This study evaluated whether obese patients who lost weight before their total joint replacement and kept it off post-operatively were at lower risk of surgical site infection (SSI) and re-admission compared with those who remained the same weight.
We reviewed 444 patients who underwent a total hip replacement and 937 with a total knee replacement who lost weight pre-operatively and sustained their weight loss after surgery. After adjustments, patients who lost weight before a total hip replacement and kept it off post-operatively had a 3.77 (95% confidence interval (CI) 1.59 to 8.95) greater likelihood of deep SSIs and those who lost weight before a total knee replacement had a 1.63 (95% CI 1.16 to 2.28) greater likelihood of re-admission compared with the reference group.
These findings raise questions about the safety of weight management before total replacement of the hip and knee joints.
Cite this article: Bone Joint J 2014;96-B:629–35.
Patient characteristics, such as obesity and its related co-morbidities, are important risk factors for osteoarthritis which, in its most severe stages, can lead to the need for total joint replacement (TJR). Obesity is also an indicator of the outcome of TJR.1,2 Survivorship of the implant and post-operative complications such as surgical site infection (SSI), thromboembolic events, and service use after the procedure, are important outcomes by which the success of TJR is evaluated; these can occur more commonly in obese patients.3-5 Obesity may make surgery more difficult for reasons of biomechanics or metabolism.6 The presence of more fat around the joint can increase operating time, which may lead to an increased risk of infection. Morbid obesity has been strongly associated with the risk of SSI after both knee7-12 and hip operations.8,11,13,14 Obesity has also been found to be associated with a higher risk of re-admission after joint replacement and a generally negative impact on the outcomes of these operations.3-5,15
It is currently recommended3-5,15 that obese patients are advised to lose weight before a TJR. Although a few small studies have evaluated how surgically-induced weight loss affects the outcome of a joint replacement, none have assessed whether non-surgically-induced weight loss 16-18 is associated with a change in risk of post-operative complications. Similarly, little is known about whether the post-operative maintenance of pre-operative weight loss is associated with a change in risk of post-operative complications.
We have characterised a group of obese patients who had undergone a primary TJR and lost weight non-surgically before their operation and kept it off post-operatively, and then determined whether those who had lost weight pre-operatively and kept it off post-operatively were at a lower risk of SSI or re-admission than obese patients who remained the same weight over the same period.
Patients and Methods
We conducted a retrospective analysis of patients who underwent a TJR between 1 January, 2008 and 31 December 2010. Two data sources were used.
Firstly, a TJR registry was used to identify a sample of patients who underwent a primary total hip replacement (THR) or total knee replacement (TKR) and any post-operative SSI and 90 day re-admissions. This data collection, structure, and coverage have previously been described.19-21 Briefly, the registry covers an integrated healthcare system membership of over nine million in the USA. Data are collected using both surgeon-reported data and the patients’ electronic health records, administrative and/or claims data and other databases within the institution. It identifies SSIs and re-admissions prospectively. Participation in the registry in 2010 was 90% for THRs and 95% for TKRs.
Subsequently, the institution’s electronic health records (EHR) were used to extract the sample patient’s weight measurements. These are recorded during most patient contact with the healthcare system. Patients were not advised or encouraged, as part of the study, to lose weight and no standard protocol for weight assessment was in effect. Weight data were extracted for the time intervals of 181 to 365 days pre-operatively; 91 to 180 days pre-operatively; 0 to 90 days pre-operatively (considered the intra-operative time); 1 to 90 days post-operatively; 91 to 180 days post-operatively and 180 to 365 days post-operatively. If more than one weight per period was recorded, the median was used.
Patients who were obese (body mass index (BMI) ≥ 30 kg/m2) one year before their primary unilateral THR or TKR for osteoarthritis were identified (n = 15 099). TJR procedures performed in the two largest geographical regions (Southern and Northern California) covered by the registry were included in the sample as information about the patient’s co-morbidities was available. Patients who had undergone surgical weight loss were excluded, as were patients who had missing one year pre- and post-operative weight measurements (n = 1436). Weight change was defined as ≥ 5% change in body weight, according to the Food and Drug Administration’s (FDA) suggestion of a clinically significant change in weight.22 Only obese patients who had lost weight non-surgically before their operation and kept it off post-operatively and those who remained the same weight throughout the study period were included in the sample (n = 9675). Patients of other weights and weight patterns during the period were not included in the analysis (n = 3988).
Pre-operative weight loss sustained or continued post-operatively was the exposure of interest. Patients who remained the same weight in the years before and after TJR acted as the reference group.
