An assessment of the POSSUM system in orthopaedic surgery
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An assessment of the POSSUM system in
orthopaedic surgery
K. Mohamed, G. P. Copeland, D. A. Boot, H. C. Casserley,
I. M. Shackleford, P. G. Sherry, G. J. Stewart
From Warrington Hospital, England
1,8
e describe the development and validation of a applicable scoring methods so far devised. Both are been
W scoring system for auditing orthopaedic surgery.
It is a minor modification of the POSSUM scoring
found to be equally applicable to subspecialties including
vascular surgery, surgical gastroenterology and urology and
9-11
system widely used in general surgery. The are used by many health-care organisations.
orthopaedic POSSUM system which we have A number of orthopaedic studies have drawn attention to
developed gives predictions for mortality and the variability in outcome after surgery, in particular for
morbidity which correlate well with the observed rates fracture of the neck of the femur, and it has been argued
in a sample of 2326 orthopaedic operations over a that this is more likely to be related to the case mix than to
12,13
period of 12 months. the hospital facilities or the surgeons. A recent report
J Bone Joint Surg [Br] 2002;84-B:735-9.
has cautioned that predicting the outcome of orthopaedic
Received 11 July 2001; Accepted after revision 17 December 2001 intervention in the elderly injured patient, merely on the
basis of the injury severity score, can be hazardous since
14
host factors appear to be of greater importance.
The assessment of outcome after surgical intervention is A system which is weighted towards physiological status
not a new science. As early as 1750 BC King Hammurabi would appear to be of potential benefit in assessing the
of Babylon issued a number of decrees relating to surgeons outcome after orthopaedic surgery. Such a system would
and their surgery. The most infamous of these codices was allow comparison based on the patient’s physiological sta-
that if a surgeon operated on a free man and the patient tus and an assessment of the magnitude of surgery and its
became blind or worse still died, the surgeon should have timing.
his operating hand cut off. While, to some a modification of Between 1996 and 1998 we used methods similar to
this codex may still seem to be in operation, many have those which we have described previously in the develop-
1
attempted to devise more reliable and robust methods for ment of the POSSUM system, in order to design a severity
1-3
assessing the outcome of surgical intervention. score which would allow the logistic regression equation
The use of raw data on mortality and morbidity from used in general surgery to be applied to orthopaedics. We
different units produces disparities in outcome which may assessed 22 surgical severity factors which were later
be explained by variations in the case mix and in tech- reduced by multivariate analysis to the minimum number
4,5
niques of surgery. Methods have been devised to allow necessary to produce an accurate estimate of mortality and
comparison between units by taking the physiological sta- morbidity. The resulting operative severity score was sim-
tus of the patients and the operative complexity into ilar to the system for general surgery, although individual
1,6,7
account. factors and weightings differed. In this study we have now
In general surgery the POSSUM and P-POSSUM sys- attempted to validate the application of this new method.
tems have proved to be the most reliable and widely
Patients and Methods
Every patient admitted to Warrington Hospital over a
K. Mohamed, FRCS, Orthopaedic Registrar
G. P. Copeland, MCh, Consultant General Surgeon period of 12 months on whom orthopaedic surgery had
D. A. Boot, FRCS, Consultant Orthopaedic Surgeon been performed, was assessed using the new orthopaedic
H. C. Casserley, FRCS, Consultant Orthopaedic Surgeon
I. M. Shackleford, FRCS, Consultant Orthopaedic Surgeon POSSUM system. Day-care patients or those treated on an
G. J. Stewart, FRCS, Consultant Orthopaedic Surgeon outpatient basis were excluded.
P. G. Sherry, FRCS, Consultant Orthopaedic Surgeon
Warrington Hospital, Lovely Lane, Warrington, Cheshire WA5 1QG, The system includes a physiological assessment and an
UK. analysis of operative severity (Table I). The former has
Correspondence should be sent to Mr G. P. Copeland. 12 variables each of which is divided into four grades
©2002 British Editorial Society of Bone and Joint Surgery with an exponentially increasing score value (1, 2, 4 and
0301-620X/02/512626 $2.00 8). Almost all the score variables were available for every
VOL. 84-B, NO. 5, JULY 2002 735736 K. MOHAMED, G. P. COPELAND, D. A. BOOT, H. C. CASSERLEY, I. M. SHACKLEFORD, P. G. SHERRY, G. J. STEWART
Table I. Physiological and operative severity assessment in the orthopaedic POSSUM system
Physiological score Operative severity score
1 2 4 8 1 2 4 8
Age (yrs) 71 Magnitude Minor Inter Major Major+
Cardiac signs Normal On cardiac drugs Oedema Raised JVP* Number of operative 1 2 >2
or steroid Warfarin variables within 30 days
Chest radiograph Normal Borderline Cardiomegaly Blood loss per 1000
cardiomegaly operation (ml)
Resp signs Normal SOB† exertion SOB stairs SOB rest Contamination None Incised wound, Minor contamination Gross contamination
i.e. stab or necrotic tissue or necrotic tissue
Chest radiograph Normal Mild COAD‡ Mod COAD Any other Presence of None 10 Node metastases Distant
change malignancy metastases
Systolic BP 110 to 130 131 to 170 >171AN ASSESSMENT OF THE POSSUM SYSTEM IN ORTHOPAEDIC SURGERY 737
Table II. Operative complexity for the operative severity score Table III. The number of patients with complica-
Minor Fasciotomy tions after operation. Some patients had multiple
Ganglion/bursa complications
Tenotomy/tendon repair Complication Number
Arthroscopic surgery Haemorrhage 14
Carpal tunnel/nerve release
Removal of metal Infection
Closed reduction of fracture Chest 59
Urinary 34
Intermediate Excision/osteotomy small bone Wound 73
Minor joint replacement Septicaemia 7
Amputation digit/digits Pyrexia of unknown origin 5
Closed reduction with external fixation Respiratory failure 17
Open reduction of fracture of small bone
Cardiac
Major Osteotomy long bone Hypotension 29
Ligamentous reconstruction + prosthesis Cardiac failure 23
Arthrodesis large joint Myocardial infarction 17
Major joint replacement Arrhythmia 6
Amputation limb Thrombotic
Disc surgery Deep-venous thrombosis 12
Open reduction of fracture of a long bone Pulmonary embolus 9
Major + Radical tumourectomy Cerebrovascular infarction 7
Major spinal reconstruction Limb occlusion 2
Revision prosthetic replacement, major joint Other vascular complications 4
Hindquarter/forequarter amputation Renal failure 15
Urinary retention 32
Other wound problems 10
Prosthetic problems 7
Miscellaneous 26
Table IV. Risk spectra for mortality and morbidity by number of
patients
Risk band (%) Mortality Morbidity Table V. The variability in surgeon workload, and the rates of mortality
90 2 25
Table VI. Comparisons between observed and predicted rates of mortal- ROC curves are illustrated in Figures 1 and 2, which show
ity and morbidity (%) for individual surgeons during the study period.
