Knee Surg Sports Traumatol Arthrosc (2014) 22:3157–3162 DOI 10.1007/s00167-014-3147-6
KNEE
Tibial component alignment and risk of loosening in unicompartmental knee arthroplasty: a radiographic and radiostereometric study P. Barbadoro • A. Ensini • A. Leardini • M. d’Amato • A. Feliciangeli • A. Timoncini F. Amadei • C. Belvedere • S. Giannini
•
Received: 11 November 2013 / Accepted: 18 June 2014 / Published online: 28 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose Unicompartmental knee arthroplasty (UKA) has shown a higher rate of revision compared with total knee arthroplasty. The success of UKA depends on prosthesis component alignment, fixation and soft tissue integrity. The tibial cut is the crucial surgical step. The hypothesis of the present study is that tibial component malalignment is correlated with its risk of loosening in UKA. Methods This study was performed in twenty-three patients undergoing primary cemented unicompartmental knee arthroplasties. Translations and rotations of the tibial component and the maximum total point motion (MTPM) were measured using radiostereometric analysis at 3, 6, 12 and 24 months. Standard radiological evaluations were also performed immediately before and after surgery. Varus/ valgus and posterior slope of the tibial component and tibial– femoral axes were correlated with radiostereometric micromotion. A survival analysis was also performed at an average of 5.9 years by contacting patients by phone. Results Varus alignment of the tibial component was significantly correlated with MTPM, anterior tibial sinking, varus rotation and anterior and medial translations from radiostereometry. The posterior slope of the tibial component was correlated with external rotation. The survival
P. Barbadoro (&) A. Ensini M. d’Amato A. Feliciangeli A. Timoncini F. Amadei S. Giannini 1st Ortopaedic-Traumatologic Clinic, Istituto Ortopedico Rizzoli, University of Bologna, Via Pupilli 1, 40136 Bologna, Bo, Italy e-mail: [email protected] A. Leardini C. Belvedere S. Giannini Movement Analysis Laboratory and Functional-Clinical Evaluation of Prostheses, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, Bologna, Bo, Italy
rate at an average of 5.9 years was 89 %. The two patients who underwent revision presented a tibial component varus angle of 10° for both. Conclusions There is correlation between varus orientation of the tibial component and MTPM from radiostereometry in unicompartmental knee arthroplasties. Particularly, a misalignment in varus larger than 5° could lead to risk of loosening the tibial component. Level of evidence Prognostic studies—retrospective study, Level II. Keywords Unicompartmental knee arthroplasty (UKA) Tibial component fixation Tibial component alignment Radiostereometry (RSA) Maximum total point motion (MTPM)
Introduction The main current problem for unicompartmental knee arthroplasty (UKA) is the apparent low survival rate, although there are conflicting reports even about interpretation of these figures in the literature and in national registries. Some authors have described survival rates of between 85 and 98 % for UKA 10 years after surgery, which is a similar rate to that of total knee arthroplasty (TKA) [16, 25]. The longest recorded follow-up for UKA in a young, active patient was 31 years [11]. However, in the Norwegian knee arthroplasty register, a 2.0 times (95 % CI, 1.6–2.5) higher risk of failure at 10 years was recorded for UKA compared with that of TKA [15] and the Finnish arthroplasty register reports a survival rate of 60 % for UKA, and 80 % for TKA at 15 years. From a survival analysis in UKA versus corresponding TKA in three different designs, UKA was not even recommended in the treatment of unicompartmental osteoarthritis [23].
