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Effect of Body Mass Index on Limb Alignment After Total Knee Arthroplasty
The Journal of Arthroplasty 28 Suppl. 1 (2013) 101–105
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Effect of Body Mass Index on Limb Alignment After Total Knee Arthroplasty Chris S. Estes, DO a, Kenneth J. Schmidt, MD a, Ryan McLemore, PhD a, Mark J. Spangehl, MD b, Henry D. Clarke, MD b a b
Banner Good Samaritan Orthopaedic Surgery Residency, Phoenix, Arizona Department of Orthopaedic Surgery, Mayo Clinic, Phoenix, Arizona
a r t i c l e
i n f o
Article history: Received 17 August 2012 Accepted 18 February 2013 Keywords: total knee arthroplasty body mass index Mal-alignment obesity complications
a b s t r a c t Prior studies have reported increased failure rates in obese patients with postoperative limb mal-alignment. This study was undertaken to determine if a relationship exists between postoperative limb alignment and BMI in patients undergoing primary TKA performed with mechanical instruments. An IRB-approved retrospective review of 196 knees was undertaken. Limb alignment was determined on full-length, standing, hip-to-ankle x-rays, preoperatively and postoperatively. The effects of gender, side, preoperative mechanical alignment and BMI on postoperative alignment were analyzed via multivariate regression analysis. Both preoperative mechanical limb alignment (P b 0.001) and BMI (P = 0.009) had a significant effect on postoperative limb alignment following TKA performed with mechanical instruments. © 2013 Elsevier Inc. All rights reserved.
Obesity has become an epidemic in the United States with the percentage of adults with a BMI over 30 kg/m 2 rising from 13% in the 1960s to 22.9% in 1988–1994, and to 33.8% in 2007–2008 [1–3]. Fehring et al [4] reported a similar increase in the rate of obese arthroplasty patients seen in their clinic from 30.4% to 52.1% in the period from 1990 to 2005. Concerns exist about the outcome of total knee arthroplasty (TKA) in obese patients. While many studies demonstrate equivalent or better improvements in functional scores after TKA in obese patients, compared to non-obese patients [5–12], increased postoperative complication rates have also been reported. Problems include the following: increased rates of wound complications, deep and superficial infections, medial collateral ligament avulsion, decreased postoperative range of motion, presence of radiolucent lines, component loosening, and revision surgery [6,7,11,13–21]. In some cases, these increased failures may be due to greater technical challenges during surgery. Theoretical concerns persist about the role that mal-alignment may play in wear and prosthetic loosening after TKA; however, this topic is controversial. One recent study suggests that factors other than mal-alignment may be more important in long-term failures [22]. Despite this important new information, laboratory data suggest that as little as 3° of mal-alignment of the mechanical axis may result in altered pressure distribution and load between compartments of
The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.02.038. Reprint request: Henry D. Clarke, MD, Department of Orthopaedic Surgery, Mayo Clinic, 5777 E. Mayo Blvd., Phoenix, AZ 85054. 0883-5403/2808-0025$36.00/0 – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.02.038
the knee [23]. Accurate component alignment in obese patients may be even more important, due to the increased stresses placed on the joint that may further accentuate abnormal loading patterns, and lead to premature failure [7,13,24]. Limited clinical data support these concerns. Two reports from the same institution have both reported that BMI and component mal-alignment are independent factors associated with increased rates of aseptic failure [25,26]. In one of these reports, Berend et al [25] identified that tibial component varus mal-alignment and BMI greater than 33.7 kg/m 2 were correlated with increased TKA failure. However, both of these studies were based on short knee radiographs, not full-length, hip-to-ankle views that demonstrate the static mechanical axis of the entire limb. The association between obesity and increased mal-alignment has also been previously investigated. The thick soft tissue envelope in obese patients may make exposure difficult, obscure bony landmarks, and obstruct accurate positioning of cutting guides [21]. All of these factors may contribute to suboptimal component positioning. Two prior studies have reported an increased incidence of component mal-alignment after TKA in obese patients, compared to the non-obese [7,18]. In distinction, three other reports noted similar component alignment in obese and non-obese patients. However, limited, or no data were presented in these publications to support these conclusions [13,15,27]. We are unaware of any adequately powered study, using standing, full-length, hip-to-ankle x-rays that has investigated this issue. The purpose of this study was to determine the effect of gender, side, BMI, and preoperative femoro-tibial angle on postoperative femoro-tibial angle. The secondary outcome measure was the incidence of mal-aligned knees in patients with a BMI b35 kg/m 2 compared to those with a BMI ≥35 kg/m 2.
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C.S. Estes et al. / The Journal of Arthroplasty 28 Suppl. 1 (2013) 101–105
Methods
Table 2 Comparison of Knees with BMI b35 kg/m2 vs. BMI ≥35 kg/m2.