The endpoints of this study were SSI and 90 day re-admissions. Deep and superficial SSIs were defined according to the criteria of the Centers for Disease Control and Prevention/National Healthcare Safety Network.23 A 90-day re-admission is defined as any inpatient re-admission for any reason within 90 days of discharge. This was only available for patients registered between 1 January 2009 and 31 December 2010 (n = 6859); therefore the analysis for 90-day re-admission has a smaller denominator.
Patient characteristics (gender, age, race), co-morbidities (defined by the Elixhauser co-morbidity algorithm),24 general health status (defined by the American Society of Anesthesiologists (ASA) score), and intra-operative BMI were evaluated. These covariates were used to characterise the study sample and were investigated as possible confounders of the relationship between weight group, SSI and 90-day re-admission.
Means and standard deviations (sd) were used to describe continuous variables and frequencies and rates were used to describe categorical variables. The cumulative incidence of SSI and 90-day re-admission were calculated. Analyses were conducted separately for TKR and THR due to differences in the samples characteristics, co-morbidities and incidence of outcomes. Binary logistic regression models were used to characterise patients who lost weight pre-operatively and kept it off or continued to lose weight post-operatively and compared with those who remained the same weight. Multivariable models that compared the likelihood of a patient being in one group rather than the other, were created by including all patient characteristics associated with the outcome (p < 0.1) in bivariate models. Binary logistic regression models were created to evaluate whether losing weight pre-operatively and maintaining that weight loss post-operatively was associated with likelihood of SSI (deep or superficial) and re-admission and compared with those who remained the same weight throughout the study period. Finally, binary logistic models included all variables found to be possible confounders (associated with being a patient who lost weight pre-operatively and kept it off post-operatively and the outcome of interest) (using p < 0.1). Odds ratios (OR), 95% confidence intervals (CI) and Wald Chi-squared p-values are reported for all models. Missing data were excluded from analysis. Both groups with complete and those with missing data were compared. Multicollinearity was evaluated and tolerance values threshold levels were set at < 0.1. Data were analysed using SAS (Version 9.2, SAS Institute, Cary, North Carolina) and alpha and p = 0.05 was used as the criterion of statistical significance.
During the study period, 444 obese patients lost weight one year before their THR and either kept it off or continued to lose it post-operatively while 2110 remained the same weight one year pre- and post-operatively (Table I). Patients who lost weight before their THR started the study period (six months to one year pre-operatively) with a mean BMI of 35.8 kg/m2 (sd 4.8) and ended the study period (six months to one year post-operatively) with a mean BMI of 31.8 kg/m2 (sd 4.5) (Fig. 1 [in kg]). Their mean weight loss was 9.2% (sd 4.5) pre-operatively and 1.5% (sd 5.4) post-operatively. Those who did not change their weight throughout the study period started the study with a mean BMI of 34.6 kg/m2 (sd 4.2) and ended with a mean BMI of 34.5 kg/m2 (sd 4.3).
|Total hip replacement||Total knee replacement|
|Pre-op losers who kept off/lost post-op||Stay the same pre- and post-op||Pre-op losers who kept off/lost post-op||Stay the same pre- and post-op|
|n (%)||n (%)||n (%)||n (%)|
|Total n||444 (17.4)||2110 (82.6)||937 (13.2)||6184 (86.8)|
|Gender||Female||289 (65.1)||1109 (52.6)||639 (68.2)||3843 (62.1)|
|Male||155 (34.9)||1001 (47.4)||298 (31.8)||2341 (37.9)|
|Age category, years||< 65||192 (43.2)||994 (47.1)||427 (45.6)||2639 (42.7)|