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Therefore, the correct indications, an accurate surgical technique and surgical experience in UKA [9, 29] are very important for the clinical and survival success of the implant. The surgical indications for UKA are as follows: osteonecrosis of the medial condyle or isolated unicompartmental tibial–femoral osteoarthritis; a correctable deformity in varus of\10° or in valgus\14°; a flexion contracture of\5° and a knee flexion[100° [10, 25]. UKA can be a good surgical solution even if there is cartilage damage to the contralateral compartment and the patella-femoral joint, even when it is asymptomatic [3]. Anterior cruciate ligament deficiency does not seem to affect UKA survivorship [4], although some authors consider it a relative contraindication to UKA surgery. A posterior slope of more than 7° should be avoided, particularly if the anterior cruciate ligament is insufficient at the time of implantation [1]. The surgical success of UKA depends on prosthesis component alignment, fixation and soft tissue integrity [2, 22, 24, 34]. The tibial cut is a crucial step. A recent multicenter retrospective study in 418 failed UKAs showed that isolated aseptic loosening of the tibial component is the main cause of UKA failure and that nearly one half of these failures occurred within the first 5 years [14]. An increased risk of UKA revision was reported at 2 and 5 years after surgery and the risk of revision at 2 years is correlated with the surgeon volume [9]. Medial UKA failure can be caused by marked under-correction (postoperative varus of more than 7°), which can lead to residual pain, or overcorrection due to deterioration of the contralateral compartment [25]. Five factors were statistically associated with UKA revision: young patients; thin polyethylene; longer polyethylene insert shelf age, varus of the tibial component and varus of the post-operative hip–knee– ankle angle [8]. In this paper, a reduction in the varus of the medial tibial plateau with respect to the pre-operative angle was found to reduce the risk of revision, but the critical value of varus beyond which the stability of the tibial component may be put at risk was not quantified. Varus– valgus misalignments of the femoral component do not seem to affect the knee kinematics [20]. In this study, the tibial component alignment was correlated with its fixation in UKA, considering RSA measures and the survival rate at mid-term follow-up. The hypothesis is that a tibial malalignment in the frontal and sagittal planes, i.e., varus and posterior slopes, compromises the tibial component fixation in medial UKA.
Materials and methods This is a retrospective study performed on the same population of a previous study from these authors [13]. The population had been composed of twenty-three patients
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who underwent the implantation of a primary OptetrakÒ UKA (Exactech, Gainesville, Florida, USA), from November 2006 to October 2008. Eighteen patients of this initial population, of which we had full RSA data collected until 2 years of follow-up, returned for the present study. The size of the patient population analysed in this study meets the criteria for achieving differences in measurements with 80 % statistical power and a-level of 0.05. The surgical technique targeted the restoration of the same varus–valgus angle and the same posterior slope of the pre-operative radiographs of the tibia; however, a reduction in the posterior slope was performed on one patient (from 8° to 2°) because of the insufficiency of the anterior cruciate ligament, to ensure better joint stability in flexion. Before cementing the final components, four tantalum spherical markers 0.8 mm were inserted into the tibial metaphysis and four into the tibial component polyethylene, for the following RSA. In the pre-operative period, a clinical evaluation using the International Knee Society (IKS) scoring system and weight-bearing anterior– posterior and lateral radiographs of the knee were performed in all patients. The clinical analysis and the radiographs were repeated together with RSA post-operatively and at 3, 6, 12, and 24 months [13]. After these assessments, clinical evaluations using IKS and standard radiological analysis via weight-bearing anterior–posterior and lateral radiographs of the knee were performed every year of follow-up. Three angles were measured on the postoperative radiographs: the anatomical, i.e. tibial–femoral, axis, the varus–valgus of the tibial component in the coronal plane and the posterior slope of the tibial component in the sagittal plane. These were correlated with the RSA data, which are indicative of tibial component migration. In particular, the maximum total point motion (MTPM), the amount of three-dimensional translation of the marker that moves the most at each follow-up, was used as the predictive index of tibial component instability and loosening. An implant with continuous migration after the first year, i.e. an MTPM increment larger than 0.2 mm between the first- and the second-year post-operative, was defined instable [30]. The accuracy of our RSA system was described elsewhere [13]. Age, sex, pre-operative and final follow-up (at an average of 5.3 years) clinical values (IKSKnee and Function score), post-surgical tibial–femoral axes, posterior slope and varus/valgus tibial angles and RSA parameters were statistically evaluated. For the latter parameters, we considered translations along and rotations about the three anatomical axes at 24 months of follow-up, in addition to MTPM at 3, 6, 12 and 24 months of followup. Patients were contacted by telephone at a mean of 5.9 years (min. 4.5, max. 6.5 years) after surgery; a survival analysis was performed at this time for all eighteen patients, taking revision surgery as the end point.
