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Effect of patient factors on long-term outcome

All knee arthroplasty patients

Chapter 4: Can short-term post-operative morbidity predict longer-term outcome?

4.1.5 Operation time and long-term outcome

4.3.2.3 Effect of patient factors on long-term outcome

4.3.2.3.1 Patient Age

The effect of patient age on SF-36, WOMAC and Oxford Knee scores is shown in Table 43. The regression coefficients are reported for a 10-year increase in age.

Table 43. Association between patient age and PROMs at 18 months post knee arthroplasty Coefficient (95% CI) P-value SF-36 0.8 (-4.8, 6.7) 0.68 WOMAC -1.9 (-6.4, 2.6) 0.40

Oxford Knee Score -1.0

(-3.4, 1.4)

187 These results show no statistical association between patient age and patient- reported outcome measures at 18 months post-surgery.

4.3.2.3.2 Patient Sex

The effect of sex on SF-36, WOMAC and Oxford Knee scores is shown in Table 44. The coefficient represents the difference in outcome between the two

categories (male and female).

Table 44. Association between sex and PROMs at 18 months post knee arthroplasty

Outcome measure

Sex Mean (SD) Coefficient (95% CI) P-value SF-36 Female 51.7 (19.8) 0 0.77 Male 49.7 (21.5) 2.0 (-14.1, 16.4) WOMAC Female 27.8 (20.0) 0 0.89 Male 28.7 (19.0) 0.9 (-11.3, 13.0) Oxford Knee Score Female 27.7 (12.1) 0 0.68 Male 26.4 (9.2) -1.3 (-7.7, 5.1)

These results show no statistical association between sex and patient-reported outcome measures at 18 months post knee arthroplasty.

4.3.2.3.3 ASA grade

The effect of ASA grade on SF-36, WOMAC and Oxford Knee scores is shown in Table 45. The coefficient represents the difference in outcome between each category and the reference category (ASA grade 1).

188

Table 45. Association between ASA grade and PROMs at 18 months post knee arthroplasty

Outcome ASA grade Mean (SD) Coefficient (95% CI) P-value SF-36 1 54.3 (16.2) 0 0.32 2 51.0 (18.1) 3.3 (-7.9, 14.6) 3 46.6 (13.8) 7.7 (-5.0, 20.9) WOMAC 1 24.3 (11.8) 0 0.42 2 27.4 (20.4) 3.1 (-11.2, 17.4) 3 35.2 (21.3) 10.9 (-6.4, 28.2) Oxford Knee Score 1 23.2 (6.1) 0 0.05 2 25.8 (10.2) 2.6 (-4.6, 9.7) 3 33.4 (10.8) 10.2 (1.5, 18.9)

These results show there is no statistical association between ASA grade and SF-36 and WOMAC scores following knee arthroplasty. There is some

evidence of an association between ASA grade and Oxford Knee Score although this is of borderline statistical significance (p = 0.05). There was a relatively small difference between patients with an ASA grade of 1 and 2. However, ASA grade 3 patients had an average Oxford Knee Score ten units higher than ASA grade 1 patients.

4.3.2.4 Association between length of operation and outcome

The effect of length of operation on SF-36, WOMAC and Oxford Knee scores is shown in Table 46. The regression coefficients are reported for a one-hour

189 increase in length of operation.

Table 46. Association between length of operation and PROMs at 18 months post knee arthroplasty

Coefficient (95% CI) P-value SF-36 2.6 (-7.9, 12.8) 0.65 WOMAC 1.8 (-10.9, 14.4) 0.78

Oxford Knee Score 0.3

(-6.4, 7.0)

0.93

These results show no statistical association between length of knee surgery and PROMs.

4.3.2.5 Multivariable analysis of results

4.3.2.5.1 SF-36

None of the studied variables (POMS scores, length of patient stay, patient factors and length of operation) had any association with SF-36 scores so no multivariable analysis was performed.

4.3.2.5.2 WOMAC

A multivariable analysis was performed using factors showing some association with the WOMAC scores on univariable analysis. These variables included POMS score on POD 3 and 5, and length of stay. Multivariable analysis suggested that only post-operative length of stay was significantly associated with WOMAC scores at 18 months post-surgery. There was no additional effect

190 of the POMS scores on POD 3 and 5 once length of stay had been adjusted for. As length of hospital stay was the only variable in the final model, the size

effects are equivalent to those found in the univariable analysis.

4.3.1.5.3 Oxford Knee Score

A multivariable analysis suggested that only ASA grade was significantly

associated with the Oxford Knee Score at 18 months post-surgery. There were no additional significant variables after accounting for this factor.

4.4 Discussion

4.3.1 Summary

This chapter investigated the association between morbidity on POD 3, 5, 8 and 15 and PROMs (SF-36, WOMAC and Oxford Hip/Knee Scores) at 18 months post surgery. For hip arthroplasty patients, univariable analysis indicated an association between morbidity on POD 15 and WOMAC and Oxford Hip Scores. However, multivariable analysis did not support this. For knee arthroplasty patients, no association was found between post-operative morbidity and PROMs. Since POMS has no association with longer-term PROMs, POMS should not be used as an early surrogate marker of surgical outcome.

The relationship between length of hospital stay and PROMs was also investigated. In the hip arthroplasty group, univariable analysis revealed an

191 association in length of stay and WOMAC and Oxford Hip Scores. In the knee arthroplasty group, univariable analysis revealed an association between length of stay and WOMAC scores. Multivariable analysis confirmed these

associations.

The relationship between patient factors (age, sex, ASA) and PROMs was assessed. There was no association between age and PROMs, or sex and PROMs. Univariable analysis revealed an association between ASA score and WOMAC scores in the hip arthroplasty group. Multivariable analysis confirmed this was not statistically significant with other variables taken into account. Univariable analysis revealed an association between ASA score and Oxford Knee Score in the knee arthroplasty group. Multivariable analysis confirmed this was statistically significant.

The relationship between operating time and PROMs was assessed. In the hip arthroplasty group univariable analysis revealed an association between

operating time and Oxford Hip Scores. Multivariable analysis confirmed this was not statistically significant when other variables were taken into account. In the knee arthroplasty group no association was found between operation length and PROMs.

4.3.2 Strengths of study

This study involved collecting a large data set. A dedicated research team recorded all POMS data, patient data, length of stay and length of operation.

192 The data was accurate and complete. A second dedicated research team collected the patient-reported outcome scores. Again, this ensured that full and accurate data sets were obtained.

4.3.3 Limitations of study

The main limitation of this study is the small sample size. This may have lead to false negative results. If a larger sample size had been used, more

significant associations between post-operative morbidity and long-term patient- reported outcome may have been found. This is an area for future research. A further limitation of the study is the fact it was performed at a single centre. Thus the results may not be transferable to other centres.