Midterm Results of Porous Tantalum Femoral Cones in Revision Total Knee Arthroplasty

Potter, G. David III MD; Abdel, Matthew P. MD; Lewallen, David G. MD; Hanssen, Arlen D. MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.15.00874
Scientific Articles
Abstract

Background: Severe bone loss during a revision total knee arthroplasty (TKA) remains a challenging problem. The purpose of this study was to determine the midterm clinical outcomes, fixation as evaluated radiographically, and survivorship of tantalum femoral cones used during revision TKAs in patients with severe femoral bone loss.

Methods: From 2003 to 2011, 159 tantalum metaphyseal femoral cones were implanted in 157 patients at a single institution. Knee Society scores, radiographic results, and implant survivorship were analyzed. Two patients were lost to follow-up. Nineteen died of causes unrelated to the surgery but had been followed for >2 years and thus were included in the study. The mean age at the time of the index surgery was 64 years, and 82 patients were male. The mean duration of follow-up was 5 years (range, 2 to 10 years).

Results: The mean Knee Society score increased from 47 preoperatively to 65 at the most recent follow-up evaluation (p = 0.1). Radiographically, all 134 unrevised femoral cones were seen to be well fixed without any evidence of loosening. At 5 years, 23 cones had been revised: 14 because of infection, 6 because of aseptic loosening of the cone (all in hinged TKAs in patients with a Type-3 defect), and 3 because of ligamentous instability. The 5-year survivorship was 96% when the end point was revision of the cone due to aseptic loosening, 84% when it was revision of the cone for any reason, and 70% when it was any reoperation.

Conclusions: In what we believe to be the largest series of such implants, femoral cones provided a durable and reliable option for metaphyseal fixation during revision TKA with severe femoral bone loss. Aseptic failure of the femoral cone was associated with use of a hinged TKA in a patient with a Type-3 bone defect.

Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Author Information

1Department of Orthopedic Surgery, Mayo Clinic, Rochester, Minnesota

E-mail address for G.D. Potter III: potter.gorden@mayo.edu

E-mail address for M.P. Abdel: abdel.matthew@mayo.edu

E-mail address for D.G. Lewallen: lewallen.david@mayo.edu

E-mail address for A.D. Hanssen: hanssen.arlen@mayo.edu

Article Outline

Severe bone loss during a revision total knee arthroplasty (TKA) remains a challenging problem. Morselized cancellous bone graft, filling with cement, small-fragment structural allograft, thicker polyethylene inserts, and modular augments attached to revision prosthetic designs can be used to address smaller osseous defects. Structural allografts, impaction bone-grafting, custom prosthetic components, and specialized segmental hinged knee components can supplement large osseous defects1-4.

The metaphyseal areas of the distal part of the femur and proximal part of the tibia are particularly attractive locations for fixation during revision TKA given that they usually have some available host bone stock and remain well vascularized. Porous tantalum metaphyseal cones have a potential advantage of improved biologic fixation because of high porosity (75% to 80%), an interconnected pore space, and a low modulus of elasticity (3 MPa) similar to that of cancellous bone5-7. These features allow tantalum cones to fill bone defects while tolerating physiological loads. Porous femoral cones have been utilized in patients with Anderson Orthopaedic Research Institute (AORI) Type-2B or Type-3 bone defects to enhance bone stock for distal fixation1,5,8. The cones allow the surgeon to size and position the implant for the best fit for the individual defect. The metaphyseal cones can be used with multiple different revision knee systems.

Short-term (2 to 4-year) clinical follow-up indicated that these femoral cones provide structural support and the potential for long-term biologic fixation and durable reconstructions5. The purpose of this paper was to determine the midterm clinical outcomes, durability of the fixation as seen radiographically, and survivorship of tantalum femoral cones used during revision TKAs with severe distal femoral bone loss.

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Materials and Methods

We retrospectively identified 159 consecutive revision TKAs (performed in 157 patients) in which a femoral cone had been utilized from January 2003 to December 2011 at a single institution. All patients had a highly porous tantalum metaphyseal femoral cone (Trabecular Metal; Zimmer) implanted by 1 of 2 senior authors (A.D.H. and D.G.L.). During the same time frame, 2,546 total knee revisions had been performed without the use of a femoral cone at our institution, and 858 of them had been done by the 2 senior authors. The mean clinical follow-up period was 5 years (range, 2 to 10 years). The study protocol was reviewed and approved by our institutional review board.

