft patellar augmentation of the extensor mechanism to span the soft tissue defect. However, allograft infection and failure resulted in removal of the allograft and well-fixed TKA, implantation of an antibiotic cement spacer, and medial gastrocnemius transposition to the extensor mechanism, followed by delayed revision TKA. The other patient was treated with removal of the trabecular metal patellar component and primary repair of the extensor mechanism with quadriceps tendon turndown. Extensor mechanism failure occurred by 6 weeks after repair. At last followup the patient had a 90° extensor lag and declined additional surgery. One patient who developed loosening of a trabecular metal component had revision to another trabecular metal implant, which subsequently loosened. The remaining four trabecular metal implants in Group 1 loosened and migrated inferiorly to the recess between the tibial component and patellar tendon. These patients have intermittent effusions and anterior knee pain after activity, but have declined additional surgery to remove the implant.

Fig 1A
Fig 1A:
E. (A) A preoperative lateral radiograph of a 71-year-old woman who had prior patellectomy and rotating hinge revTKA is shown. The patient reported anterior knee pain, difficulty rising from a seated position, and episodes of “giving way” associated with extensor mechanism weakness during walking. Quadriceps strength was diminished compared to the contralateral knee, but extensor lag was 0°. (B) A preoperative patellar radiograph shows small bony exostosis in the extensor mechanism and absence of patellar bone stock. (C) A lateral radiograph performed six weeks after surgery shows placement of a trabecular metal patellar implant into the extensor mechanism soft tissue sleeve. The patient had marked improvement in quadriceps strength, pain, and functional activities. (D) A postoperative patellar radiograph performed 6 weeks after surgery shows central placement of the patellar component in the femoral component troch-lear groove. (E) A lateral radiograph performed 6 months after surgery shows displacement of patellar component associated with pain and quadriceps weakness. (F) A patellar radiograph performed 6 months after surgery shows tilting of the patellar component that eroded through the extensor mechanism. Necrosis of the soft tissue over the patella developed, leading to extensor mechanism discontinuity treated with removal of the trabecular metal patellar component and allograft augmentation of the extensor mechanism. Allograft failure and deep infection occurred with subsequent removal of the allograft. (G) An intraoperative view of the extensor mechanism defect after failed trabecular metal patellar component and allograft is shown.

Ten of the 11 patellar implants in Group 2 have remained stable with no further revision at minimum 12-month followup. One patient in Group 2 developed patellar loosening in association with an infection. An 88-year-old man in Group 2 who had previous two-stage revTKAs for infection developed recurrent infection with loosening of the patellar component 1 year after trabecular metal patellar reconstruction. This was treated by removal of the trabecular metal implant, tibial insert exchange, soft tissue débridement, and retention of the remaining components.

Two patients in Group 2 had fractured patella at the time of reconstruction although the extensor mechanism soft tissue sleeve remained intact. Migration of the lateral patellar facet occurred in both patients, but the trabecular metal implants remained stable on the residual anterior and medial patellar bone surface.

The average preoperative Knee Society knee and function scores were 58.3 and 51.7 respectively for Group 1, and 57.2 and 64.2, respectively, for Group 2. The average preoperative extensor lag in Group 1 was 12° and the average preoperative extensor lag in Group 2 was 7°. The average preoperative range of motion in Group 1 was 106° (range, 80°-130°), and in Group 2 the average preoperative range of motion was 105° (range, 60°-120°). For Group 1, postoperative KSS scores were not calculated because of the high failure rate. For Group 2, the average postoperative Knee Society knee and function scores were 87.2 and 78.3, respectively (excluding the one failure attributable to infection). The average postoperative extensor lag was 0.5° and the average postoperative range of motion was 121° (range, 100°-130°).

DISCUSSION

We have utilized a number of techniques to reconstruct the patella for TKA associated with severe patellar bone loss, including cemented patellar component revision, morselized or structural allograft, and patelloplasty with relatively unsatisfactory results. When the trabecular metal patellar component became available for clinical use, we incorporated it into our practice in the hope it would provide a favorable solution to this difficult problem. However, our results indicate successful trabecular metal patellar reconstruction requires the implant be attached to bone rather than soft tissue.

