Distal femoral bone mineral density decreases following patellofemoral arthroplasty: 1-year follow-up study of 14 patients

Background: The bone mineral density (BMD) o f the distal fem ur decreases by 16-36% w ith in one year after total knee arthroplasty (TKA) because o f the femoral component's stress-shielding effect. The aim o f this prospective study was to determ ine the quantitative change from the baseline BMD in the distal fem ur 1 year after patellofemoral arthroplasty using dual-energy X-ray absorptiometry (DXA). Methods: Between December 2007 and December 2008, 14 patients had patellofemoral arthroplasty for isolated patellofemoral osteoarthritis. Distal femoral BMD was assessed using DXA in 2 regions o f interest (ROI) on the lateral view 2 weeks before and 12 m onths after patellofemoral arthroplasty. The contra-lateral knee was used as a control, w ith BMD measurements performed in identical ROIs. Results: The mean change from baseline BMD in the operated knees after 1 year was -0.169 g /cm 2 (95% CI: -0.293 to 0.046 g /cm 2) behind the anterior flange (-15%), and -0.076 g /cm 2 (95% CI: -0.177 to 0.024 g/cm 2) in the supracondylar area 1 cm above the prosthesis (-8%) (p = 0.01 and p = 0.13, respectively). The mean change from baseline BMD in the non-operated knees after 1 year was 0.016 g /cm 2 (95% CI: -0.152 to 0.185 g /cm 2) behind the anterior flange (2%), and 0.023 g /cm 2 (95% CI: -0.135 to 0.180 g /cm 2) in the supracondylar area 1 cm above the prosthesis (2%) (p = 0.83, and p = 0.76, respectively). Conclusions: Our findings suggest tha t patellofemoral arthroplasty results in a statistically significant decrease in BMD behind the anterior flange. Background After total knee arthroplasty (TKA), the bone mineral density (BMD) of the distal femur decreases by 16-36% within one year because of the femoral component's stress-shielding effect [1-7]. Although the femoral com­ ponent in patellofemoral arthroplasty is smaller than in TKA, the mechanical loading, and consequently the stress distribution of the distal femoral bone, is altered compared with the physiological situation. This can lead to bone remodeling, resulting in decreased BMD behind the anterior flange of the femoral component. In TKA, bone loss in the distal anterior femur can lead to supra­ condylar fractures or loosening of the implant, and may induce difficulties during revision arthroplasty [8,9]. * C orresp ond ence : va n jo n b e rg e n@ d z .n l 1 Department of Orthopedic Surgery, Deventer Hospital, Nico Bolkesteinlaan 75, 7416 SE Deventer, The Netherlands Full lis t o f a u th o r in fo rm a tio n is ava ilab le a t th e end o f th e a rtic le Since patellofemoral arthroplasty is typically used in younger patients, conversion to TKA after painful femorotibial osteoarthritis develops will eventually be per­ formed in a relatively large proportion of patients [10]. Although the clinical outcome of TKA done later does not appear to be influenced by prior patellofemoral arthroplasty [11], the results of such a revision may, how­ ever, be compromised by loss of bone stock. To date, no clinical studies have addressed the possible decrease in distal femoral BMD as a param eter of bone remodeling following patellofemoral arthroplasty. We hypothesized that because of the relative small size there is no significant stress-shielding effect behind the femoral component of a patellofemoral prosthesis resulting in a decrease in BMD in the distal femur. The prim ary objec­ tive was, therefore, to determine the change from base­ line in the BMD behind the anterior flange 1 year after © 2010 van J o nbergen e t al; licensee B ioM ed C entra l L td . Th is is an O pen Access a rtic le d is tr ib u te d u nd e r th e te rm s o f th e C rea tive Com B io M e d Central m ons A ttr ib u t io n L icense (h ttp ://c re a tiv e c o m m o n s .o rg /lic e n s e s /b y /2 .0 ), w h ic h p e rm its u n res tric te d use, d is tr ib u tio n , and re p ro