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Effect of femoral to tibial varus mismatch on the contact area of unicondylar knee prostheses
The Knee 17 (2010) 350–355
Contents lists available at ScienceDirect
The Knee
Effect of femoral to tibial varus mismatch on the contact area of unicondylar knee prostheses Christian Diezi, Stephan Wirth, Dominik C. Meyer, Peter P. Koch ⁎ Department of Orthopaedics, University of Zurich, Balgrist, Zurich, Switzerland
a r t i c l e
i n f o
Article history: Received 30 May 2009 Received in revised form 9 October 2009 Accepted 9 October 2009 Keywords: Unicondylar knee Contact area Geometry Loosening Position
a b s t r a c t In unicondylar knee prostheses, the relative angle and congruency of the femoral against the tibial component is not mechanically constrained and may vary with the surgical implantation technique. The contact area between both components was measured with increasing varus (0–20°) and flexion angles (− 20° to 90°) in five prosthesis models in the laboratory. The contact area varied with the relative position of the components and was critically reduced up to 70% at a varus range between > 5° and < 25°. The importance of relative malpositioning of the femoral and tibial components may be underestimated and reduces the contact area of unicondylar prostheses decisively, independent from the limb axis. This increases local pressure and may thus importantly contribute to increased wear and early loosening. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Unicompartmental knee replacement is a treatment option of localized unicompartmental medial or lateral osteoarthritis of the knee. These prostheses are small, require little bone resection and the surgical procedure retains the ligamentous stability and physiological movement of the knee. However the time to implant failure, characterized by polyethylene wear and implant loosening is usually considered shorter than for total knee replacement [7]. It was shown that one of the main causes for failure in total knee arthroplasty is the size of contact areas and the magnitude of contact pressure [4,15]. Small contact areas in conjunction with high pressures lead to early failure of the polyethylene [12]. Vice versa, larger contact areas may produce lower wear rates [13] due to less load per area. The influence of malalignment on contact pressures in fixed and mobile bearing total knee prostheses has been evaluated in earlier studies [5], under different conditions such as antero-posterior, medio-lateral and rotational translation of the femoral component and could confirm these assumptions. Several factors which contribute to increased contact pressure in unicondylar knee replacement have been evaluated in the literature, such as hip–knee angle, slope of the tibial plateau and horizontal malalignment, all in analogy to total knee replacement [1,5–7,10,11,13,14]. However, in total knee prostheses, the coronal (varus/valgus) contact angle of femoral
to tibial component is defined by the shape of the prosthesis, as long as both condyles are articulating (i.e. 0°). But in unicondylar implants, this angle is defined by how the components are surgically implanted and is therefore highly variable (Fig. 1). This angle is per se independent from the limb axis. Our hypothesis was, that at an unfavorable angle, not the round smooth center of the femoral component, but the in some cases sharp edge of the femoral component articulates with the tibial plateau and thus the contact area could presumably be reduced. Reduction of the contact area will lead to an increased local pressure and consecutive potentially increased polyethylene wear. This problem appears to have the potential to be of greater biomechanical importance for polyethylene wear than many other known factors such as patient weight or small varus/valgus malalignment of the entire leg. To our knowledge, this problem has hitherto not been addressed in the literature, but is without doubt a potential contributing factor for implant survival that deserves further attention. This study was therefore designed to evaluate the geometrical influence of varus/ valgus malpositioning of the femoral component relative to the tibial component at different degrees of flexion with respect to the size and shape of the tibiofemoral contact area. As a secondary goal, we intended to define a zone in which deviations from the optimal implant position can be tolerated for each model. 2. Materials and methods
⁎ Corresponding author. Uniklinik Balgrist, Forchstr. 340, CH-8008 Zürich, Switzerland. Tel.: +41 44 386 11 11. E-mail address: [email protected] (P.P. Koch). 0968-0160/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2009.10.004
We evaluated five different models of unicompartmental knee prostheses of medium size (Fig. 2A, B) with their matching inlays (six
C. Diezi et al. / The Knee 17 (2010) 350–355
Fig. 1. Schematic effect of relative varus or valgus angle between femoral and tibial component: with 0° deviation, there is appropriate congruence between the components (A), while at 20° varus angle, the femoral component contacts the tibial polyethylene (B) at the edge.
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the back surface of the inlays also a base plate for each model had to be prepared (Fig. 4). Different flexion grades in combination with different varus/valgus deviations (later simply referred to as “varus”) were tested. The flexion grades were: −20°, −10°, 0°, 45° and 90°. For each flexion grade, varus tilt was evaluated in 0, 5, 10, 15 and 20° of varus. Three prints with a load of 500 N during 30s were obtained for each combination and each prosthesis. The obtained prints were scanned together with a scale for calibration and imported into Adobe Professional 7.0 (Adobe Systems Inc., San Jose, CA, USA), where the measurements were performed with the measuring tools. We measured the sagittal and frontal diameters as well as the total contact areas (threshold of gray-scale 80 N/cm2). Calculations and tables were performed in Microsoft Office Excel (Microsoft Corp., Redmond, WA, USA). 2.2. Statistical analysis
different inlays, Fig. 2C). The models were: Allegretto® (Zimmer Inc., Warsaw, Indiana, USA), Genesis Uni® (Smith-Nephew, London, United Kingdom), Kaps® (X-Nov, Belfort, France), Link EndoModel® (Link, Berne, Switzerland) and Preservation Uni Knee® (DePuy, Warsaw, Indiana). Preservation Uni Knee® was tested with a fixed as well as a mobile inlay. The designs of the femoral components and their inlays differ slightly in radius and shape of the inlay.