A retrospective, IRB-approved review of medical records and xrays was performed for all patients who underwent primary TKA by the senior author between January 1, 2009 and November 22, 2011. During this period, all patients underwent routine preoperative and postoperative full-length standing hip-to-ankle x-rays of both lower extremities. BMI was recorded prospectively in all patients within 4 weeks prior to surgery. One hundred seventy-five consecutive knees were initially reviewed and additional consecutive knees with a BMI N35 kg/m 2 were added in order to appropriately power the study. In total, data collection was performed on 234 knees. Exclusion criteria included the following: absence of full-length preoperative or postoperative hip-ankle x-rays; poor quality x-rays (inaccurate digital stitching of the images, suboptimal limb rotation and/or knee flexion); and stemmed components. After applying these criteria, 38 knees (~16%) were removed from the data set because of missing full-length films (13), excessive knee flexion and/or rotation (21), digital stitching errors (2), and the presence of stemmed components (2). Therefore, the final study population consisted of 196 knees (Table 1). One hundred twelve knees belonged to patients with a BMI of b35 kg/m 2. Eighty-four knees belonged to patients with a BMI ≥ 35 kg/m 2 (48 knees, BMI ≥35–39 kg/m 2; 36 knees, BMI ≥40 kg/m 2) (Table 2). The same surgical technique had been used in all patients. Preoperatively, the full-length x-ray was used to measure the angle
Range Post-op alignment (degrees) Mean Post-op alignment (degrees) Total number of knees Number of knees mal-aligned Percent of knees mal-aligned
BMI b35 kg/m2
BMI ≥35 kg/m2
−4.9 to 3.9 −1.0 112 27 24.1%
−8.4 to 4.9 −2.0 84 30 35.7%
subtended by the mechanical and anatomic axes of the femur to determine the angle of the distal femoral cut, which was made at either 5° or 6° of valgus using an intra-medullary, mechanical cutting guide. The tibial cut was made perpendicular to the mechanical axis of the tibia using an extra-medullary cutting guide. Hip-to-ankle radiographs were repeated at the 2- to 3-week postoperative visit for all patients.
Table 1 Study Group With Reasons for Exclusion.
Initial group: 175 consecutive TKAs
Additional 59 consecutive TKAs in patients with BMI >35
234 TKAs reviewed
38 TKAs were excluded: 13 with no full-length x-ray 21 with excessive knee flexion and/or rotation on x-ray 2 with digital stitching errors 2 with stemmed components
Final study group: 196 TKAs Fig. 1. Femoro-tibial angle as measured on a standing, full-length hip-to-ankle x-ray.
C.S. Estes et al. / The Journal of Arthroplasty 28 Suppl. 1 (2013) 101–105
The preoperative and postoperative x-rays were evaluated in the coronal plane only. The Cobb Angle function of a medical image viewer program (QReads, Mayo Clinic, Rochester, MN) was used to quantify the angle subtending the mechanical axis of the femur and mechanical axis of the tibia (the femoro-tibial angle). A line drawn from the center of the femoral head to the center of the femoral condyles represented the mechanical axis of the femur. A line drawn from the center of the tibial plateau to the center of the talus represented the mechanical axis of the tibia (Fig. 1). The gender, operative side, and preoperative and postoperative mechanical axis of the limb were recorded for all patients. The primary outcome measure was the effect of gender, side, BMI, and preoperative femoro-tibial angle on postoperative femoro-tibial angle. The secondary outcome measure was the incidence of malaligned knees in patients with a BMI b35 kg/m 2 compared to those with a BMI ≥ 35 kg/m 2. A femoro-tibial angle greater than 3° of varus or valgus was considered mal-aligned. Statistics Statistical analysis was performed with Minitab (Minitab, Inc., State College, PA). Multivariate linear regression analysis was performed to examine the effect of gender, side, BMI, and preoperative femoro-tibial angle on postoperative femoro-tibial angle. Logistic regression was used to examine the contribution of these risk factors to the risk of mal-alignment in this patient population. The incidence of mal-alignment was compared in patients with a BMI b35 kg/m 2 versus those with a BMI ≥ 35 kg/m 2 using the chisquared test. A-priori power analysis was performed to determine the number of knees required to detect a 20% difference in the number of mal-aligned knees in patients with a BMI b 35 kg/m 2 compared to those with a BMI ≥ 35 kg/m 2. To achieve a power of 80% it was determined that 89 knees would be required in each group.
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Fig. 3. Postoperative femoro-tibial angle vs. BMI.
patients postoperatively in varus, the average angle was 2.4°. The overall average alignment of patients in the study was 1.4° of varus. The greatest mal-alignment measured was 8.4° of varus. Multivariate regression analysis revealed that preoperative mechanical limb alignment (P b 0.001) and BMI (P = 0.009) both affected postoperative limb alignment. Gender and side did not have a significant effect on postoperative alignment (Table 3). Analysis with logistic regression supported this finding, indicating that preoperative mechanical limb alignment (P = 0.003) and increasing BMI (P = 0.076) were associated with an increased risk of a patient being in mal-alignment post-surgically. Pearson chi-squared analysis revealed that there was a strong trend for increased incidence of mal-alignment in patients with BMI ≥ 35 kg/m 2 compared to those with a BMI b35 kg/m 2 (P = 0.077; OR 1.75; 95% CI = 0.94–3.26).