d u c ­ t io n in a ny m e d ium , p ro v id e d th e o rig in a l w o rk is p ro p e rly c ited. van Jonbergen et al. BMC Musculoskeletal Disorders 2010, 11:74 http://www.biomedcentral.eom/1471-2474/11/74 Page 2 of 5 patellofemoral arthroplasty using dual-energy X-ray absorptiometry (DXA). Methods In 2007, we initiated a prospective study to investigate the distal femoral BMD using DXA in patients undergoing patellofemoral arthroplasty. All patients who were planned for patellofemoral arthroplasty for isolated patel­ lofemoral osteoarthritis at Deventer Hospital, Deventer, The Netherlands, were evaluated for inclusion in the study. Patients with known rheumatic, renal, hepatic, or gastrointestinal disease, and patients using medication that interferes with mineral metabolism (i.e. treatm ent for osteoporosis or long-term steroid therapy) were excluded from the study. Additionally, patients with a previous TKA or patellofemoral arthroplasty of the con­ tra-lateral knee were excluded. The study was approved by the Regional Ethics Committee (NL16145.075.07, December 2007) and Institutional Review Board. Sample size was calculated using estimates of mean femoral BMD and standard deviation (SD) behind the anterior flange after TKA [1]. The reported mean BMD behind the anterior flange of a total knee prosthesis in the replaced knee was 0.94 g/cm2 (0.31), and 1.25 g/cm 2 (0.30) in the contra-lateral, non-replaced knee [1]. A group sample size of 13 patients achieves 95% power to detect a difference of 0.31 g/cm2 between the null hypothesis that both group means are 1.25 g/cm 2, and the alternative hypothesis that the mean of group 2 (replaced knee) is 0.94 g/cm 2 with known group SDs of 0.31 g/cm 2 and 0.30 g/cm 2 and with a significance level (alpha) of 0.05 using a two-tailed paired t-test (PASS 2008, NCSS software, Kaysville, Utah). Between December 2007 and December 2008, 2 ortho­ pedic surgeons who performed patellofemoral arthro­ plasty at Deventer Hospital recruited 14 patients. All patients provided w ritten informed consent. All eligible patients were preoperatively assessed by 1 of the 2 partic­ ipating orthopedic surgeons, who completed the Knee Society Knee Score (KSKS) and the Knee Society Func­ tional Score (KSFS). The Dutch version of the W estern Ontario and M cM aster Universities Osteoarthritis Index 3.1 (WOMAC) was completed by all patients. M easure­ m ent of the BMD in the distal femur was performed using DXA in the lateral view (GE Lunar Prodigy system, General Electrics, Oldelft Benelux B.V., Delft/ Veenendaal, The Netherlands) 2 weeks before patell­ ofemoral arthroplasty and 12 m onths after arthroplasty. M easurements of a calibration phantom were performed each day before scanning the patients. All measurements were made by an independent radiographic technician. Both the scanning procedure and positioning of the patients and knees were standardized, with the patient in the lateral decubitus position and the knee flexed 15-30 degrees to obtain a true lateral scan. Two regions of inter­ est (ROI) were selected; one in the distal anterior area just behind the anterior flange of the prosthesis (centered between the tip of the fixation peg and the proximal end of the prosthesis) (ROI 1), and the other more proximally, in the supracondylar area 1 cm superior to the anterior flange of the femoral component (ROI 2) (Figure 1). ROI 2 was selected as a reference ROI above the prosthesis, where stress-shielding was assumed to be negligible. The measured area of each ROI was 1 x 1 cm. The contra-lat­ eral, non-operated knee was used as a control, with BMD measurements in identical ROIs. We employed knee-spe­ cific software in all cases. Two similarly experienced surgeons at our institution performed patellofemoral arthroplasty with the currently commercially available Richards type II prosthesis (Smith & Nephew Inc., Memphis, Tennessee). Surgery was per­ formed under pneumatic tourniquet control and antibiFigure 