Statistical analysis was carried out with SPSS software (SPSS Inc., Chicago, Illinois, USA). A Univariate Analysis of Variance was performed to compare the interactions. This was done with respect to the prostheses, varus, flexion, prostheses and varus, prostheses and flexion, varus and flexion, as well as prostheses and varus and flexion. In a second step a Variance Components Estimation using restricted maximum likelihood estimation and a two-way ANOVA with factors varus and flexion was performed for every prosthesis.
2.1. Methods 3. Results
The tests were designed to assess the contact area of the femoral component on the tibial insert at a standardized load of 500 N, assuming a patient with a body weight of 80 kg. In order to achieve this, a suitable method needed to be found which allows visualizing the spherical contact area between the components. We chose not to use a mathematical computer-simulated model because we assumed that without taking respect to the polyethylene elasticity the computer could underestimate the contact areas on an infinitely hard plateau where theoretically the contact area would be close to zero. Furthermore, in order to avoid artificially increased contact areas with a thick pressure-sensitive film that could influence the congruity of the tested surfaces we decided to use a very thin (0.04 mm) but relatively hard material between prosthesis and inlay. For obtaining acceptable prints of the contact areas we initially tested different methods and materials such as different white papers, stickers, foils, carbon papers and stamping-inks [2,5,9,11,14]. The clearest prints — without problems with the spherical shape of the implants — could be observed with a combination of a thin airmail paper (Atlantic Clipper, Elco AG, Brugg, Switzerland) with an according extra-fine carbon paper, so that we chose this combination for the testing. Furthermore, in literature, carbon paper was shown to provide the exact location and dimension of contact areas equally to a sensor system like the T-Scan [16]. In our pre-tests this assumption was confirmed. Thickness of one sheet of our airmail/carbon paper combination was measured to be 0.044 mm. To define the area producing an appropriate print on the paper, a steel-stamp of 1 × 1 cm (1 cm2) was pressed against the carbon/paper combination with increasing load on an Instron material testing machine (model 4204, Instron, High Wycombe, UK). The according gray-scale of 80 N/cm2 was later defined as threshold to electronically define the size of the contact area. With 500 N load on the prosthesis, excellent prints could be obtained with the white and carbon paper placed directly between the femoral components and their inlays. Thereby the total contact area can be reliably reproduced, with the limitation that peak pressure within the contact area cannot be quantified (Fig. 3). A bearing for fixation of the femoral components was constructed, so that the different flexion grades as well as varus tilt could be adjusted (with an accuracy of <1°). Because of the different designs of
Statistical evaluation showed significant differences for all test conditions between models of prostheses (p < 0.05). However, we believe that a direct “quantitative” comparison between prostheses is not justified, as the actual in vivo performance of these implants will without doubt depend on many additional and individual factors and each prosthesis is differently designed. We therefore decided to analyze each implant individually and not to emphasize on differences between the different prostheses. For each prosthesis (Fig. 2A, B, and C), a separate description and a graph indicating the total contact area are given. 3.1. Allegretto The inlay has a slight meniscal shape (Fig. 5A). Contact area is decreasing with increased varus. At flexion 0° and varus 0° there is a maximal area of 22 mm2 average. Minimal contact area of 10 mm2 is found at flexion 45° and varus 20°. The biggest decrease in contact areas can be seen in varus > 10°. Variability of contact areas in 0° varus from flexion − 20° to 90° is 13%. Frontal and sagittal diameters slightly decrease with increased varus. Statistically varus was found to be the leading condition of the interactions and its contribution was 76%. 3.2. Genesis Uni (Fig. 5B) The inlay has a slight meniscal shape. Contact area is decreasing with increased varus. At flexion − 20° and varus 5° there is a maximal area of 31 mm2 average. Minimal contact area of 11 mm2 is found at flexion 90° and varus 20°. The biggest differences between areas can be seen in varus >5°. Variability of contact areas in 0° varus from flexion − 20° to 90° is 31%. Frontal and sagittal diameters clearly decrease with increased varus. Statistically, varus was found to be the leading condition of the interactions and its contribution was 79%. 3.3. Kaps (Fig. 5C) The femoral component has the largest radius of the five examined prostheses, the contact area is nearly plane. The inlay is U-shaped in both sagittal and frontal axes. The radius of the inlay is smaller than the one of the femoral component. This has the effect that the femoral component touches the inlay in two different areas medially and laterally and produces two contact areas in − 20/− 10/0/45° flexion in 0° varus (if perfectly centered in the coronal plane), what could not be seen in the other prostheses. In the varus conditions 5° to 20° for all flexion grades only one contact area could be detected. For graphical and statistical evaluation the two areas in 0° varus were summarized. Contact area is decreasing with increased varus (as one of the edges looses contact). At flexion 45° and varus 0° there is a maximal area of 43 mm2 average. Minimal contact area of 9.6 mm2 is found at flexion 90° and varus 20°. The biggest differences between areas can be seen between varus 0° and 5° in 45° and 90° of flexion. In the other flexion grades there is a slight decrease with increased varus. Variability of contact areas in 0° varus is 21%. Frontal and sagittal diameters decrease
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Fig. 2. A. Frontal view of the evaluated femoral components: A Allegretto, B Genesis Uni, C Kaps, D Link Endo-Model, E Preservation Uni Knee. There is an obvious difference of the radius especially for Genesis Uni (B, largest radius) and Link Endo-Model (D, smallest radius). B. Lateral view of the evaluated femoral components: A Allegretto, B Genesis Uni, C Kaps, D Link Endo-Model, E Preservation Uni Knee. C. Evaluated inlays: A Allegretto, B Genesis Uni, C Kaps, D Link Endo-Model, E Preservation Uni Knee (E1 fix, E2 mobile). The Link inlay appears completely plane, whereas the other inlays show a more or less slight meniscal or U-shape.
