Incresing Age Knee

The effect of obesity and increasing age on operative time and length of stayin primary hip and knee arthroplasty

B. Bradley, S. Griffiths, K. Stewart, G. Higgins, M. Hockings, D. Isaac

PII: S0883-5403(14)00399-4DOI: doi: 10.1016/j.arth.2014.06.002Reference: YARTH 54027

To appear in: Journal of Arthroplasty

Received date: 10 April 2014Revised date: 21 May 2014Accepted date: 3 June 2014

Please cite this article as: Bradley B, Griths S, Stewart K, Higgins G, Hockings M, IsaacD, The eect of obesity and increasing age on operative time and length of stay in primaryhip and knee arthroplasty, Journal of Arthroplasty (2014), doi: 10.1016/j.arth.2014.06.002

This is a PDF le of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its nal form. Please note that during the production processerrors may be discovered which could aect the content, and all legal disclaimers thatapply to the journal pertain.

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THE EFFECT OF OBESITY AND INCREASING AGE ON

OPERATIVE TIME AND LENGTH OF STAY IN PRIMARY HIP

AND KNEE ARTHROPLASTY

B. Bradley*, S. Griffiths, K. Stewart, G. Higgins, M. Hockings, D. Isaac

Dept. Trauma Orthopaedics

Torbay Hospital

South Devon Hospitals NHS Trust

Devon

UK

*Corresponding author:

Mr Ben Bradley

Specialist Registrar in Trauma Orthopaedics

4 Hicks Close

Probus

Truro

Cornwall

TR2 4NE

[email protected]

Mobile: +44 (0)7967 196025

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ABSTRACT

We retrospectively reviewed 589 patients undergoing lower-limb arthroplasty

surgery, recording age, Body Mass Index (BMI) and co-morbidities. The effect of

these on operative duration and length of stay (LOS) was analysed. For a 1 point

increase in BMI we expect LOS to increase by a factor of 2.9% and mean theatre time

to increase by 1.46 minutes. For a l-year increase in age, we expect LOS to increase

by a factor of 1.2%. We have calculated the extra financial costs associated. The

current reimbursement system underestimates the financial impact of BMI and age.

The results have been used to produce a chart that allows prediction of LOS following

lower limb arthroplasty based on BMI and age. This data is of use in planning

operating lists.

INTRODUCTION

As a population the UK is becoming older and increasingly obese. Data released in

2012 revealed that 26.2% of the UK adult population is obese (i.e. Body Mass Index

(BMI) >30 Kg/M2) compared to 13.2% in 1993

1. The Office for National Statistics

has demonstrated both increases in the median age and the proportion of older people

in the UK population2.

It is well recognized that obesity is a risk factor for developing lower limb

osteoarthritis.3,4

While obesity leads to a future increased risk of both hip and knee

arthroplasty5, knees appears more susceptible to degenerative disease in the obese

patient6 with each unit of age-adjusted BMI associated with a 4% increase in knee

osteoarthritis7 and an odds ratio of developing osteoarthritis of 9.3 with a BMI in

excess of 30 Kg/M2 (4)

.

The National Joint Registry (NJR)8 supports this observation, demonstrating that the

number of elective primary hip and knee replacement procedures performed in obese

patients (BMI >30Kg/M2) is increasing. 26% of patients undergoing primary hip

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arthroplasty in 2011 were obese compared with 21% in 2004. 32% of patients that

received a total knee replacement (TKR) in 2011 were obese compared with 28% in

20048.

The impact of obesity and increasing age on the outcome of joint replacement surgery

remains controversial. The current difficult financial climate has led to an increasing

drive towards reducing costs within the UK National Health Service (NHS) and, in a

future where we are facing the use of Patient Reported Outcome Measures (PROMs)

data to reimburse Trusts on a Payment by results Basis (PbR), it is important that the

full impact of obesity and age on arthroplasty surgery is realised.

Perceptions of poorer outcomes and increased financial costs associated with joint

replacement surgery in the obese patient has already lead to some Trusts within the

UK and abroad rationing hip and knee arthroplasty surgery, barring access to patients

with a BMI over 30 Kg/M2 (9, 6)

. Despite increased complication rates, poorer implant

survivorship and potentially worse long-term functional outcomes, obese patients do

benefit from arthroplasty surgery10,11,12

It is therefore difficult to justify withholding

this surgery based on BMI alone. Performing hip and knee arthroplasty surgery in

obese patients is associated with increased costs13,14

and it is important that Trusts

understand the reason for these increased costs, address any modifiable factors and

adjust tariffs accordingly in order to allow appropriate levels of financial

remuneration.