Results Discussion The breakdown of patients enrolled in this study by BMI is detailed in Fig. 2A. The population is well distributed with roughly half of the knees belonging to patients with a BMI b 35 kg/m2 and half of the knees of patients with a BMI ≥35 kg/m2 (Table 2). There are only a small number of patients with BMI of ≥45kg/m2. Fig. 2B demonstrates the distribution of postoperative alignment. The distribution of patient alignment versus BMI is shown in Fig. 3. In the 196 knees included in the study, the maximum valgus angle measured was 4.9°. In total, 43 patients (22%) were in some degree of valgus alignment postoperatively and 149 of the patients (76%) were in some degree of varus alignment postoperatively. Four patients (2%) were judged to be in perfect alignment (0°). For patients in valgus postoperatively, the average angle was 1.8°. For
An increased rate of obesity has been reported in patients undergoing TKA. While favorable functional improvements have been reported, concerns exist regarding the durability of TKA in these obese patients. In addition to increased short-term complications, including wound healing problems and infection, higher rates of long-term failure because of wear and loosening have been reported [6,7,11,13–21]. Malalignment may contribute to these late failures [26]. While a recent study by Parratte et al [22] questions the relationship between limb alignment that falls outside of a neutral mechanical axis ±3° and longterm failures, theoretical concerns about increased stresses in the polyethylene and at the bone–cement interface persist. In obese
Fig. 2. Histograms showing overall patient distribution by BMI (A) and postoperative femoro-tibial (F-T) angle (B). The desired target range of neutral ±3° is demonstrated by the dotted lines (B).
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Table 3 Results of Multivariate Regression Analysis: Factors Affecting Post-Surgical Alignment. Predictor
P Value
Side Gender BMI Preoperative femoro-tibial angle
0.466 0.103 0.009 b0.001
patients, where the joint forces may be greater than in non-obese patients, these concerns are even more pertinent. While prior studies have identified increased BMI and malalignment as independent risk factors for long-term failure, prior studies have produced conflicting information about the role of increased BMI on the rate of postoperative mal-alignment [7,13,15,18,25–27]. Concerns about the use of short knee x-rays, insufficient power, and other methodological problems in these studies contribute to this controversy. Krushell and Fingeroth [7] compared component alignment in 39 patients with a BMI N 40 kg/m2 to a control group of 39 matched patients with a BMI b30 kg/m2. Standard-length, weight-bearing, antero-posterior radiographs, and shoot-through, hanging-extension radiographs were obtained. The mean femoro-tibial angle was 2.6° of valgus in the morbidly obese group (range 4° varus to 8° valgus). The mean femoro-tibial angle in the control group was 5.5° valgus (range: 2° varus to 12° valgus) (P b 0.001). Both the femoral and tibial components had a tendency to be placed in more varus in the morbidly obese group [femoral component: 5.0° vs. 6.5° valgus (P b 0.05); tibial component: 2.5° vs. 1.0° varus (P b 0.05)] [7]. In a second study, Järvenpää et al [18] retrospectively compared 52 patients with a BMI N30 kg/m 2 to 48 patients with a BMI b 30 kg/ m 2. Long, weight-bearing coronal and sagittal radiographs were obtained. Deviation within ± 3° from the mechanical axis was considered acceptable. The mean of deviation from the mechanical axis was similar in both groups. However, 15 patients in the obese group deviated 3° or more from the mechanical axis while only 5 knees in the non-obese group deviated 3° or more from the mechanical axis (P = 0.028) [18]. Three other studies provide limited information. Foran et al [15] reported that knee-alignment measures between morbidly obese, obese and non-obese were virtually identical among the groups. However, the methods and data for this outcome measure were not presented in the paper [15]. In a prospective, matched study, Amin et al [13] reported that the mean postoperative total valgus angle was similar between a group of 40 knees with a BMI N 40 kg/m 2 and a matched group with a BMI b 30 kg/m 2. However, again, the methods for this portion of the study and the incidence of mal-alignment were not presented. Finally, Jackson and et al [27] performed a case-matched study, comparing outcomes between patients with a BMI N 30 kg/m 2 to those with a BMI b30 kg/m 2 undergoing cement-less TKA. Standard AP, lateral, patellar, and weight-bearing knee radiographs were evaluated with the Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System. They reported that there was no difference in femoral, or tibial component alignment in the coronal, or sagittal plane; however, quantitative data were not provided [27]. In distinction to the noted limitations of these prior studies, we believe that this is the first adequately powered study using fulllength standing, hip-to-ankle x-rays to examine the effect of BMI on postoperative alignment after TKA. We demonstrate that both greater preoperative deformity and increased BMI have a significant effect on postoperative mechanical axis alignment after TKA. This effect of BMI on mal-alignment may be due to the greater soft tissue envelope about the knee that can hinder exposure, obscure bony landmarks about the knee and ankle, and interfere with accurate placement of cutting guides. Limitations of the study include the high rate of poor quality xrays that led to the exclusion of about 16% of the study patients. In