with increased varus with the biggest steps between 0° and 5° varus. Statistically, varus was found to be the leading condition of the interactions and its contribution was 99%. 3.4. Link Endo-Model (Fig. 5D) The femoral component has the smallest radius, it is almost round. The inlay appears absolutely plane. Contact area is only slightly decreasing with increased varus. At flexion − 10° and varus 0° there is a maximal area of 21 mm2 average. Minimal contact area of 12 mm2 is found at flexion 90° and varus 20°. For flexion grades between − 20 and 45° the contact areas remain quite constant. All in all, calculated variability of contact areas in 0° varus is 30%. Frontal and sagittal diameters remain quite constant for all flexion and varus conditions, except at flexion 90° and varus 20°. In contrast to the other prostheses flexion was found to be the statistically leading condition of the interactions and its contribution was 80%. 3.5. Preservation Uni Knee with fixed inlay (Fig. 5E) The fixed inlay has a very discrete U-shape in both axes. Contact area is decreasing with increased varus. At flexion 0° and varus 5° there is a maximal area of 27 mm2
average. Minimal contact area of 12 mm2 is found at flexion − 20° and varus 20°. The biggest differences between areas can be seen between varus 5 and 10°. Variability of contact areas in 0° varus is 26%. Frontal and sagittal diameters clearly decrease with increased varus with the biggest steps between 5 and 10° varus. Statistically, varus was found to be the leading condition of the interactions and its contribution was 73%. 3.6. Preservation Uni Knee with mobile inlay (Fig. 5F) The mobile inlay is clearly U-shaped in both axes. Contact area is clearly decreasing with increased varus. At flexion 90° and varus 5° there is a maximal area of 48 mm2 average. Minimal contact area of 13 mm2 is found at flexion − 20° and varus 20°. The biggest differences between areas can be seen between varus 0 and 5°. Variability of contact areas in 0° varus is 30%. frontal and sagittal diameters also clearly decrease with increased varus with the biggest steps between 0 and 5° varus. Statistically varus was found to be the leading condition of the interactions and its contribution was 94%. In summary, it could be observed that contact areas can be reduced up to 70% with increasing varus of the femoral component. Furthermore, with flexion of the prosthesis from 0° to 90° in neutral position (0° varus), there is in most prostheses a variability of contact area of approximately 10–30% (Fig. 5A–F).
C. Diezi et al. / The Knee 17 (2010) 350–355
Fig. 3. Examples of prints obtained with the carbon/paper combination with a load of 500 N.
4. Discussion Unicondylar knee prostheses have obvious advantages such as being small, needing little bone resection and retaining the ligamentous stability and physiological movement of the knee. They come however at the cost of a usually shorter time to failure, associated with polyethylene wear [8] and consecutive implant loosening or with progressive gonarthrosis of the lateral and/or patellofemoral compartment [3]. Typical wear rates for total knee prostheses are 0.33 mm/yr versus 0.49 mm/yr for unicondylar prostheses [6,7]. Most frequently cited factors influencing polyethylene wear are high contact pressure and friction in young patients with high activity, (long) shelf age of the implant, and (high) hip–knee angle [1]. There is a large number of studies assessing factors contributing to high local stress on the implant and therefore early failure and increased wear [1,2,4–7,9,10,13–15]. However it appears that the relative position of the components of unicondylar prostheses has hitherto not been considered yet as a contributing factor, which is independent from the general limb alignment.
Fig. 4. Testing setup. The fixation for the femoral components was freely adjustable for the different flexion grades as well as varus tilt. The inlays were placed in individual base plates.