In line with the ageing UK population the number of primary joint arthroplasties

performed in the elderly patient group is set to continue to increase and the financial

implication of this needs to be fully appreciated. In addition to this escalating

requirement for surgery, advanced patient age has been shown to be associated with a

slower post-operative rehabilitation15

and an increased length of stay 16,17,18

although

the extent of this appears variable.

In our orthopaedic unit there is a perception that operative time in hip and knee

replacement surgery takes longer in obese patients and these patients take longer to

recover resulting in an increased length of stay (LOS). There is good evidence that

operative time is longer in morbidly obese patients; however, the extent of the time

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difference is controversial19,20

. It was our aim to quantify this in our unit to allow

appropriate planning of operative lists, preventing list over-runs and patient

cancellations. The evidence surrounding LOS in obese patients following arthroplasty

surgery is less clear with some studies demonstrating significantly increased stays21,22

while others report no difference23,24

.

We therefore aimed to assess the relationship between both operative duration and

LOS for obese patients in our unit. The effect of increasing age on LOS was also

investigated with the aim being to use both BMI and age to pre-operatively predict

LOS in our Trust.

METHODS

We retrospectively reviewed all primary hip and knee arthroplasties performed in our

unit in a one year period (2010). Our unit forms part of a UK training hospital

managing a trauma and elective workload. Patients were identified through our

database (CSC Galaxy Surgery), and the case notes of each patient were reviewed.

The operation notes for each procedure were reviewed and all complex primary

arthroplasty procedures were identified and excluded from this study. For total hip

replacements (THR) complex primaries were deemed to be procedures that required

acetabular bone grafting or augments or where intra-operative fractures occurred

requiring fixation. For total knee replacements (TKR) complex primary procedures

were cases involving the use of augments or more constrained prostheses and the

occurrence of intra-operative fractures. The grade of surgeon and the level of

supervision of junior surgeons were recorded. Procedures performed by all grades of

surgeon were included in this analysis.

Each patients age was recorded, and their pre-operative BMI and co-morbidities were

identified from the anaesthetic assessment. Specific co-morbidity indexes or the ASA

grade had not been routinely documented at anaesthetic assessment and therefore the

individual co-morbidities for each patient were recorded. The case notes also revealed

the date of discharge and therefore the LOS for that patient.

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Our theatre administration system (CSC Galaxy Surgery) was interrogated in order to

reveal the theatre time (not including anaesthetic time) for each procedure. This time

includes the time required to transfer the patient from the anaesthetic room, position

and drape the patient, perform the preoperative and postoperative checks, and move

the patient from theatres at the end of the case.

To assess the impact of BMI on theatre time, we fitted a linear regression model on

theatre time, with effects for BMI and type of joint replacement (hip or knee). To

assess the impact of BMI on length of stay, we fitted a linear regression model on log-

transformed (base 10) length of stay, with effects for BMI, patient age, type of joint

replacement and patient co-morbidities. Analyses were carried out using the software

package R25

RESULTS

589 consecutive primary hip and knee arthroplasties were reviewed. This included

305 THRs and 284 TKRs. 53 THRs and 30 TKRs were excluded following review of

the operation note, as they were considered to be complex primary joint replacements

as defined above.

The total number of hip arthroplasties analysed was 252. The number of knee

arthroplasties analysed was 254. The patient cohort characteristics are shown in Table

1.

TABLE 1: Patient cohort characteristics

Total

identified

Number

excluded

Involved in

study

Average

BMI

Average Age

THR 305 53 252 29.7 71.9

TKR 284 30 254 30.3 71.5

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BMI

16-18.5 18.5-

25.5

25.5-

30.5

30.5-

35.5

35.5-

40.5

40.5-

50.5

>50.5

THR 5 57 96 57 20 9 8

TKR 0 51 86 77 31 9 0

Age

90.5

THR 6 7 16 28 42 61 49 27 13 3

TKR 3 5 21 43 47 52 40 28 14 1

The procedures were performed by 15 different primary surgeons. This cohort

comprised 8 Consultant Orthopaedic Surgeons, 2 Associate Specialists and 5

Specialist Registrars operating under Consultant supervision.