many cases, these problems resulted from the knee being excessively flexed or externally rotated, despite use of a standard x-ray protocol. This may have been caused by the timing of the postoperative studies that were performed 2–3 weeks after surgery. In the early postoperative period, some of the patients may not have been able to comply with the positioning instructions because of pain, or limitation of motion. Therefore, in future studies it may be better to obtain this x-ray at a later time, perhaps 6 weeks. In addition to this problem with the x-ray quality, is the inherent limitation associated with standing x-rays; this technique provides only a single static evaluation of limb alignment. Indeed, a dynamic evaluation of joint loading, such as through gait analysis, may be necessary to draw any conclusions about optimal limb alignment after TKA. Another significant limitation is while BMI is a continuous variable representing obesity, this study has many more obese patients in the 35- to 45-kg/m 2 range than in the 45- to 55+-kg/ m 2 range. It is not known from this analysis if the risk of malalignment is further increased as a patient becomes more obese. Finally, a single surgeon performed all surgeries and the results of this study may not be applicable to other surgeons. The potential that other surgeons may be more or less accurate in restoring alignment in obese patients exists. At the current time, a question about what is the optimal limb alignment after TKA to maximize long-term prosthesis survival exists [22]. While recent work suggests that the traditional goal of a neutral axis ± 3° may not be best for all patients, the present study demonstrates that increased BMI makes it more difficult to achieve the desired goal using mechanical, intra-medullary femoral guides, and extra-medullary tibial guides [22]. We also demonstrate that increased preoperative deformity is associated with more deviation from the goal. Therefore, in patients with greater BMI and increased deformity, the surgeon should be aware that alternative methods for determining limb alignment should be considered, especially once the optimal goal has been determined. However, importantly, many of the issues that likely make the use of these standard cutting guides less accurate in obese patients may also affect the accuracy of alternative methods. Contemporary computer navigation systems may be affected both by difficulties with exposure and with referencing the pertinent bony landmarks in obese patients. Similarly, custom cutting guides, based on preoperative imaging studies, may be less accurate in patients with increased BMI because of exposure issues; however, a problem unique to these custom guides in obese patients is soft tissue artifact that could reduce the resolution of the bone and cartilage surfaces on the preoperative studies. This may cause the manufacture of poorly fitting custom guides. Certainly, these issues require further investigation. In summary, while the goal for optimal limb alignment after TKA remains controversial, this study demonstrates that in patients with increased BMI, especially in the presence of greater preoperative deformity, standard cutting guides are less accurate in achieving the desired goal. Once the desired alignment goal has been determined, alternative methods of preparing the femoral and tibial cuts may be preferable. References 1. Wang Y, Beydoun MA. The obesity epidemic in the United States—gender, age, socioeconomic, racial/ethnic, and geographic characteristics: a systematic review and meta-regression analysis. Epidemiol Rev 2007;29:6. 2. Flegal KM, Carroll MD, Ogden CL, et al. Prevalence and trends in obesity among US adults, 1999–2000. JAMA 2002;288(14):1723. 3. Flegal KM, Carroll MD, Ogden CL, et al. Prevalence and trends in obesity among US adults, 1999–2008. JAMA 2010;303(3):235. 4. Fehring TK, Odum SM, Griffin WL, et al. The obesity epidemic: its effect on total joint arthroplasty. J Arthroplasty 2007;22(6 Suppl 2):71. 5. Amin AK, Patton JT, Cook RE, et al. Does obesity influence the clinical outcome at five years following total knee replacement for osteoarthritis? J Bone Joint Surg Br 2006;88(3):335.
C.S. Estes et al. / The Journal of Arthroplasty 28 Suppl. 1 (2013) 101–105 6. Dewan A, Bertolusso R, Karastinos A, Conditt M, Noble PC, Parsley BS. Implant durability and knee function after total knee arthroplasty in the morbidly obese patient. J Arthroplasty. 2009 Sep;24(6 Suppl):89–94, 94.e1–3. 7. Krushell RJ, Fingeroth RJ. Primary total knee arthroplasty in morbidly obese patients: a 5- to 14-year follow-up study. J Arthroplasty 2007;22(6 Suppl 2):77. 8. Mont MA, Mathur SK, Krackow KA, et al. Cementless total knee arthroplasty in obese patients. A comparison with a matched control group. J Arthroplasty 1996;11(2):153. 9. Nguyen DM, El-Serag HB. The epidemiology of obesity. Gastroenterol Clin North Am 2010;39(1):1. 10. Rajgopal V, Bourne RB, Chesworth BM, et al. The impact of morbid obesity on patient outcomes after total knee arthroplasty. J Arthroplasty 2008;23(6):795. 11. Samson AJ, Mercer GE, Campbell DG. Total knee replacement in the morbidly obese: a literature review. ANZ J Surg 2010;80(9):595. 12. Singh JA, Gabriel SE, Lewallen DG. Higher body mass index is not associated with worse pain outcomes after primary or revision total knee arthroplasty. J Arthroplasty 2011;26(3):366.e1. 13. Amin AK, Clayton RAE, Patton JT, et al. Total knee replacement in morbidly obese patients. Results of a prospective, matched study. J Bone Joint Surg Br 2006;88(10):1321. 14. Dowsey MM, Liew D, Stoney JD, et al. The impact of pre-operative obesity on weight change and outcome in total knee replacement: a prospective study of 529 consecutive patients. J Bone Joint Surg Br 2010;92(4):513. 15. Foran JRH, Mont MA, Etienne G, et al. The outcome of total knee arthroplasty in obese patients. J Bone Joint Surg Am 2004;86-A(8):1609.