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In total knee prostheses, the coronal contact angle of femur to tibia is defined by the shape of the prosthesis, as long as both condyles are articulating. But in unicondylar implants, this angle is defined by how the components are surgically implanted and is therefore potentially highly variable. Our hypothesis was, that at an unfavorable angle, instead of the round smooth center of the femoral component, the potentially sharp and small edge may articulate with the tibial plateau. (Figs. 1 and 2A). The experiments performed in this study were designed to assess this effect at different degrees of flexion and varus angle in six commercially available prostheses. We were aware that a quantification of the clinical effect of the described forms of malalignment would be very difficult to predict and virtually impossible in laboratory. So we deliberately chose a simple and direct method for obtaining the prints. Nevertheless this method showed reproducible results. Previous investigators did show that carbon paper provides the exact location and dimension of contact areas equally to a sensor system like the T-Scan [16]. This method allows to place a very thin (0.04 mm) and relatively hard material between the components, where a potentially soft and thick pressure-sensitive film might tend to measure artificially increased contact areas. Another possible method would be a computer simulation of the different prosthesis models and implantation positions. We however refrained from that for the reasons explained above. In summary, the combination of carbon/airmail paper was in our setup effective to answer the questions asked. Our numbers show that indeed, at an unfavorable angle of implantation, the contact area may be reduced by more than 70%, thereby potentially increasing the local stress by a factor 3–4. Additionally, the shape of the contact area is changed to a longitudinal edge which may be more than 10 times longer than wide, a condition for which the prostheses were most likely not designed. One prosthesis even has two contact points at a specific position (Kaps). In order to define a zone of tolerance for varus mismatch for every prosthesis it should be considered that with flexion of the prosthesis from 0° to 90° in neutral position (0° varus), there is in most prostheses a variability of contact area of approximately 10–30% (Fig. 5A–F). Here we assume that a maximal variability of 30% of the size of the contact area is acceptable and foreseen by the manufacturers. Assuming that a change of maximal 30% in contact area relative to the 0° varus and 0° flexion position is acceptable, tolerated contact areas can be calculated and a threshold can be defined with respect to the according measurements. Thus a zone of tolerance for varus malpositioning for the different tested prosthesis models can be defined and may be regarded as follows: Allegretto 10°, Genesis 5°, Kaps >20° (thereby neglecting the contact of two ridges in 0° varus at 45° and 90°), Link > 20°, Preservation fix 15°, Preservation mobile <5°. Unexpectedly the “Preservation fix” inlay had a better tolerance for malpositioning than the “Preservation mobile” inlay. We observed that models with a smaller femoral radius and a flat inlay tolerated more varus malpositioning. This finding may support reports that fullpolyethylene inlays may equally achieve very favorable survival rates. Our measurements indicate that frontal component malpositioning might be clinically more relevant for polyethylene failure than a 10% increased knee angle. A limitation of our measurement technique was that we had to focus our evaluations on the combined effect of flexion (sagittal) and varus/valgus (frontal) angulation of the prostheses. It would also be possible to assess rotational malpositioning and relative translation of the components, which might not only result in an altered contact area, but also in an increased shear load instead of compression. All of these factors would then need to be theoretically combined which appears not experimentally realistic. Due to the nature of the question asked, the test setup is not applicable to all types of implants; in preliminary tests also the Oxford Prosthesis (Biomet Inc., Warsaw, Indiana, USA) was evaluated, but had to be excluded from further
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C. Diezi et al. / The Knee 17 (2010) 350–355
Fig. 5. A. Allegretto: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as 10°. B. Genesis: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as 5°. C. Kaps: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as >20° (thereby neglecting the contact of two ridges in 0° varus at 45° and 90°). D. Link: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as >20°. E. Preservation with fix inlay: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as 15°. F. Preservation with mobile inlay: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as <5°, tolerance for malpositioning is worse than with the fix inlay.
C. Diezi et al. / The Knee 17 (2010) 350–355
experimental testing. This was due to the fundamentally different geometry. The Oxford prosthesis is generally considered to be fully congruent. It is most likely very little sensitive to relative malalignment of the components and has a very large absolute contact area. Nevertheless, we found that the delivered components had a very small femoral to tibial mismatch and were not perfectly congruent. Even though there might be a small mismatch in a completely new prosthesis it will most likely become more congruent after several years of use. As our tests were successful in demonstrating already very large differences between designs of prostheses and between different implant positions, we consider our methods to be adequate to prove our working hypothesis. The results presented here describe however only a singular aspect of the properties of the tested prostheses, therefore we make no qualitative ranking or prediction of the overall performance of these implants. Further there is to consider that not all instruments delivered with the prostheses allow for equally precise implantation. Maybe computer-assisted surgery will be an appropriate way to particularly eliminate the proposed position-related problems. Because there are also tribological, chemical and biological factors involved, the next appropriate step appears to perform wear testing in vitro or in vivo to quantify the impact of the prosthetic contact area on implant wear and survival. In conclusion, the data presented here show that an unfavorable angle of implantation can lead to a reduction of contact area by more than 70%, thereby potentially increasing the local stress. This finding indicates that the relative position of femoral versus tibial component in unicondylar knee prostheses is of potentially great importance for the implant durability. This factor should not be neglected, neither surgically nor when clinically or in vitro analyzing the survival of these implants. References [1] Ashraf T, Newman JH, Desai VV, Beard D, Nevelos JE. Polyethylene wear in a noncongruous unicompartmental knee replacement: a retrieval analysis. Knee 2004;11:177–81.