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Statistical analysis
test was significant (p \ 0.05), Pearson’s correlation was performed to investigate the relationships between different parameters. Statistical analysis was carried out using the statistical package for the social sciences (SPSS) software version 15.0 (SPSS Inc., Chicago, USA).
The RSA data were correlated with radiographs based measurements, clinical parameters, age and sex. When the
Results
This study was approved by the Ethical Committee of Rizzoli Orthopaedic Institute–Bologna University (registration number 11/06).
Table 1 The anatomical axis and the tibial component alignment in sagittal and frontal plane for all the eighteen patients is reported Val?/varF–T axes
P.S. tibial component
Val?/vartibial component
1
2
3
-4
2
4
7
-10
3
2
3
-3
4
-1
4
2
5
-2
4
-10
6
1
8
-7
7
5
8
-4
8
5
3
-3
9 10
2 6
4 8
-1 -2
11
-3
9
0
12
-2
3
-4
13
5
7
-1
14
6
2
-1
15
0
3
-7
16
3
2
-4
17
4
4
-4
18
2
3
-5
A statistically significant correlation was found between the varus angle of the tibial component and MTPM, for each follow-up (all with p \ 0.005 and r [ 0.7); particularly at 24 months p was \0.0001 and r = 0.86). The posterior slope angle of the tibial component was found correlated with external rotation from RSA at 24 month follow-up (p = 0.02, r = 0.62). Correlations were also found between the varus angle of the tibial component and the RSA-based rotations in the sagittal and frontal planes, and translations along the mediolateral and the anteroposterior axes. In particular, the Pearson correlation values for the varus angle were as follow: anterior sinking (p = 0.03, r = 0.61), varus rotation (p = 0.001, r = 0.79), medial (p = 0.01, r = 0.58) and anterior (p = 0.04, r = 0.67) translations. No correlations were found between age, sex, tibial–femoral axis, clinical score and RSA data. All radiological data of the eighteen patients measured on the immediate post-operative radiographs are reported in Table 1. A trend was observed between the varus angle of the tibial component and the difference of MTPM between the first and the second year of follow-up (Fig. 1). From the correlation analysis of the present population it can be deduced, according to an established criterion [30], that the
Fig. 1 The trend line highlights the association between the varus of the tibial component and its migration between the first and the second year after surgery. The grey area can be considered acceptable for the implantation of the tibial component
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second- to first-year MTPM difference of more than 0.2 mm corresponds to varus misalignment of the tibial component larger than 5°. One patient showed MTPM higher than 1.5 mm already at 3 months follow-up, which remained higher than 1.6 mm at 12 and at 24 months of follow-up, though not accompanied initially with concerning clinical scores, IKSKnee and Function being, respectively, 70 and 60 at 2-years follow-up. However, these scores worsened to, respectively, 23 and 35 at 6 years follow-up, and the patient underwent revision of the UKA to TKA. During revision surgery, the tibial component showed to be loosened. The tibial component had been implanted with a 10° varus angle, as measured on the post-operative radiographs. According to the definition [30], another patient was at risk of failure at 2 years of follow-up; this patient underwent in fact revision surgery with a TKA at 5 years and 5 months after UKA implantation due to aseptic loosening of the tibial component. Again, after initial satisfactory IKS-Knee and Function scores, i.e. 100 at 2, 3 and 4 years of follow-up, these worsened to, respectively, 53 and 55 at 5 years. The tibial component had been implanted with a 10° varus angle also in this patient. These were the only two patients with clinical results worsening and revision surgery indications. The overall survival rate at an average of 5.9 years was 89 %.