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16. Gillespie GN, Porteous AJ. Obesity and knee arthroplasty. Knee 2007;14(2):81. 17. Griffin FM, Scuderi GR, Insall JN, et al. Total knee arthroplasty in patients who were obese with 10 years followup. Clin Orthop Relat Res 1998;356:28. 18. Järvenpää J, Kettunen J, Kröger H, et al. Obesity may impair the early outcome of total knee arthroplasty. Scand J Surg 2010;99(1):45. 19. Malinzak RA, Ritter MA, Berend ME, et al. Morbidly obese, diabetic, younger, and unilateral joint arthroplasty patients have elevated total joint arthroplasty infection rates. J Arthroplasty 2009;24(6 Suppl):84. 20. Spicer DD, Pomeroy DL, Badenhausen WE, et al. Body mass index as a predictor of outcome in total knee replacement. Int Orthop 2001;25(4):246. 21. Winiarsky R, Barth P, Lotke P. Total knee arthroplasty in morbidly obese patients. J Bone Joint Surg Am 1998;80(12):1770. 22. Parratte S, Pagnano MW, Trousdale RT, et al. Effect of postoperative mechanical axis alignment on the fifteen-year survival of modern, cemented total knee replacements. J Bone Joint Surg Am 2010;92(12):2143. 23. Werner FW, Ayers DC, Maletsky LP, et al. The effect of valgus/varus malalignment on load distribution in total knee replacements. J Biomech 2005;38(2):349. 24. Booth Jr RE. Total knee arthroplasty in the obese patient: tips and quips. J Arthroplasty 2002;17(4 Suppl 1):69. 25. Berend ME, Ritter MA, Meding JB, et al. Tibial component failure mechanisms in total knee arthroplasty. Clin Orthop Relat Res 2004;428:26. 26. Ritter MA, Davis KE, Meding JB, et al. The effect of alignment and BMI on failure of total knee replacement. J Bone Joint Surg Am 2011;93(17):1588. 27. Jackson MP, Sexton SA, Walter WL, et al. The impact of obesity on the mid-term outcome of cementless total knee replacement. J Bone Joint Surg Br 2009;91(8):1044.
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Effect of Body Mass Index on Limb Alignment After Total Knee Arthroplasty Chris S. Estes, DO a, Kenneth J. Schmidt, MD a, Ryan McLemore, PhD a, Mark J. Spangehl, MD b, Henry D. Clarke, MD b a b
Banner Good Samaritan Orthopaedic Surgery Residency, Phoenix, Arizona Department of Orthopaedic Surgery, Mayo Clinic, Phoenix, Arizona
a r t i c l e
i n f o
Article history: Received 17 August 2012 Accepted 18 February 2013 Keywords: total knee arthroplasty body mass index Mal-alignment obesity complications
a b s t r a c t Prior studies have reported increased failure rates in obese patients with postoperative limb mal-alignment. This study was undertaken to determine if a relationship exists between postoperative limb alignment and BMI in patients undergoing primary TKA performed with mechanical instruments. An IRB-approved retrospective review of 196 knees was undertaken. Limb alignment was determined on full-length, standing, hip-to-ankle x-rays, preoperatively and postoperatively. The effects of gender, side, preoperative mechanical alignment and BMI on postoperative alignment were analyzed via multivariate regression analysis. Both preoperative mechanical limb alignment (P b 0.001) and BMI (P = 0.009) had a significant effect on postoperative limb alignment following TKA performed with mechanical instruments. © 2013 Elsevier Inc. All rights reserved.
Obesity has become an epidemic in the United States with the percentage of adults with a BMI over 30 kg/m 2 rising from 13% in the 1960s to 22.9% in 1988–1994, and to 33.8% in 2007–2008 [1–3]. Fehring et al [4] reported a similar increase in the rate of obese arthroplasty patients seen in their clinic from 30.4% to 52.1% in the period from 1990 to 2005. Concerns exist about the outcome of total knee arthroplasty (TKA) in obese patients. While many studies demonstrate equivalent or better improvements in functional scores after TKA in obese patients, compared to non-obese patients [5–12], increased postoperative complication rates have also been reported. Problems include the following: increased rates of wound complications, deep and superficial infections, medial collateral ligament avulsion, decreased postoperative range of motion, presence of radiolucent lines, component loosening, and revision surgery [6,7,11,13–21]. In some cases, these increased failures may be due to greater technical challenges during surgery. Theoretical concerns persist about the role that mal-alignment may play in wear and prosthetic loosening after TKA; however, this topic is controversial. One recent study suggests that factors other than mal-alignment may be more important in long-term failures [22]. Despite this important new information, laboratory data suggest that as little as 3° of mal-alignment of the mechanical axis may result in altered pressure distribution and load between compartments of
The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.02.038. Reprint request: Henry D. Clarke, MD, Department of Orthopaedic Surgery, Mayo Clinic, 5777 E. Mayo Blvd., Phoenix, AZ 85054. 0883-5403/2808-0025$36.00/0 – see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.02.038
the knee [23]. Accurate component alignment in obese patients may be even more important, due to the increased stresses placed on the joint that may further accentuate abnormal loading patterns, and lead to premature failure [7,13,24]. Limited clinical data support these concerns. Two reports from the same institution have both reported that BMI and component mal-alignment are independent factors associated with increased rates of aseptic failure [25,26]. In one of these reports, Berend et al [25] identified that tibial component varus mal-alignment and BMI greater than 33.7 kg/m 2 were correlated with increased TKA failure. However, both of these studies were based on short knee radiographs, not full-length, hip-to-ankle views that demonstrate the static mechanical axis of the entire limb. The association between obesity and increased mal-alignment has also been previously investigated. The thick soft tissue envelope in obese patients may make exposure difficult, obscure bony landmarks, and obstruct accurate positioning of cutting guides [21]. All of these factors may contribute to suboptimal component positioning. Two prior studies have reported an increased incidence of component mal-alignment after TKA in obese patients, compared to the non-obese [7,18]. In distinction, three other reports noted similar component alignment in obese and non-obese patients. However, limited, or no data were presented in these publications to support these conclusions [13,15,27]. We are unaware of any adequately powered study, using standing, full-length, hip-to-ankle x-rays that has investigated this issue. The purpose of this study was to determine the effect of gender, side, BMI, and preoperative femoro-tibial angle on postoperative femoro-tibial angle. The secondary outcome measure was the incidence of mal-aligned knees in patients with a BMI b35 kg/m 2 compared to those with a BMI ≥35 kg/m 2.