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[2] Bachus KN, DeMarco AL, Judd KT, Horwitz DS, Brodke DS. Measuring contact area, force, and pressure for bioengineering applications: using Fuji Film and TekScan systems. Med Eng Phys 2006;28:483–8. [3] Beard DJ, Pandit H, Gill HS, Hollinghurst D, Dodd CA, Murray DW. The influence of the presence and severity of pre-existing patellofemoral degenerative changes on the outcome of the Oxford medial unicompartmental knee replacement. J Bone Jt Surg Br Vol 2007;89:1597–601. [4] Blunn GW, Joshi AB, Minns RJ, Lidgren L, Lilley P, Ryd L, et al. Wear in retrieved condylar knee arthroplasties. A comparison of wear in different designs of 280 retrieved condylar knee prostheses. J Arthroplast 1997;12:281–90. [5] Cheng CK, Huang CH, Liau JJ. The influence of surgical malalignment on the contact pressures of fixed and mobile bearing knee prostheses — a biomechanical study. Clin Biomech (Bristol, Avon) 2003;18:231–6. [6] Collier MB, Eickmann TH, Sukezaki F, McAuley JP, Eng GA. Patient, implant, and alignment factors associated with revision of medial compartment unicondylar arthroplasty. J Arthroplast 2006;21:108–15. [7] Collier MB, Engh Jr CA, McAuley JP, Engh GA. Factors associated with the loss of thickness of polyethylene tibial bearings after knee arthroplasty. J Bone Jt Surg Am Vol 2007;89:1306–14. [8] Eickmann TH, Collier MB, Sukezaki F, McAuley JP, Engh GA. Survival of medial unicondylar arthroplasties placed by one surgeon 1984–1998. Clin Orthop Relat Res 2006;452:143–9. [9] Harris ML, Morberg P, Bruce WJ, Walsh WR. An improved method for measuring tibiofemoral contact areas in total knee arthroplasty: a comparison of K-scan sensor and Fuji film. J Biomech 1999;32:951–8. [10] Hernigou P, Deschamps G. Alignment influences wear in the knee after medial unicompartmental arthroplasty. Clin Orthop Relat Res 2004:161–5. [11] Ostermeier S, Nowakowski A, Stukenborg-Colsman C. Dynamic in vitro measurement of pressure and movement with the LCS prosthetic system. Orthopade 2003;32:292–5. [12] Rostoker W, Galante JO. Contact pressure dependence of wear rates of ultra high molecular weight polyethylene. J Biomed Mater Res 1979;13:957–64. [13] Sathasivam S, Walker PS, Campbell PA, Rayner K. The effect of contact area on wear in relation to fixed bearing and mobile bearing knee replacements. J Biomed Mater Res 2001;58:282–90. [14] Villa T, Migliavacca F, Gastaldi D, Colombo M, Pietrabissa R. Contact stresses and fatigue life in a knee prosthesis: comparison between in vitro measurements and computational simulations. J Biomech 2004;37:45–53. [15] Wasielewski RC, Galante JO, Leighty RM, Natarajan RN, Rosenberg AG. Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty. Clin Orthop Relat Res 1994:31–43. [16] Cabral CWLF, Silva FA, Silva WAB, Landulpho AB, Silva LB. Comparison between two methods to record occlusal contacts in habitual maximal intercuspation. Braz J Oral Sci 2006;19:1239–43.
Contents lists available at ScienceDirect
The Knee
Effect of femoral to tibial varus mismatch on the contact area of unicondylar knee prostheses Christian Diezi, Stephan Wirth, Dominik C. Meyer, Peter P. Koch ⁎ Department of Orthopaedics, University of Zurich, Balgrist, Zurich, Switzerland
a r t i c l e
i n f o
Article history: Received 30 May 2009 Received in revised form 9 October 2009 Accepted 9 October 2009 Keywords: Unicondylar knee Contact area Geometry Loosening Position
a b s t r a c t In unicondylar knee prostheses, the relative angle and congruency of the femoral against the tibial component is not mechanically constrained and may vary with the surgical implantation technique. The contact area between both components was measured with increasing varus (0–20°) and flexion angles (− 20° to 90°) in five prosthesis models in the laboratory. The contact area varied with the relative position of the components and was critically reduced up to 70% at a varus range between > 5° and < 25°. The importance of relative malpositioning of the femoral and tibial components may be underestimated and reduces the contact area of unicondylar prostheses decisively, independent from the limb axis. This increases local pressure and may thus importantly contribute to increased wear and early loosening. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Unicompartmental knee replacement is a treatment option of localized unicompartmental medial or lateral osteoarthritis of the knee. These prostheses are small, require little bone resection and the surgical procedure retains the ligamentous stability and physiological movement of the knee. However the time to implant failure, characterized by polyethylene wear and implant loosening is usually considered shorter than for total knee replacement [7]. It was shown that one of the main causes for failure in total knee arthroplasty is the size of contact areas and the magnitude of contact pressure [4,15]. Small contact areas in conjunction with high pressures lead to early failure of the polyethylene [12]. Vice versa, larger contact areas may produce lower wear rates [13] due to less load per area. The influence of malalignment on contact pressures in fixed and mobile bearing total knee prostheses has been evaluated in earlier studies [5], under different conditions such as antero-posterior, medio-lateral and rotational translation of the femoral component and could confirm these assumptions. Several factors which contribute to increased contact pressure in unicondylar knee replacement have been evaluated in the literature, such as hip–knee angle, slope of the tibial plateau and horizontal malalignment, all in analogy to total knee replacement [1,5–7,10,11,13,14]. However, in total knee prostheses, the coronal (varus/valgus) contact angle of femoral
to tibial component is defined by the shape of the prosthesis, as long as both condyles are articulating (i.e. 0°). But in unicondylar implants, this angle is defined by how the components are surgically implanted and is therefore highly variable (Fig. 1). This angle is per se independent from the limb axis. Our hypothesis was, that at an unfavorable angle, not the round smooth center of the femoral component, but the in some cases sharp edge of the femoral component articulates with the tibial plateau and thus the contact area could presumably be reduced. Reduction of the contact area will lead to an increased local pressure and consecutive potentially increased polyethylene wear. This problem appears to have the potential to be of greater biomechanical importance for polyethylene wear than many other known factors such as patient weight or small varus/valgus malalignment of the entire leg. To our knowledge, this problem has hitherto not been addressed in the literature, but is without doubt a potential contributing factor for implant survival that deserves further attention. This study was therefore designed to evaluate the geometrical influence of varus/ valgus malpositioning of the femoral component relative to the tibial component at different degrees of flexion with respect to the size and shape of the tibiofemoral contact area. As a secondary goal, we intended to define a zone in which deviations from the optimal implant position can be tolerated for each model. 2. Materials and methods
⁎ Corresponding author. Uniklinik Balgrist, Forchstr. 340, CH-8008 Zürich, Switzerland. Tel.: +41 44 386 11 11. E-mail address: [email protected] (P.P. Koch). 0968-0160/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2009.10.004
We evaluated five different models of unicompartmental knee prostheses of medium size (Fig. 2A, B) with their matching inlays (six
C. Diezi et al. / The Knee 17 (2010) 350–355
Fig. 1. Schematic effect of relative varus or valgus angle between femoral and tibial component: with 0° deviation, there is appropriate congruence between the components (A), while at 20° varus angle, the femoral component contacts the tibial polyethylene (B) at the edge.