Discussion The most important finding of the present study was the observed correlation between the varus angle of the tibial component and its migration, i.e. MTPM from RSA, and the correlation between the posterior slope of the tibial component and its external rotation from RSA. The patient who underwent revision at about 6 years after surgery presented an early migration, higher than 1.6 mm at 12 months of follow-up, which should be considered at risk in fact by a recent systematic review, though about TKA [28]. This early migration indeed caused an early mobilization of the tibial component, thus confirming that a lack of primary fixation can lead later to implant failure. The other patient who underwent revision was already considered instable at 2 years of follow-up, according to RSA-based thresholds [30]. It is interesting here to point out that these two patients presented 10° of varus of the tibial component on the post-operative radiographs. The cemented TKA have a better early fixation, but a greater risk of future aseptic loosening than cementless TKA [26, 27]. In TKA, a few degrees of component malalignment will cause leg malalignment and consequently failure of the prosthesis; in UKA, there is a higher tolerance in range of
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alignment [19]. Varus malalignment in the mechanical axis (hip–knee–ankle angle in the frontal plane) does not effectively compromise the functional outcome [18], and the progression of osteoarthritis in the lateral compartment is not influenced by the post-operative alignment [12]. Some authors considered a moderate under-correction of both the mechanical and anatomical axes, with a residual varus deformity of 3°–5° in the latter, as the surgical goal to avoid osteoarthritis progression in the contralateral compartment and early loosening of the implant [5, 7]. It has been shown that tibial–femoral alignment can influence polyethylene wear and subsidence of the tibial component [33]. Correction of the varus mechanical axis, reducing the varus angle of the medial tibial component and reducing or reproducing the tibial plateau posterior slope is the correct surgical technique to prevent UKA failure [8]. The recommended range of implantation for the femoral component is between ±10° varus/valgus and between ±10° flexion/extension. The recommended range of implantation for the tibial component is between ±5° varus/valgus and between 2° and 12° posterior slope (±5° superior/inferior tilt, considering 7° of posterior slope as a reference value). Within these ranges, the orientation of the components does not affect the outcomes [17]. The posterior slope is important to increase flexion and femoral rollback and to improve the stress distribution at the bone–tibial component interface. Hernigou and Deschamps found a correlation between anterior tibial translation and posterior slope of the tibial component and recommended a posterior slope of between 3° and 7° [19]. Some authors considered a tibial cut perpendicular to the epiphyseal axis [5, 6]. Swienckowski and page reported poor clinical results for UKA when the tibial component was placed in varus inclination [32]. A finite element analysis about the tibial component inclination in UKA revealed that the varus angle of the tibial component increased the yielding area, due to excessive compressive stress on the proximal end of the medial diaphyseal cortex. To optimize stress distribution at the implant–bone interface, thus minimizing the yielding area, it is advisable to implant the tibial component at a slight valgus inclination (2°–4° valgus), without changing the leg alignment. An excessive posterior inclination should be avoided to minimize bone stress [21, 31]. The limitations of the present study are those typical of standard RSA, i.e. X-ray exposure, critical identifications and tracking of the tantalum markers. In addition, this is a retrospective study in a limited number of patients. This is mainly because UKA has specific surgical indications, and patients are selected accordingly, and because of the predictive power of the RSA, so that not many patients are required to assess the risk of UKA aseptic loosening. In the future, a study on a larger group of UKA with tibial component aseptic loosening could be undertaken to verify the correlations found.
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As described about TKA [28], it is here confirmed that high migration, i.e. MTPM, of the tibial component at 1 year of follow-up is another RSA indicator of UKA instability. In addition, the higher risk of aseptic loosening determined by an increase in MTPM of more than 0.2 mm between the first and the second year of follow-up [30] is also here confirmed. However, the present UKA survival rate of nearly 90 % at almost 6 years can be considered more than satisfactory. The clinical relevance of this work is mainly for UKA implantation: during surgery attention should be given to carefully avoid excessive varus and posterior slope in performing of the tibial bone cut.
Conclusion
9.
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14.
In UKA, tibial component misalignment in varus was found correlated with its overall migration in the secondyear follow-up. In particular, according also to standard recommendations, risk of loosening of the tibial component is high for misalignments in varus larger than 5°. These results were corroborated by the present clinical assessment and survival analysis of this series. Conflict of interest of interest.