Patients were identified with the use of our institutional total joint database, in which follow-up data are regularly recorded for all patients who have undergone a total joint arthroplasty. The database includes demographic characteristics, clinical follow-up dates, implant information, reoperations, and complications. If a patient is unable to return for a regularly scheduled follow-up appointment (at 1, 2, and 5 years and every 5 years thereafter), a comprehensive questionnaire form and a request to submit radiographs is sent to the patient, supplemented with a telephone questionnaire if required. Clinical outcomes were assessed with the Knee Society score, with scores of 80 to 100 classified as excellent, 70 to 79 as good, 60 to 69 as fair, and <60 as poor9.

Of the 157 patients, 75 (48%) were female and 82 (52%) were male. Their mean age was 64 years (range, 24 to 85 years) at the time of cone placement (Table I). Nineteen (12%) had died by the time of this review for reasons unrelated to the surgical intervention but had been followed for ≤2 years and were thus included in the study. Two patients (1%), who had had the operation at least 2 years before the time of our review, were not seen after their first follow-up appointment at 6 months and were designated lost to follow-up.

All 157 patients had had knee surgery prior to cone placement, with a mean of 4 (range, 1 to 23) prior procedures. The reasons for the index femoral revision included reimplantation due to prior infection (75), aseptic loosening (56), osteolysis (26), implant failure (16), instability (11), and fracture (1). Twenty-two knees had more than 1 of these indications. All revisions due to prior infection were performed in 2 stages, with 1 of the 2 senior authors also performing the first stage (prior to implantation of the cone).

Preoperative radiographic review and intraoperative assessment identified bone loss according to the AORI Bone Defect Classification system8. Type-1 defects have intact and sufficient metaphyseal cancellous bone stock to maintain the joint line. Type-2A defects have damaged metaphyseal cancellous bone of only 1 femoral condyle that requires small augments or bone graft to reconstruct the bone loss. Type-2B defects have variable amounts of metaphyseal bone loss in both the medial and the lateral femoral condyle. A defect is classified as Type 3 when the bone is deficient in a major portion of the femoral metaphysis with involvement of both condyles and at the level of, or proximal to, the epicondyles. The surgeons identified Type-2B and 3 defects at the time of femoral revision surgery and determined whether the bone loss was substantial enough to use porous tantalum cones for metaphyseal reconstruction. All bone defects in this study were identified as Type 2B (127; 80%) or Type 3 (32; 20%).

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Surgical Technique

The surgical technique has been described previously by our group5. Preoperative planning was based on clinical examination and radiographic review. Identifying the anticipated bone defect preoperatively allows for planning and for augments appropriate to address the bone loss to be made available. The intraoperative assessment of the bone defects confirms the preoperative plan and is performed with the trial sizes that correspond to the appropriate femoral cone to define where remaining bone stock requires contouring. Size-specific impactors are used to press-fit the tantalum cone into final position. Remaining voids around the implant are augmented with either bone graft putty or cancellous bone to prevent extrusion of bone cement between the cone and adjacent host bone. The femoral stem is fully fixed to the metaphyseal-diaphyseal portion of the femur through the cone with unpressurized antibiotic-impregnated cement. The stem lengths ranged from 30 to 155 mm, with 100 mm being the most common stem length (45% of the knees), in this study. No uncemented stems or hybrid fixation (cement in the metaphysis and a cementless stem) were used in this study.

The prosthetic constructs used with the cone at the time of the revision included varus-valgus constrained components in 96 cases, hinged constructs in 56 cases, and posterior-stabilized components in 7 cases. The level of constraint was dictated by ligamentous and soft-tissue status, not the mere presence of bone defects or revision components.

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Radiographic Analysis

Preoperative radiographs were reviewed to classify the defect and for preoperative planning. The immediate postoperative radiographs as well as each successive radiograph were reviewed by 2 observers for implant alignment, migration, and osseointegration as well as any associated complications such as fracture or osseous resorption. Radiographs, including full-length and short leg standing anteroposterior and lateral views, were analyzed with use of the Knee Society radiographic review criteria10,11. Bone in-growth series, in which fluoroscopy is used to ensure that the x-ray beam is not tangential to the bone-component interface, were utilized to evaluate for radiolucency around the components12. Radiographs made at the time of the latest follow-up were available and used for analysis of all patients in the cohort (Figs. 1 through 5).