351
the back surface of the inlays also a base plate for each model had to be prepared (Fig. 4). Different flexion grades in combination with different varus/valgus deviations (later simply referred to as “varus”) were tested. The flexion grades were: −20°, −10°, 0°, 45° and 90°. For each flexion grade, varus tilt was evaluated in 0, 5, 10, 15 and 20° of varus. Three prints with a load of 500 N during 30s were obtained for each combination and each prosthesis. The obtained prints were scanned together with a scale for calibration and imported into Adobe Professional 7.0 (Adobe Systems Inc., San Jose, CA, USA), where the measurements were performed with the measuring tools. We measured the sagittal and frontal diameters as well as the total contact areas (threshold of gray-scale 80 N/cm2). Calculations and tables were performed in Microsoft Office Excel (Microsoft Corp., Redmond, WA, USA). 2.2. Statistical analysis
different inlays, Fig. 2C). The models were: Allegretto® (Zimmer Inc., Warsaw, Indiana, USA), Genesis Uni® (Smith-Nephew, London, United Kingdom), Kaps® (X-Nov, Belfort, France), Link EndoModel® (Link, Berne, Switzerland) and Preservation Uni Knee® (DePuy, Warsaw, Indiana). Preservation Uni Knee® was tested with a fixed as well as a mobile inlay. The designs of the femoral components and their inlays differ slightly in radius and shape of the inlay.
Statistical analysis was carried out with SPSS software (SPSS Inc., Chicago, Illinois, USA). A Univariate Analysis of Variance was performed to compare the interactions. This was done with respect to the prostheses, varus, flexion, prostheses and varus, prostheses and flexion, varus and flexion, as well as prostheses and varus and flexion. In a second step a Variance Components Estimation using restricted maximum likelihood estimation and a two-way ANOVA with factors varus and flexion was performed for every prosthesis.
2.1. Methods 3. Results
The tests were designed to assess the contact area of the femoral component on the tibial insert at a standardized load of 500 N, assuming a patient with a body weight of 80 kg. In order to achieve this, a suitable method needed to be found which allows visualizing the spherical contact area between the components. We chose not to use a mathematical computer-simulated model because we assumed that without taking respect to the polyethylene elasticity the computer could underestimate the contact areas on an infinitely hard plateau where theoretically the contact area would be close to zero. Furthermore, in order to avoid artificially increased contact areas with a thick pressure-sensitive film that could influence the congruity of the tested surfaces we decided to use a very thin (0.04 mm) but relatively hard material between prosthesis and inlay. For obtaining acceptable prints of the contact areas we initially tested different methods and materials such as different white papers, stickers, foils, carbon papers and stamping-inks [2,5,9,11,14]. The clearest prints — without problems with the spherical shape of the implants — could be observed with a combination of a thin airmail paper (Atlantic Clipper, Elco AG, Brugg, Switzerland) with an according extra-fine carbon paper, so that we chose this combination for the testing. Furthermore, in literature, carbon paper was shown to provide the exact location and dimension of contact areas equally to a sensor system like the T-Scan [16]. In our pre-tests this assumption was confirmed. Thickness of one sheet of our airmail/carbon paper combination was measured to be 0.044 mm. To define the area producing an appropriate print on the paper, a steel-stamp of 1 × 1 cm (1 cm2) was pressed against the carbon/paper combination with increasing load on an Instron material testing machine (model 4204, Instron, High Wycombe, UK). The according gray-scale of 80 N/cm2 was later defined as threshold to electronically define the size of the contact area. With 500 N load on the prosthesis, excellent prints could be obtained with the white and carbon paper placed directly between the femoral components and their inlays. Thereby the total contact area can be reliably reproduced, with the limitation that peak pressure within the contact area cannot be quantified (Fig. 3). A bearing for fixation of the femoral components was constructed, so that the different flexion grades as well as varus tilt could be adjusted (with an accuracy of <1°). Because of the different designs of
Statistical evaluation showed significant differences for all test conditions between models of prostheses (p < 0.05). However, we believe that a direct “quantitative” comparison between prostheses is not justified, as the actual in vivo performance of these implants will without doubt depend on many additional and individual factors and each prosthesis is differently designed. We therefore decided to analyze each implant individually and not to emphasize on differences between the different prostheses. For each prosthesis (Fig. 2A, B, and C), a separate description and a graph indicating the total contact area are given. 3.1. Allegretto The inlay has a slight meniscal shape (Fig. 5A). Contact area is decreasing with increased varus. At flexion 0° and varus 0° there is a maximal area of 22 mm2 average. Minimal contact area of 10 mm2 is found at flexion 45° and varus 20°. The biggest decrease in contact areas can be seen in varus > 10°. Variability of contact areas in 0° varus from flexion − 20° to 90° is 13%. Frontal and sagittal diameters slightly decrease with increased varus. Statistically varus was found to be the leading condition of the interactions and its contribution was 76%. 3.2. Genesis Uni (Fig. 5B) The inlay has a slight meniscal shape. Contact area is decreasing with increased varus. At flexion − 20° and varus 5° there is a maximal area of 31 mm2 average. Minimal contact area of 11 mm2 is found at flexion 90° and varus 20°. The biggest differences between areas can be seen in varus >5°. Variability of contact areas in 0° varus from flexion − 20° to 90° is 31%. Frontal and sagittal diameters clearly decrease with increased varus. Statistically, varus was found to be the leading condition of the interactions and its contribution was 79%. 