|≥ 65||252 (56.8)||1116 (52.9)||510 (54.4)||3545 (57.3)|
|Race||Asian||7 (1.6)||35 (1.7)||25 (2.7)||228 (3.7)|
|Black||39 (8.8)||220 (10.4)||107 (11.4)||647 (10.5)|
|Hispanic||37 (8.3)||228 (10.8)||145 (15.5)||1243 (20.1)|
|Other/Multi*||9 (2.0)||42 (2.0)||19 (2.0)||129 (2.1)|
|Unknown||4 (0.9)||26 (1.2)||5 (0.5)||99 (1.6)|
|White||348 (78.4)||1559 (73.9)||636 (67.9)||3838 (62.1)|
|Diabetes||131 (29.5)||559 (26.5)||287 (30.6)||2032 (32.9)|
|ASA Score Category||Unknown||7 (1.6)||41 (1.9)||17 (1.8)||138 (2.2)|
|1& 2||224 (50.5)||1097 (52.0)||466 (49.7)||3181 (51.4)|
|≥ 3||213 (48.0)||972 (46.1)||454 (48.5)||2865 (46.3)|
|Intra-operative BMI, kg/m2||< 30||151 (34.0)||142 (6.7)||222 (23.7)||281 (4.5)|
|≥ 30 and < 35||191 (43.0)||1219 (57.8)||407 (43.4)||3225 (52.2)|
|≥ 35||102 (23.0)||749 (35.5)||308 (32.9)||2678 (43.3)|
|Number of co-morbidities||Unknown||42 (9.5)||257 (12.2)||117 (12.5)||907 (14.7)|
|0||32 (7.2)||154 (7.3)||64 (6.8)||343 (5.6)|
|1||66 (14.9)||398 (18.9)||138 (14.7)||933 (15.1)|
|2||104 (23.4)||502 (23.8)||199 (21.2)||1437 (23.2)|
|3||102 (23.0)||404 (19.2)||207 (22.1)||1221 (19.7)|
|4||46 (10.4)||219 (10.4)||120 (12.8)||750 (12.1)|
|≥ 5||52 (11.7)||176 (8.3)||92 (9.8)||593 (9.6)|
* Other/Multi, includes Native American, mixed race and other
Of obese patients undergoing a TKR, 937 lost weight pre-operatively and either kept it off or lost more post-operatively, while 6184 stayed the same weight during the study period (Table I). Patients who lost weight before their TKR and kept it off or lost more weight post-operatively started the study with a mean BMI of 36.9 kg/m2 (sd 5.4) and ended with a mean BMI of 33.0 kg/m2 (sd 4.8) (Fig. 2 [in kg]). The mean weight loss of these patients was 8.2% (sd 3.7) pre-operatively and 2.0% (sd 5.1) post-operatively. Those who remained the same weight throughout the study period started with a mean BMI of 35.3 kg/m2 (sd 4.5) and ended at 35.2 kg/m2 (sd 4.6).
From the THR multivariable models, men were less likely than women to lose weight and keep it off (OR = 0.59, 95% CI 0.47 to 0.74). Patients with congestive heart failure (CHF) (OR = 1.79, 95% CI 1.10 to 2.92) or a coagulopathy (OR = 2.70, 95% CI 1.30 to 5.56) were more likely to lose weight pre-operatively, and keep it off after the procedure, than those without these conditions.
Of patients undergoing TKR, men were also less likely to lose weight pre-operatively and keep it off post-operatively (OR = 0.75, 95% CI 0.64 to 0.88) than women. Racial differences were found, with Asian patients (OR = 0.60, 95% CI 0.38 to 0.93) and those of Hispanic origin (OR = 0.66, 95% CI 0.54 to 0.81) being less likely to lose weight and keep it off than Caucasians. In addition, in patients undergoing TKR, anaemia of chronic blood loss was associated with a higher likelihood (OR = 2.20, 95% CI 1.52 to 3.19) of pre-operative weight loss and maintenance, while patients with liver disease were less likely to be in the group who lost weight and kept it off (OR = 0.48, 95% CI 0.26 to 0.88) (Table II).
|Gender (male vs female)||0.59||0.47 to 0.74||< 0.001|
|Age (per one year increment)||1.01||0.99 to 1.02||0.400|
|Congestive heart failure||1.79||1.10 to 2.92||0.020|
|Coagulopathy||2.70||1.30 to 5.56||0.008|
|Gender (male vs female)||0.75||0.64 to 0.88||0.001|
|Race: Asian vs White||0.60||0.38 to 0.93||0.024|
|Race: Black vs White||0.85||0.67 to 1.08||0.186|
|Race: Hispanic vs White||0.66||0.54 to 0.81||< 0.001|
|Race: Other/Multi*vs White||0.87||0.52 to 1.47||0.610|
|Chronic blood loss anaemia||2.20||1.52 to 3.19||< 0.001|
|Liver disease||0.48||0.26 to 0.88||0.018|
* Other/Multi, includes Native American, mixed race and other
Patients undergoing THR who lost weight and kept it off had a cumulative incidence of SSIs (deep and superficial combined) of 2.7% (n = 12) and of 8.1% (n = 22) for 90-day re-admission, while patients who remained the same weight throughout the study had an incidence of 1.4% (n = 29) for SSIs and 6.6% (n = 88) for re-admission.
