Metal Backed Fixed-Bearing Unicondylar Knee Arthroplasties
Metal Backed Fixed-Bearing Unicondylar Knee Arthroplasties
From September 2006 to November 2010 a total of 132 medial UKAs were consecutively implanted in 106 patients at our institution. The study group consisted of 53 men (65 knees) and 53 women (67 knees). Half of the surgeries (66 UKAs) were performed on the left and the other half on the right knee, 26 patients received a UKA on both sides, 20 of them in a single surgery. The average age of the patients at surgery was 69 ± 9 years. Patients with other implants such as total knee arthroplasties (TKA, n = 658) or lateral UKA (n = 4) operated in the study period were not included in the study. The study was conducted in accordance with good clinical practice and ethical approval was obtained Ethikkomission Nordwest und Zentralschweiz (EKNZ 2015-229). All patients gave informed consent for the study. All patients were followed prospectively at six weeks, one, two and five years after surgery according to the in-house register documentation. Data analysis was performed retrospectively. The mean follow-up was 3.4 ± 1.0 years.
The Zimmer Unicompartmental High Flex Knee Systemâ„¢ (ZUK; Zimmer, Winterthur, Switzerland) is a fixed-bearing metal back UKA developed from the M/G Unicompartmental Knee Systemâ„¢ (Zimmer, Winterthur, Switzerland) and can be implanted with a MIS approach (spacer-block technique). However, clinical and radiological data for this new implant system is scarce.
Medial compartment arthritis was diagnosed on conventional (weight-bearing short anterior-posterior (ap), true lateral, sunrise view and full-length hip-knee-ankle (HKA)) radiographs. The function of the anterior cruciate ligament (ACL) was confirmed prior to surgery by Lachman testing and in doubt an MRI was performed to confirm ACL integrity. In case of an insufficient ACL a TKA was implanted. A tourniquet control was used in all cases. Surgery was performed or supervised by a team of nine different orthopaedic consultants. A MIS medial parapatellar approach, incising only the medial capsule from the medial patella pol down to the tibial tuberosity, was used combined with the spacer-block technique. All components were cemented using Palacos R + G bone cement (Haereus Medical, Weinheim, Germany). Postoperatively a continuous passive mobilization machine was used and full weight bearing exercises were initiated from the first postoperative day under guidance of a physical therapist.
Three different survival analyses were performed using Kaplan-Meier analysis with three different endpoints defining failure: (a) revision with exchange of any component for any reason, (b) revision due to aseptic loosening and (c), a worst case scenario, where all cases with progressive radiolucencies were additionally counted to the cases with aseptic loosening because they were assumed to lead to aseptic loosening. Revision was defined as an exchange, addition or removal of any component for any reason, reoperation was defined as any intervention even without exchanging any of the components. Any complications related to the implant were prospectively recorded. Clinical records were screened for additional information.
Clinical outcome was prospectively assessed using the American Knee Score (AKS) at 6 weeks, 1, 2 and 5 years.
The AKS is comprised of two parts: The knee score addressing pain, stability and range of movement (ROM) and the function score which examines function, with particular reference to stair climbing, walking distance or whether walking aids are needed. For each section, the maximum score is 100 points.
Furthermore an Oxford Knee Score (OKS) was assessed during the 5-year follow-up control. For patients with a follow-up less than 5 years the OKS was completed during a telephone interview. For the OKS each question is weighted between zero and four, with four being the best outcome, leading to an overall possible score between 0 and 48 points.
Radiological follow-up was scheduled according to a standardized prospective protocol at 6 weeks, 1, 2 and 5 years of follow-up including weight-bearing short anterior-posterior (ap), true lateral and patella sunrise view radiographs. Additional full-length hip-knee-ankle (HKA) radiographs were performed preoperatively as well as one and five years postoperatively. The mean radiological follow-up was 2.5 ± 1.2 years.
Radiographic evaluation was performed as described by Sarmah et al. (Fig. 1). In detail, we measured the alignment of the femoral component to the femoral axis in the ap (A) and lateral radiographs (C) as well as the alignment of the tibial component to the tibial axis in the ap (B) and lateral radiographs (D). Changes in the alignment of the component between the first and last postoperative radiographs were calculated. Additionally, the mechanical femoral axis of the leg was measured on the HKA radiograph. All measurements were performed by one independent observer (JB) within a month using digital measurement software (ims, Imagic Bildverarbeitung AG, Glattbrugg, Switzerland). To control the intraobserver reliability one independent observer (JB) remeasured ten randomized blinded radiographic series.