3.3. Kaps (Fig. 5C) The femoral component has the largest radius of the five examined prostheses, the contact area is nearly plane. The inlay is U-shaped in both sagittal and frontal axes. The radius of the inlay is smaller than the one of the femoral component. This has the effect that the femoral component touches the inlay in two different areas medially and laterally and produces two contact areas in − 20/− 10/0/45° flexion in 0° varus (if perfectly centered in the coronal plane), what could not be seen in the other prostheses. In the varus conditions 5° to 20° for all flexion grades only one contact area could be detected. For graphical and statistical evaluation the two areas in 0° varus were summarized. Contact area is decreasing with increased varus (as one of the edges looses contact). At flexion 45° and varus 0° there is a maximal area of 43 mm2 average. Minimal contact area of 9.6 mm2 is found at flexion 90° and varus 20°. The biggest differences between areas can be seen between varus 0° and 5° in 45° and 90° of flexion. In the other flexion grades there is a slight decrease with increased varus. Variability of contact areas in 0° varus is 21%. Frontal and sagittal diameters decrease
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Fig. 2. A. Frontal view of the evaluated femoral components: A Allegretto, B Genesis Uni, C Kaps, D Link Endo-Model, E Preservation Uni Knee. There is an obvious difference of the radius especially for Genesis Uni (B, largest radius) and Link Endo-Model (D, smallest radius). B. Lateral view of the evaluated femoral components: A Allegretto, B Genesis Uni, C Kaps, D Link Endo-Model, E Preservation Uni Knee. C. Evaluated inlays: A Allegretto, B Genesis Uni, C Kaps, D Link Endo-Model, E Preservation Uni Knee (E1 fix, E2 mobile). The Link inlay appears completely plane, whereas the other inlays show a more or less slight meniscal or U-shape.
with increased varus with the biggest steps between 0° and 5° varus. Statistically, varus was found to be the leading condition of the interactions and its contribution was 99%. 3.4. Link Endo-Model (Fig. 5D) The femoral component has the smallest radius, it is almost round. The inlay appears absolutely plane. Contact area is only slightly decreasing with increased varus. At flexion − 10° and varus 0° there is a maximal area of 21 mm2 average. Minimal contact area of 12 mm2 is found at flexion 90° and varus 20°. For flexion grades between − 20 and 45° the contact areas remain quite constant. All in all, calculated variability of contact areas in 0° varus is 30%. Frontal and sagittal diameters remain quite constant for all flexion and varus conditions, except at flexion 90° and varus 20°. In contrast to the other prostheses flexion was found to be the statistically leading condition of the interactions and its contribution was 80%. 3.5. Preservation Uni Knee with fixed inlay (Fig. 5E) The fixed inlay has a very discrete U-shape in both axes. Contact area is decreasing with increased varus. At flexion 0° and varus 5° there is a maximal area of 27 mm2
average. Minimal contact area of 12 mm2 is found at flexion − 20° and varus 20°. The biggest differences between areas can be seen between varus 5 and 10°. Variability of contact areas in 0° varus is 26%. Frontal and sagittal diameters clearly decrease with increased varus with the biggest steps between 5 and 10° varus. Statistically, varus was found to be the leading condition of the interactions and its contribution was 73%. 3.6. Preservation Uni Knee with mobile inlay (Fig. 5F) The mobile inlay is clearly U-shaped in both axes. Contact area is clearly decreasing with increased varus. At flexion 90° and varus 5° there is a maximal area of 48 mm2 average. Minimal contact area of 13 mm2 is found at flexion − 20° and varus 20°. The biggest differences between areas can be seen between varus 0 and 5°. Variability of contact areas in 0° varus is 30%. frontal and sagittal diameters also clearly decrease with increased varus with the biggest steps between 0 and 5° varus. Statistically varus was found to be the leading condition of the interactions and its contribution was 94%. In summary, it could be observed that contact areas can be reduced up to 70% with increasing varus of the femoral component. Furthermore, with flexion of the prosthesis from 0° to 90° in neutral position (0° varus), there is in most prostheses a variability of contact area of approximately 10–30% (Fig. 5A–F).
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Fig. 3. Examples of prints obtained with the carbon/paper combination with a load of 500 N.
4. Discussion Unicondylar knee prostheses have obvious advantages such as being small, needing little bone resection and retaining the ligamentous stability and physiological movement of the knee. They come however at the cost of a usually shorter time to failure, associated with polyethylene wear [8] and consecutive implant loosening or with progressive gonarthrosis of the lateral and/or patellofemoral compartment [3]. Typical wear rates for total knee prostheses are 0.33 mm/yr versus 0.49 mm/yr for unicondylar prostheses [6,7]. Most frequently cited factors influencing polyethylene wear are high contact pressure and friction in young patients with high activity, (long) shelf age of the implant, and (high) hip–knee angle [1]. There is a large number of studies assessing factors contributing to high local stress on the implant and therefore early failure and increased wear [1,2,4–7,9,10,13–15]. However it appears that the relative position of the components of unicondylar prostheses has hitherto not been considered yet as a contributing factor, which is independent from the general limb alignment.
Fig. 4. Testing setup. The fixation for the femoral components was freely adjustable for the different flexion grades as well as varus tilt. The inlays were placed in individual base plates.