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The authors declare that they have not conflict
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Knee Surg Sports Traumatol Arthrosc (2014) 22:3157–3162 30. Ryd L, Albrektsson BE, Carlsson L, Dansgard F, Herberts P, Lindstrand A, Regner L, Toksvig-Larsen S (1995) Roentgen stereophotogrammetric analysis as a predictor of mechanical loosening of knee prostheses. J Bone Joint Surg Br 77(3): 377–383 31. Sawatari T, Tsumura H, Iesaka K, Furushiro Y, Torisu T (2005) Three-dimensional finite element analysis of unicompartmental knee arthroplasty: the influence of tibial component inclination. J Orthop Res 23(3):549–554 32. Swienckowski J, Page BJ 2nd (1989) Medial unicompartmental arthroplasty of the knee. Use of the L-cut and comparison with the tibial inset method. Clin Orthop Relat Res 239:161–167 33. Weale AE, Murray DW, Baines J, Newman JH (2000) Radiological changes five years after unicompartmental knee replacement. J Bone Joint Surg Br 82(7):996–1000 34. Whiteside LA (2005) Making your next unicompartmental knee arthroplasty last: three keys to success. J Arthroplasty 20(4 Suppl 2):2–3
KNEE
Tibial component alignment and risk of loosening in unicompartmental knee arthroplasty: a radiographic and radiostereometric study P. Barbadoro • A. Ensini • A. Leardini • M. d’Amato • A. Feliciangeli • A. Timoncini F. Amadei • C. Belvedere • S. Giannini
•
Received: 11 November 2013 / Accepted: 18 June 2014 / Published online: 28 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose Unicompartmental knee arthroplasty (UKA) has shown a higher rate of revision compared with total knee arthroplasty. The success of UKA depends on prosthesis component alignment, fixation and soft tissue integrity. The tibial cut is the crucial surgical step. The hypothesis of the present study is that tibial component malalignment is correlated with its risk of loosening in UKA. Methods This study was performed in twenty-three patients undergoing primary cemented unicompartmental knee arthroplasties. Translations and rotations of the tibial component and the maximum total point motion (MTPM) were measured using radiostereometric analysis at 3, 6, 12 and 24 months. Standard radiological evaluations were also performed immediately before and after surgery. Varus/ valgus and posterior slope of the tibial component and tibial– femoral axes were correlated with radiostereometric micromotion. A survival analysis was also performed at an average of 5.9 years by contacting patients by phone. Results Varus alignment of the tibial component was significantly correlated with MTPM, anterior tibial sinking, varus rotation and anterior and medial translations from radiostereometry. The posterior slope of the tibial component was correlated with external rotation. The survival
P. Barbadoro (&) A. Ensini M. d’Amato A. Feliciangeli A. Timoncini F. Amadei S. Giannini 1st Ortopaedic-Traumatologic Clinic, Istituto Ortopedico Rizzoli, University of Bologna, Via Pupilli 1, 40136 Bologna, Bo, Italy e-mail: [email protected] A. Leardini C. Belvedere S. Giannini Movement Analysis Laboratory and Functional-Clinical Evaluation of Prostheses, Istituto Ortopedico Rizzoli, Via Di Barbiano 1/10, Bologna, Bo, Italy
rate at an average of 5.9 years was 89 %. The two patients who underwent revision presented a tibial component varus angle of 10° for both. Conclusions There is correlation between varus orientation of the tibial component and MTPM from radiostereometry in unicompartmental knee arthroplasties. Particularly, a misalignment in varus larger than 5° could lead to risk of loosening the tibial component. Level of evidence Prognostic studies—retrospective study, Level II. Keywords Unicompartmental knee arthroplasty (UKA) Tibial component fixation Tibial component alignment Radiostereometry (RSA) Maximum total point motion (MTPM)
Introduction The main current problem for unicompartmental knee arthroplasty (UKA) is the apparent low survival rate, although there are conflicting reports even about interpretation of these figures in the literature and in national registries. Some authors have described survival rates of between 85 and 98 % for UKA 10 years after surgery, which is a similar rate to that of total knee arthroplasty (TKA) [16, 25]. The longest recorded follow-up for UKA in a young, active patient was 31 years [11]. However, in the Norwegian knee arthroplasty register, a 2.0 times (95 % CI, 1.6–2.5) higher risk of failure at 10 years was recorded for UKA compared with that of TKA [15] and the Finnish arthroplasty register reports a survival rate of 60 % for UKA, and 80 % for TKA at 15 years. From a survival analysis in UKA versus corresponding TKA in three different designs, UKA was not even recommended in the treatment of unicompartmental osteoarthritis [23].