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Statistical Analysis

Demographic data were reported using descriptive statistics, with the number (percentage) and mean (range) as appropriate. The two patients who did not have a minimum of 2 years of follow-up were excluded from the clinical and radiographic analyses, but all joints were included in the estimation of prosthetic survival using the Kaplan-Meier method. Any patient who died or was lost to follow-up was censored from the denominator. The changes in continuous clinical outcomes between the preoperative and most recent follow-up evaluations were assessed using a paired t test. Survivorship was estimated using Kaplan-Meier survival analysis with 3 end points: revision due to aseptic loosening of the femoral cone, revision of the cone for any reason, and a reoperation for any reason. The alpha level was set at 0.05 for significance.

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Results

Clinical Outcomes

The mean Knee Society score of the entire cohort improved from 47 (range, 0 to 90) preoperatively to 65 (range, 6 to 94) at the most recent follow-up evaluation (p = 0.1). The mean preoperative flexion contracture was 1° (range, −5° of hyperextension to 20° of flexion contracture), and the mean preoperative maximum flexion was 98° (range, 40° to 130°). At the time of the most recent follow-up, the mean postoperative flexion contracture was 2° (range, −20° of hyperextension to 60° of flexion contracture) and the mean maximum flexion was 103° (range, 50° to 130°) (p = 0.14).

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Radiographic Results

Immediate postoperative radiographs demonstrated good apposition at the bone-porous cone interface in all cases. Subsequent follow-up imaging revealed evidence of osseointegration with reactive osseous trabeculation at the interface in all 134 unrevised cases (Fig. 2). At the latest follow-up evaluation, all unrevised femoral cones were well fixed and without any evidence of loosening or migration.

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Survivorship

The Kaplan-Meier estimates demonstrated a 5-year survivorship of 96% (95% confidence interval [CI] = 92% to 100%) when the end point was revision of the femoral cone due to aseptic loosening of the cone, 84% (95% CI = 78% to 92%) when it was revision of the femoral cone for any reason, and 70% (95% CI = 62% to 79%) when it was a reoperation for any reason (Fig. 6). At 5 years, 23 femoral cones had been revised: 14 because of infection, 6 because of aseptic loosening of the femoral cone, and 3 because of ligamentous instability. Of the 23 revised cones, 16 were loose (6 aseptically and 10 in association with infection). Thirteen of the 14 knees in which the cone was revised for infection had had a previous arthroplasty-related infection prior to cone placement. All 6 cones revised because of aseptic loosening were part of a hinged TKA and associated with a Type-3 bone defect. Another 22 patients underwent additional procedures that did not involve the femoral cone, which was left in place. These included reoperations for infection (7), aseptic tibial loosening (4), instability (4), traumatic fracture (3), extensor mechanism disruption (3), and flexion contracture release (1).

Of the 75 patients for whom infection was the indication for the index revision TKA, 20 underwent additional surgical procedures after placement of the porous cone; 12 of them had 1 subsequent operation and 8 had ≥2. In this group of 75 patients, the 5-year survivorship was 95% (95% CI = 87% to 100%) with revision due to aseptic loosening of the femoral cone as the end point, 81% (95% CI = 70% to 92%) with femoral cone revision for any reason as the end point, and 68% (95% CI = 57% to 82%) with a reoperation for any reason as the end point. The survivorship in the subgroup with infection was virtually the same as that in the entire cohort, suggesting that failure rates of porous femoral cones do not increase in the setting of infection.

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Complications

At the time of the most recent follow-up, 4 (2.5%) of the patients had complications, including polyethylene dissociation (2) and wound complications (2). The femoral cone was noted to be well fixed at the time of all subsequent operations for complications. Eight patients eventually underwent an amputation because of persistent infection, and 6 of the 8 cones were noted to be well fixed at the time of amputation. All patients who underwent an amputation had a history of TKA-related infection prior to the cone placement.