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In total knee prostheses, the coronal contact angle of femur to tibia is defined by the shape of the prosthesis, as long as both condyles are articulating. But in unicondylar implants, this angle is defined by how the components are surgically implanted and is therefore potentially highly variable. Our hypothesis was, that at an unfavorable angle, instead of the round smooth center of the femoral component, the potentially sharp and small edge may articulate with the tibial plateau. (Figs. 1 and 2A). The experiments performed in this study were designed to assess this effect at different degrees of flexion and varus angle in six commercially available prostheses. We were aware that a quantification of the clinical effect of the described forms of malalignment would be very difficult to predict and virtually impossible in laboratory. So we deliberately chose a simple and direct method for obtaining the prints. Nevertheless this method showed reproducible results. Previous investigators did show that carbon paper provides the exact location and dimension of contact areas equally to a sensor system like the T-Scan [16]. This method allows to place a very thin (0.04 mm) and relatively hard material between the components, where a potentially soft and thick pressure-sensitive film might tend to measure artificially increased contact areas. Another possible method would be a computer simulation of the different prosthesis models and implantation positions. We however refrained from that for the reasons explained above. In summary, the combination of carbon/airmail paper was in our setup effective to answer the questions asked. Our numbers show that indeed, at an unfavorable angle of implantation, the contact area may be reduced by more than 70%, thereby potentially increasing the local stress by a factor 3–4. Additionally, the shape of the contact area is changed to a longitudinal edge which may be more than 10 times longer than wide, a condition for which the prostheses were most likely not designed. One prosthesis even has two contact points at a specific position (Kaps). In order to define a zone of tolerance for varus mismatch for every prosthesis it should be considered that with flexion of the prosthesis from 0° to 90° in neutral position (0° varus), there is in most prostheses a variability of contact area of approximately 10–30% (Fig. 5A–F). Here we assume that a maximal variability of 30% of the size of the contact area is acceptable and foreseen by the manufacturers. Assuming that a change of maximal 30% in contact area relative to the 0° varus and 0° flexion position is acceptable, tolerated contact areas can be calculated and a threshold can be defined with respect to the according measurements. Thus a zone of tolerance for varus malpositioning for the different tested prosthesis models can be defined and may be regarded as follows: Allegretto 10°, Genesis 5°, Kaps >20° (thereby neglecting the contact of two ridges in 0° varus at 45° and 90°), Link > 20°, Preservation fix 15°, Preservation mobile <5°. Unexpectedly the “Preservation fix” inlay had a better tolerance for malpositioning than the “Preservation mobile” inlay. We observed that models with a smaller femoral radius and a flat inlay tolerated more varus malpositioning. This finding may support reports that fullpolyethylene inlays may equally achieve very favorable survival rates. Our measurements indicate that frontal component malpositioning might be clinically more relevant for polyethylene failure than a 10% increased knee angle. A limitation of our measurement technique was that we had to focus our evaluations on the combined effect of flexion (sagittal) and varus/valgus (frontal) angulation of the prostheses. It would also be possible to assess rotational malpositioning and relative translation of the components, which might not only result in an altered contact area, but also in an increased shear load instead of compression. All of these factors would then need to be theoretically combined which appears not experimentally realistic. Due to the nature of the question asked, the test setup is not applicable to all types of implants; in preliminary tests also the Oxford Prosthesis (Biomet Inc., Warsaw, Indiana, USA) was evaluated, but had to be excluded from further
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Fig. 5. A. Allegretto: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as 10°. B. Genesis: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as 5°. C. Kaps: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as >20° (thereby neglecting the contact of two ridges in 0° varus at 45° and 90°). D. Link: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as >20°. E. Preservation with fix inlay: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as 15°. F. Preservation with mobile inlay: contact area of the femoral component on the tibial insert in mm2 ± stdev at 500 N load. Zone of tolerance may be regarded as <5°, tolerance for malpositioning is worse than with the fix inlay.
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experimental testing. This was due to the fundamentally different geometry. The Oxford prosthesis is generally considered to be fully congruent. It is most likely very little sensitive to relative malalignment of the components and has a very large absolute contact area. Nevertheless, we found that the delivered components had a very small femoral to tibial mismatch and were not perfectly congruent. Even though there might be a small mismatch in a completely new prosthesis it will most likely become more congruent after several years of use. As our tests were successful in demonstrating already very large differences between designs of prostheses and between different implant positions, we consider our methods to be adequate to prove our working hypothesis. The results presented here describe however only a singular aspect of the properties of the tested prostheses, therefore we make no qualitative ranking or prediction of the overall performance of these implants. Further there is to consider that not all instruments delivered with the prostheses allow for equally precise implantation. Maybe computer-assisted surgery will be an appropriate way to particularly eliminate the proposed position-related problems. Because there are also tribological, chemical and biological factors involved, the next appropriate step appears to perform wear testing in vitro or in vivo to quantify the impact of the prosthetic contact area on implant wear and survival. In conclusion, the data presented here show that an unfavorable angle of implantation can lead to a reduction of contact area by more than 70%, thereby potentially increasing the local stress. This finding indicates that the relative position of femoral versus tibial component in unicondylar knee prostheses is of potentially great importance for the implant durability. This factor should not be neglected, neither surgically nor when clinically or in vitro analyzing the survival of these implants. References [1] Ashraf T, Newman JH, Desai VV, Beard D, Nevelos JE. Polyethylene wear in a noncongruous unicompartmental knee replacement: a retrieval analysis. Knee 2004;11:177–81.
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