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Therefore, the correct indications, an accurate surgical technique and surgical experience in UKA [9, 29] are very important for the clinical and survival success of the implant. The surgical indications for UKA are as follows: osteonecrosis of the medial condyle or isolated unicompartmental tibial–femoral osteoarthritis; a correctable deformity in varus of\10° or in valgus\14°; a flexion contracture of\5° and a knee flexion[100° [10, 25]. UKA can be a good surgical solution even if there is cartilage damage to the contralateral compartment and the patella-femoral joint, even when it is asymptomatic [3]. Anterior cruciate ligament deficiency does not seem to affect UKA survivorship [4], although some authors consider it a relative contraindication to UKA surgery. A posterior slope of more than 7° should be avoided, particularly if the anterior cruciate ligament is insufficient at the time of implantation [1]. The surgical success of UKA depends on prosthesis component alignment, fixation and soft tissue integrity [2, 22, 24, 34]. The tibial cut is a crucial step. A recent multicenter retrospective study in 418 failed UKAs showed that isolated aseptic loosening of the tibial component is the main cause of UKA failure and that nearly one half of these failures occurred within the first 5 years [14]. An increased risk of UKA revision was reported at 2 and 5 years after surgery and the risk of revision at 2 years is correlated with the surgeon volume [9]. Medial UKA failure can be caused by marked under-correction (postoperative varus of more than 7°), which can lead to residual pain, or overcorrection due to deterioration of the contralateral compartment [25]. Five factors were statistically associated with UKA revision: young patients; thin polyethylene; longer polyethylene insert shelf age, varus of the tibial component and varus of the post-operative hip–knee– ankle angle [8]. In this paper, a reduction in the varus of the medial tibial plateau with respect to the pre-operative angle was found to reduce the risk of revision, but the critical value of varus beyond which the stability of the tibial component may be put at risk was not quantified. Varus– valgus misalignments of the femoral component do not seem to affect the knee kinematics [20]. In this study, the tibial component alignment was correlated with its fixation in UKA, considering RSA measures and the survival rate at mid-term follow-up. The hypothesis is that a tibial malalignment in the frontal and sagittal planes, i.e., varus and posterior slopes, compromises the tibial component fixation in medial UKA.
Materials and methods This is a retrospective study performed on the same population of a previous study from these authors [13]. The population had been composed of twenty-three patients
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who underwent the implantation of a primary OptetrakÒ UKA (Exactech, Gainesville, Florida, USA), from November 2006 to October 2008. Eighteen patients of this initial population, of which we had full RSA data collected until 2 years of follow-up, returned for the present study. The size of the patient population analysed in this study meets the criteria for achieving differences in measurements with 80 % statistical power and a-level of 0.05. The surgical technique targeted the restoration of the same varus–valgus angle and the same posterior slope of the pre-operative radiographs of the tibia; however, a reduction in the posterior slope was performed on one patient (from 8° to 2°) because of the insufficiency of the anterior cruciate ligament, to ensure better joint stability in flexion. Before cementing the final components, four tantalum spherical markers 0.8 mm were inserted into the tibial metaphysis and four into the tibial component polyethylene, for the following RSA. In the pre-operative period, a clinical evaluation using the International Knee Society (IKS) scoring system and weight-bearing anterior– posterior and lateral radiographs of the knee were performed in all patients. The clinical analysis and the radiographs were repeated together with RSA post-operatively and at 3, 6, 12, and 24 months [13]. After these assessments, clinical evaluations using IKS and standard radiological analysis via weight-bearing anterior–posterior and lateral radiographs of the knee were performed every year of follow-up. Three angles were measured on the postoperative radiographs: the anatomical, i.e. tibial–femoral, axis, the varus–valgus of the tibial component in the coronal plane and the posterior slope of the tibial component in the sagittal plane. These were correlated with the RSA data, which are indicative of tibial component migration. In particular, the maximum total point motion (MTPM), the amount of three-dimensional translation of the marker that moves the most at each follow-up, was used as the predictive index of tibial component instability and loosening. An implant with continuous migration after the first year, i.e. an MTPM increment larger than 0.2 mm between the first- and the second-year post-operative, was defined instable [30]. The accuracy of our RSA system was described elsewhere [13]. Age, sex, pre-operative and final follow-up (at an average of 5.3 years) clinical values (IKSKnee and Function score), post-surgical tibial–femoral axes, posterior slope and varus/valgus tibial angles and RSA parameters were statistically evaluated. For the latter parameters, we considered translations along and rotations about the three anatomical axes at 24 months of follow-up, in addition to MTPM at 3, 6, 12 and 24 months of followup. Patients were contacted by telephone at a mean of 5.9 years (min. 4.5, max. 6.5 years) after surgery; a survival analysis was performed at this time for all eighteen patients, taking revision surgery as the end point.