Linear regression of theatre time showed a statistically significant change in mean

theatre time with BMI and type of joint replacement. For a patient with an

approximately average BMI of 30 kg/m2, the expected theatre time is 122 minutes for

a THR and 110 minutes for a TKR. A 5-point increase in BMI is expected to increase

theatre time by approximately 7 minutes (p

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TABLE 2: expected theatre time (minutes) with 95% prediction intervals, for sub-

groups of BMI and type of joint replacement. Predictions are estimated at the mid-

point of each group, except for the >40 BMI group, where the prediction is estimated

at 50 kg/m2.

Total hip replacements

BMI Time LPL UPL

16-18.5 103.3913 36.24559 170.5369

18.5-25 109.9405 43.01478 176.8661

25-30 117.5812 50.77821 184.3841

30-35 124.8581 58.03716 191.6789

35-40 132.1349 65.16472 199.1051

>40 151.0548 83.08978 219.0198

LPL lower prediction limit, UPL upper prediction limit

Total knee replacements

BMI Time LPL UPL

16-18.5 91.12897 23.89519 158.3628

18.5-25 97.67816 30.69971 164.6566

25-30 105.31889 38.50471 172.1331

30-35 112.59576 45.80337 179.3882

35-40 119.87264 52.97049 186.7748

>40 138.79252 70.99619 206.5889

Graph 1 demonstrates the relationship between BMI and operative time for both total

hip replacements and total knee replacements. Error bars indicate 1 SEM.

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TABLE 3: expected length of stay (days) with 95% prediction intervals, for sub-

groups of BMI and type of joint replacement for a 70-year old patient with no co-

morbidities. Predictions are estimated at the mid-point of each group, except for the

>40 BMI group, where the prediction is estimated at 50 kg/m2

Total hip replacements

BMI LOS LPL UPL

16-18.5 2.970315 1.463407 6.028925

18.5-25 3.428014 1.693230 6.940155

25-30 3.956241 1.956394 8.000354

30-35 4.565863 2.257267 9.235554

35-40 5.269423 2.600737 10.676520

>40 6.627463 3.252783 13.503288

Total knee replacements

BMI LOS LPL UPL

16-18.5 2.817077 1.386257 5.724712

18.5-25 3.251164 1.604664 6.587087

25-30 3.752139 1.854880 7.590006

30-35 4.330311 2.141087 8.757980

35-40 4.997574 2.467966 10.119972

>40 6.285553 3.088875 12.790476

40

60

80

100

120

140

160

180

200

16-18.5 18.5-25 25-30 30-35 35-40 >40

the

atr

e t

ime

(m

inu

tes)

BMI

Hips

Knees

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The results of this study have been incorporated into separate charts for total hip and

total knee replacements which incorporate both BMI and age in order to predict the

length of stay for that patient (Chart 1).

DISCUSSION

The effect of obesity and increasing age in hip and knee replacement surgery has been

extensively investigated and remains controversial.

Hip and knee arthroplasty surgery in the obese patient may be associated with

increased complications. Significantly higher rates of deep infection7,26

and superficial

wound infections10

have been reported. Long term implant survivorship is reduced

with higher rates of implant failure27,28

and revision surgery29

attributed to increased

wear rates with greater load application,30

and increased rates of aseptic loosening31

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and dislocation10

in obese THR patients. Other studies have not demonstrated a link

between complication rates and obesity, and the topic remains controversial.32,33

The overall success of joint arthroplasty surgery in improving function and quality of

life is well reported34

and, despite increased complication rates, obese patients do

benefit from arthroplasty surgery4,35

showing a significant improvement in both

condition-specific and generic well-being scores10,11,12

However, it is not clear

whether this improvement is equivalent to the non-obese population.12

While some

studies report no difference in post-operative pain, condition-specific and generic

well-being scores between the obese and non-obese patient33,35,19

others show

significant differences in Knee Society and Harris Hip Scores32,11

. It does appear,

however, that obesity does not affect improvements in post-operative quality of life

scores10

.

Increasing age has been shown to result in a significantly increased LOS following

primary hip and knee replacement surgery16,17,18

. This has been attributed to poorer

rehabilitation potential36

leading to slower rehabilitation times15

and increased co-

morbidities37

resulting in an increased risk of post-operative complications 38

.

This study has demonstrated that operative time and post-operative LOS increase

significantly with increasing BMI in primary hip and knee arthroplasty surgery.