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C.S. Estes et al. / The Journal of Arthroplasty 28 Suppl. 1 (2013) 101–105
Methods
Table 2 Comparison of Knees with BMI b35 kg/m2 vs. BMI ≥35 kg/m2.
A retrospective, IRB-approved review of medical records and xrays was performed for all patients who underwent primary TKA by the senior author between January 1, 2009 and November 22, 2011. During this period, all patients underwent routine preoperative and postoperative full-length standing hip-to-ankle x-rays of both lower extremities. BMI was recorded prospectively in all patients within 4 weeks prior to surgery. One hundred seventy-five consecutive knees were initially reviewed and additional consecutive knees with a BMI N35 kg/m 2 were added in order to appropriately power the study. In total, data collection was performed on 234 knees. Exclusion criteria included the following: absence of full-length preoperative or postoperative hip-ankle x-rays; poor quality x-rays (inaccurate digital stitching of the images, suboptimal limb rotation and/or knee flexion); and stemmed components. After applying these criteria, 38 knees (~16%) were removed from the data set because of missing full-length films (13), excessive knee flexion and/or rotation (21), digital stitching errors (2), and the presence of stemmed components (2). Therefore, the final study population consisted of 196 knees (Table 1). One hundred twelve knees belonged to patients with a BMI of b35 kg/m 2. Eighty-four knees belonged to patients with a BMI ≥ 35 kg/m 2 (48 knees, BMI ≥35–39 kg/m 2; 36 knees, BMI ≥40 kg/m 2) (Table 2). The same surgical technique had been used in all patients. Preoperatively, the full-length x-ray was used to measure the angle
Range Post-op alignment (degrees) Mean Post-op alignment (degrees) Total number of knees Number of knees mal-aligned Percent of knees mal-aligned
BMI b35 kg/m2
BMI ≥35 kg/m2
−4.9 to 3.9 −1.0 112 27 24.1%
−8.4 to 4.9 −2.0 84 30 35.7%
subtended by the mechanical and anatomic axes of the femur to determine the angle of the distal femoral cut, which was made at either 5° or 6° of valgus using an intra-medullary, mechanical cutting guide. The tibial cut was made perpendicular to the mechanical axis of the tibia using an extra-medullary cutting guide. Hip-to-ankle radiographs were repeated at the 2- to 3-week postoperative visit for all patients.
Table 1 Study Group With Reasons for Exclusion.
Initial group: 175 consecutive TKAs
Additional 59 consecutive TKAs in patients with BMI >35
234 TKAs reviewed
38 TKAs were excluded: 13 with no full-length x-ray 21 with excessive knee flexion and/or rotation on x-ray 2 with digital stitching errors 2 with stemmed components
Final study group: 196 TKAs Fig. 1. Femoro-tibial angle as measured on a standing, full-length hip-to-ankle x-ray.
C.S. Estes et al. / The Journal of Arthroplasty 28 Suppl. 1 (2013) 101–105
The preoperative and postoperative x-rays were evaluated in the coronal plane only. The Cobb Angle function of a medical image viewer program (QReads, Mayo Clinic, Rochester, MN) was used to quantify the angle subtending the mechanical axis of the femur and mechanical axis of the tibia (the femoro-tibial angle). A line drawn from the center of the femoral head to the center of the femoral condyles represented the mechanical axis of the femur. A line drawn from the center of the tibial plateau to the center of the talus represented the mechanical axis of the tibia (Fig. 1). The gender, operative side, and preoperative and postoperative mechanical axis of the limb were recorded for all patients. The primary outcome measure was the effect of gender, side, BMI, and preoperative femoro-tibial angle on postoperative femoro-tibial angle. The secondary outcome measure was the incidence of malaligned knees in patients with a BMI b35 kg/m 2 compared to those with a BMI ≥ 35 kg/m 2. A femoro-tibial angle greater than 3° of varus or valgus was considered mal-aligned. Statistics Statistical analysis was performed with Minitab (Minitab, Inc., State College, PA). Multivariate linear regression analysis was performed to examine the effect of gender, side, BMI, and preoperative femoro-tibial angle on postoperative femoro-tibial angle. Logistic regression was used to examine the contribution of these risk factors to the risk of mal-alignment in this patient population. The incidence of mal-alignment was compared in patients with a BMI b35 kg/m 2 versus those with a BMI ≥ 35 kg/m 2 using the chisquared test. A-priori power analysis was performed to determine the number of knees required to detect a 20% difference in the number of mal-aligned knees in patients with a BMI b 35 kg/m 2 compared to those with a BMI ≥ 35 kg/m 2. To achieve a power of 80% it was determined that 89 knees would be required in each group.