good correlation across the range.
Mortality Morbidity
Surgeon Observed Predicted Observed Predicted
1 15 15 62 64 Discussion
2 5 6 52 50
3 9 10 41 43 Worldwide public and political interest has been turned on
4 4 4 10 10 the assessment of quality of care and surgical outcome.
5 9 9 41 40
6 9 9 46 47 This is perhaps easier within the surgical specialties since
death after surgery is an obvious adverse outcome. This has
led many non-surgical clinicians to suggest that rates of
mortality are a suitable indicator of surgical prowess. There
rates for mortality and morbidity. When corrected for case is, however, increasing awareness that ‘raw’ data may be
mix using the POSSUM system, however, there is little both erroneous and deceptive, while morbidity is often
difference between the observed rates and those predicted ignored. Death after orthopaedic surgery is rare and usually
by POSSUM (Table VI). follows a number of antecedent complications.
The predictive accuracy of these equations was assessed The overall rates of mortality and morbidity for our
by determining the receiver-operating-characteristic curves period of study appear to be in keeping with previously
(ROC curves) and the classification matrices for different published rates for inpatient surgery in district general
1,6
levels of predicted mortality and morbidity. The resultant hospitals and the risk profile shows similarities to those
VOL. 84-B, NO. 5, JULY 2002738 K. MOHAMED, G. P. COPELAND, D. A. BOOT, H. C. CASSERLEY, I. M. SHACKLEFORD, P. G. SHERRY, G. J. STEWART
1.0 1.0
Sensitivity (true-positive rate)
0.8
Sensitivity (true-positive rate)
0.8
0.6 0.6
0.4 0.4
0.2 0.2
0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0
1- Specificity (false-positive rate) 1- Specificity (false-positive rate)
Fig. 1 Fig. 2
ROC curve for mortality. A curve approaching the linear line
indicates no predictive ability for the assessing system. The
further from the linear line the better is the predictive ability. ROC curve for morbidity.
1,6
of general surgery suggesting that our study group was risk and that the predictions for an individual patient may
comparable with the average orthopaedic case mix for a be of benefit when assessing a patient who has died or
district general hospital. In general surgery, however, there suffered a complication. We have previously drawn atten-
is a tendency to have a greater number of patients whose tion to the benefits of audit in patients who survive, but for
1,4,6,7
risk of death is greater than 70%. whom a predicted risk of death exceeds 50%, since data on
Our study shows a close correlation between the overall these patients are often of more value than those on patients
7 16
observed rates for mortality and morbidity and the predic- who die. Recently, Wilson et al have shown that poten-
tions derived from the POSSUM logistic regression equa- tially significant improvements in overall care are possible
tions. The physiological variables assessed were those by optimisation in this particular group.
shown by logistic regression analysis to be the most impor- Clearly, no regression equation for risk assessment should
tant in predicting mortality and morbidity. Other additional remain static indefinitely. We have found no need for change
variables were not found independently to improve the over the past ten years, but should dramatic changes occur in
predictive ability of the logistic regression equation. We the future the equation can be easily updated without the
have not compared the POSSUM predictions with P-POS- need to alter the score variables. Should this occur, patients
SUM. It has been shown previously that if the correct already scored would be assessed with the present equation
mathematical model is applied there are no significant and new patients with an updated format.
15
differences in the predictive ability of either method. Our study demonstrates that POSSUM can be used as an
There would appear at first sight to be significant differ- audit aid to assess the quality of orthopaedic care. Thus a
ences in outcome between the six surgeons studied. Rates of quality measurement similar to that used in general surgery,
mortality varied between 1.1% and 3.0% and of morbidity namely the ratio of observed adverse events to predicted
between 4.6% and 13.4%. Similar variations have been adverse events, may be used in orthopaedic surgery and be
shown for general surgery and can be explained on the basis more sensitive than simple rates of mortality and
of the case mix and operative complexity. The orthopaedic morbidity.
POSSUM predictions in our analysis suggest that a similar No benefits in any form have been received or will be received from a
explanation may account for apparently marked differences in commercial party related directly or indirectly to the subject of this
article.
surgical outcome in orthopaedic surgery. This is shown by the
close correlation between observed and predicted rates for
both mortality and morbidity for individual surgeons. References
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