A review of 120 consecutive total knee arthroplasty patient records was made comparing oxidized zirconium with traditional cobalt chrome alloy femoral components and assessing the clinical and radiographic outcomes of each. The direct hospital implant cost for each of these implant systems was obtained and assessed in the context of any difference in patient Knee Society Scores, knee flexion, pain, probability of metal allergy, and radiographic signs of pending implant failures. At an average of 5.6-year followup (1–11-year range in both groups), the data showed no clinical or radiological variance between the two groups, while the hospital direct implant cost with the oxidized zirconium femoral component was $1900 higher than the cobalt chrome implants. Considering the cost difference between the implants and the lack of a difference in outcomes at an average of 5-6 years of followup, this paper brings into question the ability to justify the use of oxidized zirconium femoral components based solely on improved patient outcomes.
The volume of joint arthroplasty procedures performed each year has steadily increased over time, with primary total knee arthroplasty (TKA) projected to increase by 673%, from 450,000 in 2005 to 3.48 million procedures in 2030 [
Oxidized zirconium is a relatively new material consisting of a zirconium alloy (Zr-2.5Nb) core that has been oxidized to produce a zirconia (ZrO2) ceramic-like surface. Knee simulator studies have demonstrated the proposed benefits of oxidized zirconium, which include less nickel exposure and improved wear properties, such as increased hardness, decreased surface roughness, and increased resistance to abrasion, adhesion, and delamination compared to cobalt-chromium-molybdenum (CoCrMo) [
The present study assesses clinical and radiographic outcomes in a consecutive series of osteoarthritic knee patients comparing the use of a single primary TKA design with femoral components fabricated from either oxidized zirconium or CoCrMo alloy. Due to the promising results of previously published knee simulator studies [
Between May 2001 and October 2002, 120 consecutive primary TKAs were performed in 118 patients by a single surgeon and coauthor (TBP). In this institutional review board approved study, data was retrospectively collected from each patient’s pre- and annual postoperative clinical assessments. Sixteen patients were lost to death or did not achieve the minimum follow-up time of 1 year, providing a final cohort of 104 knee surgeries in 102 patients.
All patients were implanted with the same TKA design (Profix Total Knee System, Smith & Nephew, Memphis, TN, USA). This design provides the option for using one of four different modular tibial central stems; however, all patients in this study received the same short, surface-textured stem (Figure
Gross photograph of the Profix TKA (with CoCrMo alloy femoral component).
The indication for TKA was osteoarthritis for all patients. The PCL was sacrificed in all 104 cases, and a polyethylene insert incorporating an anterior-constrained articular geometry was utilized to provide anteroposterior stability. The medial parapatellar approach was used in all but 3 cases (all CoCrMo alloy cohort), which were exposed using the subvastus medial approach. The subvastus approach was performed on patients if the distal thigh circumference was small enough that it could be practically done. Patients with a larger distal thigh circumference received the medial parapatellar approach. Ten knees (50%) in the oxidized zirconium cohort and 33 knees (39%) in the CoCrMo alloy cohort underwent lateral retinacular release. This procedure was performed based on intraoperative assessment after all prosthesis components were implanted to allow optimal patella tracking. Patella resurfacing was performed in patients if they failed to meet the criteria of having proper congruency of the natural patella, maintaining 85% of normal appearing cartilage, proper tracking in the trochlear groove, and minimum patellofemoral pain at preoperative assessment. One patient in the CoCrMo cohort underwent patella resurfacing because these criteria were not met.
All patients received a short course of low-molecular weight heparin (Enoxaparin, Sanofi-Aventis, Paris, France) as well as oral warfarin for thromboprophylaxis while hospitalized. Venous thrombosis event (VTE) prophylaxis was 5 mg oral warfarin daily starting the day of surgery with low-molecular-weight heparin injections for post-op days 2 and 3 followed by a 4-week duration of 2 mg per day mini fixed-dose oral warfarin regimen based on individual patient VTE risk assessment. For the patient without higher VTE or bleeding risk assessment, once hospital International Normalized Ratio (INR) levels reached 1.2 or higher, oral warfarin was reduced to 2 mg daily and after discharge INR monitoring was not done unless signs or symptoms of bleeding occurred. For higher-risk VTE patients, higher-dose monitored oral warfarin was used (INR target = 2.0). For patients whom postsurgical hemoglobin levels dropped to 8.0–8.5 mg/dL or lower, transfusion was considered and ensued unless patient requested or other medical conditions prohibited transfusion. Physical therapy was standardized following the hospitals’ total joint rehabilitation protocols and included partial weight bearing postoperative day one and criteria for discharge including independent ambulation, performance of activities of daily living, and passive knee flexion of 80–90 degrees. A continuous passive motion machine was not used after the primary surgery. The same therapy team performed home rehabilitation for two weeks on all patients; this was followed by outpatient therapy.