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Fig. 3. Postoperative femoro-tibial angle vs. BMI.
patients postoperatively in varus, the average angle was 2.4°. The overall average alignment of patients in the study was 1.4° of varus. The greatest mal-alignment measured was 8.4° of varus. Multivariate regression analysis revealed that preoperative mechanical limb alignment (P b 0.001) and BMI (P = 0.009) both affected postoperative limb alignment. Gender and side did not have a significant effect on postoperative alignment (Table 3). Analysis with logistic regression supported this finding, indicating that preoperative mechanical limb alignment (P = 0.003) and increasing BMI (P = 0.076) were associated with an increased risk of a patient being in mal-alignment post-surgically. Pearson chi-squared analysis revealed that there was a strong trend for increased incidence of mal-alignment in patients with BMI ≥ 35 kg/m 2 compared to those with a BMI b35 kg/m 2 (P = 0.077; OR 1.75; 95% CI = 0.94–3.26).
Results Discussion The breakdown of patients enrolled in this study by BMI is detailed in Fig. 2A. The population is well distributed with roughly half of the knees belonging to patients with a BMI b 35 kg/m2 and half of the knees of patients with a BMI ≥35 kg/m2 (Table 2). There are only a small number of patients with BMI of ≥45kg/m2. Fig. 2B demonstrates the distribution of postoperative alignment. The distribution of patient alignment versus BMI is shown in Fig. 3. In the 196 knees included in the study, the maximum valgus angle measured was 4.9°. In total, 43 patients (22%) were in some degree of valgus alignment postoperatively and 149 of the patients (76%) were in some degree of varus alignment postoperatively. Four patients (2%) were judged to be in perfect alignment (0°). For patients in valgus postoperatively, the average angle was 1.8°. For
An increased rate of obesity has been reported in patients undergoing TKA. While favorable functional improvements have been reported, concerns exist regarding the durability of TKA in these obese patients. In addition to increased short-term complications, including wound healing problems and infection, higher rates of long-term failure because of wear and loosening have been reported [6,7,11,13–21]. Malalignment may contribute to these late failures [26]. While a recent study by Parratte et al [22] questions the relationship between limb alignment that falls outside of a neutral mechanical axis ±3° and longterm failures, theoretical concerns about increased stresses in the polyethylene and at the bone–cement interface persist. In obese
Fig. 2. Histograms showing overall patient distribution by BMI (A) and postoperative femoro-tibial (F-T) angle (B). The desired target range of neutral ±3° is demonstrated by the dotted lines (B).
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Table 3 Results of Multivariate Regression Analysis: Factors Affecting Post-Surgical Alignment. Predictor
P Value
Side Gender BMI Preoperative femoro-tibial angle
0.466 0.103 0.009 b0.001
patients, where the joint forces may be greater than in non-obese patients, these concerns are even more pertinent. While prior studies have identified increased BMI and malalignment as independent risk factors for long-term failure, prior studies have produced conflicting information about the role of increased BMI on the rate of postoperative mal-alignment [7,13,15,18,25–27]. Concerns about the use of short knee x-rays, insufficient power, and other methodological problems in these studies contribute to this controversy. Krushell and Fingeroth [7] compared component alignment in 39 patients with a BMI N 40 kg/m2 to a control group of 39 matched patients with a BMI b30 kg/m2. Standard-length, weight-bearing, antero-posterior radiographs, and shoot-through, hanging-extension radiographs were obtained. The mean femoro-tibial angle was 2.6° of valgus in the morbidly obese group (range 4° varus to 8° valgus). The mean femoro-tibial angle in the control group was 5.5° valgus (range: 2° varus to 12° valgus) (P b 0.001). Both the femoral and tibial components had a tendency to be placed in more varus in the morbidly obese group [femoral component: 5.0° vs. 6.5° valgus (P b 0.05); tibial component: 2.5° vs. 1.0° varus (P b 0.05)] [7]. In a second study, Järvenpää et al [18] retrospectively compared 52 patients with a BMI N30 kg/m 2 to 48 patients with a BMI b 30 kg/ m 2. Long, weight-bearing coronal and sagittal radiographs were obtained. Deviation within ± 3° from the mechanical axis was considered acceptable. The mean of deviation from the mechanical axis was similar in both groups. However, 15 patients in the obese group deviated 3° or more from the mechanical axis while only 5 knees in the non-obese group deviated 3° or more from the mechanical axis (P = 0.028) [18]. Three other studies provide limited information. Foran et al [15] reported that knee-alignment measures between morbidly obese, obese and non-obese were virtually identical among the groups. However, the methods and data for this outcome measure were not presented in the paper [15]. In a prospective, matched study, Amin et al [13] reported that the mean postoperative total valgus angle was similar between a group of 40 knees with a BMI N 40 kg/m 2 and a matched group with a BMI b 30 kg/m 2. However, again, the methods for this portion of the study and the incidence of mal-alignment were not presented. Finally, Jackson and et al [27] performed a case-matched study, comparing outcomes between patients with a BMI N 30 kg/m 2 to those with a BMI b30 kg/m 2 undergoing cement-less TKA. Standard AP, lateral, patellar, and weight-bearing knee radiographs were evaluated with the Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System. They reported that there was no difference in femoral, or tibial component alignment in the coronal, or sagittal plane; however, quantitative data were not provided [27]. In distinction to the noted limitations of these prior studies, we believe that this is the first adequately powered study using fulllength standing, hip-to-ankle x-rays to examine the effect of BMI on postoperative alignment after TKA. We demonstrate that both greater preoperative deformity and increased BMI have a significant effect on postoperative mechanical axis alignment after TKA. This effect of BMI on mal-alignment may be due to the greater soft tissue envelope about the knee that can hinder exposure, obscure bony landmarks about the knee and ankle, and interfere with accurate placement of cutting guides. Limitations of the study include the high rate of poor quality xrays that led to the exclusion of about 16% of the study patients. In
many cases, these problems resulted from the knee being excessively flexed or externally rotated, despite use of a standard x-ray protocol. This may have been caused by the timing of the postoperative studies that were performed 2–3 weeks after surgery. In the early postoperative period, some of the patients may not have been able to comply with the positioning instructions because of pain, or limitation of motion. Therefore, in future studies it may be better to obtain this x-ray at a later time, perhaps 6 weeks. In addition to this problem with the x-ray quality, is the inherent limitation associated with standing x-rays; this technique provides only a single static evaluation of limb alignment. Indeed, a dynamic evaluation of joint loading, such as through gait analysis, may be necessary to draw any conclusions about optimal limb alignment after TKA. Another significant limitation is while BMI is a continuous variable representing obesity, this study has many more obese patients in the 35- to 45-kg/m 2 range than in the 45- to 55+-kg/ m 2 range. It is not known from this analysis if the risk of malalignment is further increased as a patient becomes more obese. Finally, a single surgeon performed all surgeries and the results of this study may not be applicable to other surgeons. The potential that other surgeons may be more or less accurate in restoring alignment in obese patients exists. At the current time, a question about what is the optimal limb alignment after TKA to maximize long-term prosthesis survival exists [22]. While recent work suggests that the traditional goal of a neutral axis ± 3° may not be best for all patients, the present study demonstrates that increased BMI makes it more difficult to achieve the desired goal using mechanical, intra-medullary femoral guides, and extra-medullary tibial guides [22]. We also demonstrate that increased preoperative deformity is associated with more deviation from the goal. Therefore, in patients with greater BMI and increased deformity, the surgeon should be aware that alternative methods for determining limb alignment should be considered, especially once the optimal goal has been determined. However, importantly, many of the issues that likely make the use of these standard cutting guides less accurate in obese patients may also affect the accuracy of alternative methods. Contemporary computer navigation systems may be affected both by difficulties with exposure and with referencing the pertinent bony landmarks in obese patients. Similarly, custom cutting guides, based on preoperative imaging studies, may be less accurate in patients with increased BMI because of exposure issues; however, a problem unique to these custom guides in obese patients is soft tissue artifact that could reduce the resolution of the bone and cartilage surfaces on the preoperative studies. This may cause the manufacture of poorly fitting custom guides. Certainly, these issues require further investigation. In summary, while the goal for optimal limb alignment after TKA remains controversial, this study demonstrates that in patients with increased BMI, especially in the presence of greater preoperative deformity, standard cutting guides are less accurate in achieving the desired goal. Once the desired alignment goal has been determined, alternative methods of preparing the femoral and tibial cuts may be preferable. References 1. Wang Y, Beydoun MA. The obesity epidemic in the United States—gender, age, socioeconomic, racial/ethnic, and geographic characteristics: a systematic review and meta-regression analysis. Epidemiol Rev 2007;29:6. 2. Flegal KM, Carroll MD, Ogden CL, et al. Prevalence and trends in obesity among US adults, 1999–2000. JAMA 2002;288(14):1723. 3. Flegal KM, Carroll MD, Ogden CL, et al. Prevalence and trends in obesity among US adults, 1999–2008. JAMA 2010;303(3):235. 4. Fehring TK, Odum SM, Griffin WL, et al. The obesity epidemic: its effect on total joint arthroplasty. J Arthroplasty 2007;22(6 Suppl 2):71. 5. Amin AK, Patton JT, Cook RE, et al. Does obesity influence the clinical outcome at five years following total knee replacement for osteoarthritis? J Bone Joint Surg Br 2006;88(3):335.
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