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3159
Statistical analysis
test was significant (p \ 0.05), Pearson’s correlation was performed to investigate the relationships between different parameters. Statistical analysis was carried out using the statistical package for the social sciences (SPSS) software version 15.0 (SPSS Inc., Chicago, USA).
The RSA data were correlated with radiographs based measurements, clinical parameters, age and sex. When the
Results
This study was approved by the Ethical Committee of Rizzoli Orthopaedic Institute–Bologna University (registration number 11/06).
Table 1 The anatomical axis and the tibial component alignment in sagittal and frontal plane for all the eighteen patients is reported Val?/varF–T axes
P.S. tibial component
Val?/vartibial component
1
2
3
-4
2
4
7
-10
3
2
3
-3
4
-1
4
2
5
-2
4
-10
6
1
8
-7
7
5
8
-4
8
5
3
-3
9 10
2 6
4 8
-1 -2
11
-3
9
0
12
-2
3
-4
13
5
7
-1
14
6
2
-1
15
0
3
-7
16
3
2
-4
17
4
4
-4
18
2
3
-5
A statistically significant correlation was found between the varus angle of the tibial component and MTPM, for each follow-up (all with p \ 0.005 and r [ 0.7); particularly at 24 months p was \0.0001 and r = 0.86). The posterior slope angle of the tibial component was found correlated with external rotation from RSA at 24 month follow-up (p = 0.02, r = 0.62). Correlations were also found between the varus angle of the tibial component and the RSA-based rotations in the sagittal and frontal planes, and translations along the mediolateral and the anteroposterior axes. In particular, the Pearson correlation values for the varus angle were as follow: anterior sinking (p = 0.03, r = 0.61), varus rotation (p = 0.001, r = 0.79), medial (p = 0.01, r = 0.58) and anterior (p = 0.04, r = 0.67) translations. No correlations were found between age, sex, tibial–femoral axis, clinical score and RSA data. All radiological data of the eighteen patients measured on the immediate post-operative radiographs are reported in Table 1. A trend was observed between the varus angle of the tibial component and the difference of MTPM between the first and the second year of follow-up (Fig. 1). From the correlation analysis of the present population it can be deduced, according to an established criterion [30], that the
Fig. 1 The trend line highlights the association between the varus of the tibial component and its migration between the first and the second year after surgery. The grey area can be considered acceptable for the implantation of the tibial component
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second- to first-year MTPM difference of more than 0.2 mm corresponds to varus misalignment of the tibial component larger than 5°. One patient showed MTPM higher than 1.5 mm already at 3 months follow-up, which remained higher than 1.6 mm at 12 and at 24 months of follow-up, though not accompanied initially with concerning clinical scores, IKSKnee and Function being, respectively, 70 and 60 at 2-years follow-up. However, these scores worsened to, respectively, 23 and 35 at 6 years follow-up, and the patient underwent revision of the UKA to TKA. During revision surgery, the tibial component showed to be loosened. The tibial component had been implanted with a 10° varus angle, as measured on the post-operative radiographs. According to the definition [30], another patient was at risk of failure at 2 years of follow-up; this patient underwent in fact revision surgery with a TKA at 5 years and 5 months after UKA implantation due to aseptic loosening of the tibial component. Again, after initial satisfactory IKS-Knee and Function scores, i.e. 100 at 2, 3 and 4 years of follow-up, these worsened to, respectively, 53 and 55 at 5 years. The tibial component had been implanted with a 10° varus angle also in this patient. These were the only two patients with clinical results worsening and revision surgery indications. The overall survival rate at an average of 5.9 years was 89 %.