A range of motion assessment was performed by the operative surgeon at all follow-up appointments and knee flexion was recorded. Patients who failed to obtain 70 degrees of flexion at 2 weeks were treated with anti-inflammatory medications and diuretics. Formal therapy was held and patients worked on knee flexion exercises at home. Radiographic assessment of the surgical knee was performed at the two- or six-week follow-up office visit. The decision to manipulate was made in the 8- to 12-week period once it was demonstrated that progress in therapy was no longer improving motion, and the patient had less than 90 degrees of flexion.
Manipulations were performed in the operating room under general or spinal anesthesia. After manipulation, patients used continuous passive motion machines at home six hours a day for a total of two weeks. Patients were seen daily by home therapy and had a follow-up visit with the surgeon two weeks after manipulation if it was required.
Patient demographics for knees implanted with femoral components fabricated from both types of materials were comparable (Table
Demographic data.
Oxidized zirconium | CoCrMo alloy | |
---|---|---|
Sample size | 20 | 84 |
Male : female | 6 : 14 | 20 : 64 |
Left : right Knee | 9 : 11 | 48 : 36 |
Age at index surgery |
|
|
Cost of entire TKA prosthesis | $5,300 | $3,400 |
In order to assess clinical and radiographic outcomes, medical records were retrospectively reviewed. Surgical approach and posterior cruciate ligament (PCL) treatment were recorded at the time of index surgery. Pre- and postoperative Knee Society Scores (KSS) [
Radiographic analysis consisted of full-length, standing anteroposterior, sunrise, and lateral view radiographs which were assessed by three independent arthroplasty surgeons for the presence of osteolytic lesions and radiolucent lines greater than 2 mm located around the femoral and tibial components. In addition to the Knee Society scoring system, poor patient outcomes were defined as probable metal allergy, persistent pain, final flexion of <90 degrees, manual manipulation, radiolucencies per serial radiographs, and revision knee surgery.
Quantitative data was analyzed using a two-sample
Implant-specific contract cost for all implants used in this study was obtained from the hospital where the surgeries were performed. The surgical technique and post-op protocol for the two groups differed only by the femoral component materials used. The cost difference was due solely to the difference in cost of the individual femoral components. The purchasing records reflected the hospital’s negotiated cost from the manufacturer for both types of prostheses implanted during the time frame in which these TKA were performed. The direct hospital-related implant cost for each type of prostheses can be found in Table
Clinical outcomes for both the oxidized zirconium and CoCrMo alloy cohorts (including a younger age subgroup within the CoCrMo alloy group) are summarized in Table
Clinical outcomes.
Oxidized zirconium age all <65 ( |
CoCrMo alloy all patients ( |
CoCrMo alloy |
|
---|---|---|---|
Followup (years) |
|
|
|
Preoperative KSS |
|
|
|
Postoperative Last followup KSS |
|
|
|
Maximum flexion (°) |
|
|
|
Knees experiencing pain at recent followup | 4 | 8 | 4 |
Patients with probable metal allergy | 1 | 0 | 0 |
Knees requiring manual manipulation | 0 | 2 | 2 |
Knees experiencing VTE | 0 | 0 | 0 |
Knees requiring revision | 0 | 0 | 0 |
Knees experiencing other complications | 0 | 2** | 1** |
**These knees experienced dehiscence following surgery, were treated with secondary wound closure, and healed uneventfully.
The summary data on knee flexion (Table
Data obtained from patients that clinically presented with pain at their most recent follow-up assessment were also recorded and are shown in Table
The records of patients reporting pain at the most recent followup were reviewed for probable metal allergy. The review consisted of the surgeon and coauthor (TBP) assessing the chart of each patient experiencing pain and noting if patients presented with at least three of the five clinical manifestations of a metal allergy defined in previous studies [
None of the patients of the present study, in either cohort, required revision surgery for any reason. Additionally, none of the patients included in the oxidized zirconium cohort were required to undergo manual manipulation, experienced a symptomatic VTE, or presented with a complication during follow-up treatment. Two knees, within the CoCrMo alloy cohort, required closed, manual manipulation following implantation. Additionally, two knees in this cohort experienced wound dehiscence two weeks following surgery. Both were treated with secondary wound closure and healed uneventfully. None of the patients in the CoCrMo alloy cohort experienced a symptomatic VTE.
Radiographic outcomes for the different material cohorts are summarized in Table
Radiographic outcomes.