(Enlarge Image)
Figure 1.
Radiograph of the lower extremity of a 68 year old male patient with a ZUK implanted, illustrating the measurement of the femoral condyle angle (a), the tibial plateau angle (b), the femoral flexion (c) and the tibial tilt (d). Positive values are defined as valgus, extension or positive tilt
Radiolucencies were divided into non-progressive and progressive radiolucencies. The non-progressive radiolucencies were defined as (1) less than two millimeters thick, (2) well defined and (3) with a parallel radiodense line. The progressive radiolucencies were defined (1) as thicker than two millimeters and (2) not well defined. The radiolucent zones were classified as defined by Kalra et al. and adapted to the UKA implant (Fig. 2). All radiographs were examined for progressive and non-progressive radiolucencies by two reviewers (JB and MC) and were defined as a consensus if both found radiolucencies. Finally all radiographs were checked for progressive osteoarthritis (OA) in the lateral or patellofemoral knee compartment as defined by Kellgren and Lawrence.
(Enlarge Image)
Figure 2.
Anterior-posterior (a) and lateral (b) radiographs of a 92 year old male patient with the ZUK in situ illustrating the radiolucent zones which were used in the present study
Statistical analysis was performed using IBM SPSS Statistics 21 (IBM Corporation, Somers, New York). A Shapiro-Wilk test was performed to verify normal distribution of the data. A dependent-sample t-test was used to compare paired metric parameters, a Wilcoxon test for not normally distributed data.
To compare outcome parameters in two groups either the independent-sample t-test or the Mann-Whitney-Test was performed. Correlation between ordinal-scaled parameters was determined using Spearman's correlation. A Chi-square test was used to compare nominal data between two groups. The level of significance was set at p < 0.05.
For the intra-observer reliability of the performed radiological values, the two-way random intraclass correlation coefficient (ICC 2,1) with single measurement and absolute agreement were calculated for each parameter and presented with 95 % confidence interval. The interpretation of the ICC values was graded using the classification scheme of Munro, as low (0.26–0.49), moderate (0.50–0.69), high (0.70–0.89) and very high (0.90–1.00).
The radiological and demographic data were expressed as a mean value ± standard deviation if the data was normally distributed, if not we used median and range.
Methods
Study Design
From September 2006 to November 2010 a total of 132 medial UKAs were consecutively implanted in 106 patients at our institution. The study group consisted of 53 men (65 knees) and 53 women (67 knees). Half of the surgeries (66 UKAs) were performed on the left and the other half on the right knee, 26 patients received a UKA on both sides, 20 of them in a single surgery. The average age of the patients at surgery was 69 ± 9 years. Patients with other implants such as total knee arthroplasties (TKA, n = 658) or lateral UKA (n = 4) operated in the study period were not included in the study. The study was conducted in accordance with good clinical practice and ethical approval was obtained Ethikkomission Nordwest und Zentralschweiz (EKNZ 2015-229). All patients gave informed consent for the study. All patients were followed prospectively at six weeks, one, two and five years after surgery according to the in-house register documentation. Data analysis was performed retrospectively. The mean follow-up was 3.4 ± 1.0 years.
Implant
The Zimmer Unicompartmental High Flex Knee Systemâ„¢ (ZUK; Zimmer, Winterthur, Switzerland) is a fixed-bearing metal back UKA developed from the M/G Unicompartmental Knee Systemâ„¢ (Zimmer, Winterthur, Switzerland) and can be implanted with a MIS approach (spacer-block technique). However, clinical and radiological data for this new implant system is scarce.
Indication, Surgical Technique and Early Postoperative Care
Medial compartment arthritis was diagnosed on conventional (weight-bearing short anterior-posterior (ap), true lateral, sunrise view and full-length hip-knee-ankle (HKA)) radiographs. The function of the anterior cruciate ligament (ACL) was confirmed prior to surgery by Lachman testing and in doubt an MRI was performed to confirm ACL integrity. In case of an insufficient ACL a TKA was implanted. A tourniquet control was used in all cases. Surgery was performed or supervised by a team of nine different orthopaedic consultants. A MIS medial parapatellar approach, incising only the medial capsule from the medial patella pol down to the tibial tuberosity, was used combined with the spacer-block technique. All components were cemented using Palacos R + G bone cement (Haereus Medical, Weinheim, Germany). Postoperatively a continuous passive mobilization machine was used and full weight bearing exercises were initiated from the first postoperative day under guidance of a physical therapist.
