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.
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).
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.
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.
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.
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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