While other studies have reported increased operative times19,20

and length of stays in

the obese and morbidly obese patient groups21,22

we have been able to quantify this as

a linear relationship with each point increase in BMI resulting in an increased theatre

time of 1.46 minutes (p

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Our study is a retrospective analysis of 506 consecutive patients undergoing simple

primary hip and knee arthroplasty and is potentially limited by the number and

varying grade of surgeons involved. We have analysed hip and knee replacements

performed by consultant surgeons, associate specialists and registrars under consultant

supervision. We would expect there to be differences in surgical time between

individual surgeons and between different grades of surgeons. However, we are

confident that our methods are valid as the same cohort of surgeons are operating on

the obese and non-obese patient groups.

The results of this study potentially have important financial implications for the

NHS. It is well recognised that arthroplasty surgery within the United States in obese

patients is associated with increased use of hospital resources and therefore higher

financial costs13,14

. No study has accurately identified the cost of performing primary

hip and knee replacements in patients with high BMI in the NHS.

Reports from the United States quantify the increased cost of THR as $299 in the

obese group and $1179 in the morbidly obese group13

. The cost TKR is increased by

$256 and $821 in the obese and morbidly obese groups respectively13

. These

increased costs are attributed to higher operating costs24

and slower patient

rehabilitation21,22

In our institution the cost of theatre time is estimated at 6.95 per minute. We

demonstrate that an increase in BMI of 1 point would result in an increased cost of

10.15 in terms of theatre time alone. For an increase in BMI of 5 points the increased

cost associating with increased surgical time would be 50.75; for a 10 point BMI

increase this would be 101.50; a 20 point increase leading to an increased cost of

203.

The increased surgical time associated with performing hip and knee arthroplasty in

patients with high BMI does not truly reflect the increased demands on theatre time

presented by these patients. These patients also present an anaesthetic challenge with

procedures often being technically more difficult and lengthy. Our data does not

include this additional anaesthetic time. Transferring and positioning processes are

also more difficult resulting in increased time utilisation.

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We believe this information is of use in planning operative lists. Patients with a high

BMI can be allocated additional theatre time directly related to their actual BMI,

reducing list overruns and patient cancellations as a result of unplanned prolonged

theatre times.

In terms of LOS we demonstrate that a 1 point increase in BMI results in the LOS

increasing by a factor of 2.9% (p30Kg/M2) is a recognised OPCS code. Patients undergoing

primary THR with obesity as their only comorbidity generate the HRG code HB11B .

The 2013/2014 tariff for this procedure is 6211. For a non-obese patient without co-

morbidities (HB11C) the tariff is 5906.

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In terms of primary TKR isolated obesity generates a HRG code of HB212B with a

tariff of 6405 as opposed to 5707 in the non-obese patient without other co-

morbidites.

The PbR system therefore reimburses local healthcare Trusts an extra 305 for a

patient with isolated obesity undergoing THR and 698 for an obese patient with no

other co-morbidities undergoing TKR.

The results of this study have suggested that a 1 point increase in BMI will present an

increased financial cost in the region of 20.05 for both primary THR and TKR in

terms of bed occupancy and increased surgical time alone. The extra anaesthetic and

logistical challenges presented by these patients are likely to further increase theatre

time utilisation in addition to the increased utilisation of resources associated with a

prolonged admission are likely to further increase financial impact. We have not

attempted to calculate the additional costs associated with treating the higher

complication rate in obese patients reported elsewhere. Our study would therefore

appear to underestimate the financial costs, but clearly it is important to ensure that

OPCS coding is undertaken correctly to take advantage of the additional remuneration

available to at least partly offset the cost of treating the obese.

This study demonstrates that the relationship between obesity and both increased

theatre time and increased LOS is linear and therefore with increasing levels of

obesity local healthcare trusts face spiralling financial costs. The current HRG tariff

system for THR and TKR does not reflect this with a standard extra payment awarded

when the BMI exceeds 30 Kg/M2. We feel that this simplistic system fails to

adequately reimburse Trusts. The increased resource utilisation by overweight

patients (i.e. BMI 25-30) is not recognised and the increased costs associated with

performing this surgery in the morbidly obese and heavier groups is underestimated

by this system.

The current PbR system also fails to recognise the increased length of stay presented

by the elderly population as highlighted in this study potentially leading to inadequate

levels of financial reimbursement to Local Healthcare Trusts.

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CONCLUSIONS

We have demonstrated that there is a relationship between obesity and operative time,

and patient age and length of admission. This data will be of relevance when planning

theatre lists and admission durations. The financial cost of operating on obese patients

can be offset by additional tariffs available, but any financial benefits may be negated

by costs incurred through the management of the increased complications reported in

this patient group. There is no additional tariff available related to patient age.

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