Oxidized zirconium ( |
CoCrMo alloy ( |
CoCrMo alloy, age ≤65 ( |
|
---|---|---|---|
Radiolucencies | 0 | 2 | 0 |
Osteolytic lesions | 0 | 0 | 0 |
Emerging surgical techniques and biomaterials give patients and surgeons new and innovative tools for improving TKA clinical outcomes and survivorship. Oxidized zirconium is a proprietary material supported as an alternative to traditional CoCrMo alloy due to its hardness, scratch resistance, and hypoallergenic properties in femoral components of TKAs. Knee simulator studies have shown promising results for improved wear properties and biocompatibility of oxidized zirconium compared to traditional metal alloys used to fabricate TKA prostheses [
After a preliminary review of patient demographics, as well as clinical and radiographic outcomes, a subgroup within the CoCr group was established in order to more directly compare clinical effectiveness between cohorts of similar age. It could be argued that since the oxidized zirconium components were implanted in younger and more active patients, they were subjected to a more rigorous in vivo environment. However, upon comparison to a patient cohort of a similar age range, no significant statistical or clinical differences were found among patient cohorts.
Similar preoperative KSS were noted for both groups, as well as the CoCrMo alloy subgroup with an age less than 65 years. Postoperative mean KSS were also similar (
In addition to KSS, maximum flexion was measured at each most recent followup. Comparing the measurements taken at the most recent follow-up visit, the values showed a difference of 2.7 degrees (
Manifestations of metal allergy, mainly to nickel, in TKA are not well defined, and there are little publications on patient presentation of a metal allergy. Gao et al. and Thomas et al. noted that metal allergies can present as dermatitis, swelling, and pain during the first three months after surgery, and continuing for years afterward [
When the twelve patients who experienced pain at the most recent followup were compared, no statistical difference was calculated between the two materials (
When considering the clinical and radiological presentations of both types of femoral components, the results show no significant differences in outcomes between the oxidized zirconium and CoCrMo alloy cohorts. Reviewing data containing Knee Society Scores, knee flexion, probability of a metal allergy, presentations of pain and other complications, and number of revisions demonstrates that there are no statistical or clinical differences in values obtained from both cohorts. This also holds true with respect to radiographic outcomes, in which neither patient group presented with osteolysis nor radiolucencies is directly associated with femoral component material or failure. Summation of this average 5.6-year in vivo study does not corroborate with the improved clinical outcomes the knee simulator studies suggest for the benefits of oxidized zirconium over CoCrMo alloy. Our statistical analysis and data provide similar data to the clinical studies reporting on the Genesis II (Smith & Nephew, Memphis, TN, USA) TKA, which showed similar early clinical outcomes between femoral components fabricated from the two materials of interest [
The cost comparison component of this study shows a substantial difference in price between the oxidized zirconium and the CoCrMo alloy femoral components. At the time of implantation, the hospital direct implant cost of the total knee implant system with an oxidized zirconium femur was $1,900 (55%) more expensive than that of the same TKA with a CoCrMo femoral component.
With no apparent added clinical benefit of the oxidized zirconium components compared to the CoCrMo alloy components, the extra costs of oxidized zirconium may not be warranted. Other medium-term studies focusing on the clinical outcomes of patients with oxidized zirconium and CoCrMo alloy TKAs have also shown similar results and that patients do not have a preference for one implant over the other in bilateral TKA series with each knee receiving implants differing only by the femoral oxidized zirconium versus CoCrMo components [
There are some inherent limitations of this retrospective review of clinical and radiographic outcomes. The number of patients in each patient cohort may not be sufficient to appreciate a potentially very small incidence of adverse events. Additionally, the statistically significant age difference between cohorts posed a challenge when comparing clinical and radiographic outcomes. This challenge was addressed by creating a subgroup of one cohort with a similar age range as the oxidized zirconium cohort in order to effectively compare two groups with hypothetically similar activity levels. The average follow-up time of both patient cohorts of approximately 5-6 years may not be sufficient to fully capture potential adverse events that occur at more long-term followup. However, wear-induced osteolysis can present radiographically within the first 3 postoperative years [
Previous in vitro wear analysis of oxidized zirconium versus CoCrMo showed an improved wear-resistant bearing surface and decreased metal ion release. This suggests that oxidized zirconium implants were likely to have a more favorable clinical outcome compared to traditional cobalt chromium implants. The present study of the two biomaterials took into account Knee Society Scores, knee flexion, incidence of a metal allergy, pain, and other reported complications. Both pre- and postoperative KSS and pain scores had no statistical variation between the two component materials. There was no clinical difference in knee flexion, number of patients experiencing pain, complications, or radiographic findings of implant related osteolysis. Long-term follow-up studies are required to conduct a true cost analysis comparison between these two biomaterials. However, considering that the hospital purchase cost for the oxidized zirconium implant at the institution where these surgeries were performed was $1900 (55%) more than the CoCrMo alloy implant, and with no added clinical benefit at early followup, the oxidized zirconium implant may not worth the additional cost.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Ms. Sandy Fowler and Mrs. Katharine A. Pace are acknowledged for their contributions to the paper and data preparation.