1 Location of regions of interest (ROI) on a lateral radio­ graph of a right knee. van Jonbergen et al. BMC Musculoskeletal Disorders 2010, 11:74 http://www.biomedcentral.com/1471-2474/11/74 Page S of S otic prophylaxis using intravenous Cefazoline 1 g, 3 times daily, for the first 24 hours with the first dose adminis­ tered 30 minutes before application of the tourniquet. All operations were performed in an identical manner according to the manufacturers' instruction, as described elsewhere [10]. No intramedullary guiding rod was used during surgery. All 14 patients received the same postop­ erative treatm ent. We allowed patients protected weight bearing with crutches immediate after surgery, and full unrestricted weight bearing was allowed 6 weeks after surgery. All patients routinely received antithrombotic prophylaxis with a low-molecular-weight heparin (Frag­ min) for 6 weeks. All patients had regular clinical follow-ups at 2 and 8 weeks to evaluate wound healing and rehabilitation, DXA was not performed at these follow-up visits. At the 1-year follow-up, patients were clinically assessed using the KSKS and KSFS, and were asked to complete the W OMAC questionnaire. During follow-up, the radiologi­ cal examinations consisted of 2 radiographs (anteroposte­ rior standing and lateral non-weight bearing) performed 6 weeks and one year post surgery (Figure 2). Radiological findings were reported using the Knee Society total knee arthroplasty roentgenographic evaluation and scoring system [12]. All pertinent data were entered in a spreadsheet pro­ gram and analyzed using PASW Statistics 18 software (SPSS Inc, Chicago, Illinois). We performed descriptive analysis using the mean and standard deviation for con­ tinuous variables, and frequencies for categorical vari­ ables. The 95% confidence intervals (CI) were calculated for the absolute changes in BMD from baseline. The two­ tailed paired t-test was used to analyze for differences in preoperative and postoperative BMD. A linear regression model was used to evaluate for influence of BMI, age, and sex on change in BMD from baseline. A p-value of less than 0.05 was considered significant in all the tests. Results Between December 2007 and December 2008, 14 patients had unilateral patellofemoral arthroplasty, receiving the Richards type II patellofemoral prosthesis. All 14 patients were available for the one year follow-up. The patient's demographic data are presented in Table 1. Mean KSKS improved from 61 (range, 50 to 78) preoperatively to 88 (range, 60 to 100) one year after surgery (p < 0.001). Mean KSFS improved from 65 (range, 50 to 80) preoperatively to 86 (range, 50 to 100) one year after sur­ gery (p = 0.004). The mean preoperative W OM AC scores improved from 50 (range, 22 to 69) to 23 (range, 4 to 39) one year after surgery (p < 0.001). No complications were noted from the surgical procedure within the study period. The 1-year radiographic follow-up showed that all prostheses were in good alignment without radiolucent lines or osteolysis. Results of the BMD measurements are summarized in Table 2. In the operated knees, there was a 15% decrease in mean BMD at 12 months in ROI 1 (p = 0.01), and an 8% decrease in ROI 2 (p = 0.13). In the non-operated knees, there was a 2% increase in the mean BMD at 12 months in both ROI 1 and ROI 2 (p = 0.83 and p = 0.76, respectively). Regression analysis of the change from baseline BMD for both regions of interest against BMI (regression coef­ ficient = 0.017, p = 0.3), age (regression coefficient = 0.002, p = 0.8), and sex (regression coefficient = -0.063, p = 0.6) demonstrated no significant relationships. Discussion The results of our prospective, 1-year DXA study dem on­ strate a statistically significant 15% decrease in BMD behind the anterior flange of the femoral component dur­ ing the first year after patellofemoral arthroplasty. To our knowledge, there has been no previous study that Table 1: Patient characteristics.