Patellar bone loss after total knee arthroplasty can be associated with patellar component loosening, patellar maltracking or subluxation, and extensor mechanism weakness or discontinuity. Treatment of a failed patellar component usually depends on the quality of the remaining bone stock. Cemented patellar component revision has been noted to provide satisfactory results when bone stock is adequate for cemented implant fixation.22 However, loss of cancellous bone often precludes cement fixation. Other alternatives include bone grafting with morselized or structural allograft, patellectomy, or retention of the bony shell with osteotomy, if necessary, to facilitate its articulation in the femoral component trochlear groove.1,2,11,12,19,20,23,25 Cementless trabecular metal patellar reconstruction has been proposed as a method to achieve biologic fixation to the remaining host patellar bone stock or soft tissue and restore patellar function.3,16,18

The trabecular metal implant consists of a rough, porous tantalum surface with favorable bone ingrowth characteristics, similar to cancellous bone.4 Clinical and animal studies have shown reliable bony fixation to the trabecular metal.3,4

Bone ingrowth into porous coatings depends on many factors including the pore size, implant material, stability of the implant bone interface, and variable biologic host factors.7,14,21 Stable ingrowth of bone into porous coated implants occurs when there is less than 40 μm of micro-motion at the bone implant interface, whereas fibrous tissue develops if there is 150 μm of micromotion leading to instability of the implant.14 However, animal studies have indicated stable soft tissue attachment to tantalum also can be achieved, which may permit use of trabecular metal in the treatment of tendon or ligament avulsion or for patellar augmentation after TKA with prior patellectomy when not enough bone stock is available for attachment to the metal surface.10,15

Because of the favorable bone ingrowth characteristics of porous tantalum, and laboratory evidence indicating direct tendon attachment to porous tantalum can be achieved, we considered this implant a viable treatment for managing severe patellar bone loss after TKA, including prior patellectomy with no residual bone stock. Our results have been favorable when some bone stock remains for biologic fixation to the implant, and support similar findings reported by other authors.3,16,18 However, we have had extremely poor results with direct soft tissue attachment to the implant. In our series of seven patients with no residual patellar bone stock, all seven implants loosened within 1 year. In two patients, both of whom had functioning extensor mechanisms before surgery, catastrophic necrosis of the extensor mechanism developed after trabecular metal patellar reconstruction. One of these became infected after attempted allograft extensor mechanism reconstruction, necessitating removal of a previously well-functioning TKA, and multiple secondary reconstructive salvage procedures.

Despite the interesting findings we have presented, this study has several limitations. First, clinical and radiographic outcomes were reviewed retrospectively, and indications for assignment to each treatment group were not specified a priori. Second, the total number of patients evaluated was small (16 patients, 18 knees). Finally, the duration of followup was relatively short (12 months). The high rate of failure in Group 1 does not, however, require longer followup. Further, our study represents the first attempt to define the appropriate indications for the use of tantalum augmentation in the setting of patellar bone loss following TKA.

The reasons for the discrepancy among animal studies of soft tissue ingrowth directly into trabecular metal and our clinical experience are unclear. Our patients had multiple prior surgeries and the periarticular soft tissues may have had poor vascularity, which could have limited the potential for ingrowth into the tantalum surface. Lewallen et al15 placed a canine patellar tendon between two tantalum washers, which may provide more stability at the tendon tantalum interface than we achieved in the clinical setting with suture repair to stabilize the implant to the extensor mechanism. However, this type of stability at the tendon metal interface may not be feasible with conventional TKA surgical techniques. The implant stability in our patients may have been improved if we had restricted postoperative range of motion. However, this may also result in knee stiffness. Our experience indicates with currently available implants and suture fixation, direct attachment of ligament or tendon to trabecular metal in revTKA patients does not seem feasible. Although we continue to use trabecular metal patellar reconstruction when residual bone is present for implant fixation, we no longer use this technique for cases in which the majority of the fixation surface is composed of soft tissue.

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