Discussion The most important finding of the present study was the observed correlation between the varus angle of the tibial component and its migration, i.e. MTPM from RSA, and the correlation between the posterior slope of the tibial component and its external rotation from RSA. The patient who underwent revision at about 6 years after surgery presented an early migration, higher than 1.6 mm at 12 months of follow-up, which should be considered at risk in fact by a recent systematic review, though about TKA [28]. This early migration indeed caused an early mobilization of the tibial component, thus confirming that a lack of primary fixation can lead later to implant failure. The other patient who underwent revision was already considered instable at 2 years of follow-up, according to RSA-based thresholds [30]. It is interesting here to point out that these two patients presented 10° of varus of the tibial component on the post-operative radiographs. The cemented TKA have a better early fixation, but a greater risk of future aseptic loosening than cementless TKA [26, 27]. In TKA, a few degrees of component malalignment will cause leg malalignment and consequently failure of the prosthesis; in UKA, there is a higher tolerance in range of
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alignment [19]. Varus malalignment in the mechanical axis (hip–knee–ankle angle in the frontal plane) does not effectively compromise the functional outcome [18], and the progression of osteoarthritis in the lateral compartment is not influenced by the post-operative alignment [12]. Some authors considered a moderate under-correction of both the mechanical and anatomical axes, with a residual varus deformity of 3°–5° in the latter, as the surgical goal to avoid osteoarthritis progression in the contralateral compartment and early loosening of the implant [5, 7]. It has been shown that tibial–femoral alignment can influence polyethylene wear and subsidence of the tibial component [33]. Correction of the varus mechanical axis, reducing the varus angle of the medial tibial component and reducing or reproducing the tibial plateau posterior slope is the correct surgical technique to prevent UKA failure [8]. The recommended range of implantation for the femoral component is between ±10° varus/valgus and between ±10° flexion/extension. The recommended range of implantation for the tibial component is between ±5° varus/valgus and between 2° and 12° posterior slope (±5° superior/inferior tilt, considering 7° of posterior slope as a reference value). Within these ranges, the orientation of the components does not affect the outcomes [17]. The posterior slope is important to increase flexion and femoral rollback and to improve the stress distribution at the bone–tibial component interface. Hernigou and Deschamps found a correlation between anterior tibial translation and posterior slope of the tibial component and recommended a posterior slope of between 3° and 7° [19]. Some authors considered a tibial cut perpendicular to the epiphyseal axis [5, 6]. Swienckowski and page reported poor clinical results for UKA when the tibial component was placed in varus inclination [32]. A finite element analysis about the tibial component inclination in UKA revealed that the varus angle of the tibial component increased the yielding area, due to excessive compressive stress on the proximal end of the medial diaphyseal cortex. To optimize stress distribution at the implant–bone interface, thus minimizing the yielding area, it is advisable to implant the tibial component at a slight valgus inclination (2°–4° valgus), without changing the leg alignment. An excessive posterior inclination should be avoided to minimize bone stress [21, 31]. The limitations of the present study are those typical of standard RSA, i.e. X-ray exposure, critical identifications and tracking of the tantalum markers. In addition, this is a retrospective study in a limited number of patients. This is mainly because UKA has specific surgical indications, and patients are selected accordingly, and because of the predictive power of the RSA, so that not many patients are required to assess the risk of UKA aseptic loosening. In the future, a study on a larger group of UKA with tibial component aseptic loosening could be undertaken to verify the correlations found.
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As described about TKA [28], it is here confirmed that high migration, i.e. MTPM, of the tibial component at 1 year of follow-up is another RSA indicator of UKA instability. In addition, the higher risk of aseptic loosening determined by an increase in MTPM of more than 0.2 mm between the first and the second year of follow-up [30] is also here confirmed. However, the present UKA survival rate of nearly 90 % at almost 6 years can be considered more than satisfactory. The clinical relevance of this work is mainly for UKA implantation: during surgery attention should be given to carefully avoid excessive varus and posterior slope in performing of the tibial bone cut.
Conclusion
9.
10.
11.
12.
13.
14.
In UKA, tibial component misalignment in varus was found correlated with its overall migration in the secondyear follow-up. In particular, according also to standard recommendations, risk of loosening of the tibial component is high for misalignments in varus larger than 5°. These results were corroborated by the present clinical assessment and survival analysis of this series. Conflict of interest of interest.
3161
15.
16.
The authors declare that they have not conflict
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