Survival Analysis
Three different survival analyses were performed using Kaplan-Meier analysis with three different endpoints defining failure: (a) revision with exchange of any component for any reason, (b) revision due to aseptic loosening and (c), a worst case scenario, where all cases with progressive radiolucencies were additionally counted to the cases with aseptic loosening because they were assumed to lead to aseptic loosening. Revision was defined as an exchange, addition or removal of any component for any reason, reoperation was defined as any intervention even without exchanging any of the components. Any complications related to the implant were prospectively recorded. Clinical records were screened for additional information.
Clinical Outcome
Clinical outcome was prospectively assessed using the American Knee Score (AKS) at 6 weeks, 1, 2 and 5 years.
The AKS is comprised of two parts: The knee score addressing pain, stability and range of movement (ROM) and the function score which examines function, with particular reference to stair climbing, walking distance or whether walking aids are needed. For each section, the maximum score is 100 points.
Furthermore an Oxford Knee Score (OKS) was assessed during the 5-year follow-up control. For patients with a follow-up less than 5 years the OKS was completed during a telephone interview. For the OKS each question is weighted between zero and four, with four being the best outcome, leading to an overall possible score between 0 and 48 points.
Radiological Outcome
Radiological follow-up was scheduled according to a standardized prospective protocol at 6 weeks, 1, 2 and 5 years of follow-up including weight-bearing short anterior-posterior (ap), true lateral and patella sunrise view radiographs. Additional full-length hip-knee-ankle (HKA) radiographs were performed preoperatively as well as one and five years postoperatively. The mean radiological follow-up was 2.5 ± 1.2 years.
Radiographic evaluation was performed as described by Sarmah et al. (Fig. 1). In detail, we measured the alignment of the femoral component to the femoral axis in the ap (A) and lateral radiographs (C) as well as the alignment of the tibial component to the tibial axis in the ap (B) and lateral radiographs (D). Changes in the alignment of the component between the first and last postoperative radiographs were calculated. Additionally, the mechanical femoral axis of the leg was measured on the HKA radiograph. All measurements were performed by one independent observer (JB) within a month using digital measurement software (ims, Imagic Bildverarbeitung AG, Glattbrugg, Switzerland). To control the intraobserver reliability one independent observer (JB) remeasured ten randomized blinded radiographic series.
(Enlarge Image)
Figure 1.
Radiograph of the lower extremity of a 68 year old male patient with a ZUK implanted, illustrating the measurement of the femoral condyle angle (a), the tibial plateau angle (b), the femoral flexion (c) and the tibial tilt (d). Positive values are defined as valgus, extension or positive tilt
Radiolucencies were divided into non-progressive and progressive radiolucencies. The non-progressive radiolucencies were defined as (1) less than two millimeters thick, (2) well defined and (3) with a parallel radiodense line. The progressive radiolucencies were defined (1) as thicker than two millimeters and (2) not well defined. The radiolucent zones were classified as defined by Kalra et al. and adapted to the UKA implant (Fig. 2). All radiographs were examined for progressive and non-progressive radiolucencies by two reviewers (JB and MC) and were defined as a consensus if both found radiolucencies. Finally all radiographs were checked for progressive osteoarthritis (OA) in the lateral or patellofemoral knee compartment as defined by Kellgren and Lawrence.
(Enlarge Image)
Figure 2.
Anterior-posterior (a) and lateral (b) radiographs of a 92 year old male patient with the ZUK in situ illustrating the radiolucent zones which were used in the present study
Statistics
Statistical analysis was performed using IBM SPSS Statistics 21 (IBM Corporation, Somers, New York). A Shapiro-Wilk test was performed to verify normal distribution of the data. A dependent-sample t-test was used to compare paired metric parameters, a Wilcoxon test for not normally distributed data.
To compare outcome parameters in two groups either the independent-sample t-test or the Mann-Whitney-Test was performed. Correlation between ordinal-scaled parameters was determined using Spearman's correlation. A Chi-square test was used to compare nominal data between two groups. The level of significance was set at p < 0.05.
For the intra-observer reliability of the performed radiological values, the two-way random intraclass correlation coefficient (ICC 2,1) with single measurement and absolute agreement were calculated for each parameter and presented with 95 % confidence interval. The interpretation of the ICC values was graded using the classification scheme of Munro, as low (0.26–0.49), moderate (0.50–0.69), high (0.70–0.89) and very high (0.90–1.00).
The radiological and demographic data were expressed as a mean value ± standard deviation if the data was normally distributed, if not we used median and range.
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