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Discussion

While there are multiple established methods for reconstruction of small to moderate-sized bone defects, there is no general consensus regarding the best method for femoral revision in the setting of very large osseous deficiencies. Highly porous cones provide the potential for biologic metaphyseal fixation and are effective secondarily as structural supports for the femoral prosthesis. The high friction coefficient at the interface of these porous cones with host bone combined with the cone geometry facilitates initial mechanical stability. The volume porosity of tantalum is similar to that of trabecular bone. Bobyn et al.6 demonstrated 70% to 80% porous ingrowth by 52 weeks, and Findlay et al.7 showed that the porous nature increases osteoblast expression and osseous ingrowth. In our series, which we believe to be the largest to date, radiographic evidence as well as intraoperative assessment at the time of subsequent surgical procedures demonstrated osseous fixation in the metaphysis. We found femoral cones to be a durable and reliable option for restoration of metaphyseal fixation during revision TKA with severe bone loss. Aseptic failure of the femoral cone was associated with use of hinged TKAs in patients with a Type-3 bone defect.

Metaphyseal sleeves and cemented or cementless stems are alternatives for fixation in revision TKA. Sleeves provide metaphyseal fixation while avoiding the stress shielding of cemented diaphyseal stems. Graichen et al.13 reported results similar to those in our study, with revision rates due to aseptic loosening of 7% and a survivorship of 98% at 3.6 years. Furthermore, they reported significant improvement in the range of motion and knee function. Fehring et al.14 found that cemented stems were more stable than press-fit stems at 61 months and reported no revisions due to aseptic loosening.

The clinical outcomes and range of motion improved in our series (although not to a statistically significant extent), which is consistent with numerous studies5,15-19. A previous study by our group5 demonstrated significant improvement in the Knee Society scores and no revisions of 24 highly porous femoral cones at a mean of 33 months. Rao et al.17 demonstrated good osseointegration and significant improvement in functional outcomes 3 years after use of tantalum cones for AORI Type-2 and 3 defects. These findings are also similar to early and midterm results with tibial cones1,18,20-22.

In a study of 33 tantalum cones in 27 patients, Lachiewicz et al.15 found an osseointegration rate of 97% and a significant improvement in Knee Society scores at a mean of 3 years. In a recent study by Schmitz et al.18, 38 patients demonstrated improvements in Knee Society scores, range of motion, and visual analog scores for pain at 37 months. Our findings were consistent with these studies, in that our cohort showed an increased range of motion postoperatively and a clinically relevant (although not statistically significant) improvement in Knee Society scores at 5 years. Osseointegration has been demonstrated radiographically at up to 3 years postoperatively in multiple prior smaller studies, and the findings were similar to our observation that all unrevised cones were well fixed at the time of the latest follow-up15,16,18.

In agreement with other published studies, we demonstrated excellent survivorship with only 4% of the cones requiring revision due to aseptic loosening. One notable trend in our study was the association of aseptic loosening with the use of a hinged TKA prosthesis for a Type-3 bone defect. Schmitz et al.18 reported that 2 of 38 cones required revision due to aseptic loosening, and Villanueva-Martínez et al.19 reported only 1 revision due to recurrent infection in a cohort of 29 cones; otherwise, all of the cones were well fixed at the time of the latest follow-up. In our patient cohort, the estimated survivorship free of any revision of the femoral cone was 84% at 5 years.

This study has limitations, which include bias associated with retrospective chart reviews. Also, although we believe that our follow-up was the longest of any study of a similar cohort, still longer follow-up is needed. Lastly, because the mortality rate was non-trivial, the Kaplan-Meier estimates may slightly overestimate failure rates as compared with the cumulative incidence of failure.

In summary, highly porous tantalum femoral cones provided reliable metaphyseal support and fixation, which was used as a base for TKA components. The femoral cones had reasonable midterm survival and provided reasonable functional outcomes for patients with a Type-2B or Type-3 bone defect. Because the rate of loosening was higher in patients with a Type-3 defect and a hinged TKA, we suggest that development of implants with different shapes, sizes, and methods of preparation be considered for Type-3 femoral defects. Continued follow-up is required to assess the long-term durability of these revisions done in patients with severe bone loss.

Investigation performed at the Mayo Clinic, Rochester, Minnesota

A commentary by Kelly G. Vince, MD, is linked to the online version of this article at jbjs.org.

Disclosure: There were no external sources of funding used in this investigation. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work and “yes” to indicate that the author had a patent and/or copyright, planned, pending, or issued, broadly relevant to this work.

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