In patients undergoing TKR who lost weight and kept it off, 1.2% (n = 11) had an SSI and 8.1% (n = 45) were re-admitted within 90 days, compared with 0.9% (n = 53) SSIs and 5.2% (n = 197) re-admissions among those who remained the same weight for the entire study period (Table III).
|Total hip replacement||Total knee replacement|
|Pre-operative losers who kept off/lost post-operation||Stay the same pre- and post-operation||Pre-operative losers who kept off/lost post-operation||Stay the same pre- and post-operation|
|n (%)||n (%)||n (%)||n (%)|
|Total (n)||444 (17.4)||2110 (82.6)||937 (13.2)||6184 (86.8)|
|Deep surgical site infection||9 (2.0)||14 (0.7)||8 (0.9)||37 (0.6)|
|Superficial surgical site infection||3 (0.7)||15 (0.7)||3 (0.3)||16 (0.3)|
|Re-admission within 90 days||22 (8.1)*||88 (6.6)*||45 (8.1)†||197 (5.2)†|
* For total hip replacement (THR): n = 1809 (≥ 2009), 206 (11%) missing re-admission data † For total knee replacement (TKR): n = 5050, 706 (14%) missing re-admission data
In patients undergoing THR, after adjusting for intra-operative BMI (the only confounder identified), there was a higher likelihood of deep SSI in patients who lost weight and kept it off post-operatively than in those who remained the same weight (OR = 3.77, 95% CI 1.59 to 8.95). No differences in the likelihood of superficial SSI or re-admission were seen in either the crude or adjusted estimates.
In patients undergoing TKR, after adjusting for age and the co-morbidity of congestive heart failure, there was a greater likelihood of 90-day re-admission in patients who lost weight and kept it off (1.63 (95% (CI 1.16 to 2.28)) compared with those who remained the same weight for the entire study period. No differences in the likelihood of SSIs were seen in either the crude or adjusted estimates (Table IV).
|THR||OR||95% CI||p-value||OR||95% CI||p-value|
|Surgical site infection deep*||3.10||1.33 to 7.20||0.009||3.77||1.59 to 8.95||0.003|
|Surgical site infection superficial†||0.95||0.27 to 3.30||0.936||0.95||0.27 to 3.30||0.936|
|Re-admission within 90 days‡||1.26||0.77 to 2.04||0.360||1.18||0.72 to 1.93||0.502|
|Surgical site infection deep§||1.43||0.66 to 3.08||0.360||1.67||0.77 to 3.61||0.195|
|Surgical site infection superficial*||1.24||0.36 to 4.26||0.734||1.41||0.41 to 4.85||0.589|
|Re-admission within 90 days**||1.60||1.14 to 2.24||0.006||1.63||1.16 to 2.28||0.005|
*Model adjusted for intra-operative BMI †No confounders – model same as bivariate ‡Model adjusted for liver disease co-morbidity §Model adjusted for intra-operative BMI and gender **Model adjusted for age and congestive heart failure co-morbidity
In a cohort of obese patients, gender, age, and race were associated with the ability to lose weight one year prior to joint replacement and keep it off post-operatively. Certain co-morbidities, such as CHF and coagulopathy in patients undergoing a THR and anaemia of chronic blood loss and liver disease in patients having a TKR, were also associated with weight loss. Most importantly, patients undergoing a THR who lost weight pre-operatively and kept it off post-operatively had a higher likelihood of deep SSI than patients who stayed the same weight throughout the study period. Obese patients who lost weight before their TKR and kept it off post-operatively had a higher likelihood of re-admission within 90 days.
Except for two previous studies which used the same data as this study,25,26 the association of patient characteristics with pre-operative weight change has not previously been reported in patients undergoing total joint replacement. However, the patterns found in the current study are consistent with the varying prevalence of obesity by age, gender and racial groups in the USA27 and the likely ability of these groups to change their weight.28
The association of co-morbidities with pre-operative weight loss which is sustained post-operatively should be considered in relation to both its pre- and post-operative presence. In a previous study, we did not find coagulopathy or CHF to be associated with weight change before a THR, which leads us to believe that these conditions are associated with the post-operative change in weight.25 Similarly, we did not find liver disease to be associated with a lower likelihood of weight loss, while in the current study it was associated with the ability of patients undergoing a TKR to lose weight. The anaemia of chronic blood loss had previously been reported as associated with susceptibility to weight loss before TKR, confirming that this weight loss is likely due to anaemia. The difference in patterns of association of these co-morbidities suggests that they could be associated with the post-operative weight of the patient, either due to associated complications or other mechanisms by which they affect the patients’ metabolism.
Four studies have evaluated post-operative outcomes associated with pre-operative weight loss alone, without reference to whether this was maintained post-operatively.16-18,25 One, from the same data source used in this study, evaluated obese patients who underwent non-operative weight loss prior to surgery and did not find any association between a 5% pre-operative weight loss and risk of SSI or re-admissions in patients undergoing total joint replacement.25 Three studies evaluated patients who underwent surgical weight loss interventions and their effect on the outcome of total joint replacement.16-18 These consistently reported higher rates of peri-operative complications regardless of weight loss, when compared with the TJR population from which they were drawn. They do not mention the amount of weight lost by the patients: this may be much higher than the amount we report, which could account for the differences. The non-surgical weight loss study considered any patient with a weight loss of 5% or more to be a ‘successful’ loser of weight but these changes are likely to be minimal, compared with those who lost weight after surgical procedures.
The unexpected results that patients who lose weight pre-operatively and continue to lose it post-operatively have a higher likelihood of deep SSI after THR and re-admission after TKR, could have occurred for several reasons. Firstly, while we tried to compare the study group to an obese control group and adjust for possible co-morbidities, which could influence the risk of complications, it is possible that there was residual confounding from unevaluated characteristics. Secondly, it is possible that post-operative weight loss occurred as a result of the complication rather than the reverse. We reviewed the amount of weight loss in patients in relation to their complications in an attempt to clarify this possibility and found that only after TKR was the weight loss different in patients who were re-admitted. There was a higher rate of post-operative weight loss in patients who were re-admitted, but the amount does not explain the temporal relationship of these events. Thirdly, patients who lost weight might still have been losing weight after their TKR. It is possible that there were unintended consequences of such weight loss, such as malnutrition, which can be associated with a poor outcome after TJR.29 Fourthly, the added trauma of a surgical procedure could have triggered post-operative metabolic stress (catabolic state or hyperglycaemia), thereby increasing a patient’s risk of complications.30,31
The limitations of the study include its non-random sample, the lack of standardisation in the collection of weight data, our inability to discern whether post-operative weight loss was due to complications or the reverse, and our inability to comment on why differences in superficial and deep infection were seen, as well as the differences in THR and TKR results. From our sample of 15 099 obese patients, 1436 (9.5%) had missing measurements of pre- and post-operative weight and were not included in the study. Consequently, we may have excluded healthier patients from our sample. This could potentially bias our estimates, but it does not explain why the potential benefit of weight loss was not seen and why the post-operative risk of the outcomes we evaluated was higher in certain cases. We were also limited in our ability to discern the temporal relationship between the patients’ post-operative weight loss and the complications reported. Additionally, we were unable to assess the different effects of weight loss and its maintenance in patients who underwent THR in relation to the risk of superficial and deep infection. And finally, the differences seen between the THR and TKR cohorts, while likely to be due to the different sample profiles and differences in risk factors for infection12,13,32,33 and re-admission,34,35 have not previously been reported and must be investigated further.
The strengths of this study include the evaluation of a representative cohort of patients undergoing TJR, the prospective determination of the outcomes evaluated, and the integrated system used for data collection. The cohort was from a sample which was socio-demographically comparable to the geographical region it represents.36-38 The TJR registry outcomes used in this study were prospectively monitored and adjudicated. The ability to link all patient records, using a unique identifier (used by the integrated health care system), within such a large sample cannot be reproduced by any of the larger national samples of TJR (Nationwide Inpatient Sample, regional TJR registries, or the American TJR registry) and decreases the possibility of any data handling bias (merging errors, record linkage bias).
In conclusion, while this study provides important information about non-surgical weight loss in obese patients and its implications for the likelihood of SSI and re-admission, it raises questions about the safety of weight management prior to total joint replacement.
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The authors would like to thank all Kaiser Permanente orthopaedic surgeons and the staff of the Department of Surgical Outcomes and Analysis who have contributed to the success of the National Total Joint Replacement Registry. We would also like to thank C. Ake, PhD for his assistance with editing this manuscript.
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
This article was primary edited by A. Ross and first proof edited by D. Rowley.
Supplementary material. A table showing total knee and total hip replacement study sample co-morbidities by weight groups, 2008-2010, is available alongside the electronic version of this article on our website www.bjj.boneandjoint.org.uk